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Pathological and functional amyloid formation orchestrated by the secretory pathway
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Pathological and functional amyloid formation orchestrated by the secretory pathway Mary E Huff, William E Balch and Jeffery W Kelly Amyloidogenesis has historically been associated with pathology in a class of neurodegenerative diseases known as amyloid diseases. Recent studies have shown that proteolysis by furin during secretion initiates both variant gelsolin amyloidogenesis, associated with the disease familial amyloidosis of Finnish type, and Pmel17 fiber formation, which is necessary for the functional biogenesis of melanosomes. Proteolysis combined with organelle-dependent environment changes orchestrate amyloidogenesis associated with both pathological processes and a functional pathway. Addresses The Scripps Research Institute, Departments of Chemistry and Cell Biology, The Skaggs Institute for Chemical Biology, 10550 North Torrey Pines Road, BCC265, La Jolla, CA 92037, USA e-mail: [email protected]
Current Opinion in Structural Biology 2003, 13:674–682 This review comes from a themed issue on Proteins Edited by Christian Cambillau and David I Stuart 0959-440X/$ – see front matter ß 2003 Elsevier Ltd. All rights reserved. DOI 10.1016/j.sbi.2003.10.010
Abbreviations AD Alzheimer’s disease ER endoplasmic reticulum FAF familial amyloidosis of Finnish type TGN trans-Golgi network
diseases is characterized by intracellular inclusions. It is not yet clear how misfolding evades the quality control systems in these disorders. Proteins deposited intracellularly include a-synuclein in Parkinson’s disease [4,5], Huntingtin in Huntington’s disease [6,7], Cu/Zn superoxide dismutase in some amyotrophic lateral sclerosis cases (Lou Gehrig’s disease) [8–10] and tau associated with Alzheimer’s disease (AD) [11–13]. Characteristics of amyloid and its associated diseases
Amyloid diseases are late-onset, degenerative disorders characterized by the aberrant aggregation and extracellular deposition of proteins or peptide fragments (Figure 1). The deposits associated with a given disease are generally composed of a single protein. Amyloid plaques contain fibrillar protein aggregates with a crossb sheet quaternary structure that exhibit selective binding towards the fluorophores thioflavin T and Congo red [14]. A subset of these proteins, including lysozyme, transthyretin, b2-microglobulin and some immunoglobulin light chains, misfold and are deposited as fulllength polypeptides [2]. The others are subject to endoproteolysis to form peptide fragments. Typically, one of the fragments subsequently deposits as amyloid, as in the case of cleavage of the amyloid precursor protein into peptides including Ab, which deposits in AD. Amyloid deposits have consistently been found to bind accessory molecules, including serum amyloid P component, proteoglycans/glycosaminoglycans and apolipoprotein E [14]. Determinants of amyloid formation
Introduction Minimizing protein misfolding and aggregation both inside and outside of cells is now appreciated to be critical to maintaining normal physiology. Chaperones provide multiple opportunities for proteins to fold within a cell and, if unsuccessful, these polypeptides are typically degraded by the proteasome. As much as 30% of the newly synthesized protein pool is degraded due to inefficient folding [1]. In recent years, it has become clear that many human diseases result from the misfolding and aggregation of proteins (see [2,3] for reviews). Misfolding in the endoplasmic reticulum (ER) and subsequent proteasome degradation causes loss-of-function disorders, including cystic fibrosis, whereas extracellular protein aggregation, outside the reaches of the chaperone and proteasome systems, leads to more than 30 distinct human amyloid diseases (Table 1). A third class of protein misfolding Current Opinion in Structural Biology 2003, 13:674–682
The fibrillogenic proteins in human amyloid diseases (Table 1) do not share a common structure or function, yet they all ultimately assemble into a cross-b sheet conformation under conditions that make the native state unfavorable (Figure 1) [2,15]. Recent work demonstrates that the ability to form amyloid fibrils is not limited to proteins known to be associated with pathology: several ‘normal’ proteins have been converted into fibrils in vitro (Table 2). The conditions required in some cases appear harsh (Table 2), but comparison with pathologic amyloidogenesis is difficult because the mechanism and location of fibril formation in humans remain unclear. Dobson and co-workers [16] have proposed that the cross-b sheet structure of amyloid fibrils is mainly stabilized by interactions between polypeptide backbones. However, in several cases, it is now clear that sidechains contribute significantly to the specificity of aggregation and to the stability of the aggregate [17,18]. www.current-opinion.com
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Figure 1
Table 1 Amyloid precursor proteins and their associated diseases.
Amyloid b precursor protein
Alzheimer’s disease Hereditary cerebral hemorrhage with amyloidosis, Dutch type Hereditary renal amyloidosis Familial amyloid polyneuropathy, type III Hereditary systemic amyloidosis Isolated atrial amyloid Familial British dementia Familial Danish dementia Medullary thyroid carcinoma Hereditary cerebral hemorrhage with amyloidosis, Icelandic type Hereditary renal amyloidosis Familial amyloidosis, Finnish type Primary systemic amyloidosis Myeloma-associated amyloidosis Primary systemic amyloidosis Myeloma-associated amyloidosis Insulin-related amyloidosis Type II diabetes
Apolipoprotein AI
Apolipoprotein AII Atrial natriuretic protein BRI precursor protein variants Calcitonin Cystatin C Fibrinogen Aa-chain variants Gelsolin Immunoglobulin heavy chain Immunoglobulin light chain Insulin Islet amyloid polypeptide (amylin) Kerato-epithelin Lactoferrin Lysozyme Medin (lactadherin fragment) b2-microglobulin Prolactin Serum amyloid A
Transthyretin
Prion protein
Lattice dystrophies of the cornea Familial corneal amyloidosis Hereditary systemic amyloidosis Hereditary renal amyloidosis Aortic medial amyloidosis Hemodialysis-related amyloidosis Aging pituitary Senile hypophyseal prolactinoma Secondary systemic amyloidosis Familial Mediterranean fever Muckle–Wells syndrome Senile systemic amyloidosis Familial amyloid polyneuropathy, types I and II Familial amyloid cardiomyopathy Kuru Creutzfeldt–Jakob disease Fatal familial insomnia Gerstmann-Stra¨ usslerScheinker disease
Non–native conformation
Native conformation Proteolysis
re St ar ru ra ct ng ur em al en t
Associated diseases
Structural rearrangement
Precursor protein
Fragment
Self assembly
Amyloidogenic intermediate
n Oligomeric intermediate
Amyloid fibrils
Current Opinion in Structural Biology
Mechanisms of amyloid formation. Slight shifts away from a protein’s native conformation induced by mutation or by changes in environment allow structural rearrangement to form an amyloidogenic assembly-competent intermediate from a full-length precursor protein. Alternatively, proteolysis of the native structure or a perturbed conformation can generate a polypeptide fragment that rearranges into the amyloidogenic intermediate. The amyloidogenic intermediates undergo self-assembly into soluble oligomers that mature into the fibrils deposited in plaques. This scheme depicts unseeded polymerization in amyloidogenesis; however, a seeded pathway also exists in which the oligomeric intermediate recruits the non-native full-length protein or proteolytic fragment through induction of the amyloidogenic conformation.
Table 2 Examples of full-length proteins that have been shown to be amyloidogenic in vitro, but are not currently associated with a human disease. Amyloidogenic protein
Protein source
Conditions
References
Myoglobin Acylphosphatase Fibronectin type III module Procarboxypeptidase A2 activation domain WW domain of FBP28 SH3 domain of phosphatidylinositol 3-kinase Type I antifreeze protein Monellin B1 IgG-binding domain of protein G HypF Methionine aminopeptidase Cytochrome c
Human Human Human Human Human Bovine Fish Plant Bacteria Bacteria Bacteria Bacteria
pH 9, 658C pH 5.5, 25% TFE pH 7.4 pH 3, thermal or chemical denaturation pH 7, 378C pH 2 Freeze/thaw pH 2.5, 858C, 100 mM NaCl Agitation; changes in pH, ionic strength or temperature pH 5.5, 30% TFE pH 3, GdmCl Removal of cofactor, extended incubation
[16] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99]
GdmCl, guanidinium chloride; TFE, trifluoroethanol.
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Current Opinion in Structural Biology 2003, 13:674–682
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What is the toxic species in amyloid fibril formation?
Amyloidogenesis can result either from partial denaturation of a secreted native protein or from the generation of aggregation-prone fragments through proteolysis of a precursor protein during or after secretion (Figure 1). Although protein aggregates, including fibrils, are invariably observed in amyloid diseases, the nature of the misfolded species leading to the gain-of-toxic-function phenotype characterizing the amyloidoses remains unclear. Krafft, Glabe, Lansbury, Teplow, Selkoe [19–24] and others have demonstrated the presence of, and in some cases the inherent toxicity of, small soluble oligomers of the Ab peptides associated with AD. Saraiva and colleagues [25] have similarly identified nonfibrillar transthyretin aggregates in nerve biopsies of individuals showing early signs of peripheral neuropathy in familial amyloid polyneuropathy. Additionally, soluble oligomers of amyloid-forming nonpathogenic proteins such as HypF and the SH3 domain (Table 2) have also revealed significant toxicity in cell culture [26]. An antibody specific to soluble Ab oligomers also recognizes the soluble oligomers of other amyloidogenic proteins and inhibits their cytotoxicity in cell culture, suggesting that the soluble oligomeric conformation may be a common link in neurodegenerative disease pathology [27]. An active immunization strategy employing soluble oligomers as the antigen is very effective in preventing Ab deposition in AD transgenic animals, although the active immunization trial in humans was halted because of severe inflammation in a few subjects [28,29]. Rethinking the role of amyloid
The realization that low molecular weight oligomers of amyloidogenic peptides and proteins are more toxic than fibrils to neuronal cultures and related cell lines calls into question the role that mature amyloid fibrils and plaques play in pathology. Although it is generally accepted that tissue displacement by amyloid contributes to late-stage pathology in many amyloidoses, fibril formation at an early stage of the disease could offer protection against the toxic oligomeric intermediates [21,30]. Numerous laboratories have demonstrated the significant stability of the amyloid cross-b sheet structure. Furthermore, amyloid fibril formation by many sequences is spontaneous under conditions found in a few organelles of eukaryotes. Hence, it would be surprising if the amyloid quaternary structure were not used for certain functional purposes. Preliminary observations discussed later in this review support the existence of functional intracellular eukaryotic amyloid. Interestingly, the proposed pathway for native amyloid formation in eukaryotes is nearly identical to the recently discovered pathway for pathogenic amyloid formation in the gelsolin and BRI familial amyloid diseases, to be discussed first in this review [31,32]. Anfinsen taught us that both sequence and environCurrent Opinion in Structural Biology 2003, 13:674–682
ment control conformational preferences. Therefore, it is not surprising that proteolysis is used to alter the sequence, which, in concert with organelle sequestration to alter the environment, facilitates the formation of functional amyloid in membrane-delimited compartments.
Gelsolin in familial amyloidosis of Finnish type Gelsolin amyloidosis
The amyloid deposits associated with familial amyloidosis of Finnish type (FAF) are composed of a proteolytic fragment(s) of one point mutant of the actin-modulating protein gelsolin. FAF is caused by a guanine to adenine or thymine substitution at nucleotide 654 (a mutational hotspot), which results in mutation of the aspartic acid residue at position 187 to asparagine (D187N) or tyrosine (D187Y), respectively. This genetic variation results in an autosomal dominant neurodegenerative disease wherein individuals present with ophthalmologic, dermatologic and neurologic symptoms in the third or fourth decade of life [33]. FAF amyloid deposits are mainly composed of a 71-residue fragment (173–243) of variant gelsolin, although a less common 53-residue peptide (173–225) has also been reported [34]. Gelsolin function
Human gelsolin is a member of a large family of actinmodulating proteins. Gelsolin is folded in the cytoplasm as an 80 kDa intracellular protein and by the secretory pathway as an 83 kDa extracellular protein; the different forms are generated from alternate transcriptional initiation sites and mRNA splicing of the same gene [35]. Gelsolin modulates actin assembly through nucleation, capping and severing activities, functions controlled by Ca2þ and phosphoinositide binding. Intracellular gelsolin regulates the motility and architecture of cells, whereas the extracellular form maintains blood viscosity by controlling the ratio of monomeric (G) to filamentous (F) actin [36]. The FAF mutations do not influence the actin-regulating activity of cytoplasmic gelsolin, implicating FAF as a gain-of-toxic-function disease, instead of a loss-of-function disorder [37]. Gelsolin-null mice display no overt phenotype; however, studies of cells derived from these mice indicate that gelsolin deficiency leads to impaired platelet activation, defective inflammatory response and reduced leukocyte mobility [38]. Gelsolin has been shown to play a critical role in the contraction and motility of fibroblasts [38,39] and neurons [40], and gelsolin deficiency results in altered signaling [41–43] and Ca2þ regulation [44] in these cells. In addition, gelsolin levels have been found to be decreased in many cancers [45–54], and gelsolin has been shown to act as a metastasis [55] and tumor [45,49,56,57] suppressor. www.current-opinion.com
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How do the gelsolin FAF variants cause amyloidosis and pathology?
