The nuclear lamina is a meshwork of intermediate-type filaments

Differential accessibility of the tail domain of nuclear lamin A in interphase and mitotic cells

Human autoantibodies reactive against the tail domain exclusive to lamin A and absent from lamin C have been used for immunofluorescence studies on human fibroblast and epithelial cells. These autoantibodies were seen to react on mitotic cells where lamin A is present in a soluble depolymerized form and to react against lamin A in assembled interphase nuclear lamina after in situ extraction of chromatin. Taken together, these results support the suggestion that the tail domain of lamin A may be involved in the putative interaction of lamin A with chromatin.

Transformation of the amphibian oocyte into the egg: structural and biochemical events

Amphibian oocytes, arrested in prophase I, are stimulated to progress to metaphase II by progesterone. This process is referred to as meiotic maturation and transforms the oocyte, which cannot support the early events of embryogenesis, into the egg, which can. Meiotic maturation entails global reorganization of cell ultrastructure: In the cell cortex, the plasma membrane flattens and the cortical granules undergo redistribution. In the cell periphery, the annulate lamellae disassemble and the mitochondria become dispersed. In the cell interior, the germinal vesicle becomes disassembled and the meiotic spindles form. Marked changes in the cytoskeleton and mRNA distribution also occur throughout the cell. All of these events are temporally correlated with intracellular signalling events: Fluctuations in cAMP levels, changes in pH, phosphorylation and dephosphorylation, and ion flux changes. Evidence suggests that specific intracellular signals are responsible for specific reorganizations of ultrastructure and mRNA distribution.

Antibodies to nuclear lamin C in chronic hepatitis delta virus infection

Sera of patients with chronic hepatitis delta virus infection stained the nuclear periphery in indirect immunofluorescence. Using proteins of isolated nuclei, isolated nuclear matrices, the nuclear pore complex-lamina fraction and purified lamins A and C as antigen source in immunoblotting experiments, nuclear lamin C was identified as the reactive antigen. Most sera tested (8 of 10) recognized nuclear lamin C exclusively, but not the nuclear lamins A and B. Antibodies reacting with both nuclear lamins A and C, which share extensive sequence homologies, have been reported to occur in autoimmune hepatitis and primary biliary cirrhosis. The present findings suggest that the novel autoantibody associated with chronic hepatitis delta virus infection recognizes an epitope localized in the short carboxyterminal region of nuclear lamin C.

The lamin B receptor of the nuclear envelope inner membrane: a polytopic protein with eight potential transmembrane domains

The lamin B receptor is a previously identified integral membrane protein in the nuclear envelope of turkey erythrocytes that associates with the nuclear intermediate filament protein lamin B (Worman, H. J., J. Yuan, G. Blobel, and S. D. Georgatos. 1988. Proc. Natl. Acad. Sci. USA. 85:8531-8534). In the present report, we use cell fractionation and antibodies against the lamin B receptor to localize it to an 8-M urea-extracted membrane fraction of chicken liver nuclei, supporting an inner nuclear membrane localization. We deduced the amino acid sequence of the chicken lamin B receptor from overlapping clones obtained by screening cDNA libraries with a probe generated by the polymerase chain reaction with primers based on the partial protein sequence of the isolated protein. The mature lamin B receptor has a calculated molecular mass of 73,375 D and eight segments of hydrophobic amino acids that could function as transmembrane domains as determined by hydropathy analysis. Preceding the first putative transmembrane segment is a highly charged 204-residue-long amino terminal region that contains two consensus sites for phosphorylation by protein kinase A. Since the lamin B receptor has been shown to be phosphorylated by protein kinase A in vitro and in vivo and this phosphorylation affects lamin B binding (Applebaum, J., G. Blobel, and S. D. Georgatos. 1990. J. Biol. Chem. 265:4181-4185), it is likely that this amino terminal region faces the nucleoplasm. The amino terminal region also contains three DNA-binding motifs that are found in gene regulatory proteins and histones, suggesting that the lamin B receptor may additionally play a role in gene regulation and/or chromatin organization.

Molecular analysis of the Drosophila nuclear lamin gene

A complete nucleotide sequence of a 4.2-kb genomic fragment containing the Drosophila lamin gene and flanking sequences is presented. Primer extension experiments and sequence analysis revealed that transcription starts from a single promoter. The lamin maternal 2.8-kb transcript and the 3.0-kb zygotic transcript are generated from two alternative polyadenylation sites. The gene contains four exons. The first intron is 7 bp upstream of the first AUG site. The two other introns are located within the alpha-helical rod domain of the protein: one in coil 1B in the 42-amino-acid domain that is absent in vertebrate cytoplasmic intermediate filament proteins and the other in coil 2 at a position different from intron positions within the vertebrate intermediate filament genes. Together with the sequence homology analysis, the data suggest either that the lamin gene was the ancestral gene of intermediate filament genes or that the lamin gene diverged from other intermediate filament genes early in evolution.

