The RanGAP1-RanBP2 complex is essential for microtubule-kinetochore interactions in vivo

B-type Plexins Regulate Mitosis via RanGTPase

Aberrant mitosis can result in aneuploidy and cancer. The small GTPase, Ras-related nuclear protein (Ran), is a key regulator of mitosis. B-type plexins regulate Ran activity by acting as RanGTPase-activating proteins and have been implicated in cancer progression. However, whether B-type plexins have a role in mitosis has not so far been investigated. We show here that Plexin B1 functions in the control of mitosis. Depletion of Plexin B1 affects mitotic spindle assembly, significantly delaying anaphase. This leads to mitotic catastrophe in some cells and prolonged application of the spindle assembly checkpoint. Plexin B1 depletion also promoted acentrosomal microtubule nucleation and defects in spindle pole refocusing and increased the number of cells with multipolar or aberrant mitotic spindles. An increase in lagging chromosomes or chromosomal bridges at anaphase was also found upon Plexin B1 depletion. Plexin B1 localizes to the mitotic spindle in dividing cells. The mitotic defects observed upon Plexin B1 depletion were rescued by an RCC1 inhibitor, indicating that Plexin B1 signals, via Ran, to affect mitosis. These errors in mitosis generated multinucleate cells and nuclei of altered morphology and abnormal karyotype. Furthermore, semaphorin 4D treatment increased the percentage of cells with micronuclei, precursors of chromothripsis. Implications: Defects in B-type plexins may contribute to the well-established role of plexins in cancer progression by inducing chromosomal instability.

Structural basis for a nucleoporin exportin complex between RanBP2, SUMO1-RanGAP1, the E2 Ubc9, Crm1 and the Ran GTPase

The human nucleoporin RanBP2/Nup358 interacts with SUMO1-modified RanGAP1 and the SUMO E2 Ubc9 at the nuclear pore complex (NPC) to promote export and disassembly of exportin Crm1/Ran(GTP)/cargo complexes. In mitosis, RanBP2/SUMO1-RanGAP1/Ubc9 remains intact after NPC disassembly and is recruited to kinetochores and mitotic spindles by Crm1 where it contributes to mitotic progression. Interestingly, RanBP2 binds SUMO1-RanGAP1/Ubc9 via motifs that also catalyze SUMO E3 ligase activity. Here, we resolve cryo-EM structures of a RanBP2 C-terminal fragment in complex with Crm1, SUMO1-RanGAP1/Ubc9, and two molecules of Ran(GTP). These structures reveal several unanticipated interactions with Crm1 including a nuclear export signal (NES) for RanGAP1, the deletion of which mislocalizes RanGAP1 and the Ran GTPase in cells. Our structural and biochemical results support models in which RanBP2 E3 ligase activity is dependent on Crm1, the RanGAP1 NES and Ran GTPase cycling.

Multifaceted roles of SUMO in DNA metabolism

Sumoylation, a process in which SUMO (small ubiquitin like modifier) is conjugated to target proteins, emerges as a post-translational modification that mediates protein-protein interactions, protein complex assembly, and localization of target proteins. The coordinated actions of SUMO ligases, proteases, and SUMO-targeted ubiquitin ligases determine the net result of sumoylation. It is well established that sumoylation can somewhat promiscuously target proteins in groups as well as selectively target individual proteins. Through changing protein dynamics, sumoylation orchestrates multi-step processes in chromatin biology. Sumoylation influences various steps of mitosis, DNA replication, DNA damage repair, and pathways protecting chromosome integrity. This review highlights examples of SUMO-regulated nuclear processes to provide mechanistic views of sumoylation in DNA metabolism.

Nup358 restricts ER-mitochondria connectivity by modulating mTORC2/Akt/GSK3β signalling

ER-mitochondria contact sites (ERMCSs) regulate processes, including calcium homoeostasis, energy metabolism and autophagy. Previously, it was shown that during growth factor signalling, mTORC2/Akt gets recruited to and stabilizes ERMCSs. Independent studies showed that GSK3β, a well-known Akt substrate, reduces ER-mitochondria connectivity by disrupting the VAPB-PTPIP51 tethering complex. However, the mechanisms that regulate ERMCSs are incompletely understood. Here we find that annulate lamellae (AL), relatively unexplored subdomains of ER enriched with a subset of nucleoporins, are present at ERMCSs. Depletion of Nup358, an AL-resident nucleoporin, results in enhanced mTORC2/Akt activation, GSK3β inhibition and increased ERMCSs. Depletion of Rictor, a mTORC2-specific subunit, or exogenous expression of GSK3β, was sufficient to reverse the ERMCS-phenotype in Nup358-deficient cells. We show that growth factor-mediated activation of mTORC2 requires the VAPB-PTPIP51 complex, whereas, Nup358's association with this tether restricts mTORC2/Akt signalling and ER-mitochondria connectivity. Expression of a Nup358 fragment that is sufficient for interaction with the VAPB-PTPIP51 complex suppresses mTORC2/Akt activation and disrupts ERMCSs. Collectively, our study uncovers a novel role for Nup358 in controlling ERMCSs by modulating the mTORC2/Akt/GSK3β axis.

Advances in the understanding of nuclear pore complexes in human diseases

BACKGROUND

Nuclear pore complexes (NPCs) are sophisticated and dynamic protein structures that straddle the nuclear envelope and act as gatekeepers for transporting molecules between the nucleus and the cytoplasm. NPCs comprise up to 30 different proteins known as nucleoporins (NUPs). However, a growing body of research has suggested that NPCs play important roles in gene regulation, viral infections, cancer, mitosis, genetic diseases, kidney diseases, immune system diseases, and degenerative neurological and muscular pathologies.

