Assembly-disassembly of the nuclear lamina

Keratins regulate Hsp70-mediated nuclear localization of p38 mitogen-activated protein kinase

Intermediate filament protein keratin 8 (K8) binds to heat shock protein 70 (Hsp70) and p38 MAPK, and is phosphorylated at Ser74 by p38α (MAPK14, hereafter p38). However, a p38 binding site on K8 and the molecular mechanism of K8-p38 interaction related to Hsp70 are unknown. Here, we identify a p38 docking site on K8 (Arg148/149 and Leu159/161) that is highly conserved in other intermediate filaments. A docking-deficient K8 mutation caused increased p38-Hsp70 interaction and enhanced p38 nuclear localization, indicating that the p38 dissociated from mutant K8 makes a complex with Hsp70, which is known as a potential chaperone for p38 nuclear translocation. Comparison of p38 MAPK binding with keratin variants associated with liver disease showed that the K18 I150V variant dramatically reduced binding with p38, which is similar to the effect of the p38 docking-deficient mutation on K8. Because the p38 docking site on K8 (Arg148/149 and Leu159/161) and the K18 Ile150 residue are closely localized in the parallel K8/K18 heterodimer, the K18 I150V mutation might interfere with K8-p38 interaction. These findings show that keratins, functioning as cytoplasmic anchors for p38, modulate p38 nuclear localization and thereby might affect a number of p38-mediated signal transduction pathways.

Baculovirus infection induces disruption of the nuclear lamina

Baculovirus nucleocapsids egress from the nucleus primarily via budding at the nuclear membrane. The nuclear lamina underlying the nuclear membrane represents a substantial barrier to nuclear egress. Whether the nuclear lamina undergoes disruption during baculovirus infection remains unknown. In this report, we generated a clonal cell line, Sf9-L, that stably expresses GFP-tagged Drosophila lamin B. GFP autofluorescence colocalized with immunofluorescent anti-lamin B at the nuclear rim of Sf9-L cells, indicating GFP-lamin B was incorporated into the nuclear lamina. Meanwhile, virus was able to replicate normally in Sf9-L cells. Next, we investigated alterations to the nuclear lamina during baculovirus infection in Sf9-L cells. A portion of GFP-lamin B localized diffusely at the nuclear rim, and some GFP-lamin B was redistributed within the nucleus during the late phase of infection, suggesting the nuclear lamina was partially disrupted. Immunoelectron microscopy revealed associations between GFP-lamin B and the edges of the electron-dense stromal mattes of the virogenic stroma, intranuclear microvesicles, and ODV envelopes and nucleocapsids within the nucleus, indicating the release of some GFP-lamin B from the nuclear lamina. Additionally, GFP-lamin B phosphorylation increased upon infection. Based on these data, baculovirus infection induced lamin B phosphorylation and disruption of the nuclear lamina.

Nuclear lamina at the crossroads of the cytoplasm and nucleus

The nuclear lamina is a protein meshwork that lines the nuclear envelope in metazoan cells. It is composed largely of a polymeric assembly of lamins, which comprise a distinct sequence homology class of the intermediate filament protein family. On the basis of its structural properties, the lamina originally was proposed to provide scaffolding for the nuclear envelope and to promote anchoring of chromatin and nuclear pore complexes at the nuclear surface. This viewpoint has expanded greatly during the past 25 years, with a host of surprising new insights on lamina structure, molecular composition and functional attributes. It has been established that the self-assembly properties of lamins are very similar to those of cytoplasmic intermediate filament proteins, and that the lamin polymer is physically associated with components of the cytoplasmic cytoskeleton and with a multitude of chromatin and inner nuclear membrane proteins. Cumulative evidence points to an important role for the lamina in regulating signaling and gene activity, and in mechanically coupling the cytoplasmic cytoskeleton to the nucleus. The significance of the lamina has been vaulted to the forefront by the discovery that mutations in lamins and lamina-associated polypeptides lead to an array of human diseases. A key future challenge is to understand how the lamina integrates pathways for mechanics and signaling at the molecular level. Understanding the structure of the lamina from the atomic to supramolecular levels will be essential for achieving this goal.

Shaping the endoplasmic reticulum into the nuclear envelope

The nuclear envelope (NE), a double membrane enclosing the nucleus of eukaryotic cells, controls the flow of information between the nucleoplasm and the cytoplasm and provides a scaffold for the organization of chromatin and the cytoskeleton. In dividing metazoan cells, the NE breaks down at the onset of mitosis and then reforms around segregated chromosomes to generate the daughter nuclei. Recent data from intact cells and cell-free nuclear assembly systems suggest that the endoplasmic reticulum (ER) is the source of membrane for NE assembly. At the end of mitosis, ER membrane tubules are targeted to chromatin via tubule ends and reorganized into flat nuclear membrane sheets by specific DNA-binding membrane proteins. In contrast to previous models, which proposed vesicle fusion to be the principal mechanism of NE formation, these new studies suggest that the nuclear membrane forms by the chromatin-mediated reshaping of the ER.

The highly conserved nuclear lamin Ig-fold binds to PCNA: its role in DNA replication

This study provides insights into the role of nuclear lamins in DNA replication. Our data demonstrate that the Ig-fold motif located in the lamin C terminus binds directly to proliferating cell nuclear antigen (PCNA), the processivity factor necessary for the chain elongation phase of DNA replication. We find that the introduction of a mutation in the Ig-fold, which alters its structure and causes human muscular dystrophy, inhibits PCNA binding. Studies of nuclear assembly and DNA replication show that lamins, PCNA, and chromatin are closely associated in situ. Exposure of replicating nuclei to an excess of the lamin domain containing the Ig-fold inhibits DNA replication in a concentration-dependent fashion. This inhibitory effect is significantly diminished in nuclei exposed to the same domain bearing the Ig-fold mutation. Using the crystal structures of the lamin Ig-fold and PCNA, molecular docking simulations suggest probable interaction sites. These findings also provide insights into the mechanisms underlying the numerous disease-causing mutations located within the lamin Ig-fold.

Dynamics of lamin A/C in porcine embryos produced by nuclear transfer

This study was conducted to investigate the presence of lamin A/C in porcine nuclear transfer embryos and to determine whether lamin A/C can serve as a potential marker for nuclear reprogramming. First, lamin A/C was studied in oocytes and embryos produced by fertilization or parthenogenetic oocyte activation. We found that lamin A/C was present in the nuclear lamina of oocytes at the germinal vesicle stage while it was absent in mature oocytes. Lamin A/C was detected throughout preimplantation development in both in vivo-derived and parthenogenetic embryos. Incubation of the activated oocytes in the presence of alpha-amanitin (an inhibitor of RNA polymerase II), or cycloheximide (a protein synthesis inhibitor) did not perturb lamin A/C assembly, indicating that the assembly resulted from solubilized lamins dispersed in the cytoplasm. In nuclear transfer embryos, the lamin A/C signal that had previously been identified in fibroblast nuclei disappeared soon after fusion. It became detectable again after the formation of the pronucleus-like structure, and all nuclear transfer embryos displayed lamin A/C staining during early development. Olfactory bulb progenitor cells lacked lamin A/C; however, when such cells were fused with enucleated oocytes, the newly formed nuclear envelopes stained positive for lamin A/C. These findings suggest that recipient oocytes remodel the donor nuclei using type A lamins dispersed in the ooplasm. The results also indicate that lamin A/C is present in the nuclear envelope of pig oocytes and early embryos and unlike in some other species, its presence after nuclear transfer is not an indicator of erroneous reprogramming.

