- Email: [email protected]
Converging views of endocytosis in yeast and mammals
Available online at www.sciencedirect.com
Converging views of endocytosis in yeast and mammals Elizabeth Conibear Receptor-mediated endocytosis is important for the selective internalization of membrane proteins. In mammals, clathrin, adaptors, and dynamin play prominent roles in regulating cargo selection and vesicle formation. Endocytosis in yeast is generally conserved, but exhibits significant and perplexing differences in the relative importance of clathrin adaptors, dynamin-like proteins, and actin. Recent studies are now reconciling divergent views of endocytic processes in yeast and mammals. The discovery of cargo-specific functions for yeast homologs of mammalian clathrin adaptors has rapidly expanded the number of endocytic adaptors in yeast. Moreover, unifying models have been advanced to explain how dynamin, actin, and membrane-deforming proteins drive membrane scission. While differences remain, discoveries from each system will continue to inform the other. Address Centre for Molecular Medicine and Therapeutics, Child and Family Research Institute, Department of Medical Genetics, University of British Columbia, Vancouver, BC V5Z 4H4, Canada Corresponding author: Conibear, Elizabeth ([email protected])
Current Opinion in Cell Biology 2010, 22:513–518 This review comes from a themed issue on Membranes and organelles Edited by Suzanne Pfeffer and Peter Novick Available online 10th June 2010 0955-0674/$ – see front matter # 2010 Elsevier Ltd. All rights reserved. DOI 10.1016/j.ceb.2010.05.009
Introduction Clathrin-mediated endocytosis can be broken down into three main steps: the clustering of coat proteins and cargo at the future endocytic site, membrane deformation, and a scission event that releases the fully formed endocytic vesicle. Many aspects of this process are evolutionarily conserved. Components of the mammalian endocytosis machinery have homologs in yeast, and two-color live-cell imaging studies have shown these proteins are recruited in a similar temporal sequence in yeast and mammals [1,2]. Despite these similarities, significant differences have been noted (for reviews see [3–5]). Clathrin adaptors such as AP-2 that are central in linking cargo to the vesicle coat in mammals are present yet are dispensable in yeast. And while dynamin is crucial for vesicle scission in mammals, yeast dynamin-like proteins do not play a similar role www.sciencedirect.com
[6,7]. Finally, actin is not absolutely required for uptake in many mammalian cell types, yet is essential for endocytosis in yeast. This review will highlight recent findings that address these apparent differences, with a focus on the yeast model system. Studies of the past year have uncovered cargo-specific roles for yeast clathrin adaptors, and have identified new families of conserved endocytic adaptor proteins present in yeast and man. Meanwhile, new models have been advanced to explain differences in the relative importance of actin and dynamin for vesicle scission in various systems. These studies reconcile views of endocytosis in different organisms and suggest underlying processes are fundamentally conserved.
Emerging roles for cargo-specific adaptors in yeast Clathrin adaptors: AP-2 and AP180
Given the well-established role of AP-2 adaptors in mammalian cells, it has been perplexing that AP-2 mutations have no discernable effect in yeast. A recent large-scale screen provides the first indication that the yeast AP-2 complex may in fact be important for the uptake of specific cargo [8]. Mutants lacking any of the four AP-2 subunits are resistant to K28, a virally encoded yeast killer toxin that enters cells after binding and endocytosis of an unidentified receptor. AP-2 mutants are unable to take up toxin from the media, and because yeast AP-2 is recruited to endocytic sites concurrent with clathrin and other early coat components, a direct role in receptor uptake seems likely. Identifying the K28 receptor and characterizing its interactions with AP-2 will help rule out indirect effects on toxin uptake. A cargo-specific role for the yeast homologs of the clathrin adaptor complex AP180/CALM was also uncovered in a genome-wide screen of yeast deletion mutants [9]. Cells lacking both AP180 homologs are severely defective in the uptake of the VAMP/synaptobrevinlike vSNARE Snc1, but not other commonly studied cargo. Because AP180 mediates the uptake of synaptobrevin in worms, flies, and mammals this sorting role appears to be highly conserved [10–12]. The signal recognized by AP180 has not been determined for any cargo, though mutation of two residues in Snc1 blocks its internalization [9,13]. It is not known if these residues form a small sorting motif or are part of a larger conformational epitope, as seen for other SNARE– adaptor interactions [14]. Current Opinion in Cell Biology 2010, 22:513–518
514 Membranes and organelles
The muniscins: a new family of mu homology domain adaptors
A recent structural analysis identified yeast Syp1 and the related mammalian proteins FCHO1/2 and SGIP1 as members of a new family of conserved endocytic adaptor proteins with homology to the cargo-binding medium (mu) chain of clathrin adaptor complexes [15]. The Syp1 mu homology domain (uHD) binds directly to the candidate cargo protein Mid2 and also interacts with the endocytic scaffold protein Ede1, which may stabilize coat formation [15,16]. In Syp1, the uHD is separated from a membrane-binding F-BAR domain by an unstructured linker region that negatively regulates actin polymerization by inhibiting Las17/WASP [15,17]. This arrangement could link cargo recruitment to vesicle budding [17]. The shallow curvature of the Syp1 F-BAR domain may favor association at initial stages of coat formation [15,18]. As membrane curvature increases, dissociation of Syp1 would activate actin assembly and further accelerate invagination. The timing of Syp1 recruitment is consistent in this model: Syp1 and clathrin arrive early at endocytic sites, but unlike clathrin, Syp1 remains at the cell cortex and does not associate with the invaginating vesicle [16]. However, loss of Syp1 does not alter the progression of endocytic vesicle formation [16,17], suggesting other actin regulatory proteins play a redundant role in vivo. The mammalian Syp1-like proteins FCHO1/2 and SGIP1 are also recruited to clathrin-coated pits, have an Nterminal membrane-binding domain, and interact with the Ede1-like protein Eps15 [15,16,19]. It remains to be seen if mammalian muniscin proteins have similar effects on actin assembly through the Las17 homolog WASP, which is subject to different regulatory mechanisms, including auto-inhibition.
Arrestin-like proteins
In mammals, clathrin associated sorting proteins often work in conjunction with clathrin adaptors [20]. For example, b-arrestins link G protein-coupled receptors (GPCRs) to the endocytic machinery to downregulate signaling. When b-arrestin binds a ligand-activated, phosphorylated GPCR it undergoes a conformational change that exposes a C-terminal tail with motifs for adaptin and clathrin binding. This recruits the GPCR to clathrincoated pits and promotes receptor endocytosis. Yeast do not have canonical arrestins, but do contain representatives of a conserved family of arrestin-like proteins, the arrestin-related trafficking adaptors (ARTs; [21,22,23]). ART C-terminal tails lack adaptin and clathrin-binding sequences, and instead contain PxY motifs that recruit the ubiquitin ligase Rsp5 [21]. Binding of ARTs to activated receptors results in their ubiquitination and internalization. ARTs regulate the uptake of a diverse group of nutrient transporters whose expression at the cell surface is tightly controlled in response to extracellular cues [21,22,24]. A high level of a particular substrate signals clearance of its cognate transporter from the cell surface, whereas cell stress triggers uptake of many different transporters. Although ARTs can act redundantly, several have been matched to the substrate-induced uptake of specific transporters (Table 1). Surprisingly, stress signaling often involves a different ART, typically one member of the related Art2/Art8 pair [21,24]. For example, Art4 regulates the glucose-induced uptake of the glucose transporter Hxt6, whereas its stress-induced internalization is mediated by Art8 [24]. Because linking PxY motifs to the arginine transporter Can1 tail bypasses the need for Art1 [21], ARTs function only as cargo-specific ubiquitin ligase adaptors and
Table 1 Cargo-specific endocytic adaptors in yeast Adaptor Mu homology domain
Arrestin-like
*
AP-2 complex SYP1 YAP1801/2 SLA1 ART1 ART2 ART3 ART4 ART5 ART6 ART7 ART8 ART9 ART10 BUL1/2
Alias
LDB19, CVS7 ECM21 ALY2 ROD1 YGR068C ALY1 ROG3 CSR2 RIM8 YLR392C
Homologs
Cargo
Reference
AP-2 complex SGIP1, FCHO1 AP180, CALM CIN85 Mammalian ADC proteins
Putative K28 receptor MID2 SNC1 WSC1, STE2 CAN1, LYP1, MUP1 LYP1*, SMF1 *
[8] [15,16,17] [9] [34,51] [21] [21,22]
HXT6 ITR1
[24] [24]
SMF1*, HXT6 * RIM21
[22,24] [52]
GAP1, CTR1
[25]
Stress-stimulated uptake.