PURPOSE

In this review, we introduce the structure and function of NPCs. Then We described the physiological and pathological effects of each component of NPCs which provide a direction for future clinical applications.

METHODS

The literatures from PubMed have been reviewed for this article.

CONCLUSION

This review summarizes current studies on the implications of NPCs in human physiology and pathology, highlighting the mechanistic underpinnings of NPC-associated diseases.

METTL3-mediated RanGAP1 promotes colorectal cancer progression through the MAPK pathway by recruiting YTHDF1

Ran GTPase activating protein 1 (RanGAP1) has been implicated in various diseases, but its role in colorectal cancer (CRC) progression remains unclear. Using tumor tissues and public databases, we found that RanGAP1 was significantly upregulated in CRC tissues and was associated with poor prognosis of patients. N6-methyladenosine (m6A) was found to play an important role in higher expression of RanGAP1. MeRIP-seq, RIP-qPCR, Luciferase reporter assays and other related experiment elucidated the molecular mechanism underlying m6A modification of RanGAP1. Besides, cell function experiments and xenograft tumor models corroborated the function of RanGAP1 in CRC progression. By RNA-seq and related analysis, RanGAP1 was verified to influent CRC progression via the Mitogen-Activated Protein Kinase (MAPK) signaling pathway. Therefore, N6-methyladenosine modification of RanGAP1 by METTL3/YTHDF1 plays a role in CRC progression through the MAPK pathway and could be a potential biomarker and therapeutic target for CRC. Schematic diagram showed that N6-methyladenosine modification of RanGAP1 promotes CRC progression via the MAPK signaling pathway.

RanBP2/Nup358 Mediates Sumoylation of STAT1 and Antagonizes Interferon-α-Mediated Antiviral Innate Immunity

Type I interferon (IFN-I)-induced signaling plays a critical role in host antiviral innate immune responses. Despite this, the mechanisms that regulate this signaling pathway have yet to be fully elucidated. The nucleoporin Ran Binding Protein 2 (RanBP2) (also known as Nucleoporin 358 KDa, Nup358) has been implicated in a number of cellular processes, including host innate immune signaling pathways, and is known to influence viral infection. In this study, we documented that RanBP2 mediates the sumoylation of signal transducers and activators of transcription 1 (STAT1) and inhibits IFN-α-induced signaling. Specifically, we found that RanBP2-mediated sumoylation inhibits the interaction of STAT1 and Janus kinase 1 (JAK1), as well as the phosphorylation and nuclear accumulation of STAT1 after IFN-α stimulation, thereby antagonizing the IFN-α-mediated antiviral innate immune signaling pathway and promoting viral infection. Our findings not only provide insights into a novel function of RanBP2 in antiviral innate immunity but may also contribute to the development of new antiviral therapeutic strategies.

SUMOylation regulates Lem2 function in centromere clustering and silencing

Regulation by the small modifier SUMO is heavily dependent on spatial control of enzymes that mediate the attachment and removal of SUMO on substrate proteins. Here, we show that in the fission yeast Schizosaccharomyces pombe, delocalisation of the SUMO protease Ulp1 from the nuclear envelope results in centromeric defects that can be attributed to hyper-SUMOylation at the nuclear periphery. Unexpectedly, we find that although this localised hyper-SUMOylation impairs centromeric silencing, it can also enhance centromere clustering. Moreover, both effects are at least partially dependent on SUMOylation of the inner nuclear membrane protein Lem2. Lem2 has previously been implicated in diverse biological processes, including the promotion of both centromere clustering and silencing, but how these distinct activities are coordinated was unclear; our observations suggest a model whereby SUMOylation serves as a regulatory switch, modulating Lem2 interactions with competing partner proteins to balance its roles in alternative pathways. Our findings also reveal a previously unappreciated role for SUMOylation in promoting centromere clustering.

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.

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.

RANBP2 evolution and human disease

Ran-binding protein 2 (RANBP2)/Nup358 is a nucleoporin and a key component of the nuclear pore complex. Through its multiple functions (e.g., SUMOylation, regulation of nucleocytoplasmic transport) and subcellular localizations (e.g., at the nuclear envelope, kinetochores, annulate lamellae), it is involved in many cellular processes. RANBP2 dysregulation or mutation leads to the development of human pathologies, such as acute necrotizing encephalopathy 1, cancer, neurodegenerative diseases, and it is also involved in viral infections. The chromosomal region containing the RANBP2 gene is highly dynamic, with high structural variation and recombination events that led to the appearance of a gene family called RANBP2 and GCC2 Protein Domains (RGPD), with multiple gene loss/duplication events during ape evolution. Although RGPD homoplasy and maintenance during evolution suggest they might confer an advantage to their hosts, their functions are still unknown and understudied. In this review, we discuss the appearance and importance of RANBP2 in metazoans and its function-related pathologies, caused by an alteration of its expression levels (through promotor activity, post-transcriptional, or post-translational modifications), its localization, or genetic mutations.