Characterization of a second highly conserved B-type lamin present in cells previously thought to contain only a single B-type lamin

Previous analyses of the nuclear lamina of mammalian cells have revealed three major protein components (lamins A, B and C) that have been identified by protein sequence homology as members of the intermediate filament (IF) protein family. It has been claimed that mammalian cells contain either all three lamins or lamin B alone. Using monoclonal antibodies specific for B-type lamins and cDNA cloning we identified a second major mammalian B-type lamin (murine lamin B2), thus showing that lamin composition in mammals is more complex than previously thought. Lamin B2 is coexpressed with lamin B1 (formerly termed lamin B) in all somatic cells and mammalian species that we analysed, including a variety of cells currently believed to contain only a single lamin. This suggests that two B-type lamins are necessary to form a functional lamina in mammalian somatic cells. By cDNA cloning we found that Xenopus laevis lamin LII is the amphibian homolog of mammalian lamin B2. Lamin expression during embryogenesis of amphibians and mammals shows striking similarities. The first lamins expressed in the early embryo are the two B-type lamins, while A-type lamins are only detected much later in development. These findings indicate that the genomic differentiation into two B-type lamins occurred early in vertebrate evolution and has been maintained in both their primary structure and pattern of expression.

Lamins A and C bind and assemble at the surface of mitotic chromosomes

To study a possible interaction of nuclear lamins with chromatin, we examined assembly of lamins A and C at mitotic chromosome surfaces in vitro. When a postmicrosomal supernatant of metaphase CHO cells containing disassembled lamins A and C is incubated with chromosomes isolated from mitotic Chinese hamster ovary cells, lamins A and C undergo dephosphorylation and uniformly coat the chromosome surfaces. Furthermore, when purified rat liver lamins A and C are dialyzed with mitotic chromosomes into a buffer of physiological ionic strength and pH, lamins A and C coat chromosomes in a similar fashion. In both cases a lamin-containing supramolecular structure is formed that remains intact when the chromatin is removed by digestion with micrococcal nuclease and extraction with 0.5 M KCl. Lamins associate with chromosomes at concentrations approximately eightfold lower than the critical concentration at which they self-assemble into insoluble structures in the absence of chromosomes, indicating that chromosome surfaces contain binding sites that promote lamin assembly. These binding sites are destroyed by brief treatment of chromosomes with trypsin or micrococcal nuclease. Together, these data suggest the existence of a specific lamin-chromatin interaction in cells that may be important for nuclear envelope reassembly and interphase chromosome structure.

Autoantibodies to lamins A and C in sera of patients showing peripheral fluorescent antinuclear antibody pattern on HEP-2 cells

Lamins A, B, and C are the major proteins of a polymeric structure called nuclear lamina, which is intercalated between chromatin and the inner membrane of the nuclear envelope. Using immunofluorescence on HEp-2 cells, specific enzyme-linked immunosorbent assay, and Western blotting performed against nuclear lamina preparation from Ehrlich ascites tumor cells, we characterized three patients, whose sera contained antibodies to nuclear lamins. The reaction pattern observed in two of the patients may result from single or combined occurrence of anti-lamin A and C antibodies. The third patient had antibodies that probably recognized an epitope in the carboxy-terminal region of lamin C. The sera were donated by a heterogeneous group of patients, and no common clinical or laboratory signs seemed to link them together.

Visualization of transport-related configurations of the nuclear pore transporter

The transport of macromolecules between the cytoplasm and nucleus of the cell is mediated by the nuclear pore complex (NPC). In this study, details of the central transporter assembly within NPCs have been examined by cryoelectron microscopy, image processing, and classification analysis. The NPC transporter in isolated amphibian nuclei appears to adopt a minimum of four transport-related configurations including: (a) a putative closed form with a 90-100 A diameter central pore, (b) a docked form with material aligned over the pore, (c) an open form with substrates apparently caught "in transit," and (d) an open form with an enlarged pore. This data confirms previous observations on NPC transporters labeled with nucleoplasmin-gold (Akey, C.W., and D.S. Goldfarb. 1989. J. Cell Biol. 109:971-982) and allows a working model of the central NPC transporter to be proposed. The model is comprised of two supramolecular irislike assemblies which open asynchronously to provide an expanded pore for translocation while maintaining transport fidelity.

Expression and characterization of human lamin C

We have expressed human lamin C cDNA in E. coli using a modification of the pLcII vector system. Protein produced in this way had seven additional amino acids at its N-terminus, but retained key lamin structural and assembly properties. The modified vector we produced may prove useful when difficulties are encountered in removal of the cII fusion peptide by factor X cleavage in the pLcII system. Shadowed preparations of expressed lamin C showed the presence of 50-nm rod-like particles that closely resembled those observed for native material. Isolated molecules had two globular domains at one end, indicating that they were constructed from two parallel polypeptide chains. The expressed material also formed paracrystals with a characteristic 22.5 nm axial repeat, indicating that its assembly properties had also been retained. We also used site-specific mutagenesis to engineer a lamin fragment that lacked the C-terminal non-helical domain of the molecule. This material formed paracrystals similar to those obtained with the intact molecule, indicating that the large C-terminal non-helical domain did not contain information vital for lamin assembly.