Current Opinion in Cell Biology 2010, 22:513–518
www.sciencedirect.com
Converging views of endocytosis in yeast and mammals Conibear 515
are not needed for subsequent interactions with the endocytic machinery. The ubiquitin ligase adaptor Bul1 can act alone or redundantly with ARTs to mediate transporter uptake [24–26]. Interestingly, N-terminal domains in Bul1 and arrestins are members of a larger ‘Arrestin N-like’ clan [24,27]. Thus, the number of proteins distantly related to arrestins that act as ubiquitin ligase adaptors during endocytosis may be greater than currently recognized. How do ARTs recognize activated cargo? Phosphorylation is important, but not sufficient, for ART binding to the metal transporter Smf1 [22]. Nikko and Pelham have speculated that signaling releases transporters from membrane microdomains, allowing them to access both arrestins and the endocytic machinery [24]. In fact, addition of its substrate releases Can1 from ergosterol-dependent clusters that are distinct from sites of endocytosis [28], and mutations that disrupt formation of these clusters trigger Can1 endocytosis. It will be interesting to determine if this requires Art1. Further studies are needed to clarify the determinants recognized by ARTs and related ubiquitin adaptors, and to understand how different stimuli confer regulation by distinct ARTs. Other classes of membrane cargo may also require ARTs for internalization; the pheromone receptor Ste2 is a GPCR that requires phosphorylation and ubiquitination for uptake yet does not recruit Rsp5 directly [29]. The arrestin-like ART proteins are similar to arrestins in many ways, yet b-arrestins lack PxY motifs, and must use different mechanisms to recruit ubiquitin ligases [30]. These PxY motifs are conserved in mammal ART homologs, the ADC (arrestin domain containing) proteins, and it will be interesting to determine if mammalian ADCs have conserved roles as ubiquitin ligase adaptors for receptor uptake. Ubiquitin-binding adaptors
If arrestin-like proteins are important only for addition of the ubiquitin signal, how is the signal recognized by the endocytic machinery? Three coat components, the epsins Ent1 and Ent2 and the Eps15-like scaffold protein Ede1, have ubiquitin-binding domains (UBDs) believed to recruit ubiquitinated cargo [31]. Live-cell imaging shows that the pheromone receptor Ste2 moves to preexisting clathrin-coated pits after ligand binding and ubiquitination, and this clustering depends on Ent1, Ent2, and Ede1 [29]. Surprisingly, mutations that remove these UBDs while leaving other portions of these proteins intact do not prevent Ste2 uptake [32], arguing against an obligatory role in cargo recognition. It is likely these UBDs contribute to coated pit assembly [32], and that other proteins participate in recruitment of ubiquitinated cargo. The SH3 domains from several endocytic proteins, including the clathrin adaptor Sla1 and its mammalian homolog CIN85, bind ubiquitin in vitro [33] and may www.sciencedirect.com
serve as ubiquitin adaptors. The in vivo relevance of SH3ubiquitin interactions for yeast endocytosis remains to be demonstrated in vivo. However, it is interesting to note that the SHD1 domain of Sla1, which interacts with cargo containing an NPFxD signal, has an SH3-like fold [34,35]. How many more?
These studies have revealed unexpected diversity in the number of cargo-specific adaptors involved in yeast endocytosis. The functional importance of yeast proteins related to AP-2, AP180 and arrestins is clear, although few of the corresponding cargo proteins have been identified, and their internalization signals have not yet been mapped. Large-scale functional screens directed at additional cargo proteins may yet uncover new adaptor families, although exhaustive identification of all possible cargo-specific sorting proteins will be difficult. Additional work will also be needed to determine if all cargo-specific uptake requires clathrin, or if endocytic adaptors link to other components of the vesicle coat.
Reconciling different requirements for actin and dynamin Essential requirement for actin in yeast
The apparent differences between yeast and mammalian endocytosis extend to the invagination and scission reactions. Although a burst of actin polymerization precedes vesicle fission in mammalian cells as it does in yeast, inhibitor studies suggest actin is not absolutely required for the budding of clathrin-coated vesicles in most cell types [36,37,38]. Why, then, is actin so important for invagination and scission in yeast? The answer may have to do with the turgor pressure present in yeast cells, which provides an outwardly directed force on the plasma membrane. Actin polymerization has been proposed to generate additional force that counteracts this pressure. In support of this model, increasing turgor pressure has recently been shown to hinder endocytosis, whereas reducing the pressure difference suppresses the internalization defects of actin-bundling mutants and cells treated with the actin de-polymerizing drug Latrunculin A [39]. Interestingly, large clathrin-coated plaques found on the basal surface of adherent mammalian cells also require actin for their internalization [38]. It is tempting to speculate that in these cells actin polymerization overcomes the adhesive forces that anchor the basal membrane to the substrate [40]. Role of actin and dynamin in generating morphological differences
Endocytic structures in yeast and mammals are also morphologically distinct (Figure 1). In yeast, endocytic invaginations are wide-necked (30–50 nm) tubules covered by BAR-domain proteins, surrounded by an actin matrix, and coated with clathrin only at the tip [41]. By contrast, mammalian cells display round, clathrin-coated Current Opinion in Cell Biology 2010, 22:513–518
516 Membranes and organelles
Figure 1
Endocytic invaginations in yeast and mammals. In mammalian cells, dynamin encircles the short, narrow neck of budding clathrin-coated vesicles, which have lower levels of polymerized actin (left). Dynamin double knockout cells accumulate long, 36 nm diameter tubules decorated with BARdomain proteins, actin regulatory factors, and F-actin, and capped with a clathrin-coated bud (center). These are reminiscent of the tubular endocytic invaginations in yeast, which are attached to the membrane by a wide (30–50 nM) neck, and contain BAR-domain proteins, actin regulatory proteins, and are surrounded by a matrix of polymerized actin (right). Formation of elongated tubules is actin-dependent in both yeast and mammals.
