Remodelling the walls of the nucleus

Nuclear matrix, nuclear envelope and premature aging syndromes in a translational research perspective

Lamin A-related progeroid syndromes are genetically determined, extremely rare and severe. In the past ten years, our knowledge and perspectives for these diseases has widely progressed, through the progressive dissection of their pathophysiological mechanisms leading to precocious and accelerated aging, from the genes mutations discovery until therapeutic trials in affected children. A-type lamins are major actors in several structural and functional activities at the nuclear periphery, as they are major components of the nuclear lamina. However, while this is usually poorly considered, they also play a key role within the rest of the nucleoplasm, whose defects are related to cell senescence. Although nuclear shape and nuclear envelope deformities are obvious and visible events, nuclear matrix disorganization and abnormal composition certainly represent the most important causes of cell defects with dramatic pathological consequences. Therefore, lamin-associated diseases should be better referred as laminopathies instead of envelopathies, this later being too restrictive, considering neither the key structural and functional roles of soluble lamins in the entire nucleoplasm, nor the nuclear matrix contribution to the pathophysiology of lamin-associated disorders and in particular in defective lamin A processing-associated aging diseases. Based on both our understanding of pathophysiological mechanisms and the biological and clinical consequences of progeria and related diseases, therapeutic trials have been conducted in patients and were terminated less than 10 years after the gene discovery, a quite fast issue for a genetic disease. Pharmacological drugs have been repurposed and used to decrease the toxicity of the accumulated, unprocessed and truncated prelaminA in progeria. To date, none of them may be considered as a cure for progeria and these clinical strategies were essentially designed toward reducing a subset of the most dramatic and morbid features associated to progeria. New therapeutic strategies under study, in particular targeting the protein expression pathway at the mRNA level, have shown a remarkable efficacy both in vitro in cells and in vivo in mice models. Strategies intending to clear the toxic accumulated proteins from the nucleus are also under evaluation. However, although exceedingly rare, improving our knowledge of genetic progeroid syndromes and searching for innovative and efficient therapies in these syndromes is of paramount importance as, even before they can be used to save lives, they may significantly (i) expand the affected childrens' lifespan and preserve their quality of life; (ii) improve our understanding of aging-related disorders and other more common diseases; and (iii) expand our fundamental knowledge of physiological aging and its links with major physiological processes such as those involved in oncogenesis.

Depletion of the protein kinase VRK1 disrupts nuclear envelope morphology and leads to BAF retention on mitotic chromosomes

The VRK1 protein kinase regulates the phosphorylation of BAF, which binds to dsDNA and LEM domain–containing proteins. VRK1 depletion increases the immobile fraction of BAF at the nuclear periphery and disturbs nuclear envelope architecture. It also leads to the retention of BAF on chromosomes as cells enter and progress through mitosis.

Barrier to autointegration factor (BAF), which is encoded by the BANF1 gene, binds with high-affinity to double-stranded DNA and LEM domain–containing proteins at the nuclear periphery. A BANF1 mutation has recently been associated with a novel human progeria syndrome, and cells from these patients have aberrant nuclear envelopes. The interactions of BAF with its DNA- and protein-binding partners are known to be regulated by phosphorylation, and previously we validated BAF as a highly efficient substrate for the VRK1 protein kinase. Here we show that depletion of VRK1 in MCF10a and MDA-MB-231 cells results in aberrant nuclear architecture. The immobile fraction of green fluorescent protein (GFP)–BAF at the nuclear envelope (NE) is elevated, suggesting that prolonged interactions of BAF with its binding partners is likely responsible for the aberrant NE architecture. Because detachment of BAF from its binding partners is associated with NE disassembly, we performed live-imaging analysis of control and VRK1-depleted cells to visualize GFP-BAF dynamics during mitosis. In the absence of VRK1, BAF does not disperse but instead remains chromosome bound from the onset of mitosis. VRK1 depletion also increases the number of anaphase bridges and multipolar spindles. Thus phosphorylation of BAF by VRK1 is essential both for normal NE architecture and proper dynamics of BAF–chromosome interactions during mitosis. These results are consistent with previous studies of the VRK/BAF signaling axis in Caenorhabditis elegans and Drosophila melanogaster and validate VRK1 as a key regulator of NE architecture and mitotic chromosome dynamics in mammalian cells.

Catastrophic nuclear envelope collapse in cancer cell micronuclei

During mitotic exit, missegregated chromosomes can recruit their own nuclear envelope (NE) to form micronuclei (MN). MN have reduced functioning compared to primary nuclei in the same cell, although the two compartments appear to be structurally comparable. Here we show that over 60% of MN undergo an irreversible loss of compartmentalization during interphase due to NE collapse. This disruption of the MN, which is induced by defects in nuclear lamina assembly, drastically reduces nuclear functions and can trigger massive DNA damage. MN disruption is associated with chromatin compaction and invasion of endoplasmic reticulum (ER) tubules into the chromatin. We identified disrupted MN in both major subtypes of human non-small-cell lung cancer, suggesting that disrupted MN could be a useful objective biomarker for genomic instability in solid tumors. Our study shows that NE collapse is a key event underlying MN dysfunction and establishes a link between aberrant NE organization and aneuploidy.

Defective nuclear import of Tpr in Progeria reflects the Ran sensitivity of large cargo transport

Nuclear transport of large protein cargoes such as Tpr is more sensitive to the alteration of the ratio of nuclear to cytoplasmic Ran that occurs in Progeria.

The RanGTPase acts as a master regulator of nucleocytoplasmic transport by controlling assembly and disassembly of nuclear transport complexes. RanGTP is required in the nucleus to release nuclear localization signal (NLS)–containing cargo from import receptors, and, under steady-state conditions, Ran is highly concentrated in the nucleus. We previously showed the nuclear/cytoplasmic Ran distribution is disrupted in Hutchinson-Gilford Progeria syndrome (HGPS) fibroblasts that express the Progerin form of lamin A, causing a major defect in nuclear import of the protein, translocated promoter region (Tpr). In this paper, we show that Tpr import was mediated by the most abundant import receptor, KPNA2, which binds the bipartite NLS in Tpr with nanomolar affinity. Analyses including NLS swapping revealed Progerin did not cause global inhibition of nuclear import. Rather, Progerin inhibited Tpr import because transport of large protein cargoes was sensitive to changes in the Ran nuclear/cytoplasmic distribution that occurred in HGPS. We propose that defective import of large protein complexes with important roles in nuclear function may contribute to disease-associated phenotypes in Progeria.