Super-resolution microscopy reveals focal organization of ER-associated Y-complexes in mitosis

During mitotic entry of vertebrate cells, nuclear pore complexes (NPCs) are rapidly disintegrated. NPC disassembly is initiated by hyperphosphorylation of linker nucleoporins (Nups), which leads to the dissociation of FG repeat Nups and relaxation of the nuclear permeability barrier. However, less is known about disintegration of the huge nuclear and cytoplasmic rings, which are formed by annular assemblies of Y-complexes that are dissociated from NPCs as intact units. Surprisingly, we observe that Y-complex Nups display slower dissociation kinetics compared with other Nups during in vitro NPC disassembly, indicating a mechanistic difference in the disintegration of Y-based rings. Intriguingly, biochemical experiments reveal that a fraction of Y-complexes remains associated with mitotic ER membranes, supporting recent microscopic observations. Visualization of mitotic Y-complexes by super-resolution microscopy demonstrates that they form two classes of higher order assemblies: large clusters at kinetochores and small, focal ER-associated assemblies. These, however, lack features qualifying them as persisting ring-shaped subassemblies previously proposed to serve as structural templates for NPC reassembly during mitotic exit, which helps to refine current models of nuclear reassembly.

Loss of RanGAP1 drives chromosome instability and rapid tumorigenesis of osteosarcoma

Chromothripsis is a catastrophic event of chromosomal instability that involves intensive fragmentation and rearrangements within localized chromosomal regions. However, its cause remains unclear. Here, we show that reduction and inactivation of Ran GTPase-activating protein 1 (RanGAP1) commonly occur in human osteosarcoma, which is associated with a high rate of chromothripsis. In rapidly expanding mouse osteoprogenitors, RanGAP1 deficiency causes chromothripsis in chr1q, instant inactivation of Rb1 and degradation of p53, consequent failure in DNA damage repair, and ultrafast osteosarcoma tumorigenesis. During mitosis, RanGAP1 anchors to the kinetochore, where it recruits PP1-γ to counteract the activity of the spindle-assembly checkpoint (SAC) and prevents TOP2A degradation, thus safeguarding chromatid decatenation. Loss of RanGAP1 causes SAC hyperactivation and chromatid decatenation failure. These findings demonstrate that RanGAP1 maintains mitotic chromosome integrity and that RanGAP1 loss drives tumorigenesis through its direct effects on SAC and decatenation and secondary effects on DNA damage surveillance.

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.

Workshop on RanBP2/Nup358 and acute necrotizing encephalopathy

Dominant missense mutations in RanBP2/Nup358 cause Acute Necrotizing Encephalopathy (ANE), a pediatric disease where seemingly healthy individuals develop a cytokine storm that is restricted to the central nervous system in response to viral infection. Untreated, this condition leads to seizures, coma, long-term neurological damage and a high rate of mortality. The exact mechanism by which RanBP2 mutations contribute to the development of ANE remains elusive. In November 2021, a number of clinicians and basic scientists presented their work on this disease and on the interactions between RanBP2/Nup358, viral infections, the innate immune response and other cellular processes.

Roles of Nucleoporin RanBP2/Nup358 in Acute Necrotizing Encephalopathy Type 1 (ANE1) and Viral Infection

Ran Binding Protein 2 (RanBP2 or Nucleoporin358) is one of the main components of the cytoplasmic filaments of the nuclear pore complex. Mutations in the RANBP2 gene are associated with acute necrotizing encephalopathy type 1 (ANE1), a rare condition where patients experience a sharp rise in cytokine production in response to viral infection and undergo hyperinflammation, seizures, coma, and a high rate of mortality. Despite this, it remains unclear howRanBP2 and its ANE1-associated mutations contribute to pathology. Mounting evidence has shown that RanBP2 interacts with distinct viruses to regulate viral infection. In addition, RanBP2 may regulate innate immune response pathways. This review summarizes recent advances in our understanding of how mutations in RANBP2 contribute to ANE1 and discusses how RanBP2 interacts with distinct viruses and affects viral infection. Recent findings indicate that RanBP2 might be an important therapeutic target, not only in the suppression of ANE1-driven cytokine storms, but also to combat hyperinflammation in response to viral infections.

Structure, Maintenance, and Regulation of Nuclear Pore Complexes: The Gatekeepers of the Eukaryotic Genome

In eukaryotic cells, the genetic material is segregated inside the nucleus. This compartmentalization of the genome requires a transport system that allows cells to move molecules across the nuclear envelope, the membrane-based barrier that surrounds the chromosomes. Nuclear pore complexes (NPCs) are the central component of the nuclear transport machinery. These large protein channels penetrate the nuclear envelope, creating a passage between the nucleus and the cytoplasm through which nucleocytoplasmic molecule exchange occurs. NPCs are one of the largest protein assemblies of eukaryotic cells and, in addition to their critical function in nuclear transport, these structures also play key roles in many cellular processes in a transport-independent manner. Here we will review the current knowledge of the NPC structure, the cellular mechanisms that regulate their formation and maintenance, and we will provide a brief description of a variety of processes that NPCs regulate.

SUMOylation modulates the function of DDX19 in mRNA export

Nuclear export of mRNAs is a critical regulatory step in eukaryotic gene expression. The mRNA transcript undergoes extensive processing, and is loaded with a set of RNA-binding proteins (RBPs) to form export-competent messenger ribonucleoprotein particles (mRNPs) in the nucleus. During the transit of mRNPs through the nuclear pore complex (NPC), the DEAD-box ATPase - DDX19 - remodels mRNPs at the cytoplasmic side of the NPC, by removing a subset of RNA-binding proteins to terminate mRNP export. This requires the RNA-dependent ATPase activity of DDX19 and its dynamic interactions with Gle1 and Nup214. However, the regulatory mechanisms underlying these interactions are unclear. We find that DDX19 gets covalently attached with a small ubiquitin-like modifier (SUMO) at lysine 26, which enhances its interaction with Gle1. Furthermore, a SUMOylation-defective mutant of human DDX19B, K26R, failed to provide a complete rescue of the mRNA export defect caused by DDX19 depletion. Collectively, our results suggest that SUMOylation fine-tunes the function of DDX19 in mRNA export by regulating its interaction with Gle1. This study identifies SUMOylation of DDX19 as a modulatory mechanism during the mRNA export process.