budding profiles that are attached to the membrane by narrow necks and have much less associated actin. Recent work suggests dynamin could be responsible for these morphological differences [42]. Cells lacking both dynamin 1 and 2 accumulate long clathrin-capped tubular structures enriched for actin and F-BAR proteins strikingly similar to those in yeast [42] (Figure 1). When dynamin is present, scission may happen too quickly for high levels of actin to accumulate. Conversely, in yeast, the absence of dynamin could allow prolonged actin polymerization and the formation of tubular structures. This suggests that, despite the different appearance of endocytic profiles in yeast and mammals, the underlying processes that regulate invagination are fundamentally similar. A unifying model for vesicle invagination and scission
How can scission proceed in the absence of dynamin? Recent studies of clathrin-independent uptake of Shiga toxin provide evidence that actin promotes changes in the organization of the underlying lipids in endocytic tubules, and the resulting tension at membrane domain boundaries drives scission [43]. Drubin and colleagues have developed a quantitative theoretical model of endocytosis that suggests the membrane domains that drive scission in yeast result from differences in the distribution of PIP2, rather than cholesterol [44,45,46]. In this model, initial membrane curvature generated by Current Opinion in Cell Biology 2010, 22:513–518
coat protein assembly and actin/myosin pulling forces promotes recruitment of BAR-domain proteins, which form a tubule. The resulting increase in curvature activates the synaptojanin-like PIP2 phosphatase Inp52, but because BAR-domain proteins protect the tubular neck, PIP2 hydrolysis is limited to the bud. The resulting lipid phase boundary between bud and neck creates a lateral force that constricts the neck, leading to scission. The basic tenets of this model can be extended to mammalian cells, where dynamin could have a greater role in deforming the membrane and in preventing accumulation of synaptojanin at the neck of the budding vesicle by competing for binding partners [46,47,48]. This would protect PIP2 at the neck, causing phase segregation and scission. Several key features of this model remain to be tested experimentally. It will be important to define which lipid species are important for generating membrane domains and line tension, and to determine how PIP2 phosphatases are recruited and their activity regulated. Other factors may be at work in mammalian cells; after all, scission does not occur in dynamin knockouts despite abundant actin polymerization and BAR-domain recruitment [42]. It is important to note these models do not exclude roles for actin and dynamin at other steps in vesicle formation that precede the final scission event [49,50]. www.sciencedirect.com
Converging views of endocytosis in yeast and mammals Conibear 517
Conclusions Yeast is an important model for understanding protein trafficking in higher cells, and the underlying mechanisms that govern budding of vesicles from cellular membranes are generally well conserved. Recent work provides explanations for some of the apparent discrepancies between endocytic processes in yeast and man. Others are less easily reconciled, and may reflect the plasticity of uptake mechanisms in various organisms and cell types. A single mammalian cell can employ a large spectrum of endocytic mechanisms to internalize different cargo, and each has different requirements for clathrin, dynamin, actin regulators, and lipids. Elucidating the unique and conserved features of these processes will be essential in understanding the fundamental principles that underlie cargo selection, membrane deformation, and vesicle scission in all organisms and cell types.
10. Bao H, Daniels RW, MacLeod GT, Charlton MP, Atwood HL, Zhang B: AP180 maintains the distribution of synaptic and vesicle proteins in the nerve terminal and indirectly regulates the efficacy of Ca2+-triggered exocytosis. J Neurophysiol 2005, 94:1888-1903. 11. Dittman JS, Kaplan JM: Factors regulating the abundance and localization of synaptobrevin in the plasma membrane. Proc Natl Acad Sci USA 2006, 103:11399-11404. 12. Harel A, Wu F, Mattson MP, Morris CM, Yao PJ: Evidence for CALM in directing VAMP2 trafficking. Traffic 2008, 9:417-429. 13. Lewis MJ, Nichols BJ, Prescianotto-Baschong C, Riezman H, Pelham HR: Specific retrieval of the exocytic SNARE Snc1p from early yeast endosomes. Mol Biol Cell 2000, 11:23-38. 14. Miller SE, Collins BM, McCoy AJ, Robinson MS, Owen DJ: A SNARE–adaptor interaction is a new mode of cargo recognition in clathrin-coated vesicles. Nature 2007, 450:570-574.
Acknowledgements
15. Reider A, Barker SL, Mishra SK, Im YJ, Maldonado-Ba´ez L, Hurley JH, Traub LM, Wendland B: Syp1 is a conserved endocytic adaptor that contains domains involved in cargo selection and membrane tubulation. EMBO J 2009, 28:3103-3116. Solving the structure of yeast Syp1 identifies a membrane-tubulating FBAR domain and a cargo-binding mu homology domain, defining a new family of conserved adaptor proteins.
I regret that many excellent papers could not be discussed owing to space limitations. I thank Helen Burston, Beverly Wendland, and Margaret Robinson for interesting discussions and helpful comments. Funding was provided by the Canadian Institutes of Health Research (FRN#64394).
16. Stimpson H, Toret C, Cheng A, Pauly B, Drubin D: Early-arriving Syp1p and Ede1p function in endocytic site placement and formation in budding yeast. Mol Biol Cell 2009:E09-050429v0421.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
Perrais D, Merrifield CJ: Dynamics of endocytic vesicle creation. Dev Cell 2005, 9:581-592.
2.
Kaksonen M, Toret CP, Drubin DG: Harnessing actin dynamics for clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 2006, 7:404-414.
3.
Baggett JJ, Wendland B: Clathrin function in yeast endocytosis. Traffic 2001, 2:297-302.
4.
Galletta B, Cooper J: Actin and endocytosis: mechanisms and phylogeny. Curr Opin Cell Biol 2009.
5.
Robertson AS, Smythe E, Ayscough KR: Functions of actin in endocytosis. Cell Mol Life Sci 2009, 66:2049-2065.
6.
Yu X, Cai M: The yeast dynamin-related GTPase Vps1p functions in the organization of the actin cytoskeleton via interaction with Sla1p. J Cell Sci 2004, 117:3839-3853.
7.
Nannapaneni S, Wang D, Jain S, Schroeder B, Highfill C, Reustle L, Pittsley D, Maysent A, Moulder S, McDowell R et al.: The yeast dynamin-like protein Vps1:vps1 mutations perturb the internalization and the motility of endocytic vesicles and endosomes via disorganization of the actin cytoskeleton. Eur J Cell Biol 2010, 89:499-508.
8.
Carroll SY, Stirling PC, Stimpson HEM, Giesselmann E, Schmitt MJ, Drubin DG: A yeast killer toxin screen provides insights into a/b toxin entry, trafficking, and killing mechanisms. Dev Cell 2009, 17:552-560. This report provides the first evidence of a role for yeast AP-2 in protein uptake.
9.
Burston HE, Maldonado-Baez L, Davey M, Montpetit B, Schluter C, Wendland B, Conibear E: Regulators of yeast endocytosis identified by systematic quantitative analysis. J Cell Biol 2009, 185:1097-1110. A systematic survey of yeast knockout mutants identifies genes with general and cargo-specific roles in the endocytosis of SNARE-based reporter proteins.
