MAN1, an inner nuclear membrane protein that shares the LEM domain with lamina-associated polypeptide 2 and emerin

Caenorhabditis elegans nucleoporins Nup93 and Nup205 determine the limit of nuclear pore complex size exclusion in vivo

Nuclear pore complexes (NPCs) span the nuclear envelope and mediate communication between the nucleus and the cytoplasm. To obtain insight into the structure and function of NPCs of multicellular organisms, we have initiated an extensive analysis of Caenorhabditis elegans nucleoporins. Of 20 assigned C. elegans nucleoporin genes, 17 were found to be essential for embryonic development either alone or in combination. In several cases, depletion of nucleoporins by RNAi caused severe defects in nuclear appearance. More specifically, the C. elegans homologs of vertebrate Nup93 and Nup205 were each found to be required for normal NPC distribution in the nuclear envelope in vivo. Depletion of Nup93 or Nup205 caused a failure in nuclear exclusion of nonnuclear macromolecules of approximately 70 kDa without preventing active nuclear protein import or the assembly of the nuclear envelope. The defects in NPC exclusion were accompanied by abnormal chromatin condensation and early embryonic arrest. Thus, the contribution to NPC structure of Nup93 and Nup205 is essential for establishment of normal NPC function and for cell viability.

Nuclear membrane proteins with potential disease links found by subtractive proteomics

To comprehensively identify integral membrane proteins of the nuclear envelope (NE), we prepared separately NEs and organelles known to cofractionate with them from liver. Proteins detected by multidimensional protein identification technology in the cofractionating organelles were subtracted from the NE data set. In addition to all 13 known NE integral proteins, 67 uncharacterized open reading frames with predicted membrane-spanning regions were identified. All of the eight proteins tested targeted to the NE, indicating that there are substantially more integral proteins of the NE than previously thought. Furthermore, 23 of these mapped within chromosome regions linked to a variety of dystrophies.

The lamina-associated polypeptide 2 (LAP2) isoforms beta, gamma and omega of zebrafish: developmental expression and behavior during the cell cycle

Zebrafish lamina-associated polypeptides 2 (ZLAP2) beta, gamma and omega have in common an N-terminal region with a LEM domain, and in the C-terminal half of the molecule a lamina binding domain and a membrane spanning sequence. The maternally synthesized omega is the largest isoform and the only LAP2 present in the rapidly dividing embryonic cells up to the gastrula stage. ZLAP2omega levels decrease during development, concomitant with the increase of the somatic isoforms ZLAP2beta and gamma. In somatic zebrafish cells ZLAP2gamma is the predominant isoform, whereas only small amounts of ZLAP2beta are present. During early embryonic development, ZLAP2omega becomes associated with mitotic chromosomes before anaphase. The surface of these chromosomes is decorated with vesicles, and each chromosome assembles its own nuclear envelope at the end of mitosis (karyomere formation). Ectopically expressed ZLAP2omega-green fluorescent protein (GFP) fusion protein targets vesicles to mitotic chromosomes in Xenopus A6 cells, suggesting that ZLAP2omega is involved in karyomere formation during early zebrafish development. When ZLAP2beta and gamma were expressed as GFP fusion proteins in Xenopus A6 cells, the beta- but not the gamma-isoform was found in association with mitotic chromosomes, and ZLAP2beta-containing chromosomes were decorated with vesicles. Further analysis of ZLAP2-GFP fusion proteins containing only distinct domains of the ZLAP2 isoforms revealed that the common N-terminal region in conjunction with beta- or omega-specific sequences mediate binding to mitotic chromosomes in vivo.

Emerin interacts in vitro with the splicing-associated factor, YT521-B

Emerin is a nuclear membrane protein that interacts with lamin A/C at the nuclear envelope. Mutations in either emerin or lamin A/C cause Emery-Dreifuss muscular dystrophy (EDMD). The functions of emerin are poorly understood, but EDMD affects mainly skeletal and cardiac muscle. We used a high-stringency yeast two-hybrid method to screen a human heart cDNA library, with full-length emerin as bait. Four out of five candidate interactors identified were nuclear proteins: lamin A, splicing factor YT521-B, proteasome subunit PA28 gamma and transcription factor vav-1. Specific binding between emerin and the functional C-terminal domain of YT521-B was confirmed by pull-down assays and biomolecular interaction analysis (BIAcore). Inhibition by emerin of YT521-B-dependent splice site selection in vivo suggests that the interaction is physiologically significant. A 'bipartite' binding site for YT521-B in emerin was identified using alanine substitution or disease-associated mutations in emerin. The transcription factor GCL (germ cell-less) has previously been shown to bind to the same site. The results are consistent with an emerging view that lamins and lamina-associated proteins, like emerin, have a regulatory role, as well as a structural role in the nucleus. YT521-B joins a growing list of candidates for a role in a gene expression model of the pathogenesis of EDMD.

Participation of a fusogenic protein, glyceraldehyde-3-phosphate dehydrogenase, in nuclear membrane assembly

We found an autoimmune serum, K199, that strongly suppresses nuclear membrane assembly in a cell-free system involving a Xenopus egg extract. Four different antibodies that suppress nuclear assembly were affinity-purified from the serum using Xenopus egg cytosol proteins. Three proteins recognized by these antibodies were identified by partial amino acid sequencing to be glyceraldehyde-3-phosphate dehydrogenase (GAPDH), fructose-1,6-bisphosphate aldolase, and the regulator of chromatin condensation 1. GAPDH is known to be a fusogenic protein. To verify the participation of GAPDH in nuclear membrane fusion, authentic antibodies against human and rat GAPDH were applied, and strong suppression of nuclear assembly at the nuclear membrane fusion step was observed. The nuclear assembly activity suppressed by antibodies was recovered on the addition of purified chicken GAPDH. A peptide with the sequence of amino acid residues 70-94 of GAPDH, which inhibits GAPDH-induced phospholipid vesicle fusion, inhibited nuclear assembly at the nuclear membrane fusion step. We propose that GAPDH plays a crucial role in the membrane fusion step in nuclear assembly in a Xenopus egg extract cell-free system.

A hydrophilic lamin-binding domain from the Drosophila YA protein can target proteins to the nuclear envelope

The nuclear lamina provides an architectural framework for the nuclear envelope and an attachment site for interphase chromatin. In Drosophila eggs and early embryos its major constituent, lamin Dm0, interacts with a lamina protein called YA. When the lamin-interaction region of YA is deleted, YA still enters nuclei but fails to localize to nuclear envelopes, suggesting that lamin interaction targets YA to the nuclear envelope. Here, we show that C-terminal lamin-interacting region of YA is sufficient to target the heterologous soluble protein GFP-NLS to the nuclear periphery in Drosophila tissue culture cells. Yeast two-hybrid analysis and transient transfection assays further defined this domain: residues 556-696 of YA are sufficient for both lamin Dm0 interaction and the targeting of GFP-NLS to the nuclear periphery. This region of YA is hydrophilic and lacks any transmembrane domain or known membrane-targeting motifs. We propose that the localization of YA to the nuclear lamina involves interaction with polymerized lamin Dm0 mediated by the lamin-targeting domain of YA. This hydrophilic YA domain might provide a useful molecular tool for targeting heterologous non-membrane-associated proteins to the nuclear envelope.

