Nup84, a novel nucleoporin that is associated with CAN/Nup214 on the cytoplasmic face of the nuclear pore complex

TDP43 aggregation at ER-exit sites impairs ER-to-Golgi transport

Protein aggregation plays key roles in age-related degenerative diseases, but how different proteins coalesce to form inclusions that vary in composition, morphology, molecular dynamics and confer physiological consequences is poorly understood. Here we employ a general reporter based on mutant Hsp104 to identify proteins forming aggregates in human cells under common proteotoxic stress. We identify over 300 proteins that form different inclusions containing subsets of aggregating proteins. In particular, TDP43, implicated in Amyotrophic Lateral Sclerosis (ALS), partitions dynamically between two distinct types of aggregates: stress granule and a previously unknown non-dynamic (solid-like) inclusion at the ER exit sites (ERES). TDP43-ERES co-aggregation is induced by diverse proteotoxic stresses and observed in the motor neurons of ALS patients. Such aggregation causes retention of secretory cargos at ERES and therefore delays ER-to-Golgi transport, providing a link between TDP43 aggregation and compromised cellular function in ALS patients.

From the sideline: Tissue-specific nucleoporin function in health and disease, an update

The subcellular compartmentalisation of eukaryotic cells requires selective exchange between the cytoplasm and the nucleus. Intact nucleocytoplasmic transport is vital for normal cell function and mutations in the executing machinery have been causally linked to human disease. Central players in nucleocytoplasmic exchange are nuclear pore complexes (NPCs), which are built from ~30 distinct proteins collectively termed nucleoporins. Aberrant nucleoporin expression was detected in human cancers and autoimmune diseases since quite some time, while it was through the increasing use of next generation sequencing that mutations in nucleoporin genes associated with mainly rare hereditary diseases were revealed. The number of newly identified mutations is steadily increasing, as is the number of diseases. Mutational hotspots have emerged: mutations in the scaffold nucleoporins seemingly affect primarily inner organs, such as heart, kidney, and ovaries, whereas genetic alterations in peripheral, cytoplasmic nucleoporins affect primarily the central nervous system and development. In this review, we summarise latest insights on altered nucleoporin function in the context of human hereditary disorders, with a focus on those where mechanistic insights are beginning to emerge.

Cryo-electron microscopy reveals the structure of the nuclear pore complex

The nuclear pore complex (NPC) is a giant protein assembly that penetrates the double layers of the nuclear membrane. The overall structure of the NPC has approximately eightfold symmetry and is formed by approximately 30 nucleoporins. The great size and complexity of the NPC have hindered the study of its structure for many years until recent breakthroughs were achieved by integrating the latest high-resolution cryo-electron microscopy (cryo-EM), the emerging artificial intelligence-based modeling and all other available structural information from crystallography and mass spectrometry. Here, we review our latest knowledge of the NPC architecture and the history of its structural study from in vitro to in situ with progressively improved resolutions by cryo-EM, with a particular focus on the latest subnanometer-resolution structural studies. The future directions for structural studies of NPCs are also discussed.

Overexpressed Nup88 stabilized through interaction with Nup62 promotes NF-κB dependent pathways in cancer

Bidirectional nucleo-cytoplasmic transport, regulating several vital cellular processes, is mediated by the Nuclear Pore Complex (NPC) comprising the nucleoporin (Nup) proteins. Nup88, a constituent nucleoporin, is overexpressed in many cancers, and a positive correlation exists between progressive stages of cancer and Nup88 levels. While a significant link of Nup88 overexpression in head and neck cancer exists but mechanistic details of Nup88 roles in tumorigenesis are sparse. Here, we report that Nup88 and Nup62 levels are significantly elevated in head and neck cancer patient samples and cell lines. We demonstrate that the elevated levels of Nup88 or Nup62 impart proliferation and migration advantages to cells. Interestingly, Nup88-Nup62 engage in a strong interaction independent of Nup-glycosylation status and cell-cycle stages. We report that the interaction with Nup62 stabilizes Nup88 by inhibiting the proteasome-mediated degradation of overexpressed Nup88. Overexpressed Nup88 stabilized by interaction with Nup62 can interact with NF-κB (p65) and sequesters p65 partly into nucleus of unstimulated cells. NF-κB targets like Akt, c-myc, IL-6 and BIRC3 promoting proliferation and growth are induced under Nup88 overexpression conditions. In conclusion, our data indicates that simultaneous overexpression of Nup62 and Nup88 in head and neck cancer stabilizes Nup88. Stabilized Nup88 interacts and activates p65 pathway, which perhaps is the underlying mechanism in Nup88 overexpressing tumors.

Phase separation of FG-nucleoporins in nuclear pore complexes

The nuclear envelope (NE) is a bilayer membrane that separates and physically isolates the genetic material from the cytoplasm. Nuclear pore complexes (NPCs) are cylindrical structures embedded in the NE and remain the sole channel of communication between the nucleus and the cytoplasm. The interior of NPCs contains densely packed intrinsically disordered FG-nucleoporins (FG-Nups), consequently forming a permeability barrier. This barrier facilitates the selection and specificity of the cargoes that are imported, exported, or shuttled through the NPCs. Recent studies have revealed that FG-Nups undergo the process of liquid-liquid phase separation into liquid droplets. Moreover, these liquid droplets mimic the permeability barrier observed in the interior of NPCs. This review highlights the phase separation of FG-Nups occurring inside the NPCs rooted in the NE. We discuss the phase separation of FG-Nups and compare the different aspects contributing to their phase separation. Furthermore, several diseases caused by the aberrant phase separation of the proteins are examined with respect to NEs. By understanding the fundamental process of phase separation at the nuclear membrane, the review seeks to explore the parameters influencing this phenomenon as well as its importance, ultimately paving the way for better research on the structure-function relationship of biomolecular condensates.

A nuclear export sequence promotes CRM1-dependent targeting of the nucleoporin Nup214 to the nuclear pore complex

Nup214 is a major nucleoporin on the cytoplasmic side of the nuclear pore complex with roles in late steps of nuclear protein and mRNA export. It interacts with the nuclear export receptor CRM1 (also known as XPO1) via characteristic phenylalanine-glycine (FG) repeats in its C-terminal region. Here, we identify a classic nuclear export sequence (NES) in Nup214 that mediates Ran-dependent binding to CRM1. Nup214 versions with mutations in the NES, as well as wild-type Nup214 in the presence of the selective CRM1 inhibitor leptomycin B, accumulate in the nucleus of Nup214-overexpressing cells. Furthermore, physiological binding partners of Nup214, such as Nup62 and Nup88, are recruited to the nucleus together with Nup214. Nuclear export of mutant Nup214 can be rescued by artificial nuclear export sequences at the C-terminal end of Nup214, leading also to a correct localization of Nup88. Our results suggest a function of the Nup214 NES in the biogenesis of the nuclear pore complex and/or in terminal steps of CRM1-dependent protein export.