Varicella-zoster virus infection of human foreskin fibroblast cells results in atypical cyclin expression and cyclin-dependent kinase activity

In its course of human infection, varicella-zoster virus (VZV) infects rarely dividing cells such as dermal fibroblasts, differentiated keratinocytes, mature T cells, and neurons, none of which are actively synthesizing DNA; however, VZV is able to productively infect them and use their machinery to replicate the viral genome. We hypothesized that VZV alters the intracellular environment to favor viral replication by dysregulating cell cycle proteins and kinases. Cyclin-dependent kinases (CDKs) and cyclins displayed a highly unusual profile in VZV-infected confluent fibroblasts: total amounts of CDK1, CDK2, cyclin B1, cyclin D3, and cyclin A protein increased, and kinase activities of CDK2, CDK4, and cyclin B1 were strongly and simultaneously induced. Cyclins B1 and D3 increased as early as 24 h after infection, concurrent with VZV protein synthesis. Confocal microscopy indicated that cyclin D3 overexpression was limited to areas of IE62 production, whereas cyclin B1 expression was irregular across the VZV plaque. Downstream substrates of CDKs, including pRb, p107, and GM130, did not show phosphorylation by immunoblotting, and p21 and p27 protein levels were increased following infection. Finally, although the complement of cyclin expression and high CDK activity indicated a progression through the S and G(2) phases of the cell cycle, DNA staining and flow cytometry indicated a possible G(1)/S blockade in infected cells. These data support earlier studies showing that pharmacological CDK inhibitors can inhibit VZV replication in cultured cells.

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.

A mechanism of AP-1 suppression through interaction of c-Fos with lamin A/C

AP-1 (Activating Protein 1) transcription factor activity is tightly regulated at multiple levels, including dimer formation (i.e., Fos/Jun). Here we show that the intermediate filament protein lamin A/C suppresses AP-1 function through direct interaction with c-Fos, and that both proteins can interact and colocalize at the nuclear envelope (NE) in mammalian cells. Perinuclear localization of c-Fos is absent in Lmna-null cells but can be restored by lamin A overexpression. In vitro, preincubation of c-Fos with lamin A prior to the addition of c-Jun inhibits AP-1 DNA-binding activity. In vivo, overexpression of lamin A reduces the formation of c-Fos/c-Jun heterodimers, and suppresses AP-1 DNA-binding and transcriptional activity. Notably, c-Fos colocalizes with lamin A/C at the NE in starvation-synchronized quiescent cells lacking detectable AP-1 DNA binding. In contrast, serum-induced AP-1 DNA-binding activity coincides with abundant nucleoplasmic c-Fos expression without changes in lamin A/C localization. We also found that Lmna-null cells display enhanced proliferation. In contrast, lamin A overexpression causes growth arrest, and ectopic c-Fos partially overcomes lamin A/C-induced cell cycle alterations. We propose lamin A/C-mediated c-Fos sequestration at the NE as a novel mechanism of transcriptional and cell cycle control.

Ca2+-dependent nuclear contraction in the heliozoon Actinophrys sol

Ca2+-dependent contractility was found to exist in the nucleus of the heliozoon protozoan Actinophrys sol. Upon addition of Ca2+ ([Ca2+]free = 2.0 x 10(-3) M), diameters of isolated and detergent-extracted nuclei became reduced from 16.5+/-1.7 microm to 11.0+/-1.3 microm. The threshold level of [Ca2+]free for the nuclear contraction was 2.9 x 10(-7) M. The nuclear contraction was not induced by Mg2+, and was not inhibited by colchicine or cytochalasin B. Contracted nuclei became expanded when Ca2+ was removed by EGTA; thus cycles of contraction and expansion could be repeated many times by alternating addition of Ca2+ and EGTA. The Ca2+-dependent nuclear contractility remained even after high salt treatment, suggesting a possible involvement of nucleoskeletal components in the nuclear contraction. Electron microscopy showed that, in the relaxed state, filamentous structures were observed to spread in the nucleus to form a network. After addition of Ca2+, they became aggregated and constructed a mass of thicker filaments, followed by re-distribution of the filaments spread around inside of the nucleus when Ca2+ was removed. These results suggest that the nuclear contraction is induced by Ca2+-dependent transformation of the filamentous structures in the nucleus.

Endoplasmic reticulum positioning and partitioning in mitotic HeLa cells

Cell division serves to distribute chromosomes and organelles into two daughter cells, but the mechanism of rough endoplasmic reticulum (RER) segregation in animal cell mitosis is poorly understood. Here we study the distribution of RER in mitotic HeLa cells and its relation to the cytoskeleton. At metaphase, the RER was located in the cell cortex and was most concentrated in two locations. Close to the plasma membrane the RER was closely associated with cortical actin, and after treatment with Latrunculin A RER elements retracted to the deep cortex and became more tubular. Positioning was therefore dependent on cortical F-actin. Deeper in the cortex cisternae were wrapped tightly around the contours of the spindle body and orientated along microtubules close the spindle poles. Stereology revealed a close correlation between RER volume and cell volume in telophase daughter cells. These results suggest that the RER is positioned at the outer and inner regions of metaphase cortex by association with cytoskeleton. This arrangement combined with a disposition in concentric layers, deep to the plasma membrane, appears to distribute the RER evenly in the cortex and may help to couple quantities of RER and cell constituents.

Inhibiting MAP kinase activity prevents calcium transients and mitosis entry in early sea urchin embryos

A transient calcium increase triggers nuclear envelope breakdown (mitosis entry) in sea urchin embryos. Cdk1/cyclin B kinase activation is also known to be required for mitosis entry. More recently, MAP kinase activity has also been shown to increase during mitosis. In sea urchin embryos, both kinases show a similar activation profile, peaking at the time of mitosis entry. We tested whether the activity of both kinases is required for mitosis entry and whether either kinase controls mitotic calcium signals. We found that reducing the activity of either mitotic kinase prevents nuclear envelope breakdown, despite the presence of a calcium transient, when cdk1/cyclin B kinase activity is alone inhibited. When MAP kinase activity alone was inhibited, the calcium signal was absent, suggesting that MAP kinase activity is required to generate the calcium transient that triggers nuclear envelope breakdown. However, increasing intracellular free calcium by microinjection of calcium buffers or InsP(3) while MAP kinase was inhibited did not itself induce nuclear envelope breakdown, indicating that additional MAP kinase-regulated events are necessary. After MAP kinase inhibition early in the cell cycle, the early events of the cell cycle (pronuclear migration/fusion and DNA synthesis) were unaffected, but chromosome condensation and spindle assembly are prevented. These data indicate that in sea urchin embryos, MAP kinase activity is part of a signaling complex alongside two components previously shown to be essential for entry into mitosis: the calcium transient and the increase in cdk1/cyclinB kinase activity.

