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The nuclear envelope
The nuclear
envelope
E.A. Nigg Swiss Institute
for Experimental Current
Opinion
Cancer
in Cell Biology
Introduction
composition
Epalinges,
Switzerland
1989, 1:435-440
ing nuclear protein import (for review see Dingwall and Laskey, Annu Rev Cell Bioll986,2:367-390; Goldfarb, in this issue pp 441-446). In brief, large nuclear proteins contain sequence motifs (nuclear location signals) that specify their nuclear accumulation in a process that involves at least two steps, i.e. signal-dependent binding by as yet unidentified receptors, followed by energy-dependent translocation across the pore [4,5]. The latter step is sensitive to inhibition bywheat germ agglutinin (WGA) [ 51, a lectin that binds to N-acetylglucosamine (GlcNAc) and sialic acids. This inhibition is probably related to the fact that many pore proteins contain O-linked GlcNAc residues.
The nuclear envelope separates nuclear and cytoplasmic activities, and it controls all macromolecular exchange between nucleus and cytoplasm. III addition, it helps to determine the spatial organization of the interphase nucleus. The major structural components of the nuclear envelope are the outer and inner membranes, nuclear pore complexes, and the nuclear lamina. Nuclear pore complexes are elaborate supramolecular assemblies that join and penetrate the two nuclear membranes. On the one hand, they provide diffusion channels for ions and small macromolecules (e.g. globular proteins of up to about 40 kD); on the other hand, they mediate selective, energy-dependent translocation of larger proteins and ribonucleoprotein particles. Little is known about the biochemical differences between outer and inner nuclear membranes, but they differ functionally: the outer membrane is active in membrane-bound protein synthesis and in most cells is morphologically continuous with the endoplasmic reticulum (ER); the inner membrane is lined by a fibrillar protein meshwork known as the nuclear lamina. This karyoskeletal structure is believed to function in the attachment and spatial organization of interphase chromatin. In addition, it is thought to be important for dynamic changes in nuclear envelope structure during the cell cycle. This review highlights recent progress in our understanding of the structure and function of the nuclear envelope, and offers some speculative interpretations of these data (for earlier reviews, see Franke et al, J Cell Biol1981, 91:39s-50s; [l-3]).
The molecular pore complex
Research,
The first pore-associated protein to be identied was a 190kD integral membrane glycoprotein. This protein contains high-mannose oligosaccharides, and, considering its abundance (up to 25 copies per pore), may function in anchoring pore complexes to the lipid bilayer (Gerace et al, J Cell Biol 1982, 95ZJ26-837). More recently, several laboratories have raised monoclonal antibodies against a number of pore proteins of lower abundance. These proteins range in molecular mass between 45-210 kD, and they all contain multiple GlcNAc residues in an 0-glycosidic linkage (Davis and Blobel, Cell 1986, 45:69’+709; Snow et al, J Cell Bioll987, 104:11431156; Holt et al, J Cell Biol 1987, 104:1157-1164) [6,7]. This modification must involve a novel mechanism of glycosylation, because, in contrast to the lumenal disposition of most common carbohydrate modifications, the envelopeassociated 0-GlcNAc residues are accessible from the cy toplastic and nucleoplasmic compartments (Snow et al, 1987; [6,8]; for review, see [9]). It is important to note that 0-GlcNAc-modi6ed proteins are not exclusive to the nuclear pore, but occur throughout the ceil, most notably the nucleus and the cytoplasm [9]. Thus, it is likely that this type of glycosylation influences multiple cellular processes.
of the nuclear
Nuclear pore complexes are large, highly symmetrical structures that control the transport of macromolecules in and out of the nucleus (reviewed in [3] ). Whereas information on nuclear export mechanisms remains limited, major progress has been made toward understand-
Little is known about the precise arrangement or function of individual pore complex proteins. This is partly due to the fact that most of the available monoclonal antibodies recognize epitopes comprising O-GlcNAc-modi-
Abbreviations cDNA-complementary ClcNAc-N-acetylglucosamine;
@ Current
DNA; IF-intermediate
Science
ER-endoplasmic filaments;
reticulum; WCA-wheat
Ltd ISSN 0955-0674
germ
agglutinin.
435
436
Nucleus
dnd gene expression
.
iied residues, and, therefore, react with multiple proteins. Interestingly, however, one monoclonal antibody labeled the nucleoplasmic side of pore complexes exclusively, suggesting a role of the corresponding antigen in vectorial transport (Snow ef al, 1987). Another monoclonal antibody, when microinjected into Xenopus oocytes, inhibited mediated transport of both protein and RNA; it did not affect diffusion of small proteins into the nucleus 1101. I
The nuclear lamina: type karyoskeleton
vent co-polymerization of cytoplasmic and nuclear IF subunits. In this context, it is interesting that B-type lamins have recently been implicated in providing intranuclear attachment points for cytoplasmic IF [ 19,201. Whereas morphological studies confirm an interaction between cytoplasmic IF and the nucleus (Franke, Pm tophsmu 1971, 73:263-292; Granger and Iazarides, Cell 1982,30:263275; Goldman et al, Ann NYAcud Sci 1985, 455:1-17) evidence for a direct linkage to nuclear lamins is presently based on in U&O reassociation studies only, and will need to be confirmed by independent techniques.
an intermediate-filament
The nuclear lamina consists of intermediate filament (IF) type proteins, the nuclear lamins (Aebi et al, Nature 1986, 323:560-564; Fisher et al, Proc Nat1 Acud Sci USA 1986, 83645W54; McKeon et al., Nature 1986, 319:463-468; [ 11-141). As shown in Fig. 1, lamins share with cytoplasmic IF proteins a tripartite molecular organization comprising a central a-helical ‘rod’ domain, flanked by N- and C-terminal non-a-helical ‘head and ‘tail’ domains (see legend to Fig. 1). The increased rod length originally thought to be characteristic of lamin proteins (Fig. 1) was recently also observed in invertebrate (cytoplasrnic) IF proteins [ 151, supporting the view that lamins may have arisen early in evolution. Distinctive features of lamin proteins include a nuclear location signal [ 161 (Fig. 1, arrowed), an acidic domain [Fig. 1, (E/D) region], and a C-terminal tetrapeptide motif CXXM (Fig. 1; see also below). In vertebrates, the lamina is composed of several distinct polypeptides that range in molecular mass from 60-75 kD. Iamins have been tentatively grouped into two major subfamilies, i.e. A-type and B-type lamins. In the absence of more extensive structural information, the most meaningful criterion for classiiication probably remains an operational one, i.e. the fact that A-type lamins are completely solubilized during mitosis, whereas Btype lamins remain membrane-associated throughout the cell cycle (e.g. Burke and Gerace, Cell 1986, 44:6X)-652) [17]. These observations have contributed to the notion that A- and B-type lamins may exhibit partially different functions (for reviews, see [2,3]). Seemingly diagnostic structural differences between A- and B-type lamins include the exclusive presence of a histidine-rich domain in the tail region of A-type lamins and, conversely, the conservation of a cysteine residue in the rod of B-type lamins (Fig. 1). Also, it is probably signiiicant that most A-type lamins have a longer C-terminal end domain than B-type lamins. It may be expected that site-directed mutagenesis will be useful in assessing the functional signiiicance of individual motifs. Available evidence indicates that lamins readily form parallel unstaggered dimers, but little is known about the mode of assembly of iilamentous structures [3]. To what extent lamins form homo- or heteropolymeric assemblies is an important unanswered question [12,18], and it is still not clear which structural differences pre-
Differential
expression
embryonic chromatin
development: compaction?
