Protein kinase activity associated with the nuclear lamina

The LEM-ESCRT toolkit: Repair and maintenance of the nucleus

The eukaryotic genome is enclosed in a nuclear envelope that protects it from potentially damaging cellular activities and physically segregates transcription and translation.Transport across the NE is highly regulated and occurs primarily via the macromolecular nuclear pore complexes.Loss of nuclear compartmentalization due to defects in NPC function and NE integrity are tied to neurological and ageing disorders like Alzheimer’s, viral pathogenesis, immune disorders, and cancer progression.Recent work implicates inner-nuclear membrane proteins of the conserved LEM domain family and the ESCRT machinery in NE reformation during cell division and NE repair upon rupture in migrating cancer cells, and generating seals over defective NPCs. In this review, we discuss the recent in-roads made into defining the molecular mechanisms and biochemical networks engaged by LEM and many other integral inner nuclear membrane proteins to preserve the nuclear barrier.

Lamin B2 promotes the malignant phenotype of non-small cell lung cancer cells by upregulating dimethylation of histone 3 lysine 9

The relationship between Lamin B2 and tumor proliferation and migration is unclear. We explored the impact of Lamin B2 on non-small cell lung cancer (NSCLC) cells. Tissue microarray and immunohistochemistry were combined to evaluate Lamin B2 expression and its relationship with the clinicopathological factors found in NSCLC. Western blotting, immunofluorescence analysis, and bioinformatics were used to investigate the effects of Lamin B2 on various regulatory pathways in cancer. Cytological experiments were conducted to evaluate Lamin B2 expression in tumor cells. We conducted co-immunoprecipitation and chromatin immunoprecipitation to explore the molecular mechanisms underlying the relationship between Lamin B2 and NSCLC and evaluate the results of rescue experiments. Lamin B2 was highly expressed in NSCLC and positively correlated with lymph node metastasis. In NSCLC, Lamin B2 interacted with Cyclin D1, upregulating G9α expression, thus increasing H3K9me2 levels. H3K9me2 binds to the promoter region of the E-cadherin gene (CDH1) to induce CDH1 silencing and promotes cancer cell migration. Thus, we found that Lamin B2 was highly expressed in NSCLC cells and promoted their migration by increasing H3K9me2 levels, which induced E-cadherin gene silencing.

LITTLE NUCLEI 1 and 4 regulate nuclear morphology in Arabidopsis thaliana

The morphology of plant nuclei varies among different species, organs, tissues and cell types. However, mechanisms and factors involved in the maintenance of nuclear morphology are poorly understood. Because nuclei retain their shapes even after cytoskeletal inhibitor treatments both in vivo and in vitro, we assumed involvement of the nuclear lamina, which plays a critical role in the regulation of nuclear morphology in animals. The crude nuclear lamina fraction isolated from Arabidopsis thaliana leaves was analyzed by mass spectrometry, and putative nuclear lamina proteins were identified. Among their T-DNA insertion lines, nuclei of little nuclei1 (linc1) and linc4 disruptants were more spherical than those of wild-type plants. Because A. thaliana harbors four LINC genes, we prepared all single and linc1/4 and linc2/3 double disruptants. In leaf epidermal cells, the circularity index of the nucleus in all linc disruptants except linc3 was significantly higher than that in the wild-type plants. The extent of the effects of LINC1 and/or LINC4 disruption was significantly higher than that of the effects of LINC2 disruption. The nuclear area was significantly smaller in the linc1, linc4 and linc1/4 disruptants than in the wild-type plants. Regardless of the defects in nuclear morphology, all linc disruptants exhibited a normal ploidy level. In interphase cells, LINC1 and LINC4 were mainly localized to the nuclear periphery, whereas LINC2 was in the nucleoplasm and LINC3 was detected in both regions. From prometaphase to anaphase in mitotic root tip cells, LINC1 was co-localized with chromosomes, whereas other LINCs were dispersed in the cytoplasm.