Gelsolin is composed of six homologous domains, each consisting of a five- or six-stranded b sheet sandwiched between one long and one short a helix [58–60]. Mutation of residue D187 appears to cause FAF by allowing aberrant proteolysis in domain 2, a hypothesis supported by studies showing that the 71-residue amyloid fragments of wild-type and D187N gelsolin are equally amyloidogenic in vitro under acidic conditions [61]. In other words, the wild-type protein is probably not amyloidogenic in vivo because its proteolytic fragment is not generated in the secretory pathway. The serum of heterozygous FAF patients contains fulllength wild-type gelsolin, full-length disease-associated variant gelsolin, 68 and 60 kDa fragments of the C-terminal region of variant gelsolin (termed C68 and C60, respectively), and other minor products of aberrant processing of the variants [62–65] (Figure 2). C68 is the product of the initial cleavage of the FAF variant at the 172–173 amide bond by a-gelsolinase [62–64], whereas C60 is the C-terminal fragment(s) of gelsolin that remains after the amyloidogenic fragment (173–243 or 173–225) is cleaved from C68 by b-gelsolinase [62–64]. It is unlikely that full-length variant gelsolin has a significantly nonnative structure under conditions found in the ER, as misfolded proteins generally do not escape the quality control checks within the ER. Recent studies show that cleavage of variant gelsolin occurs during secretion in the trans-Golgi network (TGN), just beyond the influence of the quality control systems [31]. This study also identified a-gelsolinase as the protease furin [31] (Figures 2 and 3a). Furin is a Figure 2
D187N/Y 1
755
Full length 173
Furin C68
755
173 243 244
β-gelsolinase(s) C60
755
173 225 226
755
C60 Amyloid
Excretion/Degradation Current Opinion in Structural Biology
Schematic representation of variant gelsolin processing in FAF. FAF variant plasma gelsolin is cleaved by furin (a-gelsolinase) in the TGN and by one or more b-gelsolinases after secretion. The 71- and 53-residue peptides (in white) liberated by these two cleavage events self-assemble into amyloid fibrils. The full-length, C68 and C60 species shaded gray are present in the plasma of FAF patients. www.current-opinion.com
member of a family of Ca2þ-dependent proteases known as proprotein convertases, which function to activate proproteins and prohormones during secretion [66]. Furin cleaves after an –R-x-x-R– consensus sequence, perfectly matching the –R169-x-x-R172-A173– a-gelsolinase cleavage site in gelsolin. Co-transfection of furin and variant gelsolin in COS cells results in nearly complete cleavage of the full-length protein to C68, whereas co-transfection of furin and wild-type gelsolin causes a fraction of the wildtype protein to be processed to C68 [31]. This result emphasizes that even wild-type gelsolin is a substrate for furin, but that normally this site is masked. The susceptibility of the FAF variants to furin cleavage in the slightly acidic environment of the TGN results from the inability of the variants to bind Ca2þ within a recently discovered domain 2 binding site [31]. Wild-type gelsolin domain 2 binds Ca2þ with a Kd of 650 nM, and removal of Ca2þ significantly reduces its stability to denaturation by heat or chaotropes [31,67] and increases its susceptibility to general proteolysis [68]. The D187N and D187Y FAF variant domain 2 constructs do not bind Ca2þ at micromolar concentrations according to calorimetry, an observation strongly supported by denaturation studies revealing no significant difference in stability or general proteolytic susceptibility in the absence or presence of Ca2þ [31,68]. Wild-type domain 2 is not susceptible to furin cleavage in vitro in the presence of Ca2þ, but the FAF variants are significantly proteolyzed under the same conditions [31]. A crystal structure of wildtype domain 2 confirms that D187 contributes to a metalbinding site, together with the backbone carbonyl of G186 and the sidechain carboxylate groups of E209 and D259 [69]. A comparison of the metallated [69] and metal-free [59] gelsolin domain 2 structures reveals the basis of Ca2þ-binding-mediated resistance to furin proteolysis: metal binding induces structure into an otherwise disordered loop, burying the –R-x-x-R– sequence required for furin recognition [68]. That the inability to bind Ca2þ is the factor allowing aberrant proteolysis of the FAF variants was recently confirmed by mutation of E209, a non-FAF-related binding residue to Ca2þ in the same binding site [68]. This protein exhibits characteristics of the FAF variants in transfected cells and in vitro. Gelsolin amyloidogenesis in FAF requires furin proteolysis of the variants to initiate formation of the amyloidogenic 71- and 53-residue peptides. Wild-type gelsolin is not cleaved by furin in normal cells and therefore has never been observed in amyloid deposits or in soluble processed forms. Furin proteolysis is necessary but not sufficient for FAF amyloidogenesis; the C68 product of furin cleavage must undergo further proteolytic processing to enable amyloid formation. Although the roles of Ca2þ binding and furin proteolysis in the initiation of FAF are now clear, the subsequent steps, such as the identity of bgelsolinase(s) and the conditions that promote FAF fibril Current Opinion in Structural Biology 2003, 13:674–682
678 Proteins
Figure 3
(a)
(b)
Wild-type gelsolin
Full-length wild type
N u c l e u s
C68 FAF gelsolin Furin
Pmel17 N u c l e u s
Furin
Full-length FAF gelsolin
Stage I
FAF gelsolin
Stage IV
Stage II ER
Golgi
Plasma
ER
Stage III
Golgi
Plasma
Current Opinion in Structural Biology
Gelsolin and Pmel17 processing in the mammalian secretory pathway. (a) Amyloidogenesis in FAF is initiated when a fraction of secreted variant gelsolin (green pathway), which is unable to bind and be stabilized by Ca2þ within domain 2, is cleaved by furin in the TGN. Furin-processed variant gelsolin subsequently undergoes cleavage by b-gelsolinase and self-assembly into amyloid fibrils. Wild-type gelsolin (purple pathway) binds Ca2þ in the secretory pathway and escapes furin cleavage, resulting in the secretion of only the full-length form and precluding amyloidogenesis. (b) Melanosome biogenesis requires synthesis and maturation of Pmel17. Pmel17 is cleaved by furin in a post-Golgi compartment that may be endosomally derived, allowing the liberated fragments to form fibers during the early stages of melanosome maturation.
formation in vivo, remain unclear. Interestingly, furin also processed the BRI protein to initiate amyloidogenesis in familial British dementia [32].
Pmel17 fiber formation in melanosome biogenesis Melanosome biogenesis
Melanosomes are specialized lysosome-related organelles that generate and store the pigment melanin in melanocytes and retinal pigment epithelial cells (recently reviewed in [70]). Melanosome biogenesis occurs through distinct sequential morphological steps [70,71] (Figure 3b). Stage I premelanosomes are endosomally derived vesicular structures that lack melanin. As they mature into stage II premelanosomes, they develop a striated appearance due to the formation of a parallel array of intralumenal protein fibers. The synthesis and deposition of melanin onto the protein fibers causes stage III melanosomes to have characteristically darkened and thickened striations. In mature stage IV melanosomes, the accumulation of melanin masks all intralumenal structure. Melanosomal maturation depends on the presence of specific cargo proteins that are delivered to the appropriate-stage melanosome through highly regulated sorting mechanisms [72]. Pmel17
Pmel17 is a type I integral membrane protein that localizes to the melanosomal matrix. Expression of Pmel17 Current Opinion in Structural Biology 2003, 13:674–682
correlates closely with melanin production [73] and truncations result in pigmentation defects in transgenic mice [74], highlighting its significant role in melanosome structure and function. Pmel17 is associated with intralumenal vesicles in early endosomes and premelanosomes, but becomes significantly enriched in stage II premelanosomes in association with the developing striations [72,75]. Exogenous expression of Pmel17 in nonpigmented cell types results in the formation of Pmel17-containing striations within multivesicular bodies [75]. Pmel17 in melanosome maturation
The pathway through which Pmel17 matures into melanosomal fibers (Figure 3b) bears a striking resemblance to the mechanism of gelsolin variant amyloidogenesis in FAF (Figure 3a) [76]. Like the gelsolin FAF variants, which are cleaved in the TGN, Pmel17 is proteolytically processed in a post-Golgi, pre-lysosomal compartment into an 80 kDa lumenal N-terminal fragment (Ma) that is covalently linked by a disulfide bond to the 28 kDa membrane-associated C-terminal fragment (Mb) [75]. Elegant studies by Raposo, Marks and co-workers [76] have recently demonstrated that, like the aberrant processing of gelsolin FAF variants, Pmel17 is cleaved into Ma and Mb by furin or a related proprotein convertase. Cleavage of Pmel17 is required for striation formation. The appearance of striations was blocked by inhibition of furin cleavage by mutation of its Pmel17 www.current-opinion.com
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cleavage and recognition site, or by exogenous expression of a proprotein convertase inhibitor [76]. As in the case of the FAF variants of gelsolin, furin processing is probably necessary but not sufficient for Pmel17 fiber formation. In addition to cleavage, it appears that Ma must be released from Mb through disulfide bond reduction, and a conformational change probably ensues to facilitate selfassociation into melanosomal fibers [76]. Assembly of Ma is most likely facilitated by the low lumenal pH (<5) of the maturing melanosome [72,76], as disease-associated peptides are known to exhibit increased amyloidogenicity at low pH [61].
Amyloid as a functional entity The analogies between gelsolin amyloidogenesis in FAF and Pmel17 fiber formation in melanosome biogenesis suggest that proteolysis, in combination with environment changes within an organelle, can mediate amyloid fibril formation in both disease-associated and functional contexts. The hypothesis that amyloid can be a functional quaternary structure represents a significant departure from current thinking, but it should not be surprising that pathology helps clarify and support a native pathway [77]. The functional use of amyloid seems likely, as it is a spontaneously formed stable structure that could be envisioned to have evolved from a scaffold- or membrane-type role in early organisms [78,79]. Numerous examples, in addition to Pmel17 fiber formation, exist to support the functional role of amyloid-like structures in nature [77]. The eggshell of the silkmoth is almost exclusively composed of chorion, an amyloid-type structure that appears to function to protect the oocyte and developing embryo from environmental hazards [80]. Additionally, curli are extracellular amyloid fibers produced by Escherichia coli to facilitate binding and colonization of surfaces. Recent data illustrate that curli are biophysically indistinguishable from eukaryotic pathogenic amyloid and that E. coli use at least four accessory proteins to carefully regulate curli assembly, so as to avoid the toxicity that could result from cytoplasmic amyloidogenesis [81]. The filamentous bacterium Streptomyces coelicolor produces a set of small proteins known as chaplins, which polymerize into amyloid-like fibrils, coating the surface of the bacterium and altering its hydrophobicity [82]. A similar hydrophobicity-modulation mechanism is utilized by filamentous fungi, which produce an amyloid-like structure composed of members of a class of more than 40 proteins known as hydrophobins [83]. Yeast prion proteins are highly conserved, and several studies demonstrate that their capacity to form amyloid gives the host yeast a selective growth advantage over prion-free yeast under some conditions [84–86]. Similarly, the filamentous fungus Podospora anserina forms infectious amyloid from the HET-s protein that appears to be required for normal biological function [87,88]. www.current-opinion.com
Conclusions The mechanisms that orchestrate disease-associated gelsolin amyloid fibril formation and functional Pmel17 fiber formation are strikingly similar. The full-length precursor proteins are proteolyzed by furin in the secretory pathway, and the resulting fragments undergo further processing and/or conformational change before assembling into fibers, probably induced by an acidic environment in Pmel17 fiber formation and possibly gelsolin amyloidogenesis. Although amyloidogenesis has historically been associated with disease, the recently discovered similarities between pathological and functional fibril formation highlight that the probable role of the amyloid quaternary structure in biological function must now be considered. Functional amyloid appears to play an important role in mammalian melanosome biogenesis, as well as in insect, bacterial, fungal and yeast function. Ironically, amyloid formation may be beneficial in the early stages of human amyloid diseases by sequestering small toxic oligomers, although tissue displacement in the late stages is clearly deleterious.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1.