Prevalence and functionality of intrinsic disorder in human FG-nucleoporins

The nuclear-cytoplasmic transport of biomolecules is assisted by the nuclear pores composed of evolutionarily conserved proteins termed nucleoporins (Nups). The central Nups, characterized by multiple FG-repeats, are highly dynamic and contain a high level of intrinsically disordered regions (IDPRs). FG-Nups bind several protein partners and play critical roles in molecular interactions and the regulation of cellular functions through their IDPRs. In the present study, we performed a multiparametric bioinformatics analysis to characterize the prevalence and functionality of IDPRs in human FG-Nups. These analyses revealed that the sequence of all FG-Nups contained >50% IDPRs (except Nup54 and Nup358). Nup98, Nup153, and POM121 were extremely disordered with ~80% IDPRs. The functional disorder-based binding regions in the FG-Nups were identified. The phase separation behavior of FG-Nups indicated that all FG-Nups have the potential to undergo liquid-to-liquid phase separation that could stabilize their liquid state. The inherent structural flexibility in FG-Nups is mechanistically and functionally advantageous. Since certain FG-Nups interact with disease-relevant protein aggregates, their complexes can be exploited for drug design. Furthermore, consideration of the FG-Nups from the intrinsic disorder perspective provides critical information that can guide future experimental studies to uncover novel pathways associated with diseases linked with protein misfolding and aggregation.

Multi-layered proteomic analyses decode compositional and functional effects of cancer mutations on kinase complexes

Rapidly increasing availability of genomic data and ensuing identification of disease associated mutations allows for an unbiased insight into genetic drivers of disease development. However, determination of molecular mechanisms by which individual genomic changes affect biochemical processes remains a major challenge. Here, we develop a multilayered proteomic workflow to explore how genetic lesions modulate the proteome and are translated into molecular phenotypes. Using this workflow we determine how expression of a panel of disease-associated mutations in the Dyrk2 protein kinase alter the composition, topology and activity of this kinase complex as well as the phosphoproteomic state of the cell. The data show that altered protein-protein interactions caused by the mutations are associated with topological changes and affected phosphorylation of known cancer driver proteins, thus linking Dyrk2 mutations with cancer-related biochemical processes. Overall, we discover multiple mutation-specific functionally relevant changes, thus highlighting the extensive plasticity of molecular responses to genetic lesions.

Nucleocytoplasmic shuttling of the glucocorticoid receptor is influenced by tetratricopeptide repeat-containing proteins

It has been demonstrated that tetratricopeptide-repeat (TPR) domain proteins regulate the subcellular localization of glucocorticoid receptor (GR). This study analyses the influence of the TPR domain of high molecular weight immunophilins in the retrograde transport and nuclear retention of GR. Overexpression of the TPR peptide prevented efficient nuclear accumulation of the GR by disrupting the formation of complexes with the dynein-associated immunophilin FKBP52 (also known as FKBP4), the adaptor transporter importin-β1 (KPNB1), the nuclear pore-associated glycoprotein Nup62 and nuclear matrix-associated structures. We also show that nuclear import of GR was impaired, whereas GR nuclear export was enhanced. Interestingly, the CRM1 (exportin-1) inhibitor leptomycin-B abolished the effects of TPR peptide overexpression, although the drug did not inhibit GR nuclear export itself. This indicates the existence of a TPR-domain-dependent mechanism for the export of nuclear proteins. The expression balance of those TPR domain proteins bound to the GR-Hsp90 complex may determine the subcellular localization and nucleocytoplasmic properties of the receptor, and thereby its pleiotropic biological properties in different tissues and cell types.

NUP214 deficiency causes severe encephalopathy and microcephaly in humans

Nuclear pore complex (NPC) is a fundamental component of the nuclear envelope and is key to the nucleocytoplasmic transport. Mutations in several NUP genes that encode individual components of NPC known as nucleoporins have been identified in recent years among patients with static encephalopathies characterized by developmental delay and microcephaly. We describe a multiplex consanguineous family in which four affected members presented with severe neonatal hypotonia, profound global developmental delay, progressive microcephaly and early death. Autozygome and linkage analysis revealed that this phenotype is linked to a founder disease haplotype (chr9:127,113,732-135,288,807) in which whole exome sequencing revealed the presence of a novel homozygous missense variant in NUP214. Functional analysis of patient-derived fibroblasts recapitulated the dysmorphic phenotype of nuclei that was previously described in NUP214 knockdown cells. In addition, the typical rim staining of NUP214 is largely displaced, further supporting the deleterious effect of the variant. Our data expand the list of NUP genes that are mutated in encephalopathy disorders in humans.

Biallelic mutations in nucleoporin NUP88 cause lethal fetal akinesia deformation sequence

Nucleoporins build the nuclear pore complex (NPC), which, as sole gate for nuclear-cytoplasmic exchange, is of outmost importance for normal cell function. Defects in the process of nucleocytoplasmic transport or in its machinery have been frequently described in human diseases, such as cancer and neurodegenerative disorders, but only in a few cases of developmental disorders. Here we report biallelic mutations in the nucleoporin NUP88 as a novel cause of lethal fetal akinesia deformation sequence (FADS) in two families. FADS comprises a spectrum of clinically and genetically heterogeneous disorders with congenital malformations related to impaired fetal movement. We show that genetic disruption of nup88 in zebrafish results in pleiotropic developmental defects reminiscent of those seen in affected human fetuses, including locomotor defects as well as defects at neuromuscular junctions. Phenotypic alterations become visible at distinct developmental stages, both in affected human fetuses and in zebrafish, whereas early stages of development are apparently normal. The zebrafish phenotypes caused by nup88 deficiency are rescued by expressing wild-type Nup88 but not the disease-linked mutant forms of Nup88. Furthermore, using human and mouse cell lines as well as immunohistochemistry on fetal muscle tissue, we demonstrate that NUP88 depletion affects rapsyn, a key regulator of the muscle nicotinic acetylcholine receptor at the neuromuscular junction. Together, our studies provide the first characterization of NUP88 in vertebrate development, expand our understanding of the molecular events causing FADS, and suggest that variants in NUP88 should be investigated in cases of FADS.

Fetal movement is a prerequisite for normal fetal development and growth. Fetal akinesia deformation sequence (FADS) is the result of decreased fetal movement coinciding with congenital malformations related to impaired fetal movement. FADS may be caused by heterogenous defects at any point along the motor system pathway and genes encoding components critical to the neuromuscular junction and acetylcholine receptor clustering represent a major class of FADS disease genes. We report here biallelic, loss-of-function mutations in the nucleoporin NUP88 that result in lethal FADS and with this the first lethal human developmental disorder due to mutations in a nucleoporin gene. We show that loss of Nup88 in zebrafish results in defects reminiscent of those seen in affected human fetuses and loss of NUP88 affects distinct developmental stages, both during human and zebrafish development. Consistent with the notion that a primary cause for FADS is impaired formation of the neuromuscular junction, loss of Nup88 in zebrafish coincides with abnormalities in acetylcholine receptor clustering, suggesting that defective NUP88 function in FADS impairs neuromuscular junction formation.