XMAN1, an inner nuclear membrane protein, antagonizes BMP signaling by interacting with Smad1 in Xenopus embryos

A family of inner nuclear membrane proteins is implicated in gene regulation by interacting with chromatin, nuclear lamina and intranuclear proteins; however, the physiological functions of these proteins are largely unknown. Using a Xenopus expression screening approach with an anterior neuroectoderm cDNA library, we have identified an inner nuclear membrane protein, XMAN1, as a novel neuralizing factor that is encoded by the Xenopus ortholog of human MAN1. XMAN1 mRNA is expressed maternally, and appears to be restricted to the entire ectoderm at the early gastrula stage, then to the anterior neuroectoderm at the neurula stage. XMAN1 induces anterior neural markers without mesoderm induction in ectodermal explants, and a partial secondary axis when expressed ventrally by dorsalizing the ventral mesoderm. Importantly, XMAN1 antagonizes bone morphogenetic protein (BMP) signaling downstream of its receptor Alk3, as judged by animal cap assays, in which XMAN1 blocks expression of downstream targets of BMP signaling (Xhox3 and Msx1), and by luciferase reporter assays, in which XMAN1 suppresses BMP-dependent activation of the Xvent2 promoter. Deletion mutant analyses reveal that the neuralizing and BMP-antagonizing activities of XMAN1 reside in the C-terminal region, and that the C-terminal region binds to Smad1, Smad5 and Smad8, which are intracellular mediators of the BMP pathway. Interference with endogenous XMAN1 functions with antisense morpholino oligos leads to the reduction of anterior neuroectoderm. These results provide the first evidence that the nuclear envelope protein XMAN1 acts as a Smad-interacting protein to antagonize BMP signaling during Xenopus embryogenesis.

MAN1 and emerin have overlapping function(s) essential for chromosome segregation and cell division in Caenorhabditis elegans

Emerin and MAN1 are LEM domain-containing integral membrane proteins of the vertebrate nuclear envelope. The function of MAN1 is unknown, whereas emerin is known to interact with nuclear lamins, barrier-to-autointegration factor (BAF), nesprin-1 alpha, and a transcription repressor. Mutations in emerin cause X-linked recessive Emery-Dreifuss muscular dystrophy. Emerin and MAN1 homologs are both conserved in Caenorhabditis elegans, but loss of Ce-emerin has no detectable phenotype. We therefore used C. elegans to test the hypothesis that Ce-MAN1 overlaps functionally with Ce-emerin. Supporting this model, Ce-MAN1 interacted directly with Ce-lamin and Ce-BAF in vitro and required Ce-lamin for its nuclear envelope localization. Interestingly, RNA interference-mediated removal of approximately 90% of Ce-MAN1 was lethal to approximately 15% of embryos. However, in the absence of Ce-emerin, approximately 90% reduction of Ce-MAN1 was lethal to all embryos by the 100-cell stage, with a phenotype involving repeated cycles of anaphase chromosome bridging and cytokinesis ["cell untimely torn" (cut) phenotype]. Immunostaining showed that the anaphase-bridged chromatin specifically retained a mitosis-specific phosphohistone H3 epitope and failed to recruit detectable Ce-lamin or Ce-BAF. These findings show that LEM domain proteins are essential for cell division and that Ce-emerin and Ce-MAN1 share at least one and possibly multiple overlapping functions, which may be relevant to Emery-Dreifuss muscular dystrophy.

Nuclear envelope proteins and neuromuscular diseases

Several neuromuscular diseases are caused by mutations in emerin and A-type lamins, proteins of the nuclear envelope. Emery-Dreifuss muscular dystrophy is caused by mutations in emerin (X-linked) or A-type lamins (autosomal dominant). Mutations in A-type lamins also cause limb-girdle muscular dystrophy type 1B, dilated cardiomyopathy with conduction defect, and Charcot-Marie-Tooth disorder type 2B1. They also cause partial lipodystrophy syndromes. The functions of emerin and A-type lamins and the mechanisms of how mutations in these proteins cause tissue-specific diseases are not well understood. The mutated proteins may cause structural damage to cells but may also affect processes such as gene regulation. This review gives an overview of this topic and describes recent advances in identification of disease-causing mutations, studies of cells and tissues from subjects with these diseases, and animal and cell culture models.

Lamina-associated polypeptide 2beta (LAP2beta) is contained in a protein complex together with A- and B-type lamins

Lamina-associated polypeptide 2beta (LAP2beta) of vertebrates is an integral membrane protein of the inner nuclear membrane that is generated by alternative splicing from the LAP2 gene. In the majority of Xenopus somatic cells including cultured kidney epithelial cells (A6 cells) there is only one major LAP2 isoform expressed that has the highest similarities with the mammalian LAP2beta whereas isoforms corresponding in size to the mammalian LAP2gamma and alpha are not detectable. We selected A6 cells and A6 cells stably expressing GFP fusion proteins of Xenopus LAP2beta (XLAP2Pbeta) as a model system to study interactions between LAP2beta and lamins. In vitro binding experiments with GST-XLAP2beta fusion proteins and immunoprecipitations with antibodies to GFP revealed that XLAP2beta is part of a complex that contains A- and B-type lamins. For the targeting to the nuclear envelope and the in vivo formation of this complex, GFP fusion proteins were sufficient comprising only the carboxyterminal 135 amino acids of XLAP2beta or the comparable region of zebrafish LAP2beta. A highly conserved 36 amino acids long sequence is located in this region of LAP2beta that is part of the lamina-binding domain previously identified in rat LAP2beta. GFP-LAP2beta fusion proteins of Xenopus, zebrafish, and rat that contained this sequence do compete with endogenous LAP2 in transfected cells for the same binding sites in the lamina. Our data indicate that the lamina-binding site of LAP2beta has been highly conserved during vertebrate evolution and suggests that this region of LAP2beta mediates the interactions between polymers of A- and B-type lamins.