Microdissection of the sequence and structure of intermediate filament chains

A large number of intermediate filament (IF) chains have now been sequenced. From these data, it has been possible to deduce the main elements of the secondary structure, especially those lying within the central rod domain of the molecule. These conclusions, allied to results obtained from crosslinking studies, have shown that at least four unique but related structures are adopted by the class of structures known generically as intermediate filaments: (1) epidermal and reduced trichocyte keratin; (2) oxidized trichocyte keratin; (3) desmin, vimentin, neurofilaments, and related Type III and IV proteins; and (4) lamin molecules. It would be expected that local differences in sequences of the proteins in these four groups would occur, and that this would ultimately relate to assembly. Site-directed mutagenesis and theoretical methods have now made it possible to investigate these ideas further. In particular, new data have been obtained that allow the role played by some individual amino acids or a short stretch of sequence to be determined. Among the observations catalogued here are the key residues involved in intra- and interchain ionic interactions, as well as those involved in stabilizing some modes of molecular aggregation; the structure and role of subdomains in the head and tail domains; the repeat sequences occurring along the length of the chain and their structural significance; trigger motifs in coiled-coil segments; and helix initiation and termination motifs that terminate the rod domain. Much more remains to be done, not least of which is gaining an increased understanding of the many subtle differences that exist between different IF chains at the sequence level.

Characterization of the tight junction protein ZO-2 localized at the nucleus of epithelial cells

ZO-2 is a MAGUK protein that in confluent epithelial sheets localizes at tight junctions (TJ) whereas in sparse cultures accumulates in clusters at the nucleus. Here, we have characterized several nuclear properties of ZO-2. We observe that ZO-2 is present in the nuclear matrix and co-immunoprecipitates with lamin B(1) and actin from the nuclei of sparse cultures. We show that ZO-2 presents several NLS at its amino region, that when deleted, diminish the nuclear import of the ZO-2 amino segment and impair the ability of the region to regulate the transcriptional activity of promoters controlled by AP-1. Several RS repeats are detected in the ZO-2 amino segment, however, their deletion does not preclude the display of a speckled nuclear pattern. ZO-2 displays two putative NES. However, only the second one appears to be functional, as when conjugated to ovalbumin (OV), it is able to translocate this protein from the nucleus to the cytoplasm in a leptomycin B-sensitive way.

Integral membrane proteins and dynamic organization of the nuclear envelope

The nuclear envelope is a complex structure consisting of nuclear membranes, nuclear pore complexes and lamina. Several integral membrane proteins specific to the nuclear pore membrane and the inner nuclear membrane are known. Pore membrane proteins are probably important for organization and assembly of the nuclear pore complex, while proteins of the inner nuclear membrane are likely to play major roles in the structure and dynamics of the nuclear lamina and chromatin. Biochemical studies are now identifying potential binding partners for some of these integral membrane proteins, and analysis of nuclear envelope assembly at the end of mitosis is providing important insights into their functions.

Nuclear envelope breakdown in starfish oocytes proceeds by partial NPC disassembly followed by a rapidly spreading fenestration of nuclear membranes

Breakdown of the nuclear envelope (NE) was analyzed in live starfish oocytes using a size series of fluorescently labeled dextrans, membrane dyes, and GFP-tagged proteins of the nuclear pore complex (NPC) and the nuclear lamina. Permeabilization of the nucleus occurred in two sequential phases. In phase I the NE became increasingly permeable for molecules up to approximately 40 nm in diameter, concurrent with a loss of peripheral nuclear pore components over a time course of 10 min. The NE remained intact on the ultrastructural level during this time. In phase II the NE was completely permeabilized within 35 s. This rapid permeabilization spread as a wave from one epicenter on the animal half across the nuclear surface and allowed free diffusion of particles up to approximately 100 nm in diameter into the nucleus. While the lamina and nuclear membranes appeared intact at the light microscopic level, a fenestration of the NE was clearly visible by electron microscopy in phase II. We conclude that NE breakdown in starfish oocytes is triggered by slow sequential disassembly of the NPCs followed by a rapidly spreading fenestration of the NE caused by the removal of nuclear pores from nuclear membranes still attached to the lamina.

Molecular cloning of one isotype of human lamina-associated polypeptide 1s and a topological analysis using its deletion mutants

LAP1s (lamina-associated polypeptide 1s) are type 2 integral membrane proteins with a single membrane-spanning region of the inner nuclear membrane. We report here on the cloning of the full-length cDNA of human LAP1B (huLAP1B) that encodes 584 amino acids. The sequence homology between the predicted rat LAP1B and huLAP1B was found to be 73.6%. A topological analysis was carried out by transiently expressing N-terminal GFP fused deletion mutants of huLAP1B in cells. The transmembrane (TM) domain (aa 346-368) is required for the localization of the nuclear and endoplasmic reticulum membrane and that the TM domain and the C-terminal half of the nucleoplasmic domain (aa 190-331) are sufficient for the proper localization of LAP1B. In contrast, the well-conserved lumenal domain of the nuclear membrane is not required for its topological function. Biochemical analysis showed that huLAP1B is retained within the nucleus via interactions of the nucleoplasmic portion with nuclear components.

The cell cycle dependent mislocalisation of emerin may contribute to the Emery-Dreifuss muscular dystrophy phenotype

Emerin is the nuclear membrane protein defective in X-linked Emery-Dreifuss muscular dystrophy (X-EDMD). The majority of X-EDMD patients have no detectable emerin. However, there are cases that produce mutant forms of emerin, which can be used to study its function. Our previous studies have shown that the emerin mutants S54F, P183T, P183H, Del95-99, Del236-241 (identified in X-EDMD patients) are targeted to the nuclear membrane but to a lesser extent than wild-type emerin. In this paper, we have studied how the mislocalisation of these mutant emerins may affect nuclear functions associated with the cell cycle using flow cytometry and immunofluorescence microscopy. We have established that cells expressing the emerin mutant Del236-241 (a deletion in the transmembrane domain), which was mainly localised in the cytoplasm, exhibited an aberrant cell cycle length. Thereafter, by examining the intracellular localisation of endogenously expressed lamin A/C and exogenously expressed wild-type and mutant forms of emerin after a number of cell divisions, we determined that the mutant forms of emerin redistributed endogenous lamin A/C. The extent of lamin A/C redistribution correlated with the amount of EGFP-emerin that was mislocalised. The amount of EGFP-emerin mislocalized, in turn, was associated with alterations in the nuclear envelope morphology. The nuclear morphology and redistribution of lamin A/C was most severely affected in the cells expressing the emerin mutant Del236-241.

