Nup358 integrates nuclear envelope breakdown with kinetochore assembly

Interplay between the cyclophilin homology domain of RANBP2 and MX2 regulates HIV-1 capsid dependencies on nucleoporins

Interlinked interactions between the viral capsid (CA), nucleoporins (Nups), and the antiviral protein myxovirus resistance 2 (MX2/MXB) influence human immunodeficiency virus 1 (HIV-1) nuclear entry and the outcome of infection. Although RANBP2/NUP358 has been repeatedly identified as a critical player in HIV-1 nuclear import and MX2 activity, the mechanism by which RANBP2 facilitates HIV-1 infection is not well understood. To explore the interactions between MX2, the viral CA, and RANBP2, we utilized CRISPR-Cas9 to generate cell lines expressing RANBP2 from its endogenous locus but lacking the C-terminal cyclophilin (Cyp) homology domain and found that both HIV-1 and HIV-2 infections were reduced significantly in RANBP2ΔCyp cells. Importantly, although MX2 still localized to the nuclear pore complex in RANBP2ΔCyp cells, antiviral activity against HIV-1 was decreased. By generating cells expressing specific point mutations in the RANBP2-Cyp domain, we determined that the effect of the RANBP2-Cyp domain on MX2 anti-HIV-1 activity is due to direct interactions between RANBP2 and CA. We further determined that CypA and RANBP2-Cyp have similar effects on HIV-1 integration targeting. Finally, we found that the Nup requirements for HIV infection and MX2 activity were altered in cells lacking the RANBP2-Cyp domain. These findings demonstrate that the RANBP2-Cyp domain affects viral infection and MX2 sensitivity by altering CA-specific interactions with cellular factors that affect nuclear import and integration targeting.

IMPORTANCE

Human immunodeficiency virus 1 (HIV-1) entry into the nucleus is an essential step in viral replication that involves complex interactions between the viral capsid (CA) and multiple cellular proteins, including nucleoporins (Nups) such as RANBP2. Nups also mediate the function of the antiviral protein myxovirus resistance 2 (MX2); however, determining the precise role of Nups in HIV infection has proved challenging due to the complex nature of the nuclear pore complex (NPC) and significant pleiotropic effects elicited by Nup depletion. We have used precise gene editing to assess the role of the cyclophilin domain of RANBP2 in HIV-1 infection and MX2 activity. We find that this domain affects viral infection, nucleoporin requirements, MX2 sensitivity, and integration targeting in a CA-specific manner, providing detailed insights into how RANBP2 contributes to HIV-1 infection.

Nucleoporin Nup358 drives the differentiation of myeloid-biased multipotent progenitors by modulating HDAC3 nuclear translocation

Nucleoporins, the components of nuclear pore complexes (NPCs), can play cell type- and tissue-specific functions. Yet, the physiological roles and mechanisms of action for most NPC components have not yet been established. We report that Nup358, a nucleoporin linked to several myeloid disorders, is required for the developmental progression of early myeloid progenitors. We found that Nup358 ablation in mice results in the loss of myeloid-committed progenitors and mature myeloid cells and the accumulation of myeloid-primed multipotent progenitors (MPPs) in bone marrow. Accumulated MPPs in Nup358 knockout mice are greatly restricted to megakaryocyte/erythrocyte-biased MPP2, which fail to progress into committed myeloid progenitors. Mechanistically, we found that Nup358 is required for histone deacetylase 3 (HDAC3) nuclear import and function in MPP2 cells and established that this nucleoporin regulates HDAC3 nuclear translocation in a SUMOylation-independent manner. Our study identifies a critical function for Nup358 in myeloid-primed MPP2 differentiation and uncovers an unexpected role for NPCs in the early steps of myelopoiesis.

Distinct Mitotic Functions of Nucleolar and Spindle-Associated Protein 1 (NuSAP1) Are Controlled by Two Consensus SUMOylation Sites

Nucleolar and Spindle-Associated Protein 1 (NuSAP1) is an important mitotic regulator, implicated in control of mitotic microtubule stability and chromosome segregation. NuSAP1 regulates these processes by interacting with several protein partners. Its abundance, activity and interactions are therefore tightly regulated during mitosis. Protein conjugation with SUMO (Small Ubiquitin-like MOdifier peptide) is a reversible post-translational modification that modulates rapid changes in the structure, interaction(s) and localization of proteins. NuSAP1 was previously found to interact with RANBP2, a nucleoporin with SUMO ligase and SUMO-stabilizing activity, but how this interaction affects NuSAP1 activity has remained elusive. Here, we show that NuSAP1 interacts with RANBP2 and forms proximity ligation products with SUMO2/3 peptides in a RANBP2-dependent manner at key mitotic sites. A bioinformatic search identified two putative SUMO consensus sites in NuSAP1, within the DNA-binding and the microtubule-binding domains, respectively. Site-specific mutagenesis, and mitotic phenotyping in cell lines expressing each NuSAP1 mutant version, revealed selective roles of each individual site in control of NuSAP1 localization and in generation of specific mitotic defects and distinct fates in daughter cells. These results identify therefore two new regulatory sites for NuSAP1 functions and implicate RANBP2 in control of NuSAP1 activity.

A novel auxin-inducible degron system for rapid, cell cycle-specific targeted proteolysis

The discrimination of protein biological functions in different phases of the cell cycle is limited by the lack of experimental approaches that do not require pre-treatment with compounds affecting the cell cycle progression. Therefore, potential cycle-specific biological functions of a protein of interest could be biased by the effects of cell treatments. The OsTIR1/auxin-inducible degron (AID) system allows "on demand" selective and reversible protein degradation upon exposure to the phytohormone auxin. In the current format, this technology does not allow to study the effect of acute protein depletion selectively in one phase of the cell cycle, as auxin similarly affects all the treated cells irrespectively of their proliferation status. Therefore, the AID system requires coupling with cell synchronization techniques, which can alter the basal biological status of the studied cell population, as with previously available approaches. Here, we introduce a new AID system to Regulate OsTIR1 Levels based on the Cell Cycle Status (ROLECCS system), which induces proteolysis of both exogenously transfected and endogenous gene-edited targets in specific phases of the cell cycle. We validated the ROLECCS technology by down regulating the protein levels of TP53, one of the most studied tumor suppressor genes, with a widely known role in cell cycle progression. By using our novel tool, we observed that TP53 degradation is associated with increased number of micronuclei, and this phenotype is specifically achieved when TP53 is lost in S/G2/M phases of the cell cycle, but not in G1. Therefore, we propose the use of the ROLECCS system as a new improved way of studying the differential roles that target proteins may have in specific phases of the cell cycle.