Prelamin A accelerates vascular calcification via activation of the DNA damage response and senescence-associated secretory phenotype in vascular smooth muscle cells

RATIONALE

Vascular calcification is prevalent in the aging population, yet little is known of the mechanisms driving age-associated vascular smooth muscle cell (VSMC) phenotypic change.

OBJECTIVE

To investigate the role of nuclear lamina disruption, a specific hallmark of VSMC aging, in driving VSMC osteogenic differentiation.

METHODS AND RESULTS

Prelamin A, the unprocessed form of the nuclear lamina protein lamin A, accumulated in calcifying human VSMCs in vitro and in vivo, and its overexpression promoted VSMC osteogenic differentiation and mineralization. During VSMC aging in vitro, prelamin A accumulation occurred concomitantly with increased p16 expression and osteogenic differentiation and was associated with increased levels of DNA damage. Microarray analysis showed that DNA damage repair pathways were significantly impaired in VSMCs expressing prelamin A and that chemical inhibition and siRNA depletion of the DNA damage response kinases ataxia-telangiectasia mutated/ataxia-telangiectasia- and Rad3-related effectively blocked VSMC osteogenic differentiation and mineralization. In coculture experiments, prelamin A-expressing VSMCs induced alkaline phosphatase activity in mesenchymal progenitor cells, and this was abrogated by inhibition of ataxia-telangiectasia-mutated signaling, suggesting that DNA damage induces the secretion of pro-osteogenic factors by VSMCs. Cytokine array analysis identified several ataxia-telangiectasia mutated-dependent senescence-associated secretory phenotype factors/cytokines released by prelamin A-positive VSMCs, including the calcification regulators bone morphogenetic protein 2, osteoprotegerin, and interleukin 6.

CONCLUSIONS

Prelamin A promotes VSMC calcification and aging by inducing persistent DNA damage signaling, which acts upstream of VSMC osteogenic differentiation and the senescence-associated secretory phenotype. Agents that target the DNA damage response and prelamin A toxicity may be potential therapies for the treatment of vascular calcification.

Microtubule dynamics alter the interphase nucleus

Microtubules are known to drive chromosome movements and to induce nuclear envelope breakdown during mitosis and meiosis. Here we show that microtubules can enforce nuclear envelope folding and alter the levels of nuclear envelope-associated heterochromatin during interphase, when the nuclear envelope is intact. Microtubule reassembly, after chemically induced depolymerization led to folding of the nuclear envelope and to a transient accumulation of condensed chromatin at the site nearest the microtubule organizing center (MTOC). This microtubule-dependent chromatin accumulation next to the MTOC is dependent on the composition of the nuclear lamina and the activity of the dynein motor protein. We suggest that forces originating from simultaneous polymerization of microtubule fibers deform the nuclear membrane and the underlying lamina. Whereas dynein motor complexes localized to the nuclear envelope that slide along the microtubules transfer forces and/or signals into the nucleus to induce chromatin reorganization and accumulation at the nuclear membrane folds. Thus, our study identified a molecular mechanism by which mechanical forces generated in the cytoplasm reshape the nuclear envelope, alter the intranuclear organization of chromatin, and affect the architecture of the interphase nucleus.

LITTLE NUCLEI 1 and 4 regulate nuclear morphology in Arabidopsis thaliana

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

Protein homeostasis: live long, won't prosper

Protein turnover is an effective way of maintaining a functional proteome, as old and potentially damaged polypeptides are destroyed and replaced by newly synthesized copies. An increasing number of intracellular proteins, however, have been identified that evade this turnover process and instead are maintained over a cell's lifetime. This diverse group of long-lived proteins might be particularly prone to accumulation of damage and thus have a crucial role in the functional deterioration of key regulatory processes during ageing.

The GIP gamma-tubulin complex-associated proteins are involved in nuclear architecture in Arabidopsis thaliana

During interphase, the microtubular cytoskeleton of cycling plant cells is organized in both cortical and perinuclear arrays. Perinuclear microtubules (MTs) are nucleated from γ-Tubulin Complexes (γ-TuCs) located at the surface of the nucleus. The molecular mechanisms of γ-TuC association to the nuclear envelope (NE) are currently unknown. The γ-TuC Protein 3 (GCP3)-Interacting Protein 1 (GIP1) is the smallest γ-TuC component identified so far. AtGIP1 and its homologous protein AtGIP2 participate in the localization of active γ-TuCs at interphasic and mitotic MT nucleation sites. Arabidopsis gip1gip2 mutants are impaired in establishing a fully functional mitotic spindle and exhibit severe developmental defects. In this study, gip1gip2 knock down mutants were further characterized at the cellular level. In addition to defects in both the localization of γ-TuC core proteins and MT fiber robustness, gip1gip2 mutants exhibited a severe alteration of the nuclear shape associated with an abnormal distribution of the nuclear pore complexes. Simultaneously, they showed a misorganization of the inner nuclear membrane protein AtSUN1. Furthermore, AtGIP1 was identified as an interacting partner of AtTSA1 which was detected, like the AtGIP proteins, at the NE. These results provide the first evidence for the involvement of a γ-TuC component in both nuclear shaping and NE organization. Functional hypotheses are discussed in order to propose a model for a GIP-dependent nucleo-cytoplasmic continuum.

The human TREX-2 complex is stably associated with the nuclear pore basket

In eukaryotes mRNA export involves many evolutionarily conserved factors that carry the nascent transcript to the nuclear pore complex (NPC). The THO/TREX complex couples transcription to mRNA export and recruits the mRNA export receptor NXF1 for the transport of mRNP particles to the NPC. The transcription and export complex 2 (TREX-2) was suggested to interact with NXF1 and to shuttle between transcription sites and the NPC. Here, we characterize the dynamics of human TREX-2 and show that it stably associates with the NPC basket. Moreover, the association of TREX-2 with the NPC requires the basket nucleoporins NUP153 and TPR, but is independent of transcription. Differential profiles of mRNA nuclear accumulation reveal that TREX-2 functions similarly to basket nucleoporins, but differently from NXF1. Thus, our results show that TREX-2 is an NPC-associated complex in mammalian cells and suggest that it is involved in putative NPC basket-related functions.