It is believed that emerin is part of a novel nuclear protein complex consisting of the barrier-to-autointegration factor (BAF), the nuclear lamina, nuclear actin and other associated proteins. The data presented here show that lamin A/C localisation is dominantly directed by its interaction with certain emerin mutants and perhaps wild-type emerin as well. These results suggest that emerin links A-type lamins to the nuclear envelope and that the correct localisation of these nuclear proteins is important for maintaining cell cycle timing.

Fate of the inner nuclear membrane protein lamin B receptor and nuclear lamins in herpes simplex virus type 1 infection

During herpesvirus egress, capsids bud through the inner nuclear membrane. Underlying this membrane is the nuclear lamina, a meshwork of intermediate filaments with which it is tightly associated. Details of alterations to the lamina and the inner nuclear membrane during infection and the mechanisms involved in capsid transport across these structures remain unclear. Here we describe the fate of key protein components of the nuclear envelope and lamina during herpes simplex virus type 1 (HSV-1) infection. We followed the distribution of the inner nuclear membrane protein lamin B receptor (LBR) and lamins A and B(2) tagged with green fluorescent protein (GFP) in live infected cells. Together with additional results from indirect immunofluorescence, our studies reveal major morphologic distortion of nuclear-rim LBR and lamins A/C, B(1), and B(2). By 8 h p.i., we also observed a significant redistribution of LBR-GFP to the endoplasmic reticulum, where it colocalized with a subpopulation of cytoplasmic glycoprotein B by immunofluorescence. In addition, analysis by fluorescence recovery after photobleaching reveals that LBR-GFP exhibited increased diffusional mobility within the nuclear membrane of infected cells. This is consistent with the disruption of interactions between LBR and the underlying lamina. In addition to studying stably expressed GFP-lamins by fluorescence microscopy, we studied endogenous A- and B-type lamins in infected cells by Western blotting. Both approaches reveal a loss of lamins associated with virus infection. These data indicate major disruption of the nuclear envelope and lamina of HSV-1-infected cells and are consistent with a virus-induced dismantling of the nuclear lamina, possibly in order to gain access to the inner nuclear membrane.

Vaccinia virus DNA replication occurs in endoplasmic reticulum-enclosed cytoplasmic mini-nuclei

Vaccinia virus (vv), a member of the poxvirus family, is unique among most DNA viruses in that its replication occurs in the cytoplasm of the infected host cell. Although this viral process is known to occur in distinct cytoplasmic sites, little is known about its organization and in particular its relation with cellular membranes. The present study shows by electron microscopy (EM) that soon after initial vv DNA synthesis at 2 h postinfection, the sites become entirely surrounded by membranes of the endoplasmic reticulum (ER). Complete wrapping requires ~45 min and persists until virion assembly is initiated at 6 h postinfection, and the ER dissociates from the replication sites. [(3)H]Thymidine incorporation at different infection times shows that efficient vv DNA synthesis coincides with complete ER wrapping, suggesting that the ER facilitates viral replication. Proteins known to be associated with the nuclear envelope in interphase cells are not targeted to these DNA-surrounding ER membranes, ruling out a role for these molecules in the wrapping process. By random green fluorescent protein-tagging of vv early genes of unknown function with a putative transmembrane domain, a novel vv protein, the gene product of E8R, was identified that is targeted to the ER around the DNA sites. Antibodies raised against this vv early membrane protein showed, by immunofluorescence microscopy, a characteristic ring-like pattern around the replication site. By electron microscopy quantitation the protein concentrated in the ER surrounding the DNA site and was preferentially targeted to membrane facing the inside of this site. These combined data are discussed in relation to nuclear envelope assembly/disassembly as it occurs during the cell cycle.

Paternal chromosome incorporation into the zygote nucleus is controlled by maternal haploid in Drosophila

maternal haploid (mh) is a strict maternal effect mutation that causes the production of haploid gynogenetic embryos (eggs are fertilized but only maternal chromosomes participate in development). We conducted a cytological analysis of fertilization and early development in mh eggs to elucidate the mechanism of paternal chromosome elimination. In mh eggs, as in wild-type eggs, male and female pronuclei migrate and appose, the first mitotic spindle forms, and both parental sets of chromosomes congress on the metaphase plate. In contrast to control eggs, mh paternal sister chromatids fail to separate in anaphase of the first division. As a consequence the paternal chromatin stretches and forms a bridge in telophase. During the first three embryonic divisions, damaged paternal chromosomes are progressively eliminated from the spindles that organize around maternal chromosomes. A majority of mh embryos do not survive the deleterious presence of aneuploid nuclei and rapidly arrest their development. The rest of mh embryos develop as haploid gynogenetic embryos and die before hatching. The mh phenotype is highly reminiscent of the early developmental defects observed in eggs fertilized by ms(3)K81 mutant males and in eggs produced in incompatible crosses of Drosophila harboring the endosymbiont bacteria Wolbachia.

Polo-like kinase 1 phosphorylates cyclin B1 and targets it to the nucleus during prophase

In vertebrate cells, the nuclear entry of Cdc2-cyclin B1 (MPF) during prophase is thought to be essential for the induction and coordination of M-phase events. Phosphorylation of cyclin B1 is central to its nuclear translocation, but the kinases that are responsible remain unknown. Here we have purified a protein kinase from Xenopus M-phase extracts that phosphorylates a crucial serine residue (S147) in the middle of the nuclear export signal sequence of cyclin B1. We have identified this kinase as Plx1 (ref. 16), a Xenopus homologue of Polo-like kinase (Plk)-1. During cell-cycle progression in HeLa cells, a change in the kinase activity of endogenous Plk1 toward S147 and/or S133 correlates with a kinase activity in the cell extracts. An anti-Plk1 antibody depletes the M-phase extracts of the kinase activity toward S147 and/or S133. An anti-phospho-S147 antibody reacts specifically with cyclin B1 only during G2/M phase. A mutant cyclin B1 in which S133 and S147 are replaced by alanines remains in the cytoplasm, whereas wild-type cyclin B1 accumulates in the nucleus during prophase. Co-expression of constitutively active Plk1 stimulates nuclear entry of cyclin B1. Our results indicate that Plk1 may be involved in targeting MPF to the nucleus during prophase.

CUG-initiated FGF-2 induces chromatin compaction in cultured cardiac myocytes and in vitro

Fibroblast growth factor-2 (FGF-2) is a mitogen found in CUG-initiated 21-25 kDa ("hi") or AUG-initiated 16-18 kDa ("lo") forms. Previously we demonstrated that "hi"-but not "lo"-FGF-2 caused a distinct nuclear phenotype characterized by apparently condensed chromatin present as separate clumps in the nucleus of cardiac myocytes. In this manuscript we investigated whether these effects were related to apoptosis or mitosis and whether they reflected a direct effect of "hi" FGF-2 on chromatin. Myocytes overexpressing "hi" FGF-2 and presenting the clumped chromatin phenotype: (i) were not labeled above background with antibodies to phosphorylated histones H1 and H3 used as indicators of mitotic chromatin condensation; (ii) did not stain positive for TUNEL; (iii) their nuclear lamina, visualized by anti-laminB immunofluorescence, appeared intact; (iv) neither caspase inhibitors, nor Bcl-2 or "lo" FGF-2 overexpression prevented the manifestation of the compacted nuclear phenotype. Purified recombinant "hi" FGF-2 was more potent than "lo" FGF-2 in promoting the condensation/aggregation of chick erythrocyte chromatin partially reconstituted with histone H1 in vitro. We conclude that the DNA phenotype induced by "hi" FGF-2 in cardiac myocytes likely reflects a direct effect on chromatin structure that does not require the engagement of mitosis or apoptosis. By affecting chromatin compaction "hi" FGF-2 may contribute to the regulation of gene expression.