Aurora B SUMOylation Is Restricted to Centromeres in Early Mitosis and Requires RANBP2

Conjugation with the small ubiquitin-like modifier (SUMO) modulates protein interactions and localisation. The kinase Aurora B, a key regulator of mitosis, was previously identified as a SUMOylation target in vitro and in assays with overexpressed components. However, where and when this modification genuinely occurs in human cells was not ascertained. Here, we have developed intramolecular Proximity Ligation Assays (PLA) to visualise SUMO-conjugated Aurora B in human cells in situ. We visualised Aurora B-SUMO products at centromeres in prometaphase and metaphase, which declined from anaphase onwards and became virtually undetectable at cytokinesis. In the mitotic window in which Aurora B/SUMO products are abundant, Aurora B co-localised and interacted with NUP358/RANBP2, a nucleoporin with SUMO ligase and SUMO-stabilising activity. Indeed, in addition to the requirement for the previously identified PIAS3 SUMO ligase, we found that NUP358/RANBP2 is also implicated in Aurora B-SUMO PLA product formation and centromere localisation. In summary, SUMOylation marks a distinctive window of Aurora B functions at centromeres in prometaphase and metaphase while being dispensable for functions exerted in cytokinesis, and RANBP2 contributes to this control, adding a novel layer to modulation of Aurora B functions during mitosis.

ahctf1 and kras mutations combine to amplify oncogenic stress and restrict liver overgrowth in a zebrafish model of hepatocellular carcinoma

The nucleoporin (NUP) ELYS, encoded by AHCTF1, is a large multifunctional protein with essential roles in nuclear pore assembly and mitosis. Using both larval and adult zebrafish models of hepatocellular carcinoma (HCC), in which the expression of an inducible mutant kras transgene (kras^G12V) drives hepatocyte-specific hyperplasia and liver enlargement, we show that reducing ahctf1 gene dosage by 50% markedly decreases liver volume, while non-hyperplastic tissues are unaffected. We demonstrate that in the context of cancer, ahctf1 heterozygosity impairs nuclear pore formation, mitotic spindle assembly, and chromosome segregation, leading to DNA damage and activation of a Tp53-dependent transcriptional programme that induces cell death and cell cycle arrest. Heterozygous expression of both ahctf1 and ranbp2 (encoding a second nucleoporin), or treatment of heterozygous ahctf1 larvae with the nucleocytoplasmic transport inhibitor, Selinexor, completely blocks kras^G12V-driven hepatocyte hyperplasia. Gene expression analysis of patient samples in the liver hepatocellular carcinoma (LIHC) dataset in The Cancer Genome Atlas shows that high expression of one or more of the transcripts encoding the 10 components of the NUP107-160 subcomplex, which includes AHCTF1, is positively correlated with worse overall survival. These results provide a strong and feasible rationale for the development of novel cancer therapeutics that target ELYS function and suggest potential avenues for effective combinatorial treatments.

Architecture of the cytoplasmic face of the nuclear pore

INTRODUCTION The subcellular compartmentalization of eukaryotic cells requires selective transport of folded proteins and protein-nucleic acid complexes. Embedded in nuclear envelope pores, which are generated by the circumscribed fusion of the inner and outer nuclear membranes, nuclear pore complexes (NPCs) are the sole bidirectional gateways for nucleocytoplasmic transport. The ~110-MDa human NPC is an ~1000-protein assembly that comprises multiple copies of ~34 different proteins, collectively termed nucleoporins. The symmetric core of the NPC is composed of an inner ring encircling the central transport channel and outer rings formed by Y‑shaped coat nucleoporin complexes (CNCs) anchored atop both sides of the nuclear envelope. The outer rings are decorated with compartment‑specific asymmetric nuclear basket and cytoplasmic filament nucleoporins, which establish transport directionality and provide docking sites for transport factors and the small guanosine triphosphatase Ran. The cytoplasmic filament nucleoporins also play an essential role in the irreversible remodeling of messenger ribonucleoprotein particles (mRNPs) as they exit the central transport channel. Unsurprisingly, the NPC's cytoplasmic face represents a hotspot for disease‑associated mutations and is commonly targeted by viral virulence factors. RATIONALE Previous studies established a near-atomic composite structure of the human NPC's symmetric core by combining (i) biochemical reconstitution to elucidate the interaction network between symmetric nucleoporins, (ii) crystal and single-particle cryo-electron microscopy structure determination of nucleoporins and nucleoporin complexes to reveal their three-dimensional shape and the molecular details of their interactions, (iii) quantitative docking in cryo-electron tomography (cryo-ET) maps of the intact human NPC to uncover nucleoporin stoichiometry and positioning, and (iv) cell‑based assays to validate the physiological relevance of the biochemical and structural findings. In this work, we extended our approach to the cytoplasmic filament nucleoporins to reveal the near-atomic architecture of the cytoplasmic face of the human NPC. RESULTS Using biochemical reconstitution, we elucidated the protein-protein and protein-RNA interaction networks of the human and Chaetomium thermophilum cytoplasmic filament nucleoporins, establishing an evolutionarily conserved heterohexameric cytoplasmic filament nucleoporin complex (CFNC) held together by a central heterotrimeric coiled‑coil hub that tethers two separate mRNP‑remodeling complexes. Further biochemical analysis and determination of a series of crystal structures revealed that the metazoan‑specific cytoplasmic filament nucleoporin NUP358 is composed of 16 distinct domains, including an N‑terminal S‑shaped α‑helical solenoid followed by a coiled‑coil oligomerization element, numerous Ran‑interacting domains, an E3 ligase domain, and a C‑terminal prolyl‑isomerase domain. Physiologically validated quantitative docking into cryo-ET maps of the intact human NPC revealed that pentameric NUP358 bundles, conjoined by the oligomerization element, are anchored through their N‑terminal domains to the central stalk regions of the CNC, projecting flexibly attached domains as far as ~600 Å into the cytoplasm. Using cell‑based assays, we demonstrated that NUP358 is dispensable for the architectural integrity of the assembled interphase NPC and RNA export but is required for efficient translation. After NUP358 assignment, the remaining 4-shaped cryo‑ET density matched the dimensions of the CFNC coiled‑coil hub, in close proximity to an outer-ring NUP93. Whereas the N-terminal NUP93 assembly sensor motif anchors the properly assembled related coiled‑coil channel nucleoporin heterotrimer to the inner ring, biochemical reconstitution confirmed that the NUP93 assembly sensor is reused in anchoring the CFNC to the cytoplasmic face of the human NPC. By contrast, two C. thermophilum CFNCs are anchored by a divergent mechanism that involves assembly sensors located in unstructured portions of two CNC nucleoporins. Whereas unassigned cryo‑ET density occupies the NUP358 and CFNC binding sites on the nuclear face, docking of the nuclear basket component ELYS established that the equivalent position on the cytoplasmic face is unoccupied, suggesting that mechanisms other than steric competition promote asymmetric distribution of nucleoporins. CONCLUSION We have substantially advanced the biochemical and structural characterization of the asymmetric nucleoporins' architecture and attachment at the cytoplasmic and nuclear faces of the NPC. Our near‑atomic composite structure of the human NPC's cytoplasmic face provides a biochemical and structural framework for elucidating the molecular basis of mRNP remodeling, viral virulence factor interference with NPC function, and the underlying mechanisms of nucleoporin diseases at the cytoplasmic face of the NPC. [Figure: see text].