Association of RanGAP to nuclear pore complex component, RanBP2/Nup358, is required for pupal development in Drosophila

Ran's GTPase-activating protein (RanGAP) is tethered to the nuclear envelope (NE) in multicellular organisms. We investigated the consequences of RanGAP localization in human tissue culture cells and Drosophila. In tissue culture cells, disruption of RanGAP1 NE localization surprisingly has neither obvious impacts on viability nor nucleocytoplasmic transport of a model substrate. In Drosophila, we identified a region within nucleoporin dmRanBP2 required for direct tethering of dmRanGAP to the NE. A dmRanBP2 mutant lacking this region shows no apparent growth defects during larval stages but arrests at the early pupal stage. A direct fusion of dmRanGAP to the dmRanBP2 mutant rescues this arrest, indicating that dmRanGAP recruitment to dmRanBP2 per se is necessary for the pupal ecdysis sequence. Our results indicate that while the NE localization of RanGAP is widely conserved in multicellular organisms, the targeting mechanisms are not. Further, we find a requirement for this localization during pupal development.

RANBP2 mutation causing autosomal dominant acute necrotizing encephalopathy attenuates its interaction with COX11

PURPOSE

Autosomal dominant acute necrotizing encephalopathy (ADANE) is caused by missense mutations in the gene encoding Ran-binding protein 2 (RANBP2), a nuclear pore protein regulating mitochondrial localization and function. Previous studies have found that RANBP2 binds to COX11 and suppresses its inhibitory activity over hexokinase1. To further elucidate mitochondrial dysfunction in ADANE, we analyzed the interaction between mutated RANBP2 and COX11.

METHODS

We extracted cDNA from a patient and constructed pGEX wild-type or mutant-type vectors including RANBP2 c.1754C>T, the commonest variant in ADANE. We transformed E. coli competent cells with the vectors and had them express GST-RANBP2 recombinant protein, and conducted a pull-down assay of RANBP2 and COX11.

RESULTS

The amount of COX11 bound to mutated RANBP2 was significantly smaller than that bound to the wild-type RANBP2.

CONCLUSION

Mutated RANBP2 had an attenuated binding ability to COX11. Whether this change indeed decreases ATP production remains to be further explored.

Mechanism and function of DNA replication-independent DNA-protein crosslink repair via the SUMO-RNF4 pathway

DNA-protein crosslinks (DPCs) obstruct essential DNA transactions, posing a serious threat to genome stability and functionality. DPCs are proteolytically processed in a ubiquitin- and DNA replication-dependent manner by SPRTN and the proteasome but can also be resolved via targeted SUMOylation. However, the mechanistic basis of SUMO-mediated DPC resolution and its interplay with replication-coupled DPC repair remain unclear. Here, we show that the SUMO-targeted ubiquitin ligase RNF4 defines a major pathway for ubiquitylation and proteasomal clearance of SUMOylated DPCs in the absence of DNA replication. Importantly, SUMO modifications of DPCs neither stimulate nor inhibit their rapid DNA replication-coupled proteolysis. Instead, DPC SUMOylation provides a critical salvage mechanism to remove DPCs formed after DNA replication, as DPCs on duplex DNA do not activate interphase DNA damage checkpoints. Consequently, in the absence of the SUMO-RNF4 pathway cells are able to enter mitosis with a high load of unresolved DPCs, leading to defective chromosome segregation and cell death. Collectively, these findings provide mechanistic insights into SUMO-driven pathways underlying replication-independent DPC resolution and highlight their critical importance in maintaining chromosome stability and cellular fitness.

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.

Novel insights into the impact of the SUMOylation pathway in hematological malignancies (Review)

The small ubiquitin-like modifier (SUMO) system serves an important role in the regulation of protein stability and function. SUMOylation sustains the homeostatic equilibrium of protein function in normal tissues and numerous types of tumor. Accumulating evidence has revealed that SUMO enzymes participate in carcinogenesis via a series of complex cellular or extracellular processes. The present review outlines the physiological characteristics of the SUMOylation pathway and provides examples of SUMOylation participation in different cancer types, including in hematological malignancies (leukemia, lymphoma and myeloma). It has been indicated that the SUMO pathway may influence chromosomal instability, cell cycle progression, apoptosis and chemical drug resistance. The present review also discussed the possible relationship between SUMOylation and carcinogenic mechanisms, and evaluated their potential as biomarkers and therapeutic targets in the diagnosis and treatment of hematological malignancies. Developing and investigating inhibitors of SUMO conjugation in the future may offer promising potential as novel therapeutic strategies.

Multifunctionality of F-rich nucleoporins

Nucleoporins (Nups) represent a range of proteins most known for composing the macromolecular assembly of the nuclear pore complex (NPC). Among them, the family of intrinsically disordered proteins (IDPs) phenylalanine-glycine (FG) rich Nups, form the permeability barrier and coordinate the high-speed nucleocytoplasmic transport in a selective way. Those FG-Nups have been demonstrated to participate in various biological processes besides nucleocytoplasmic transport. The high number of accessible hydrophobic motifs of FG-Nups potentially gives rise to this multifunctionality, enabling them to form unique microenvironments. In this review, we discuss the multifunctionality of disordered and F-rich Nups and the diversity of their localizations, emphasizing the important roles of those Nups in various regulatory and metabolic processes.