Schubert U, Anton LC, Gibbs J, Norbury CC, Yewdell JW, Bennink JR: Rapid degradation of a large fraction of newly synthesized proteins by proteasomes. Nature 2000, 404:770-774.
2.
Kelly JW: Alternative conformations of amyloidogenic proteins govern their behavior. Curr Opin Struct Biol 1996, 6:11-17.
3.
Dobson CM: Protein folding and disease: a view from the first Horizon Symposium. Nat Rev Drug Discov 2003, 2:154-160.
4.
Polymeropoulos MH, Lavedan C, Leroy E, Ide SE, Dehejia A, Dutra A, Pike B, Root H, Rubenstein J, Boyer R et al.: Mutation in the alpha-synuclein gene identified in families with Parkinson’s disease. Science 1997, 276:2045-2047.
5.
Giasson BI, Uryu K, Trojanowski JQ, Lee VM: Mutant and wild type human alpha-synucleins assemble into elongated filaments with distinct morphologies in vitro. J Biol Chem 1999, 274:7619-7622.
6.
The Huntington’s Disease Collaborative Research Group: A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington’s disease chromosomes. Cell 1993, 72:971-983.
7.
Scherzinger E, Lurz R, Turmaine M, Mangiarini L, Hollenbach B, Hasenbank R, Bates GP, Davies SW, Lehrach H, Wanker EE: Huntingtin-encoded polyglutamine expansions form amyloid-like protein aggregates in vitro and in vivo. Cell 1997, 90:549-558.
8.
Rosen DR, Siddique T, Patterson D, Figlewicz DA, Sapp P, Hentati A, Donaldson D, Goto J, O’Regan JP, Deng HX et al.: Mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis. Nature 1993, 362:59-62.
9.
Bruijn LI, Houseweart MK, Kato S, Anderson KL, Anderson SD, Ohama E, Reaume AG, Scott RW, Cleveland DW: Aggregation and motor neuron toxicity of an ALS-linked SOD1 mutant independent from wild-type SOD1. Science 1998, 281:1851-1854.
10. DiDonato M, Craig L, Huff ME, Thayer MM, Cardoso RMF, Kassmann CJ, Lo TP, Bruns CK, Powers ET, Kelly JW et al.: ALS Current Opinion in Structural Biology 2003, 13:674–682
680 Proteins
mutants of human superoxide dismutase form fibrous aggregates via framework destabilization. J Mol Biol 2003, 332:601-615. 11. Goedert M, Wischik CM, Crowther RA, Walker JE, Klug A: Cloning and sequencing of the cDNA encoding a core protein of the paired helical filament of Alzheimer disease: identification as the microtubule-associated protein tau. Proc Natl Acad Sci USA 1988, 85:4051-4055. 12. Crowther RA, Olesen OF, Jakes R, Goedert M: The microtubule binding repeats of tau protein assemble into filaments like those found in Alzheimer’s disease. FEBS Lett 1992, 309:199-202. 13. Crowther RA, Olesen OF, Smith MJ, Jakes R, Goedert M: Assembly of Alzheimer-like filaments from full-length tau protein. FEBS Lett 1994, 337:135-138. 14. Sipe JD, Cohen AS: Review: history of the amyloid fibril. J Struct Biol 2000, 130:88-98. 15. Colon W, Kelly JW: Partial denaturation of transthyretin is sufficient for amyloid fibril formation in vitro. Biochemistry 1992, 31:8654-8660. 16. Fandrich M, Fletcher MA, Dobson CM: Amyloid fibrils from muscle myoglobin. Nature 2001, 410:165-166. Myoglobin, a compact globular protein composed almost entirely of ahelical structure, can be induced to form insoluble aggregates that display all of the characteristics of amyloid. The authors propose that the amyloid tertiary structure is dominated by interactions between the backbone of the polypeptide chain, providing an explanation for the formation of amyloid by many proteins that have no common sequence or native state structure.
toxicity arises from the structural nature of the aggregates and is independent of the primary sequence of the amyloidogenic protein. 27. Kayed R, Head E, Thompson JL, McIntire TM, Milton SC, Cotman CW, Glabe CG: Common structure of soluble amyloid oligomers implies common mechanism of pathogenesis. Science 2003, 300:486-489. An antibody was developed that is specific to the soluble oligomeric state of Ab, but not the monomeric or fibrillar states; the antibody inhibited the toxicity of the Ab soluble oligomers in cell culture. The antibody also recognizes and inhibits the toxicity of the soluble oligomeric states of several other amyloidogenic proteins, suggesting that amyloid diseases proceed through similar intermediate structures and may have a common mechanism of toxicity. 28. Janus C: Vaccines for Alzheimer’s disease: how close are we? CNS Drugs 2003, 17:457-474. 29. Nath A, Hall E, Tuzova M, Dobbs M, Jons M, Anderson C, Woodward J, Guo Z, Fu W, Kryscio R et al.: Autoantibodies to amyloid beta-peptide (Abeta) are increased in Alzheimer’s disease patients and Abeta antibodies can enhance Abeta neurotoxicity: implications for disease pathogenesis and vaccine development. Neuromolecular Med 2003, 3:29-39. 30. Lansbury PT Jr: Evolution of amyloid: what normal protein folding may tell us about fibrillogenesis and disease. Proc Natl Acad Sci USA 1999, 96:3342-3344.
17. Broome BM, Hecht MH: Nature disfavors sequences of alternating polar and non-polar amino acids: implications for amyloidogenesis. J Mol Biol 2000, 296:961-968.
31. Chen CD, Huff ME, Matteson J, Page L, Phillips R, Kelly JW, Balch WE: Furin initiates gelsolin familial amyloidosis in the Golgi through a defect in Ca2R stabilization. EMBO J 2001, 20:6277-6287. Gelsolin amyloid disease is initiated by aberrant cleavage of the diseaseassociated variant protein by furin in the secretory pathway. Susceptibility to proteolysis occurs because of the mutation-induced disruption of a Ca2þ-binding site near the site of cleavage. Loss of Ca2þ binding causes a significant decrease in the stability of gelsolin near the cleavage site, unmasking a furin cleavage site that is protected in the wild-type protein.
18. West MW, Wang W, Patterson J, Mancias JD, Beasley JR, Hecht MH: De novo amyloid proteins from designed combinatorial libraries. Proc Natl Acad Sci USA 1999, 96:11211-11216.
32. Kim SH, Wang R, Gordon DJ, Bass J, Steiner DF, Lynn DG, Thinakaran G, Meredith SC, Sisodia SS: Furin mediates enhanced production of fibrillogenic ABri peptides in familial British dementia. Nat Neurosci 1999, 2:984-988.
19. Lambert MP, Barlow AK, Chromy BA, Edwards C, Freed R, Liosatos M, Morgon TE, Rozovsky I, Trommer B, Viola KL et al.: Diffusable, nonfibrillar ligands derived from Ab1-42 are potent central nervous system toxins. Proc Natl Acad Sci USA 1998, 95:6448-6453.
33. Kiuru S: Gelsolin-related familial amyloidosis, Finnish type (FAF), and its variants found worldwide. Amyloid 1998, 5:55-66.
20. Hartley DM, Walsh DM, Ye CP, Diehl T, Vasquez S, Vassilev PM, Teplow DB, Selkoe DJ: Protofibrillar intermediates of amyloid beta-protein induce acute electrophysiological changes and progressive neurotoxicity in cortical neurons. J Neurosci 1999, 19:8876-8884.
34. Maury CP: Gelsolin-related amyloidosis. Identification of the amyloid protein in Finnish hereditary amyloidosis as a fragment of variant gelsolin. J Clin Invest 1991, 87:1195-1199. 35. Kwiatkowski DJ, Stossel TP, Orkin SH, Mole JE, Colten HR, Yin HL: Plasma and cytoplasmic gelsolins are encoded by a single gene and contain a duplicated actin-binding domain. Nature 1986, 323:455-458.
21. Hardy J, Selkoe DJ: The amyloid hypothesis of Alzheimer’s disease: progress and problems on the road to therapeutics. Science 2002, 297:353-356.
36. Yin HL: Gelsolin: calcium- and polyphosphoinositide-regulated actin-modulating protein. Bioessays 1987, 7:176-179.
22. Volles MJ, Lansbury PT Jr: Zeroing in on the pathogenic form of alpha-synuclein and its mechanism of neurotoxicity in Parkinson’s disease. Biochemistry 2003, 42:7871-7878.
37. Kangas H, Ulmanen I, Paunio T, Kwiatkowski DJ, Lehtovirta M, Jalanko A, Peltonen L: Functional consequences of amyloidosis mutation for gelsolin polypeptide - analysis of gelsolin-actin interaction and gelsolin processing in gelsolin knock-out fibroblasts. FEBS Lett 1999, 454:233-239.
23. Caughey B, Lansbury PT: Protofibrils, pores, fibrils, and neurodegeneration: separating the responsible protein aggregates from the innocent bystanders. Annu Rev Neurosci 2003, 26:267-298. 24. Goldberg MS, Lansbury PT Jr: Is there a cause-and-effect relationship between alpha-synuclein fibrillization and Parkinson’s disease? Nat Cell Biol 2000, 2:E115-E119. 25. Sousa MM, Cardoso I, Fernandes R, Guimaraes A, Saraiva MJ: Deposition of transthyretin in early stages of familial amyloidotic polyneuropathy: evidence for toxicity of nonfibrillar aggregates. Am J Pathol 2001, 159:1993-2000. 26. Bucciantini M, Giannoni E, Chiti F, Baroni F, Formigli L, Zurdo J, Taddei N, Ramponi G, Dobson CM, Stefani M: Inherent toxicity of aggregates implies a common mechanism for protein misfolding diseases. Nature 2002, 416:507-511. Small soluble aggregates of two non-disease-associated proteins are significantly more toxic to cultured cells than the monomeric soluble or mature fibrillar proteins. These data suggest that amyloid-associated Current Opinion in Structural Biology 2003, 13:674–682
38. Witke W, Sharpe AH, Hartwig JH, Azuma T, Stossel TP, Kwiatkowski DJ: Hemostatic, inflammatory, and fibroblast responses are blunted in mice lacking gelsolin. Cell 1995, 81:41-51. 39. Arora PD, Janmey PA, McCulloch CA: A role for gelsolin in stress fiber-dependent cell contraction. Exp Cell Res 1999, 250:155-167. 40. Lu M, Witke W, Kwiatkowski DJ, Kosik KS: Delayed retraction of filopodia in gelsolin null mice. J Cell Biol 1997, 138:1279-1287. 41. Furukawa K, Fu W, Li Y, Witke W, Kwiatkowski DJ, Mattson MP: The actin-severing protein gelsolin modulates calcium channel and NMDA receptor activities and vulnerability to excitotoxicity in hippocampal neurons. J Neurosci 1997, 17:8178-8186. 42. Singh SS, Chauhan A, Murakami N, Chauhan VP: Profilin and gelsolin stimulate phosphatidylinositol 3-kinase activity. Biochemistry 1996, 35:16544-16549. www.current-opinion.com
Pathological and functional amyloid formation Huff, Balch and Kelly 681
43. Sun HQ, Yamamoto M, Mejillano M, Yin HL: Gelsolin, a multifunctional actin regulatory protein. J Biol Chem 1999, 274:33179-33182. 44. Endres M, Fink K, Zhu J, Stagliano NE, Bondada V, Geddes JW, Azuma T, Mattson MP, Kwiatkowski DJ, Moskowitz MA: Neuroprotective effects of gelsolin during murine stroke. J Clin Invest 1999, 103:347-354. 45. Tanaka M, Mullauer L, Ogiso Y, Fujita H, Moriya S, Furuuchi K, Harabayashi T, Shinohara N, Koyanagi T, Kuzumaki N: Gelsolin: a candidate for suppressor of human bladder cancer. Cancer Res 1995, 55:3228-3232.
amyloidosis of the Finnish type. In Amyloid and Amyloidosis. Edited by Kisilevsky R, Benson MD, Frangione B, Gauldie J, Muckle TJ, Young ID. London: Parthenon; 1993:623-625. 63. Maury CP, Sletten K, Totty N, Kangas H, Liljestrom M: Identification of the circulating amyloid precursor and other gelsolin metabolites in patients with G654A mutation in the gelsolin gene (Finnish familial amyloidosis): pathogenetic and diagnostic implications. Lab Invest 1997, 77:299-304. 64. Maury CPJ, Rossi H: Demonstration of a circulating 65K gelsolin variant specific for familial amyloidosis, Finnish type. Biochem Biophys Res Commun 1993, 191:41-44.