The Nuclear Lamina

Lamins interact with a host of nuclear membrane proteins, transcription factors, chromatin regulators, signaling molecules, splicing factors, and even chromatin itself to form a nuclear subcompartment, the nuclear lamina, that is involved in a variety of cellular processes such as the governance of nuclear integrity, nuclear positioning, mitosis, DNA repair, DNA replication, splicing, signaling, mechanotransduction and -sensation, transcriptional regulation, and genome organization. Lamins are the primary scaffold for this nuclear subcompartment, but interactions with lamin-associated peptides in the inner nuclear membrane are self-reinforcing and mutually required. Lamins also interact, directly and indirectly, with peripheral heterochromatin domains called lamina-associated domains (LADs) and help to regulate dynamic 3D genome organization and expression of developmentally regulated genes.

Lamin C is required to establish genome organization after mitosis

BACKGROUND

The dynamic 3D organization of the genome is central to gene regulation and development. The nuclear lamina influences genome organization through the tethering of lamina-associated domains (LADs) to the nuclear periphery. Evidence suggests that lamins A and C are the predominant lamins involved in the peripheral association of LADs, potentially serving different roles.

RESULTS

Here, we examine chromosome architecture in mouse cells in which lamin A or lamin C are downregulated. We find that lamin C, and not lamin A, is required for the 3D organization of LADs and overall chromosome organization. Striking differences in localization are present as cells exit mitosis and persist through early G1 and are linked to differential phosphorylation. Whereas lamin A associates with the nascent nuclear envelope (NE) during telophase, lamin C remains in the interior, surrounding globular LAD aggregates enriched on euchromatic regions. Lamin C association with the NE is delayed until several hours into G1 and correlates temporally and spatially with the post-mitotic NE association of LADs. Post-mitotic LAD association with the NE, and global 3D genome organization, is perturbed only in cells depleted of lamin C, and not lamin A.

CONCLUSIONS

Lamin C regulates LAD dynamics during exit from mitosis and is a key regulator of genome organization in mammalian cells. This reveals an unexpectedly central role for lamin C in genome organization, including inter-chromosomal LAD-LAD segregation and LAD scaffolding at the NE, raising intriguing questions about the individual and overlapping roles of lamin A/C in cellular function and disease.

Evolutionary Dynamics of the Spindle Assembly Checkpoint in Eukaryotes

The tremendous diversity in eukaryotic life forms can ultimately be traced back to evolutionary modifications at the level of molecular networks. Deep understanding of these modifications will not only explain cellular diversity, but will also uncover different ways to execute similar processes and expose the evolutionary 'rules' that shape the molecular networks. Here, we review the evolutionary dynamics of the spindle assembly checkpoint (SAC), a signaling network that guards fidelity of chromosome segregation. We illustrate how the interpretation of divergent SAC systems in eukaryotic species is facilitated by combining detailed molecular knowledge of the SAC and extensive comparative genome analyses. Ultimately, expanding this to other core cellular systems and experimentally interrogating such systems in organisms from all major lineages may start outlining the routes to and eventual manifestation of the cellular diversity of eukaryotic life.

YTHDF1 Aggravates the Progression of Cervical Cancer Through m6A-Mediated Up-Regulation of RANBP2

N6-methyladenosine (m⁶A) is the most common post-transcriptional modification of RNA in eukaryotes, which has been demonstrated to play important roles in various cancers. YTHDF1 acts as a crucial m⁶A “reader” and regulates the fate of m⁶A modified mRNA. However, its role in cervical cancer remains unknown. In this study, we showed that YTHDF1 was highly expressed in cervical cancer, and was closely associated with the poor prognosis of cervical cancer patients. YTHDF1 knockdown suppressed the growth, migration and invasion, and induced apoptosis of cervical cancer cells. Moreover, YTHDF1 knockdown inhibited tumorigenesis of cervical cancer cells in vivo. Through combined on-line data analysis of RIP-seq, meRIP-seq and Ribo-seq upon YTHDF1 knockdown, RANBP2 was identified as the key target of YTHDF1 in cervical cancer cells. YTHDF1 regulated RANBP2 translation in an m⁶A-dependent manner without effect on its mRNA expression. RANBP2 potentiated the growth, migration and invasion of cervical cancer cells. Our study demonstrated the oncogenic role of YTHDF1 in cervical cancer by regulating RANBP2 expression and YTHDF1 represents a potential target for cervical cancer therapy.

NUP62 is required for the maintenance of the spindle assembly checkpoint and chromosomal stability

The nuclear pore protein NUP62 localizes to spindle poles in mitosis and plays a role in maintaining centrosome homeostasis. In this study, we found that NUP62-depleted cells exhibited a defective spindle assembly checkpoint (SAC) and that depletion of NUP62 caused a slight decrease in MAD2 protein levels after nocodazole treatment. However, depletion of NUP62 did not cause a failure in kinetochore localization of the SAC proteins BUBR1, MAD1, and MAD2 in prometaphase. NUP62 depletion slightly prolonged mitotic timing but did not affect cell doubling time. In addition, NUP62 depletion caused a SAC defect and induced aneuploidy in human neural stem cells. Furthermore, overexpression of NUP62Q391P, a mutant protein causing autosomal recessive infantile bilateral striatal necrosis, resulted in a defect in the SAC, indicating that the amino acid residue Q391 in NUP62 is crucial for its effect on the SAC. Overall, we conclude that NUP62 maintains the SAC downstream of kinetochores and thereby ensures maintenance of chromosomal stability.

From the Nuclear Pore to the Fibrous Corona: A MAD Journey to Preserve Genome Stability

The relationship between kinetochores and nuclear pore complexes (NPCs) is intimate but poorly understood. Several NPC components and associated proteins are relocated to mitotic kinetochores to assist in different activities that ensure faithful chromosome segregation. Such is the case of the Mad1-c-Mad2 complex, the catalytic core of the spindle assembly checkpoint (SAC), a surveillance pathway that delays anaphase until all kinetochores are attached to spindle microtubules. Mad1-c-Mad2 is recruited to discrete domains of unattached kinetochores from where it promotes the rate-limiting step in the assembly of anaphase-inhibitory complexes. SAC proficiency further requires Mad1-c-Mad2 to be anchored at NPCs during interphase. However, the mechanistic relevance of this arrangement for SAC function remains ill-defined. Recent studies uncover the molecular underpinnings that coordinate the release of Mad1-c-Mad2 from NPCs with its prompt recruitment to kinetochores. Here, current knowledge on Mad1-c-Mad2 function and spatiotemporal regulation is reviewed and the critical questions that remain unanswered are highlighted.

Nucleoporins in cardiovascular disease

Cardiovascular disease is a pressing health problem with significant global health, societal, and financial burdens. Understanding the molecular basis of polygenic cardiac pathology is thus essential to devising novel approaches for management and treatment. Recent identification of uncharacterized regulatory functions for a class of nuclear envelope proteins called nucleoporins offers the opportunity to understand novel putative mechanisms of cardiac disease development and progression. Consistent reports of nucleoporin deregulation associated with ischemic and dilated cardiomyopathies, arrhythmias and valvular disorders suggests that nucleoporin impairment may be a significant but understudied variable in cardiopathologic disorders. This review discusses and converges existing literature regarding nuclear pore complex proteins and their association with cardiac pathologies, and proposes a role for nucleoporins as facilitators of cardiac disease.