T-cell receptor (TCR) signaling promotes the assembly of RanBP2/RanGAP1-SUMO1/Ubc9 nuclear pore subcomplex via PKC-θ-mediated phosphorylation of RanGAP1

The nuclear pore complex (NPC) is the sole and selective gateway for nuclear transport, and its dysfunction has been associated with many diseases. The metazoan NPC subcomplex RanBP2, which consists of RanBP2 (Nup358), RanGAP1-SUMO1, and Ubc9, regulates the assembly and function of the NPC. The roles of immune signaling in regulation of NPC remain poorly understood. Here, we show that in human and murine T cells, following T-cell receptor (TCR) stimulation, protein kinase C-θ (PKC-θ) directly phosphorylates RanGAP1 to facilitate RanBP2 subcomplex assembly and nuclear import and, thus, the nuclear translocation of AP-1 transcription factor. Mechanistically, TCR stimulation induces the translocation of activated PKC-θ to the NPC, where it interacts with and phosphorylates RanGAP1 on Ser⁵⁰⁴ and Ser⁵⁰⁶. RanGAP1 phosphorylation increases its binding affinity for Ubc9, thereby promoting sumoylation of RanGAP1 and, finally, assembly of the RanBP2 subcomplex. Our findings reveal an unexpected role of PKC-θ as a direct regulator of nuclear import and uncover a phosphorylation-dependent sumoylation of RanGAP1, delineating a novel link between TCR signaling and assembly of the RanBP2 NPC subcomplex.

Acute necrotizing encephalopathy-linked mutations in Nup358 impair interaction of Nup358 with TNRC6/GW182 and miRNA function

MicroRNA (miRNA)-mediated translational suppression of mRNAs is involved in the regulation of multiple cellular processes. A recent study showed that Nup358, a protein mutated in a neurological disorder called acute necrotizing encephalopathy 1 (ANE1), helps in the coupling of miRNA-induced silencing complex (miRISC) - consisting of miRNA, AGO and GW182/TNRC6 proteins - with the target mRNA. Here we provide a detailed characterization of the interaction between Nup358 and GW182. We identified that the N-terminal region of Nup358 directly interacts with the C-terminal silencing domain of GW182. Interestingly, ANE1-associated Nup358 mutants display reduced interaction with GW182. Consistent with this, one of the prevalent ANE1 mutations, 585th threonine (T) residue changed to methionine (M) [T585M] compromised Nup358's ability to function in the miRNA pathway. Collectively, these results suggest that the ANE1-associated mutations in Nup358 might affect the miRNA pathway and contribute to the development of ANE1.

Poly-SUMO-2/3 chain modification of Nuf2 facilitates CENP-E kinetochore localization and chromosome congression during mitosis

SUMO modification is required for the kinetochore localization of the kinesin-like motor protein CENP-E, which subsequently mediates the alignment of chromosomes to the spindle equator during mitosis. However, the underlying mechanisms by which sumoylation regulates CENP-E kinetochore localization are still unclear. In this study, we first elucidate that the kinetochore protein Nuf2 is not only required for CENP-E kinetochore localization but also preferentially modified by poly-SUMO-2/3 chains. In addition, poly-SUMO-2/3 modification of Nuf2 is significantly upregulated during mitosis, which is temporally correlated to the kinetochore localization of CENP-E during mitosis. We further show that the mitotic defects in CENP-E kinetochore localization and chromosome congression caused by global inhibition of sumoylation can be rescued by expressing a fusion protein between Nuf2 and the SUMO-conjugating enzyme Ubc9 for stimulating Nuf2 SUMO-2/3 modification. Moreover, the expression of another fusion protein between Nuf2 and three SUMO-2 moieties (SUMO-2 trimer), which mimics the trimeric SUMO-2/3 chain modification of Nuf2, can also rescue the mitotic defects due to global inhibition of sumoylation. Conversely, expressing the other forms of Nuf2-SUMO fusion proteins, which imitate Nuf2 modifications by SUMO-2/3 monomer, SUMO-2/3 dimer, and SUMO-1 trimer, respectively, cannot rescue the same mitotic defects. Lastly, compared to Nuf2, the fusion protein simulating the trimeric SUMO-2 chain-modified Nuf2 exhibits a significantly higher binding affinity to CENP-E wild type containing a functional SUMO-interacting motif (SIM) but not the CENP-E SIM mutant. Hence, our results support a model that poly-SUMO-2/3 chain modification of Nuf2 facilitates CENP-E kinetochore localization and chromosome congression during mitosis.Abbreviations: CENP-E, centromere-associated protein E; SUMO, small ubiquitin-related modifier; SIM, SUMO-interacting motif.

Nuclear pore complexes in development and tissue homeostasis

Nuclear pore complexes are multiprotein channels that span the nuclear envelope, which connects the nucleus to the cytoplasm. In addition to their main role in the regulation of nucleocytoplasmic molecule exchange, it has become evident that nuclear pore complexes and their components also have multiple transport-independent functions. In recent years, an increasing number of studies have reported the involvement of nuclear pore complex components in embryogenesis, cell differentiation and tissue-specific processes. Here, we review the findings that highlight the dynamic nature of nuclear pore complexes and their roles in many cell type-specific functions during development and tissue homeostasis.