Building a nuclear envelope at the end of mitosis: coordinating membrane reorganization, nuclear pore complex assembly, and chromatin de-condensation

The metazoan nucleus is disassembled and re-built at every mitotic cell division. The nuclear envelope, including nuclear pore complexes, breaks down at the beginning of mitosis to accommodate the capture of massively condensed chromosomes by the spindle apparatus. At the end of mitosis, a nuclear envelope is newly formed around each set of segregating and de-condensing chromatin. We review the current understanding of the membrane restructuring events involved in the formation of the nuclear membrane sheets of the envelope, the mechanisms governing nuclear pore complex assembly and integration in the nascent nuclear membranes, and the regulated coordination of these events with chromatin de-condensation.

Nesprin interchain associations control nuclear size

Nesprins-1/-2/-3/-4 are nuclear envelope proteins, which connect nuclei to the cytoskeleton. The largest nesprin-1/-2 isoforms (termed giant) tether F-actin through their N-terminal actin binding domain (ABD). Nesprin-3, however, lacks an ABD and associates instead to plectin, which binds intermediate filaments. Nesprins are integrated into the outer nuclear membrane via their C-terminal KASH-domain. Here, we show that nesprin-1/-2 ABDs physically and functionally interact with nesprin-3. Thus, both ends of nesprin-1/-2 giant are integrated at the nuclear surface: via the C-terminal KASH-domain and the N-terminal ABD-nesprin-3 association. Interestingly, nesprin-2 ABD or KASH-domain overexpression leads to increased nuclear areas. Conversely, nesprin-2 mini (contains the ABD and KASH-domain but lacks the massive nesprin-2 giant rod segment) expression yields smaller nuclei. Nuclear shrinkage is further enhanced upon nesprin-3 co-expression or microfilament depolymerization. Our findings suggest that multivariate intermolecular nesprin interactions with the cytoskeleton form a lattice-like filamentous network covering the outer nuclear membrane, which determines nuclear size.

Division of the intermediate compartment at the onset of mitosis provides a mechanism for Golgi inheritance

As mammalian cells prepare for mitosis, the Golgi ribbon is first unlinked into its constituent stacks and then transformed into spindle-associated, pleiomorphic membrane clusters in a process that remains enigmatic. Also, it remains unclear whether Golgi inheritance involves the incorporation of Golgi enzymes into a pool of coat protein I (COPI) vesicles, or their COPI-independent transfer to the endoplasmic reticulum (ER). Based on the observation that the intermediate compartment (IC) at the ER-Golgi boundary is connected to the centrosome, we examined its mitotic fate and possible role in Golgi breakdown. The use of multiple imaging techniques and markers revealed that the IC elements persist during the M phase, maintain their compositional and structural properties and remain associated with the mitotic spindle, forming circular arrays at the spindle poles. At G2/M transition, the movement of the pericentrosomal domain of the IC (pcIC) to the cell centre and its expansion coincide with the unlinking of the Golgi ribbon. At prophase, coupled to centrosome separation, the pcIC divides together with recycling endosomes, providing novel landmarks for mitotic entry. We provide evidence that the permanent IC elements function as way stations during the COPI-dependent dispersal of Golgi components at prometa- and metaphase, indicating that they correspond to the previously described Golgi clusters. In addition, they continue to communicate with the vesicular 'Golgi haze' and thus are likely to provide templates for Golgi reassembly. These results implicate the IC in mitotic Golgi inheritance, resulting in a model that integrates key features of the two previously proposed pathways.

Microtubule-associated nuclear envelope proteins in interphase and mitosis

The LINC (linker of nucleoskeleton and cytoskeleton) complex forms a transcisternal bridge across the NE (nuclear envelope) that connects the cytoskeleton with the nuclear interior. This enables some proteins of the NE to communicate with the centrosome and the microtubule cytoskeleton. The position of the centrosome relative to the NE is of vital importance for many cell functions, such as cell migration and division, and centrosomal dislocation is a frequent phenotype in laminopathic disorders. Also in mitosis, a small group of transmembrane NE proteins associate with microtubules when they concentrate in a specific membrane domain associated with the mitotic spindle. The present review discusses structural and functional aspects of microtubule association with NE proteins and how this association may be maintained over the cell cycle.

Sm protein down-regulation leads to defects in nuclear pore complex disassembly and distribution in C. elegans embryos

Nuclear pore complexes (NPCs) are large macromolecular structures embedded in the nuclear envelope (NE), where they facilitate exchange of molecules between the cytoplasm and the nucleoplasm. In most cell types, NPCs are evenly distributed around the NE. However, the mechanisms dictating NPC distribution are largely unknown. Here, we used the model organism Caenorhabditis elegans to identify genes that affect NPC distribution during early embryonic divisions. We found that down-regulation of the Sm proteins, which are core components of the spliceosome, but not down-regulation of other splicing factors, led to clustering of NPCs. Down-regulation of Sm proteins also led to incomplete disassembly of NPCs during mitosis, but had no effect on lamina disassembly, suggesting that the defect in NPC disassembly was not due to a general defect in nuclear envelope breakdown. We further found that these mitotic NPC remnants persisted on an ER membrane that juxtaposes the mitotic spindle. At the end of mitosis, the remnant NPCs moved toward the chromatin and the reforming NE, where they ultimately clustered by forming membrane stacks perforated by NPCs. Our results suggest a novel, splicing-independent, role for Sm proteins in NPC disassembly, and point to a possible link between NPC disassembly in mitosis and NPC distribution in the subsequent interphase.

The nuclear envelope environment and its cancer connections

Because of the association between aberrant nuclear structure and tumour grade, nuclear morphology is an indispensible criterion in the current pathological assessment of cancer. Components of the nuclear envelope environment have central roles in many aspects of cell function that affect tumour development and progression. As the roles of the nuclear envelope components, including nuclear pore complexes and nuclear lamina, are being deciphered in molecular detail there are opportunities to harness this knowledge for cancer therapeutics and biomarker development. In this Review, we summarize the progress that has been made in our understanding of the nuclear envelope and the implications of changes in this environment for cancer biology.