Interphase nuclear envelope lamins form a discontinuous network that interacts with only a fraction of the chromatin in the nuclear periphery

Antibodies directed against nuclear envelope lamin proteins have been used in conjunction with three-dimensional light and electron microscope methodologies to determine the spatial organization of lamins in diploid interphase nuclei and to relate this organization to the positions of chromatin in the nuclear periphery. Using Drosophila early embryos, Drosophila Kc cells, and human HeLa cells, it is qualitatively and quantitatively observed that lamins are organized as a highly discontinuous, apparently fibrillar network that leaves large voids in the nuclear periphery containing little or no lamin. Using fluorescence microscopy to compare and quantitate the relationship between chromatin and the lamin network, it is found that although there is a strong tendency for the most peripheral chromatin to be positioned directly underneath a lamin fiber, only a small fraction of the chromatin in the nuclear periphery is sufficiently close to a lamin fiber to possibly be in direct contact.

Elucidating the early stages of keratin filament assembly

Because of extraordinarily tight coiled-coil associations of type I and type II keratins, the composition and structure of keratin subunits has been difficult to determine. We report here the use of novel genetic and biochemical methods to explore the early stages of keratin filament assembly. Using bacterially expressed humans K5 and K14, we show that remarkably, these keratins behave as 1:1 complexes even in 9 M urea and in the presence of a reducing agent. Gel filtration chromatography and chemical cross-linking were used to identify heterodimers and heterotetramers as the most stable building blocks of keratin filament assembly. EM suggested that the dimer consists of a coiled-coil of K5 and K14 aligned in register and in parallel fashion, and the tetramer consists of two dimers in antiparallel fashion, without polarity. In 4 M urea, both end-to-end and lateral packing of tetramers occurred, leading to a variety of larger heteromeric complexes. The coexistence of multiple, higher-ordered associations under strongly denaturing conditions suggests that there may not be a serial sequence of events leading to the assembly of keratin intermediate filaments, but rather a number of associations may take place in parallel.

Association of gold-labelled nucleoplasmin with the centres of ring components of Xenopus oocyte nuclear pore complexes

We have used heavy-metal shadowing to study the interaction of morphological components of Xenopus oocyte nuclear pore complexes with nucleoplasmin conjugated to colloidal gold. When microinjected into Xenopus oocytes, gold-labelled nucleoplasmin accumulated on the axis of the pores. Envelopes partially disrupted by treatment with low ionic strength buffer produced isolated islands of pores together with substantial quantities of rings deriving from the cytoplasmic and nucleoplasmic faces of the pores. In preparations from oocytes in which nucleoplasmin-gold had been microinjected, most (238/288) of the rings examined had also been labelled and, in the majority of these (60%), the label was located centrally within isolated rings. The central positioning of the nucleoplasmin-gold in isolated rings indicated that these morphological components of the pores were probably involved in the transport of nucleoplasmin into the nucleus, either by way of the initial binding of the molecule or by way of its translocation across the nuclear envelope. Although more work is required to resolve the precise stage at which the rings are involved, a number of lines of evidence suggested that they were more likely to be involved in the translocation step rather than in initial binding of nucleoplasmin.

A polypeptide of 59 kDa is associated with bundles of cytoplasmic filaments in Neurospora crassa

Complex arrangements of filamentous structures have been isolated from vegetative cells of the fungus Neurospora crassa. They were enriched by differential centrifugation and purified by permeation chromatography. The filamentous structures are made up of units of 8-10 nm diameter and were isolated in bundles of up to six to nine units. The main constituent of these structures is a polypeptide with an apparent molecular mass of 59 kDa (P59Nc), which represents 4-5% of the total N. crassa proteins. The filamentous structures are cold-stable and are not affected by high-ionic-strength solutions or by the presence of 10 mM-EDTA or 1% (w/v) Triton X-100; they were disassembled by raising the pH of the solution or by using Tris-based buffers. The disassembled form assembled into structures sedimentable at 105,000 g after dialysis against the isolation buffer. The sedimentable structures were organized in the form of regular aggregates of 42-45 nm polypeptides and reacted weakly with anti-IFA, a monoclonal antibody which recognizes an epitope common to many of the higher-eukaryote intermediate-filament polypeptides. Immunofluorescence examination of wall-digested hyphae of N. crassa using affinity-purified antibodies prepared against P59Nc showed immunostaining of abundant filamentous and dot-shaped structures distributed in the cytoplasm.

In vitro disassembly of the nuclear lamina and M phase-specific phosphorylation of lamins by cdc2 kinase

The nuclear lamina is an intermediate filament-type network underlying the inner nuclear membrane. Phosphorylation of lamin proteins is believed to cause lamina disassembly during meiotic and mitotic M phase, but the M phase-specific lamin kinase has not been identified. Here we show that the cdc2 kinase, a major element implicated in controlling the eukaryotic cell cycle, phosphorylates chicken B-type lamins in vitro on sites that are specifically phosphorylated during M phase in vivo. Concomitantly, cdc2 kinase is capable of inducing lamina depolymerization upon incubation with isolated nuclei. One of the target sites of cdc2 kinase is identified as a motif (SPTR) conserved in the N-terminal domain of all lamin proteins. These results lead us to propose that mitotic disassembly of the nuclear lamina results from direct phosphorylation of lamins by cdc2 kinase.