of lamin
proteins
a role for lamin
during A in
The spatial organization of the nucleus has long been considered important for differential gene expression, timed chromatin replication, and efficient karyokinesis. Morphological evidence indicates that interphase chromatin interacts with the nuclear envelope (for reviews, see [2,3]), but the molecular nature of these interactions remains to be defined. It is likely (albeit not proven) that the nuclear lamina plays a major role in mediating interactions between interphase chromatin and the inner nuclear membrane. A key issue to be addressed at this point, therefore, is the molecular basis of lamina-chromatin interactions. Indirect support for a role of the lamina in determining nuclear architecture stems from the observation that lamin isoforms are differentially expressed during vertebrate embryogenesis (for early references, see Krohne and Benavente, Exp Cell Res 1986, 162:1-10; Stick and Hausen, Cell 1985, 41:191-200). Particularly intriguing in the present context is the finding that no A-type lamins are detectable in early vertebrate embtyos [ 11,21,22]. Reduced expression of A-type lamins was also observed in undifferentiated embryonal carcinoma cells [ 22,231, and in certain lymphoid cell lines [ 241. These results raise the possibility that expression of A-type lamins may be important for establishing or stabilizing cell-type speciiic differences in nuclear organization (see Fig. 2 for a specific, speculative model). This hypothesis may be tested by examining the functional consequences, if any, of ectopic expression of A-type lamins. Irrespective of the functional significance of the developmental expression of lamin isoforms, it will be interesting to determine at what levels their expression is controlled.
Membrane-association of lamin proteins: a role for membrane receptors, lipid modification and precursor processing?
Association of lamin proteins with the inner nuclear membrane is likely to involve interactions with integral
The nuclear
I
Head
I I
I
(a-helical
I
domain)
I Vertebrate cytoplasmic intermediate filament protein
A-type
B-type
I
Central Rod (Heptad repeats)
envelope
Nigg
Tail
! ‘4’2
lamin
NH,
lamin
NH, i
i
-53nm-j 8
Fig. 1. Schematic
view of the structural organization of nuclear lamins and cytoplasmic intermediate filament (IO proteins. Models are based on secondary structure predictions (Gamier et al., / MO/ Biol1978, 120:97-120) and partly supported by experimental evidence (for review see Osborn and Weber, Trends in Biochem Sci 1986, 11:469-472; Franke Cell 1987,48:3-4; Steinert and Roop, Annu Rev Biochem 1988, 57:593-625). Information on lamin proteins was compiled from the sequence of three A- and three B-type lamins, namely human lamin A (Fisher et a/., Proc Nat/ Acad Sci USA 1986, 83:645C-6454), Xenopus lamins A Ill1 and LI (a purported B-type lamin 11211, and chicken lamins A, B, and B2 (Peter et a/., / MO/ Viol 1989, in press; Vorburger et al., / MO/ Biol 1989, in press). The central rod domains of all IF proteins contain a-helical regions (coils la, lb and 2) that are joined by short linker segments. In vertebrate lamins, these linkers do not contain strong a-helix breakers (i.e. prolines), raising the possibility that lamin rod domains may be a-helical throughout their entire lengths. Coils la, lb and 2 of all IF proteins consist of repeated.heptad motifs that favour the formation of two-stranded parallel coiled-coil structures. Typical of all lamins sequenced so far are a nuclear location signal, an acidic domain (E/D) and four conserved tryptophan residue (A) in the tail region. The C-terminal tetrapeptide motif -CXXM is also conserved in all lamins (except human lamin CL Major differences between A- and B-type lamins include the presence of a cysteine residue (C) in coil lb of B-type lamins, and, conversely, the presence of a stretch of histidine residues (HI in A-type lamins.
membrane proteins and/or post-translational modifkations. Recent results indicate that both mechanisms may in fact co-operate. In support of the former possibility, Senior and Gerace [25] have identified in rat liver nuclear envelopes three integral membrane proteins of 75, 68 and 55kD. These proteins remained tightly associated with the insoluble lamina under a variety of extraction conditions, and, as shown by immunoelectronmicroscopy, were located exclusively at the inner nuclear membrane [25]. Simifarly, on the basis of results obtained by a ligand-blotting assay, Worman et al [26] proposed an avian nuclear envelope 58kD protein to function as a lamin B receptor. In support of this proposal, binding of radioiodinated lamin B to lamin-depleted envelopes was partially inhibited by antibodies against the putative receptor [ 261. Possible structural relationships between these various proteins remain to be explored,
and rigorous proof for a lamin-binding function will require in situ cross-linking experiments. The notion that lamin attachment to the lipid bilayer might involve post-translational modifications has received support from the observation that lamin proteins incorporate a derivative of mevalonic acid; this derivative is presumed to be very hydrophobic and of isoprenoid nature [ 27,281. To test the hypothesis that isoprenylation may contribute to establish or stabilize the membrane association typical of B-type lamin proteins, it will be important to identify (and subsequently mutagenize) the target sites for isoprenylation. Possible sites include the cysteine residue present in the rod domain of B- but not A-type lamins (Fig. l), and/or the one conserved in the Cterminal motif -CXXM. The latter possibility is particularly intriguing because a very similar C-terminal sequence is
437
438
Nucleus
ahd
gene
expression
Progenitor cell (early embryo) I
Ir-
B-type
lamins
only
Extended ”
I
1 Differentiation
I
well known to serve as a major site for post-translational modi6cations in the case of ras proteins (for review see Magee and Hanley, Nature 1988, 335114-115). Isoprenylation of the cysteine present in the motif CXXM might provide an explanation for the observation that lamin A is synthesized as a precursor that is processed only after reaching the nuclear envelope (Gerace et al, JCell Sci 1984, Suppl 1: 137-160; Dagenais et al, Exp Cell Res 1985, 161~269-276; Lehner et al, Proc Nat1 Acad Sci USA 1986, 83:20962099). In one possible scenario, processing might involve proteolytic removal of an isoprenylated C-terminal peptide from the lamin A precursor; this hypothesis might explain why only the precursor, but not mature lamin A, is isoprenylated [28], and it would be consistent with preferential membrane-attachment of Btype lamins. Although the functional significance of A-type lamin precursor processing may thus be emerging, the significance of processing of other types of lamin precursors remains obscure (Lehner et al, 1986; [14,29]). In the case of the Drasophih lamin, processing was proposed to involve proteolytic cleavage at the N-terminus and phosphotylation [ 291.
chromatin
Fig. 2. Highly speculative model proposing a contribution of A-type lamins to the establishment or stabilization of cell type specific differences in nuclear architecture. Specifically, as illustrated by depicting A- and B-type lamins in adjacent layers, it is presumed that Atype lamins interact more tightly with chromatin than B-type lamins. As a consequence, the absence of A-type lamins in undifferentiated cell types is proposed to favour dynamic changes in chromatin architecture that may occur during differentiation. Conversely, expression of A-type lamins in differentiated cells may help to establish or stabilize chromatin compaction patterns typical of specific cell types. A uniform surface distribution of A-type lamins would be consistent with a stabilizing role; asymmetrical distributions might be expected if lamin A were to induce particular chromatin organizations.