Differential turnover of Nup188 controls its levels at centrosomes and role in centriole duplication

NUP188 encodes a scaffold component of the nuclear pore complex (NPC) and has been implicated as a congenital heart disease gene through an ill-defined function at centrioles. Here, we explore the mechanisms that physically and functionally segregate Nup188 between the pericentriolar material (PCM) and NPCs. Pulse-chase fluorescent labeling indicates that Nup188 populates centrosomes with newly synthesized protein that does not exchange with NPCs even after mitotic NPC breakdown. In addition, the steady-state levels of Nup188 are controlled by the sensitivity of the PCM pool, but not the NPC pool, to proteasomal degradation. Proximity-labeling and super-resolution microscopy show that Nup188 is vicinal to the inner core of the interphase centrosome. Consistent with this, we demonstrate direct binding between Nup188 and Cep152. We further show that Nup188 functions in centriole duplication at or upstream of Sas6 loading. Together, our data establish Nup188 as a component of PCM needed to duplicate the centriole with implications for congenital heart disease mechanisms.

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.

Cyclin B1-Cdk1 facilitates MAD1 release from the nuclear pore to ensure a robust spindle checkpoint

How the cell rapidly and completely reorganizes its architecture when it divides is a problem that has fascinated researchers for almost 150 yr. We now know that the core regulatory machinery is highly conserved in eukaryotes, but how these multiple protein kinases, protein phosphatases, and ubiquitin ligases are coordinated in space and time to remodel the cell in a matter of minutes remains a major question. Cyclin B1-Cdk is the primary kinase that drives mitotic remodeling; here we show that it is targeted to the nuclear pore complex (NPC) by binding an acidic face of the kinetochore checkpoint protein, MAD1, where it coordinates NPC disassembly with kinetochore assembly. Localized cyclin B1-Cdk1 is needed for the proper release of MAD1 from the embrace of TPR at the nuclear pore so that it can be recruited to kinetochores before nuclear envelope breakdown to maintain genomic stability.

The Small GTPase Superfamily in Plants: A Conserved Regulatory Module with Novel Functions

Small GTP-binding proteins represent a highly conserved signaling module in eukaryotes that regulates diverse cellular processes such as signal transduction, cytoskeletal organization and cell polarity, cell proliferation and differentiation, intracellular membrane trafficking and transport vesicle formation, and nucleocytoplasmic transport. These proteins function as molecular switches that cycle between active and inactive states, and this cycle is linked to GTP binding and hydrolysis. In this review, the roles of the plant complement of small GTP-binding proteins in these cellular processes are described, as well as accessory proteins that control their activity, and current understanding of the functions of individual members of these families in plants-with a focus on the model organism Arabidopsis-is presented. Some potential novel roles of these GTPases in plants, relative to their established roles in yeast and/or animal systems, are also discussed.

The implication of the SUMOylation pathway in breast cancer pathogenesis and treatment

Breast cancer is the most commonly diagnosed malignancy in woman worldwide, and is the second most common cause of death in developed countries. The transformation of a normal cell into a malignant derivate requires the acquisition of diverse genomic and proteomic changes, including enzymatic post-translational modifications (PTMs) on key proteins encompassing critical cell signaling events. PTMs occur on proteins after translation, and regulate several aspects of proteins activity, including their localization, activation and turnover. Deregulation of PTMs can potentially lead to tumorigenesis, and several de-regulated PTM pathways contribute to abnormal cell proliferation during breast tumorigenesis. SUMOylation is a PTM that plays a pivotal role in numerous aspects of cell physiology, including cell cycle regulation, protein trafficking and turnover, and DNA damage repair. Consistently with this, the deregulation of the SUMO pathway is observed in different human pathologies, including breast cancer. In this review we will describe the role of SUMOylation in breast tumorigenesis and its implication for breast cancer therapy.

Nuclear Pores Assemble from Nucleoporin Condensates During Oogenesis

The molecular events that direct nuclear pore complex (NPC) assembly toward nuclear envelopes have been conceptualized in two pathways that occur during mitosis or interphase, respectively. In gametes and embryonic cells, NPCs also occur within stacked cytoplasmic membrane sheets, termed annulate lamellae (AL), which serve as NPC storage for early development. The mechanism of NPC biogenesis at cytoplasmic membranes remains unknown. Here, we show that during Drosophila oogenesis, Nucleoporins condense into different precursor granules that interact and progress into NPCs. Nup358 is a key player that condenses into NPC assembly platforms while its mRNA localizes to their surface in a translation-dependent manner. In concert, Microtubule-dependent transport, the small GTPase Ran and nuclear transport receptors regulate NPC biogenesis in oocytes. We delineate a non-canonical NPC assembly mechanism that relies on Nucleoporin condensates and occurs away from the nucleus under conditions of cell cycle arrest.

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.

Importin-β/karyopherin-β1 modulates mitotic microtubule function and taxane sensitivity in cancer cells via its nucleoporin-binding region

The nuclear transport receptor importin-β/karyopherin-β1 is overexpressed in cancers that display genomic instability. It is regarded as a promising cancer target and inhibitors are being developed. In addition to its role in nucleo-cytoplasmic transport, importin-β regulates mitosis, but the programmes and pathways in which it operates are defined only in part. To unravel importin-β's mitotic functions we have developed cell lines expressing either wild-type or a mutant importin-β form in characterised residues required for nucleoporin binding. Both forms similarly disrupted spindle pole organisation, while only wild-type importin-β affected microtubule plus-end function and microtubule stability. A proteome-wide search for differential interactors identified a set of spindle regulators sensitive to mutations in the nucleoporin-binding region. Among those, HURP (hepatoma up-regulated protein) is an importin-β interactor and a microtubule-stabilising factor. We found that induction of wild type, but not mutant importin-β, under the same conditions that destabilise mitotic microtubules, delocalised HURP, indicating that the spatial distribution of HURP along the spindle requires importin-β's nucleoporin-binding residues. Concomitantly, importin-β overexpression sensitises cells to taxanes and synergistically increases mitotic cell death. Thus, the nucleoporin-binding domain is dispensable for importin-β function in spindle pole organisation, but regulates microtubule stability, at least in part via HURP, and renders cells vulnerable to certain microtubule-targeting drugs.