Centrioles resist forces applied on centrosomes during G2/M transition

BACKGROUND INFORMATION

Centrosome movements at the onset of mitosis result from a balance between the pulling and pushing forces mediated by microtubules. The structural stability of the centrosome core structure, the centriole pair, is correlated with a heavy polyglutamylation of centriole tubulin.

RESULTS

Using HeLa cells stably expressing centrin-green fluorescent protein as a centriole marker, we monitored the effect of microinjecting an anti-(polyglutamylated tubulin) monoclonal antibody, GT335, in G1/S or G2 cells. In contrast with the slow effect of the monoclonal antibody GT335 during interphase, a dramatic and rapid centrosome fragmentation occurred in cells microinjected in G2 that was both Eg5- and dynein-dependent. Inhibition of either one of these two motors significantly decreased the scattering of centrosome fragments, and inhibition of centrosome segregation by impairing microtubule dynamics abolished centrosome fragmentation.

CONCLUSIONS

Our results demonstrate that the compact structure of the mitotic centrosome is capable of absorbing most of the pulling and pushing forces during G2/M transition and suggest that centrosomes could act as mechanosensors integrating tensions during cell division.

Structure of Sad1-UNC84 homology (SUN) domain defines features of molecular bridge in nuclear envelope

The SUN (Sad1-UNC-84 homology) domain is conserved in a number of nuclear envelope proteins involved in nuclear migration, meiotic telomere tethering, and antiviral responses. The LINC (linker of nucleoskeleton and cytoskeleton) complex, formed by the SUN and the nesprin proteins at the nuclear envelope, serves as a mechanical linkage across the nuclear envelope. Here we report the crystal structure of the SUN2 protein SUN domain, which reveals a homotrimer. The SUN domain is sufficient to mediate binding to the KASH (Klarsicht, ANC-1, and Syne homology) domain of nesprin 2, and the regions involved in the interaction have been identified. Binding of the SUN domain to the KASH domain is abolished by deletion of a region important for trimerization or by point mutations associated with nuclear migration failure. We propose a model of the LINC complex, where the SUN and the KASH domains form a higher ordered oligomeric network in the nuclear envelope. These findings provide the structural basis for understanding the function and the regulation of the LINC complex.

A dominant-negative form of POM121 binds chromatin and disrupts the two separate modes of nuclear pore assembly

Nuclear pore complexes (NPCs) are formed during two separate stages of the metazoan cell cycle. They are assembled into the re-forming nuclear envelope (NE) at the exit from mitosis and into an intact, expanding NE during interphase. Here, we show that a soluble internal fragment of the membrane nucleoporin POM121 has a dominant-negative effect on both modes of assembly in a cell-free reconstitution system. The soluble POM121 fragment binds chromatin at sites that are distinct from ELYS-Nup107-160 'seeding' sites and prevents membrane enclosure and NPC formation. Importin-β negatively regulates chromatin binding by the POM121 fragment through a conserved NLS motif and is also shown to affect the recruitment of the endogenous membrane protein to chromatin in the full assembly system. When an intact NE is present before the addition of the dominant-negative fragment, NPCs are inserted into the NE but membrane expansion is inhibited. This results in densely packed NPCs with no intervening membrane patches, as visualized by scanning electron microscopy. We conclude that POM121 plays an important role in both modes of assembly and links nuclear membrane formation and expansion to nuclear pore biogenesis.

Formation of the postmitotic nuclear envelope from extended ER cisternae precedes nuclear pore assembly

Live-cell imaging and electron tomography show that nuclear pore complexes only assemble on a previously formed nuclear envelope.

During mitosis, the nuclear envelope merges with the endoplasmic reticulum (ER), and nuclear pore complexes are disassembled. In a current model for reassembly after mitosis, the nuclear envelope forms by a reshaping of ER tubules. For the assembly of pores, two major models have been proposed. In the insertion model, nuclear pore complexes are embedded in the nuclear envelope after their formation. In the prepore model, nucleoporins assemble on the chromatin as an intermediate nuclear pore complex before nuclear envelope formation. Using live-cell imaging and electron microscope tomography, we find that the mitotic assembly of the nuclear envelope primarily originates from ER cisternae. Moreover, the nuclear pore complexes assemble only on the already formed nuclear envelope. Indeed, all the chromatin-associated Nup107–160 complexes are in single units instead of assembled prepores. We therefore propose that the postmitotic nuclear envelope assembles directly from ER cisternae followed by membrane-dependent insertion of nuclear pore complexes.

The defective nuclear lamina in Hutchinson-gilford progeria syndrome disrupts the nucleocytoplasmic Ran gradient and inhibits nuclear localization of Ubc9

The mutant form of lamin A responsible for the premature aging disease Hutchinson-Gilford progeria syndrome (termed progerin) acts as a dominant negative protein that changes the structure of the nuclear lamina. How the perturbation of the nuclear lamina in progeria is transduced into cellular changes is undefined. Using patient fibroblasts and a variety of cell-based assays, we determined that progerin expression in Hutchinson-Gilford progeria syndrome inhibits the nucleocytoplasmic transport of several factors with key roles in nuclear function. We found that progerin reduces the nuclear/cytoplasmic concentration of the Ran GTPase and inhibits the nuclear localization of Ubc9, the sole E2 for SUMOylation, and of TPR, the nucleoporin that forms the basket on the nuclear side of the nuclear pore complex. Forcing the nuclear localization of Ubc9 in progerin-expressing cells rescues the Ran gradient and TPR import, indicating that these pathways are linked. Reducing nuclear SUMOylation decreases the nuclear mobility of the Ran nucleotide exchange factor RCC1 in vivo, and the addition of SUMO E1 and E2 promotes the dissociation of RCC1 and Ran from chromatin in vitro. Our data suggest that the cellular effects of progerin are transduced, at least in part, through reduced function of the Ran GTPase and SUMOylation pathways.

The nuclear envelope

The nuclear envelope (NE) is a highly regulated membrane barrier that separates the nucleus from the cytoplasm in eukaryotic cells. It contains a large number of different proteins that have been implicated in chromatin organization and gene regulation. Although the nuclear membrane enables complex levels of gene expression, it also poses a challenge when it comes to cell division. To allow access of the mitotic spindle to chromatin, the nucleus of metazoans must completely disassemble during mitosis, generating the need to re-establish the nuclear compartment at the end of each cell division. Here, I summarize our current understanding of the dynamic remodeling of the NE during the cell cycle.