Phosphorylation of vimentin head domain inhibits interaction with the carboxyl-terminal end of alpha-helical rod domain studied by surface plasmon resonance measurements

The amino-terminal head domain of vimentin is the target site for several protein kinases and phosphorylation induces disassembly of the vimentin intermediate filaments in vivo and in vitro. To better understand molecular mechanisms involved in phosphorylation-dependent disassembly, we examined domain interactions involving the head domain and the effect of phosphorylation on the interaction, using surface plasmon resonance. We observed that the head domain binds to the carboxyl-terminal helix 2B in the rod domain, under physiological ionic strength. This interaction was interfered with by A-kinase phosphorylation of the head domain. Deletion of the carboxyl-terminal 20 amino acids of helix 2B resulted in loss of the interaction. Furthermore, peptide representing the carboxyl-terminal 20 residues of helix 2B had a substantial affinity with the head domain but not with the phosphorylated one. These findings support the idea that the interaction between the head domain and the last 20 residues of helix 2B is essential for association of vimentin tetramers into the intermediate filaments and that the phosphorylation-dependent disassembly is the result of loss of the interaction.

Identification and molecular cloning of germinal vesicle lamin B3 in goldfish (Carassius auratus) oocytes

A bulk isolation method was developed to collect a large number of germinal vesicles (GV) from postvitellogenic oocytes of goldfish (Carassius auratus). Using this method, we obtained GV lamina which are resistant to high salt and nonionic detergent. 2D PAGE revealed that the goldfish GV lamina contained several spots with similar molecular masses (67 kDa) and slightly different neutral isoelectrofocusing values (pI 5.8-6.2). After trypsin digestion and extraction of a major spot (pI 6.1), the peptide was subjected to RP-HPLC and sequenced. A homology search identified this spot as a nuclear lamin. A cDNA encoding goldfish GV lamin was isolated by RT-PCR using degenerate primers designed from the GV lamin tryptic peptide sequence. The goldfish GV lamin cDNA encodes a predicted molecular mass of 67 455 Da with a pI of 5.84. Phylogenetic analysis indicates that the amino-acid sequence is most similar to Xenopus oocyte-specific GV lamin B3, but differs from somatic lamins (A, B1 or B2). In contrast to somatic lamins, neither goldfish nor Xenopus GV lamin contain conserved phosphorylation sites for nuclear transport, except the nuclear localization sequence. Therefore, we conclude that the goldfish oocyte GV is mainly comprised of GV-type lamin (the homolog of Xenopus lamin B3).

The ALG-2/AIP-complex, a modulator at the interface between cell proliferation and cell death? A hypothesis

During the development of an organism cell proliferation, differentiation and cell death are tightly balanced, and are controlled by a number of different regulators. Alterations in this balance are often observed in a variety of human diseases. The role of Ca(2+) as one of the key regulators of the cell is discussed with respect to two recently discovered proteins, ALG-2 and AIP, of which the former is a Ca(2+)-binding protein, and the latter is substrate to various kinases. The two proteins interact with each other in a Ca(2+)-dependent manner, and the role of the complex ALG-2/AIP as a possible modulator at the interface between cell proliferation and cell death is discussed.

Identification of a novel retinoic acid-responsive element within the lamin A/C promoter

A-type lamins are not present in either early embryos or the embryonal carcinoma (EC) cell line. P19 cells, which are EC cell line, are able to express A-type lamins upon retinoic acid (RA) treatment. Here we report that a novel RA-responsive element, termed lamin A/C-RA-responsive element (L-RARE), is located within the lamin A/C promoter. RA activated the luciferase activity of the reporter which had four tandem repeats of the wild-type L-RARE, while a loss of function mutant, which altered CACCCCC to CACtatC within L-RARE, did not respond. Four specific binding complexes of L-RARE, Complexes-A, -B, -C, and -D, were detected in protein extracts obtained from P19 cells treated with and without RA. Specific antibodies revealed that Sp1 and Sp3 were included in Complex-A and Complexes-B and -C, respectively. Thus, L-RARE was important in the RA-mediated activation of the lamin A/C promoter and was recognized by DNA binding proteins.

Dynamics of the nuclear lamina as monitored by GFP-tagged A-type lamins

The behavior of chimeric proteins consisting of A-type lamins and green fluorescent protein (GFP) was studied to investigate the localization and dynamics of nuclear lamins in living cells. Cell line CHO-K1 was transfected with cDNA constructs encoding fusion proteins of lamin A-GFP, lamin Adelta10-GFP, or lamin C-GFP. In the interphase nucleus lamin-GFP fluorescence showed a perinuclear localization and incorporation into the lamina for all three constructs. Our findings show for the first time that the newly discovered lamin A 10 protein is localized to the nuclear membrane. The GFP-tagged lamins were processed and behaved similarly to the endogenous lamin molecules, at least in cells that expressed physiological levels of the GFP-lamins. In addition to the typical perinuclear localization, in the majority of transfected cells each individual A-type lamin-GFP revealed an extensive collection of branching intra- and trans-nuclear tubular structures, which showed a clear preference for a vertical orientation. Time-lapse studies of 3-D reconstructed interphase cells showed a remarkable stability in both number and location of these structures over time, while the lamina showed considerable dynamic movements, consisting of folding and indentation of large parts of the lamina. Fluorescence recovery after bleaching studies revealed a low protein turnover of both tubular and lamina-associated lamins. Repetitive bleaching of intranuclear areas revealed the presence of an insoluble intranuclear fraction of A-type lamins. Time-lapse studies of mitotic cells showed that reformation of the lamina and the tubular structures consisting of A-type lamins did not occur until after cytokinesis was completed.

LAP2 binding protein 1 (L2BP1/BAF) is a candidate mediator of LAP2-chromatin interaction

Lamina-associated polypeptide (LAP) 2, which directly interacts with B-type lamins and chromosomes, is an integral membrane protein specifically distributed along the inner nuclear membrane of the nuclear envelope. The chromatin- and lamin-binding activity of LAP2 suggests that LAP2 plays an important role in targeting mitotic vesicles to chromosomes and reorganizing the nuclear structure at the end of mitosis. Here I identified a LAP2 interacting protein, termed L2BP1 (LAP2 binding protein 1). The rat L2BP1 cDNA sequence is predicted to encode a protein of 89 amino acids which turns out to be a rat homolog of mouse and human BAF (Barrier-to-Autointegration Factor). L2BP1 is distributed diffusely throughout the nucleus in interphase cells. It is, however, highly concentrated at the chromosomes during the M-phase. Further, the L2BP1 binding domain of LAP2 overlaps its chromosome-binding region. These findings suggest that L2BP1 is a candidate mediator of LAP2-chromosome interaction at the end of mitosis.