Transcriptional repressor germ cell-less (GCL) and barrier to autointegration factor (BAF) compete for binding to emerin in vitro

Emerin belongs to the "LEM domain" family of nuclear proteins, which contain a characteristic approximately 40-residue LEM motif. The LEM domain mediates direct binding to barrier to autointegration factor (BAF), a conserved 10-kDa chromatin protein essential for embryogenesis in Caenorhabditis elegans. In mammalian cells, BAF recruits emerin to chromatin during nuclear assembly. BAF also mediates chromatin decondensation during nuclear assembly. The LEM domain and central region of emerin are essential for binding to BAF and lamin A, respectively. However, two other conserved regions of emerin lacked ascribed functions, suggesting that emerin could have additional partners. We discovered that these "unascribed" domains of emerin mediate direct binding to a transcriptional repressor, germ cell-less (GCL). GCL co-immunoprecipitates with emerin from HeLa cells. We determined the binding affinities of emerin for GCL, BAF, and lamin A and analyzed their oligomeric interactions. We showed that emerin forms stable complexes with either lamin A plus GCL or lamin A plus BAF. Importantly, BAF competed with GCL for binding to emerin in vitro, predicting that emerin can form at least two distinct types of complexes in vivo. Loss of emerin causes Emery-Dreifuss muscular dystrophy, a tissue-specific inherited disease that affects skeletal muscles, major tendons, and the cardiac conduction system. Although GCL alone cannot explain the disease mechanism, our results strongly support gene expression models for Emery-Dreifuss muscular dystrophy by showing that emerin binds directly to a transcriptional repressor, GCL, and by suggesting that emerin-repressor complexes might be regulated by BAF. Biochemical roles for emerin in gene expression are discussed.

Mouse germ cell-less as an essential component for nuclear integrity

A mouse homologue of the Drosophila melanogaster germ cell-less (mgcl-1) gene is expressed ubiquitously, and its gene product is localized to the nuclear envelope based on its binding to LAP2 beta (lamina-associated polypeptide 2 beta). To elucidate the role of mgcl-1, we analyzed two mutant mouse lines that lacked mgcl-1 gene expression. Abnormal nuclear morphologies that were probably due to impaired nuclear envelope integrity were observed in the liver, exocrine pancreas, and testis. In particular, functional abnormalities were observed in testis in which the highest expression of mgcl-1 was detected. Fertility was significantly impaired in mgcl-1-null male mice, probably as a result of severe morphological abnormalities in the sperm. Electron microscopic observations showed insufficient chromatin condensation and abnormal acrosome structures in mgcl-1-null sperm. In addition, the expression patterns of transition proteins and protamines, both of which are essential for chromatin remodeling during spermatogenesis, were aberrant. Considering that the first abnormality during the process of spermatogenesis was abnormal nuclear envelope structure in spermatocytes, the mgcl-1 gene product appears to be essential for appropriate nuclear-lamina organization, which in turn is essential for normal sperm morphogenesis and chromatin remodeling.

HA95 and LAP2 beta mediate a novel chromatin-nuclear envelope interaction implicated in initiation of DNA replication

HA95 is a chromatin-associated protein that interfaces the nuclear envelope (NE) and chromatin. We report an interaction between HA95 and the inner nuclear membrane protein lamina-associated polypeptide (LAP) 2 beta, and a role of this association in initiation of DNA replication. Precipitation of GST-LAP2 beta fusion proteins and overlays of immobilized HA95 indicate that a first HA95-binding region lies within amino acids 137-242 of LAP2 beta. A second domain sufficient to bind HA95 colocalizes with the lamin B-binding domain of LAP2beta at residues 299-373. HA95-LAP2 beta interaction is not required for NE formation. However, disruption of the association of HA95 with the NH2-terminal HA95-binding domain of LAP2 beta abolishes the initiation, but not elongation, of DNA replication in purified G1 phase nuclei incubated in S-phase extract. Inhibition of replication initiation correlates with proteasome-mediated proteolysis of Cdc6, a component of the prereplication complex. Rescue of Cdc6 degradation with proteasome inhibitors restores replication. We propose that an interaction of LAP2beta, or LAP2 proteins, with HA95 is involved in the control of initiation of DNA replication.

SANE, a novel LEM domain protein, regulates bone morphogenetic protein signaling through interaction with Smad1

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGF-beta) superfamily that play important roles in bone formation, embryonic patterning, and epidermal-neural cell fate decisions. BMPs signal through pathway specific mediators such as Smads1 and 5, but the upstream regulation of BMP-specific Smads has not been fully characterized. Here we report the identification of SANE (Smad1 Antagonistic Effector), a novel protein with significant sequence similarity to nuclear envelop proteins such as MAN1. SANE binds to Smad1/5 and to BMP type I receptors and regulates BMP signaling. SANE specifically blocks BMP-dependent signaling in Xenopus embryos and in a mammalian model of bone formation but does not inhibit the TGF-beta/Smad2 pathway. Inhibition of BMP signaling by SANE requires interaction between SANE and Smad1, because a SANE mutant that does not bind Smad1 does not inhibit BMP signaling. Furthermore, inhibition appears to be mediated by inhibition of BMP-induced Smad1 phosphorylation, blocking ligand-dependent nuclear translocation of Smad1. These studies define a new mode of regulation for intracellular BMP/Smad1 signaling.

Bidder's organ in the toad Bufo marinus: effects of orchidectomy on the morphology and expression of lamina-associated polypeptide 2

The Bidder's organ is an undeveloped ovary located anterior to the testis in male true toads (Bufonidae). The presence of sperm and oocytes, derived from the primordial germ cells of the male toad, provides an exceptional condition for the study of germ cell differentiation in vertebrates. In this study, the effects of orchidectomy on morphology and on lamina-associated polypeptide 2 (LAP2) expression were investigated in the Bidder's organ of Bufo marinus. To characterize bidderian oocytes, oogenesis in the ovary was divided into six stages. It was found that ovarian and bidderian oocytes were morphologically identical. To determine the expression of LAP2 isoforms, oocytes from the ovary and the Bidder's organ were examined by sodium dodecylsulfate-polyacrylamide gel electrophoresis and immunoblotting. In amphibians, LAP2beta is expressed in somatic cells, whereas LAP2omega is expressed in oocytes. The Bidder's organ normally contained previtellogenic oocytes of less than 150 micro m in diameter that expressed LAP2beta. However, the organ in some male toads contained a few larger oocytes, resulting in the faint detection of LAP2omega. After orchidectomy, bidderian oocytes grew and strongly expressed LAP2omega. Moreover, as in the ovary, LAP2omega was upregulated in bidderian oocytes of 240 microm in diameter. This work represents the first demonstration of the molecular similarity between ovarian and bidderian oocytes.