46. Moriya S, Yanagihara K, Fujita H, Kuzumaki N: Differential expression of HSP90, gelsolin and GST-p in human gastric carcinoma cell lines. Int J Oncol 1994, 5:1347-1351.
65. Paunio T, Kiuru S, Karonen S-L, Palo J, Peltonen L: Quantification of serum and cerebrospinal fluid gelsolin in familial amyloidosis, Finnish type (AGel). Amyloid 1994, 1:80-89.
47. Afify AM, Werness B A: Decreased expression of the actinbinding protein gelsolin in endometrial and ovarian adenocarcinomas. Appl Immunohistochem Mol Morphol 1998, 6:30-34.
66. Steiner DF: The proprotein convertases. Curr Opin Chem Biol 1998, 2:31-39.
48. Asch HL, Winston JS, Edge SB, Stomper PC, Asch BB: Downregulation of gelsolin expression in human breast ductal carcinoma in situ with and without invasion. Breast Cancer Res Treat 1999, 55:179-188. 49. Asch HL, Head K, Dong Y, Natoli F, Winston JS, Connolly JL, Asch BB: Widespread loss of gelsolin in breast cancers of humans, mice, and rats. Cancer Res 1996, 56:4841-4845. 50. Furuuchi K, Fujita H, Tanaka M, Shichinohe T, Senmaru N, Ogiso Y, Moriya S, Hamada M, Kato H, Kuzumaki N: Gelsolin as a suppressor of malignant phenotype in human colon cancer. Tumor Targeting 1996, 2:277-283.
67. Zapun A, Grammatyka S, Deral G, Vernet T: Calcium-dependent conformational stability of modules 1 and 2 of human gelsolin. Biochem J 2000, 350:873-881. 68. Huff ME, Page LJ, Balch WE, Kelly JW: Gelsolin domain 2 Ca2R affinity determines susceptibility to furin proteolysis and familial amyloidosis of Finnish type. J Mol Biol 2003, 334:119-127. 69. Kazmirski SL, Isaacson RL, An C, Buckle A, Johnson CM, Daggett V, Fersht AR: Loss of a metal-binding site in gelsolin leads to familial amyloidosis-Finnish type. Nat Struct Biol 2002, 9:112-116. 70. Marks MS, Seabra MC: The melanosome: membrane dynamics in black and white. Nat Rev Mol Cell Biol 2001, 2:738-748.
51. Dosaka-Akita H, Hommura F, Fujita H, Kinoshita I, Nishi M, Morikawa T, Katoh H, Kawakami Y, Kuzumaki N: Frequent loss of gelsolin expression in non-small cell lung cancers of heavy smokers. Cancer Res 1998, 58:322-327.
71. Raposo G, Marks MS: The dark side of lysosome-related organelles: specialization of the endocytic pathway for melanosome biogenesis. Traffic 2002, 3:237-248.
52. Lee HK, Driscoll D, Asch H, Asch B, Zhang PJ: Downregulated gelsolin expression in hyperplastic and neoplastic lesions of the prostate. Prostate 1999, 40:14-19.
72. Raposo G, Tenza D, Murphy DM, Berson JF, Marks MS: Distinct protein sorting and localization to premelanosomes, melanosomes, and lysosomes in pigmented melanocytic cells. J Cell Biol 2001, 152:809-824.
53. Chaponnier C, Gabbiani G: Gelsolin modulation in epithelial and stromal cells of mammary carcinoma. Am J Pathol 1989, 134:597-603. 54. Porter RM, Holme TC, Newman EL, Hopwood D, Wilkinson JM, Cuschieri A: Monoclonal antibodies to cytoskeletal proteins: an immunohistochemical investigation of human colon cancer. J Pathol 1993, 170:435-440. 55. Fujita H, Okada F, Hamada J, Hosokawa M, Moriuchi T, Koya RC, Kuzumaki N: Gelsolin functions as a metastasis suppressor in B16-BL6 mouse melanoma cells and requirement of the carboxyl-terminus for its effect. Int J Cancer 2001, 93:773-780. 56. Mullauer L, Fujita H, Ishizaki A, Kuzumaki N: Tumor-suppressive function of mutated gelsolin in ras-transformed cells. Oncogene 1993, 8:2531-2536. 57. Sagawa N, Fujita H, Banno Y, Nozawa Y, Katoh H, Kuzumaki N: Gelsolin suppresses tumorigenicity through inhibiting PKC activation in a human lung cancer cell line, PC10. Br J Cancer 2003, 88:606-612. 58. McLaughlin PJ, Gooch JT, Mannherz HG, Weeds AG: Structure of gelsolin segment 1-actin complex and the mechanism of filament severing. Nature 1993, 364:685-692. 59. Burtnick LD, Koepf EK, Grimes J, Jones EY, Stuart DI, Mclaughlin PJ, Robinson RC: The crystal structure of plasma gelsolin: implications for actin severing, capping, and nucleation. Cell 1997, 90:661-670.
73. Kwon BS, Halaban R, Kim GS, Usack L, Pomerantz S, Haq AK: A melanocyte-specific complementary DNA clone whose expression is inducible by melanotropin and isobutylmethyl xanthine. Mol Biol Med 1987, 4:339-355. 74. Martinez-Esparza M, Jimenez-Cervantes C, Bennett DC, Lozano JA, Solano F, Garcia-Borron JC: The mouse silver locus encodes a single transcript truncated by the silver mutation. Mamm Genome 1999, 10:1168-1171. 75. Berson JF, Harper DC, Tenza D, Raposo G, Marks MS: Pmel17 initiates premelanosome morphogenesis within multivesicular bodies. Mol Biol Cell 2001, 12:3451-3464. The melanosomal protein Pmel17 is cleaved into two subunits in a postGolgi compartment during secretion. Overexpression of Pmel17 in nonpigmented cells induces the formation of striations within multivesicular bodies, indicating that the presence of Pmel17 is directly involved in striation formation during melanosomal maturation. 76. Berson JF, Theos AC, Harper DC, Tenza D, Raposo G, Marks MS: Proprotein convertase cleavage liberates a fibrillogenic fragment of a resident glycoprotein to initiate melanosome biogenesis. J Cell Biol 2003, 161:521-533. The development of melanosomes is dependent on the formation of lumenal fibrous striations composed of Pmel17. Furin proteolysis of Pmel17 late in the secretory pathway is required for the initiation of striation formation and melanosomal maturation. 77. Kelly JW, Balch WE: Amyloid as a natural product. J Cell Biol 2003, 161:461-462.
60. Robinson RC, Mejillano M, Le VP, Burtnick LD, Yin HL, Choe S: Domain movement in gelsolin: a calcium-activated switch. Science 1999, 286:1939-1942.
78. Brack A, Orgel LE: Beta structures of alternating polypeptides and their possible prebiotic significance. Nature 1975, 256:383-387.
61. Ratnaswamy G, Koepf E, Bekele H, Yin H, Kelly JW: The amyloidogenicity of gelsolin is controlled by proteolysis and pH. Chem Biol 1999, 6:293-304.
79. Zhang S, Holmes T, Lockshin C, Rich A: Spontaneous assembly of a self-complementary oligopeptide to form a stable macroscopic membrane. Proc Natl Acad Sci USA 1993, 90:3334-3338.
62. Sunada Y, Sakurai T, Nomomura Y, Kanazawa I: Abnormal gelsolin fragments in the plasma of patients with familial
80. Iconomidou VA, Vriend G, Hamodrakas SJ: Amyloids protect the silkmoth oocyte and embryo. FEBS Lett 2000, 479:141-145.
www.current-opinion.com
Current Opinion in Structural Biology 2003, 13:674–682
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81. Chapman MR, Robinson LS, Pinkner JS, Roth R, Heuser J, Hammar M, Normark S, Hultgren SJ: Role of Escherichia coli curli operons in directing amyloid fiber formation. Science 2002, 295:851-855. E. coli use highly aggregated protein fibers known as curli to mediate binding to surfaces and other proteins; these fibers are indistinguishable from eukaryotic pathogenic amyloid. Formation of curli fibers is carefully regulated by specific assembly machinery expressed as multiple proteins from the same operon.
89. Chiti F, Webster P, Taddei N, Clark A, Stefani M, Ramponi G, Dobson CM: Designing conditions for in vitro formation of amyloid protofilaments and fibrils. Proc Natl Acad Sci USA 1999, 96:3590-3594.
82. Claessen D, Rink R, de Jong W, Siebring J, de Vreugd P, Boersma FGH, Dijkhuizen L, Wosten HAB: A novel class of secreted hydrophobic proteins is involved in aerial hyphae formation in Streptomyces coelicolor by forming amyloid-like fibrils. Genes Dev 2003, 17:1714-1726. S. coelicolor produce amyloid-type fibrils that assemble on the surface of the bacterium and modulate its ability to grow in water or air.
91. Villegas V, Zurdo J, Filimonov V, Aviles F, Dobson C, Serrano L: Protein engineering as a strategy to avoid formation of amyloid fibrils. Protein Sci 2000, 9:1700-1708.
83. Mackay JP, Matthews JM, Winefield RD, Mackay LG, Haverkamp RG, Templeton MD: The hydrophobin EAS is largely unstructured in solution and functions by forming amyloid-like structures. Structure 2001, 9:83-91.
93. Guijarro JI, Sunde M, Jones JA, Campbell ID, Dobson CM: Amyloid fibril formation by an SH3 domain. Proc Natl Acad Sci USA 1998, 95:4224-4228.
84. Eaglestone SS, Cox BS, Tuite MF: Translation termination efficiency can be regulated in Saccharomyces cerevisiae by environmental stress through a prion-mediated mechanism. EMBO J 1999, 18:1974-1981. 85. True HL, Lindquist SL: A yeast prion provides a mechanism for genetic variation and phenotypic diversity. Nature 2000, 407:477-483. 86. Osherovich LZ, Weissman JS: The utility of prions. Dev Cell 2002, 2:143-151. 87. Dos Reis S, Coulary-Salin B, Forge V, Lascu I, Begueret J, Saupe SJ: The HET-s prion protein of the filamentous fungus Podospora anserina aggregates in vitro into amyloid-like fibrils. J Biol Chem 2002, 277:5703-5706. HET-s is a fungal prion protein that aggregates into fibrils exhibiting all of the characteristics of amyloid. HET-s infection across strains may serve to transmit a necessary biological function. 88. Maddelein M-L, Dos Reis S, Duvezin-Caubet S, Coulary-Salin B, Saupe SJ: Amyloid aggregates of the HET-s prion protein are infectious. Proc Natl Acad Sci USA 2002, 99:7402-7407. Amyloid aggregates of recombinant HET-s formed in vitro are infectious when introduced into fungal cells, demonstrating the protein-only mechanism characteristic of prion propagation.
Current Opinion in Structural Biology 2003, 13:674–682
90. Litvinovich SV, Brew SA, Aota S, Akiyama SK, Haudenschild C, Ingham KC: Formation of amyloid-like fibrils by self-association of a partially unfolded fibronectin type III module. J Mol Biol 1998, 280:245-258.
92. Ferguson N, Berriman J, Petrovich M, Sharpe TD, Finch JT, Fersht AR: Rapid amyloid fiber formation from the fast-folding WW domain FBP28. Proc Natl Acad Sci USA 2003, 100:9814-9819.
94. Graether SP, Slupsky CM, Sykes BD: Freezing of a fish antifreeze protein results in amyloid fibril formation. Biophys J 2003, 84:552-557. 95. Konno T, Murata K, Nagayama K: Amyloid-like aggregates of a plant protein: a case of a sweet-tasting protein, monellin. FEBS Lett 1999, 454:122-126. 96. Ramirez-Alvarado M, Merkel JS, Regan L: A systematic exploration of the influence of the protein stability on amyloid fibril formation in vitro. Proc Natl Acad Sci USA 2000, 97:8979-8984. 97. Chiti F, Bucciantini M, Capanni C, Taddei N, Dobson CM, Stefani M: Solution conditions can promote formation of either amyloid protofilaments or mature fibrils from the HypF N-terminal domain. Protein Sci 2001, 10:2541-2547. 98. Yutani K, Takayama G, Goda S, Yamagata Y, Maki S, Namba K, Tsunasawa S, Ogasahara K: The process of amyloid-like fibril formation by methionine aminopeptidase from a hyperthermophile, Pyrococcus furiosus. Biochemistry 2000, 39:2769-2777. 99. Pertinhez TA, Bouchard M, Tomlinson EJ, Wain R, Ferguson SJ, Dobson CM, Smith LJ: Amyloid fibril formation by a helical cytochrome. FEBS Lett 2001, 495:184-186.