Subcellular localization and partial purification of prelamin A endoprotease: an enzyme which catalyzes the conversion of farnesylated prelamin A to mature lamin A

The nuclear lamina protein, lamin A is produced by proteolytic cleavage of a 74 kDa precursor protein, prelamin A. The conversion of this precursor to mature lamin A is mediated by a specific endoprotease, prelamin A endoprotease. Subnuclear fractionation indicates that the prelamin A endoprotease is localized at the nuclear membrane. The enzyme appears to be an integral membrane protein, as it can only be removed from the nuclear envelope with detergent. It is effectively solubilized by the detergent n-octyl-beta-D-glucopyranoside and can be partially-purified (approximately 1200-fold) by size exclusion and cation exchange (Mono S) chromatography. Prelamin A endoprotease from HeLa cells was eluted from Mono S with 0.3 M sodium chloride as a single peak of activity. SDS-PAGE analysis of this prelamin A endoprotease preparation shows that it contains one major polypeptide at 65 kDa and smaller amounts of a second 68 kDa polypeptide. Inhibition of the enzyme activity in this preparation by specific serine protease inhibitors is consistent with the enzyme being a serine protease.

Lamin B is a prompt heat shock protein

The cellular response to hyperthermia involves the increased synthesis of heat shock proteins (HSPs) within several hours after treatment. In addition, a subset of proteins has been shown to be increased immediately after heating. These "prompt" HSPs are predominantly found in the nuclear matrix-intermediate filament fraction and are not present or detectable in unheated cells. Since the nuclear matrix has been suggested to be a target for heat-induced cell killing, prompt HSPs may play a prominent role in the heat shock response. Using Western blotting and flow cytometry, we found that an increase in the synthesis of lamin B, one of the major proteins of the nuclear lamina, is induced during heating at 45.5 degrees C but not during heating at 42 degrees C. Since it is an abundant protein which is constitutively expressed in mammalian cells, lamin B appears to be a unique member of the prompt HSP family. The kinetics of induction of lamin B during 45.5 degrees C heating did not correlate with the dose-dependent reduction in cell survival. While increased levels of lamin B during 45.5 degrees C heating do not appear to confer a survival advantage directly, a possible role for lamin B in cellular recovery after heat shock cannot be discounted.

Chromatin binding and polymerization of the endogenous Xenopus egg lamins: the opposing effects of glycogen and ATP

We have previously identified and quantitated three B-type lamin isoforms present in the nuclei of mature Xenopus laevis oocytes, and in cell-free egg extracts. As Xenopus egg extracts are frequently used to analyze nuclear envelope assembly and lamina functions, we felt it was imperative that the polymerization and chromatin-binding properties of the endogenous B-type egg lamins be investigated. While we have demonstrated that soluble B-type lamins bind to chromatin, we have also observed that the polymerization of egg lamins does not require membranes or chromatin. Lamin assembly is enhanced by the addition of glycogen/glucose, or by the depletion of ATP from the extract. Moreover, the polymerization of egg cytosol lamins and their binding to demembranated sperm or chromatin assembled from naked lambda-DNA is inhibited by an ATP regeneration system. These ATP-dependent inhibitory activities can be overcome by the coaddition of glycogen to egg cytosol. We have observed that glycogen does not alter ATP levels during cytosol incubation, but rather, as glycogen-enhanced lamin polymerization is inhibited by okadaic acid, we conclude that glycogen activates protein phosphatases. Because protein phosphatase 1 (PP1) is the only phosphatase known to be specifically regulated by glycogen our data indicate that PP1 is involved in lamin polymerization. Our results show that ATP and glycogen effect lamin polymerization and chromatin binding by separate and opposing mechanisms.

The equine herpesvirus 1 IR6 protein that colocalizes with nuclear lamins is involved in nucleocapsid egress and migrates from cell to cell independently of virus infection

The equine herpesvirus 1 (EHV-1) IR6 protein forms typical rod-like structures in infected cells, influences virus growth at elevated temperatures, and determines the virulence of EHV-1 Rac strains (Osterrieder et al., Virology 226:243-251, 1996). Experiments to further elucidate the functions and properties of the IR6 protein were conducted. It was shown that the IR6 protein of wild-type RacL11 virus colocalizes with nuclear lamins very late in infection as demonstrated by confocal laser scan microscopy and coimmunoprecipitation experiments. In contrast, the mutated IR6 protein encoded by the RacM24 strain did not colocalize with the lamin proteins at any time postinfection (p.i.). Electron microscopical examinations of ultrathin sections were performed on cells infected at 37 and 40 degreesC, the latter being a temperature at which the IR6-negative RacH virus and the RacM24 virus are greatly impaired in virus replication. These analyses revealed that nucleocapsid formation is efficient at 40 degreesC irrespective of the virus strain. However, whereas cytoplasmic virus particles were readily observed at 16 h p.i. in cells infected with the wild-type EHV-1 RacL11 or an IR6-recombinant RacH virus (HIR6-1) at 40 degreesC, virtually no capsid translocation to the cytoplasm was obvious in RacH- or RacM24-infected cells at the elevated temperature, demonstrating that the IR6 protein is involved in nucleocapsid egress. Transient transfection assays using RacL11 or RacM24 IR6 plasmid DNA and COS7 or Rk13 cells, infection studies using a gB-negative RacL11 mutant (L11DeltagB) which is deficient in direct cell-to-cell spread, and studies using lysates of IR6-transfected cells demonstrated that the wild-type IR6 protein is transported from cell to cell in the absence of virus infection and can enter cells by a yet unknown mechanism.

Assembly of Drosophila lamin Dm0 and C mutant proteins studied with the baculovirus system

Despite extensive knowledge of the in vitro polymerization properties of nuclear lamins, it is still not well understood how the nuclear lamina assembles in vivo. To learn more about the relationship between in vitro and in vivo polymerization of nuclear lamins, we expressed Drosophila lamin Dm0, mutant proteins, having well defined alterations of their in vitro polymerization properties, in Sf9 cells using the baculovirus system. All lamin Dm0 mutants assembled into fibrillar aggregates indistinguishable in morphology from those assembled by the wild-type protein. However, in contrast to wild-type lamin Dm0, mutant proteins were extracted with buffers of physiological ionic strength and pH containing Triton X-100. These results indicate that various types of lamin dimer-dimer interactions can be disrupted without affecting the morphology of the lamin Dm0 polymer. However, all types of dimer-dimer interactions tested appear to be important for full polymer stability. In addition, we analyzed the polymer formation of two Drosophila lamin C mutants and found that a segment in the carboxy-terminal tail domain is required for assembly of lamin C paracrystals at the nuclear lamina.