Cell-cycle regulation of nuclear envelope structure and phosphorylation of lamin proteins
When the nuclear envelope disassembles during mitosis, soluble nuclear proteins mix with the contents of the cytoplasm. Recent results indicate that sorting of nuclear and cytoplasmic components after mitosis may be facilitated by reformation of an intact nuclear envelope prior to chromatin decondensation and nuclear expansion [30,31]. Moreover, as suggested by Blow and Iaskey [ 311, reformation of a complete nuclear envelope may be essential for limiting DNA replication to a single round for each cell cycle. Mechanisms controlling nuclear membrane breakdown and reassembly remain poorly understood. In contrast, there is good evidence that the reversible disassembly of the lamina is triggered, at least in part, by hyperphosphorylation of lamin proteins by an as yet unidentikd kinase(s) (reviewed in [l-3]). However, a contribution of other types of cell-cycle dependent modiiications, specifically methylation, to the regulation of lamina dynamics is not excluded (Chelsky et al, J Biol Chem
The nuclear
1987, 262:4303-4309). Considering that both phosphoryiation and demethylation of carboxyl groups lead to an increase in net negative charge, it is conceivable that the two mechanisms may co-operate to induce lamina disassembly. Although lamin proteins are phosphotylated to much higher levels during mitosis than during interphase, they are phosphorylated throughout the cell cycle (Gerace and Blobel, Cell 1980,19:277-287; Ottaviano and Gerace, J Biol C&em 1985, 260:624-632). Recent studies raise the possibility that subtle reorganization of the nuclear periphery may result from lamin phosphorylation under the control of hormones and growth factors [32-341. Alteration in the structure of the interphase lamina may facilitate the incorporation of newly synthesized protein into an expanding nuclear envelope. Alternatively, lamin phosphoryfation may modulate interactions between the lamina and chromatin, and, as a consequence, may influence transcriptional activity or chromatin replication.
6. l
envelope
Nigg
DMS Ll, BL~BEL G: Nuclear pore complex contains a family of glycoproteins that include p62: glycosylation through a previously unidentified cellular pathway. Pra: Nut1 Acud Sci
USA 1987, 84:7552-7556. Shows that O-linked GlcNAc attachment onto newly synthesized pore complex protein p62 occurs in the cytoplasm, conlirming glycosylation by a novel mechanism. 7. l
PAM MK, D’ONOFRIO M, WILLINGHAM MC, HANOVER JA: A monoclonal antibody against a family of nuclear pore proteins (nucleoporins): O-Linked N-acetylglucosanke is part of the immunodeterminant. Proc Nurl AC& Sci USA 1987, 8464626466.
Identilication of a series of pore complex proteins by a monoclonal antibody; demonstration that O-linked GlcNAc residues are part of the immunogenic epitope. 8. l e
HANOVER JA, COHEN N-acetylghtcosamine pore. J Biol Chem
CK, WIUINGHAM
is attached
hiC,
PARK MK:
to proteins
of the
O-linked
nuclear
1987, 262:9887+394. Convincing demonstration of cytoplasmic and nucleoplasmic accessibility of O-linked GlcNAc residues. 9. l e
HART GW. HOLT GD, HALTlwANGER RS: Nuclear and cytoplasmic glycosylation: novel saccharide linkages in unexpected places. Trends Bic&em Sci 1988, 13380-384.
Review on nuclear and cytoplasmk 0-GlcNAc glycosylation. 10.
Acknowledgements
l
Work in the authors’ laboratory was supported by the Swiss National Science Foundation and the Swiss Cancer Ieague.
Annotated reading l l e
references
and recommended
1.
NEwTOaT JW, FORFJES DJ: The nucleus: me and dynamics. Annu Rev Biodem 1987, A review on nuclear organization, emphasizing
NIGG EA tial
Nuclear immunochemical
of
56:535-565.
cell-free systems for
function
and organization:
approaches.
Inf
Rev
the potenCylol 1988,
110:27-92. A comprehensive review of nuclear structure and function. Emphasis is on information obtained by immuncchemkal approaches. 3.
GERACE L BURKE B:
Functional Rev Cell Biol
organization
of the
nuclear
1988, 4~335-374. The latest review on the nuclear envelope. Annu
l e
envelope.
4.
RICHARDSON WD,
l e
C: Nuclear protein migration biding at the nuclear envelope location through nuclear pores.
MU.IS AD, DILWORTH
SM, L&TKF( RA, DINGWAU
involves two steps: followed by slower Ceil 1988, 52:65%64.
[51. NEWMEYER
l e
rated into translocation.
DD,
FORBES DJ:
WOUN
l
pus somatic tissues displays strong lamh A. EMBO J 1987, 633809-3818.
Sl+ KROHNE G, KIR~CHNER Mw:
A new lamin in Xene homology to human
Nuclear
import
12.
KROHNE
l e
MW: Nuclear lamin Lt of Xenoplrs luevia cDNA cloning, amino acid sequence and biding specificity of a member of the lamin B subfamily. EMBO J 1987, 6:3801-3808.
can
be
sepa-
distinct steps in vitro: nuclear pore binding and Cell 1988, 52641-652. based on an in vitro nuclear transpon system resolve 2
Experiments steps in nuclear protein migration: the first step, i.e. protein binding to the envelope, is signal-dependent; the second step, ttanslocation through the pores, requires adenosine ttiphosphate, and is WGA-inhibitable. See also [4].
G, WOUN
SL, MCKEON
FD,
FRANKE WW,
KWCHNER
Cloning and determination of the primaty structure of Xenopus Lt, a putative B-type lamin. Evidence (by crosslinkIng) for formation of dImers consisting of unstaggered parallel polypeptide chains. 13.
STICK R
cDNA cloning of the developmentaUy Lm of Xenopus luevis EMBO J 1988,
regulated
7:3189-3197. Characterization of cDNA and developmental analysis of RNA levels of Xenopus L, (i.e. the unique lamin of Xen0pu.s oocytes and early cleav age embryos). Ptimaty structure analysis suggests that Lttt may not belong to A- or B-type subfamilies, but may represent a specialized isoform. l
lamin
14.
GRUENBAUM Y, L~NDESMAN H, PADDY M, SEDAT JW, PA: Drosophila nuclear
l
rapid trans-
Results from protein microinjection Indicate 2 steps in nuclear protein migration: first, signal-dependent (rapid) binding, second, energy-dependent (slower) translocation. Cytoplasmk fibrib, attached to nuclear pore complexes, are identified as candidate signal receptors. See also 5.
11.
function,
SUUCNre,
studying nuclear transport and nuclear dynamks. l e
1988, 107:12891297. Demonstration that a monoclonal antibody directed against O.GlcNAc mod&d pore proteins inhibits signal-mediated protein and RNA translocation across the nuclear envelope.
Identification, by complementary DNA (cDNA) cloning of Xenopus lamIn A Demonstration of developmental changes in expression. See aIs3 121,221.
Of interest Of outstanding interest
2.
FEATHERSTONE C, DAREZY MK, GERACE L A monoclonal antibody against the nuclear pore complex inhibits nucleocytoplasmic transport of protein and RNA in vivo. J Celf Biol
lated from either species apparently
Y, DREES B, BARE JW, SAUM~EBER SMITH DE, BREWON BM, FISHER
lamin precursor of two developmentally encoded by a single
Dm, is transregulated mRNA gene. J Cell Bid
1988. 106:58>5%. Cloning and sequencing of cDNA for Dra@ih lamIn Dq. Despite the identiiication of 2 messenger RNAs ditfering in 3’.untranslated regions, evidence is presented to suggest the existence of a single-copy lamin gene in mih 15.
WEBER
H, KOSSMAGK-STEPHAN
K:
l
Ammo acid sequences and homopolymer-formhtg abiity the intermediate hlament proteins from an invertebrate ithelium. EMBO J 1988, 7:29953001.