www.sciencedirect.com
17. Boettner DR, D’Agostino JL, Torres OT, Daugherty-Clarke K, Uygur A, Reider A, Wendland B, Lemmon SK, Goode BL: The FBAR protein Syp1 negatively regulates WASp-Arp2/3 complex activity during endocytic patch formation. Curr Biol 2009, 19:1979-1987. This paper shows that Syp1 negatively regulates actin assembly by inhibiting Las17/WASp activation of the Arp2/3 complex. 18. Suetsugu S, Toyooka K, Senju Y: Subcellular membrane curvature mediated by the BAR domain superfamily proteins. Semin Cell Dev Biol 2009. 19. Uezu A, Horiuchi A, Kanda K, Kikuchi N, Umeda K, Tsujita K, Suetsugu S, Araki N, Yamamoto H, Takenawa T et al.: SGIP1alpha is an endocytic protein that directly interacts with phospholipids and Eps15. J Biol Chem 2007, 282:26481-26489. 20. Traub LM, Traub LM: Tickets to ride: selecting cargo for clathrin-regulated internalization. Nat Rev Mol Cell Biol 2009, 10:583. 21. Lin C, Macgurn J, Chu T, Stefan C, Emr S: Arrestin-related ubiquitin-ligase adaptors regulate endocytosis and protein turnover at the cell surface. Cell 2008. This report identifies a family of arrestin-related proteins that recruit the ubiquitin ligase Rsp5 to specific plasma membrane proteins to promote their internalization. 22. Nikko E, Sullivan J, Pelham H: Arrestin-like proteins mediate ubiquitination and endocytosis of the yeast metal transporter Smf1. EMBO Rep 2008. 23. Aubry L, Guetta D, Klein G: The arrestin fold: variations on a theme. Curr Genomics 2009, 10:133-142. 24. Nikko E, Pelham HRB: Arrestin-mediated endocytosis of yeast plasma membrane transporters. Traffic 2009, 10:1856-1867. 25. Liu J, Sitaram A, Burd CG: Regulation of copper-dependent endocytosis and vacuolar degradation of the yeast copper transporter, Ctr1p, by the Rsp5 ubiquitin ligase. Traffic 2007, 8:1375-1384. 26. Risinger AL, Kaiser CA: Different ubiquitin signals act at the golgi and plasma membrane to direct GAP1 trafficking. Mol Biol Cell 2008, 19:2962-2972. 27. Finn R, Mistry J, Tate J, Coggill P, Heger A, Pollington J, Gavin O, Gunasekaran P, Ceric G, Forslund K et al.: The Pfam protein families database. Nucleic Acids Res 2010, 38:D211. Current Opinion in Cell Biology 2010, 22:513–518
518 Membranes and organelles
28. Grossmann G, Malinsky J, Stahlschmidt W, Loibl M, Weig-Meckl I, Frommer WB, Opekarova´ M, Tanner W: Plasma membrane microdomains regulate turnover of transport proteins in yeast. J Cell Biol 2008, 183:1075-1088.
41. Idrissi F-Z, Gro¨tsch H, Ferna´ndez-Golbano IM, PresciattoBaschong C, Riezman H, Geli M-I: Distinct acto/myosin-I structures associate with endocytic profiles at the plasma membrane. J Cell Biol 2008, 180:1219-1232.
29. Toshima JY, Nakanishi J-i, Mizuno K, Toshima J, Drubin DG: Requirements for recruitment of a G protein-coupled receptor to clathrin-coated pits in budding yeast. Mol Biol Cell 2009, 20:5039-5050.
42. Ferguson S, Raimondi A, Paradise S, Shen H, Mesaki K, Ferguson A, Destaing O, Ko G, Takasaki J, Cremona O et al.: Coordinated actions of actin and BAR proteins upstream of dynamin at endocytic clathrin-coated pits. Dev Cell 2009, 17:811-822. Analysis of cells derived from dynamin 1 and 2 double conditional knockout mice shows that actin, actin regulators, and F-BAR proteins accumulate on arrested endocytic structures. This emphasizes the evolutionary similarity of endocytosis in yeast and mammals.
30. Shenoy SK, Xiao K, Venkataramanan V, Snyder PM, Freedman NJ, Weissman AM: Nedd4 mediates agonist-dependent ubiquitination, lysosomal targeting, and degradation of the beta2-adrenergic receptor. J Biol Chem 2008, 283:22166-22176. 31. Shih SC, Katzmann DJ, Schnell JD, Sutanto M, Emr SD, Hicke L: Epsins and Vps27p/Hrs contain ubiquitin-binding domains that function in receptor endocytosis. Nat Cell Biol 2002, 4:389-393. 32. Dores MR, Schnell JD, Maldonado-Baez L, Wendland B, Hicke L: The function of yeast epsin and Ede1 ubiquitin-binding domains during receptor internalization. Traffic 2010, 11:151-160. 33. Stamenova SD, French ME, He Y, Francis SA, Kramer ZB, Hicke L: Ubiquitin binds to and regulates a subset of SH3 domains. Mol Cell 2007, 25:273-284. 34. Howard JP, Hutton JL, Olson JM, Payne GS: Sla1p serves as the targeting signal recognition factor for NPFX(1,2)D-mediated endocytosis. J Cell Biol 2002, 157:315-326. 35. Mahadev RK, Di Pietro SM, Olson JM, Piao HL, Payne GS, Overduin M: Structure of Sla1p homology domain 1 and interaction with the NPFxD endocytic internalization motif. EMBO J 2007, 26:1963-1971. 36. Moskowitz HS, Yokoyama CT, Ryan TA: Highly cooperative control of endocytosis by clathrin. Mol Biol Cell 2005, 16:1769-1776. 37. Yarar D, Waterman-Storer CM, Schmid SL: A dynamic actin cytoskeleton functions at multiple stages of clathrin-mediated endocytosis. Mol Biol Cell 2005, 16:964-975. 38. Saffarian S, Cocucci E, Kirchhausen T: Distinct dynamics of endocytic clathrin-coated pits and coated plaques. PLoS Biol 2009, 7:e1000191. Quantitative live-cell imaging of adherent cultured cells demonstrates that actin is required for the internalization of flat clathrin-coated plaques on the basal surface, but is not required for the uptake of clathrin-coated buds at the ventral surface. 39. Aghamohammadzadeh S, Ayscough K: Differential requirements for actin during yeast and mammalian endocytosis. Nat Cell Biol 2009. This work presents evidence that actin is required at early stages of endocytosis in yeast to counteract turgor pressure, thus providing an explanation for the differing requirement for actin in yeast and mammals. 40. Traub LM: Clathrin couture: fashioning distinctive membrane coats at the cell surface. PLoS Biol 2009, 7:e1000192.
Current Opinion in Cell Biology 2010, 22:513–518
43. Ro¨mer W, Pontani L-L, Sorre B, Rentero C, Berland L, Chambon V, Lamaze C, Bassereau P, Sykes C, Gaus K et al.: Actin dynamics drive membrane reorganization and scission in clathrin-independent endocytosis. Cell 2010, 140:540-553. This paper shows that scission of Shiga toxin-induced tubules requires actin and is correlated with the formation of membrane domains. 44. Liu J, Kaksonen M, Drubin DG, Oster G: Endocytic vesicle scission by lipid phase boundary forces. Proc Natl Acad Sci USA 2006, 103:10277-10282. 45. Sun Y, Carroll S, Kaksonen M, Toshima JY, Drubin DG: PtdIns(4,5)P2 turnover is required for multiple stages during clathrin- and actin-dependent endocytic internalization. J Cell Biol 2007, 177:355-367. 46. Liu J, Sun Y, Drubin DG, Oster GF: The mechanochemistry of endocytosis. PLoS Biol 2009, 7:e1000204. In this paper, the authors present a theoretical model of endocytosis to explain the sequence and spatial organization of endocytic events in yeast. 47. McPherson PS: Regulatory role of SH3 domain-mediated protein-protein interactions in synaptic vesicle endocytosis. Cell Signal 1999, 11:229-238. 48. Krendel M, Osterweil EK, Mooseker MS: Myosin 1E interacts with synaptojanin-1 and dynamin and is involved in endocytosis. FEBS Lett 2007, 581:644-650. 49. Loerke D, Mettlen M, Yarar D, Jaqaman K, Jaqaman H, Danuser G, Schmid SL: Cargo and dynamin regulate clathrin-coated pit maturation. PLoS Biol 2009, 7:e57. 50. Mettlen M, Pucadyil T, Ramachandran R, Schmid SL: Dissecting dynamin’s role in clathrin-mediated endocytosis. Biochem Soc Trans 2009, 37:1022-1026. 51. Piao HL, Machado IM, Payne GS: NPFXD-mediated endocytosis is required for polarity and function of a yeast cell wall stress sensor. Mol Biol Cell 2007, 18:57-65. 52. Herrador A, Herranz S, Lara D, Vincent O: Recruitment of the ESCRT machinery to a putative seven-transmembranedomain receptor is mediated by an arrestin-related protein. Mol Cell Biol 2010, 30:897-907.