Colocalization of emerin and lamins in interphase nuclei and changes during mitosis

Emerin is a nuclear membrane protein which is affected by mutation in X-linked Emery-Dreifuss muscular dystrophy. We have previously suggested that emerin is a member of a family of type II integral membrane proteins which associate with the nuclear lamina and which include lamina-associated proteins and the lamin B receptor. We now show that emerin in COS cells is not restricted to the nuclear rim but is also found at intranuclear sites, where it colocalizes with nuclear lamins B1, B2 and A/C. During mitosis, emerin is dispersed throughout the cell and then participates in the reconstitution of membranes around the daughter nuclei. Although emerin and lamins do not remain colocalized during mitosis, they all show some association with the midbody of the mitotic spindle.

Detergent-salt resistance of LAP2alpha in interphase nuclei and phosphorylation-dependent association with chromosomes early in nuclear assembly implies functions in nuclear structure dynamics

Lamina-associated polypeptide (LAP) 2 of the inner nuclear membrane (now LAP2beta) and LAP2alpha are related proteins produced by alternative splicing, and contain a common 187 amino acid N-terminal domain. We show here that, unlike LAP2beta, LAP2alpha behaved like a nuclear non-membrane protein in subcellular fractionation studies and was localized throughout the nuclear interior in interphase cells. It co-fractionated with LAP2beta in nuclear lamina/matrix-enriched fractions upon extraction of nuclei with detergent, salt and nucleases. During metaphase LAP2alpha dissociated from chromosomes and became concentrated around the spindle poles. Furthermore, LAP2alpha was mitotically phosphorylated, and phosphorylation correlated with increased LAP2alpha solubility upon extraction of cells in physiological buffers. LAP2alpha relocated to distinct sites around chromosomes at early stages of nuclear reassembly and intermediarily co-localized with peripheral lamin B and intranuclear lamin A structures at telophase. During in vitro nuclear assembly LAP2alpha was dephosphorylated and assembled into insoluble chromatin-associated structures, and recombinant LAP2alpha was found to interact with chromosomes in vitro. Some LAP2alpha may also associate with membranes prior to chromatin attachment. Altogether the data suggest a role of LAP2alpha in post-mitotic nuclear assembly and in the dynamic structural organization of the nucleus.

Partitioning of cytoplasmic organelles during mitosis with special reference to the Golgi complex

During mitosis, not only the genetic material stored in the nucleus but also the constituents of the cytoplasm should be equally partitioned between the daughter cells. For this sake, the dividing cell goes through an extensive structural reorganization and transport along the endocytic and exocytic pathways is temporarily arrested. Early in prophase, the radiating array of cytoplasmic microtubules disassembles and the membrane systems of the secretory apparatus start to split up. In metaphase, the nuclear envelope fragments and the condensing chromosomes associate with the forming mitotic spindle. The cisternal and tubular elements of the endoplasmic reticulum and the Golgi complex break down into small vesicles, presumably as the result of an imbalance between vesicle budding and fusion. In anaphase, the two sets of chromosomes are pulled apart and a cleavage furrow forms halfway between the spindle poles. Since most organelles occur in multiple and widely dispersed copies at this stage, they will be evenly distributed between the daughter cells. During telophase and cytokinesis, the preceding fragmentation process is reversed. A nuclear envelope reappears around the chromosomes and cytoplasmic microtubules reassemble. The endoplasmic reticulum is rebuilt as a continuous system of flattened cisternae and tubules. Stacks of Golgi cisternae arise from small vesicles and are rearranged in an interconnected network. In parallel, the biosynthetic functions of the cell are normalized and intracellular membrane traffic is resumed.

The major nuclear envelope targeting domain of LAP2 coincides with its lamin binding region but is distinct from its chromatin interaction domain

LAP2 is an integral protein of the inner nuclear membrane which binds lamins and chromosomes and is suggested to have an important role in nuclear envelope organization. In a previous study we identified an internal 76-amino acid region of LAP2 which is required for stable targeting of the protein to the nuclear envelope. Here, we have mapped the lamin binding region of LAP2 and demonstrate that it coincides with this nuclear envelope targeting domain. In contrast, we found that the portion of LAP2 involved in binding to chromosomes resides in a separate region of the protein near its NH2 terminus. The minimal lamin binding region of LAP2 is capable of conferring stable nuclear envelope localization when attached to the transmembrane and partial lumenal domains of a protein that shows no nuclear envelope targeting activity. This directly supports the notion that a major mechanism for localization of integral membrane proteins at the inner nuclear membrane involves binding to lamins, which would constrain diffusion through the continuous nuclear envelope/endoplasmic reticulum membrane system.

Functional dissection of YA, an essential, developmentally regulated nuclear lamina protein in Drosophila melanogaster

The Drosophila YA protein is a nuclear lamina component whose function is essential to initiate embryonic development. To identify regions of YA required for its action in its normal cellular context, we made targeted mutations in the YA protein and tested their consequences in flies and embryos in vivo. We found that critical amino acids are distributed along the length of the YA molecule, with functionally important regions including the N- and the C-terminal ends, the cysteine residues in YA's two potential zinc fingers, a serine/threonine-rich region, and a potential maturation-promoting factor or mitogen-activated protein kinase phosphorylation target site, ITPIR. In addition, several Ya mutations showed intragenic complementation, with N-terminal mutations complementing C-terminal mutations, suggesting that YA proteins interact with one another. In support of this interaction, we demonstrated by immunoprecipitation that YA molecules are present in complexes with each other. Finally, we showed that the C-terminal 179 amino acids of YA are necessary to target, or retain, YA in the nuclear envelope.

A proline residue in the alpha-helical rod domain of type I keratin 16 destabilizes keratin heterotetramers

The type I keratins 14 (K14) and 16 (K16) are distinct in their assembly properties and their expression pattern despite a high degree of sequence identity. Understanding K16 function and regulation is of interest, given its strong induction in keratinocytes located at the wound edge after injury to stratified epithelia. We reported previously that, compared with K14, K16 forms unstable heterotetramers with either K5 or K6 as the type II keratin pairing partner (Paladini, R. D., Takahashi, K., Bravo, N. S., and Coulombe, P. A. (1996) J. Cell Biol. 132, 381-397). We show here that yet another related type I keratin, K17, forms stable heterotetramers with a variety of type II keratins, further accentuating the unique nature of K16. Analysis of chimeric K14-K16 proteins in a heterotetramer formation assay indicated that the instability determinant resides in a 220-amino acid segment within the alpha-helical rod domain of K16. Site-directed mutagenesis revealed that Pro188, an amino acid residue located in subdomain 1B of the rod, accounts quantitatively for the instability of K16-containing heterotetramers under denaturing conditions. In vitro polymerization studies suggest that the presence of Pro188 correlates with a reduction in assembly efficiency. In addition to their implications for the stable conformation of the keratin heterotetramers, these findings suggest that the tetramer-forming properties of K16 may influence its partitioning between the soluble and polymer pools, and hence contribute to its regulation in epithelial cells under resting and wound repair conditions.