Mutations of phosphorylation sites in lamin A that prevent nuclear lamina disassembly in mitosis

The nuclear envelope is a dynamic structure that completely disassembles in response to MPF/cdc2 activity in mitosis. A key feature of this process is the hyperphosphorylation of the major structural proteins of the envelope, the nuclear lamins A, B, and C. Two highly conserved serine residues of the lamin protein (Ser-22 and Ser-392 of lamins A and C) are symmetrically positioned 5 amino acids from the ends of the large alpha-helical domain and are shown in the accompanying paper by Ward and Kirschner to be among four sites phosphorylated during nuclear envelope breakdown. Mutations in Ser-22 and Ser-392 that prevent phosphorylation at these sites block the disassembly of the nuclear lamina during mitosis. We propose a model for the regulation of lamin assembly in which phosphorylation just outside the ends of the alpha-helical domain controls the assembly dynamics of the lamin coiled-coil dimers.

Identification of cell cycle-regulated phosphorylation sites on nuclear lamin C

The mechanism by which MPF induces nuclear lamin disassembly and nuclear envelope breakdown during mitosis was studied in a frog egg extract in which the transition from interphase to mitosis can be induced by the addition of MPF. Bacterially expressed human nuclear lamin C, assembled in vitro into filaments, showed increased phosphorylation on specific sites in the extract in response to MPF. Phosphorylation was accompanied by disassembly of the lamin filaments. We determined the sequences of the sites phosphorylated both in the presence and absence of MPF. The sequence data suggest that multiple protein kinases act on the lamins, and S6 kinase II was identified as one potentially important lamin kinase.

Correlation between structure and mass distribution of the nuclear pore complex and of distinct pore complex components

Nuclear pore complexes (NPCs) prepared from Xenopus laevis oocyte nuclear envelopes were studied in "intact" form (i.e., unexposed to detergent) and after detergent treatment by a combination of conventional transmission electron microscopy (CTEM) and quantitative scanning transmission electron microscopy (STEM). In correlation-averaged CTEM pictures of negatively stained intact NPCs and of distinct NPC components (i.e., "rings," "spoke" complexes, and "plug-spoke" complexes), several fine structural features arranged with octagonal symmetry about a central axis could reproducibly be identified. STEM micrographs of unstained/freeze-dried intact NPCs as well as of their components yielded comparable but less distinct features. Mass determination by STEM revealed the following molecular masses: intact NPC with plug, 124 +/- 11 MD; intact NPC without plug, 112 +/- 11 MD; heavy ring, 32 +/- 5 MD; light ring, 21 +/- 4 MD; plug-spoke complex, 66 +/- 8 MD; and spoke complex, 52 +/- 3 MD. Based on these combined CTEM and STEM data, a three-dimensional model of the NPC exhibiting eightfold centrosymmetry about an axis perpendicular to the plane of the nuclear envelope but asymmetric along this axis is proposed. This structural polarity of the NPC across the nuclear envelope is in accord with its well-documented functional polarity facilitating mediated nucleocytoplasmic exchange of molecules and particles.

The coiled coil of in vitro assembled keratin filaments is a heterodimer of type I and II keratins: use of site-specific mutagenesis and recombinant protein expression

Recombinant DNA technology has been used to analyze the first step in keratin intermediate filament (IF) assembly; i.e., the formation of the double stranded coiled coil. Keratins 8 and 18, lacking cysteine, were subjected to site specific in vitro mutagenesis to change one amino acid in the same relative position of the alpha-helical rod domain of both keratins to a cysteine. The mutations lie at position -36 of the rod in a "d" position of the heptad repeat pattern, and thus air oxidation can introduce a zero-length cystine cross-link. Mutant keratins 8 and 18 purified separately from Escherichia coli readily formed cystine homodimers in 2 M guanidine-HCl, and could be separated from the monomers by gel filtration. Heterodimers with a cystine cross-link were obtained when filaments formed by the two reduced monomers were allowed to oxidize. Subsequent ion exchange chromatography in 8.5 M urea showed that only a single dimer species had formed. Diagonal electrophoresis and reverse phase HPLC identified the dimer as the cystine containing heterodimer. This heterodimer readily assembled again into IF indistinguishable from those obtained from the nonmutant counterparts or from authentic keratins. In contrast, the mixture of cystine-stabilized homodimers formed only large aberrant aggregates. However, when a reducing agent was added, filaments formed again and yielded the heterodimer after oxidation. Thus, the obligatory heteropolymer step in keratin IF assembly seems to occur preferentially at the dimer level and not during tetramer formation. Our results also suggest that keratin I and II homodimers, once formed, are at least in 2 M guanidine-HCl a metastable species as their mixtures convert spontaneously into heterodimers unless the homodimers are stabilized by the cystine cross-link. This previously unexpected property of homodimers explains major discrepancies in the literature on the keratin dimer.