Nuclear lamins

The nuclear lamins are type V intermediate filament proteins that are critically important for the structural properties of the nucleus. In addition, they are involved in the regulation of numerous nuclear processes, including DNA replication, transcription and chromatin organization. The developmentally regulated expression of lamins suggests that they are involved in cellular differentiation. Their assembly dynamic properties throughout the cell cycle, particularly in mitosis, are influenced by posttranslational modifications. Lamins may regulate nuclear functions by direct interactions with chromatin and determining the spatial organization of chromosomes within the nuclear space. They may also regulate chromatin functions by interacting with factors that epigenetically modify the chromatin or directly regulate replication or transcription.

Phosphorylation of the p34(cdc2) target site on goldfish germinal vesicle lamin B3 before oocyte maturation

The nuclear membranes surrounding fish and frog oocyte germinal vesicles (GVs) are supported by the lamina, an internal, mesh-like structure that consists of the protein lamin B3. The mechanisms by which lamin B3 is transported into GVs and is assembled to form the nuclear lamina are not well understood. In this study, we developed a heterogeneous microinjection system in which wild-type or mutated goldfish GV lamin B3 (GFLB3) was expressed in Escherichia coli, biotinylated, and microinjected into Xenopus oocytes. The localization of the biotinylated GFLB3 was visualized by fluorescence confocal microscopy. The results of these experiments indicated that the N-terminal domain plays important roles in both nuclear transport and assembly of lamin B3 to form the nuclear lamina. The N-terminal domain includes a major consensus phosphoacceptor site for the p34(cdc2) kinase at amino acid residue Ser-28. To investigate nuclear lamin phosphorylation, we generated a monoclonal antibody (C7B8D) against Ser-28-phosphorylated GFLB3. Two-dimensional (2-D) electrophoresis of GV protein revealed two major spots of lamin B3 with different isoelectric points (5.9 and 6.1). The C7B8D antibody recognized the pI-5.9 spot but not the pI-6.1 spot. The former spot disappeared when the native lamina was incubated with lambda phage protein phosphatase (lambda-PP), indicating that a portion of the lamin protein was already phosphorylated in the goldfish GV-stage oocytes. GFLB3 that had been microinjected into Xenopus oocytes was also phosphorylated in Xenopus GV lamina, as judged by Western blotting with C7B8D. Thus, lamin phosphorylation appears to occur prior to oocyte maturation in vivo in both these species. Taken together, our results suggest that the balance between phosphorylation by interphase lamin kinases and dephosphorylation by phosphatases regulates the conformational changes in the lamin B3 N-terminal head domain that in turn regulates the continual in vivo rearrangement and remodeling of the oocyte lamina.

Anti-phosphopeptide antibody, P-STM as a novel tool for detecting mitotic phosphoproteins: identification of lamins A and C as two major targets

A polyclonal, phospho-epitope-specific antibody (P-STM) was generated to detect the activated p21-activated kinase 2 (PAK2), based on the regulatory autophosphorylation site Thr(402) of PAK2 [Yu et al., 1998]. In this report, we show that this antibody can also recognize many phosphoproteins in mitotic HeLa and A431 cells. Signal of these phosphoproteins emerged after treating the cells with nocodazole and okadaic acid, and was highly detected in G2-M phase transition of HeLa cells released from double thymidine block. Immunofluorescence analysis revealed that P-STM strongly stained HeLa cells at prometaphase and metaphase, but not at interphase and anaphase. Interestingly, this staining pattern was almost identical to that obtained by staining with MPM2, a monoclonal antibody known to react with phosphoproteins in mitotic HeLa cells. However, the phosphoproteins detected by the two antibodies are quite different. Two-dimensional gel electrophoresis (2DE) and tryptic peptide fingerprint analysis by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry were employed to identify lamins A and C as two of the mitotic cell-specific phosphoproteins recognized by P-STM. Lamins A and C immunoprecipitated from nocodazole-treated cells, but not from untreated cells showed strong reactivity to P-STM, and this reactivity lost completely after protein phosphatase 2A treatment. In summary, our results show that P-STM represents a novel tool for detecting mitotic phosphoproteins, which are different from those recognized by MPM2, and that lamins A and C are the two prominent mitotic phosphoproteins detected by P-STM.