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Pathological and functional amyloid formation orchestrated by the secretory pathway Mary E Huff, William E Balch and Jeffery W Kelly Amyloidogenesis has historically been associated with pathology in a class of neurodegenerative diseases known as amyloid diseases. Recent studies have shown that proteolysis by furin during secretion initiates both variant gelsolin amyloidogenesis, associated with the disease familial amyloidosis of Finnish type, and Pmel17 fiber formation, which is necessary for the functional biogenesis of melanosomes. Proteolysis combined with organelle-dependent environment changes orchestrate amyloidogenesis associated with both pathological processes and a functional pathway. Addresses The Scripps Research Institute, Departments of Chemistry and Cell Biology, The Skaggs Institute for Chemical Biology, 10550 North Torrey Pines Road, BCC265, La Jolla, CA 92037, USA e-mail: [email protected]
Current Opinion in Structural Biology 2003, 13:674–682 This review comes from a themed issue on Proteins Edited by Christian Cambillau and David I Stuart 0959-440X/$ – see front matter ß 2003 Elsevier Ltd. All rights reserved. DOI 10.1016/j.sbi.2003.10.010
Abbreviations AD Alzheimer’s disease ER endoplasmic reticulum FAF familial amyloidosis of Finnish type TGN trans-Golgi network
diseases is characterized by intracellular inclusions. It is not yet clear how misfolding evades the quality control systems in these disorders. Proteins deposited intracellularly include a-synuclein in Parkinson’s disease [4,5], Huntingtin in Huntington’s disease [6,7], Cu/Zn superoxide dismutase in some amyotrophic lateral sclerosis cases (Lou Gehrig’s disease) [8–10] and tau associated with Alzheimer’s disease (AD) [11–13]. Characteristics of amyloid and its associated diseases
Amyloid diseases are late-onset, degenerative disorders characterized by the aberrant aggregation and extracellular deposition of proteins or peptide fragments (Figure 1). The deposits associated with a given disease are generally composed of a single protein. Amyloid plaques contain fibrillar protein aggregates with a crossb sheet quaternary structure that exhibit selective binding towards the fluorophores thioflavin T and Congo red [14]. A subset of these proteins, including lysozyme, transthyretin, b2-microglobulin and some immunoglobulin light chains, misfold and are deposited as fulllength polypeptides [2]. The others are subject to endoproteolysis to form peptide fragments. Typically, one of the fragments subsequently deposits as amyloid, as in the case of cleavage of the amyloid precursor protein into peptides including Ab, which deposits in AD. Amyloid deposits have consistently been found to bind accessory molecules, including serum amyloid P component, proteoglycans/glycosaminoglycans and apolipoprotein E [14]. Determinants of amyloid formation
Introduction Minimizing protein misfolding and aggregation both inside and outside of cells is now appreciated to be critical to maintaining normal physiology. Chaperones provide multiple opportunities for proteins to fold within a cell and, if unsuccessful, these polypeptides are typically degraded by the proteasome. As much as 30% of the newly synthesized protein pool is degraded due to inefficient folding [1]. In recent years, it has become clear that many human diseases result from the misfolding and aggregation of proteins (see [2,3] for reviews). Misfolding in the endoplasmic reticulum (ER) and subsequent proteasome degradation causes loss-of-function disorders, including cystic fibrosis, whereas extracellular protein aggregation, outside the reaches of the chaperone and proteasome systems, leads to more than 30 distinct human amyloid diseases (Table 1). A third class of protein misfolding Current Opinion in Structural Biology 2003, 13:674–682
The fibrillogenic proteins in human amyloid diseases (Table 1) do not share a common structure or function, yet they all ultimately assemble into a cross-b sheet conformation under conditions that make the native state unfavorable (Figure 1) [2,15]. Recent work demonstrates that the ability to form amyloid fibrils is not limited to proteins known to be associated with pathology: several ‘normal’ proteins have been converted into fibrils in vitro (Table 2). The conditions required in some cases appear harsh (Table 2), but comparison with pathologic amyloidogenesis is difficult because the mechanism and location of fibril formation in humans remain unclear. Dobson and co-workers [16] have proposed that the cross-b sheet structure of amyloid fibrils is mainly stabilized by interactions between polypeptide backbones. However, in several cases, it is now clear that sidechains contribute significantly to the specificity of aggregation and to the stability of the aggregate [17,18]. www.current-opinion.com
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Figure 1
Table 1 Amyloid precursor proteins and their associated diseases.
Amyloid b precursor protein
Alzheimer’s disease Hereditary cerebral hemorrhage with amyloidosis, Dutch type Hereditary renal amyloidosis Familial amyloid polyneuropathy, type III Hereditary systemic amyloidosis Isolated atrial amyloid Familial British dementia Familial Danish dementia Medullary thyroid carcinoma Hereditary cerebral hemorrhage with amyloidosis, Icelandic type Hereditary renal amyloidosis Familial amyloidosis, Finnish type Primary systemic amyloidosis Myeloma-associated amyloidosis Primary systemic amyloidosis Myeloma-associated amyloidosis Insulin-related amyloidosis Type II diabetes
Apolipoprotein AI
Apolipoprotein AII Atrial natriuretic protein BRI precursor protein variants Calcitonin Cystatin C Fibrinogen Aa-chain variants Gelsolin Immunoglobulin heavy chain Immunoglobulin light chain Insulin Islet amyloid polypeptide (amylin) Kerato-epithelin Lactoferrin Lysozyme Medin (lactadherin fragment) b2-microglobulin Prolactin Serum amyloid A
Transthyretin
Prion protein
Lattice dystrophies of the cornea Familial corneal amyloidosis Hereditary systemic amyloidosis Hereditary renal amyloidosis Aortic medial amyloidosis Hemodialysis-related amyloidosis Aging pituitary Senile hypophyseal prolactinoma Secondary systemic amyloidosis Familial Mediterranean fever Muckle–Wells syndrome Senile systemic amyloidosis Familial amyloid polyneuropathy, types I and II Familial amyloid cardiomyopathy Kuru Creutzfeldt–Jakob disease Fatal familial insomnia Gerstmann-Stra¨ usslerScheinker disease
Non–native conformation
Native conformation Proteolysis
re St ar ru ra ct ng ur em al en t
Associated diseases
Structural rearrangement
Precursor protein
Fragment
Self assembly
Amyloidogenic intermediate
n Oligomeric intermediate
Amyloid fibrils
Current Opinion in Structural Biology
Mechanisms of amyloid formation. Slight shifts away from a protein’s native conformation induced by mutation or by changes in environment allow structural rearrangement to form an amyloidogenic assembly-competent intermediate from a full-length precursor protein. Alternatively, proteolysis of the native structure or a perturbed conformation can generate a polypeptide fragment that rearranges into the amyloidogenic intermediate. The amyloidogenic intermediates undergo self-assembly into soluble oligomers that mature into the fibrils deposited in plaques. This scheme depicts unseeded polymerization in amyloidogenesis; however, a seeded pathway also exists in which the oligomeric intermediate recruits the non-native full-length protein or proteolytic fragment through induction of the amyloidogenic conformation.
Table 2 Examples of full-length proteins that have been shown to be amyloidogenic in vitro, but are not currently associated with a human disease. Amyloidogenic protein
Protein source
Conditions
References
Myoglobin Acylphosphatase Fibronectin type III module Procarboxypeptidase A2 activation domain WW domain of FBP28 SH3 domain of phosphatidylinositol 3-kinase Type I antifreeze protein Monellin B1 IgG-binding domain of protein G HypF Methionine aminopeptidase Cytochrome c
Human Human Human Human Human Bovine Fish Plant Bacteria Bacteria Bacteria Bacteria
pH 9, 658C pH 5.5, 25% TFE pH 7.4 pH 3, thermal or chemical denaturation pH 7, 378C pH 2 Freeze/thaw pH 2.5, 858C, 100 mM NaCl Agitation; changes in pH, ionic strength or temperature pH 5.5, 30% TFE pH 3, GdmCl Removal of cofactor, extended incubation
[16] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99]
GdmCl, guanidinium chloride; TFE, trifluoroethanol.
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What is the toxic species in amyloid fibril formation?
Amyloidogenesis can result either from partial denaturation of a secreted native protein or from the generation of aggregation-prone fragments through proteolysis of a precursor protein during or after secretion (Figure 1). Although protein aggregates, including fibrils, are invariably observed in amyloid diseases, the nature of the misfolded species leading to the gain-of-toxic-function phenotype characterizing the amyloidoses remains unclear. Krafft, Glabe, Lansbury, Teplow, Selkoe [19–24] and others have demonstrated the presence of, and in some cases the inherent toxicity of, small soluble oligomers of the Ab peptides associated with AD. Saraiva and colleagues [25] have similarly identified nonfibrillar transthyretin aggregates in nerve biopsies of individuals showing early signs of peripheral neuropathy in familial amyloid polyneuropathy. Additionally, soluble oligomers of amyloid-forming nonpathogenic proteins such as HypF and the SH3 domain (Table 2) have also revealed significant toxicity in cell culture [26]. An antibody specific to soluble Ab oligomers also recognizes the soluble oligomers of other amyloidogenic proteins and inhibits their cytotoxicity in cell culture, suggesting that the soluble oligomeric conformation may be a common link in neurodegenerative disease pathology [27]. An active immunization strategy employing soluble oligomers as the antigen is very effective in preventing Ab deposition in AD transgenic animals, although the active immunization trial in humans was halted because of severe inflammation in a few subjects [28,29]. Rethinking the role of amyloid
The realization that low molecular weight oligomers of amyloidogenic peptides and proteins are more toxic than fibrils to neuronal cultures and related cell lines calls into question the role that mature amyloid fibrils and plaques play in pathology. Although it is generally accepted that tissue displacement by amyloid contributes to late-stage pathology in many amyloidoses, fibril formation at an early stage of the disease could offer protection against the toxic oligomeric intermediates [21,30]. Numerous laboratories have demonstrated the significant stability of the amyloid cross-b sheet structure. Furthermore, amyloid fibril formation by many sequences is spontaneous under conditions found in a few organelles of eukaryotes. Hence, it would be surprising if the amyloid quaternary structure were not used for certain functional purposes. Preliminary observations discussed later in this review support the existence of functional intracellular eukaryotic amyloid. Interestingly, the proposed pathway for native amyloid formation in eukaryotes is nearly identical to the recently discovered pathway for pathogenic amyloid formation in the gelsolin and BRI familial amyloid diseases, to be discussed first in this review [31,32]. Anfinsen taught us that both sequence and environCurrent Opinion in Structural Biology 2003, 13:674–682
ment control conformational preferences. Therefore, it is not surprising that proteolysis is used to alter the sequence, which, in concert with organelle sequestration to alter the environment, facilitates the formation of functional amyloid in membrane-delimited compartments.
Gelsolin in familial amyloidosis of Finnish type Gelsolin amyloidosis
The amyloid deposits associated with familial amyloidosis of Finnish type (FAF) are composed of a proteolytic fragment(s) of one point mutant of the actin-modulating protein gelsolin. FAF is caused by a guanine to adenine or thymine substitution at nucleotide 654 (a mutational hotspot), which results in mutation of the aspartic acid residue at position 187 to asparagine (D187N) or tyrosine (D187Y), respectively. This genetic variation results in an autosomal dominant neurodegenerative disease wherein individuals present with ophthalmologic, dermatologic and neurologic symptoms in the third or fourth decade of life [33]. FAF amyloid deposits are mainly composed of a 71-residue fragment (173–243) of variant gelsolin, although a less common 53-residue peptide (173–225) has also been reported [34]. Gelsolin function
Human gelsolin is a member of a large family of actinmodulating proteins. Gelsolin is folded in the cytoplasm as an 80 kDa intracellular protein and by the secretory pathway as an 83 kDa extracellular protein; the different forms are generated from alternate transcriptional initiation sites and mRNA splicing of the same gene [35]. Gelsolin modulates actin assembly through nucleation, capping and severing activities, functions controlled by Ca2þ and phosphoinositide binding. Intracellular gelsolin regulates the motility and architecture of cells, whereas the extracellular form maintains blood viscosity by controlling the ratio of monomeric (G) to filamentous (F) actin [36]. The FAF mutations do not influence the actin-regulating activity of cytoplasmic gelsolin, implicating FAF as a gain-of-toxic-function disease, instead of a loss-of-function disorder [37]. Gelsolin-null mice display no overt phenotype; however, studies of cells derived from these mice indicate that gelsolin deficiency leads to impaired platelet activation, defective inflammatory response and reduced leukocyte mobility [38]. Gelsolin has been shown to play a critical role in the contraction and motility of fibroblasts [38,39] and neurons [40], and gelsolin deficiency results in altered signaling [41–43] and Ca2þ regulation [44] in these cells. In addition, gelsolin levels have been found to be decreased in many cancers [45–54], and gelsolin has been shown to act as a metastasis [55] and tumor [45,49,56,57] suppressor. www.current-opinion.com
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How do the gelsolin FAF variants cause amyloidosis and pathology?