K, PLESSMANN U, D~DEMONT
of ep-
Ammo acid sequences of invertebrate (Helrjcpomatia) cytoplasmic IF proteins reveal strucNr’al feaNreS (rod IengthS) PreViOUSiy thought t0 be typical of nuclear lamin proteins. This provides support for the notion that lamins may have arisen early in evolution.
439
440
Nuclewand
gene expression I
LONGER L, MCKEON F: Mutations in the nuclear lamin l proteins resulting in their aberrant assembly in the cytoplasm. EMBO / 1988, 7:2301-2309. Mutations in lamin A are shown to induce aberrant subcellular distributions and assemblies. Experimental support for a role of the putative nuclear location signal (KKRKLE) in nuclear targeting of iamins.
Identification of mammalian integral nuclear membrane proteins by a monoclonal antibody. Exclusive immuno-electron microscopic localizetion to the inner nuclear membrane, and biochemical co-fractionation with the insoluble lamina suggest a role for these proteins in membrane attachment of nuclear lamins. See also [ 261.
of chicken l nuclear lamin proteins during mitosis: evidence for a reversible redistribution of lamin B2 between inner nuclear membrane and elements of the endoplasmic reticulum. / Cell Bid 1988: 107~397-406. Demonstration that both chicken lamins B, and B2 remain associated with nuclear envelope-derived membranes during mitosis and thus represent bonufide B-type lamins. Surprisingly, during mitosis lamin B2 is found to redistribute (reversibly) between inner nuclear membrane and the ER
l
16.
17.
STUXK R, ANGRES B, LEHNER CF, Nrcx
EA
The
fates
18.
GEORGATOS SD, STOURNARAS C, BLOBEL G: Heterotypic and homotypic associations between the nuclear lamins sitespecificity and control by phosphorylation. Proc Na!l Acad Sci USA 1988, 854325-4329. Biochemical SNdieS aimed at the important question of pairing preferences between individual lamin isoforms. Results need to be compared with those obtained by other techniques. See 131. l
GEORGATOS SD, BARBEL G: l diate filament attachment Cell Eiol 1987, 105:117-125. see [201. 19.
Lamin B constitutes site at the nuclear
an intermeenvelope. J
20.
GEORGATOS SD, WEBER K, GEISIER N, BUJBEL G: Binding of two desmin derivatives to the plasma membrane and the nuclear envelope of avian erythrocytes: evidence for a conserved site-specilicity in intermediate filament-membrane interactions. Proc Nat1 Acad Sci US4 1987, 84:67&784. Results from in uitm binding assays are interpreted to indicate a role for B-type lamins in directly binding cytoplasmic lF proteins. l
21. me
IEHNER CF, STICK R, EPPENBERGER HM, NIGG EA: Differential expression of nuclear lamin proteins during chicken development. J Cell Bid 1987, 105:577-587. Demonstration of dUTerential expression of A- and B-type lamins during avian embrogenesis: A-type lamins are undetectable in early embryonic tisSUeS.
22.
STEWART C, BURKE B: Teratocarcinoma stem cells and early mouse embryos contain only a single major lamin polypeptide closely resembling lamin B. Cell 1987, 51:383-392. Findings as in [ 211, for mouse embryogenesis and in vitro tierentiating embtyonal carcinoma cells.
l e
23.
REBEL S, IAMPRON C, ROYAL 4 RAVMOND Y: L.aIIlilY.SA and C appear during retinoic acid-induced differentiation of mouse embryonal carcinoma cells. J Cell Biol 1987, 105:109%1104. Demonstration of de nova synthesis of A-type lamins during in vitro differentiation of F9 embryonal carcinoma cells. See also [ 21,221.
l
24.
GUIUY MN, BENSUS-SAN A, BOURGE JF, BORNENS M. COURVAUN JC: A human T lymphoblastic cell line lacks lamins A and C. EMBO J 1987, 63795-3799. Evidence for dilferential expression of A- and B-type lamiis in human lymphoid cells. See also [21-231. l
26.
HJ, YUAN J, BIOBEL G, GEORGATOS SD: A lamin B receptor in the nuclear envelope. Proc Nat1 Acad Sci USA 1988, 85:85314534. Identitication, by a Ugand blotting assay, of a putative B-type lamin receptor in the avian nuclear envelope. See also [25].
27.
Sl GLOMSET JA: Evidence for modilication of lamin B by a product of mevalonic acid. J Biol Cbem 1988, 263:5997+&00. Identification of lamin B as a target of post-translational modiJication by a derivative of mevalonic acid. The modification is presumed to be isoprenoid in naNre and very hydrophobic; conceivably, it may play a role in anchoring B-type lamins to the Upid bilayer. See also 1281.
l e
SEMOR A, GERACE L to the inner nuclear nuclear lamina. J &II
Integral membrane proteins membrane and associated Biol 1988, 107:2029-2036.
specific with the
WOLDA
l e
28.
BECK IA, HOSICK
TH, SINENSKY M: Incorporation of a product of mevalonic acid metabolism into proteins of Chinese hamster ovary cell nuclei. J Cell Biol 1988, 107~1307-1316. Results similar to those described in 1271; in addition, evidence is presented for isoprenylation of the lamin A precursor. l e
29.
SMITH DE, GRUENBAUM
Y. BERRIOS M. FISHER PA:
Biosynthesis
and interconversion of Drosophila nuclear lamin isoforms during normal growth and in response to heat shock. J Cell Biol 1987, 105:771-790. Delineation of the post-translational processing pathway resulting in the production of 2 distinguishable lamin polypeptides (Dmt and Dm2) from a single primary translation product (Dm,). Processing is interpreted to involve both proteolysis and phosphorylation.
l
PL: Nuclear reassembly excludes large macromolecules. Science 1987, 238:54%550. Demonstration that reformation of the nuclear envelope precedes chromatin decondensation and nuclear expansion during postmitotic reassembly of nuc!ei. For possible functional consequences see [31].
30.
SWANXXN JA, MCNEIL
l
BLOW JJ, LUKEY RAz (Letter to the Editor) A role for the nuclear envelope in controlling DNA replication witbin the cell cycle. Nature 1988, 332:54&548. study on the role of the nuclear envelope in controlling DNA replication. An intriguing model is proposed, according to which the nuclear envelope determines the nucleocytoplasmic compartmentation of an essential replication factor (‘licensing factor’) whose exclusive access to DNA during mitosis (and inactivation’ tier replication) would result in single rounds of replication during each ceU cycle. 31.
l e
32. l
see
FIELDS AP, PETIIT GR, MAY WS: Phosphorylation at the nuclear membrane by activated protein Biol &em 1988, 263:82538260. 1331.
of lamin B kinase C. J
P, HUANG K-P, PAUL WE: Lamin B is rapidly phosphorylated in lymphocytes after activation of protein kinase C. Proc Null Acad Sci USA 1988, 85:2279-2283. Iamin proteins are shown to become rapidly phosphorylated in response to certain extracellular stimuli that are known to activate protein kinase C. The data raise the possibility (but do not prove) that signal transfer from cytoplasm to nucleus may involve nuclear translocation of active protein kinase C (or a proteolytic fragment of the kinase).
33.
HORNBECK
l e
FRtEDMAN DL, KEN R Insulin StimtiteS inCOrporatiOn Of s2Pt into nuclear lamins A and C in quiescent BHK-21 cells. J Biol Cbem 1988, 263:11031106. Demonstration that insulin induces phosphorylation of lamins upon mitogenic stimulation of quiescent fibroblasts. 34. l
25.