Ankyrin-G induces nucleoporin Nup358 to associate with the axon initial segment of neurons

Nup358 (also known as RanBP2) is a member of the large nucleoporin family that constitutes the nuclear pore complex. Depending on the cell type and the physiological state, Nup358 interacts with specific partner proteins and influences distinct mechanisms independent of its role in nucleocytoplasmic transport. Here, we provide evidence that Nup358 associates selectively with the axon initial segment (AIS) of mature neurons, mediated by the AIS scaffold protein ankyrin-G (AnkG, also known as Ank3). The N-terminus of Nup358 is demonstrated to be sufficient for its localization at the AIS. Further, we show that Nup358 is expressed as two isoforms, one full-length and another shorter form of Nup358. These isoforms differ in their subcellular distribution in neurons and expression level during neuronal development. Overall, the present study highlights an unprecedented localization of Nup358 within the AIS and suggests its involvement in neuronal function.This article has an associated First Person interview with the first author of the paper.

The Role of Phosphatases in Nuclear Envelope Disassembly and Reassembly and Their Relevance to Pathologies

The role of kinases in the regulation of cell cycle transitions is very well established, however, over the past decade, studies have identified the ever-growing importance of phosphatases in these processes. It is well-known that an intact or otherwise non-deformed nuclear envelope (NE) is essential for maintaining healthy cells and any deviation from this can result in pathological conditions. This review aims at assessing the current understanding of how phosphatases contribute to the remodelling of the nuclear envelope during its disassembling and reformation after cell division and how errors in this process may lead to the development of diseases.

Linking Nucleoporins, Mitosis, and Colon Cancer

Suppression of a nuclear pore protein Nup358/RanBP2 is linked to mitotic cell death, but the clinical relevance of this link is unknown. In a recent issue of Cell, Vecchione et al. (2016) show that in approximately 10% of BRAF-like colorectal cancer (CC) patients, Nup358/RanBP2 is critical for survival. Treatment with vinorelbine, a microtubule-depolymerizing drug that inhibits mitosis, might be a potential treatment for these CCs.

Impact of dynamin 2 on adenovirus nuclear entry

The large GTPase dynamin 2 controls both endosomal fission and microtubule acetylation. Here we report that dynamin 2 alters microtubules and regulates the trafficking of human adenovirus type 37. Dynamin 2 knockdown by siRNA in infected cells resulted in accumulation of acetylated tubulin, repositioning of microtubule organizing centers (MTOCs) closer to cell nuclei, increased virus in the cytosol (with a compensatory decrease in endosomal virus), reduced proinflammatory cytokine induction, and increased binding of virus to the nucleoporin, Nup358. These events led to increased viral DNA nuclear entry and viral replication. Overexpression of dynamin 2 generated opposite effects. Therefore, dynamin 2 inhibits adenovirus replication and promotes innate immune responses by the infected cell. MTOC transposition in dynamin 2 knockdown promotes a closer association with nuclear pore complexes to facilitate viral DNA delivery. Dynamin 2 plays a key role in adenoviral trafficking and influences host responses to infection.

Regulation of Cenp-F localization to nuclear pores and kinetochores

In metazoans, the assembly of kinetochores on centrometric chromatin and the dismantling of nuclear pore complexes are processes that have to be tightly coordinated to ensure the proper assembly of the mitotic spindle and a successful mitosis. It is therefore noteworthy that these two macromolecular assemblies share a subset of constituents. One of these multifaceted components is Cenp-F, a protein implicated in cancer and developmental pathologies. During the cell cycle, Cenp-F localizes in multiple cellular structures including the nuclear envelope in late G2/early prophase and kinetochores throughout mitosis. We recently characterized the molecular determinants of Cenp-F interaction with Nup133, a structural nuclear pore constituent. In parallel with two other independent studies, we further elucidated the mechanisms governing Cenp-F kinetochore recruitment that mainly relies on its interaction with Bub1, with redundant contribution of Cenp-E upon acute microtubule depolymerisation. Here we synthesize the current literature regarding the dual location of Cenp-F at nuclear pores and kinetochores and extend our discussion to the regulation of these NPC and kinetochore localizations by mitotic kinase and spindle microtubules.

Zap70 and downstream RanBP2 are required for the exact timing of the meiotic cell cycle in oocytes

In previous studies, we observed that Zeta-chain-associated protein kinase 70 (Zap70) regulates spindle assembly and chromosome alignment in mouse oocyte and that Ran binding protein 2 (RanBP2) is a highly associated gene with Zap70 based on a microarray analysis. Because RanBP2 is related to nuclear envelope breakdown (NEBD) during mitosis, the aim of the present study was to elucidate the molecular mechanism of Zap70 with respect to RanBP2 in the germinal vesicle breakdown (GVBD) of oocytes. Results indicated that RanBP2 expression was regulated by Zap70 and that depletion of RanBP2 using RanBP2 RNAi manifested comparable phenotypes to those observed in Zap70 RNAi-treated oocytes, which presented faster processing of GVBD. Additionally, Zap70 RNAi-treated oocytes showed faster meiotic resumption with premature activation of maturation-promoting factor (MPF), premature division of chromosomes at approximately 6-8 h and more rapid degradation of securin. In conclusion, we report that Zap70 is a crucial factor for controlling the exact timing of meiotic progression in mouse oocytes.

Importin-β and CRM1 control a RANBP2 spatiotemporal switch essential for mitotic kinetochore function

Protein conjugation with small ubiquitin-related modifier (SUMO) is a post-translational modification that modulates protein interactions and localisation. RANBP2 is a large nucleoporin endowed with SUMO E3 ligase and SUMO-stabilising activity, and is implicated in some cancer types. RANBP2 is part of a larger complex, consisting of SUMO-modified RANGAP1, the GTP-hydrolysis activating factor for the GTPase RAN. During mitosis, the RANBP2-SUMO-RANGAP1 complex localises to the mitotic spindle and to kinetochores after microtubule attachment. Here, we address the mechanisms that regulate this localisation and how they affect kinetochore functions. Using proximity ligation assays, we find that nuclear transport receptors importin-β and CRM1 play essential roles in localising the RANBP2-SUMO-RANGAP1 complex away from, or at kinetochores, respectively. Using newly generated inducible cell lines, we show that overexpression of nuclear transport receptors affects the timing of RANBP2 localisation in opposite ways. Concomitantly, kinetochore functions are also affected, including the accumulation of SUMO-conjugated topoisomerase-IIα and stability of kinetochore fibres. These results delineate a novel mechanism through which nuclear transport receptors govern the functional state of kinetochores by regulating the timely deposition of RANBP2.

Nuclear pore complex tethers to the cytoskeleton

The nuclear envelope is tethered to the cytoskeleton. The best known attachments of all elements of the cytoskeleton are via the so-called LINC complex. However, the nuclear pore complexes, which mediate the transport of soluble and membrane bound molecules, are also linked to the microtubule network, primarily via motor proteins (dynein and kinesins) which are linked, most importantly, to the cytoplasmic filament protein of the nuclear pore complex, Nup358, by the adaptor BicD2. The evidence for such linkages and possible roles in nuclear migration, cell cycle control, nuclear transport and cell architecture are discussed.