www.sciencedirect.com
Converging views of endocytosis in yeast and mammals Elizabeth Conibear Receptor-mediated endocytosis is important for the selective internalization of membrane proteins. In mammals, clathrin, adaptors, and dynamin play prominent roles in regulating cargo selection and vesicle formation. Endocytosis in yeast is generally conserved, but exhibits significant and perplexing differences in the relative importance of clathrin adaptors, dynamin-like proteins, and actin. Recent studies are now reconciling divergent views of endocytic processes in yeast and mammals. The discovery of cargo-specific functions for yeast homologs of mammalian clathrin adaptors has rapidly expanded the number of endocytic adaptors in yeast. Moreover, unifying models have been advanced to explain how dynamin, actin, and membrane-deforming proteins drive membrane scission. While differences remain, discoveries from each system will continue to inform the other. Address Centre for Molecular Medicine and Therapeutics, Child and Family Research Institute, Department of Medical Genetics, University of British Columbia, Vancouver, BC V5Z 4H4, Canada Corresponding author: Conibear, Elizabeth ([email protected])
Current Opinion in Cell Biology 2010, 22:513–518 This review comes from a themed issue on Membranes and organelles Edited by Suzanne Pfeffer and Peter Novick Available online 10th June 2010 0955-0674/$ – see front matter # 2010 Elsevier Ltd. All rights reserved. DOI 10.1016/j.ceb.2010.05.009
Introduction Clathrin-mediated endocytosis can be broken down into three main steps: the clustering of coat proteins and cargo at the future endocytic site, membrane deformation, and a scission event that releases the fully formed endocytic vesicle. Many aspects of this process are evolutionarily conserved. Components of the mammalian endocytosis machinery have homologs in yeast, and two-color live-cell imaging studies have shown these proteins are recruited in a similar temporal sequence in yeast and mammals [1,2]. Despite these similarities, significant differences have been noted (for reviews see [3–5]). Clathrin adaptors such as AP-2 that are central in linking cargo to the vesicle coat in mammals are present yet are dispensable in yeast. And while dynamin is crucial for vesicle scission in mammals, yeast dynamin-like proteins do not play a similar role www.sciencedirect.com
[6,7]. Finally, actin is not absolutely required for uptake in many mammalian cell types, yet is essential for endocytosis in yeast. This review will highlight recent findings that address these apparent differences, with a focus on the yeast model system. Studies of the past year have uncovered cargo-specific roles for yeast clathrin adaptors, and have identified new families of conserved endocytic adaptor proteins present in yeast and man. Meanwhile, new models have been advanced to explain differences in the relative importance of actin and dynamin for vesicle scission in various systems. These studies reconcile views of endocytosis in different organisms and suggest underlying processes are fundamentally conserved.
Emerging roles for cargo-specific adaptors in yeast Clathrin adaptors: AP-2 and AP180
Given the well-established role of AP-2 adaptors in mammalian cells, it has been perplexing that AP-2 mutations have no discernable effect in yeast. A recent large-scale screen provides the first indication that the yeast AP-2 complex may in fact be important for the uptake of specific cargo [8]. Mutants lacking any of the four AP-2 subunits are resistant to K28, a virally encoded yeast killer toxin that enters cells after binding and endocytosis of an unidentified receptor. AP-2 mutants are unable to take up toxin from the media, and because yeast AP-2 is recruited to endocytic sites concurrent with clathrin and other early coat components, a direct role in receptor uptake seems likely. Identifying the K28 receptor and characterizing its interactions with AP-2 will help rule out indirect effects on toxin uptake. A cargo-specific role for the yeast homologs of the clathrin adaptor complex AP180/CALM was also uncovered in a genome-wide screen of yeast deletion mutants [9]. Cells lacking both AP180 homologs are severely defective in the uptake of the VAMP/synaptobrevinlike vSNARE Snc1, but not other commonly studied cargo. Because AP180 mediates the uptake of synaptobrevin in worms, flies, and mammals this sorting role appears to be highly conserved [10–12]. The signal recognized by AP180 has not been determined for any cargo, though mutation of two residues in Snc1 blocks its internalization [9,13]. It is not known if these residues form a small sorting motif or are part of a larger conformational epitope, as seen for other SNARE– adaptor interactions [14]. Current Opinion in Cell Biology 2010, 22:513–518
514 Membranes and organelles
The muniscins: a new family of mu homology domain adaptors
A recent structural analysis identified yeast Syp1 and the related mammalian proteins FCHO1/2 and SGIP1 as members of a new family of conserved endocytic adaptor proteins with homology to the cargo-binding medium (mu) chain of clathrin adaptor complexes [15]. The Syp1 mu homology domain (uHD) binds directly to the candidate cargo protein Mid2 and also interacts with the endocytic scaffold protein Ede1, which may stabilize coat formation [15,16]. In Syp1, the uHD is separated from a membrane-binding F-BAR domain by an unstructured linker region that negatively regulates actin polymerization by inhibiting Las17/WASP [15,17]. This arrangement could link cargo recruitment to vesicle budding [17]. The shallow curvature of the Syp1 F-BAR domain may favor association at initial stages of coat formation [15,18]. As membrane curvature increases, dissociation of Syp1 would activate actin assembly and further accelerate invagination. The timing of Syp1 recruitment is consistent in this model: Syp1 and clathrin arrive early at endocytic sites, but unlike clathrin, Syp1 remains at the cell cortex and does not associate with the invaginating vesicle [16]. However, loss of Syp1 does not alter the progression of endocytic vesicle formation [16,17], suggesting other actin regulatory proteins play a redundant role in vivo. The mammalian Syp1-like proteins FCHO1/2 and SGIP1 are also recruited to clathrin-coated pits, have an Nterminal membrane-binding domain, and interact with the Ede1-like protein Eps15 [15,16,19]. It remains to be seen if mammalian muniscin proteins have similar effects on actin assembly through the Las17 homolog WASP, which is subject to different regulatory mechanisms, including auto-inhibition.
Arrestin-like proteins
In mammals, clathrin associated sorting proteins often work in conjunction with clathrin adaptors [20]. For example, b-arrestins link G protein-coupled receptors (GPCRs) to the endocytic machinery to downregulate signaling. When b-arrestin binds a ligand-activated, phosphorylated GPCR it undergoes a conformational change that exposes a C-terminal tail with motifs for adaptin and clathrin binding. This recruits the GPCR to clathrincoated pits and promotes receptor endocytosis. Yeast do not have canonical arrestins, but do contain representatives of a conserved family of arrestin-like proteins, the arrestin-related trafficking adaptors (ARTs; [21,22,23]). ART C-terminal tails lack adaptin and clathrin-binding sequences, and instead contain PxY motifs that recruit the ubiquitin ligase Rsp5 [21]. Binding of ARTs to activated receptors results in their ubiquitination and internalization. ARTs regulate the uptake of a diverse group of nutrient transporters whose expression at the cell surface is tightly controlled in response to extracellular cues [21,22,24]. A high level of a particular substrate signals clearance of its cognate transporter from the cell surface, whereas cell stress triggers uptake of many different transporters. Although ARTs can act redundantly, several have been matched to the substrate-induced uptake of specific transporters (Table 1). Surprisingly, stress signaling often involves a different ART, typically one member of the related Art2/Art8 pair [21,24]. For example, Art4 regulates the glucose-induced uptake of the glucose transporter Hxt6, whereas its stress-induced internalization is mediated by Art8 [24]. Because linking PxY motifs to the arginine transporter Can1 tail bypasses the need for Art1 [21], ARTs function only as cargo-specific ubiquitin ligase adaptors and
Table 1 Cargo-specific endocytic adaptors in yeast Adaptor Mu homology domain
Arrestin-like
*
AP-2 complex SYP1 YAP1801/2 SLA1 ART1 ART2 ART3 ART4 ART5 ART6 ART7 ART8 ART9 ART10 BUL1/2
Alias
LDB19, CVS7 ECM21 ALY2 ROD1 YGR068C ALY1 ROG3 CSR2 RIM8 YLR392C
Homologs
Cargo
Reference
AP-2 complex SGIP1, FCHO1 AP180, CALM CIN85 Mammalian ADC proteins
Putative K28 receptor MID2 SNC1 WSC1, STE2 CAN1, LYP1, MUP1 LYP1*, SMF1 *
[8] [15,16,17] [9] [34,51] [21] [21,22]
HXT6 ITR1
[24] [24]
SMF1*, HXT6 * RIM21
[22,24] [52]
GAP1, CTR1
[25]
Stress-stimulated uptake.