Moonlighting nuclear pore proteins: tissue-specific nucleoporin function in health and disease

The nuclear pore complex is the main transportation hub for exchange between the cytoplasm and the nucleus. It is built from nucleoporins that form distinct subcomplexes to establish this huge protein complex in the nuclear envelope. Malfunctioning of nucleoporins is well known in human malignancies, such as gene fusions of NUP214 and NUP98 in hematological neoplasms and overexpression of NUP88 in a variety of human cancers. In the past decade, the incremental utilization of next-generation sequencing has unraveled mutations in nucleoporin genes in the context of an increasing number of hereditary diseases, often in a tissue-specific manner. It emerges that, on one hand, the central nervous system and the heart are particularly sensitive to mutations in nucleoporin genes. On the other hand, nucleoporins forming the scaffold structure of the nuclear pore complex are eminently mutation-prone. These novel and exciting associations between nucleoporins and human diseases emphasize the need to shed light on these unanticipated tissue-specific roles of nucleoporins that may go well beyond their role in nucleocytoplasmic transport. In this review, the current insights into altered nucleoporin function associated with human hereditary disorders will be discussed.

The Oncogenic Fusion Proteins SET-Nup214 and Sequestosome-1 (SQSTM1)-Nup214 Form Dynamic Nuclear Bodies and Differentially Affect Nuclear Protein and Poly(A)+ RNA Export

Genetic rearrangements are a hallmark of several forms of leukemia and can lead to oncogenic fusion proteins. One example of an affected chromosomal region is the gene coding for Nup214, a nucleoporin that localizes to the cytoplasmic side of the nuclear pore complex (NPC). We investigated two such fusion proteins, SET-Nup214 and SQSTM1 (sequestosome)-Nup214, both containing C-terminal portions of Nup214. SET-Nup214 nuclear bodies containing the nuclear export receptor CRM1 were observed in the leukemia cell lines LOUCY and MEGAL. Overexpression of SET-Nup214 in HeLa cells leads to the formation of similar nuclear bodies that recruit CRM1, export cargo proteins, and certain nucleoporins and concomitantly affect nuclear protein and poly(A)+ RNA export. SQSTM1-Nup214, although mostly cytoplasmic, also forms nuclear bodies and inhibits nuclear protein but not poly(A)+ RNA export. The interaction of the fusion proteins with CRM1 is RanGTP-dependent, as shown in co-immunoprecipitation experiments and binding assays. Further analysis revealed that the Nup214 parts mediate the inhibition of nuclear export, whereas the SET or SQSTM1 part determines the localization of the fusion protein and therefore the extent of the effect. SET-Nup214 nuclear bodies are highly mobile structures, which are in equilibrium with the nucleoplasm in interphase and disassemble during mitosis or upon treatment of cells with the CRM1-inhibitor leptomycin B. Strikingly, we found that nucleoporins can be released from nuclear bodies and reintegrated into existing NPC. Our results point to nuclear bodies as a means of preventing the formation of potentially insoluble and harmful protein aggregates that also may serve as storage compartments for nuclear transport factors.

Complex Commingling: Nucleoporins and the Spindle Assembly Checkpoint

The segregation of the chromosomes during mitosis is an important process, in which the replicated DNA content is properly allocated into two daughter cells. To ensure their genomic integrity, cells present an essential surveillance mechanism known as the spindle assembly checkpoint (SAC), which monitors the bipolar attachment of the mitotic spindle to chromosomes to prevent errors that would result in chromosome mis-segregation and aneuploidy. Multiple components of the nuclear pore complex (NPC), a gigantic protein complex that forms a channel through the nuclear envelope to allow nucleocytoplasmic exchange of macromolecules, were shown to be critical for faithful cell division and implicated in the regulation of different steps of the mitotic process, including kinetochore and spindle assembly as well as the SAC. In this review, we will describe current knowledge about the interconnection between the NPC and the SAC in an evolutional perspective, which primarily relies on the two mitotic checkpoint regulators, Mad1 and Mad2. We will further discuss the role of NPC constituents, the nucleoporins, in kinetochore and spindle assembly and the formation of the mitotic checkpoint complex during mitosis and interphase.

Nuclear Pore Complexes and Nucleocytoplasmic Transport: From Structure to Function to Disease

Nucleocytoplasmic transport is an essential cellular activity and occurs via nuclear pore complexes (NPCs) that reside in the double membrane of the nuclear envelope. Significant progress has been made during the past few years in unravelling the ultrastructural organization of NPCs and their constituents, the nucleoporins, by cryo-electron tomography and X-ray crystallography. Mass spectrometry and genomic approaches have provided deeper insight into the specific regulation and fine tuning of individual nuclear transport pathways. Recent research has also focused on the roles nucleoporins play in health and disease, some of which go beyond nucleocytoplasmic transport. Here we review emerging results aimed at understanding NPC architecture and nucleocytoplasmic transport at the atomic level, elucidating the specific function individual nucleoporins play in nuclear trafficking, and finally lighting up the contribution of nucleoporins and nuclear transport receptors in human diseases, such as cancer and certain genetic disorders.

Nucleoporin Gene Fusions and Hematopoietic Malignancies

Nuclear pore complexes (NPCs) are the sole gateways between the nucleus and the cytoplasm of eukaryotic cells and they mediate all macromolecular trafficking between these cellular compartments. Nucleocytoplasmic transport is highly selective and precisely regulated and as such an important aspect of normal cellular function. Defects in this process or in its machinery have been linked to various human diseases, including cancer. Nucleoporins, which are about 30 proteins that built up NPCs, are critical players in nucleocytoplasmic transport and have also been shown to be key players in numerous other cellular processes, such as cell cycle control and gene expression regulation. This review will focus on the three nucleoporins Nup98, Nup214, and Nup358. Common to them is their significance in nucleocytoplasmic transport, their multiple other functions, and being targets for chromosomal translocations that lead to haematopoietic malignancies, in particular acute myeloid leukaemia. The underlying molecular mechanisms of nucleoporin-associated leukaemias are only poorly understood but share some characteristics and are distinguished by their poor prognosis and therapy outcome.

Several phenylalanine-glycine motives in the nucleoporin Nup214 are essential for binding of the nuclear export receptor CRM1

Nucleoporins containing phenylalanine glycine (FG) repeats play an important role in nucleocytoplasmic transport as they bind to transport receptors and mediate translocation of transport complexes across the nuclear pore complex (NPC). Nup214/CAN, a nucleoporin that is found at the cytoplasmic side of the NPC, interacts with both import and export receptors. In functional assays, dominant-negative fragments of Nup214 inhibited CRM1-dependent nuclear export, as the export receptor became rate-limiting. Several nuclear import pathways, by contrast, were not affected by the Nup214 fragments. We now characterize the CRM1-binding region of Nup214 in detail and identify several FG motives that are required for this interaction. Our results support a model where CRM1, like other transport receptors, contacts FG-Nups via multiple binding sites.

Systematic analysis of barrier-forming FG hydrogels from Xenopus nuclear pore complexes

Nuclear pore complexes (NPCs) control the traffic between cell nucleus and cytoplasm. While facilitating translocation of nuclear transport receptors (NTRs) and NTR·cargo complexes, they suppress passive passage of macromolecules ⩾30 kDa. Previously, we reconstituted the NPC barrier as hydrogels comprising S. cerevisiae FG domains. We now studied FG domains from 10 Xenopus nucleoporins and found that all of them form hydrogels. Related domains with low FG motif density also substantially contribute to the NPC's hydrogel mass. We characterized all these hydrogels and observed the strictest sieving effect for the Nup98-derived hydrogel. It fully blocks entry of GFP-sized inert objects, permits facilitated entry of the small NTR NTF2, but arrests importin β-type NTRs at its surface. O-GlcNAc modification of the Nup98 FG domain prevented this arrest and allowed also large NTR·cargo complexes to enter. Solid-state NMR spectroscopy revealed that the O-GlcNAc-modified Nup98 gel lacks amyloid-like β-structures that dominate the rigid regions in the S. cerevisiae Nsp1 FG hydrogel. This suggests that FG hydrogels can assemble through different structural principles and yet acquire the same NPC-like permeability.