SUMO conjugation - a mechanistic view

The regulation of protein fate by modification with the small ubiquitin-related modifier (SUMO) plays an essential and crucial role in most cellular pathways. Sumoylation is highly dynamic due to the opposing activities of SUMO conjugation and SUMO deconjugation. SUMO conjugation is performed by the hierarchical action of E1, E2 and E3 enzymes, while its deconjugation involves SUMO-specific proteases. In this review, we summarize and compare the mechanistic principles of how SUMO gets conjugated to its substrate. We focus on the interplay of the E1, E2 and E3 enzymes and discuss how specificity could be achieved given the limited number of conjugating enzymes and the thousands of substrates.

Nup358 binds to AGO proteins through its SUMO-interacting motifs and promotes the association of target mRNA with miRISC

MicroRNA (miRNA)-guided mRNA repression, mediated by the miRNA-induced silencing complex (miRISC), is an important component of post-transcriptional gene silencing. However, how miRISC identifies the target mRNA in vivo is not well understood. Here, we show that the nucleoporin Nup358 plays an important role in this process. Nup358 localizes to the nuclear pore complex and to the cytoplasmic annulate lamellae (AL), and these structures dynamically associate with two mRNP granules: processing bodies (P bodies) and stress granules (SGs). Nup358 depletion disrupts P bodies and concomitantly impairs the miRNA pathway. Furthermore, Nup358 interacts with AGO and GW182 proteins and promotes the association of target mRNA with miRISC A well-characterized SUMO-interacting motif (SIM) in Nup358 is sufficient for Nup358 to directly bind to AGO proteins. Moreover, AGO and PIWI proteins interact with SIMs derived from other SUMO-binding proteins. Our study indicates that Nup358-AGO interaction is important for miRNA-mediated gene silencing and identifies SIM as a new interacting motif for the AGO family of proteins. The findings also support a model wherein the coupling of miRISC with the target mRNA could occur at AL, specialized domains within the ER, and at the nuclear envelope.

Nup62, associated with spindle microtubule rather than spindle matrix, is involved in chromosome alignment and spindle assembly during mitosis

An increasing number of the active mitotic functions of nucleoporins in the distinct steps of mitosis have been assigned over the past few years. As one of FG-repeats containing nucleoporins, Nup62 has been found to be involved in nuclear transport, cell migration, virus infection, and cell cycle regulation. However, the role and mechanism of Nup62 in mitotic regulation have not been fully revealed. In this paper, it was revealed that a fraction of Nup62 was associated with mitotic spindle microtubule instead of spindle matrix, and the localization of Nup62 in the mitotic spindle depended on its three coiled-coil domains rather than Crm1, although Nup62 strongly interacted with Crm1 during mitosis. Moreover, depletion of Nup62 by small interference of RNA seriously induced the defects of chromosome alignment and spindle assembly although the bipolar spindle was observed in most of the Nup62 knock-down cells. Notably, congression of polar chromosome defect was observed in more than 30% of Nup62 knock-down cells. These findings revealed that Nup62 was a novel mitotic spindle associated nucleoporin and involved in chromosome alignment and spindle assembly.

A Vulnerability of a Subset of Colon Cancers with Potential Clinical Utility

BRAF(V600E) mutant colon cancers (CCs) have a characteristic gene expression signature that is also found in some tumors lacking this mutation. Collectively, they are referred to as "BRAF-like" tumors and represent some 20% of CCs. We used a shRNA-based genetic screen focused on genes upregulated in BRAF(V600E) CCs to identify vulnerabilities of this tumor subtype that might be exploited therapeutically. Here, we identify RANBP2 (also known as NUP358) as essential for survival of BRAF-like, but not for non-BRAF-like, CC cells. Suppression of RANBP2 results in mitotic defects only in BRAF-like CC cells, leading to cell death. Mechanistically, RANBP2 silencing reduces microtubule outgrowth from the kinetochores, thereby inducing spindle perturbations, providing an explanation for the observed mitotic defects. We find that BRAF-like CCs display far greater sensitivity to the microtubule poison vinorelbine both in vitro and in vivo, suggesting that vinorelbine is a potential tailored treatment for BRAF-like CCs.

The GTPase RAN regulates multiple steps of the centrosome life cycle

Growing lines of evidence implicate the small GTPase RAN, its regulators and effectors--predominantly, nuclear transport receptors--in practically all aspects of centrosome biology in mammalian cells. These include duplication licensing, cohesion, positioning, and microtubule-nucleation capacity. RAN cooperates with the protein nuclear export vector exportin 1/CRM1 to recruit scaffolding proteins containing nuclear export sequences that play roles in the structural organization of centrosomes. Together, they also limit centrosome reduplication by regulating the localization of key "licensing" proteins during the centrosome duplication cycle. In parallel, RAN also regulates the capacity of centrosomes to nucleate and organize functional microtubules, and this predominanlty involves importin vectors: many factors regulating microtubule nucleation or function harbor nuclear localization sequences that interact with importin molecules and such interaction inhibits their activity. Active RANGTP binding to importin molecules removes the inhibition and releases microtubule regulatory factors in the free productive form. A dynamic scenario emerges, in which RAN is pivotal in linking spatiotemporal control of centrosome regulators to the cell cycle machinery.

The Ran Pathway in Drosophila melanogaster Mitosis

Over the last two decades, the small GTPase Ran has emerged as a central regulator of both mitosis and meiosis, particularly in the generation, maintenance, and regulation of the microtubule (MT)-based bipolar spindle. Ran-regulated pathways in mitosis bear many similarities to the well-characterized functions of Ran in nuclear transport and, as with transport, the majority of these mitotic effects are mediated through affecting the physical interaction between karyopherins and Spindle Assembly Factors (SAFs)—a loose term describing proteins or protein complexes involved in spindle assembly through promoting nucleation, stabilization, and/or depolymerization of MTs, through anchoring MTs to specific structures such as centrosomes, chromatin or kinetochores, or through sliding MTs along each other to generate the force required to achieve bipolarity. As such, the Ran-mediated pathway represents a crucial functional module within the wider spindle assembly landscape. Research into mitosis using the model organism Drosophila melanogaster has contributed substantially to our understanding of centrosome and spindle function. However, in comparison to mammalian systems, very little is known about the contribution of Ran-mediated pathways in Drosophila mitosis. This article sets out to summarize our understanding of the roles of the Ran pathway components in Drosophila mitosis, focusing on the syncytial blastoderm embryo, arguing that it can provide important insights into the conserved functions on Ran during spindle formation.

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 envelope and chromatin, lock and key of genome integrity

More than as an inert separation between the inside and outside of the nucleus, the nuclear envelope (NE) constitutes an active toll, which controls the import and export of molecules, and also a hub for a diversity of genomic processes, such as transcription, DNA repair, and chromatin dynamics. Proteins localized at the inner surface of the NE (such as lamins, nuclear pore proteins, lamin-associated proteins) interact with chromatin in a dynamic manner, contributing to the establishment of topological domains. In this review, we address the complex interplay between chromatin and NE. We discuss the divergence of this cross talk during evolution and comment both on the current established models and the most recent findings. In particular, we focus our attention on how the NE cooperates with chromatin in protecting the genome integrity.