Barrier to autointegration factor interacts with the cone-rod homeobox and represses its transactivation function

Crx (cone-rod homeobox) is a homeodomain transcription factor implicated in regulating the expression of photoreceptor and pineal genes. To identify proteins that interact with Crx in the retina, we carried out a yeast two-hybrid screen of a retinal cDNA library. One of the identified clones encodes Baf (barrier to autointegration factor), which was previously shown to have a role in mitosis and retroviral integration. Additional biochemical assays provided supporting evidence for a Baf-Crx interaction. The Baf protein is detectable in all nuclear layers of the mouse retina, including the photoreceptors and the bipolar cells where Crx is expressed. Transient transfection assays with a rhodopsin-luciferase reporter in HEK293 cells demonstrate that overexpression of Baf represses Crx-mediated transactivation, suggesting that Baf acts as a negative regulator of Crx. Consistent with this role for Baf, an E80A mutation of CRX associated with cone-rod dystrophy has a higher than normal transactivation potency but a reduced interaction with Baf. Although our studies did not identify a causative Baf mutation in retinopathies, we suggest that Baf may contribute to the phenotype of a photoreceptor degenerative disease by modifying the activity of Crx. In view of the ubiquitous expression of Baf, we hypothesize that it may play a role in regulating tissue- or cell type-specific gene expression by interacting with homeodomain transcription factors.

Nesprin-1alpha self-associates and binds directly to emerin and lamin A in vitro

Nesprin-1alpha is a spectrin repeat (SR)-containing, transmembrane protein of the inner nuclear membrane, and is highly expressed in muscle cells. A yeast two-hybrid screen for nesprin-1alpha-interacting proteins showed that nesprin-1alpha interacted with itself. Blot overlay experiments revealed that nesprin-1alpha's third SR binds the fifth SR. The carboxy-terminal half of nesprin-1alpha directly bound lamin A, a nuclear intermediate filament protein. Biochemical analysis demonstrated that nesprin-1alpha dimers bind directly to the nucleoplasmic domain of emerin, an inner nuclear membrane protein, with an affinity of 4 nM. Binding was optimal for full nucleoplasmic dimers of nesprin-1alpha, since nesprin fragments SR1-5 and SR5-7 bound emerin as monomers with affinities of 53 nM and 250 mM, respectively. We propose that membrane-anchored nesprin-1alpha antiparallel dimers interact with both emerin and lamin A to provide scaffolding at the inner nuclear membrane.

Barrier-to-autointegration factor: major roles in chromatin decondensation and nuclear assembly

Barrier-to-autointegration factor (BAF) is a DNA-bridging protein, highly conserved in metazoans. BAF binds directly to LEM (LAP2, emerin, MAN1) domain nuclear membrane proteins, including LAP2 and emerin. We used site-directed mutagenesis and biochemical analysis to map functionally important residues in human BAF, including those required for direct binding to DNA or emerin. We also tested wild-type BAF and 25 point mutants for their effects on nuclear assembly in Xenopus egg extracts, which contain approximately 12 microM endogenous BAF dimers. Exogenous BAF caused two distinct effects: at low added concentrations, wild-type BAF enhanced chromatin decondensation and nuclear growth; at higher added concentrations, wild-type BAF completely blocked chromatin decondensation and nuclear growth. Mutants fell into four classes, including one that defines a novel functional surface on the BAF dimer. Our results suggest that BAF, unregulated, potently compresses chromatin structure, and that BAF interactions with both DNA and LEM proteins are critical for membrane recruitment and chromatin decondensation during nuclear assembly.

Life at the edge: the nuclear envelope and human disease

A group of human diseases, known as 'laminopathies', are associated with defects in proteins of the nuclear envelope. Most laminopathy mutations have been mapped to the A-type lamin gene, which is expressed in most adult cell types. So, why should different mutations in a near-ubiquitously expressed gene be associated with various discrete tissue-restricted diseases? Attempts to resolve this paradox are uncovering new molecular interactions #151; both inside the nucleus and at its periphery -- which indicate that the nuclear envelope has functions that go beyond mere housekeeping.

NUANCE, a giant protein connecting the nucleus and actin cytoskeleton

NUANCE (NUcleus and ActiN Connecting Element) was identified as a novel protein with an α-actinin-like actin-binding domain. A human 21.8 kb cDNA of NUANCE spreads over 373 kb on chromosome 14q22.1-q22.3. The cDNA sequence predicts a 796 kDa protein with an N-terminal actin-binding domain, a central coiled-coil rod domain and a predicted C-terminal transmembrane domain. High levels of NUANCE mRNA were detected in the kidney, liver,stomach, placenta, spleen, lymphatic nodes and peripheral blood lymphocytes. At the subcellular level NUANCE is present predominantly at the outer nuclear membrane and in the nucleoplasm. Domain analysis shows that the actin-binding domain binds to Factin in vitro and colocalizes with the actin cytoskeleton in vivo as a GFP-fusion protein. The C-terminal transmembrane domain is responsible for the targeting the nuclear envelope. Thus, NUANCE is the firstα-actinin-related protein that has the potential to link the microfilament system with the nucleus.

The nuclear envelope, lamins and nuclear assembly

The nuclear lamina is composed of both A- and B-type lamins and lamin-binding proteins. Many lamin-binding proteins are integral proteins of the inner nuclear membrane. Lamins and inner nuclear membrane proteins are important for a variety of cell functions, including nuclear assembly, replication, transcription, and nuclear integrity. Recent advances in the field in the past year include the identification of a family of spectrin-repeat-containing inner nuclear membrane proteins and other novel inner-membrane proteins, and the discovery of a nuclear membrane fusion complex. There is also growing evidence that A- and B-type lamins and their binding partners have distinct roles during nuclear assembly and interphase.

The nuclear muscular dystrophies

Nuclear muscular dystrophies are referred to as inherited muscular dystrophies caused by mutations in genes--(STA) or lamina (LMNA)--encoding components of the nuclear envelope. Phenotypically, they present as Emery-Dreifuss muscular dystrophy (EDMD), limb-girdle muscle dystrophy 1B (LGMD1B), or dilated cardiomyopathy with conduction defects (DCM-CD). Genetically related are the Dunnigan-type of familial partial lipodystrophy (FPLD) and Charcot-Marie-Tooth neuropathy type 2 (CMT2B). Until now, approximately 70 unique STA mutations, leading to X-linked EDMD or DCM-CD, have resulted mostly in a complete lack of emerin. Further 50 mostly missense mutations in LMNA result in autosomal-dominant EDMD, autosomal-recessive EDMD, LGMD1B, DCM-CD, FPLD, or CMT2B. Independent of type or location of the mutations, emerinopathies and laminopathies show wide clinical intrafamilial and interfamilial variability. Although structural abnormalities of nuclei in animal and cell models have been observed, the molecular pathology of the nuclear muscular dystrophies needs still to be elucidated.