Current Opinion in Cell Biology 2010, 22:513–518
www.sciencedirect.com
Converging views of endocytosis in yeast and mammals Conibear 515
are not needed for subsequent interactions with the endocytic machinery. The ubiquitin ligase adaptor Bul1 can act alone or redundantly with ARTs to mediate transporter uptake [24–26]. Interestingly, N-terminal domains in Bul1 and arrestins are members of a larger ‘Arrestin N-like’ clan [24,27]. Thus, the number of proteins distantly related to arrestins that act as ubiquitin ligase adaptors during endocytosis may be greater than currently recognized. How do ARTs recognize activated cargo? Phosphorylation is important, but not sufficient, for ART binding to the metal transporter Smf1 [22]. Nikko and Pelham have speculated that signaling releases transporters from membrane microdomains, allowing them to access both arrestins and the endocytic machinery [24]. In fact, addition of its substrate releases Can1 from ergosterol-dependent clusters that are distinct from sites of endocytosis [28], and mutations that disrupt formation of these clusters trigger Can1 endocytosis. It will be interesting to determine if this requires Art1. Further studies are needed to clarify the determinants recognized by ARTs and related ubiquitin adaptors, and to understand how different stimuli confer regulation by distinct ARTs. Other classes of membrane cargo may also require ARTs for internalization; the pheromone receptor Ste2 is a GPCR that requires phosphorylation and ubiquitination for uptake yet does not recruit Rsp5 directly [29]. The arrestin-like ART proteins are similar to arrestins in many ways, yet b-arrestins lack PxY motifs, and must use different mechanisms to recruit ubiquitin ligases [30]. These PxY motifs are conserved in mammal ART homologs, the ADC (arrestin domain containing) proteins, and it will be interesting to determine if mammalian ADCs have conserved roles as ubiquitin ligase adaptors for receptor uptake. Ubiquitin-binding adaptors
If arrestin-like proteins are important only for addition of the ubiquitin signal, how is the signal recognized by the endocytic machinery? Three coat components, the epsins Ent1 and Ent2 and the Eps15-like scaffold protein Ede1, have ubiquitin-binding domains (UBDs) believed to recruit ubiquitinated cargo [31]. Live-cell imaging shows that the pheromone receptor Ste2 moves to preexisting clathrin-coated pits after ligand binding and ubiquitination, and this clustering depends on Ent1, Ent2, and Ede1 [29]. Surprisingly, mutations that remove these UBDs while leaving other portions of these proteins intact do not prevent Ste2 uptake [32], arguing against an obligatory role in cargo recognition. It is likely these UBDs contribute to coated pit assembly [32], and that other proteins participate in recruitment of ubiquitinated cargo. The SH3 domains from several endocytic proteins, including the clathrin adaptor Sla1 and its mammalian homolog CIN85, bind ubiquitin in vitro [33] and may www.sciencedirect.com
serve as ubiquitin adaptors. The in vivo relevance of SH3ubiquitin interactions for yeast endocytosis remains to be demonstrated in vivo. However, it is interesting to note that the SHD1 domain of Sla1, which interacts with cargo containing an NPFxD signal, has an SH3-like fold [34,35]. How many more?
These studies have revealed unexpected diversity in the number of cargo-specific adaptors involved in yeast endocytosis. The functional importance of yeast proteins related to AP-2, AP180 and arrestins is clear, although few of the corresponding cargo proteins have been identified, and their internalization signals have not yet been mapped. Large-scale functional screens directed at additional cargo proteins may yet uncover new adaptor families, although exhaustive identification of all possible cargo-specific sorting proteins will be difficult. Additional work will also be needed to determine if all cargo-specific uptake requires clathrin, or if endocytic adaptors link to other components of the vesicle coat.
Reconciling different requirements for actin and dynamin Essential requirement for actin in yeast
The apparent differences between yeast and mammalian endocytosis extend to the invagination and scission reactions. Although a burst of actin polymerization precedes vesicle fission in mammalian cells as it does in yeast, inhibitor studies suggest actin is not absolutely required for the budding of clathrin-coated vesicles in most cell types [36,37,38]. Why, then, is actin so important for invagination and scission in yeast? The answer may have to do with the turgor pressure present in yeast cells, which provides an outwardly directed force on the plasma membrane. Actin polymerization has been proposed to generate additional force that counteracts this pressure. In support of this model, increasing turgor pressure has recently been shown to hinder endocytosis, whereas reducing the pressure difference suppresses the internalization defects of actin-bundling mutants and cells treated with the actin de-polymerizing drug Latrunculin A [39]. Interestingly, large clathrin-coated plaques found on the basal surface of adherent mammalian cells also require actin for their internalization [38]. It is tempting to speculate that in these cells actin polymerization overcomes the adhesive forces that anchor the basal membrane to the substrate [40]. Role of actin and dynamin in generating morphological differences
Endocytic structures in yeast and mammals are also morphologically distinct (Figure 1). In yeast, endocytic invaginations are wide-necked (30–50 nm) tubules covered by BAR-domain proteins, surrounded by an actin matrix, and coated with clathrin only at the tip [41]. By contrast, mammalian cells display round, clathrin-coated Current Opinion in Cell Biology 2010, 22:513–518
516 Membranes and organelles
Figure 1
Endocytic invaginations in yeast and mammals. In mammalian cells, dynamin encircles the short, narrow neck of budding clathrin-coated vesicles, which have lower levels of polymerized actin (left). Dynamin double knockout cells accumulate long, 36 nm diameter tubules decorated with BARdomain proteins, actin regulatory factors, and F-actin, and capped with a clathrin-coated bud (center). These are reminiscent of the tubular endocytic invaginations in yeast, which are attached to the membrane by a wide (30–50 nM) neck, and contain BAR-domain proteins, actin regulatory proteins, and are surrounded by a matrix of polymerized actin (right). Formation of elongated tubules is actin-dependent in both yeast and mammals.