The nucleoporin Mlp2 is involved in chromosomal distribution during mitosis in trypanosomatids

Nucleoporins are evolutionary conserved proteins mainly involved in the constitution of the nuclear pores and trafficking between the nucleus and cytoplasm, but are also increasingly viewed as main actors in chromatin dynamics and intra-nuclear mitotic events. Here, we determined the cellular localization of the nucleoporin Mlp2 in the 'divergent' eukaryotes Leishmania major and Trypanosoma brucei. In both protozoa, Mlp2 displayed an atypical localization for a nucleoporin, essentially intranuclear, and preferentially in the periphery of the nucleolus during interphase; moreover, it relocated at the mitotic spindle poles during mitosis. In T. brucei, where most centromeres have been identified, TbMlp2 was found adjacent to the centromeric sequences, as well as to a recently described unconventional kinetochore protein, in the periphery of the nucleolus, during interphase and from the end of anaphase onwards. TbMlp2 and the centromeres/kinetochores exhibited a differential migration towards the poles during mitosis. RNAi knockdown of TbMlp2 disrupted the mitotic distribution of chromosomes, leading to a surprisingly well-tolerated aneuploidy. In addition, diploidy was restored in a complementation assay where LmMlp2, the orthologue of TbMlp2 in Leishmania, was expressed in TbMlp2-RNAi-knockdown parasites. Taken together, our results demonstrate that Mlp2 is involved in the distribution of chromosomes during mitosis in trypanosomatids.

Inhibition of nucleoporin member Nup214 expression by miR-133b perturbs mitotic timing and leads to cell death

Background

Nucleoporins mediate nucleocytoplasmic exchange of macromolecules and several have been assigned active mitotic functions. Nucleoporins can participate in various mitotic functions like spindle assembly, kinetochore organisation and chromosome segregation- important for genome integrity. Pathways to genome integrity are frequently deregulated in cancer and many are regulated in part by microRNAs. Indeed, altered levels of numerous microRNAs have frequently been associated with tumorigenesis. Here, we unveil a microRNA-mediated regulation of the nucleoporin Nup214 and its downstream effect on genome integrity.

Methods

Databases/bioinformatic tools such as miRBase, Oncomine and RNAhybrid predicted Nup214 as a miR-133b target. To validate this, we used luciferase reporter assays, Real-Time PCR and immuno-blotting. Flow cytometry and immuno-blots of mitotic markers were used to analyse cell cycle pattern upon thymidine synchronization and miR-133b treatment. Mitotic indices and chromosomal abnormalities were assessed by immuno-fluorescence for FITC-tagged phospho-H3 as well as video-microscopy for GFP-tagged histone H4. Annexin V/propidium iodide staining, caspase3/PARP cleavage and colony formation assays were done to investigate cell death upon either miR-133b transfection or NUP214 knockdown by siRNA. UPCI:SCC084, HCT116, HeLa-H4-pEGFP and HEK293 (human oral squamous cell carcinoma, colorectal, cervical carcinomas and embryonic kidney cell lines, respectively) were used. miR-133b and NUP214 expressions were validated in cancer cell lines and tissues by Real-Time PCR.

Results

Examination of head and neck tumour tissues and cancer cell lines revealed that Nup214 and miR-133b expressions are negatively correlated. In vitro, Nup214 was significantly downregulated by ectopic miR-133b. This downregulation elevated mitotic indices and delayed degradation of mitotic marker proteins cyclinB1 and cyclinA and dephosphorylation of H3. Moreover, this mitotic delay enhanced chromosomal abnormalities and apoptosis.

Conclusions

We have identified NUP214, a member of the massive nuclear pore complex, as a novel miR-133b target. Thus, we have shown a hitherto unknown microRNA regulation of mitosis mediated by a member of the nucleoporin family. Based on observations, we also raise some hypotheses regarding transport-dependent/independent functions of Nup214 in this study. Our results hence attempt to explain why miR-133b is generally downregulated in tumours and lay out the potential for Nup214 as a therapeutic target in the treatment of cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0299-z) contains supplementary material, which is available to authorized users.

Selective impairment of a subset of Ran-GTP-binding domains of ran-binding protein 2 (Ranbp2) suffices to recapitulate the degeneration of the retinal pigment epithelium (RPE) triggered by Ranbp2 ablation

Retinal pigment epithelium (RPE) degeneration underpins diseases triggered by disparate genetic lesions, noxious insults, or both. The pleiotropic Ranbp2 controls the expression of intrinsic and extrinsic pathological stressors impinging on cellular viability. However, the physiological targets and mechanisms controlled by Ranbp2 in tissue homeostasis, such as RPE, are ill defined. We show that mice, RPE-cre::Ranbp2(-/-), with selective Ranbp2 ablation in RPE develop pigmentary changes, syncytia, hypoplasia, age-dependent centrifugal and non-apoptotic degeneration of the RPE, and secondary leakage of choriocapillaris. These manifestations are accompanied by the development of F-actin clouds, metalloproteinase-11 activation, deregulation of expression or subcellular localization of critical RPE proteins, atrophic cell extrusions into the subretinal space, and compensatory proliferation of peripheral RPE. To gain mechanistic insights into what Ranbp2 activities are vital to the RPE, we performed genetic complementation analyses of transgenic lines of bacterial artificial chromosomes of Ranbp2 harboring loss of function of selective Ranbp2 domains expressed in a Ranbp2(-/-) background. Among the transgenic lines produced, only Tg(RBD2/3*-HA)::RPE-cre::Ranbp2(-/-)-expressing mutations, which selectively impair binding of RBD2/3 (Ran-binding domains 2 and 3) of Ranbp2 to Ran-GTP, recapitulate RPE degeneration, as observed with RPE-cre::Ranbp2(-/-). By contrast, Tg(RBD2/3*-HA) expression rescues the degeneration of cone photoreceptors lacking Ranbp2. The RPE of RPE-cre::Ranbp2(-/-) and Tg(RBD2/3*-HA)::RPE-cre::Ranbp2(-/-) share proteostatic deregulation of Ran GTPase, serotransferrin, and γ-tubulin and suppression of light-evoked electrophysiological responses. These studies unravel selective roles of Ranbp2 and its RBD2 and RBD3 in RPE survival and functions. We posit that the control of Ran GTPase by Ranbp2 emerges as a novel therapeutic target in diseases promoting RPE degeneration.

Gathering up meiotic telomeres: a novel function of the microtubule-organizing center

During meiosis, telomeres cluster and promote homologous chromosome pairing. Telomere clustering depends on conserved SUN and KASH domain nuclear membrane proteins, which form a complex called the linker of nucleoskeleton and cytoskeleton (LINC) and connect telomeres with the cytoskeleton. It has been thought that LINC-mediated cytoskeletal forces induce telomere clustering. However, how cytoskeletal forces induce telomere clustering is not fully understood. Recent study of fission yeast has shown that the LINC complex forms the microtubule-organizing center (MTOC) at the telomere, which has been designated as the "telocentrosome", and that microtubule motors gather telomeres via telocentrosome-nucleated microtubules. This MTOC-dependent telomere clustering might be conserved in other eukaryotes. Furthermore, the MTOC-dependent clustering mechanism appears to function in various other biological events. This review presents an overview of the current understanding of the mechanism of meiotic telomere clustering and discusses the universality of the MTOC-dependent clustering mechanism.

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.