WORMAN
envelope
E.A. Nigg Swiss Institute
for Experimental Current
Opinion
Cancer
in Cell Biology
Introduction
composition
Epalinges,
Switzerland
1989, 1:435-440
ing nuclear protein import (for review see Dingwall and Laskey, Annu Rev Cell Bioll986,2:367-390; Goldfarb, in this issue pp 441-446). In brief, large nuclear proteins contain sequence motifs (nuclear location signals) that specify their nuclear accumulation in a process that involves at least two steps, i.e. signal-dependent binding by as yet unidentified receptors, followed by energy-dependent translocation across the pore [4,5]. The latter step is sensitive to inhibition bywheat germ agglutinin (WGA) [ 51, a lectin that binds to N-acetylglucosamine (GlcNAc) and sialic acids. This inhibition is probably related to the fact that many pore proteins contain O-linked GlcNAc residues.
The nuclear envelope separates nuclear and cytoplasmic activities, and it controls all macromolecular exchange between nucleus and cytoplasm. III addition, it helps to determine the spatial organization of the interphase nucleus. The major structural components of the nuclear envelope are the outer and inner membranes, nuclear pore complexes, and the nuclear lamina. Nuclear pore complexes are elaborate supramolecular assemblies that join and penetrate the two nuclear membranes. On the one hand, they provide diffusion channels for ions and small macromolecules (e.g. globular proteins of up to about 40 kD); on the other hand, they mediate selective, energy-dependent translocation of larger proteins and ribonucleoprotein particles. Little is known about the biochemical differences between outer and inner nuclear membranes, but they differ functionally: the outer membrane is active in membrane-bound protein synthesis and in most cells is morphologically continuous with the endoplasmic reticulum (ER); the inner membrane is lined by a fibrillar protein meshwork known as the nuclear lamina. This karyoskeletal structure is believed to function in the attachment and spatial organization of interphase chromatin. In addition, it is thought to be important for dynamic changes in nuclear envelope structure during the cell cycle. This review highlights recent progress in our understanding of the structure and function of the nuclear envelope, and offers some speculative interpretations of these data (for earlier reviews, see Franke et al, J Cell Biol1981, 91:39s-50s; [l-3]).
The molecular pore complex
Research,
The first pore-associated protein to be identied was a 190kD integral membrane glycoprotein. This protein contains high-mannose oligosaccharides, and, considering its abundance (up to 25 copies per pore), may function in anchoring pore complexes to the lipid bilayer (Gerace et al, J Cell Biol 1982, 95ZJ26-837). More recently, several laboratories have raised monoclonal antibodies against a number of pore proteins of lower abundance. These proteins range in molecular mass between 45-210 kD, and they all contain multiple GlcNAc residues in an 0-glycosidic linkage (Davis and Blobel, Cell 1986, 45:69’+709; Snow et al, J Cell Bioll987, 104:11431156; Holt et al, J Cell Biol 1987, 104:1157-1164) [6,7]. This modification must involve a novel mechanism of glycosylation, because, in contrast to the lumenal disposition of most common carbohydrate modifications, the envelopeassociated 0-GlcNAc residues are accessible from the cy toplastic and nucleoplasmic compartments (Snow et al, 1987; [6,8]; for review, see [9]). It is important to note that 0-GlcNAc-modi6ed proteins are not exclusive to the nuclear pore, but occur throughout the ceil, most notably the nucleus and the cytoplasm [9]. Thus, it is likely that this type of glycosylation influences multiple cellular processes.
of the nuclear
Nuclear pore complexes are large, highly symmetrical structures that control the transport of macromolecules in and out of the nucleus (reviewed in [3] ). Whereas information on nuclear export mechanisms remains limited, major progress has been made toward understand-
Little is known about the precise arrangement or function of individual pore complex proteins. This is partly due to the fact that most of the available monoclonal antibodies recognize epitopes comprising O-GlcNAc-modi-
Abbreviations cDNA-complementary ClcNAc-N-acetylglucosamine;
@ Current
DNA; IF-intermediate
Science
ER-endoplasmic filaments;
reticulum; WCA-wheat
Ltd ISSN 0955-0674
germ
agglutinin.
435
436
Nucleus
dnd gene expression
.
iied residues, and, therefore, react with multiple proteins. Interestingly, however, one monoclonal antibody labeled the nucleoplasmic side of pore complexes exclusively, suggesting a role of the corresponding antigen in vectorial transport (Snow ef al, 1987). Another monoclonal antibody, when microinjected into Xenopus oocytes, inhibited mediated transport of both protein and RNA; it did not affect diffusion of small proteins into the nucleus 1101. I
The nuclear lamina: type karyoskeleton
vent co-polymerization of cytoplasmic and nuclear IF subunits. In this context, it is interesting that B-type lamins have recently been implicated in providing intranuclear attachment points for cytoplasmic IF [ 19,201. Whereas morphological studies confirm an interaction between cytoplasmic IF and the nucleus (Franke, Pm tophsmu 1971, 73:263-292; Granger and Iazarides, Cell 1982,30:263275; Goldman et al, Ann NYAcud Sci 1985, 455:1-17) evidence for a direct linkage to nuclear lamins is presently based on in U&O reassociation studies only, and will need to be confirmed by independent techniques.
an intermediate-filament
The nuclear lamina consists of intermediate filament (IF) type proteins, the nuclear lamins (Aebi et al, Nature 1986, 323:560-564; Fisher et al, Proc Nat1 Acud Sci USA 1986, 83645W54; McKeon et al., Nature 1986, 319:463-468; [ 11-141). As shown in Fig. 1, lamins share with cytoplasmic IF proteins a tripartite molecular organization comprising a central a-helical ‘rod’ domain, flanked by N- and C-terminal non-a-helical ‘head and ‘tail’ domains (see legend to Fig. 1). The increased rod length originally thought to be characteristic of lamin proteins (Fig. 1) was recently also observed in invertebrate (cytoplasrnic) IF proteins [ 151, supporting the view that lamins may have arisen early in evolution. Distinctive features of lamin proteins include a nuclear location signal [ 161 (Fig. 1, arrowed), an acidic domain [Fig. 1, (E/D) region], and a C-terminal tetrapeptide motif CXXM (Fig. 1; see also below). In vertebrates, the lamina is composed of several distinct polypeptides that range in molecular mass from 60-75 kD. Iamins have been tentatively grouped into two major subfamilies, i.e. A-type and B-type lamins. In the absence of more extensive structural information, the most meaningful criterion for classiiication probably remains an operational one, i.e. the fact that A-type lamins are completely solubilized during mitosis, whereas Btype lamins remain membrane-associated throughout the cell cycle (e.g. Burke and Gerace, Cell 1986, 44:6X)-652) [17]. These observations have contributed to the notion that A- and B-type lamins may exhibit partially different functions (for reviews, see [2,3]). Seemingly diagnostic structural differences between A- and B-type lamins include the exclusive presence of a histidine-rich domain in the tail region of A-type lamins and, conversely, the conservation of a cysteine residue in the rod of B-type lamins (Fig. 1). Also, it is probably signiiicant that most A-type lamins have a longer C-terminal end domain than B-type lamins. It may be expected that site-directed mutagenesis will be useful in assessing the functional signiiicance of individual motifs. Available evidence indicates that lamins readily form parallel unstaggered dimers, but little is known about the mode of assembly of iilamentous structures [3]. To what extent lamins form homo- or heteropolymeric assemblies is an important unanswered question [12,18], and it is still not clear which structural differences pre-
Differential
expression
embryonic chromatin
development: compaction?