Cytomegalovirus recruitment of cellular kinases to dissolve the nuclear lamina

The passage of large-sized herpesviral capsids through the nuclear lamina and the inner nuclear membrane to leave the nucleus requires a dissolution of the nuclear lamina. Here, we report on the functions of M50/p35, a beta-herpesviral protein of murine cytomegalovirus. M50/p35 inserts into the inner nuclear membrane and is aggregated by a second viral protein, M53/p38, to form the capsid docking site. M50/p35 recruits the cellular protein kinase C for phosphorylation and dissolution of the nuclear lamina, suggesting that herpesviruses target a critical element of nuclear architecture.

Breakdown of cytoskeletal proteins during meiosis of starfish oocytes and proteolysis induced by calpain

Meiosis reinitiation in starfish oocytes is characterized by Ca(2+) transients in the cytosol and in the nucleus and is accompanied by the disassembly of the nuclear envelope, a process which is likely to be mediated by the cleavage of selected proteins. We have used mass spectrometry analysis (mass profile fingerprinting) on 2D polyacrylamide gels of extracts of oocytes in which meiosis resumption was induced by 1-methyladenine and have identified five proteins that were specifically degraded: alpha-tubulin, lamin B, dynamin, and two kinds of actin. They are all components of the cytoskeleton or associated with it. We then investigated whether calpain, which is activated by the increase in cell Ca(2+), could cleave the same proteins that became degraded under the influence of 1-methyladenine and thus be involved in nuclear membrane breakdown. The investigation was prompted by the finding that microinjection of calpain into the nuclei of prophase arrested oocytes induced meiosis in the absence of 1-methyladenine. Incubation of prophase arrested (disrupted) oocytes with calpain produced a 2D gel protein pattern in which some of the degradation products coincided with those seen in oocytes challenged with 1-methyladenine.

Molecular basis for tissue expansion: clinical implications for the surgeon

A wide variety of tissue expansion techniques have been used for breast reconstruction, craniofacial surgery, and burn care in plastic reconstructive surgery. However, the basic mechanism by which skin and surrounding tissue respond to mechanical expansion remains unclear. Recent studies have revealed the biomechanical aspects of cells subjected to strain and various factors involved in the stretch-induced signal transduction pathway. In this regard, we have reported previously that mechanical force increases keratinocyte growth and protein synthesis and alters cell morphology. The mechanism by which strain causes an enhancement of cellular growth appears to be a network of several integrated cascades, implicating growth factors, cytoskeleton, and the protein kinase family. Recently, additional evidence has accumulated that mechanical strain stimulates signal transduction pathways that could trigger a series of cascades eventually leading to a new skin production. For example, we have evidence suggesting a key role for protein kinase C (PKC) in mechanosignaling as PKC is activated and translocated in keratinocytes subjected to strain in an isoform-specific manner. In this report, molecular mechanisms leading to enhancement of skin surface area are reviewed, and possible future applications are discussed.

Purification of intact nuclear lamina and identification of novel laminlike proteins in Raji, a cell line devoid of lamins A and C

Research on the structure of the nuclear lamina and the nuclear matrix of cells devoid of lamins A and C has been hampered by the fact that intact residual nuclear structures are difficult to isolate from such cells. In this paper, we show that some extraction parameters, such as buffer composition and the nature of the detergent used to remove nuclear membranes, are critical for achieving isolation of whole nuclear residual structures from the lymphoblastic cell line Raji, used as a model for cells without lamins A and C. Electron microscopic analysis shows that the nuclear lamina of Raji cells is formed by a network of intermediate-sized filaments interrupted with circular discontinuities. Both lamins B1 and B2, and lamin D/E, are present in this structure. In addition, a group of 45-kDa proteins or intermediate filament protein - reacting proteins (IFA-RPs), located uniquely in the lamina, were found to exhibit the same immunological and chemical characteristics as lamins. Although they behave like nuclear lamins, microsequencing analysis of the IFA-RPs has revealed no homology with known lamins. These IFA-RPs may contribute to the formation of the nuclear lamina filament network in the absence of lamins A and C. Key words: nuclear lamina, intermediate filaments, lamin.