The phenylalanine-glycine (FG) domains of vertebrate nucleoporins assemble into hydrogels with different sieving characteristics for macromolecules. Nup98 forms the tightest filter, which is relieved by O-linked glycosylation.

LONO1 encoding a nucleoporin is required for embryogenesis and seed viability in Arabidopsis

Early embryogenesis in Arabidopsis (Arabidopsis thaliana) is distinguished by a predictable pattern of cell divisions and is a good system for investigating mechanisms of developmental pattern formation. Here, we identified a gene called LONO1 (LNO1) in Arabidopsis in which mutations can abolish the first asymmetrical cell division of the zygote, alter planes and number of cell divisions in early embryogenesis, and eventually arrest embryo development. LNO1 is highly expressed in anthers of flower buds, stigma papilla of open flowers, and embryo and endosperm during early embryogenesis, which is correlated with its functions in reproductive development. The homozygous lno1-1 seed is not viable. LNO1, a homolog of the nucleoporin NUP214 in human (Homo sapiens) and Nup159 in yeast (Saccharomyces cerevisiae), encodes a nucleoporin protein containing phenylalanine-glycine repeats in Arabidopsis. We demonstrate that LNO1 can functionally complement the defect in the yeast temperature-sensitive nucleoporin mutant nup159. We show that LNO1 specifically interacts with the Arabidopsis DEAD-box helicase/ATPase LOS4 in the yeast two-hybrid assay. Furthermore, mutations in AtGLE1, an Arabidopsis homolog of the yeast Gle1 involved in the same poly(A) mRNA export pathway as Nup159, also result in seed abortion. Our results suggest that LNO1 is a component of the nuclear pore complex required for mature mRNA export from the nucleus to the cytoplasm, which makes LNO1 essential for embryogenesis and seed viability in Arabidopsis.

mRNA export and cancer

Studies in the past several years highlight important features of the messenger RNA (mRNA) export process. For instance, groups of mRNAs acting in the same biochemical processes can be retained or exported in a coordinated manner thereby impacting on specific biochemistries and ultimately on cell physiology. mRNAs can be transported by either bulk export pathways involving NXF1/TAP or more specialized pathways involving chromosome region maintenance 1 (CRM1). Studies on primary tumor specimens indicate that many common and specialized mRNA export factors are dysregulated in cancer including CRM1, eukaryotic translation initiation factor 4E (eIF4E), HuR, nucleoporin 88, REF/Aly, and THO. This positions these pathways as potential therapeutic targets. Recently, specific targeting of the eIF4E-dependent mRNA export pathway in a phase II proof-of-principle trial with ribavirin led to impaired eIF4E-dependent mRNA export correlating with clinical responses including remissions in leukemia patients. Here, we provide an overview of these mRNA export pathways and highlight their relationship to cancer.

Alterations in nuclear pore architecture allow cancer cell entry into or exit from drug-resistant dormancy

Phenotypic diversity arises in tumors just as it does in developing organisms, and tumor recurrence frequently manifests from the selective survival of divergent drug-resistant cells. Although the expanding tumor cell population may be successfully targeted, drug-resistant cells may persist and sustain the tumor or enter dormancy before igniting a future relapse. Herein, we show that partial knockdown of nucleoporin p62 (NUP62) by small-interfering RNA confers cisplatin resistance to cultured high-grade ovarian carcinoma cells. Treatment with NUP62 small-interfering RNA and cisplatin leaves resistant cells in a state of dormancy; some dormant cells can be induced to proliferate by transient induction of NUP62 expression from an ectopic expression construct. In addition to suggesting functional links between nuclear pore complex architecture and cancer cell survival, the culture system provides a novel experimental window into the dynamics of tumor cell drug resistance and dormancy.

The role of nuclear pore complex in tumor microenvironment and metastasis

One of the main reasons for cancer mortality is caused by the highly invasive behavior of cancer cells, which often due to aggressive metastasis. Metastasis is mediated by various growth factors and cytokines, operating through numerous signaling pathways. Remarkably, all these metastatic signaling pathways must enter the nucleus through a single gatekeeper, the nuclear pore complex (NPC). NPCs are the only gateway between the cytoplasm and the nucleus. NPCs are among the largest proteinaceous assemblies in the cell and are composed of multiple copies of around 30 different proteins called nucleoporins. Here, we review what is currently known about the NPC, and its role in the mechanisms of tumor progression. We will also explore potential strategies to target metastatic pathways by manipulating the karyopherins (importins/exportins) of nucleocytoplasmic traffic through NPCs.

The nucleoporin Nup88 is interacting with nuclear lamin A

Nuclear pore complexes (NPCs) are embedded in the nuclear envelope (NE) and mediate bidirectional nucleocytoplasmic transport. Their spatial distribution in the NE is organized by the nuclear lamina, a meshwork of nuclear intermediate filament proteins. Major constituents of the nuclear lamina are A- and B-type lamins. In this work we show that the nuclear pore protein Nup88 binds lamin A in vitro and in vivo. The interaction is mediated by the N-terminus of Nup88, and Nup88 specifically binds the tail domain of lamin A but not of lamins B1 and B2. Expression of green fluorescent protein-tagged lamin A in cells causes a masking of binding sites for Nup88 antibodies in immunofluorescence assays, supporting the interaction of lamin A with Nup88 in a cellular context. The epitope masking disappears in cells expressing mutants of lamin A that are associated with laminopathic diseases. Consistently, an interaction of Nup88 with these mutants is disrupted in vitro. Immunoelectron microscopy using Xenopus laevis oocyte nuclei further revealed that Nup88 localizes to the cytoplasmic and nuclear face of the NPC. Together our data suggest that a pool of Nup88 on the nuclear side of the NPC provides a novel, unexpected binding site for nuclear lamin A.

Nup88 expression is associated with myometrial invasion in endometrial carcinoma

BACKGROUND

The nuclear pore complex protein Nup88 has been shown in previous studies to be overexpressed in tumor cells and to be associated in breast cancer with all clinical and biological features defining a more aggressive phenotype.

METHODS

In this pilot study, Nup88-mRNA expression was studied in a series of 29 endometrial carcinomas, of which 27 belonged to the endometrioid variety, the remaining 2 being papillary serous tumors, to verify if Nup88 plays a similar role in endometrial carcinoma as the one described in breast cancer. The tumor samples were obtained directly at the operating suite and fresh frozen at the time of hysterectomy. Nup88-mRNA expression was studied in them by means of differential reverse transcriptase-polymerase chain reaction. Nup88-mRNA expression was correlated with the clinical features of the tumors.

RESULTS

A significant correlation (r = 0.41, P = 0.027) was found between growing levels of Nup88-mRNA expression and depth of myometrial invasion. There was no correlation between Nup88-mRNA expression and the other 2 available clinical parameters, that is, tumor grade (r = 0.05, P = 0.79) and surgical stage (r = -0.18, P = 0.34).