Effect of aluminum and other multivalent cations on neurofilaments in vitro: an electron microscopic study

Using electron microscopy (EM) of negatively stained samples, we have systematically explored the effect of aluminum and other multivalent cations on neurofilaments (NFs) in vitro. Interactions of these cations were investigated with bovine, rabbit, and rat spinal cord native NFs, and with 10-nm filaments reconstituted from the 68-kDa subunit (NF-L) isolated from bovine spinal cord. Our results indicated that, as has been observed with other classes of intermediate filaments (IFs), all multivalent cations caused significant aggregation of native NFs, suggesting that this phenomenon is a rather general one and not limited to aluminum. In addition, all cations tried caused significant lateral aggregation of filaments reconstituted from NF-L. Aluminum lactate had an identical effect on bovine, rabbit, and rat NFs. Because aluminum causes strong aggregation of NFs in vitro, a similar phenomenon may occur in vivo leading to the observed accumulation of NFs in neuronal perikarya of rabbits after intoxication with aluminum. These in vitro observations support the concept that some human neurological diseases characterized by the accumulation of NFs may be related to abnormal levels of multivalent cations.

On the cell-free association of lamins A and C with metaphase chromosomes

Nuclear envelopes have previously been shown to assemble spontaneously around endogenous chromosomes in cell-free homogenates of mitotic Chinese hamster ovary cells. In order to further analyze the mechanisms underlying nuclear envelope reformation and the functions of the individual nuclear lamin polypeptides, a fractionated cell-free nuclear envelope reassembly system involving purified chromosomes and either a postchromosomal supernatant or a cytosol fraction from mitotic cells has been devised. Results obtained with this fractionated system show that lamins A and C will associate with the surfaces of chromosomes in the absence of lamin B and membranes, this association being inhibitable by ATP-gamma-S. However, in the absence of membranes chromatin decondensation never occurs. Using the reversible swelling of chromosomes in low ionic strength buffers lacking divalent cations as the basis of a simple assay, it is demonstrated that the association of lamins A and C with the surfaces of chromosomes has a pronounced and easily observable effect on chromatin organization.

Transfection of human lamins A and C into mouse embryonal carcinoma cells possessing only lamin B

The peripheral lamina of eukaryotic nuclei is composed of polypeptides called lamins that vary in number from one to four according to organism, cell type, and differentiated state of the cells. Early embryonic cells and stem cells of mammals generally possess only lamin B while lamins A and C appear later during differentiation. To study the role of the late appearance of lamins A and C in the differentiated phenotype, we have performed transfection of cDNAs coding for human lamins A or C into mouse embryonal carcinoma (EC) cell lines F9 and P19 lacking these two lamins. Transient transfections have shown that lamins A or C could be expressed, translocated to the peripheral lamina, and distributed into daughter cell nuclei after mitosis. These results demonstrated that EC cells devoid of lamins A and C nevertheless possessed the appropriate mechanisms for the localization and mitotic redistribution of exogenous lamins A and C.

Interactions of intermediate filaments with cell structures

Intermediate filaments (IF) are unique components of the cytoskeleton of most eukaryotic cells. Also the nuclear lamins are now recognized to be IF-like proteins, providing the nucleus with a putative skeleton for chromatin attachment. Immunofluorescence and whole-mount electron microscopic studies reveal that IF form a cytoplasmic network that surrounds the nucleus and extends to cell surface, as 'mechanical integrators of cellular space'. It seems however unlikely that IF in the cell accomplish a merely structural role, considering the diversity of IF proteins and the complex regulation of their gene expression. In this work we primarily present electron microscopic data that points to the presence of interactions between IF and several cellular components, namely the nucleus, plasma membrane, other cytoskeletal elements, cytoplasmic organelles and ribonucleoproteins. Although the functional significance of such interactions remains to be demonstrated, assumptions like involvement of IF in information transfer or cytoskeleton-dependent control of gene expression represent attractive hypothesis for future research.

Modification of nuclear lamin proteins by a mevalonic acid derivative occurs in reticulocyte lysates and requires the cysteine residue of the C-terminal CXXM motif

The C-terminus of nuclear lamins (CXXM) resembles a C-terminal motif (the CAAX box) of fungal mating factors and ras-related proteins. The CAAX box is subject to different types of post-translational modifications, including proteolytic processing, isoprenylation and carboxyl methylation. By peptide mapping we show that both chicken lamins A and B2 are processed proteolytically in vivo. However, whereas the entire CXXM motif is cleaved from lamin A, at most three C-terminal amino acids are removed from lamin B2. Following translation of cDNA-derived RNAs in reticulocyte lysates, lamin proteins specifically incorporate a derivative of [14C]mevalonic acid (MV), i.e. the precursor of a putative isoprenoid modification. Remarkably, no MV is incorporated into lamin B2 translated from a mutant cDNA encoding alanine instead of cysteine in the C-terminal CXXM motif. These results implicate this particular cysteine residue as the target for modification of lamin proteins by an isoprenoid MV derivative, and they indicate that isoprenylation is amenable to studies in cell-free systems. Moreover, our observations suggest that C-terminal processing of newly synthesized nuclear lamins is a multi-step process highly reminiscent of the pathway elaborated recently for ras-related proteins.