Distinct functions of the unique C terminus of LAP2alpha in cell proliferation and nuclear assembly

The non-membrane-bound lamina-associated polypeptide 2 isoform, LAP2alpha, forms nucleoskeletal structures with A-type lamins and interacts with chromosomes in a cell cycle-dependent manner. LAP2alpha contains a LEM (LAP2, emerin, and MAN1) domain in the constant N terminus that binds to chromosomal barrier-to-autointegration factor, and a C-terminal unique region that is essential for chromosome binding. Here we show that C-terminal LAP2alpha fragment efficiently bound to mitotic chromosomes and inhibited assembly of endogenous LAP2alpha, nuclear membranes, and lamins A/C in in vitro nuclear assembly assays. Full-length recombinant LAP2alpha, which bound to chromosomes, and N-terminal fragment, which did not bind, had no effect on assembly. This suggested an essential role for the LAP2alpha C terminus in chromosome association and for the N-terminal LEM domain in subsequent assembly stages. In vivo analysis upon transient expression of GFP-tagged LAP2alpha fragments confirmed that, unlike the N-terminal fragment, the C-terminal fragment was able to bind to chromosomes during mitosis, if expressed weakly. At higher expression levels, C-terminal LAP2alpha fragment and full-length protein led to cell cycle arrest in interphase and apoptosis, as shown by fluorescence-activated cell sorter analysis, time lapse microscopy, and BrdUrd incorporation assays. These data indicated distinct functions of LAP2alpha in cell cycle progression during interphase and in nuclear reassembly during mitosis.

Caspase-6 gene disruption reveals a requirement for lamin A cleavage in apoptotic chromatin condensation

To study the role of caspase-6 during nuclear disassembly, we generated a chicken DT40 cell line in which both alleles of the caspase-6 gene were disrupted. No obvious morphological differences were observed in the apoptotic process in caspase-6- deficient cells compared with the wild type. However, examination of apoptosis in a cell-free system revealed a block in chromatin condensation and apoptotic body formation when nuclei from HeLa cells expressing lamin A or lamin A-transfected Jurkat cells were incubated in caspase-6-deficient apoptotic extracts. Transfection of exogenous caspase-6 into the clone reversed this phenotype. Lamins A and C, which are caspase-6-only substrates, were cleaved by the wild-type and heterozygous apoptotic extracts but not by the extracts lacking caspase-6. Furthermore, the caspase-6 inhibitor z-VEID-fmk mimicked the effects of caspase-6 deficiency and prevented the cleavage of lamin A. Taken together, these observations indicate that caspase-6 activity is essential for lamin A cleavage and that when lamin A is present it must be cleaved in order for the chromosomal DNA to undergo complete condensation during apoptotic execution.

Intracellular trafficking of MAN1, an integral protein of the nuclear envelope inner membrane

MAN1 is an integral protein of the inner nuclear membrane that shares the LEM domain, a conserved globular domain of approximately 40 amino acids, with lamina-associated polypeptide (LAP) 2 and emerin. Confocal immuofluorescence microscopy studies of the intracellular targeting of truncated forms of MAN1 showed that the nucleoplasmic, N-terminal domain is necessary for inner nuclear membrane retention. A protein containing the N-terminal domain with the first transmembrane segment of MAN1 is retained in the inner nuclear membrane, whereas the transmembrane segments with the C-terminal domain of MAN1 is not targeted to the inner nuclear membrane. The N-terminal domain of MAN1 is also sufficient for inner nuclear membrane targeting as it can target a chimeric type II integral protein to this subcellular location. Deletion mutants of the N-terminal of MAN1 are not efficiently retained in the inner nuclear membrane. When the N-terminal domain of MAN1 is increased in size from∼50 kDa to ∼100 kDa, the protein cannot reach the inner nuclear membrane. Fluorescence recovery after photobleaching experiments of MAN1 fused to green fluorescent protein show that the fusion protein is relatively immobile in the nuclear envelope compared with the endoplasmic reticulum of interphase cells, suggesting binding to a nuclear component. These results are in agreement with the `diffusion-retention' model for targeting integral proteins to the inner nuclear membrane.

A three-residue signal confers localization of a reporter protein in the inner nuclear membrane

By alignment to the carboxy-terminal-deduced aa sequence of human cytomegalovirus glycoprotein B (gB), conserved hexameric aa motifs with putative function for localization in the inner nuclear membrane (INM) were identified in the nucleoplasmic tails of herpes simplex virus type 1 gB and of the cellular lamin B receptor. Fusion of the respective hexamers to CD8 as a reporter redirected transport of the chimeras into the INM, suggesting their functioning as modular signal motifs. Consecutive experiments showed that the three-residue motif RxR represents a consensus sequence which is sufficient for localization of the CD8 reporter in the INM.

The binding of lamin B receptor to chromatin is regulated by phosphorylation in the RS region

Binding of lamin B receptor (LBR) to chromatin was studied by means of an in vitro assay system involving recombinant fragments of human LBR and Xenopus sperm chromatin. Glutathione-S-transferase (GST)-fused proteins including LBR fragments containing the N-terminal region (residues 1-53) and arginine-serine repeat-containing region (residues 54-89) bound to chromatin. The binding of GST-fusion proteins incorporating the N-terminal and arginine-serine repeat-containing regions to chromatin was suppressed by mild trypsinization of the chromatin and by pretreatment with a DNA solution. A new cell-free system for analyzing the cell cycle-dependent binding of a protein to chromatin was developed from recombinant proteins, a Xenopus egg cytosol fraction and sperm chromatin. The system was applied to analyse the binding of LBR to chromatin. It was shown that the binding of LBR fragments to chromatin was stimulated by phosphorylation in the arginine-serine repeat-containing region by a protein kinase(s) in a synthetic phase egg cytosol. However, the binding of LBR fragments was suppressed by phosphorylation at different residues in the same region by a kinase(s) in a mitotic phase cytosol. These results suggested that the cell cycle-dependent binding of LBR to chromatin is regulated by phosphorylation in the arginine-serine repeat-containing region by multiple kinases.

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.

Myne-1, a spectrin repeat transmembrane protein of the myocyte inner nuclear membrane, interacts with lamin A/C

Mutations in the genes encoding the inner nuclear membrane proteins lamin A/C and emerin produce cardiomyopathy and muscular dystrophy in humans and mice. The mechanism by which these broadly expressed gene products result in tissue-specific dysfunction is not known. We have identified a protein of the inner nuclear membrane that is highly expressed in striated and smooth muscle. This protein, myne-1 (myocyte nuclear envelope), is predicted to have seven spectrin repeats, an interrupted LEM domain and a single transmembrane domain at its C-terminus. We found that myne-1 is expressed upon early muscle differentiation in multiple intranuclear foci concomitant with lamin A/C expression. In mature muscle, myne-1 and lamin A/C are perfectly colocalized, although colocalization with emerin is only partial. Moreover, we show that myne-1 and lamin A/C coimmunoprecipitate from differentiated muscle in vitro. The muscle-specific inner nuclear envelope expression of myne-1, along with its interaction with lamin A/C, indicates that this gene is a potential mediator of cardiomyopathy and muscular dystrophy.