CONCLUSIONS

From these results, it is concluded that Nup88 expression seems indeed to be associated with a distinct feature of tumor aggressiveness (myometrial invasion) in endometrial carcinoma and that larger studies are therefore probably worthwhile.

The mobile FG nucleoporin Nup98 is a cofactor for Crm1-dependent protein export

Nup98 is a mobile nucleoporin that forms distinct dots in the nucleus, and, although a role for Nup98 in nuclear transport has been suggested, its precise function remains unclear. Here, we show that Nup98 plays an important role in Crm1-mediated nuclear protein export. Nuclear, but not cytoplasmic, dots of EGFP-tagged Nup98 disappeared rapidly after cell treatment with leptomycin B, a specific inhibitor of the nuclear export receptor, Crm1. Mutational analysis demonstrated that Nup98 physically and functionally interacts with Crm1 in a RanGTP-dependent manner through its N-terminal phenylalanine-glycine (FG) repeat region. Moreover, the activity of the Nup98-Crm1 complex was modulated by RanBP3, a known cofactor for Crm1-mediated nuclear export. Finally, cytoplasmic microinjection of anti-Nup98 inhibited the Crm1-dependent nuclear export of proteins, concomitant with the accumulation of anti-Nup98 in the nucleus. These results clearly demonstrate that Nup98 functions as a novel shuttling cofactor for Crm1-mediated nuclear export in conjunction with RanBP3.

Nuclear pore proteins and cancer

Nucleocytoplasmic trafficking of macromolecules, a highly specific and tightly regulated process, occurs exclusively through the nuclear pore complex. This immense structure is assembled from approximately 30 proteins, termed nucleoporins. Here we discuss the four nucleoporins that have been linked to cancers, either through elevated expression in tumors (Nup88) or through involvement in chromosomal translocations that encode chimeric fusion proteins (Tpr, Nup98, Nup214). In each case we consider the normal function of the nucleoporin and its translocation partners, as well as what is known about their mechanistic contributions to carcinogenesis, particularly in leukemias. Studies of nucleoporin-linked cancers have revealed novel mechanisms of oncogenesis and in the future, should continue to expand our understanding of cancer biology.

The role of the nuclear transport system in cell differentiation

The eukaryotic cell nuclear transport system selectively mediates molecular trafficking to facilitate the regulation of cellular processes. The components of this system include diverse transport factors such as importins and nuclear pore components that are precisely organized to coordinate cellular events. A number of studies have demonstrated that the nuclear transport system is indispensible in many types of cellular responses. In particular, the nuclear transport machinery has been shown to be an important regulator of development, organogenesis, and tissue formation, wherein altered nuclear transport of key transcription factors can lead to disease. Importantly, precise switching between distinct forms of importin alpha is central to neural lineage specification, consistent with the hypothesis that importin expression can be a key mediator of cell differentiation.

Herpes simplex virus replication: roles of viral proteins and nucleoporins in capsid-nucleus attachment

Replication of herpes simplex virus type 1 (HSV-1) involves a step in which a parental capsid docks onto a host nuclear pore complex (NPC). The viral genome then translocates through the nuclear pore into the nucleoplasm, where it is transcribed and replicated to propagate infection. We investigated the roles of viral and cellular proteins in the process of capsid-nucleus attachment. Vero cells were preloaded with antibodies specific for proteins of interest and infected with HSV-1 containing a green fluorescent protein-labeled capsid, and capsids bound to the nuclear surface were quantified by fluorescence microscopy. Results showed that nuclear capsid attachment was attenuated by antibodies specific for the viral tegument protein VP1/2 (UL36 gene) but not by similar antibodies specific for UL37 (a tegument protein), the major capsid protein (VP5), or VP23 (a minor capsid protein). Similar studies with antibodies specific for nucleoporins demonstrated attenuation by antibodies specific for Nup358 but not Nup214. The role of nucleoporins was further investigated with the use of small interfering RNA (siRNA). Capsid attachment to the nucleus was attenuated in cells treated with siRNA specific for either Nup214 or Nup358 but not TPR. The results are interpreted to suggest that VP1/2 is involved in specific attachment to the NPC and/or in migration of capsids to the nuclear surface. Capsids are suggested to attach to the NPC by way of the complex of Nup358 and Nup214, with high-resolution immunofluorescence studies favoring binding to Nup358.

Kinase activation and transformation by NUP214-ABL1 is dependent on the context of the nuclear pore

Genetic alterations causing constitutive tyrosine kinase activation are observed in a broad spectrum of cancers. Thus far, these mutant kinases have been localized to the plasma membrane or cytoplasm, where they engage proliferation and survival pathways. We report that the NUP214-ABL1 fusion is unique among these because of its requisite localization to the nuclear pore complex for its transforming potential. We show that NUP214-ABL1 displays attenuated transforming capacity as compared to BCR-ABL1 and that NUP214-ABL1 preferentially transforms T cells, which is in agreement with its unique occurrence in T cell acute lymphoblastic leukemia. Furthermore, NUP214-ABL1 differs from BCR-ABL1 in subcellular localization, initiation of kinase activity, and signaling and lacks phosphorylation on its activation loop. In addition to delineating an unusual mechanism for kinase activation, this study provides new insights into the spectrum of chromosomal translocations involving nucleoporins by indicating that the nuclear pore context itself may play a central role in transformation.

Tumor marker nucleoporin 88 kDa regulates nucleocytoplasmic transport of NF-kappaB

Nucleoporin 88 kDa (Nup88) is a tumor marker, overexpressed in various types of cancer. In Drosophila Nup88 (mbo) was reported to selectively mediate the nucleocytoplasmic transport of NF-kappaB, an ubiquitous transcription factor involved in immune responses, apoptosis, and cancer. We addressed the function of Nup88 in mammalian cells. Selective depletion of Nup88 by small interfering RNA (siRNA) inhibited NF-kappaB-dependent reporter gene activation and the nuclear translocation of NF-kappaB without affecting the upstream activation pathway in NIH3T3 cells. In contrast, nuclear translocation of glucocorticoid receptor was not reduced by the depletion of Nup88. In metastatic melanoma cells overexpressing Nup88, constitutive activation of NF-kappaB was found both in nucleus and cytoplasm. Nup88 depletion in these cells reduced TNF-induced nuclear accumulation of NF-kappaB subunits. We conclude that Nup88 regulates the activity of NF-kappaB at the level of nucleocytoplasmic transport. Overexpression of Nup88 in tumor cells may, thus be involved in the constitutive NF-kappaB activation.

Nup53 is required for nuclear envelope and nuclear pore complex assembly

Transport across the nuclear envelope (NE) is mediated by nuclear pore complexes (NPCs). These structures are composed of various subcomplexes of proteins that are each present in multiple copies and together establish the eightfold symmetry of the NPC. One evolutionarily conserved subcomplex of the NPC contains the nucleoporins Nup53 and Nup155. Using truncation analysis, we have defined regions of Nup53 that bind to neighboring nucleoporins as well as those domains that target Nup53 to the NPC in vivo. Using this information, we investigated the role of Nup53 in NE and NPC assembly using Xenopus egg extracts. We show that both events require Nup53. Importantly, the analysis of Nup53 fragments revealed that the assembly activity of Nup53 depleted extracts could be reconstituted using a region of Nup53 that binds specifically to its interacting partner Nup155. On the basis of these results, we propose that the formation of a Nup53-Nup155 complex plays a critical role in the processes of NPC and NE assembly.