Antisense oligonucleotides targeting human protein kinase C-alpha inhibit phorbol ester-induced reduction of bradykinin-evoked calcium mobilization in A549 cells

Regulation of the bradykinin-evoked increase in intracellular Ca2+ concentration by protein kinase C (PKC)-alpha was investigated in A549 human lung carcinoma cells. Bradykinin, a potent and selective kinin B2 receptor agonist, induces calcium mobilization in a concentration-dependent fashion in this cell line. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent activator of PKC, is known to reduce the amplitude of agonist-induced calcium mobilization in various cell lines. Because PKC-alpha is a major PKC isozyme in A549 cells, we investigated whether this isozyme plays a role in this process. A 20-mer phosphorothioate oligonucleotide targeting the 3'-untranslated region of the human PKC-alpha mRNA, which contains 2'-methoxyethyl modifications incorporated into the 5' and 3' segments of the oligonucleotide, was used to assess the putative role of PKC-alpha in the receptor regulation. ISIS 9606 reduced PKC-alpha mRNA for > or = 72 hr after the initial treatment and the reduction was concentration dependent, whereas the mismatch control, ISIS 13009, had no effect. Concentrations of ISIS 9606 of 150 nM specifically reduced the level of immunoreactive PKC-alpha protein by 66.3 +/- 2.5% at 72 hr after treatment, without an effect on immunoreactive PKC-delta protein. This reduction in PKC-alpha was sufficient to inhibit the reduction of bradykinin-induced calcium mobilization by TPA. This finding is corroborated by the use of staurosporine, a nonselective PKC inhibitor, that prevented the effect of TPA. These results suggest that PKC-alpha is involved in kinin B2 receptor regulation by phorbol esters in A549 cells.

Tyrosine kinase-dependent release of an adenovirus preterminal protein complex from the nuclear matrix

Adenovirus (Ad) replicative complexes form at discrete sites on the nuclear matrix (NM) through the interaction of Ad preterminal protein (pTP). The NM is a highly salt-resistant fibrillar network which is known to anchor transcription, mRNA splicing, and DNA replication complexes. Incubation of rATP with NM to which pTP was bound caused the release of pTP as a pTP-NM complex with a size of 220 to 230 kDa; incubation with 5' adenylylimidodiphosphate (rAMP-PNP) showed no significant release, indicating that rATP hydrolysis was required. With NM extracts, it was shown that a pTP-NM complex which was capable of binding Ad origin DNA could be reconstituted in vitro. A number of high-molecular-weight NM proteins ranging in size from 120 to 200 kDa were identified on Far Western blots for their ability to bind pTP. rATP-dependent release of pTP from the NM was inhibited in a dose-dependent fashion by the addition of tyrosine kinase inhibitors, such as quercetin, methyl-2,5-dihydroxycinnamate, or genistein. NM-mediated phosphorylation of a poly(Glu, Tyr) substrate was also significantly abrogated by the addition of these compounds. rATP-dependent release of Ad DNA termini bound to the NM via pTP was also blocked by the addition of these inhibitors. These results indicate that a tyrosine kinase mechanism controls the release of pTP from its binding sites on the NM. These data support the concept that phosphorylation may play a key role in the modulation of pTP binding sites on the NM.

The dynamic properties and possible functions of nuclear lamins

The nuclear lamins are thought to form a thin fibrous layer called the nuclear lamina, underlying the inner nuclear envelope membrane. In this review, we summarize data on the dynamic properties of nuclear lamins during the cell cycle and during development. We discuss the implications of dynamics for lamin functions. The lamins may be involved in DNA replication, chromatin organization, differentiation, nuclear structural support, and nuclear envelope reassembly. Emphasis is placed on recent data that indicate that the lamina, contrary to previous views, is not a static structure. For example, the lamins form nucleoplasmic foci, distinct from the peripheral lamina, which vary in their patterns of distribution as well as their composition in a cell cycle-dependent manner. During the S phase, these foci colocalize with chromatin and sites of DNA replication. At other points during the cell cycle, they may represent sites of lamin post-translation processing that take place prior to incorporation into the lamina. Secondary modifications of the lamins such as isoprenylation and phosphorylation are involved in the regulation of the dynamic properties and the assembly of lamins. In addition, a number of lamin-associated proteins have been recently identified and these are described along with their potential functions.