Gelsolin is composed of six homologous domains, each consisting of a five- or six-stranded b sheet sandwiched between one long and one short a helix [58–60]. Mutation of residue D187 appears to cause FAF by allowing aberrant proteolysis in domain 2, a hypothesis supported by studies showing that the 71-residue amyloid fragments of wild-type and D187N gelsolin are equally amyloidogenic in vitro under acidic conditions [61]. In other words, the wild-type protein is probably not amyloidogenic in vivo because its proteolytic fragment is not generated in the secretory pathway. The serum of heterozygous FAF patients contains fulllength wild-type gelsolin, full-length disease-associated variant gelsolin, 68 and 60 kDa fragments of the C-terminal region of variant gelsolin (termed C68 and C60, respectively), and other minor products of aberrant processing of the variants [62–65] (Figure 2). C68 is the product of the initial cleavage of the FAF variant at the 172–173 amide bond by a-gelsolinase [62–64], whereas C60 is the C-terminal fragment(s) of gelsolin that remains after the amyloidogenic fragment (173–243 or 173–225) is cleaved from C68 by b-gelsolinase [62–64]. It is unlikely that full-length variant gelsolin has a significantly nonnative structure under conditions found in the ER, as misfolded proteins generally do not escape the quality control checks within the ER. Recent studies show that cleavage of variant gelsolin occurs during secretion in the trans-Golgi network (TGN), just beyond the influence of the quality control systems [31]. This study also identified a-gelsolinase as the protease furin [31] (Figures 2 and 3a). Furin is a Figure 2
D187N/Y 1
755
Full length 173
Furin C68
755
173 243 244
β-gelsolinase(s) C60
755
173 225 226
755
C60 Amyloid
Excretion/Degradation Current Opinion in Structural Biology
Schematic representation of variant gelsolin processing in FAF. FAF variant plasma gelsolin is cleaved by furin (a-gelsolinase) in the TGN and by one or more b-gelsolinases after secretion. The 71- and 53-residue peptides (in white) liberated by these two cleavage events self-assemble into amyloid fibrils. The full-length, C68 and C60 species shaded gray are present in the plasma of FAF patients. www.current-opinion.com
member of a family of Ca2þ-dependent proteases known as proprotein convertases, which function to activate proproteins and prohormones during secretion [66]. Furin cleaves after an –R-x-x-R– consensus sequence, perfectly matching the –R169-x-x-R172-A173– a-gelsolinase cleavage site in gelsolin. Co-transfection of furin and variant gelsolin in COS cells results in nearly complete cleavage of the full-length protein to C68, whereas co-transfection of furin and wild-type gelsolin causes a fraction of the wildtype protein to be processed to C68 [31]. This result emphasizes that even wild-type gelsolin is a substrate for furin, but that normally this site is masked. The susceptibility of the FAF variants to furin cleavage in the slightly acidic environment of the TGN results from the inability of the variants to bind Ca2þ within a recently discovered domain 2 binding site [31]. Wild-type gelsolin domain 2 binds Ca2þ with a Kd of 650 nM, and removal of Ca2þ significantly reduces its stability to denaturation by heat or chaotropes [31,67] and increases its susceptibility to general proteolysis [68]. The D187N and D187Y FAF variant domain 2 constructs do not bind Ca2þ at micromolar concentrations according to calorimetry, an observation strongly supported by denaturation studies revealing no significant difference in stability or general proteolytic susceptibility in the absence or presence of Ca2þ [31,68]. Wild-type domain 2 is not susceptible to furin cleavage in vitro in the presence of Ca2þ, but the FAF variants are significantly proteolyzed under the same conditions [31]. A crystal structure of wildtype domain 2 confirms that D187 contributes to a metalbinding site, together with the backbone carbonyl of G186 and the sidechain carboxylate groups of E209 and D259 [69]. A comparison of the metallated [69] and metal-free [59] gelsolin domain 2 structures reveals the basis of Ca2þ-binding-mediated resistance to furin proteolysis: metal binding induces structure into an otherwise disordered loop, burying the –R-x-x-R– sequence required for furin recognition [68]. That the inability to bind Ca2þ is the factor allowing aberrant proteolysis of the FAF variants was recently confirmed by mutation of E209, a non-FAF-related binding residue to Ca2þ in the same binding site [68]. This protein exhibits characteristics of the FAF variants in transfected cells and in vitro. Gelsolin amyloidogenesis in FAF requires furin proteolysis of the variants to initiate formation of the amyloidogenic 71- and 53-residue peptides. Wild-type gelsolin is not cleaved by furin in normal cells and therefore has never been observed in amyloid deposits or in soluble processed forms. Furin proteolysis is necessary but not sufficient for FAF amyloidogenesis; the C68 product of furin cleavage must undergo further proteolytic processing to enable amyloid formation. Although the roles of Ca2þ binding and furin proteolysis in the initiation of FAF are now clear, the subsequent steps, such as the identity of bgelsolinase(s) and the conditions that promote FAF fibril Current Opinion in Structural Biology 2003, 13:674–682
678 Proteins
Figure 3
(a)
(b)
Wild-type gelsolin
Full-length wild type
N u c l e u s
C68 FAF gelsolin Furin
Pmel17 N u c l e u s
Furin
Full-length FAF gelsolin
Stage I
FAF gelsolin
Stage IV
Stage II ER
Golgi
Plasma
ER
Stage III
Golgi
Plasma
Current Opinion in Structural Biology
Gelsolin and Pmel17 processing in the mammalian secretory pathway. (a) Amyloidogenesis in FAF is initiated when a fraction of secreted variant gelsolin (green pathway), which is unable to bind and be stabilized by Ca2þ within domain 2, is cleaved by furin in the TGN. Furin-processed variant gelsolin subsequently undergoes cleavage by b-gelsolinase and self-assembly into amyloid fibrils. Wild-type gelsolin (purple pathway) binds Ca2þ in the secretory pathway and escapes furin cleavage, resulting in the secretion of only the full-length form and precluding amyloidogenesis. (b) Melanosome biogenesis requires synthesis and maturation of Pmel17. Pmel17 is cleaved by furin in a post-Golgi compartment that may be endosomally derived, allowing the liberated fragments to form fibers during the early stages of melanosome maturation.
formation in vivo, remain unclear. Interestingly, furin also processed the BRI protein to initiate amyloidogenesis in familial British dementia [32].
Pmel17 fiber formation in melanosome biogenesis Melanosome biogenesis
Melanosomes are specialized lysosome-related organelles that generate and store the pigment melanin in melanocytes and retinal pigment epithelial cells (recently reviewed in [70]). Melanosome biogenesis occurs through distinct sequential morphological steps [70,71] (Figure 3b). Stage I premelanosomes are endosomally derived vesicular structures that lack melanin. As they mature into stage II premelanosomes, they develop a striated appearance due to the formation of a parallel array of intralumenal protein fibers. The synthesis and deposition of melanin onto the protein fibers causes stage III melanosomes to have characteristically darkened and thickened striations. In mature stage IV melanosomes, the accumulation of melanin masks all intralumenal structure. Melanosomal maturation depends on the presence of specific cargo proteins that are delivered to the appropriate-stage melanosome through highly regulated sorting mechanisms [72]. Pmel17
Pmel17 is a type I integral membrane protein that localizes to the melanosomal matrix. Expression of Pmel17 Current Opinion in Structural Biology 2003, 13:674–682
correlates closely with melanin production [73] and truncations result in pigmentation defects in transgenic mice [74], highlighting its significant role in melanosome structure and function. Pmel17 is associated with intralumenal vesicles in early endosomes and premelanosomes, but becomes significantly enriched in stage II premelanosomes in association with the developing striations [72,75]. Exogenous expression of Pmel17 in nonpigmented cell types results in the formation of Pmel17-containing striations within multivesicular bodies [75]. Pmel17 in melanosome maturation
The pathway through which Pmel17 matures into melanosomal fibers (Figure 3b) bears a striking resemblance to the mechanism of gelsolin variant amyloidogenesis in FAF (Figure 3a) [76]. Like the gelsolin FAF variants, which are cleaved in the TGN, Pmel17 is proteolytically processed in a post-Golgi, pre-lysosomal compartment into an 80 kDa lumenal N-terminal fragment (Ma) that is covalently linked by a disulfide bond to the 28 kDa membrane-associated C-terminal fragment (Mb) [75]. Elegant studies by Raposo, Marks and co-workers [76] have recently demonstrated that, like the aberrant processing of gelsolin FAF variants, Pmel17 is cleaved into Ma and Mb by furin or a related proprotein convertase. Cleavage of Pmel17 is required for striation formation. The appearance of striations was blocked by inhibition of furin cleavage by mutation of its Pmel17 www.current-opinion.com
Pathological and functional amyloid formation Huff, Balch and Kelly 679
cleavage and recognition site, or by exogenous expression of a proprotein convertase inhibitor [76]. As in the case of the FAF variants of gelsolin, furin processing is probably necessary but not sufficient for Pmel17 fiber formation. In addition to cleavage, it appears that Ma must be released from Mb through disulfide bond reduction, and a conformational change probably ensues to facilitate selfassociation into melanosomal fibers [76]. Assembly of Ma is most likely facilitated by the low lumenal pH (<5) of the maturing melanosome [72,76], as disease-associated peptides are known to exhibit increased amyloidogenicity at low pH [61].
Amyloid as a functional entity The analogies between gelsolin amyloidogenesis in FAF and Pmel17 fiber formation in melanosome biogenesis suggest that proteolysis, in combination with environment changes within an organelle, can mediate amyloid fibril formation in both disease-associated and functional contexts. The hypothesis that amyloid can be a functional quaternary structure represents a significant departure from current thinking, but it should not be surprising that pathology helps clarify and support a native pathway [77]. The functional use of amyloid seems likely, as it is a spontaneously formed stable structure that could be envisioned to have evolved from a scaffold- or membrane-type role in early organisms [78,79]. Numerous examples, in addition to Pmel17 fiber formation, exist to support the functional role of amyloid-like structures in nature [77]. The eggshell of the silkmoth is almost exclusively composed of chorion, an amyloid-type structure that appears to function to protect the oocyte and developing embryo from environmental hazards [80]. Additionally, curli are extracellular amyloid fibers produced by Escherichia coli to facilitate binding and colonization of surfaces. Recent data illustrate that curli are biophysically indistinguishable from eukaryotic pathogenic amyloid and that E. coli use at least four accessory proteins to carefully regulate curli assembly, so as to avoid the toxicity that could result from cytoplasmic amyloidogenesis [81]. The filamentous bacterium Streptomyces coelicolor produces a set of small proteins known as chaplins, which polymerize into amyloid-like fibrils, coating the surface of the bacterium and altering its hydrophobicity [82]. A similar hydrophobicity-modulation mechanism is utilized by filamentous fungi, which produce an amyloid-like structure composed of members of a class of more than 40 proteins known as hydrophobins [83]. Yeast prion proteins are highly conserved, and several studies demonstrate that their capacity to form amyloid gives the host yeast a selective growth advantage over prion-free yeast under some conditions [84–86]. Similarly, the filamentous fungus Podospora anserina forms infectious amyloid from the HET-s protein that appears to be required for normal biological function [87,88]. www.current-opinion.com
Conclusions The mechanisms that orchestrate disease-associated gelsolin amyloid fibril formation and functional Pmel17 fiber formation are strikingly similar. The full-length precursor proteins are proteolyzed by furin in the secretory pathway, and the resulting fragments undergo further processing and/or conformational change before assembling into fibers, probably induced by an acidic environment in Pmel17 fiber formation and possibly gelsolin amyloidogenesis. Although amyloidogenesis has historically been associated with disease, the recently discovered similarities between pathological and functional fibril formation highlight that the probable role of the amyloid quaternary structure in biological function must now be considered. Functional amyloid appears to play an important role in mammalian melanosome biogenesis, as well as in insect, bacterial, fungal and yeast function. Ironically, amyloid formation may be beneficial in the early stages of human amyloid diseases by sequestering small toxic oligomers, although tissue displacement in the late stages is clearly deleterious.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1.