Crystal structure of the N-terminal domain of the human protooncogene Nup214/CAN

The mammalian nuclear pore complex (NPC) is an approximately 120-MDa proteinaceous assembly consisting of approximately 30 proteins and is the sole gate in the nuclear envelope. The human protooncogene Nup214 was first identified as a target for chromosomal translocation involved in leukemogenesis. Nup214 is located on the cytoplasmic face of the NPC and is implicated in anchoring the cytoplasmic filaments of the NPC and recruiting the RNA helicase Ddx19. Here, we present the crystal structure of the human Nup214 N-terminal domain at 1.65-A resolution. The structure reveals a seven-bladed beta-propeller followed by a 30-residue C-terminal extended peptide segment, which folds back onto the beta-propeller and binds to its bottom face. The beta-propeller repeats lack any recognizable sequence motif and are distinguished by extensive insertions between the canonical beta-strands. We propose a mechanism by which the C-terminal peptide extension is involved in NPC assembly.

Nup88 expression in normal mucosa, adenoma, primary adenocarcinoma and lymph node metastasis in the colorectum

OBJECTIVES

It was the aim of this study to investigate Nup88 expression in normal colorectal mucosa, adenoma, adenocarcinoma and lymph node metastasis, as well as the relationship between Nup88 expression and clinicopathological features.

METHODS

Nup88 expression was examined by immunohistochemistry in 84 normal mucosa samples, 32 adenomas, 181 primary adenocarcinomas, and 18 lymph node metastases from colorectal tumour patients.

RESULTS

Nup88 expression was observed in normal epithelial and tumour cells. The frequency of strong Nup88 expression was increased from normal mucosa or adenoma to primary tumour and lymph node metastasis (p < 0.0001). There was no significant difference in the expression between normal mucosa and adenoma (p = 0.41). The frequency of strong Nup88 expression was higher in ulcerated tumours (40%) than in polypoid/large fungating tumours (23%; p = 0.048). The frequency of strong Nup88 expression was significantly different among tumours with good (21%), moderate (42%) and poor differentiation (48%; p = 0.01). Nup88 expression was not related to the patients' gender, age, tumour location, size, histological type, invasive depth, lymph node status and Dukes stage (p > 0.05).

CONCLUSION

Our results suggest that Nup88 may play a role during the development, aggressiveness and differentiation of colorectal tumours.

Nup214 is required for CRM1-dependent nuclear protein export in vivo

Nucleoporins mediate transport of macromolecules across the nuclear pore complex, yet the function of many individual nucleoporins is largely unresolved. To address this question, we depleted cells of the cytoplasmic nucleoporins Nup214/CAN and Nup358/RanBP2 by RNA interference. Depletion of Nup214 resulted in codepletion of its binding partner, Nup88. Nuclear pore complexes assembled in the absence of Nup214/Nup88 or Nup358 were fully functional in nuclear protein import, whereas nuclear mRNA export was slightly impaired. Depletion of Nup358 had only a minor effect on nuclear protein export. In contrast, depletion of Nup214/Nup88 led to strongly reduced CRM1-mediated export of the shuttling transcription factor NFAT as well as a human immunodeficiency virus-Rev derivative. A specific role of Nup214 in protein export is furthered by the biochemical properties of a high-affinity complex containing Nup214, CRM1, RanGTP, and an export cargo. Our results show that the Nup214/Nup88 complex is required for efficient CRM1-mediated transport, supporting a model involving a high-affinity binding site for CRM1 at Nup214 in the terminal steps of export.

Nup214-Nup88 nucleoporin subcomplex is required for CRM1-mediated 60 S preribosomal nuclear export

The nuclear pore complex (NPC) conducts macromolecular transport to and from the nucleus and provides a kinetic/hydrophobic barrier composed of phenylalanine-glycine (FG) repeats. Nuclear transport is achieved through permeation of this barrier by transport receptors. The transport receptor CRM1 facilitates export of a large variety of cargoes. Export of the preribosomal 60 S subunit follows this pathway through the adaptor protein NMD3. Using RNA interference, we depleted two FG-containing cytoplasmically oriented NPC complexes, Nup214-Nup88 and Nup358, and investigated CRM1-mediated export. A dramatic defect in NMD3-mediated export of preribosomes was found in Nup214-Nup88-depleted cells, whereas only minor export defects were evident in other CRM1 cargoes or upon depletion of Nup358. We show that the large C-terminal FG domain of Nup214 is not accessible to freely diffusing molecules from the nucleus, indicating that it does not conduct 60 S preribosomes through the NPC. Consistently, derivatives of Nup214 lacking the FG-repeat domain rescued the 60 S export defect. We show that the coiled-coil region of Nup214 is sufficient for 60 S nuclear export, coinciding with recruitment of Nup88 to the NPC. Our data indicate that Nup214 plays independent roles in NPC function by participating in the kinetic/hydrophobic barrier through its FG-rich domain and by enabling NPC gating through association with Nup88.

Functional Analysis of Nuclear Pore Complex Protein Nup62/p62 Using Monoclonal Antibodies

The nuclear pore complex (NPC) is an enormous structure embedded in the double membrane of the nuclear envelope that acts as a passageway for nucleocytoplasmic transport. The vertebrate NPC is comprised of about 30 unique proteins. Nup62/p62, a major component of the NPC, has been reported to interact directly with several nuclear transport factors, including importin-beta and NTF2. However, it has not been shown how the interaction of Nup62/p62 with transport factors is involved in nucleocytoplasmic transport. The present study reports on the preparation of monoclonal antibodies (MAbs) directed against human Nup62/p62 and a functional analysis of Nup62/p62 using antibodies in living cells. Hybridomas producing the antibodies were produced by the hybridization of mouse myeloma cells with medial iliac lymph node cells from an immunized rat. These MAbs specifically recognized Nup62/p62 as evidenced by immunoblotting analysis using a nuclear membrane fraction. In the immunostaining using MAbs, a punctuate nuclear rim staining pattern was observed. Moreover, cytoplasmic injected-anti-Nup62/p62 MAbs were rapidly targeted to the nuclear pore of cultured cells and some of them inhibited normal cell division, causing the formation of abnormal nuclei. The antibodies described in this study provide the means for immunochemical analyses of the NPC protein Nup62/p62 in mammalian cells, and represent useful molecular tools that should permit a better understanding of the biological roles and cellular dynamics of this protein in nucleocytoplasmic transport, cell division, and nuclear organization.

Caspases target only two architectural components within the core structure of the nuclear pore complex

Caspases were recently implicated in the functional impairment of the nuclear pore complex during apoptosis, affecting its dual activity as nucleocytoplasmic transport channel and permeability barrier. Concurrently, electron microscopic data indicated that nuclear pore morphology is not overtly altered in apoptotic cells, raising the question of how caspases may deactivate nuclear pore function while leaving its overall structure largely intact. To clarify this issue we have analyzed the fate of all known nuclear pore proteins during apoptotic cell death. Our results show that only two of more than 20 nuclear pore core structure components, namely Nup93 and Nup96, are caspase targets. Both proteins are cleaved near their N terminus, disrupting the domains required for interaction with other nucleoporins actively involved in transport and providing the permeability barrier but dispensable for maintaining the nuclear pore scaffold. Caspase-mediated proteolysis of only few nuclear pore complex components may exemplify a general strategy of apoptotic cells to efficiently disable huge macromolecular machines.