A 62-kDa mitotic apparatus protein required for mitotic progression is sequestered to the interphase nucleus by associating with the chromosomes during anaphase

A protein component of 62-kDa (p62) in the mitotic apparatus of the sea urchin embryo has been shown to be important for the proper progression of mitosis [Dinsmore and Sloboda, 1989: Cell 57:127-134]. To study the subcellular distribution of p62 during the cell cycle of sea urchin embryos, indirect immunofluorescence microscopy was used coupled to a modified detergent extraction procedure. The improved fluorescent images obtained by this procedure provide new information concerning the subcellular localization of p62 during the cell cycle that could not be obtained with previous conventional staining procedures [Johnston and Sloboda, 1992: J. Cell Biol. 119:843-854]. Using affinity purified antibodies to p62, we observed a cell cycle-dependent localization of p62 to the chromosomes/chromatin. Prior to nuclear envelope breakdown of the first or second cell cycle, p62 localizes to chromatin in the nucleus. During mitosis, p62 associates with the region of the spindle occupied by the microtubules of the mitotic apparatus. As anaphase proceeds, but before the nuclear envelope reforms, p62 becomes progressively associated with the chromosomes. Thus, p62 is incorporated into the forming interphase nucleus due to its association with chromosomes during late anaphase, rather than by active translocation into the newly formed daughter nuclei through the nuclear pores. The protein is not unique to marine embryos, as demonstrated by immunofluorescence of Y-1 cells, a mouse adrenal tumor cell line. In these cells, the localization of p62 is similar to the localization of the protein in echinoderm embryos, suggesting its possible function in mitotic progression in mammalian somatic cells as well.

Towards an understanding of nuclear morphogenesis

In the age of "virtual reality," the imperfect microscopic silhouettes of cells and organelles are gradually being replaced by calligraphic computer drawings. In this context, textbooks and introductory slides often depict the cell nucleus as a smooth-shaped, featureless object. However, in reality, the nuclei of different cells possess distinct sizes and morphological features which develop in a programmed fashion as each cell differentiates. To dissect this complex morphogenetic process, we need to identify the basic elements that determine nuclear architecture and the regulatory factors involved. Recently, clues about the identity of these components have been obtained both by systematic analysis and by serendipity. This review summarizes a few recent findings and ideas that may serve as a first forum for future discussions and, I hope, for further work on this topic.

Identification of novel phosphorylation sites in murine A-type lamins

We report the distribution of phosphorylation sites in murine lamins A and C (A-type lamins) in vitro and in vivo followed by reverse-phase high-performance liquid chromatography and microsequencing of peptides spanning the almost complete lamin sequence. We show that two distinct protein kinases, cell-division-cycle-2 kinase (cdc2 kinase) and protein kinase C (PKC), phosphorylate murine A-type lamins at the non-alpha-helical amino- and carboxy-terminal domains in vitro and in vivo. Cdc2 kinase, but not PKC, is capable of inducing depolymerization of the nuclear lamina in permeabilized cells. Accordingly, lamins were proposed to be direct in vivo substrates of cdc2 kinase and PKC with different effects on the lamina dynamics. Analysis of the original A-type lamins revealed phosphorylation of residues Ser5 and Ser392. Residue Ser392 was substoichiometrically phosphorylated in the substrate and by cdc2 kinase in vitro. PKC phosphorylated peptides with its kinase-specific motifs surrounding Ser5, Thr199, Thr416, Thr480 and Ser625. In vivo, a mitosis-specific phosphorylation at the cdc2-kinase-specific phosphoacceptor site Ser392 and of the N-terminal peptide was identified. An interphase-specific phosphorylation at Ser525 matching the PKC consensus sequence and of peptides phosphorylated by unknown kinases was determined. The results lead us to propose that different cyclin-dependent kinase activities act as lamin kinases in mitosis and in interphase. Other kinases may cooperate with cdc2 kinase during reversible disassembly in mitosis and may modulate the supramolecular assembly of lamin filaments.