of lamin
proteins
a role for lamin
during A in
The spatial organization of the nucleus has long been considered important for differential gene expression, timed chromatin replication, and efficient karyokinesis. Morphological evidence indicates that interphase chromatin interacts with the nuclear envelope (for reviews, see [2,3]), but the molecular nature of these interactions remains to be defined. It is likely (albeit not proven) that the nuclear lamina plays a major role in mediating interactions between interphase chromatin and the inner nuclear membrane. A key issue to be addressed at this point, therefore, is the molecular basis of lamina-chromatin interactions. Indirect support for a role of the lamina in determining nuclear architecture stems from the observation that lamin isoforms are differentially expressed during vertebrate embryogenesis (for early references, see Krohne and Benavente, Exp Cell Res 1986, 162:1-10; Stick and Hausen, Cell 1985, 41:191-200). Particularly intriguing in the present context is the finding that no A-type lamins are detectable in early vertebrate embtyos [ 11,21,22]. Reduced expression of A-type lamins was also observed in undifferentiated embryonal carcinoma cells [ 22,231, and in certain lymphoid cell lines [ 241. These results raise the possibility that expression of A-type lamins may be important for establishing or stabilizing cell-type speciiic differences in nuclear organization (see Fig. 2 for a specific, speculative model). This hypothesis may be tested by examining the functional consequences, if any, of ectopic expression of A-type lamins. Irrespective of the functional significance of the developmental expression of lamin isoforms, it will be interesting to determine at what levels their expression is controlled.
Membrane-association of lamin proteins: a role for membrane receptors, lipid modification and precursor processing?
Association of lamin proteins with the inner nuclear membrane is likely to involve interactions with integral
The nuclear
I
Head
I I
I
(a-helical
I
domain)
I Vertebrate cytoplasmic intermediate filament protein
A-type
B-type
I
Central Rod (Heptad repeats)
envelope
Nigg
Tail
! ‘4’2
lamin
NH,
lamin
NH, i
i
-53nm-j 8
Fig. 1. Schematic
view of the structural organization of nuclear lamins and cytoplasmic intermediate filament (IO proteins. Models are based on secondary structure predictions (Gamier et al., / MO/ Biol1978, 120:97-120) and partly supported by experimental evidence (for review see Osborn and Weber, Trends in Biochem Sci 1986, 11:469-472; Franke Cell 1987,48:3-4; Steinert and Roop, Annu Rev Biochem 1988, 57:593-625). Information on lamin proteins was compiled from the sequence of three A- and three B-type lamins, namely human lamin A (Fisher et a/., Proc Nat/ Acad Sci USA 1986, 83:645C-6454), Xenopus lamins A Ill1 and LI (a purported B-type lamin 11211, and chicken lamins A, B, and B2 (Peter et a/., / MO/ Viol 1989, in press; Vorburger et al., / MO/ Biol 1989, in press). The central rod domains of all IF proteins contain a-helical regions (coils la, lb and 2) that are joined by short linker segments. In vertebrate lamins, these linkers do not contain strong a-helix breakers (i.e. prolines), raising the possibility that lamin rod domains may be a-helical throughout their entire lengths. Coils la, lb and 2 of all IF proteins consist of repeated.heptad motifs that favour the formation of two-stranded parallel coiled-coil structures. Typical of all lamins sequenced so far are a nuclear location signal, an acidic domain (E/D) and four conserved tryptophan residue (A) in the tail region. The C-terminal tetrapeptide motif -CXXM is also conserved in all lamins (except human lamin CL Major differences between A- and B-type lamins include the presence of a cysteine residue (C) in coil lb of B-type lamins, and, conversely, the presence of a stretch of histidine residues (HI in A-type lamins.
membrane proteins and/or post-translational modifkations. Recent results indicate that both mechanisms may in fact co-operate. In support of the former possibility, Senior and Gerace [25] have identified in rat liver nuclear envelopes three integral membrane proteins of 75, 68 and 55kD. These proteins remained tightly associated with the insoluble lamina under a variety of extraction conditions, and, as shown by immunoelectronmicroscopy, were located exclusively at the inner nuclear membrane [25]. Simifarly, on the basis of results obtained by a ligand-blotting assay, Worman et al [26] proposed an avian nuclear envelope 58kD protein to function as a lamin B receptor. In support of this proposal, binding of radioiodinated lamin B to lamin-depleted envelopes was partially inhibited by antibodies against the putative receptor [ 261. Possible structural relationships between these various proteins remain to be explored,
and rigorous proof for a lamin-binding function will require in situ cross-linking experiments. The notion that lamin attachment to the lipid bilayer might involve post-translational modifications has received support from the observation that lamin proteins incorporate a derivative of mevalonic acid; this derivative is presumed to be very hydrophobic and of isoprenoid nature [ 27,281. To test the hypothesis that isoprenylation may contribute to establish or stabilize the membrane association typical of B-type lamin proteins, it will be important to identify (and subsequently mutagenize) the target sites for isoprenylation. Possible sites include the cysteine residue present in the rod domain of B- but not A-type lamins (Fig. l), and/or the one conserved in the Cterminal motif -CXXM. The latter possibility is particularly intriguing because a very similar C-terminal sequence is
437
438
Nucleus
ahd
gene
expression
Progenitor cell (early embryo) I
Ir-
B-type
lamins
only
Extended ”
I
1 Differentiation
I
well known to serve as a major site for post-translational modi6cations in the case of ras proteins (for review see Magee and Hanley, Nature 1988, 335114-115). Isoprenylation of the cysteine present in the motif CXXM might provide an explanation for the observation that lamin A is synthesized as a precursor that is processed only after reaching the nuclear envelope (Gerace et al, JCell Sci 1984, Suppl 1: 137-160; Dagenais et al, Exp Cell Res 1985, 161~269-276; Lehner et al, Proc Nat1 Acad Sci USA 1986, 83:20962099). In one possible scenario, processing might involve proteolytic removal of an isoprenylated C-terminal peptide from the lamin A precursor; this hypothesis might explain why only the precursor, but not mature lamin A, is isoprenylated [28], and it would be consistent with preferential membrane-attachment of Btype lamins. Although the functional significance of A-type lamin precursor processing may thus be emerging, the significance of processing of other types of lamin precursors remains obscure (Lehner et al, 1986; [14,29]). In the case of the Drasophih lamin, processing was proposed to involve proteolytic cleavage at the N-terminus and phosphotylation [ 291.
chromatin
Fig. 2. Highly speculative model proposing a contribution of A-type lamins to the establishment or stabilization of cell type specific differences in nuclear architecture. Specifically, as illustrated by depicting A- and B-type lamins in adjacent layers, it is presumed that Atype lamins interact more tightly with chromatin than B-type lamins. As a consequence, the absence of A-type lamins in undifferentiated cell types is proposed to favour dynamic changes in chromatin architecture that may occur during differentiation. Conversely, expression of A-type lamins in differentiated cells may help to establish or stabilize chromatin compaction patterns typical of specific cell types. A uniform surface distribution of A-type lamins would be consistent with a stabilizing role; asymmetrical distributions might be expected if lamin A were to induce particular chromatin organizations.