budding profiles that are attached to the membrane by narrow necks and have much less associated actin. Recent work suggests dynamin could be responsible for these morphological differences [42]. Cells lacking both dynamin 1 and 2 accumulate long clathrin-capped tubular structures enriched for actin and F-BAR proteins strikingly similar to those in yeast [42] (Figure 1). When dynamin is present, scission may happen too quickly for high levels of actin to accumulate. Conversely, in yeast, the absence of dynamin could allow prolonged actin polymerization and the formation of tubular structures. This suggests that, despite the different appearance of endocytic profiles in yeast and mammals, the underlying processes that regulate invagination are fundamentally similar. A unifying model for vesicle invagination and scission
How can scission proceed in the absence of dynamin? Recent studies of clathrin-independent uptake of Shiga toxin provide evidence that actin promotes changes in the organization of the underlying lipids in endocytic tubules, and the resulting tension at membrane domain boundaries drives scission [43]. Drubin and colleagues have developed a quantitative theoretical model of endocytosis that suggests the membrane domains that drive scission in yeast result from differences in the distribution of PIP2, rather than cholesterol [44,45,46]. In this model, initial membrane curvature generated by Current Opinion in Cell Biology 2010, 22:513–518
coat protein assembly and actin/myosin pulling forces promotes recruitment of BAR-domain proteins, which form a tubule. The resulting increase in curvature activates the synaptojanin-like PIP2 phosphatase Inp52, but because BAR-domain proteins protect the tubular neck, PIP2 hydrolysis is limited to the bud. The resulting lipid phase boundary between bud and neck creates a lateral force that constricts the neck, leading to scission. The basic tenets of this model can be extended to mammalian cells, where dynamin could have a greater role in deforming the membrane and in preventing accumulation of synaptojanin at the neck of the budding vesicle by competing for binding partners [46,47,48]. This would protect PIP2 at the neck, causing phase segregation and scission. Several key features of this model remain to be tested experimentally. It will be important to define which lipid species are important for generating membrane domains and line tension, and to determine how PIP2 phosphatases are recruited and their activity regulated. Other factors may be at work in mammalian cells; after all, scission does not occur in dynamin knockouts despite abundant actin polymerization and BAR-domain recruitment [42]. It is important to note these models do not exclude roles for actin and dynamin at other steps in vesicle formation that precede the final scission event [49,50]. www.sciencedirect.com
Converging views of endocytosis in yeast and mammals Conibear 517
Conclusions Yeast is an important model for understanding protein trafficking in higher cells, and the underlying mechanisms that govern budding of vesicles from cellular membranes are generally well conserved. Recent work provides explanations for some of the apparent discrepancies between endocytic processes in yeast and man. Others are less easily reconciled, and may reflect the plasticity of uptake mechanisms in various organisms and cell types. A single mammalian cell can employ a large spectrum of endocytic mechanisms to internalize different cargo, and each has different requirements for clathrin, dynamin, actin regulators, and lipids. Elucidating the unique and conserved features of these processes will be essential in understanding the fundamental principles that underlie cargo selection, membrane deformation, and vesicle scission in all organisms and cell types.
10. Bao H, Daniels RW, MacLeod GT, Charlton MP, Atwood HL, Zhang B: AP180 maintains the distribution of synaptic and vesicle proteins in the nerve terminal and indirectly regulates the efficacy of Ca2+-triggered exocytosis. J Neurophysiol 2005, 94:1888-1903. 11. Dittman JS, Kaplan JM: Factors regulating the abundance and localization of synaptobrevin in the plasma membrane. Proc Natl Acad Sci USA 2006, 103:11399-11404. 12. Harel A, Wu F, Mattson MP, Morris CM, Yao PJ: Evidence for CALM in directing VAMP2 trafficking. Traffic 2008, 9:417-429. 13. Lewis MJ, Nichols BJ, Prescianotto-Baschong C, Riezman H, Pelham HR: Specific retrieval of the exocytic SNARE Snc1p from early yeast endosomes. Mol Biol Cell 2000, 11:23-38. 14. Miller SE, Collins BM, McCoy AJ, Robinson MS, Owen DJ: A SNARE–adaptor interaction is a new mode of cargo recognition in clathrin-coated vesicles. Nature 2007, 450:570-574.
Acknowledgements
15. Reider A, Barker SL, Mishra SK, Im YJ, Maldonado-Ba´ez L, Hurley JH, Traub LM, Wendland B: Syp1 is a conserved endocytic adaptor that contains domains involved in cargo selection and membrane tubulation. EMBO J 2009, 28:3103-3116. Solving the structure of yeast Syp1 identifies a membrane-tubulating FBAR domain and a cargo-binding mu homology domain, defining a new family of conserved adaptor proteins.
I regret that many excellent papers could not be discussed owing to space limitations. I thank Helen Burston, Beverly Wendland, and Margaret Robinson for interesting discussions and helpful comments. Funding was provided by the Canadian Institutes of Health Research (FRN#64394).
16. Stimpson H, Toret C, Cheng A, Pauly B, Drubin D: Early-arriving Syp1p and Ede1p function in endocytic site placement and formation in budding yeast. Mol Biol Cell 2009:E09-050429v0421.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
Perrais D, Merrifield CJ: Dynamics of endocytic vesicle creation. Dev Cell 2005, 9:581-592.
2.
Kaksonen M, Toret CP, Drubin DG: Harnessing actin dynamics for clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 2006, 7:404-414.
3.
Baggett JJ, Wendland B: Clathrin function in yeast endocytosis. Traffic 2001, 2:297-302.
4.
Galletta B, Cooper J: Actin and endocytosis: mechanisms and phylogeny. Curr Opin Cell Biol 2009.
5.
Robertson AS, Smythe E, Ayscough KR: Functions of actin in endocytosis. Cell Mol Life Sci 2009, 66:2049-2065.
6.
Yu X, Cai M: The yeast dynamin-related GTPase Vps1p functions in the organization of the actin cytoskeleton via interaction with Sla1p. J Cell Sci 2004, 117:3839-3853.
7.
Nannapaneni S, Wang D, Jain S, Schroeder B, Highfill C, Reustle L, Pittsley D, Maysent A, Moulder S, McDowell R et al.: The yeast dynamin-like protein Vps1:vps1 mutations perturb the internalization and the motility of endocytic vesicles and endosomes via disorganization of the actin cytoskeleton. Eur J Cell Biol 2010, 89:499-508.
8.
Carroll SY, Stirling PC, Stimpson HEM, Giesselmann E, Schmitt MJ, Drubin DG: A yeast killer toxin screen provides insights into a/b toxin entry, trafficking, and killing mechanisms. Dev Cell 2009, 17:552-560. This report provides the first evidence of a role for yeast AP-2 in protein uptake.
9.
Burston HE, Maldonado-Baez L, Davey M, Montpetit B, Schluter C, Wendland B, Conibear E: Regulators of yeast endocytosis identified by systematic quantitative analysis. J Cell Biol 2009, 185:1097-1110. A systematic survey of yeast knockout mutants identifies genes with general and cargo-specific roles in the endocytosis of SNARE-based reporter proteins.