Assembly of snRNP-containing coiled bodies is regulated in interphase and mitosis--evidence that the coiled body is a kinetic nuclear structure

Coiled bodies (CBs) are nuclear organelles in which splicing snRNPs concentrate. While CBs are sometimes observed in association with the nucleolar periphery, they are shown not to contain 5S or 28S rRNA or the U3 snoRNA. This argues against CBs playing a role in rRNA maturation or transport as previously suggested. We present evidence here that CBs are kinetic structures and demonstrate that the formation of snRNP-containing CBs is regulated in interphase and mitosis. The coiled body antigen, p80 coilin, was present in all cell types studied, even when CBs were not prominent. Striking changes in the formation of CBs could be induced by changes in cellular growth temperature without a concomitant change in the intracellular p80 coilin level. During mitosis, CBs disassemble, coinciding with a mitotic-specific phosphorylation of p80 coilin. Coilin is shown to be a phosphoprotein that is phosphorylated on at least two additional sites during mitosis. CBs reform in daughter nuclei after a lag period during which they are not detected. CBs are thus, dynamic nuclear organelles and we propose that cycling interactions of splicing snRNPs with CBs may be important for their participation in the processing or transport of pre-mRNA in mammalian cells.

A 62-kD protein required for mitotic progression is associated with the mitotic apparatus during M-phase and with the nucleus during interphase

A protein of 62 kD is a substrate of a calcium/calmodulin-dependent protein kinase, and both proteins copurify with isolated mitotic apparatuses (Dinsmore, J. H., and R. D. Sloboda. 1988. Cell. 53:769-780). Phosphorylation of the 62-kD protein increases after fertilization; maximum incorporation of phosphate occurs during late metaphase and anaphase and correlates directly with microtubule disassembly as determined by in vitro experiments with isolated mitotic apparatuses. Because 62-kD protein phosphorylation occurs in a pattern similar to the accumulation of the mitotic cyclin proteins, experiments were performed to determine the relationship between cyclin and the 62-kD protein. Continuous labeling of marine embryos with [35S]methionine, as well as immunoblots of marine embryo proteins using specific antibodies, were used to identify both cyclin and the 62-kD protein. These results clearly demonstrate that the 62-kD protein is distinct from cyclin and, unlike cyclin, is a constant member of the cellular protein pool during the first two cell cycles in sea urchin and surf clam embryos. Similar results were obtained using immunofluorescence microscopy of intact eggs and embryos. In addition, immunogold electron microscopy reveals that the 62-kD protein associates with the microtubules of the mitotic apparatus in dividing cells. Interestingly, the protein changes its subcellular distribution with respect to microtubules during the cell cycle. Specifically, during mitosis the 62-kD protein associates with the mitotic apparatus; before nuclear envelope breakdown, however, the 62-kD protein is confined to the nucleus. After anaphase, the 62-kD protein returns to the nucleus, where it resides until nuclear envelope disassembly of the next cell cycle.

The inner nuclear membrane protein p58 associates in vivo with a p58 kinase and the nuclear lamins

p58, also referred to as the lamin B receptor, is an intrinsic protein of the inner nuclear membrane that binds in vitro to lamin B. Previous studies have demonstrated that p58 is phosphorylated in vivo and removal of its phosphate moieties affects lamin B binding. Using affinity-purified antipeptide antibodies, we have now immunoisolated p58 from bird erythrocyte lysates under isotonic, non-denaturing conditions. Analysis of the immunopurified material shows that five distinct proteins are tightly and specifically associated with p58. Two of these polypeptides can be identified as nuclear lamins A and B. The immunoisolate also contains a kinase activity that phosphorylates p58 in vivo and in vitro, exclusively at serine residues, as indicated by phosphoamino acid analysis and two-dimensional phosphopeptide mapping. Cell fractionation experiments and in vitro phosphorylation assays demonstrate that the p58 kinase resides in the nuclear envelope and is distinct from protein kinase A and cdc2 kinase, for both of which p58 is an in vitro substrate. These data suggest that p58 is interacting in vivo with a p58 kinase and the nuclear lamins.