Genome-nuclear lamina interactions and gene regulation

The nuclear lamina, a filamentous protein network that coats the inner nuclear membrane, has long been thought to interact with specific genomic loci and regulate their expression. Molecular mapping studies have now identified large genomic domains that are in contact with the lamina. Genes in these domains are typically repressed, and artificial tethering experiments indicate that the lamina can actively contribute to this repression. Furthermore, the lamina indirectly controls gene expression in the nuclear interior by sequestration of certain transcription factors. A variety of DNA-binding and chromatin proteins may anchor specific loci to the lamina, while histone-modifying enzymes partly mediate the local repressive effect of the lamina. Experimental tools are now available to begin to unravel the underlying molecular mechanisms.

The intricacy of nuclear membrane dynamics during nucleophagy

The cell nucleus is an organelle bounded by a double-membrane which undergoes drastic reorganization during major cellular events such as cell division and apoptosis. Maintenance of proper nuclear structure, function and dynamics is central to organelle vitality. Over recent years growing evidence has shown that parts of the nucleus can be specifically degraded by an autophagic process termed nucleophagy. The process is best described in the yeast, Saccharomyces cerevisiae, where piecemeal microautophagy of the nucleus or nucleophagy (micronucleophagy) requires direct interaction of the nuclear membrane with that of the vacuole (the yeast lytic compartment). Here, we review the process of nucleophagy in the context of nuclear membrane dynamics, and examine the evidence for autophagic degradation of the nucleus in mammalian cells. Finally, we discuss the importance of nucleophagy as a 'housecleaning' mechanism for the nucleus under both normal and disease conditions.

A moonlighting function of Plasmodium falciparum histone 3, mono-methylated at lysine 9?

Background

In the human malaria parasites Plasmodium falciparum, histone modifications have been implicated in the transcriptional regulation. The acetylation and methylation status of the histones have been linked with transcriptional regulation of the parasite surface virulence factors as well as other genes with stage specific expression. In P. falciparum as well as other eukaryotes, different histone modifications were found to be compartmentalized to distinct regions in the nuclei. This compartmentalization is believed to be one of the main prerequisites for their function in epigenetic regulation of gene expression.

Methodology/Principal Findings

Here we investigate intracellular distributions of five previously uncharacterized histone modifications including histone 4 acetylation on lysine residue 5 (H4K5Ac), H4K8Ac, H3K9Ac, H4Ac4 and H3K9Me1 during the asexual developmental stages. With the exception of H3K9Me1, the modified histones were localized to the nuclear periphery. This provides a strong indication that the P. falciparum nuclear periphery is one of the most active regions in epigenetic regulation of gene expression. Interestingly, H3K9Me1 is not associated with the nuclei but instead resides in the parasitophorous vacuole (PV), the double membrane compartments surrounding the parasite cell within the host erythrocyte. In this compartment, H3K9Me1 partially co-localizes with Etramp proteins. The localization of H3K9Me1 in the PV is conserved in the other species including P. yoelii and P. vivax.

Conclusions

Similar to other eukaryotes, the periphery of the P. falciparum nuclei is likely one of the most active areas in epigenetic regulation of gene expression involving multiple histone modifications. On the other hand, H3K9Me1 evolved a new function that is linked with the PV. This functional role appears to be evolutionarily conserved in Plasmodium species.

Lipin is required for efficient breakdown of the nuclear envelope in Caenorhabditis elegans

The nuclear envelope (NE) is a double lipid bilayer that separates nucleus and cytoplasm. In metazoa, NE breakdown (NEBD) occurs during prophase and NE reformation around segregated chromatids occurs at anaphase-telophase. We identified Caenorhabditis elegans Lipin homologue (called Lpin-1) as an essential factor with roles in endoplasmic reticulum (ER) organization and NEBD. RNAi-mediated downregulation of Lpin-1 had no effect on timely entry into mitosis or on the early steps of NEBD, but Lpin-1 was required for disassembly of the nuclear lamina during late NEBD. This Lpin-1 requirement appears to be separable from the effect of Lpin-1 on the peripheral ER.

Transportin regulates major mitotic assembly events: from spindle to nuclear pore assembly

Mitosis in higher eukaryotes is marked by the sequential assembly of two massive structures: the mitotic spindle and the nucleus. Nuclear assembly itself requires the precise formation of both nuclear membranes and nuclear pore complexes. Previously, importin alpha/beta and RanGTP were shown to act as dueling regulators to ensure that these assembly processes occur only in the vicinity of the mitotic chromosomes. We now find that the distantly related karyopherin, transportin, negatively regulates nuclear envelope fusion and nuclear pore assembly in Xenopus egg extracts. We show that transportin-and importin beta-initiate their regulation as early as the first known step of nuclear pore assembly: recruitment of the critical pore-targeting nucleoporin ELYS/MEL-28 to chromatin. Indeed, each karyopherin can interact directly with ELYS. We further define the nucleoporin subunit targets for transportin and importin beta and find them to be largely the same: ELYS, the Nup107/160 complex, Nup53, and the FG nucleoporins. Equally importantly, we find that transportin negatively regulates mitotic spindle assembly. These negative regulatory events are counteracted by RanGTP. We conclude that the interplay of the two negative regulators, transportin and importin beta, along with the positive regulator RanGTP, allows precise choreography of multiple cell cycle assembly events.

Recruitment of functionally distinct membrane proteins to chromatin mediates nuclear envelope formation in vivo

Formation of the nuclear envelope (NE) around segregated chromosomes occurs by the reshaping of the endoplasmic reticulum (ER), a reservoir for disassembled nuclear membrane components during mitosis. In this study, we show that inner nuclear membrane proteins such as lamin B receptor (LBR), MAN1, Lap2beta, and the trans-membrane nucleoporins Ndc1 and POM121 drive the spreading of ER membranes into the emerging NE via their capacity to bind chromatin in a collaborative manner. Despite their redundant functions, decreasing the levels of any of these trans-membrane proteins by RNAi-mediated knockdown delayed NE formation, whereas increasing the levels of any of them had the opposite effect. Furthermore, acceleration of NE formation interferes with chromosome separation during mitosis, indicating that the time frame over which chromatin becomes membrane enclosed is physiologically relevant and regulated. These data suggest that functionally distinct classes of chromatin-interacting membrane proteins, which are present at nonsaturating levels, collaborate to rapidly reestablish the nuclear compartment at the end of mitosis.