Specific disruption of intermediate filaments and the nuclear lamina by the 19-kDa product of the adenovirus E1B oncogene

The 19-kDa protein encoded within the adenovirus E1B gene is essential for transformation by adenovirus and for proper regulation of viral early gene transcription. In order to investigate the biological function of the 19-kDa E1B protein, vectors were constructed to produce the 19-kDa protein in mammalian cells under the direction of heterologous promoters. Surprisingly, during transient expression, the E1B 19-kDa protein specifically associated with and disrupted the organization of intermediate filaments and the nuclear lamina, without disturbing the organization of other cytoskeletal networks. These results directly demonstrate an effect of a viral transforming protein on the cytoskeleton and suggest a role for intermediate filaments and the nuclear lamina in modulation of viral gene expression and the process of oncogenic transformation.

Effect of Clostridium difficile enterotoxin A on ultrastructure of Chinese hamster ovary cells

Electron microscopical immunocytochemistry and light microscopy were used to study the effect of Clostridium difficile enterotoxin A (EA) on cultured Chinese hamster ovary (CHO) cells. At 4 degrees C, immunocytochemically detected EA was randomly distributed along the plasma membrane; when cells were subsequently transferred to 37 degrees C, the EA moved into coated pits and coated vesicles within 2 min. Within 2 h of incubation at 37 degrees C with EA, the perinuclear cytoplasm of the CHO cells became highly refractile, and in 4 h, most of the cells were round; however, the majority of rounded cells excluded erythrosin B while remaining firmly attached to the culture dish plastic. When EA was removed from the cultured cells within 2 h, the cells returned to a normal morphology and formed a confluent monolayer. The nuclei of rounded cells were irregularly shaped; cytoplasmic intermediate filaments were collapsed toward the nucleus. Long bundles of parallel, 11-nm-diameter filaments appeared in the nuclei after 3 h of incubation with EA and disappeared by the fourth hour of incubation. Intoxicated cells did not undergo mitosis. Thus, EA was internalized, at least in part, by receptor-mediated endocytosis and subsequently affected the nuclei of CHO cells.

The nucleoskeleton and the topology of transcription

Transcription is conventionally believed to occur by passage of a mobile polymerase along a fixed template. Evidence for this model is derived almost entirely from material prepared using hypotonic salt concentrations. Studies on subnuclear structures isolated using hypertonic conditions, and more recently using conditions closer to the physiological, suggest an alternative. Transcription occurs as the template moves past a polymerase attached to a nucleoskeleton; this skeleton is the active site of transcription. Evidence for the two models is summarised. Much of it is consistent with the polymerase being attached and not freely diffusible. Some consequences of such a model are discussed.

The conserved carboxy-terminal cysteine of nuclear lamins is essential for lamin association with the nuclear envelope

We have analyzed the interaction of soluble nuclear lamins with the nuclear envelope by microinjection of normal and mutated lamins into the cytoplasm of Xenopus laevis oocytes. Our results demonstrate that the conserved cysteine of the carboxy-terminal tetrapeptide Cys Ala/Ser Ile Met of lamins is essential for their association with the nuclear envelope. Removal of this sequence or replacement of the cysteine by serine resulted in Xenopus lamin L1 remaining in a soluble, non-envelope-associated state within the nucleus. Similar mutations of Xenopus lamin A resulted in only partial reduction of nuclear envelope association, indicating that lamin A contains additional signals that can partially compensate for the lack of the cysteine. Mammalian lamin C lacks this tetrapeptide and is not associated with the nuclear envelope in our experimental system. Cloning of the tetrapeptide Cys Ala Ile Met to the carboxy terminus of human lamin C resulted in lamin being found in a nuclear envelope-associated form in oocytes. Mutations at the amino terminus and in the alpha-helical region of lamin L1 revealed that the carboxy terminus mediates the association of lamins with the nuclear envelope; however, this alone is insufficient for maintenance of a stable association with the nuclear envelope.

Interactions and structure of the nuclear pore complex revealed by cryo-electron microscopy

Nuclear pore complexes (NPCs) play a central role in mediating nucleocytoplasmic transport and exchange processes in eukaryotic cells. The arrangement and interactions of NPCs within amphibian nuclear envelopes have been studied using cryo-electron microscopy of unfixed and frozen hydrated specimens. The nuclear lamina in Necturus forms an orthogonal network with crossover distances which vary between 1,600 and 4,000 A and which may be related to the basic filament repeat of lamins. Furthermore, the NPCs are attached randomly within the confines of the lamin network, presumably by their nucleoplasmic rings. Image analysis of edge-on and en face projections of detergent-extracted NPCs has been combined with data on the coaxial thin rings to provide a quantitative evaluation of the triple ring model of NPC architecture proposed previously (Unwin, P. N. T., and R. Milligan. 1982. J. Cell Biol. 93:63-75). Additional details of the complex have been visualized including an intimate association of the inner spoke domains as an inner spoke ring, extensive domains within the spokes and coaxial thin rings, and interestingly, a central channel-like feature. Membrane-associated NPCs and detergent-extracted NPCs both possess peripherally located radial arms resulting in an effective diameter of approximately 1,450-1,500 A. In projection, the radial arms possess approximate mirror symmetry suggesting that they originate from both sides of the assembly. Moreover, membrane-associated NPCs are asymmetric at most radii and right-handed as viewed from the cytoplasm; detergent-extracted NPCs appear to be symmetric and have approximately 822 symmetry. Taken together, the data suggests that the framework of membrane-associated NPCs is perturbed from a symmetrical configuration, either during isolation of nuclei or by interactions with the lamina and the nuclear envelope in vivo. However, detergent extraction of nuclei appears to result in a more symmetrical alignment of components in apposing halves of the assembly.