Aberrant intracellular localization of SET-CAN fusion protein, associated with a leukemia, disorganizes nuclear export

The SET-CAN fusion gene is the product of a chromosomal rearrangement found on 9q34 associated with an acute undifferentiated leukemia. SET-CAN encodes an almost complete SET protein fused to the C-terminal two-thirds of CAN. SET is also known as TAF-Ibeta, a histone chaperone and intracellular inhibitor of protein phosphatase 2A, whereas CAN is identical to Nup214, a nucleoporin protein. To obtain insight into the leukemogenic function of SET/TAF-Ibeta-CAN/Nup214, we have examined its subcellular localization. Immunofluorescence analyses showed that SET/TAF-Ibeta and CAN/Nup214 are found in the nucleus and the nuclear envelope, respectively, whereas the majority of SET/TAF-Ibeta-CAN/Nup214 is localized in the nucleus. SET/TAF-Ibeta-CAN/Nup214 interacted with hCRM1, one of the nuclear export factors, and caused aberrant intracellular localization of hCRM1. In cells expressing SET/TAF-Ibeta-CAN/Nup214, a protein containing a nuclear export signal accumulated in the nucleus. The export of this protein was partially restored by overexpression of hCRM1. These results suggest that aberrantly localized molecules associated with SET/TAF-Ibeta-CAN/Nup214 may be involved in oncogenesis.

The nuclear pore complex protein Tpr is a common autoantigen in sera that demonstrate nuclear envelope staining by indirect immunofluorescence

We studied the autoantigen targets of 75 human sera that had antibodies to the nuclear envelope (NE) as identified by indirect immunofluorescence (IIF) on HEp-2 cells. Several different IIF staining patterns could be identified when antibodies to different components of the nuclear membrane (NM) and nuclear pore complexes (NuPC) were identified: a smooth membrane pattern characteristic of antibodies to nuclear lamins, a punctate pattern typical of antibodies to the nuclear pore complex and more complex patterns that included antibodies to nuclear and cytoplasmic organelles. Western immunoblotting of isolated nuclear and NE proteins and immunoprecipitation of radiolabelled recombinant proteins prepared by using the full-length cDNAs of the Translocated promoter region (Tpr), gp210 and p62 were used to identify specific autoantibody targets. Fifty-two of the 75 (70%) sera bound to Tpr, 25 (33%) bound to lamins A, B or C, 15 (20%) reacted with gp210 and none reacted with p62. Sixteen (21%) did not react with any of the NE components tested in our assays. The clinical features of 37 patients with anti-NE showed that there were 34 females and three males with an age range of 16-88 years (mean 59 years). The most frequent clinical diagnosis (9/37 = 24%) was autoimmune liver disease (ALD; two with primary biliary cirrhosis), followed by seven (19%) with systemic lupus erythematosus (SLE), four (11%) with a motor and/or sensory neuropathy, three (8%) with anti-phospholipid syndrome (APS), two with systemic sclerosis (SSc), two with Sjögren's syndrome (SjS), and others with a variety of diagnoses. This report indicates that Tpr, a component of the NuPC, is a common target of human autoantibodies that react with the NE.

Autoantigens of the nuclear pore complex

The nuclear envelope (NE) is one of many intracellular targets of the autoimmune response in patients with autoimmune liver disease, systemic lupus erythematosus, and related conditions. In eukaryotic organisms the NE consists of five interconnected regions: an outer nuclear membrane (ONM) that is continuous with the endoplasmic reticulum, an intermembrane or perinuclear space, an inner nuclear membrane (INM) with a unique set of integral membrane proteins, the underlying nuclear lamina, and the pore domains that are regions where the ONM and INM come together. The pore domains are sites of regulated continuity between the cytoplasm and nucleus that are occupied by supramolecular structures, termed nuclear pore complexes (NPCs). Human autoantibodies identified to date bind to specific components in three of the five NE compartments. Autoantigen targets include the lamins A, B, and C of the nuclear lamina, gp210, p62 complex proteins, Nup153, and Tpr within the NPC, and LBR, MAN1, LAP1, and LAP2 that are integral proteins of the INM. Autoantibodies to these NE targets have been shown to be correlated with various autoimmune diseases such as primary biliary cirrhosis, other autoimmune liver diseases and systemic rheumatic diseases. Now that the proteome of the NE is more clearly defined, other autoantibodies to components in this cell compartment are likely to be defined.

Nup88 mRNA overexpression is associated with high aggressiveness of breast cancer

The nuclear pore complex protein Nup88 is overexpressed in tumor cells. Immunohistochemical studies have shown that this overexpression is linked to higher aggressiveness of colorectal carcinoma and to enhanced metastatic potential of melanoma cells. However, the antibodies so far developed against Nup88 have the drawback of recognizing a number of other, up to now unspecified antigens besides Nup88. For this reason, we devised the present study on Nup88 expression at the mRNA level. RNA was extracted from fresh tumor tissue corresponding to 122 breast cancer patients. Nup88 mRNA expression was measured by means of differential RT-PCR, standardizing against a constitutive internal control gene (beta-actin). The results were dichotomized into "high" and "low" expression levels, using the median value as cut-off. High Nup88 mRNA expression levels correlated significantly with ductal and tubular histology (p = 0.012), histologic and nuclear grade 3 of tumors (p < 0.001), absence of hormone receptor expression (p < 0.001), expression of the c-erb-B2 oncogene (p < 0.001), expression of mutant p53 protein (p < 0.001), high proliferation (defined by Ki67 labeling index >20%, p < 0.001), DNA aneuploidy (p < 0.001) as well as the most important ominous clinical prognostic factor, axillary node invasion (p < 0.001). We also found an inverse correlation (p < 0.001) with expression of the H-MAM (mammaglobin) gene, a marker of low biologic and clinical aggressiveness of breast cancer. All of these factors, without exception, define a highly aggressive tumor phenotype. These findings appear to be specific to Nup88 and not to nuclear pore proteins in general. Indeed, analysis of Nup107 (which is a limiting component of the nuclear pore complex) under the same conditions in the same tumors did not yield comparable results.

Nup358/RanBP2 attaches to the nuclear pore complex via association with Nup88 and Nup214/CAN and plays a supporting role in CRM1-mediated nuclear protein export

Nuclear pore complexes (NPCs) traverse the nuclear envelope (NE), providing a channel through which nucleocytoplasmic transport occurs. Nup358/RanBP2, Nup214/CAN, and Nup88 are components of the cytoplasmic face of the NPC. Here we show that Nup88 localizes midway between Nup358 and Nup214 and physically interacts with them. RNA interference of either Nup88 or Nup214 in human cells caused a strong reduction of Nup358 at the NE. Nup88 and Nup214 showed an interdependence at the NPC and were not affected by the absence of Nup358. These data indicate that Nup88 and Nup214 mediate the attachment of Nup358 to the NPC. We show that localization of the export receptor CRM1 at the cytoplasmic face of the NE is Nup358 dependent and represents its empty state. Also, removal of Nup358 causes a distinct reduction in nuclear export signal-dependent nuclear export. We propose that Nup358 provides both a platform for rapid disassembly of CRM1 export complexes and a binding site for empty CRM1 recycling into the nucleus.