www.sciencedirect.com
17. Boettner DR, D’Agostino JL, Torres OT, Daugherty-Clarke K, Uygur A, Reider A, Wendland B, Lemmon SK, Goode BL: The FBAR protein Syp1 negatively regulates WASp-Arp2/3 complex activity during endocytic patch formation. Curr Biol 2009, 19:1979-1987. This paper shows that Syp1 negatively regulates actin assembly by inhibiting Las17/WASp activation of the Arp2/3 complex. 18. Suetsugu S, Toyooka K, Senju Y: Subcellular membrane curvature mediated by the BAR domain superfamily proteins. Semin Cell Dev Biol 2009. 19. Uezu A, Horiuchi A, Kanda K, Kikuchi N, Umeda K, Tsujita K, Suetsugu S, Araki N, Yamamoto H, Takenawa T et al.: SGIP1alpha is an endocytic protein that directly interacts with phospholipids and Eps15. J Biol Chem 2007, 282:26481-26489. 20. Traub LM, Traub LM: Tickets to ride: selecting cargo for clathrin-regulated internalization. Nat Rev Mol Cell Biol 2009, 10:583. 21. Lin C, Macgurn J, Chu T, Stefan C, Emr S: Arrestin-related ubiquitin-ligase adaptors regulate endocytosis and protein turnover at the cell surface. Cell 2008. This report identifies a family of arrestin-related proteins that recruit the ubiquitin ligase Rsp5 to specific plasma membrane proteins to promote their internalization. 22. Nikko E, Sullivan J, Pelham H: Arrestin-like proteins mediate ubiquitination and endocytosis of the yeast metal transporter Smf1. EMBO Rep 2008. 23. Aubry L, Guetta D, Klein G: The arrestin fold: variations on a theme. Curr Genomics 2009, 10:133-142. 24. Nikko E, Pelham HRB: Arrestin-mediated endocytosis of yeast plasma membrane transporters. Traffic 2009, 10:1856-1867. 25. Liu J, Sitaram A, Burd CG: Regulation of copper-dependent endocytosis and vacuolar degradation of the yeast copper transporter, Ctr1p, by the Rsp5 ubiquitin ligase. Traffic 2007, 8:1375-1384. 26. Risinger AL, Kaiser CA: Different ubiquitin signals act at the golgi and plasma membrane to direct GAP1 trafficking. Mol Biol Cell 2008, 19:2962-2972. 27. Finn R, Mistry J, Tate J, Coggill P, Heger A, Pollington J, Gavin O, Gunasekaran P, Ceric G, Forslund K et al.: The Pfam protein families database. Nucleic Acids Res 2010, 38:D211. Current Opinion in Cell Biology 2010, 22:513–518
518 Membranes and organelles
28. Grossmann G, Malinsky J, Stahlschmidt W, Loibl M, Weig-Meckl I, Frommer WB, Opekarova´ M, Tanner W: Plasma membrane microdomains regulate turnover of transport proteins in yeast. J Cell Biol 2008, 183:1075-1088.
41. Idrissi F-Z, Gro¨tsch H, Ferna´ndez-Golbano IM, PresciattoBaschong C, Riezman H, Geli M-I: Distinct acto/myosin-I structures associate with endocytic profiles at the plasma membrane. J Cell Biol 2008, 180:1219-1232.
29. Toshima JY, Nakanishi J-i, Mizuno K, Toshima J, Drubin DG: Requirements for recruitment of a G protein-coupled receptor to clathrin-coated pits in budding yeast. Mol Biol Cell 2009, 20:5039-5050.
42. Ferguson S, Raimondi A, Paradise S, Shen H, Mesaki K, Ferguson A, Destaing O, Ko G, Takasaki J, Cremona O et al.: Coordinated actions of actin and BAR proteins upstream of dynamin at endocytic clathrin-coated pits. Dev Cell 2009, 17:811-822. Analysis of cells derived from dynamin 1 and 2 double conditional knockout mice shows that actin, actin regulators, and F-BAR proteins accumulate on arrested endocytic structures. This emphasizes the evolutionary similarity of endocytosis in yeast and mammals.
30. Shenoy SK, Xiao K, Venkataramanan V, Snyder PM, Freedman NJ, Weissman AM: Nedd4 mediates agonist-dependent ubiquitination, lysosomal targeting, and degradation of the beta2-adrenergic receptor. J Biol Chem 2008, 283:22166-22176. 31. Shih SC, Katzmann DJ, Schnell JD, Sutanto M, Emr SD, Hicke L: Epsins and Vps27p/Hrs contain ubiquitin-binding domains that function in receptor endocytosis. Nat Cell Biol 2002, 4:389-393. 32. Dores MR, Schnell JD, Maldonado-Baez L, Wendland B, Hicke L: The function of yeast epsin and Ede1 ubiquitin-binding domains during receptor internalization. Traffic 2010, 11:151-160. 33. Stamenova SD, French ME, He Y, Francis SA, Kramer ZB, Hicke L: Ubiquitin binds to and regulates a subset of SH3 domains. Mol Cell 2007, 25:273-284. 34. Howard JP, Hutton JL, Olson JM, Payne GS: Sla1p serves as the targeting signal recognition factor for NPFX(1,2)D-mediated endocytosis. J Cell Biol 2002, 157:315-326. 35. Mahadev RK, Di Pietro SM, Olson JM, Piao HL, Payne GS, Overduin M: Structure of Sla1p homology domain 1 and interaction with the NPFxD endocytic internalization motif. EMBO J 2007, 26:1963-1971. 36. Moskowitz HS, Yokoyama CT, Ryan TA: Highly cooperative control of endocytosis by clathrin. Mol Biol Cell 2005, 16:1769-1776. 37. Yarar D, Waterman-Storer CM, Schmid SL: A dynamic actin cytoskeleton functions at multiple stages of clathrin-mediated endocytosis. Mol Biol Cell 2005, 16:964-975. 38. Saffarian S, Cocucci E, Kirchhausen T: Distinct dynamics of endocytic clathrin-coated pits and coated plaques. PLoS Biol 2009, 7:e1000191. Quantitative live-cell imaging of adherent cultured cells demonstrates that actin is required for the internalization of flat clathrin-coated plaques on the basal surface, but is not required for the uptake of clathrin-coated buds at the ventral surface. 39. Aghamohammadzadeh S, Ayscough K: Differential requirements for actin during yeast and mammalian endocytosis. Nat Cell Biol 2009. This work presents evidence that actin is required at early stages of endocytosis in yeast to counteract turgor pressure, thus providing an explanation for the differing requirement for actin in yeast and mammals. 40. Traub LM: Clathrin couture: fashioning distinctive membrane coats at the cell surface. PLoS Biol 2009, 7:e1000192.
Current Opinion in Cell Biology 2010, 22:513–518
43. Ro¨mer W, Pontani L-L, Sorre B, Rentero C, Berland L, Chambon V, Lamaze C, Bassereau P, Sykes C, Gaus K et al.: Actin dynamics drive membrane reorganization and scission in clathrin-independent endocytosis. Cell 2010, 140:540-553. This paper shows that scission of Shiga toxin-induced tubules requires actin and is correlated with the formation of membrane domains. 44. Liu J, Kaksonen M, Drubin DG, Oster G: Endocytic vesicle scission by lipid phase boundary forces. Proc Natl Acad Sci USA 2006, 103:10277-10282. 45. Sun Y, Carroll S, Kaksonen M, Toshima JY, Drubin DG: PtdIns(4,5)P2 turnover is required for multiple stages during clathrin- and actin-dependent endocytic internalization. J Cell Biol 2007, 177:355-367. 46. Liu J, Sun Y, Drubin DG, Oster GF: The mechanochemistry of endocytosis. PLoS Biol 2009, 7:e1000204. In this paper, the authors present a theoretical model of endocytosis to explain the sequence and spatial organization of endocytic events in yeast. 47. McPherson PS: Regulatory role of SH3 domain-mediated protein-protein interactions in synaptic vesicle endocytosis. Cell Signal 1999, 11:229-238. 48. Krendel M, Osterweil EK, Mooseker MS: Myosin 1E interacts with synaptojanin-1 and dynamin and is involved in endocytosis. FEBS Lett 2007, 581:644-650. 49. Loerke D, Mettlen M, Yarar D, Jaqaman K, Jaqaman H, Danuser G, Schmid SL: Cargo and dynamin regulate clathrin-coated pit maturation. PLoS Biol 2009, 7:e57. 50. Mettlen M, Pucadyil T, Ramachandran R, Schmid SL: Dissecting dynamin’s role in clathrin-mediated endocytosis. Biochem Soc Trans 2009, 37:1022-1026. 51. Piao HL, Machado IM, Payne GS: NPFXD-mediated endocytosis is required for polarity and function of a yeast cell wall stress sensor. Mol Biol Cell 2007, 18:57-65. 52. Herrador A, Herranz S, Lara D, Vincent O: Recruitment of the ESCRT machinery to a putative seven-transmembranedomain receptor is mediated by an arrestin-related protein. Mol Cell Biol 2010, 30:897-907.
www.sciencedirect.com