Cell-cycle regulation of nuclear envelope structure and phosphorylation of lamin proteins
When the nuclear envelope disassembles during mitosis, soluble nuclear proteins mix with the contents of the cytoplasm. Recent results indicate that sorting of nuclear and cytoplasmic components after mitosis may be facilitated by reformation of an intact nuclear envelope prior to chromatin decondensation and nuclear expansion [30,31]. Moreover, as suggested by Blow and Iaskey [ 311, reformation of a complete nuclear envelope may be essential for limiting DNA replication to a single round for each cell cycle. Mechanisms controlling nuclear membrane breakdown and reassembly remain poorly understood. In contrast, there is good evidence that the reversible disassembly of the lamina is triggered, at least in part, by hyperphosphorylation of lamin proteins by an as yet unidentikd kinase(s) (reviewed in [l-3]). However, a contribution of other types of cell-cycle dependent modiiications, specifically methylation, to the regulation of lamina dynamics is not excluded (Chelsky et al, J Biol Chem
The nuclear
1987, 262:4303-4309). Considering that both phosphoryiation and demethylation of carboxyl groups lead to an increase in net negative charge, it is conceivable that the two mechanisms may co-operate to induce lamina disassembly. Although lamin proteins are phosphotylated to much higher levels during mitosis than during interphase, they are phosphorylated throughout the cell cycle (Gerace and Blobel, Cell 1980,19:277-287; Ottaviano and Gerace, J Biol C&em 1985, 260:624-632). Recent studies raise the possibility that subtle reorganization of the nuclear periphery may result from lamin phosphorylation under the control of hormones and growth factors [32-341. Alteration in the structure of the interphase lamina may facilitate the incorporation of newly synthesized protein into an expanding nuclear envelope. Alternatively, lamin phosphoryfation may modulate interactions between the lamina and chromatin, and, as a consequence, may influence transcriptional activity or chromatin replication.
6. l
envelope
Nigg
DMS Ll, BL~BEL G: Nuclear pore complex contains a family of glycoproteins that include p62: glycosylation through a previously unidentified cellular pathway. Pra: Nut1 Acud Sci
USA 1987, 84:7552-7556. Shows that O-linked GlcNAc attachment onto newly synthesized pore complex protein p62 occurs in the cytoplasm, conlirming glycosylation by a novel mechanism. 7. l
PAM MK, D’ONOFRIO M, WILLINGHAM MC, HANOVER JA: A monoclonal antibody against a family of nuclear pore proteins (nucleoporins): O-Linked N-acetylglucosanke is part of the immunodeterminant. Proc Nurl AC& Sci USA 1987, 8464626466.
Identilication of a series of pore complex proteins by a monoclonal antibody; demonstration that O-linked GlcNAc residues are part of the immunogenic epitope. 8. l e
HANOVER JA, COHEN N-acetylghtcosamine pore. J Biol Chem
CK, WIUINGHAM
is attached
hiC,
PARK MK:
to proteins
of the
O-linked
nuclear
1987, 262:9887+394. Convincing demonstration of cytoplasmic and nucleoplasmic accessibility of O-linked GlcNAc residues. 9. l e
HART GW. HOLT GD, HALTlwANGER RS: Nuclear and cytoplasmic glycosylation: novel saccharide linkages in unexpected places. Trends Bic&em Sci 1988, 13380-384.
Review on nuclear and cytoplasmk 0-GlcNAc glycosylation. 10.
Acknowledgements
l
Work in the authors’ laboratory was supported by the Swiss National Science Foundation and the Swiss Cancer Ieague.
Annotated reading l l e
references
and recommended
1.
NEwTOaT JW, FORFJES DJ: The nucleus: me and dynamics. Annu Rev Biodem 1987, A review on nuclear organization, emphasizing
NIGG EA tial
Nuclear immunochemical
of
56:535-565.
cell-free systems for
function
and organization:
approaches.
Inf
Rev
the potenCylol 1988,
110:27-92. A comprehensive review of nuclear structure and function. Emphasis is on information obtained by immuncchemkal approaches. 3.
GERACE L BURKE B:
Functional Rev Cell Biol
organization
of the
nuclear
1988, 4~335-374. The latest review on the nuclear envelope. Annu
l e
envelope.
4.
RICHARDSON WD,
l e
C: Nuclear protein migration biding at the nuclear envelope location through nuclear pores.
MU.IS AD, DILWORTH
SM, L&TKF( RA, DINGWAU
involves two steps: followed by slower Ceil 1988, 52:65%64.
[51. NEWMEYER
l e
rated into translocation.
DD,
FORBES DJ:
WOUN
l
pus somatic tissues displays strong lamh A. EMBO J 1987, 633809-3818.
Sl+ KROHNE G, KIR~CHNER Mw:
A new lamin in Xene homology to human
Nuclear
import
12.
KROHNE
l e
MW: Nuclear lamin Lt of Xenoplrs luevia cDNA cloning, amino acid sequence and biding specificity of a member of the lamin B subfamily. EMBO J 1987, 6:3801-3808.
can
be
sepa-
distinct steps in vitro: nuclear pore binding and Cell 1988, 52641-652. based on an in vitro nuclear transpon system resolve 2
Experiments steps in nuclear protein migration: the first step, i.e. protein binding to the envelope, is signal-dependent; the second step, ttanslocation through the pores, requires adenosine ttiphosphate, and is WGA-inhibitable. See also [4].
G, WOUN
SL, MCKEON
FD,
FRANKE WW,
KWCHNER
Cloning and determination of the primaty structure of Xenopus Lt, a putative B-type lamin. Evidence (by crosslinkIng) for formation of dImers consisting of unstaggered parallel polypeptide chains. 13.
STICK R
cDNA cloning of the developmentaUy Lm of Xenopus luevis EMBO J 1988,
regulated
7:3189-3197. Characterization of cDNA and developmental analysis of RNA levels of Xenopus L, (i.e. the unique lamin of Xen0pu.s oocytes and early cleav age embryos). Ptimaty structure analysis suggests that Lttt may not belong to A- or B-type subfamilies, but may represent a specialized isoform. l
lamin
14.
GRUENBAUM Y, L~NDESMAN H, PADDY M, SEDAT JW, PA: Drosophila nuclear
l
rapid trans-
Results from protein microinjection Indicate 2 steps in nuclear protein migration: first, signal-dependent (rapid) binding, second, energy-dependent (slower) translocation. Cytoplasmk fibrib, attached to nuclear pore complexes, are identified as candidate signal receptors. See also 5.
11.
function,
SUUCNre,
studying nuclear transport and nuclear dynamks. l e
1988, 107:12891297. Demonstration that a monoclonal antibody directed against O.GlcNAc mod&d pore proteins inhibits signal-mediated protein and RNA translocation across the nuclear envelope.
Identification, by complementary DNA (cDNA) cloning of Xenopus lamIn A Demonstration of developmental changes in expression. See aIs3 121,221.
Of interest Of outstanding interest
2.
FEATHERSTONE C, DAREZY MK, GERACE L A monoclonal antibody against the nuclear pore complex inhibits nucleocytoplasmic transport of protein and RNA in vivo. J Celf Biol
lated from either species apparently
Y, DREES B, BARE JW, SAUM~EBER SMITH DE, BREWON BM, FISHER
lamin precursor of two developmentally encoded by a single
Dm, is transregulated mRNA gene. J Cell Bid
1988. 106:58>5%. Cloning and sequencing of cDNA for Dra@ih lamIn Dq. Despite the identiiication of 2 messenger RNAs ditfering in 3’.untranslated regions, evidence is presented to suggest the existence of a single-copy lamin gene in mih 15.
WEBER
H, KOSSMAGK-STEPHAN
K:
l
Ammo acid sequences and homopolymer-formhtg abiity the intermediate hlament proteins from an invertebrate ithelium. EMBO J 1988, 7:29953001.