New mitotic regulators released from chromatin

Faithful action of the mitotic spindle segregates duplicated chromosomes into daughter cells. Perturbations of this process result in chromosome mis-segregation, leading to chromosomal instability and cancer development. Chromosomes are not simply passengers segregated by spindle microtubules but rather play a major active role in spindle assembly. The GTP bound form of the Ran GTPase (RanGTP), produced around chromosomes, locally activates spindle assembly factors. Recent studies have uncovered that chromosomes organize mitosis beyond spindle formation. They distinctly regulate other mitotic events, such as spindle maintenance in anaphase, which is essential for chromosome segregation. Furthermore, the direct function of chromosomes is not only to produce RanGTP but, in addition, to release key mitotic regulators from chromatin. Chromatin-remodeling factors and nuclear pore complex proteins, which have established functions on chromatin in interphase, dissociate from mitotic chromatin and function in spindle assembly or maintenance. Thus, chromosomes actively organize their own segregation using chromatin-releasing mitotic regulators as well as RanGTP.

Down-modulation of nucleoporin RanBP2/Nup358 impaired chromosomal alignment and induced mitotic catastrophe

Chromosomal missegregation is a common feature of many human tumors. Recent studies have indicated a link between nucleoporin RanBP2/Nup358 and chromosomal segregation during mitosis; however, the molecular details have yet to be fully established. Observed through live cell imaging and flow cytometry, here we show that RNA interference-mediated knockdown of RanBP2 induced G2/M phase arrest, metaphase catastrophe and mitotic cell death. Furthermore, RanBP2 down-modulation disrupted importin/karyopherin β1 as well as the expression and localization of the Ran GTPase activating protein 1. We found that N-terminal of RanBP2 interacted with the N-terminal of importin β1. Moreover, at least a portion of RanBP2 partially localizes at the centrosome during mitosis. Notably, we also found that GTPase Ran is also involved in the regulation of RanBP2-importin β1 interaction. Overall, our results suggest that mitotic arrest and the following cell death were caused by depletion of RanBP2. Our findings point to a crucial role for RanBP2 in proper mitotic progression and faithful chromosomal segregation.

Nucleoporin Nup62 maintains centrosome homeostasis

Centrosomes are comprised of 2 orthogonally arranged centrioles surrounded by the pericentriolar material (PCM), which serves as the main microtubule organizing center of the animal cell. More importantly, centrosomes also control spindle polarity and orientation during mitosis. Recently, we and other investigators discovered that several nucleoporins play critical roles during cell division. Here, we show that nucleoporin Nup62 plays a novel role in centrosome integrity. Knockdown of Nup62 induced mitotic arrest in G₂/M phases and mitotic cell death. Depletion of Nup62 using RNA interference results in defective centrosome segregation and centriole maturation during the G₂ phase. Moreover, Nup62 depletion in human cells leads to the appearance of multinucleated cells and induces the formation of multipolar centrosomes, centriole synthesis defects, dramatic spindle orientation defects, and centrosome component rearrangements that impair cell bi-polarity. Our results also point to a potential role of Nup62 in targeting gamma-tubulin and SAS-6 to the centrioles.

Esophageal cancer alters the expression of nuclear pore complex binding protein Hsc70 and eIF5A-1

Nuclear pore complex (NPC) is the only corridor for macromolecules exchange between nucleus and cytoplasm. NPC and its components, nucleoporins, play important role in the diverse physiological processes including macromolecule exchange, chromosome segregation, apoptosis and gene expression. Recent reports also suggest involvement of nucleoporins in carcinogenesis. Applying proteomics, we analyzed expression pattern of the NPC components in a newly established esophageal cancer cell line from Persia (Iran), the high-risk region for esophageal cancer. Our results indicate overexpression of Hsc70 and downregulation of subunit alpha type-3 of proteasome, calpain small subunit 1, and eIF5A-1. Among these proteins, Hsc70 and eIF5A-1 are in direct interaction with NPC and involved in the nucleocytoplasmic exchange. Hsc70 plays a critical role as a chaperone in the formation of a cargo-receptor complex in nucleocytoplasmic transport. On the other hand, it is an NPC-associated protein that binds to nucleoporins and contributes in recycling of the nucleocytoplasmic transport receptors in mammals and affects transport of proteins between nucleus and cytoplasm. The other nuclear pore interacting protein: eIF5A-1 binds to the several nucleoporins and participates in nucleocytoplasmic transport. Altered expression of Hsc70 and eIF5A-1 may cause defects in nucleocytoplasmic transport and play a role in esophageal carcinogenesis.

Nuclear pore complexes in the maintenance of genome integrity

Maintaining genome integrity is crucial for successful organismal propagation and for cell and tissue homeostasis. Several processes contribute to safeguarding the genomic information of cells. These include accurate replication of genetic information, detection and repair of DNA damage, efficient segregation of chromosomes, protection of chromosome ends, and proper organization of genome architecture. Interestingly, recent evidence shows that nuclear pore complexes, the channels connecting the nucleus with the cytoplasm, play important roles in these processes suggesting that these multiprotein platforms are key regulators of genome integrity.

Kinesin-1 and mitochondrial motility control by discrimination of structurally equivalent but distinct subdomains in Ran-GTP-binding domains of Ran-binding protein 2

The pleckstrin homology (PH) domain is a versatile fold that mediates a variety of protein–protein and protein–phosphatidylinositol lipid interactions. The Ran-binding protein 2 (RanBP2) contains four interspersed Ran GTPase-binding domains (RBD_{n= 1–4}) with close structural homology to the PH domain of Bruton's tyrosine kinase. The RBD₂, kinesin-binding domain (KBD) and RBD₃ comprise a tripartite domain (R₂KR₃) of RanBP2 that causes the unfolding, microtubule binding and biphasic activation of kinesin-1, a crucial anterograde motor of mitochondrial motility. However, the interplay between Ran GTPase and R₂KR₃ of RanBP2 in kinesin-1 activation and mitochondrial motility is elusive. We use structure–function, biochemical, kinetic and cell-based assays with time-lapse live-cell microscopy of over 260 000 mitochondrial-motility-related events to find mutually exclusive subdomains in RBD₂ and RBD₃ towards Ran GTPase binding, kinesin-1 activation and mitochondrial motility regulation. The RBD₂ and RBD₃ exhibit Ran-GTP-independent, subdomain and stereochemical-dependent discrimination on the biphasic kinetics of kinesin-1 activation or regulation of mitochondrial motility. Further, KBD alone and R₂KR₃ stimulate and suppress, respectively, multiple biophysical parameters of mitochondrial motility. The regulation of the bidirectional transport of mitochondria by either KBD or R₂KR₃ is highly coordinated, because their kinetic effects are accompanied always by changes in mitochondrial motile events of either transport polarity. These studies uncover novel roles in Ran GTPase-independent subdomains of RBD₂ and RBD₃, and KBD of RanBP2, that confer antagonizing and multi-modal mechanisms of kinesin-1 activation and regulation of mitochondrial motility. These findings open new venues towards the pharmacological harnessing of cooperative and competitive mechanisms regulating kinesins, RanBP2 or mitochondrial motility in disparate human disorders.