Nuclear envelope disassembly and nuclear lamina depolymerization during germinal vesicle breakdown in starfish

During germinal vesicle breakdown (GVBD) in starfish, the nuclear envelope disassembles before the nuclear lamina completely depolymerizes, judging from correlative ultrastructural, immunolabeling, and light microscopic analyses. At 13 degrees C, prophase-arrested oocytes of Pisaster ochraceus begin GVBD and rapidly undergo nuclear envelope disassembly about 50 min after addition of the maturation-inducing hormone 1-methyladenine (1-MA). The nuclear lamina of these oocytes, however, remains present for 10-20 min following the vesiculation of the nuclear envelope. Completion of GVBD, as evidenced by a blending of the nuclear contents with the surrounding cytoplasm, occurs within about 15 min after the nuclear lamina has fully depolymerized. Immunofluorescence studies also indicate that a marked increase in the phosphorylations of nuclear proteins precedes the structural reorganizations of the nuclear envelope and nuclear lamina during GVBD.

A new monoclonal antibody recognizing the amino-terminal consensus sequence of vertebrate intermediate filament proteins

The mouse monoclonal antibody ME 101 raised against human peripherin, an intermediate filament protein (IFP) specific to well defined neuronal populations, recognizes all the major classes of vertebrate IFP in immunoblotting assays. Desmin, GFAP, vimentin, peripherin and the lightest neurofilament protein (NF-L) were cleaved into carboxy- and amino-terminal halves by N-chlorosuccinimide at their unique trytophan residue. Whereas the antibody directed against the epitope common to every IFP (intermediate filament antigen or IFA) and located on the carboxy-terminal end of the rod domain recognizes the carboxy-terminal half, the ME 101 antibody, as the present study illustrates, recognizes specifically the amino-terminal half. From the amino acid sequence data of IFP, it is deduced that the cognate epitope is localized on the amino-terminal part of coil la.

Cloning and sequencing of cDNA clones encoding chicken lamins A and B1 and comparison of the primary structures of vertebrate A- and B-type lamins

Nuclear lamins are intermediate-filament-type proteins forming a fibrillar meshwork underlying the inner nuclear membrane. The existence of multiple isoforms of lamin proteins in vertebrates is believed to reflect functional specializations during cell division and differentiation. Although biochemical criteria may be used to classify many lamin isoforms into A- and B-type subfamilies, the structural features distinguishing the members of these subfamilies remain to be characterized fully. Here, we report the complete primary structures of chicken lamins A and B1, as they are deduced from cloned cDNAs; in the accompanying paper we present the complete sequence of lamin B2, a second avian B-type lamin. Comparisons of the chicken lamin sequences with each other and with those of other lamins allow us to establish structural features that are common to members of both subfamilies. Conversely, multiple sequence alignments make it possible to identify a number of structural motifs that clearly differentiate B-type lamins from A-type lamins. With this information at hand, we attempt to correlate different biochemical properties of A- and B-type lamins with the presence or absence of specific sequence motifs.

Expression of nuclear lamin A and muscle-specific proteins in differentiating muscle cells in ovo and in vitro

Primary cultures and tissue samples of chicken embryonic muscle were immunologically probed for the expression of muscle-specific proteins, such as myosin heavy chain and the tropomyosins, as well as for the nuclear lamina protein, lamin A. As determined by quantitative immunoblotting, the expression of lamin A and the muscle-specific proteins were at low levels or absent in predifferentiation myoblasts both in vitro and in ovo. During differentiation, an increase of lamin A expression preceded the induction to high levels of expression of muscle-specific proteins. Immunofluorescence staining of chicken embryonic muscle cells in culture also indicates an accumulation of lamin A before the induction of muscle-specific proteins expression. Furthermore, the accumulation of lamin A reached a plateau before the muscle-specific proteins during muscle development. In two dimensional NEPHGE gel analysis of immunoprecipitated lamin A, no detectable change in the ratio of the acidic/basic isoelectric variants of lamin A was observed during myogenesis. A potential role for lamin A in the mechanisms which underlie the differential and coordinate expression of muscle-specific genes is proposed.

Organization of the nuclear pore complex in Saccharomyces cerevisiae

Fractions enriched for nuclear pore complexes (NPCs) have been isolated from Saccharomyces cerevisiae. The sequential extraction of nuclei with detergent, nucleases, and salt reveals an organization of the yeast NPC similar to other eukaryotes. Yeast NPCs contain a 30-nm "ring" structure not previously described in other organisms. This structure appears to organize 10-nm filaments into an assembly which exhibits an eight-fold rotational symmetry. Some proteins in the NPC fraction are capable of forming intermediate-sized filaments. These studies suggest that some component of the nuclear pore complex organizes an interaction between nuclear and cytoplasmic networks of intermediate filaments.