Peering through the pore: nuclear pore complex structure, assembly, and function

Nuclear pore complexes (NPCs) are large proteinaceous assemblies that provide the only known portals for exchanging macromolecules between the nucleus and cytoplasm. This includes the movement of small molecules and the selective, facilitated transport of large proteins and RNAs. Faithful, continuous NPC assembly is key for maintaining normal physiological function and is closely tied to proper cell division. This review focuses on the most outstanding issues involving NPC structure, assembly, and function.

Direct interaction with nup153 mediates binding of Tpr to the periphery of the nuclear pore complex

Tpr is a 267-kDa protein forming coiled coil-dominated homodimers that locate at the nucleoplasmic side of the nuclear pore complex (NPC). The proteins that tether Tpr to this location are unknown. Moreover, the question whether Tpr itself might act as a scaffold onto which other NPC components need to be assembled has not been answered to date. To assess Tpr's role as an architectural element of the NPC, we have studied the sequential disassembly and reassembly of NPCs in mitotic cells, paralleled by studies of cells depleted of Tpr as a result of posttranscriptional tpr gene silencing by RNA interference (RNAi). NPC assembly and recruitment of several nucleoporins, including Nup50, Nup93, Nup96, Nup98, Nup107, and Nup153, in anaphase/early telophase is shown to precede NPC association of Tpr in late telophase. In accordance, cellular depletion of Tpr by RNAi does not forestall binding of these nucleoporins to the NPC. In a search for proteins that moor Tpr to the NPC, we have combined the RNAi approach with affinity-chromatography and yeast two-hybrid interaction studies, leading to the identification of nucleoporin Nup153 as the binding partner for Tpr. The specificity of this interaction is demonstrated by its sensitivity to Tpr amino acid substitution mutations that abolish Tpr's ability to adhere to the NPC and affect the direct binding of Tpr to Nup153. Accordingly, cellular depletion of Nup153 by RNAi is shown to result in mislocalization of Tpr to the nuclear interior. Nup153 deficiency also causes mislocalization of Nup50 but has no direct effect on NPC localization of the other nucleoporins studied in this investigation. In summary, these results render Tpr a protein only peripherally attached to the NPC that does not act as an essential scaffold for other nucleoporins.

Tamo selectively modulates nuclear import in Drosophila

BACKGROUND

The NF-kappaB/Rel pathway functions in the establishment of dorsal-ventral polarity and in the innate humoral and cellular immune response in Drosophila. An important aspect of all NF-kappaB/Rel pathways is the translocation of the Rel proteins from the cytoplasm to the nucleus, where they function as transcription factors.

RESULTS

We have identified a new protein, Tamo, which binds to Drosophila Rel protein Dorsal, but not to Dorsal lacking the nuclear localization sequence. Tamo does not bind to the other Drosophila Rel proteins, Dif and Relish. The Tamo-Dorsal complex forms in the cytoplasm and Tamo also interacts with a cytoplasmically orientated nucleoporin. In addition Tamo binds the Ras family small GTPase, Ran. Tamo functions during oogenesis and, based on phenotypic analysis, controls the levels of nuclear Dorsal in early embryos. It further regulates the accumulation of Dorsal in the nucleus after immune challenge.

CONCLUSIONS

Tamo has an essential function during oogenesis. Tamo interacts with Dorsal and proteins that are part of the nuclear import machinery. We propose that tamo modulates the levels of import of Dorsal and other proteins.

Nup98 localizes to both nuclear and cytoplasmic sides of the nuclear pore and binds to two distinct nucleoporin subcomplexes

The vertebrate nuclear pore is an enormous structure that spans the double membrane of the nuclear envelope. In yeast, most nucleoporins are found symmetrically on both the nuclear and cytoplasmic sides of the structure. However, in vertebrates most nucleoporins have been localized exclusively to one side of the nuclear pore. Herein, we show, by immunofluorescence and immunoelectron microscopy, that Nup98 is found on both sides of the pore complex. Additionally, we find that the pore-targeting domain of Nup98 interacts directly with the cytoplasmic nucleoporin Nup88, a component of the Nup214, Nup88, Nup62 subcomplex. Nup98 was previously described to interact with the nuclear-oriented Nup160, 133, 107, 96 complex through direct binding to Nup96. Interestingly, the same site within Nup98 is involved in binding to both Nup88 and Nup96. Autoproteolytic cleavage of the Nup98 C terminus is required for both of these binding interactions. When cleavage is blocked by a point mutation, a minimal eight amino acids downstream of the cleavage site is sufficient to prevent most binding to either Nup96 or Nup88. Thus, Nup98 interacts with both faces of the nuclear pore, a localization in keeping with its previously described nucleocytoplasmic shuttling activity.

The cytoplasmic filaments of the nuclear pore complex are dispensable for selective nuclear protein import

The nuclear pore complex (NPC) mediates bidirectional macromolecular traffic between the nucleus and cytoplasm in eukaryotic cells. Eight filaments project from the NPC into the cytoplasm and are proposed to function in nuclear import. We investigated the localization and function of two nucleoporins on the cytoplasmic face of the NPC, CAN/Nup214 and RanBP2/Nup358. Consistent with previous data, RanBP2 was localized at the cytoplasmic filaments. In contrast, CAN was localized near the cytoplasmic coaxial ring. Unexpectedly, extensive blocking of RanBP2 with gold-conjugated antibodies failed to inhibit nuclear import. Therefore, RanBP2-deficient NPCs were generated by in vitro nuclear assembly in RanBP2-depleted Xenopus egg extracts. NPCs were formed that lacked cytoplasmic filaments, but that retained CAN. These nuclei efficiently imported nuclear localization sequence (NLS) or M9 substrates. NPCs lacking CAN retained RanBP2 and cytoplasmic filaments, and showed a minor NLS import defect. NPCs deficient in both CAN and RanBP2 displayed no cytoplasmic filaments and had a strikingly immature cytoplasmic appearance. However, they showed only a slight reduction in NLS-mediated import, no change in M9-mediated import, and were normal in growth and DNA replication. We conclude that RanBP2 is the major nucleoporin component of the cytoplasmic filaments of the NPC, and that these filaments do not have an essential role in importin alpha/beta- or transportin-dependent import.

Regulated nuclear targeting of cauliflower mosaic virus

The mature cauliflower mosaic virus (CaMV) capsid protein (CP), if expressed in the absence of other viral proteins, is transported into the plant cell nucleus by the action of a nuclear localization signal (NLS) close to the N terminus. In contrast, virus particles do not enter the nucleus, but dock at the nuclear membrane, a process inhibited by anti-NLS antibodies or by GTP gamma S, and apparently mediated by interaction of CP with host importin alpha. The very acidic N-terminal extension of the viral CP precursor inhibits nuclear targeting of the protein and hence the precursor is localized in the cytoplasm. We hypothesize that this provides a control mechanism which ensures that the CP precursor is used for virus assembly in the cytoplasm and that only mature virus particles reach the nuclear pore.