Beclin-1 is required for chromosome congression and proper outer kinetochore assembly

The functions of Beclin-1 in macroautophagy, tumorigenesis and cytokinesis are thought to be mediated by its association with the PI3K-III complex. Here, we describe a new role for Beclin-1 in mitotic chromosome congression that is independent of the PI3K-III complex and its role in autophagy. Beclin-1 depletion in HeLa cells leads to a significant reduction of the outer kinetochore proteins CENP-E, CENP-F and ZW10, and, consequently, the cells present severe problems in chromosome congression. Beclin-1 associates with kinetochore microtubules and forms discrete foci near the kinetochores of attached chromosomes. We show that Beclin-1 interacts directly with Zwint-1-a component of the KMN (KNL-1/Mis12/Ndc80) complex-which is essential for kinetochore-microtubule interactions. This suggests that Beclin-1 acts downstream of the KMN complex to influence the recruitment of outer kinetochore proteins and promotes accurate kinetochore anchoring to the spindle during mitosis.

Structural and functional analysis of the C-terminal domain of Nup358/RanBP2

The nuclear pore complex is the sole mediator of bidirectional transport between the nucleus and cytoplasm. Nup358 is a metazoan-specific nucleoporin that localizes to the cytoplasmic filaments and provides several binding sites for the mobile nucleocytoplasmic transport machinery. Here we present the crystal structure of the C-terminal domain (CTD) of Nup358 at 1.75Å resolution. The structure reveals that the CTD adopts a cyclophilin-like fold with a non-canonical active-site configuration. We determined biochemically that the CTD possesses weak peptidyl-prolyl isomerase activity and show that the active-site cavity mediates a weak association with the human immunodeficiency virus-1 capsid protein, supporting its role in viral infection. Overall, the surface is evolutionarily conserved, suggesting that the CTD serves as a protein-protein interaction platform. However, we demonstrate that the CTD is dispensable for nuclear envelope localization of Nup358, suggesting that the CTD does not interact with other nucleoporins.

SUMOylation in control of accurate chromosome segregation during mitosis

Posttranslational protein modification by small ubiquitin-related modifier (SUMO) has emerged as an important regulatory mechanism for chromosome segregation during mitosis. This review focuses on how SUMOylation regulates the centromere and kinetochore activities to achieve accurate chromosome segregation during mitosis. Kinetochores are assembled on the specialized chromatin domains called centromeres and serve as the sites for attaching spindle microtubule to segregate sister chromatids to daughter cells. Many proteins associated with mitotic centromeres and kinetochores have been recently found to be modified by SUMO. Although we are still at the early stage of elucidating how SUMOylation controls chromosome segregation during mitosis, a substantial progress has been achieved over the past decade. Furthermore, a major theme that has emerged from the recent studies of SUMOylation in mitosis is that both SUMO conjugation and deconjugation are critical for kinetochore assembly and disassembly. Lastly, we propose a model that SUMOylation coordinates multiple centromere and kinetochore activities to ensure accurate chromosome segregation.

Human nucleoporins promote HIV-1 docking at the nuclear pore, nuclear import and integration

The nuclear pore complex (NPC) mediates nucleo-cytoplasmic transport of macromolecules and is an obligatory point of passage and functional bottleneck in the replication of some viruses. The Human Immunodeficiency Virus (HIV) has evolved the required mechanisms for active nuclear import of its genome through the NPC. However the mechanisms by which the NPC allows or even assists HIV translocation are still unknown. We investigated the involvement of four key nucleoporins in HIV-1 docking, translocation, and integration: Nup358/RanBP2, Nup214/CAN, Nup98 and Nup153. Although all induce defects in infectivity when depleted, only Nup153 actually showed any evidence of participating in HIV-1 translocation through the nuclear pore. We show that Nup358/RanBP2 mediates docking of HIV-1 cores on NPC cytoplasmic filaments by interacting with the cores and that the C-terminus of Nup358/RanBP2 comprising a cyclophilin-homology domain contributes to binding. We also show that Nup214/CAN and Nup98 play no role in HIV-1 nuclear import per se: Nup214/CAN plays an indirect role in infectivity read-outs through its effect on mRNA export, while the reduction of expression of Nup98 shows a slight reduction in proviral integration. Our work shows the involvement of nucleoporins in diverse and functionally separable steps of HIV infection and nuclear import.

Crystal structure of the N-terminal domain of Nup358/RanBP2

Key steps in mRNA export are the nuclear assembly of messenger ribonucleoprotein particles (mRNPs), the translocation of mRNPs through the nuclear pore complex (NPC), and the mRNP remodeling events at the cytoplasmic side of the NPC. Nup358/RanBP2 is a constituent of the cytoplasmic filaments of the NPC specific to higher eukaryotes and provides a multitude of binding sites for the nucleocytoplasmic transport machinery. Here, we present the crystal structure of the Nup358 N-terminal domain (NTD) at 0.95Å resolution. The structure reveals an α-helical domain that harbors three central tetratricopeptide repeats (TPRs), flanked on each side by an additional solvating amphipathic α helix. Overall, the NTD adopts an unusual extended conformation that lacks the characteristic peptide-binding groove observed in canonical TPR domains. Strikingly, the vast majority of the NTD surface exhibits an evolutionarily conserved, positive electrostatic potential, and we demonstrate that the NTD possesses the capability to bind single-stranded RNA in solution. Together, these data suggest that the NTD contributes to mRNP remodeling events at the cytoplasmic face of the NPC.

The nucleoporin Nup153 affects spindle checkpoint activity due to an association with Mad1

The nucleoporin Nup153 is known to play pivotal roles in nuclear import and export in interphase cells and as the cell transitions into mitosis, Nup153 is involved in nuclear envelope breakdown. In this study, we demonstrate that the interaction of Nup153 with the spindle assembly checkpoint protein Mad1 is important in the regulation of the spindle checkpoint. Overexpression of human Nup153 in HeLa cells leads to the appearance of multinucleated cells and induces the formation of multipolar spindles. Importantly, it causes inactivation of the spindle checkpoint due to hypophosphorylation of Mad1. Depletion of Nup153 using RNA interference results in the decline of Mad1 at nuclear pores during interphase and more significantly causes a delayed dissociation of Mad1 from kinetochores in metaphase and an increase in the number of unresolved midbodies. In the absence of Nup153 the spindle checkpoint remains active. In vitro studies indicate direct binding of Mad1 to the N-terminal domain of Nup153. Importantly, Nup153 binding to Mad1 affects Mad1's phosphorylation status, but not its ability to interact with Mad2. Our data suggest that Nup153 levels regulate the localization of Mad1 during the metaphase/anaphase transition thereby affecting its phoshorylation status and in turn spindle checkpoint activity and mitotic exit.

Dissecting the NUP107 complex: multiple components and even more functions

The Nuclear Pore Complex (NPC) is a fascinating structure whose functional relevance and complexity attract significant interest. Within the NPC, several different subcomplexes interact with each other to form a highly conserved and stable structure. One of these subcomplexes is the NUP107 complex, constituted by 7-9 members. A wide variety of functions have been ascribed to the NUP107 complex, ranging from NPC assembly to mRNA export to cell differentiation. Recently, genetic dissection of the NUP107 complex has provided novel insight to the assembly of the complex and has, moreover, revealed an unexpected connection with the mitotic spindle assembly checkpoint protein MAD1.