Oncogenic MYC amplifies mitotic perturbations

The oncogenic transcription factor MYC modulates vast arrays of genes, thereby influencing numerous biological pathways including biogenesis, metabolism, proliferation, apoptosis and pluripotency. When deregulated, MYC drives genomic instability via several mechanisms including aberrant proliferation, replication stress and ROS production. Deregulated MYC also promotes chromosome instability, but less is known about how MYC influences mitosis. Here, we show that deregulating MYC modulates multiple aspects of mitotic chromosome segregation. Cells overexpressing MYC have altered spindle morphology, take longer to align their chromosomes at metaphase and enter anaphase sooner. When challenged with a variety of anti-mitotic drugs, cells overexpressing MYC display more anomalies, the net effect of which is increased micronuclei, a hallmark of chromosome instability. Proteomic analysis showed that MYC modulates multiple networks predicted to influence mitosis, with the mitotic kinase PLK1 identified as a central hub. In turn, we show that MYC modulates several PLK1-dependent processes, namely mitotic entry, spindle assembly and SAC satisfaction. These observations thus underpin the pervasive nature of oncogenic MYC and provide a mechanistic rationale for MYC's ability to drive chromosome instability.

How Does SUMO Participate in Spindle Organization?

The ubiquitin-like protein SUMO is a regulator involved in most cellular mechanisms. Recent studies have discovered new modes of function for this protein. Of particular interest is the ability of SUMO to organize proteins in larger assemblies, as well as the role of SUMO-dependent ubiquitylation in their disassembly. These mechanisms have been largely described in the context of DNA repair, transcriptional regulation, or signaling, while much less is known on how SUMO facilitates organization of microtubule-dependent processes during mitosis. Remarkably however, SUMO has been known for a long time to modify kinetochore proteins, while more recently, extensive proteomic screens have identified a large number of microtubule- and spindle-associated proteins that are SUMOylated. The aim of this review is to focus on the possible role of SUMOylation in organization of the spindle and kinetochore complexes. We summarize mitotic and microtubule/spindle-associated proteins that have been identified as SUMO conjugates and present examples regarding their regulation by SUMO. Moreover, we discuss the possible contribution of SUMOylation in organization of larger protein assemblies on the spindle, as well as the role of SUMO-targeted ubiquitylation in control of kinetochore assembly and function. Finally, we propose future directions regarding the study of SUMOylation in regulation of spindle organization and examine the potential of SUMO and SUMO-mediated degradation as target for antimitotic-based therapies.

Nup358 regulates microridge length by controlling SUMOylation-dependent activity of aPKC in zebrafish epidermis

The Par polarity complex, consisting of Par3, Par6 and atypical protein kinase C (aPKC), plays a crucial role in the establishment and maintenance of cell polarity. Although activation of aPKC is critical for polarity, how this is achieved is unclear. The developing zebrafish epidermis, along with its apical actin-based projections, called microridges, offers a genetically tractable system for unraveling the mechanisms of the cell polarity control. The zebrafish aPKC regulates elongation of microridges by controlling levels of apical Lgl, which acts as a pro-elongation factor. Here, we show that the nucleoporin Nup358 (also known as RanBP2) - a component of the nuclear pore complex and a part of cytoplasmic annulate lamellae (AL) - SUMOylates zebrafish aPKC. Nup358-mediated SUMOylation controls aPKC activity to regulate Lgl-dependent microridge elongation. Our data further suggest that cytoplasmic AL structures are the possible site for Nup358-mediated aPKC SUMOylation. We have unraveled a hitherto unappreciated contribution of Nup358-mediated aPKC SUMOylation in cell polarity regulation.This article has an associated First Person interview with the first author of the paper.

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.

Structure and Assembly of the Nuclear Pore Complex

Nuclear pore complexes (NPCs) mediate nucleocytoplasmic exchange. They are exceptionally large protein complexes that fuse the inner and outer nuclear membranes to form channels across the nuclear envelope. About 30 different protein components, termed nucleoporins, assemble in multiple copies into an intricate cylindrical architecture. Here, we review our current knowledge of the structure of nucleoporins and how those come together in situ. We delineate architectural principles on several hierarchical organization levels, including isoforms, posttranslational modifications, nucleoporins, and higher-order oligomerization of nucleoporin subcomplexes. We discuss how cells exploit this modularity to faithfully assemble NPCs.