Importin beta regulates the seeding of chromatin with initiation sites for nuclear pore assembly

The nuclear envelope of higher eukaryotic cells reforms at the exit from mitosis, in concert with the assembly of nuclear pore complexes (NPCs). The first step in postmitotic NPC assembly involves the "seeding" of chromatin with ELYS and the Nup107-160 complex. Subsequent steps in the assembly process are poorly understood and different mechanistic models have been proposed to explain the formation of the full supramolecular structure. Here, we show that the initial step of chromatin seeding is negatively regulated by importin beta. Direct imaging of the chromatin attachment sites reveals single sites situated predominantly on the highest substructures of chromatin surface and lacking any sign of annular structures or oligomerized pre-NPCs. Surprisingly, the inhibition by importin beta is only partially reversed by RanGTP. Importin beta forms a high-molecular-weight complex with both ELYS and the Nup107-160 complex in cytosol. We suggest that initiation sites for NPC assembly contain single copies of chromatin-bound ELYS/Nup107-160 and that the lateral oligomerization of these subunits depends on the recruitment of membrane components. We predict that additional regulators, besides importin beta and Ran, may be involved in coordinating the initial seeding of chromatin with subsequent steps in the NPC assembly pathway.

Nuclear envelope-limited chromatin sheets (ELCS) and heterochromatin higher order structure

The interphase nucleus and nuclear envelope can acquire a myriad of shapes in normal or pathological cell states. There exist a wide variety of indentations and invaginations, of protrusions and evaginations. It has been difficult to classify and name all of these nuclear shapes and, consequently, a barrier to understanding the biochemical and biophysical causes. This review focuses upon one type of nuclear envelope shape change, named "nuclear envelope-limited chromatin sheets" (ELCS), which appears to involve exaggerated nuclear envelope growth, carrying with it one or more layers of approximately 30 nm diameter heterochromatin. A hypothesis on the formation of ELCS is proposed, relating higher order heterochromatin structure in an interphase nucleus, nuclear envelope growth, and nuclear envelope-heterochromatin interactions.

Lipins, lipids and nuclear envelope structure

The lipid composition of biological membranes is crucial for many aspects of organelle function, including growth, signalling, and transport. Lipins represent a novel family of lipid phosphatases that dephosphorylate phosphatidic acid (PA) to produce diacylglycerol (DAG), and perform key functions in phospholipid and triacylglycerol biosynthesis and gene expression. In addition to its role in lipid biosynthesis, the yeast lipin Pah1p and its regulators are required for the maintenance of a spherical nuclear shape. This review summarizes recent advances in our understanding of the yeast lipin Pah1p and highlights the possible roles of phospholipid metabolism in nuclear membrane biogenesis.

Orchestrating nuclear envelope disassembly and reassembly during mitosis

Cell division in eukaryotes requires extensive architectural changes of the nuclear envelope (NE) to ensure that segregated DNA is finally enclosed in a single cell nucleus in each daughter cell. Higher eukaryotic cells have evolved 'open' mitosis, the most extreme mechanism to solve the problem of nuclear division, in which the NE is initially completely disassembled and then reassembled in coordination with DNA segregation. Recent progress in the field has now started to uncover mechanistic and molecular details that underlie the changes in NE reorganization during open mitosis. These studies reveal a tight interplay between NE components and the mitotic machinery.

Assembly of nuclear pore complexes mediated by major vault protein

During interphase growth of eukaryotic cells, nuclear pore complexes (NPCs) are continuously incorporated into the intact nuclear envelope (NE) by mechanisms that are largely unknown. De novo formation of NPCs involves local fusion events between the inner and outer nuclear membrane, formation of a transcisternal membranous channel of defined diameter and the coordinated assembly of hundreds of nucleoporins into the characteristic NPC structure. Here we have used a cell-free system based on Xenopus egg extract, which allows the experimental separation of nuclear-membrane assembly and NPC formation. Nuclei surrounded by a closed double nuclear membrane, but devoid of NPCs, were first reconstituted from chromatin and a specific membrane fraction. Insertion of NPCs into the preformed pore-free nuclei required cytosol containing soluble nucleoporins or nucleoporin subcomplexes and, quite unexpectedly, major vault protein (MVP). MVP is the main component of vaults, which are ubiquitous barrel-shaped particles of enigmatic function. Our results implicate MVP, and thus also vaults, in NPC biogenesis and provide a functional explanation for the association of a fraction of vaults with the NE and specifically with NPCs in intact cells.

Nucleic acid binding of the RTN1-C C-terminal region: toward the functional role of a reticulon protein

RTN1-C protein is a membrane protein localized in the ER and expressed in the nervous system. Its biological role is still unclear, although interactions of the N-terminal region of RTN1-C with proteins involved in vesicle trafficking have been observed, but the role of the C-terminal region of this family protein remains to be investigated. By a homology analysis of the amino acid sequence, we identified in the C-terminal region of RTN1-C a unique consensus sequence characteristic of H4 histone protein. Thus, a 23-mer peptide (RTN1-C(CT)) corresponding to residues 186-208 of RTN1-C was synthesized, and its conformation and its interaction with nucleic acids were investigated. Here we demonstrate the strong ability of RTN1-C(CT) peptide to bind and condense the nucleic acids using electrophoretic and spectroscopic techniques. To determine if the binding of RTN1-C to nucleic acids could be regulated in vivo by an acetylation-deacetylation mechanism, as for the histone proteins, we studied the interaction of RTN1-C with one zinc-dependent histone deacetylase (HDAC) enzyme, HDAC8, with fluorescence and kinetic techniques using an acetylated form of RTN1-C(CT). The results reported here allow us to propose that the nucleic acid binding property of RTN1-C may have an important role in the biological function of this protein, the function of which could be regulated by an acetylation-deacetylation mechanism.