BAF is required for emerin assembly into the reforming nuclear envelope

Mutations in emerin cause the X-linked recessive form of Emery-Dreifuss muscular dystrophy (EDMD). Emerin localizes at the inner membrane of the nuclear envelope (NE) during interphase, and diffuses into the ER when the NE disassembles during mitosis. We analyzed the recruitment of wildtype and mutant GFP-tagged emerin proteins during nuclear envelope assembly in living HeLa cells. During telophase, emerin accumulates briefly at the ‘core’ region of telophase chromosomes, and later distributes over the entire nuclear rim. Barrier-to-autointegration factor (BAF), a protein that binds nonspecifically to double-stranded DNA in vitro, co-localized with emerin at the ‘core’ region of chromosomes during telophase. An emerin mutant defective for binding to BAF in vitro failed to localize at the ‘core’ in vivo, and subsequently failed to localize at the reformed NE. In HeLa cells that expressed BAF mutant G25E, which did not show ‘core’ localization, the endogenous emerin proteins failed to localize at the ‘core’ region during telophase, and did not assemble into the NE during the subsequent interphase. BAF mutant G25E also dominantly dislocalized LAP2β and lamin A from the NE, but had no effect on the localization of lamin B. We conclude that BAF is required for the assembly of emerin and A-type lamins at the reforming NE during telophase, and may mediate their stability in the subsequent interphase.

Nesprins: a novel family of spectrin-repeat-containing proteins that localize to the nuclear membrane in multiple tissues

In search of vascular smooth muscle cell differentiation markers, we identified two genes encoding members of a new family of type II integral membrane proteins. Both are ubiquitously expressed, and tissue-specific alternative mRNA initiation and splicing generate at least two major isoforms of each protein, with the smaller isoforms being truncated at the N-terminus. We have named these proteins nesprin-1 and -2 for nuclear envelope spectrin repeat, as they are characterized by the presence of multiple, clustered spectrin repeats, bipartite nuclear localization sequences and a conserved C-terminal, single transmembrane domain. Transient transfection of EGFP-fusion expression constructs demonstrated their localization to the nuclear membrane with a novel C-terminal, TM-domain-containing sequence essential for perinuclear localization. Using antibodies to nesprin-1, we documented its colocalization with LAP1, emerin and lamins at the nuclear envelope, and immunogold labeling confirmed its presence at the nuclear envelope and in the nucleus where it colocalized with heterochromatin. Nesprin-1 is developmentally regulated in both smooth and skeletal muscle and is re-localized from the nuclear envelope to the nucleus and cytoplasm during C2C12 myoblast differentiation. These data and structural analogies with other proteins suggest that nesprins may function as ‘dystrophins of the nucleus’ to maintain nuclear organization and structural integrity.

Nuclear envelope disorganization in fibroblasts from lipodystrophic patients with heterozygous R482Q/W mutations in the lamin A/C gene

Dunnigan-type familial partial lipodystrophy (FPLD), characterized by an abnormal body fat redistribution with insulin resistance, is caused by missense heterozygous mutations in A-type lamins (lamins A and C). A- and B-type lamins are ubiquitous intermediate filament proteins that polymerize at the inner face of the nuclear envelope. We have analyzed primary cultures of skin fibroblasts from three patients harboring R482Q or R482W mutations. These cells were euploid and able to cycle and divide. A subpopulation of these cells had abnormal blebbing nuclei with A-type lamins forming a peripheral meshwork, which was frequently disorganized. Inner nuclear membrane protein emerin, an A-type lamin-binding protein, strictly colocalized with this abnormal meshwork. Cells from lipodystrophic patients often had other nuclear envelope defects, mainly consisting of nuclear envelope herniations that were deficient in B-type lamins, nuclear pore complexes, lamina-associated protein 2 beta, and chromatin. The mechanical properties of nuclear envelopes were altered, as judged from the extensive deformations observed in nuclei from heat-shocked cells, and from the low stringency of extraction of their components. These structural nuclear alterations were caused by the lamins A/C mutations, as the same changes were introduced in human control fibroblasts by ectopic expression of R482W mutated lamin A.

The role of the nuclear envelope in Emery-Dreifuss muscular dystrophy

The X-linked form of Emery-Dreifuss muscular dystrophy (X-EDMD) is caused by absence, or greatly reduced amounts, of the inner nuclear-membrane protein, emerin. The autosomal dominant form (AD-EDMD) is caused by missense mutations in lamins A and C, two components of the nuclear lamina that interact directly with emerin. Lamin A/C mutations also cause one form of dilated cardiomyopathy (CMD1A) and one form of limb-girdle muscular dystrophy (LGMD1B), both of which have clinical features in common with EDMD, as well as a rare, unrelated form of lipodystrophy (FPLD). Evidence is now emerging that defective assembly of the nuclear lamina is a feature of all these diseases, although not necessarily the direct cause. Why only heart and skeletal muscle, and possibly connective tissue, are affected in EDMD and why expression of the disease is so extremely variable between individuals remains to be explained.

Virology. HIV--breaking the rules for nuclear entry

How does human immunodeficiency virus (HIV) gain access to the carefully guarded nucleus of the host cell? In a Perspective, Segura-Totten and Wilson elaborate on new findings (de Noronha et al.) showing that the HIV protein Vpr is crucial for causing transient herniations in the host cell nuclear envelope. These ruptures are sufficient to enable the preintegration complexes of invading virions to enter the nucleus and to integrate with host cell DNA.

Microtubule-dependent assembly of the nuclear envelope in Xenopus laevis egg extract

Microtubules take part in several mechanisms of intracellular motility, including organelle transport and mitosis. We have studied the ability of Xenopus egg extract to support nuclear membrane and pore complex formation when microtubule dynamics are manipulated. In this report we show that the formation of a nuclear envelope surrounding sperm chromatin requires polymerized microtubules. We have observed that microtubule-depolymerizing reagents, and AS-2, a known inhibitor of the microtubule motor protein kinesin, do not inhibit the formation of a double nuclear membrane. However these double membranes contain no morphologically identifiable nuclear pore complexes and do not support the accumulation of karyophilic proteins. In contrast, the assembly of annulate lamellae, cytoplasmic structures containing a subset of pore complex proteins, was not affected. Our data show that not only polymerized microtubules, but also the microtubule motor protein kinesin, are involved in the formation of the nuclear envelope. These results support the conclusion that multiple nuclear envelope-forming mitotic vesicle populations exist, that microtubules play an essential and selective role in the transport of nuclear envelope-forming vesicle population(s), and that separate mechanisms are involved in nuclear envelope and annulate lamellae formation.