Lamin-binding fragment of LAP2 inhibits increase in nuclear volume during the cell cycle and progression into S phase

Lamina-associated polypeptide 2 (LAP2) is an integral membrane protein of the inner nuclear membrane that binds to both lamin B and chromatin and has a putative role in nuclear envelope (NE) organization. We found that microinjection of a recombinant polypeptide comprising the nucleoplasmic domain of rat LAP2 (residues 1-398) into metaphase HeLa cells does not affect the reassembly of transport-competent nuclei containing NEs and lamina, but strongly inhibits nuclear volume increase. This effect appears to be specifically due to lamin binding, because it also is caused by microinjection of the minimal lamin-binding region of LAP2 (residues 298-373) but not by the chromatin-binding domain (residues 1-88). Injection of the lamin-binding region of rat LAP2 into early G1 phase HeLa cells also strongly affects nuclear growth; it almost completely prevents the threefold nuclear volume increase that normally occurs during the ensuing 10 h. Moreover, injection of the fragment during early G1 phase strongly inhibits entry of cells into S phase, whereas injection during S phase has no apparent effect on ongoing DNA replication. Since the lamin-binding fragment of LAP2 most likely acts by inhibiting dynamics of the nuclear lamina, our results suggest that a normal function of LAP2 involves regulation of nuclear lamina growth. These data also suggest that lamina dynamics are required for growth of the NE and for nuclear volume increase during the cell cycle, and that progression into S phase is dependent on the acquisition of a certain nuclear volume.

Identification of protein phosphatase 1 as a mitotic lamin phosphatase

At the onset of mitosis, the nuclear lamins are hyperphosphorylated leading to nuclear lamina disassembly, a process required for nuclear envelope breakdown and entry into mitosis. Multiple lamin kinases have been identified, including protein kinase C, that mediate mitotic lamin phosphorylation and mitotic nuclear lamina disassembly. Conversely, lamin dephosphorylation is required for nuclear lamina reassembly at the completion of mitosis. However, the protein phosphatase(s) responsible for the removal of mitotic phosphates from the lamins is unknown. In this study, we use human lamin B phosphorylated at mitosis-specific sites as a substrate to identify and characterize a lamin phosphatase activity from mitotic human cells. Several lines of evidence demonstrate that the mitotic lamin phosphatase corresponds to type 1 protein phosphatase (PP1). First, mitotic lamin phosphatase activity is inhibited by high nanomolar concentrations of okadaic acid and the specific PP1 peptide inhibitor, inhibitor-2. Second, mitotic lamin phosphatase activity cofractionates with PP1 after ion exchange chromatography. Third, microcystin-agarose depletes mitotic extracts of both PP1 and lamin phosphatase activity. Our results demonstrate that PP1 is the major mitotic lamin phosphatase responsible for removal of mitotic phosphates from lamin B, a process required for nuclear lamina reassembly.

Regulation of prelamin A endoprotease activity by prelamin A

The maturation of lamin A is completed by the endoproteolytic cleavage of its farnesylated precursor protein, prelamin A. In the absence of this cleavage, prelamin A can neither give rise to lamin A nor assemble into the nuclear lamina. We call the enzyme which catalyzes this endoproteolytic step the 'prelamin A endoprotease'. In this study, we begin characterization of the regulation of prelamin A endoprotease. In particular, we address the question as to whether prelamin A endoprotease activity is constitutive in cells or responds to expression of prelamin A. To do this, we compared the activity of this novel endoprotease in cells which express prelamin A with those that do not. Our data shows that the enzymatic activity of prelamin A endoprotease is enhanced by the expression of prelamin A.

The inner nuclear membrane protein LAP1 forms a native complex with B-type lamins and partitions with spindle-associated mitotic vesicles

We have examined the in situ organization and nearest neighbours of the 'lamina-associated polypeptide-1' (LAP1), a type II membrane protein and a major constituent of the mammalian nuclear envelope. We show here that, during interphase, LAP1 forms multimeric assemblies which are suspended in the inner nuclear membrane and are specifically associated with B-type lamins. The LAP1-lamin B complex is distinct from analogous complexes formed by the 'lamina-associated polypeptide-2' (LAP2), another inner nuclear membrane protein, and includes a protein kinase. Upon nuclear envelope breakdown, LAP1 partitions with mitotic vesicles which carry nuclear lamin B. The LAP1 vesicles can be distinguished from fragments of the nuclear envelope containing LAP2 and exhibit a striking co-alignment with spindle microtubules. These observations suggest that the inner nuclear membrane comprises discrete territories which accommodate specific integral membrane proteins and are differentially disassembled during mitosis.

Integral membrane proteins of the nuclear envelope are dispersed throughout the endoplasmic reticulum during mitosis

We have analyzed the fate of several integral membrane proteins of the nuclear envelope during mitosis in cultured mammalian cells to determine whether nuclear membrane proteins are present in a vesicle population distinct from bulk ER membranes after mitotic nuclear envelope disassembly or are dispersed throughout the ER. Using immunofluorescence staining and confocal microscopy, we compared the localization of two inner nuclear membrane proteins (laminaassociated polypeptides 1 and 2 [LAP1 and LAP2]) and a nuclear pore membrane protein (gp210) to the distribution of bulk ER membranes, which was determined with lipid dyes (DiOC6 and R6) and polyclonal antibodies. We found that at the resolution of this technique, the three nuclear envelope markers become completely dispersed throughout ER membranes during mitosis. In agreement with these results, we detected LAP1 in most membranes containing ER markers by immunogold electron microscopy of metaphase cells. Together, these findings indicate that nuclear membranes lose their identity as a subcompartment of the ER during mitosis. We found that nuclear lamins begin to reassemble around chromosomes at the end of mitosis at the same time as LAP1 and LAP2 and propose that reassembly of the nuclear envelope at the end of mitosis involves sorting of integral membrane proteins to chromosome surfaces by binding interactions with lamins and chromatin.

Insertional mutation of the Drosophila nuclear lamin Dm0 gene results in defective nuclear envelopes, clustering of nuclear pore complexes, and accumulation of annulate lamellae

Nuclear lamins are thought to play an important role in disassembly and reassembly of the nucleus during mitosis. Here, we describe a Drosophila lamin Dm0 mutant resulting from a P element insertion into the first intron of the Dm0 gene. Homozygous mutant animals showed a severe phenotype including retardation in development, reduced viability, sterility, and impaired locomotion. Immunocytochemical and ultrastructural analysis revealed that reduced lamin Dm0 expression caused an enrichment of nuclear pore complexes in cytoplasmic annulate lamellae and in nuclear envelope clusters. In several cells, particularly the densely packed somata of the central nervous system, defective nuclear envelopes were observed in addition. All aspects of the mutant phenotype were rescued upon P element-mediated germline transformation with a lamin Dm0 transgene. These data constitute the first genetic proof that lamins are essential for the structural organization of the cell nucleus.