Schubert U, Anton LC, Gibbs J, Norbury CC, Yewdell JW, Bennink JR: Rapid degradation of a large fraction of newly synthesized proteins by proteasomes. Nature 2000, 404:770-774.
2.
Kelly JW: Alternative conformations of amyloidogenic proteins govern their behavior. Curr Opin Struct Biol 1996, 6:11-17.
3.
Dobson CM: Protein folding and disease: a view from the first Horizon Symposium. Nat Rev Drug Discov 2003, 2:154-160.
4.
Polymeropoulos MH, Lavedan C, Leroy E, Ide SE, Dehejia A, Dutra A, Pike B, Root H, Rubenstein J, Boyer R et al.: Mutation in the alpha-synuclein gene identified in families with Parkinson’s disease. Science 1997, 276:2045-2047.
5.
Giasson BI, Uryu K, Trojanowski JQ, Lee VM: Mutant and wild type human alpha-synucleins assemble into elongated filaments with distinct morphologies in vitro. J Biol Chem 1999, 274:7619-7622.
6.
The Huntington’s Disease Collaborative Research Group: A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington’s disease chromosomes. Cell 1993, 72:971-983.
7.
Scherzinger E, Lurz R, Turmaine M, Mangiarini L, Hollenbach B, Hasenbank R, Bates GP, Davies SW, Lehrach H, Wanker EE: Huntingtin-encoded polyglutamine expansions form amyloid-like protein aggregates in vitro and in vivo. Cell 1997, 90:549-558.
8.
Rosen DR, Siddique T, Patterson D, Figlewicz DA, Sapp P, Hentati A, Donaldson D, Goto J, O’Regan JP, Deng HX et al.: Mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis. Nature 1993, 362:59-62.
9.
Bruijn LI, Houseweart MK, Kato S, Anderson KL, Anderson SD, Ohama E, Reaume AG, Scott RW, Cleveland DW: Aggregation and motor neuron toxicity of an ALS-linked SOD1 mutant independent from wild-type SOD1. Science 1998, 281:1851-1854.
10. DiDonato M, Craig L, Huff ME, Thayer MM, Cardoso RMF, Kassmann CJ, Lo TP, Bruns CK, Powers ET, Kelly JW et al.: ALS Current Opinion in Structural Biology 2003, 13:674–682
680 Proteins
mutants of human superoxide dismutase form fibrous aggregates via framework destabilization. J Mol Biol 2003, 332:601-615. 11. Goedert M, Wischik CM, Crowther RA, Walker JE, Klug A: Cloning and sequencing of the cDNA encoding a core protein of the paired helical filament of Alzheimer disease: identification as the microtubule-associated protein tau. Proc Natl Acad Sci USA 1988, 85:4051-4055. 12. Crowther RA, Olesen OF, Jakes R, Goedert M: The microtubule binding repeats of tau protein assemble into filaments like those found in Alzheimer’s disease. FEBS Lett 1992, 309:199-202. 13. Crowther RA, Olesen OF, Smith MJ, Jakes R, Goedert M: Assembly of Alzheimer-like filaments from full-length tau protein. FEBS Lett 1994, 337:135-138. 14. Sipe JD, Cohen AS: Review: history of the amyloid fibril. J Struct Biol 2000, 130:88-98. 15. Colon W, Kelly JW: Partial denaturation of transthyretin is sufficient for amyloid fibril formation in vitro. Biochemistry 1992, 31:8654-8660. 16. Fandrich M, Fletcher MA, Dobson CM: Amyloid fibrils from muscle myoglobin. Nature 2001, 410:165-166. Myoglobin, a compact globular protein composed almost entirely of ahelical structure, can be induced to form insoluble aggregates that display all of the characteristics of amyloid. The authors propose that the amyloid tertiary structure is dominated by interactions between the backbone of the polypeptide chain, providing an explanation for the formation of amyloid by many proteins that have no common sequence or native state structure.
toxicity arises from the structural nature of the aggregates and is independent of the primary sequence of the amyloidogenic protein. 27. Kayed R, Head E, Thompson JL, McIntire TM, Milton SC, Cotman CW, Glabe CG: Common structure of soluble amyloid oligomers implies common mechanism of pathogenesis. Science 2003, 300:486-489. An antibody was developed that is specific to the soluble oligomeric state of Ab, but not the monomeric or fibrillar states; the antibody inhibited the toxicity of the Ab soluble oligomers in cell culture. The antibody also recognizes and inhibits the toxicity of the soluble oligomeric states of several other amyloidogenic proteins, suggesting that amyloid diseases proceed through similar intermediate structures and may have a common mechanism of toxicity. 28. Janus C: Vaccines for Alzheimer’s disease: how close are we? CNS Drugs 2003, 17:457-474. 29. Nath A, Hall E, Tuzova M, Dobbs M, Jons M, Anderson C, Woodward J, Guo Z, Fu W, Kryscio R et al.: Autoantibodies to amyloid beta-peptide (Abeta) are increased in Alzheimer’s disease patients and Abeta antibodies can enhance Abeta neurotoxicity: implications for disease pathogenesis and vaccine development. Neuromolecular Med 2003, 3:29-39. 30. Lansbury PT Jr: Evolution of amyloid: what normal protein folding may tell us about fibrillogenesis and disease. Proc Natl Acad Sci USA 1999, 96:3342-3344.
17. Broome BM, Hecht MH: Nature disfavors sequences of alternating polar and non-polar amino acids: implications for amyloidogenesis. J Mol Biol 2000, 296:961-968.
31. Chen CD, Huff ME, Matteson J, Page L, Phillips R, Kelly JW, Balch WE: Furin initiates gelsolin familial amyloidosis in the Golgi through a defect in Ca2R stabilization. EMBO J 2001, 20:6277-6287. Gelsolin amyloid disease is initiated by aberrant cleavage of the diseaseassociated variant protein by furin in the secretory pathway. Susceptibility to proteolysis occurs because of the mutation-induced disruption of a Ca2þ-binding site near the site of cleavage. Loss of Ca2þ binding causes a significant decrease in the stability of gelsolin near the cleavage site, unmasking a furin cleavage site that is protected in the wild-type protein.
18. West MW, Wang W, Patterson J, Mancias JD, Beasley JR, Hecht MH: De novo amyloid proteins from designed combinatorial libraries. Proc Natl Acad Sci USA 1999, 96:11211-11216.
32. Kim SH, Wang R, Gordon DJ, Bass J, Steiner DF, Lynn DG, Thinakaran G, Meredith SC, Sisodia SS: Furin mediates enhanced production of fibrillogenic ABri peptides in familial British dementia. Nat Neurosci 1999, 2:984-988.
19. Lambert MP, Barlow AK, Chromy BA, Edwards C, Freed R, Liosatos M, Morgon TE, Rozovsky I, Trommer B, Viola KL et al.: Diffusable, nonfibrillar ligands derived from Ab1-42 are potent central nervous system toxins. Proc Natl Acad Sci USA 1998, 95:6448-6453.
33. Kiuru S: Gelsolin-related familial amyloidosis, Finnish type (FAF), and its variants found worldwide. Amyloid 1998, 5:55-66.
20. Hartley DM, Walsh DM, Ye CP, Diehl T, Vasquez S, Vassilev PM, Teplow DB, Selkoe DJ: Protofibrillar intermediates of amyloid beta-protein induce acute electrophysiological changes and progressive neurotoxicity in cortical neurons. J Neurosci 1999, 19:8876-8884.
34. Maury CP: Gelsolin-related amyloidosis. Identification of the amyloid protein in Finnish hereditary amyloidosis as a fragment of variant gelsolin. J Clin Invest 1991, 87:1195-1199. 35. Kwiatkowski DJ, Stossel TP, Orkin SH, Mole JE, Colten HR, Yin HL: Plasma and cytoplasmic gelsolins are encoded by a single gene and contain a duplicated actin-binding domain. Nature 1986, 323:455-458.
21. Hardy J, Selkoe DJ: The amyloid hypothesis of Alzheimer’s disease: progress and problems on the road to therapeutics. Science 2002, 297:353-356.
36. Yin HL: Gelsolin: calcium- and polyphosphoinositide-regulated actin-modulating protein. Bioessays 1987, 7:176-179.
22. Volles MJ, Lansbury PT Jr: Zeroing in on the pathogenic form of alpha-synuclein and its mechanism of neurotoxicity in Parkinson’s disease. Biochemistry 2003, 42:7871-7878.
37. Kangas H, Ulmanen I, Paunio T, Kwiatkowski DJ, Lehtovirta M, Jalanko A, Peltonen L: Functional consequences of amyloidosis mutation for gelsolin polypeptide - analysis of gelsolin-actin interaction and gelsolin processing in gelsolin knock-out fibroblasts. FEBS Lett 1999, 454:233-239.
23. Caughey B, Lansbury PT: Protofibrils, pores, fibrils, and neurodegeneration: separating the responsible protein aggregates from the innocent bystanders. Annu Rev Neurosci 2003, 26:267-298. 24. Goldberg MS, Lansbury PT Jr: Is there a cause-and-effect relationship between alpha-synuclein fibrillization and Parkinson’s disease? Nat Cell Biol 2000, 2:E115-E119. 25. Sousa MM, Cardoso I, Fernandes R, Guimaraes A, Saraiva MJ: Deposition of transthyretin in early stages of familial amyloidotic polyneuropathy: evidence for toxicity of nonfibrillar aggregates. Am J Pathol 2001, 159:1993-2000. 26. Bucciantini M, Giannoni E, Chiti F, Baroni F, Formigli L, Zurdo J, Taddei N, Ramponi G, Dobson CM, Stefani M: Inherent toxicity of aggregates implies a common mechanism for protein misfolding diseases. Nature 2002, 416:507-511. Small soluble aggregates of two non-disease-associated proteins are significantly more toxic to cultured cells than the monomeric soluble or mature fibrillar proteins. These data suggest that amyloid-associated Current Opinion in Structural Biology 2003, 13:674–682
38. Witke W, Sharpe AH, Hartwig JH, Azuma T, Stossel TP, Kwiatkowski DJ: Hemostatic, inflammatory, and fibroblast responses are blunted in mice lacking gelsolin. Cell 1995, 81:41-51. 39. Arora PD, Janmey PA, McCulloch CA: A role for gelsolin in stress fiber-dependent cell contraction. Exp Cell Res 1999, 250:155-167. 40. Lu M, Witke W, Kwiatkowski DJ, Kosik KS: Delayed retraction of filopodia in gelsolin null mice. J Cell Biol 1997, 138:1279-1287. 41. Furukawa K, Fu W, Li Y, Witke W, Kwiatkowski DJ, Mattson MP: The actin-severing protein gelsolin modulates calcium channel and NMDA receptor activities and vulnerability to excitotoxicity in hippocampal neurons. J Neurosci 1997, 17:8178-8186. 42. Singh SS, Chauhan A, Murakami N, Chauhan VP: Profilin and gelsolin stimulate phosphatidylinositol 3-kinase activity. Biochemistry 1996, 35:16544-16549. www.current-opinion.com
Pathological and functional amyloid formation Huff, Balch and Kelly 681
43. Sun HQ, Yamamoto M, Mejillano M, Yin HL: Gelsolin, a multifunctional actin regulatory protein. J Biol Chem 1999, 274:33179-33182. 44. Endres M, Fink K, Zhu J, Stagliano NE, Bondada V, Geddes JW, Azuma T, Mattson MP, Kwiatkowski DJ, Moskowitz MA: Neuroprotective effects of gelsolin during murine stroke. J Clin Invest 1999, 103:347-354. 45. Tanaka M, Mullauer L, Ogiso Y, Fujita H, Moriya S, Furuuchi K, Harabayashi T, Shinohara N, Koyanagi T, Kuzumaki N: Gelsolin: a candidate for suppressor of human bladder cancer. Cancer Res 1995, 55:3228-3232.
amyloidosis of the Finnish type. In Amyloid and Amyloidosis. Edited by Kisilevsky R, Benson MD, Frangione B, Gauldie J, Muckle TJ, Young ID. London: Parthenon; 1993:623-625. 63. Maury CP, Sletten K, Totty N, Kangas H, Liljestrom M: Identification of the circulating amyloid precursor and other gelsolin metabolites in patients with G654A mutation in the gelsolin gene (Finnish familial amyloidosis): pathogenetic and diagnostic implications. Lab Invest 1997, 77:299-304. 64. Maury CPJ, Rossi H: Demonstration of a circulating 65K gelsolin variant specific for familial amyloidosis, Finnish type. Biochem Biophys Res Commun 1993, 191:41-44.