Endoplasmic reticulum of animal cells and its organization into structural and functional domains

The endoplasmic reticulum (ER) in animal cells is an extensive, morphologically continuous network of membrane tubules and flattened cisternae. The ER is a multifunctional organelle; the synthesis of membrane lipids, membrane and secretory proteins, and the regulation of intracellular calcium are prominent among its array of functions. Many of these functions are not homogeneously distributed throughout the ER but rather are confined to distinct ER subregions or domains. This review describes the structural and functional organization of the ER and highlights the dynamic properties of the ER network and the mechanisms that support the positioning of ER membranes within the cell. Furthermore, we outline processes involved in the establishment and maintenance of an anisotropic distribution of ER-resident proteins and, thus, in the organization of the ER into functionally and morphologically different subregions.

Nuclear envelope proteomics: novel integral membrane proteins of the inner nuclear membrane

The nuclear envelope (NE) is one of the least characterized structures of eukaryotic cells. The study of its functional roles is hampered by the small number of proteins known to be specifically located to it. Here, we present a comprehensive characterization of the NE proteome. We applied different fractionation procedures and isolated protein subsets derived from distinct NE compartments. We identified 148 different proteins by 16-benzyl dimethyl hexadecyl ammonium chloride (16-BAC) gel electrophoresis and matrix-assisted laser desorption ionization (MALDI) mass spectrometry; among them were 19 previously unknown or noncharacterized. The identification of known proteins in particular NE fractions enabled us to assign novel proteins to NE substructures. Thus, our subcellular proteomics approach retains the screening character of classical proteomic studies, but also allows a number of predictions about subcellular localization and interactions of previously noncharacterized proteins. We demonstrate this result by showing that two novel transmembrane proteins, a 100-kDa protein with similarity to Caenorhabditis elegans Unc-84A and an unrelated 45-kDa protein we named LUMA, reside in the inner nuclear membrane and likely interact with the nuclear lamina. The utility of our approach is not restricted to the investigation of the NE. Our approach should be applicable to the analysis of other complex membrane structures of the cell as well.

The nuclear envelope in muscular dystrophy and cardiovascular diseases

Considerable interest has been focused on the nuclear envelope in recent years following the realization that several human diseases are linked to defects in genes encoding nuclear envelope specific proteins, most notably A-type lamins and emerin. These disorders, described as laminopathies or nuclear envelopathies, include both X-linked and autosomal dominant forms of Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy with conduction system defects, limb girdle muscular dystrophy 1B with atrioventricular conduction disturbances, and Dunnigan-type familial partial lipodystrophy. Certain of these diseases are associated with nuclear structural abnormalities that can be seen in a variety of cells and tissues. These observations clearly demonstrate that A-type lamins in particular play a central role, not only in the maintenance of nuclear envelope integrity but also in the large-scale organization of nuclear architecture. What is not obvious, however, is why defects in nuclear envelope proteins that are found in most adult cell types should give rise to pathologies associated predominantly with skeletal and cardiac muscle and adipocytes. The recognition of these various disorders now raises the novel possibility that the nuclear envelope may have functions that go beyond housekeeping and which impact upon cell-type specific nuclear processes.

Nuclear envelope dynamics

The nuclear envelope (NE) provides a semi permeable barrier between the nucleus and cytoplasm and plays a central role in the regulation of macromolecular trafficking between these two compartments. In addition to this transport function, the NE is a key determinant of interphase nuclear architecture. Defects in NE proteins such as A-type lamins and the inner nuclear membrane protein, emerin, result in several human diseases that include cardiac and skeletal myopathies as well as lipodystrophy. Certain disease-linked A-type lamin defects cause profound changes in nuclear organization such as loss of peripheral heterochromatin and redistribution of other nuclear envelope components. While clearly essential in maintenance of nuclear integrity, the NE is a highly dynamic organelle. In interphase it is constantly remodeled to accommodate nuclear growth. During mitosis it must be completely dispersed so that the condensed chromosomes may gain access to the mitotic spindle. Upon completion of mitosis, dispersed NE components are reutilized in the assembly of nuclei within each daughter cell. These complex NE rearrangements are under precise temporal and spatial control and involve interactions with microtubules, chromatin, and a variety of cell-cycle regulatory molecules.Key words: nuclear envelope, lamin, nuclear pore complex, nuclear membranes, mitosis.

Nuclear membrane protein LAP2β mediates transcriptional repression alone and together with its binding partner GCL (germ-cell-less)

LAP2β is an integral membrane protein of the nuclear envelope involved in chromatin and nuclear architecture. Using the yeast two-hybrid system, we have cloned a novel LAP2β-binding protein, mGCL, which contains a BTB/POZ domain and is the mouse homologue of the Drosophila germ-cell-less (GCL) protein. In Drosophila embryos, GCL was shown to be essential for germ cell formation and was localized to the nuclear envelope. Here, we show that, in mammalian cells, GCL is co-localized with LAP2β to the nuclear envelope. Nuclear fractionation studies reveal that mGCL acts as a nuclear matrix component and not as an integral protein of the nuclear envelope. Recently, mGCL was found to interact with the DP3α component of the E2F transcription factor. This interaction reduced the transcriptional activity of the E2F-DP heterodimer, probably by anchoring the complex to the nuclear envelope. We demonstrate here that LAP2β is also capable of reducing the transcriptional activity of the E2F-DP complex and that it is more potent than mGCL in doing so. Co-expression of both LAP2β and mGCL with the E2F-DP complex resulted in a reduced transcriptional activity equal to that exerted by the pRb protein.

Solution structure of the constant region of nuclear envelope protein LAP2 reveals two LEM-domain structures: one binds BAF and the other binds DNA

The nuclear envelope proteins LAP2, emerin and MAN1 share a conserved approximately 40-residue 'LEM' motif. Loss of emerin causes Emery-Dreifuss muscular dystrophy. We have solved the solution NMR structure of the constant region of human LAP2 (residues 1-168). Human LAP2(1-168) has two structurally independent, non-interacting domains located at residues 1-50 ('LAP2-N') and residues 111-152 (LEM-domain), connected by an approximately 60-residue flexible linker. The two domains are structurally homologous, comprising a helical turn followed by two helices connected by an 11-12-residue loop. This motif is shared by subdomains of T4 endonuclease VII and transcription factor rho, despite negligible (< or =15%) sequence identity. NMR chemical shift mapping demonstrated that the LEM-domain binds BAF (barrier-to-autointegration factor), whereas LAP2-N binds DNA. Both binding surfaces comprise helix 1, the N-terminus of helix 2 and the inter-helical loop. Binding selectivity is determined by the nature of the surface residues in these binding sites, which are predominantly positively charged for LAP2-N and hydrophobic for the LEM-domain. Thus, LEM and LEM-like motifs form a common structure that evolution has customized for binding to BAF or DNA.