Age-dependent deterioration of nuclear pore complexes causes a loss of nuclear integrity in postmitotic cells

In dividing cells, nuclear pore complexes (NPCs) disassemble during mitosis and reassemble into the newly forming nuclei. However, the fate of nuclear pores in postmitotic cells is unknown. Here, we show that NPCs, unlike other nuclear structures, do not turn over in differentiated cells. While a subset of NPC components, like Nup153 and Nup50, are continuously exchanged, scaffold nucleoporins, like the Nup107/160 complex, are extremely long-lived and remain incorporated in the nuclear membrane during the entire cellular life span. Besides the lack of nucleoporin expression and NPC turnover, we discovered an age-related deterioration of NPCs, leading to an increase in nuclear permeability and the leaking of cytoplasmic proteins into the nucleus. Our finding that nuclear "leakiness" is dramatically accelerated during aging and that a subset of nucleoporins is oxidatively damaged in old cells suggests that the accumulation of damage at the NPC might be a crucial aging event.

The nucleoporin Nup153 has separable roles in both early mitotic progression and the resolution of mitosis

Accurate inheritance of genomic content during cell division is dependent on synchronized changes in cellular organization and chromosome dynamics. Elucidating how these events are coordinated is necessary for a complete understanding of cell proliferation. Previous in vitro studies have suggested that the nuclear pore protein Nup153 is a good candidate for participating in mitotic coordination. To decipher whether this is the case in mammalian somatic cells, we reduced the levels of Nup153 in HeLa cells and monitored consequences on cell growth. Reduction of Nup153 resulted in a delay during the late stages of mitosis accompanied by an increase in unresolved midbodies. Depletion of Nup153 to an even lower threshold led to a pronounced defect early in mitosis and an accumulation of cells with multilobed nuclei. Although global nucleocytoplasmic transport was not significantly altered under these depletion conditions, the FG-rich region of Nup153 was required to rescue defects in late mitosis. Thus, this motif may play a specialized role as cells exit mitosis. Rescue of the multilobed nuclei phenotype, in contrast, was independent of the FG-domain, revealing two separable roles for Nup153 in the execution of mitosis.

Knockdown of p180 eliminates the terminal differentiation of a secretory cell line

We have previously reported that the expression in yeast of an integral membrane protein (p180) of the endoplasmic reticulum (ER), isolated for its ability to mediate ribosome binding, is capable of inducing new membrane biogenesis and an increase in secretory capacity. To demonstrate that p180 is necessary and sufficient for terminal differentiation and acquisition of a secretory phenotype in mammalian cells, we studied the differentiation of a secretory cell line where p180 levels had been significantly reduced using RNAi technology and by transiently expressing p180 in nonsecretory cells. A human monocytic (THP-1) cell line, that can acquire macrophage-like properties, failed to proliferate rough ER when p180 levels were lowered. The Golgi compartment and the secretion of apolipoprotein E (Apo E) were dramatically affected in cells expressing reduced p180 levels. On the other hand, expression of p180 in a human embryonic kidney nonsecretory cell line (HEK293) showed a significant increase in proliferation of rough ER membranes and Golgi complexes. The results obtained from knockdown and overexpression experiments demonstrate that p180 is both necessary and sufficient to induce a secretory phenotype in mammalian cells. These findings support a central role for p180 in the terminal differentiation of secretory cells and tissues.

The classical nuclear localization signal receptor, importin-alpha, is required for efficient transition through the G1/S stage of the cell cycle in Saccharomyces cerevisiae

There is significant evidence linking nucleocytoplasmic transport to cell cycle control. The budding yeast, Saccharomyces cerevisiae, serves as an ideal model system for studying transport events critical to cell cycle progression because the nuclear envelope remains intact throughout the cell cycle. Previous studies linked the classical nuclear localization signal (cNLS) receptor, importin-alpha/Srp1, to the G(2)/M transition of the cell cycle. Here, we utilize two engineered mutants of importin-alpha/Srp1 with specific molecular defects to explore how protein import affects cell cycle progression. One mutant, Srp1-E402Q, is defective in binding to cNLS cargoes that contain two clusters of basic residues termed a bipartite cNLS. The other mutant, Srp1-55, has defects in release of cNLS cargoes into the nucleus. Consistent with distinct in vivo functional consequences for each of the Srp1 mutants analyzed, we find that overexpression of different nuclear transport factors can suppress the temperature-sensitive growth defects of each mutant. Studies aimed at understanding how each of these mutants affects cell cycle progression reveal a profound defect at the G(1) to S phase transition in both srp1-E402Q and srp1-55 mutants as well as a modest G(1)/S defect in the temperature-sensitive srp1-31 mutant, which was previously implicated in G(2)/M. We take advantage of the characterized defects in the srp1-E402Q and srp1-55 mutants to predict candidate cargo proteins likely to be affected in these mutants and provide evidence that three of these cargoes, Cdc45, Yox1, and Mcm10, are not efficiently localized to the nucleus in importin-alpha mutants. These results reveal that the classical nuclear protein import pathway makes important contributions to the G(1)/S cell cycle transition.

Mutation of the zebrafish nucleoporin elys sensitizes tissue progenitors to replication stress

The recessive lethal mutation flotte lotte (flo) disrupts development of the zebrafish digestive system and other tissues. We show that flo encodes the ortholog of Mel-28/Elys, a highly conserved gene that has been shown to be required for nuclear integrity in worms and nuclear pore complex (NPC) assembly in amphibian and mammalian cells. Maternal elys expression sustains zebrafish flo mutants to larval stages when cells in proliferative tissues that lack nuclear pores undergo cell cycle arrest and apoptosis. p53 mutation rescues apoptosis in the flo retina and optic tectum, but not in the intestine, where the checkpoint kinase Chk2 is activated. Chk2 inhibition and replication stress induced by DNA synthesis inhibitors were lethal to flo larvae. By contrast, flo mutants were not sensitized to agents that cause DNA double strand breaks, thus showing that loss of Elys disrupts responses to selected replication inhibitors. Elys binds Mcm2-7 complexes derived from Xenopus egg extracts. Mutation of elys reduced chromatin binding of Mcm2, but not binding of Mcm3 or Mcm4 in the flo intestine. These in vivo data indicate a role for Elys in Mcm2-chromatin interactions. Furthermore, they support a recently proposed model in which replication origins licensed by excess Mcm2-7 are required for the survival of human cells exposed to replication stress.