46. Moriya S, Yanagihara K, Fujita H, Kuzumaki N: Differential expression of HSP90, gelsolin and GST-p in human gastric carcinoma cell lines. Int J Oncol 1994, 5:1347-1351.
65. Paunio T, Kiuru S, Karonen S-L, Palo J, Peltonen L: Quantification of serum and cerebrospinal fluid gelsolin in familial amyloidosis, Finnish type (AGel). Amyloid 1994, 1:80-89.
47. Afify AM, Werness B A: Decreased expression of the actinbinding protein gelsolin in endometrial and ovarian adenocarcinomas. Appl Immunohistochem Mol Morphol 1998, 6:30-34.
66. Steiner DF: The proprotein convertases. Curr Opin Chem Biol 1998, 2:31-39.
48. Asch HL, Winston JS, Edge SB, Stomper PC, Asch BB: Downregulation of gelsolin expression in human breast ductal carcinoma in situ with and without invasion. Breast Cancer Res Treat 1999, 55:179-188. 49. Asch HL, Head K, Dong Y, Natoli F, Winston JS, Connolly JL, Asch BB: Widespread loss of gelsolin in breast cancers of humans, mice, and rats. Cancer Res 1996, 56:4841-4845. 50. Furuuchi K, Fujita H, Tanaka M, Shichinohe T, Senmaru N, Ogiso Y, Moriya S, Hamada M, Kato H, Kuzumaki N: Gelsolin as a suppressor of malignant phenotype in human colon cancer. Tumor Targeting 1996, 2:277-283.
67. Zapun A, Grammatyka S, Deral G, Vernet T: Calcium-dependent conformational stability of modules 1 and 2 of human gelsolin. Biochem J 2000, 350:873-881. 68. Huff ME, Page LJ, Balch WE, Kelly JW: Gelsolin domain 2 Ca2R affinity determines susceptibility to furin proteolysis and familial amyloidosis of Finnish type. J Mol Biol 2003, 334:119-127. 69. Kazmirski SL, Isaacson RL, An C, Buckle A, Johnson CM, Daggett V, Fersht AR: Loss of a metal-binding site in gelsolin leads to familial amyloidosis-Finnish type. Nat Struct Biol 2002, 9:112-116. 70. Marks MS, Seabra MC: The melanosome: membrane dynamics in black and white. Nat Rev Mol Cell Biol 2001, 2:738-748.
51. Dosaka-Akita H, Hommura F, Fujita H, Kinoshita I, Nishi M, Morikawa T, Katoh H, Kawakami Y, Kuzumaki N: Frequent loss of gelsolin expression in non-small cell lung cancers of heavy smokers. Cancer Res 1998, 58:322-327.
71. Raposo G, Marks MS: The dark side of lysosome-related organelles: specialization of the endocytic pathway for melanosome biogenesis. Traffic 2002, 3:237-248.
52. Lee HK, Driscoll D, Asch H, Asch B, Zhang PJ: Downregulated gelsolin expression in hyperplastic and neoplastic lesions of the prostate. Prostate 1999, 40:14-19.
72. Raposo G, Tenza D, Murphy DM, Berson JF, Marks MS: Distinct protein sorting and localization to premelanosomes, melanosomes, and lysosomes in pigmented melanocytic cells. J Cell Biol 2001, 152:809-824.
53. Chaponnier C, Gabbiani G: Gelsolin modulation in epithelial and stromal cells of mammary carcinoma. Am J Pathol 1989, 134:597-603. 54. Porter RM, Holme TC, Newman EL, Hopwood D, Wilkinson JM, Cuschieri A: Monoclonal antibodies to cytoskeletal proteins: an immunohistochemical investigation of human colon cancer. J Pathol 1993, 170:435-440. 55. Fujita H, Okada F, Hamada J, Hosokawa M, Moriuchi T, Koya RC, Kuzumaki N: Gelsolin functions as a metastasis suppressor in B16-BL6 mouse melanoma cells and requirement of the carboxyl-terminus for its effect. Int J Cancer 2001, 93:773-780. 56. Mullauer L, Fujita H, Ishizaki A, Kuzumaki N: Tumor-suppressive function of mutated gelsolin in ras-transformed cells. Oncogene 1993, 8:2531-2536. 57. Sagawa N, Fujita H, Banno Y, Nozawa Y, Katoh H, Kuzumaki N: Gelsolin suppresses tumorigenicity through inhibiting PKC activation in a human lung cancer cell line, PC10. Br J Cancer 2003, 88:606-612. 58. McLaughlin PJ, Gooch JT, Mannherz HG, Weeds AG: Structure of gelsolin segment 1-actin complex and the mechanism of filament severing. Nature 1993, 364:685-692. 59. Burtnick LD, Koepf EK, Grimes J, Jones EY, Stuart DI, Mclaughlin PJ, Robinson RC: The crystal structure of plasma gelsolin: implications for actin severing, capping, and nucleation. Cell 1997, 90:661-670.
73. Kwon BS, Halaban R, Kim GS, Usack L, Pomerantz S, Haq AK: A melanocyte-specific complementary DNA clone whose expression is inducible by melanotropin and isobutylmethyl xanthine. Mol Biol Med 1987, 4:339-355. 74. Martinez-Esparza M, Jimenez-Cervantes C, Bennett DC, Lozano JA, Solano F, Garcia-Borron JC: The mouse silver locus encodes a single transcript truncated by the silver mutation. Mamm Genome 1999, 10:1168-1171. 75. Berson JF, Harper DC, Tenza D, Raposo G, Marks MS: Pmel17 initiates premelanosome morphogenesis within multivesicular bodies. Mol Biol Cell 2001, 12:3451-3464. The melanosomal protein Pmel17 is cleaved into two subunits in a postGolgi compartment during secretion. Overexpression of Pmel17 in nonpigmented cells induces the formation of striations within multivesicular bodies, indicating that the presence of Pmel17 is directly involved in striation formation during melanosomal maturation. 76. Berson JF, Theos AC, Harper DC, Tenza D, Raposo G, Marks MS: Proprotein convertase cleavage liberates a fibrillogenic fragment of a resident glycoprotein to initiate melanosome biogenesis. J Cell Biol 2003, 161:521-533. The development of melanosomes is dependent on the formation of lumenal fibrous striations composed of Pmel17. Furin proteolysis of Pmel17 late in the secretory pathway is required for the initiation of striation formation and melanosomal maturation. 77. Kelly JW, Balch WE: Amyloid as a natural product. J Cell Biol 2003, 161:461-462.
60. Robinson RC, Mejillano M, Le VP, Burtnick LD, Yin HL, Choe S: Domain movement in gelsolin: a calcium-activated switch. Science 1999, 286:1939-1942.
78. Brack A, Orgel LE: Beta structures of alternating polypeptides and their possible prebiotic significance. Nature 1975, 256:383-387.
61. Ratnaswamy G, Koepf E, Bekele H, Yin H, Kelly JW: The amyloidogenicity of gelsolin is controlled by proteolysis and pH. Chem Biol 1999, 6:293-304.
79. Zhang S, Holmes T, Lockshin C, Rich A: Spontaneous assembly of a self-complementary oligopeptide to form a stable macroscopic membrane. Proc Natl Acad Sci USA 1993, 90:3334-3338.
62. Sunada Y, Sakurai T, Nomomura Y, Kanazawa I: Abnormal gelsolin fragments in the plasma of patients with familial
80. Iconomidou VA, Vriend G, Hamodrakas SJ: Amyloids protect the silkmoth oocyte and embryo. FEBS Lett 2000, 479:141-145.
www.current-opinion.com
Current Opinion in Structural Biology 2003, 13:674–682
682 Proteins
81. Chapman MR, Robinson LS, Pinkner JS, Roth R, Heuser J, Hammar M, Normark S, Hultgren SJ: Role of Escherichia coli curli operons in directing amyloid fiber formation. Science 2002, 295:851-855. E. coli use highly aggregated protein fibers known as curli to mediate binding to surfaces and other proteins; these fibers are indistinguishable from eukaryotic pathogenic amyloid. Formation of curli fibers is carefully regulated by specific assembly machinery expressed as multiple proteins from the same operon.
89. Chiti F, Webster P, Taddei N, Clark A, Stefani M, Ramponi G, Dobson CM: Designing conditions for in vitro formation of amyloid protofilaments and fibrils. Proc Natl Acad Sci USA 1999, 96:3590-3594.
82. Claessen D, Rink R, de Jong W, Siebring J, de Vreugd P, Boersma FGH, Dijkhuizen L, Wosten HAB: A novel class of secreted hydrophobic proteins is involved in aerial hyphae formation in Streptomyces coelicolor by forming amyloid-like fibrils. Genes Dev 2003, 17:1714-1726. S. coelicolor produce amyloid-type fibrils that assemble on the surface of the bacterium and modulate its ability to grow in water or air.
91. Villegas V, Zurdo J, Filimonov V, Aviles F, Dobson C, Serrano L: Protein engineering as a strategy to avoid formation of amyloid fibrils. Protein Sci 2000, 9:1700-1708.
83. Mackay JP, Matthews JM, Winefield RD, Mackay LG, Haverkamp RG, Templeton MD: The hydrophobin EAS is largely unstructured in solution and functions by forming amyloid-like structures. Structure 2001, 9:83-91.
93. Guijarro JI, Sunde M, Jones JA, Campbell ID, Dobson CM: Amyloid fibril formation by an SH3 domain. Proc Natl Acad Sci USA 1998, 95:4224-4228.
84. Eaglestone SS, Cox BS, Tuite MF: Translation termination efficiency can be regulated in Saccharomyces cerevisiae by environmental stress through a prion-mediated mechanism. EMBO J 1999, 18:1974-1981. 85. True HL, Lindquist SL: A yeast prion provides a mechanism for genetic variation and phenotypic diversity. Nature 2000, 407:477-483. 86. Osherovich LZ, Weissman JS: The utility of prions. Dev Cell 2002, 2:143-151. 87. Dos Reis S, Coulary-Salin B, Forge V, Lascu I, Begueret J, Saupe SJ: The HET-s prion protein of the filamentous fungus Podospora anserina aggregates in vitro into amyloid-like fibrils. J Biol Chem 2002, 277:5703-5706. HET-s is a fungal prion protein that aggregates into fibrils exhibiting all of the characteristics of amyloid. HET-s infection across strains may serve to transmit a necessary biological function. 88. Maddelein M-L, Dos Reis S, Duvezin-Caubet S, Coulary-Salin B, Saupe SJ: Amyloid aggregates of the HET-s prion protein are infectious. Proc Natl Acad Sci USA 2002, 99:7402-7407. Amyloid aggregates of recombinant HET-s formed in vitro are infectious when introduced into fungal cells, demonstrating the protein-only mechanism characteristic of prion propagation.
Current Opinion in Structural Biology 2003, 13:674–682
90. Litvinovich SV, Brew SA, Aota S, Akiyama SK, Haudenschild C, Ingham KC: Formation of amyloid-like fibrils by self-association of a partially unfolded fibronectin type III module. J Mol Biol 1998, 280:245-258.
92. Ferguson N, Berriman J, Petrovich M, Sharpe TD, Finch JT, Fersht AR: Rapid amyloid fiber formation from the fast-folding WW domain FBP28. Proc Natl Acad Sci USA 2003, 100:9814-9819.
94. Graether SP, Slupsky CM, Sykes BD: Freezing of a fish antifreeze protein results in amyloid fibril formation. Biophys J 2003, 84:552-557. 95. Konno T, Murata K, Nagayama K: Amyloid-like aggregates of a plant protein: a case of a sweet-tasting protein, monellin. FEBS Lett 1999, 454:122-126. 96. Ramirez-Alvarado M, Merkel JS, Regan L: A systematic exploration of the influence of the protein stability on amyloid fibril formation in vitro. Proc Natl Acad Sci USA 2000, 97:8979-8984. 97. Chiti F, Bucciantini M, Capanni C, Taddei N, Dobson CM, Stefani M: Solution conditions can promote formation of either amyloid protofilaments or mature fibrils from the HypF N-terminal domain. Protein Sci 2001, 10:2541-2547. 98. Yutani K, Takayama G, Goda S, Yamagata Y, Maki S, Namba K, Tsunasawa S, Ogasahara K: The process of amyloid-like fibril formation by methionine aminopeptidase from a hyperthermophile, Pyrococcus furiosus. Biochemistry 2000, 39:2769-2777. 99. Pertinhez TA, Bouchard M, Tomlinson EJ, Wain R, Ferguson SJ, Dobson CM, Smith LJ: Amyloid fibril formation by a helical cytochrome. FEBS Lett 2001, 495:184-186.
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