Molecular interactions in paracrystals of a fragment corresponding to the alpha-helical coiled-coil rod portion of glial fibrillary acidic protein: evidence for an antiparallel packing of molecules and polymorphism related to intermediate filament structure

We have expressed in Escherichia coli a fragment of c-DNA that broadly corresponds to the alpha-helical coiled-coil rod section of glial fibrillary acidic protein (GFAP) and have used the resultant protein to prepare paracrystals in which molecular interactions can be investigated. An engineered fragment of mouse GFAP c-DNA was inserted into a modified version of the E. coli expression vector pLcII, from which large quantities of a lambda cII-GFAP rod fusion protein were prepared. A protein fragment corresponding to the GFAP rod was then obtained by proteolysis with thrombin. Paracrystals of this material were produced using divalent cations (Mg, Ca, Ba) in the presence of a chaotrophic agent such as thiocyanate. These paracrystals showed a number of polymorphic patterns that were based on a fundamental pattern that had dyad symmetry and an axial repeat of 57 nm. Analysis of both positive and negative staining patterns showed that this fundamental pattern was consistent with a unit cell containing two 48-nm-long molecules in an antiparallel arrangement with their NH2 termini overlapping by approximately 34 nm. More complicated patterns were produced by stacking the fundamental pattern with staggers of approximately 1/5, 2/5, and 1/2 the axial repeat. The molecular packing the unit cell was consistent with a range of solution studies on intermediate filaments that have indicated that a molecular dimer (i.e., a tetramer containing four chains or two coiled-coil molecules) is an intermediate in filament assembly. Moreover, these paracrystals allow the molecular interactions involved in the tetramer to be investigated in some detail.

Major nucleolar proteins shuttle between nucleus and cytoplasm

Nucleolin is a 92 kd nucleolar protein implicated in regulating polymerase I transcription and binding of preribosomal RNA. Another abundant nucleolar protein of 38 kd (B23/No38) is thought to be involved in intranuclear packaging of preribosomal particles. Although both proteins have previously been detected only in nuclei, we conclude that they shuttle constantly between nucleus and cytoplasm. This conclusion is based on monitoring the equilibration of these proteins between nuclei present in interspecies heterokaryons, and on observing the antigen-mediated nuclear accumulation of cytoplasmically injected antibodies. Our unexpected results suggest a role for these major nucleolar proteins in the nucleocytoplasmic transport of ribosomal components. Moreover, they suggest that transient exposure of shuttling proteins to the cytoplasm may provide a mechanism for cytoplasmic regulation of nuclear activities.

Specificity of human anti-neurofilament autoantibodies

The specificities and isotypes of human antibodies that react with neurofilament (NF) proteins were examined by Western blot analysis. Two-thirds of the subjects tested had antibodies to the 200 kDa high molecular weight neurofilament protein (NF-H), and fewer had antibodies to the low and middle molecular weight neurofilament proteins (NF-L and NF-M respectively). Human autoantibodies bound to both native and dephosphorylated NF-H, but some antibodies bound to dephosphorylated NF-H only, indicating the presence of at least two target epitopes. They also recognized a fusion protein containing a segment of the NF-H protein produced by a cDNA clone in Escherichia coli, indicating that they bind to unmodified peptide epitopes. The anti-NF-H antibodies were mostly IgG, but were frequently complexed to IgA or IgM antibodies, possibly with rheumatoid factor or anti-idiotypic activity. These characteristics of anti-NF-H antibodies are most consistent with a secondary immune response that is antigen driven and T-cell dependent.

The protein composition of the nuclear matrix of murine P19 embryonal carcinoma cells is differentiation-stage dependent

The protein composition of the nuclear matrix of murine P19 embryonal carcinoma (EC) cells was compared with that of clonal derivatives of P19 EC differentiated in vitro, and with that of P19 EC cells induced to differentiate with retinoic acid (RA). Several major differences in nuclear matrix protein composition were found between the cell lines tested. Some polypeptides were found to occur only in EC cells, whereas others proved to be restricted to one or more of the differentiated derivatives. During RA treatment of EC cells a transient expression of some matrix proteins was observed. Several new proteins appeared, and others disappeared. Our data indicate that the protein composition of the nuclear matrix is a sensitive gauge for the differentiation state of cells.

Interconversion of Drosophila nuclear lamin isoforms during oogenesis, early embryogenesis, and upon entry of cultured cells into mitosis

Two isoforms of a single nuclear lamin, distinguishable on one-dimensional SDS-polyacrylamide gels, have previously been identified in Drosophila nuclei during interphase. A third species, designated lamin Dmmit, has now been identified as soluble in extracts of Drosophila tissue culture cells blocked in mitosis by drugs. An apparently identical form is the only lamin species detectable in late-stage egg chambers and early embryos. Phosphoamino acid analyses suggest that the conversion of lamins Dm1 and Dm2 to lamin Dmmit is brought about by a specific rearrangement of phosphate groups rather than by dramatic net changes in the levels of lamin phosphorylation. The residues involved in these phosphorylation/dephosphorylation reactions have been tentatively mapped to a 17.8-kD cyanogen bromide fragment containing amino acids 385-547. This represents a potential "hinge" domain in the lamin structure between the end of coil 2 and the globular COOH terminus. These results have implications for understanding the regulation of nuclear envelope breakdown during mitosis and karyoskeletal dynamics during oogenesis and early embryogenesis.