Structural analysis of emerin, an inner nuclear membrane protein mutated in X-linked Emery-Dreifuss muscular dystrophy

Like Duchenne and Becker muscular dystrophies, Emery-Dreifuss muscular dystrophy (EDMD) is characterized by myopathic and cardiomyopathic abnormalities. EDMD has the particularity of being linked to mutations in nuclear proteins. The X-linked form of EDMD is caused by mutations in the emerin gene, whereas autosomal dominant EDMD is caused by mutations in the lamin A/C gene. Emerin colocalizes with lamin A/C in interphase cells, and binds in vitro to lamin A/C. Recent work suggests that lamin A/C might serve as a receptor for emerin. We have undertaken a structural analysis of emerin, and in particular of its N-terminal domain, which is comprised in the emerin segment critical for binding to lamin A/C. We show that region 2-54 of emerin adopts the LEM fold. This fold was originally described in the two N-terminal domains of another inner nuclear membrane protein called lamina-associated protein 2 (LAP2). The existence of a conserved solvent-exposed surface on the LEM domains of LAP2 and emerin is discussed, as well as the nature of a possible common target.

Structural characterization of the LEM motif common to three human inner nuclear membrane proteins

BACKGROUND

Integral membrane proteins of the inner nuclear membrane are involved in chromatin organization and postmitotic reassembly of the nucleus. The discovery that mutations in the gene encoding emerin causes X-linked Emery-Dreifuss muscular dystrophy has enhanced interest in such proteins. A common structural domain of 50 residues, called the LEM domain, has been identified in emerin MAN1, and lamina-associated polypeptide (LAP) 2. In particular, all LAP2 isoforms share an N-terminal segment composed of such a LEM domain that is connected to a highly divergent LEM-like domain by a linker that is probably unstructured.

RESULTS

We have determined the three-dimensional structures of the LEM and LEM-like domains of LAP2 using nuclear magnetic resonance and molecular modeling. Both domains adopt the same fold, mainly composed of two large parallel alpha helices.

CONCLUSIONS

The structural LEM motif is found in human inner nuclear membrane proteins and in protein-protein interaction domains from bacterial multienzyme complexes. This suggests that LEM and LEM-like domains are protein-protein interaction domains. A region conserved in all LEM domains, at the surface of helix 2, could mediate interaction between LEM domains and a common protein partner.

LAP2 binds to BAF.DNA complexes: requirement for the LEM domain and modulation by variable regions

LAP2 belongs to a family of nuclear membrane proteins sharing a 43 residue LEM domain. All LAP2 isoforms have the same N-terminal 'constant' region (LAP2-c), which includes the LEM domain, plus a C-terminal 'variable' region. LAP2-c polypeptide inhibits nuclear assembly in Xenopus extracts, and binds in vitro to barrier-to-autointegration factor (BAF), a DNA-bridging protein. We tested 17 Xenopus LAP2-c mutants for nuclear assembly inhibition, and binding to BAF and BAF small middle dotDNA complexes. LEM domain mutations disrupted all activities tested. Some mutations outside the LEM domain had no effect on binding to BAF, but disrupted activity in Xenopus extracts, suggesting that LAP2-c has an additional unknown function required to inhibit nuclear assembly. Mutagenesis results suggest that BAF changes conformation when complexed with DNA. The binding affinity of LAP2 was higher for BAF small middle dotDNA complexes than for BAF, suggesting that these interactions are physiologically relevant. Nucleoplasmic domains of Xenopus LAP2 isoforms varied 9-fold in their affinities for BAF, but all isoforms supershifted BAF small middle dotDNA complexes. We propose that the LEM domain is a core BAF-binding domain that can be modulated by the variable regions of LAP2 isoforms.

Transcriptional repression, apoptosis, human disease and the functional evolution of the nuclear lamina

The number and complexity of genes encoding nuclear lamina proteins has increased during metazoan evolution. Emerging evidence reveals that transcriptional repressors such as the retinoblastoma protein, and apoptotic regulators such as CED-4, have functional and dynamic interactions with the lamina. The discovery that mutations in nuclear lamina proteins cause heritable tissue-specific diseases, including Emery-Dreifuss muscular dystrophy, is prompting a fresh look at the nuclear lamina to devise models that can account for its diverse functions and dynamics, and to understand its enigmatic structure.

Identification of tyrosine-phosphorylated proteins associated with the nuclear envelope

The nuclear envelope separates the nucleoplasm from the rest of the cell. Throughout the cell cycle, its structural integrity is controlled by reversible protein phosphorylation. Whereas its phosphorylation-dependent disassembly during mitosis is well characterized, little is known about phosphorylation events at this structure during interphase. The few characterized examples cover protein phosphorylation at serine and threonine residues, but not tyrosine phosphorylation at the nuclear envelope. Here, we demonstrate that tyrosine phosphorylation and dephosphorylation occur at the nuclear envelope of intact Neuro2a mouse neuroblastoma cells. Tyrosine kinase and phosphatase activities remain associated with purified nuclear envelopes. A similar pattern of tyrosine-phosphorylated nuclear envelope proteins suggests that the same tyrosine kinases act at the nuclear envelope of intact cells and at the purified nuclear envelope. We have also identified eight tyrosine-phosphorylated nuclear envelope proteins by 2D BAC/SDS/PAGE, immunoblotting with phosphotyrosine-specific antibodies, tryptic in-gel digestion, and MS analysis of tryptic peptides. These proteins are the lamina proteins lamin A, lamin B1, and lamin B2, the inner nuclear membrane protein LAP2beta, the heat shock protein hsc70, and the DNA/RNA-binding proteins PSF, hypothetical 16-kDa protein, and NonO, which copurify with the nuclear envelope.

Nuclear lamins A and B1: different pathways of assembly during nuclear envelope formation in living cells

At the end of mitosis, the nuclear lamins assemble to form the nuclear lamina during nuclear envelope formation in daughter cells. We have fused A- and B-type nuclear lamins to the green fluorescent protein to study this process in living cells. The results reveal that the A- and B-type lamins exhibit different pathways of assembly. In the early stages of mitosis, both lamins are distributed throughout the cytoplasm in a diffusible (nonpolymerized) state, as demonstrated by fluorescence recovery after photobleaching (FRAP). During the anaphase-telophase transition, lamin B1 begins to become concentrated at the surface of the chromosomes. As the chromosomes reach the spindle poles, virtually all of the detectable lamin B1 has accumulated at their surfaces. Subsequently, this lamin rapidly encloses the entire perimeter of the region containing decondensing chromosomes in each daughter cell. By this time, lamin B1 has assembled into a relatively stable polymer, as indicated by FRAP analyses and insolubility in detergent/high ionic strength solutions. In contrast, the association of lamin A with the nucleus begins only after the major components of the nuclear envelope including pore complexes are assembled in daughter cells. Initially, lamin A is found in an unpolymerized state throughout the nucleoplasm of daughter cell nuclei in early G1 and only gradually becomes incorporated into the peripheral lamina during the first few hours of this stage of the cell cycle. In later stages of G1, FRAP analyses suggest that both green fluorescent protein lamins A and B1 form higher order polymers throughout interphase nuclei.