K, PLESSMANN U, D~DEMONT
of ep-
Ammo acid sequences of invertebrate (Helrjcpomatia) cytoplasmic IF proteins reveal strucNr’al feaNreS (rod IengthS) PreViOUSiy thought t0 be typical of nuclear lamin proteins. This provides support for the notion that lamins may have arisen early in evolution.
439
440
Nuclewand
gene expression I
LONGER L, MCKEON F: Mutations in the nuclear lamin l proteins resulting in their aberrant assembly in the cytoplasm. EMBO / 1988, 7:2301-2309. Mutations in lamin A are shown to induce aberrant subcellular distributions and assemblies. Experimental support for a role of the putative nuclear location signal (KKRKLE) in nuclear targeting of iamins.
Identification of mammalian integral nuclear membrane proteins by a monoclonal antibody. Exclusive immuno-electron microscopic localizetion to the inner nuclear membrane, and biochemical co-fractionation with the insoluble lamina suggest a role for these proteins in membrane attachment of nuclear lamins. See also [ 261.
of chicken l nuclear lamin proteins during mitosis: evidence for a reversible redistribution of lamin B2 between inner nuclear membrane and elements of the endoplasmic reticulum. / Cell Bid 1988: 107~397-406. Demonstration that both chicken lamins B, and B2 remain associated with nuclear envelope-derived membranes during mitosis and thus represent bonufide B-type lamins. Surprisingly, during mitosis lamin B2 is found to redistribute (reversibly) between inner nuclear membrane and the ER
l
16.
17.
STUXK R, ANGRES B, LEHNER CF, Nrcx
EA
The
fates
18.
GEORGATOS SD, STOURNARAS C, BLOBEL G: Heterotypic and homotypic associations between the nuclear lamins sitespecificity and control by phosphorylation. Proc Na!l Acad Sci USA 1988, 854325-4329. Biochemical SNdieS aimed at the important question of pairing preferences between individual lamin isoforms. Results need to be compared with those obtained by other techniques. See 131. l
GEORGATOS SD, BARBEL G: l diate filament attachment Cell Eiol 1987, 105:117-125. see [201. 19.
Lamin B constitutes site at the nuclear
an intermeenvelope. J
20.
GEORGATOS SD, WEBER K, GEISIER N, BUJBEL G: Binding of two desmin derivatives to the plasma membrane and the nuclear envelope of avian erythrocytes: evidence for a conserved site-specilicity in intermediate filament-membrane interactions. Proc Nat1 Acad Sci US4 1987, 84:67&784. Results from in uitm binding assays are interpreted to indicate a role for B-type lamins in directly binding cytoplasmic lF proteins. l
21. me
IEHNER CF, STICK R, EPPENBERGER HM, NIGG EA: Differential expression of nuclear lamin proteins during chicken development. J Cell Bid 1987, 105:577-587. Demonstration of dUTerential expression of A- and B-type lamins during avian embrogenesis: A-type lamins are undetectable in early embryonic tisSUeS.
22.
STEWART C, BURKE B: Teratocarcinoma stem cells and early mouse embryos contain only a single major lamin polypeptide closely resembling lamin B. Cell 1987, 51:383-392. Findings as in [ 211, for mouse embryogenesis and in vitro tierentiating embtyonal carcinoma cells.
l e
23.
REBEL S, IAMPRON C, ROYAL 4 RAVMOND Y: L.aIIlilY.SA and C appear during retinoic acid-induced differentiation of mouse embryonal carcinoma cells. J Cell Biol 1987, 105:109%1104. Demonstration of de nova synthesis of A-type lamins during in vitro differentiation of F9 embryonal carcinoma cells. See also [ 21,221.
l
24.
GUIUY MN, BENSUS-SAN A, BOURGE JF, BORNENS M. COURVAUN JC: A human T lymphoblastic cell line lacks lamins A and C. EMBO J 1987, 63795-3799. Evidence for dilferential expression of A- and B-type lamiis in human lymphoid cells. See also [21-231. l
26.
HJ, YUAN J, BIOBEL G, GEORGATOS SD: A lamin B receptor in the nuclear envelope. Proc Nat1 Acad Sci USA 1988, 85:85314534. Identitication, by a Ugand blotting assay, of a putative B-type lamin receptor in the avian nuclear envelope. See also [25].
27.
Sl GLOMSET JA: Evidence for modilication of lamin B by a product of mevalonic acid. J Biol Cbem 1988, 263:5997+&00. Identification of lamin B as a target of post-translational modiJication by a derivative of mevalonic acid. The modification is presumed to be isoprenoid in naNre and very hydrophobic; conceivably, it may play a role in anchoring B-type lamins to the Upid bilayer. See also 1281.
l e
SEMOR A, GERACE L to the inner nuclear nuclear lamina. J &II
Integral membrane proteins membrane and associated Biol 1988, 107:2029-2036.
specific with the
WOLDA
l e
28.
BECK IA, HOSICK
TH, SINENSKY M: Incorporation of a product of mevalonic acid metabolism into proteins of Chinese hamster ovary cell nuclei. J Cell Biol 1988, 107~1307-1316. Results similar to those described in 1271; in addition, evidence is presented for isoprenylation of the lamin A precursor. l e
29.
SMITH DE, GRUENBAUM
Y. BERRIOS M. FISHER PA:
Biosynthesis
and interconversion of Drosophila nuclear lamin isoforms during normal growth and in response to heat shock. J Cell Biol 1987, 105:771-790. Delineation of the post-translational processing pathway resulting in the production of 2 distinguishable lamin polypeptides (Dmt and Dm2) from a single primary translation product (Dm,). Processing is interpreted to involve both proteolysis and phosphorylation.
l
PL: Nuclear reassembly excludes large macromolecules. Science 1987, 238:54%550. Demonstration that reformation of the nuclear envelope precedes chromatin decondensation and nuclear expansion during postmitotic reassembly of nuc!ei. For possible functional consequences see [31].
30.
SWANXXN JA, MCNEIL
l
BLOW JJ, LUKEY RAz (Letter to the Editor) A role for the nuclear envelope in controlling DNA replication witbin the cell cycle. Nature 1988, 332:54&548. study on the role of the nuclear envelope in controlling DNA replication. An intriguing model is proposed, according to which the nuclear envelope determines the nucleocytoplasmic compartmentation of an essential replication factor (‘licensing factor’) whose exclusive access to DNA during mitosis (and inactivation’ tier replication) would result in single rounds of replication during each ceU cycle. 31.
l e
32. l
see
FIELDS AP, PETIIT GR, MAY WS: Phosphorylation at the nuclear membrane by activated protein Biol &em 1988, 263:82538260. 1331.
of lamin B kinase C. J
P, HUANG K-P, PAUL WE: Lamin B is rapidly phosphorylated in lymphocytes after activation of protein kinase C. Proc Null Acad Sci USA 1988, 85:2279-2283. Iamin proteins are shown to become rapidly phosphorylated in response to certain extracellular stimuli that are known to activate protein kinase C. The data raise the possibility (but do not prove) that signal transfer from cytoplasm to nucleus may involve nuclear translocation of active protein kinase C (or a proteolytic fragment of the kinase).
33.
HORNBECK
l e
FRtEDMAN DL, KEN R Insulin StimtiteS inCOrporatiOn Of s2Pt into nuclear lamins A and C in quiescent BHK-21 cells. J Biol Cbem 1988, 263:11031106. Demonstration that insulin induces phosphorylation of lamins upon mitogenic stimulation of quiescent fibroblasts. 34. l
25.
WORMAN