PKC epsilon-related kinase associates with and phosphorylates cytokeratin 8 and 18

A 40-kD protein kinase C (PKC)epsilon related activity was found to associate with human epithelial specific cytokeratin (CK) polypeptides 8 and 18. The kinase activity coimmunoprecipitated with CK8 and 18 and phosphorylated immunoprecipitates of the CK. Immunoblot analysis of CK8/18 immunoprecipitates using an anti-PKC epsilon specific antibody showed that the 40-kD species, and not native PKC epsilon (90 kD) associated with the cytokeratins. Reconstitution experiments demonstrated that purified CK8 or CK18 associated with a 40-kD tryptic fragment of purified PKC epsilon, or with a similar species obtained from cells that express the fragment constitutively but do not express CK8/18. A peptide pseudosubstrate specific for PKC epsilon inhibited phosphorylation of CK8/18 in intact cells or in a kinase assay with CK8/18 immunoprecipitates. Tryptic peptide map analysis of the cytokeratins that were phosphorylated by purified rat brain PKC epsilon or as immunoprecipitates by the associated kinase showed similar phosphopeptides. Furthermore, PKC epsilon immunoreactive species and CK8/18 colocalized using immunofluorescent double staining. We propose that a kinase related to the catalytic fragment of PKC epsilon physically associates with and phosphorylates cytokeratins 8 and 18.

Expression of chicken lamin B2 in Escherichia coli: characterization of its structure, assembly, and molecular interactions

Chicken lamin B2, a nuclear member of the intermediate-type filament (IF) protein family, was expressed as a full-length protein in Escherichia coli. After purification, its structure and assembly properties were explored by EM, using both glycerol spraying/low-angle rotary metal shadowing and negative staining for preparation, as well as by analytical ultracentrifugation. At its first level of structural organization, lamin B2 formed "myosin-like" 3.1S dimers consisting of a 52-nm-long tail flanked at one end by two globular heads. These myosin-like molecules are interpreted to represent two lamin polypeptides interacting via their 45-kD central rod domains to form a segmented, parallel and unstaggered 52-nm-long two-stranded alpha-helical coiled-coil, and their COOH-terminal end domains folding into globular heads. At the second level of organization, lamin B2 dimers associated longitudinally to form polar head-to-tail polymers. This longitudinal mode of association of laminin dimers is in striking contrast to the lateral mode of association observed previously for cytoplasmic IF dimers. At the third level of organization, these polar head-to-tail polymers further associated laterally, in an approximately half-staggered fashion, to form filamentous and eventually paracrystal-like structures revealing a pronounced 24.5-nm axial repeat. Finally, following up on recent studies implicating the mitotic cdc2 kinase in the control of lamin polymerization (Peter, M., J. Nakagawa, M. Dorée, J. C. Labbé, and E. A. Nigg. 1990. Cell. 61:591-602), we have examined the effect of phosphorylation by purified cdc2 kinase on the assembly properties and molecular interactions of the bacterially expressed lamin B2. Phosphorylation of chicken lamin B2 by cdc2 kinase interferes with the head-to-tail polymerization of the lamin dimers. This finding supports the notion that cdc2 kinase plays a major, direct role in triggering mitotic disassembly of the nuclear lamina.

A complex containing p34cdc2 and cyclin B phosphorylates the nuclear lamin and disassembles nuclei of clam oocytes in vitro

Cell-free extracts prepared from activated clam oocytes contain factors which induce phosphorylation of the single 67-kD lamin (L67), disassemble clam oocyte nuclei, and cause chromosome condensation in vitro (Dessev, G., R. Palazzo, L. Rebhun, and R. Goldman. 1989. Dev. Biol. 131:469-504). To identify these factors, we have fractionated the oocyte extracts. The nuclear lamina disassembly (NLD) activity, together with a protein kinase activity specific for L67, appear as a single peak throughout a number of purification steps. This peak also contains p34cdc2, cyclin B, and histone H1-kinase activity, which are components of the M-phase promoting factor (MPF). The NLD/L67-kinase activity is depleted by exposure of this purified material to Sepharose conjugated to p13suc1, and is restored upon addition of a p34cdc2/p62 complex from HeLa cells. The latter complex phosphorylates L67 and induces NLD in the absence of other clam oocyte proteins. Our results suggest that a single protein kinase activity (p34cdc2-H1 kinase, identical with MPF) phosphorylates the lamin and is involved in the meiotic breakdown of the nuclear envelope in clam oocytes.

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.