DNA replication is a complex process that requires activation of cell cycle checkpoints and DNA repair pathways. Genetic analyses in fungi have suggested that nucleoporins, the proteins that make up the nuclear pore complex (NPC), play a role in the cellular response to agents that disrupt cell proliferation or damage DNA. Here we show that mutation of the Elys nucleoporin causes widespread apoptosis in the intestine and other tissues of zebrafish flotte lotte (flo) mutants. Intestinal apoptosis occurs in the absence of the DNA damage marker γH2X, and levels of chromatin bound Mcm2, a component of the DNA replication helicase, were also reduced in flo mutants. These findings suggested that flo intestinal cells cannot repair endogenous replication errors. Consistent with this idea, flo mutants were highly sensitized to treatment with DNA replication inhibitors such as hydroxyurea, UV irradiation, or cisplatin, but not agents that cause DNA double strand breaks, such as γ-irradiation or camptothecin. These data point to a conserved role for nucleoporins in the cellular response to replication stress in eukaryote cells.

Structure, dynamics and function of nuclear pore complexes

Nuclear pore complexes are large aqueous channels that penetrate the nuclear envelope, thereby connecting the nuclear interior with the cytoplasm. Until recently, these macromolecular complexes were viewed as static structures, the only function of which was to control the molecular trafficking between the two compartments. It has now become evident that this simplistic scenario is inaccurate and that nuclear pore complexes are highly dynamic multiprotein assemblies involved in diverse cellular processes ranging from the organization of the cytoskeleton to gene expression. In this review, we discuss the most recent developments in the nuclear-pore-complex field, focusing on the assembly, disassembly, maintenance and function of this macromolecular structure.

Granulocytic nuclear differentiation of lamin B receptor-deficient mouse EPRO cells

OBJECTIVE

Lamin B receptor (LBR) is an integral protein of the inner nuclear membrane. Recent studies have demonstrated that genetic deficiency of LBR during granulopoiesis results in hypolobulation of the mature neutrophil nucleus, as observed in human Pelger-Huët anomaly and mouse ichthyosis (ic). In this study, we utilized differentiated early promyelocytes (EPRO cells) that were derived from the bone marrow of homozygous and heterozygous ichthyosis mice to examine changes to the expression of nuclear envelope proteins and heterochromatin structure that result from deficient LBR expression.

MATERIALS AND METHODS

Wild-type (+/+), heterozygous (+/ic), and homozygous (ic/ic) granulocytic forms of EPRO cells were analyzed for the expression of multiple lamins and inner nuclear envelope proteins by immunostaining and immunoblotting techniques. The heterochromatin architecture was also examined by immunostaining for histone lysine methylation.

RESULTS

Wild-type (+/+) and heterozygous (+/ic) granulocytic forms revealed ring-shaped nuclei and contained LBR within the nuclear envelope; ic/ic granulocytes exhibited smaller ovoid nuclei devoid of LBR. The pericentric heterochromatin of undifferentiated and granulocytic ic/ic cells was condensed into larger spots and shifted away from the nuclear envelope, compared to +/+ and +/ic cell forms. Lamin A/C, which is normally not present in mature granulocytes, was significantly elevated in LBR-deficient EPRO cells.

CONCLUSIONS

Our observations suggest roles for LBR during granulopoiesis, which can involve augmenting nuclear membrane growth, facilitating compartmentalization of heterochromatin, and promoting downregulation of lamin A/C expression.

Capture of AT-rich chromatin by ELYS recruits POM121 and NDC1 to initiate nuclear pore assembly

Assembly of the nuclear pore, gateway to the genome, from its component subunits is a complex process. In higher eukaryotes, nuclear pore assembly begins with the binding of ELYS/MEL-28 to chromatin and recruitment of the large critical Nup107-160 pore subunit. The choreography of steps that follow is largely speculative. Here, we set out to molecularly define early steps in nuclear pore assembly, beginning with chromatin binding. Point mutation analysis indicates that pore assembly is exquisitely sensitive to the change of only two amino acids in the AT-hook motif of ELYS. The dependence on AT-rich chromatin for ELYS binding is borne out by the use of two DNA-binding antibiotics. AT-binding Distamycin A largely blocks nuclear pore assembly, whereas GC-binding Chromomycin A(3) does not. Next, we find that recruitment of vesicles containing the key integral membrane pore proteins POM121 and NDC1 to the forming nucleus is dependent on chromatin-bound ELYS/Nup107-160 complex, whereas recruitment of gp210 vesicles is not. Indeed, we reveal an interaction between the cytoplasmic domain of POM121 and the Nup107-160 complex. Our data thus suggest an order for nuclear pore assembly of 1) AT-rich chromatin sites, 2) ELYS, 3) the Nup107-160 complex, and 4) POM121- and NDC1-containing membrane vesicles and/or sheets, followed by (5) assembly of the bulk of the remaining soluble pore subunits.

In vivo dynamics of Drosophila nuclear envelope components

Nuclear pore complexes (NPCs) are multisubunit protein entities embedded into the nuclear envelope (NE). Here, we examine the in vivo dynamics of the essential Drosophila nucleoporin Nup107 and several other NE-associated proteins during NE and NPCs disassembly and reassembly that take place within each mitosis. During both the rapid mitosis of syncytial embryos and the more conventional mitosis of larval neuroblasts, Nup107 is gradually released from the NE, but it remains partially confined to the nuclear (spindle) region up to late prometaphase, in contrast to nucleoporins detected by wheat germ agglutinin and lamins. We provide evidence that in all Drosophila cells, a structure derived from the NE persists throughout metaphase and early anaphase. Finally, we examined the dynamics of the spindle checkpoint proteins Mad2 and Mad1. During mitotic exit, Mad2 and Mad1 are actively imported back from the cytoplasm into the nucleus after the NE and NPCs have reformed, but they reassociate with the NE only later in G1, concomitantly with the recruitment of the basket nucleoporin Mtor (the Drosophila orthologue of vertebrate Tpr). Surprisingly, Drosophila Nup107 shows no evidence of localization to kinetochores, despite the demonstrated importance of this association in mammalian cells.

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.