GCN5L1 interacts with αTAT1 and RanBP2 to regulate hepatic α-tubulin acetylation and lysosome trafficking

Although GCN5L1 (also known as BLOC1S1) facilitates mitochondrial protein acetylation and controls endosomal-lysosomal trafficking, the mechanisms underpinning these disparate effects are unclear. As microtubule acetylation modulates endosome-lysosome trafficking, we reasoned that exploring the role of GCN5L1 in this biology may enhance our understanding of GCN5L1-mediated protein acetylation. We show that α-tubulin acetylation is reduced in GCN5L1-knockout hepatocytes and restored by GCN5L1 reconstitution. Furthermore, GCN5L1 binds to the α-tubulin acetyltransferase αTAT1, and GCN5L1-mediated α-tubulin acetylation is dependent on αTAT1. Given that cytosolic GCN5L1 has been identified as a component of numerous multiprotein complexes, we explored whether novel interacting partners contribute to this regulation. We identify RanBP2 as a novel interacting partner of GCN5L1 and αTAT1. Genetic silencing of RanBP2 phenocopies GCN5L1 depletion by reducing α-tubulin acetylation, and we find that RanBP2 possesses a tubulin-binding domain, which recruits GCN5L1 to α-tubulin. Finally, we find that genetic depletion of GCN5L1 promotes perinuclear lysosome accumulation and histone deacetylase inhibition partially restores lysosomal positioning. We conclude that the interactions of GCN5L1, RanBP2 and αTAT1 function in concert to control α-tubulin acetylation and may contribute towards the regulation of cellular lysosome positioning. This article has an associated First Person interview with the first author of the paper.

Dynamic SUMO remodeling drives a series of critical events during the meiotic divisions in Caenorhabditis elegans

Chromosome congression and segregation in C. elegans oocytes depend on a complex of conserved proteins that forms a ring around the center of each bivalent during prometaphase; these complexes are then removed from chromosomes at anaphase onset and disassemble as anaphase proceeds. Here, we uncover mechanisms underlying the dynamic regulation of these ring complexes (RCs), revealing a strategy by which protein complexes can be progressively remodeled during cellular processes. We find that the assembly, maintenance, and stability of RCs is regulated by a balance between SUMO conjugating and deconjugating activity. During prometaphase, the SUMO protease ULP-1 is targeted to the RCs but is counteracted by SUMO E2/E3 enzymes; then in early anaphase the E2/E3 enzymes are removed, enabling ULP-1 to trigger RC disassembly and completion of the meiotic divisions. Moreover, we found that SUMO regulation is essential to properly connect the RCs to the chromosomes and then also to fully release them in anaphase. Altogether, our work demonstrates that dynamic remodeling of SUMO modifications facilitates key meiotic events and highlights how competition between conjugation and deconjugation activity can modulate SUMO homeostasis, protein complex stability, and ultimately, progressive processes such as cell division.

Most cells have two sets of chromosomes, one from each parent. Meiosis is a specialized form of cell division where chromosomes are duplicated once and segregated twice, in order to generate eggs (oocytes) or sperm with only one copy of every chromosome. This is necessary so that fertilization will produce an embryo that once again contains two complete copies of the genome. Using C. elegans as a model system, we have uncovered regulatory mechanisms important for the fidelity of these meiotic divisions. C. elegans oocytes use a kinetochore-independent chromosome segregation mechanism that relies on a large protein complex that localizes to the chromosomes. These protein complexes facilitate chromosome congression during metaphase and then are released from chromosomes in anaphase and progressively disassemble as the chromosomes segregate. We find that the stability and disassembly of these complexes is regulated by a protein modification called SUMO and by competition between enzymes that localize to the protein complex to either add or remove SUMO modifications. These findings provide insight into the mechanisms by which SUMO and SUMO enzymes regulate progression through cell division and illustrate a general strategy by which large protein complexes can be rapidly assembled and disassembled during dynamic cellular processes.

Pervasive Protein Thermal Stability Variation during the Cell Cycle

Quantitative mass spectrometry has established proteome-wide regulation of protein abundance and post-translational modifications in various biological processes. Here, we used quantitative mass spectrometry to systematically analyze the thermal stability and solubility of proteins on a proteome-wide scale during the eukaryotic cell cycle. We demonstrate pervasive variation of these biophysical parameters with most changes occurring in mitosis and G1. Various cellular pathways and components vary in thermal stability, such as cell-cycle factors, polymerases, and chromatin remodelers. We demonstrate that protein thermal stability serves as a proxy for enzyme activity, DNA binding, and complex formation in situ. Strikingly, a large cohort of intrinsically disordered and mitotically phosphorylated proteins is stabilized and solubilized in mitosis, suggesting a fundamental remodeling of the biophysical environment of the mitotic cell. Our data represent a rich resource for cell, structural, and systems biologists interested in proteome regulation during biological transitions.

Visualization of human karyopherin beta-1/importin beta-1 interactions with protein partners in mitotic cells by co-immunoprecipitation and proximity ligation assays

Karyopherin beta-1/Importin beta-1 is a conserved nuclear transport receptor, acting in protein nuclear import in interphase and as a global regulator of mitosis. These pleiotropic functions reflect its ability to interact with, and regulate, different pathways during the cell cycle, operating as a major effector of the GTPase RAN. Importin beta-1 is overexpressed in cancers characterized by high genetic instability, an observation that highlights the importance of identifying its partners in mitosis. Here we present the first comprehensive profile of importin beta-1 interactors from human mitotic cells. By combining co-immunoprecipitation and proteome-wide mass spectrometry analysis of synchronized cell extracts, we identified expected (e.g., RAN and SUMO pathway factors) and novel mitotic interactors of importin beta-1, many with RNA-binding ability, that had not been previously associated with importin beta-1. These data complement interactomic studies of interphase transport pathways. We further developed automated proximity ligation assay (PLA) protocols to validate selected interactors. We succeeded in obtaining spatial and temporal resolution of genuine importin beta-1 interactions, which were visualized and localized in situ in intact mitotic cells. Further developments of PLA protocols will be helpful to dissect importin beta-1-orchestrated pathways during mitosis.

Nucleolar and spindle-associated protein 1 (NUSAP1) interacts with a SUMO E3 ligase complex during chromosome segregation

The mitotic spindle is composed of dynamic microtubules and associated proteins that together direct chromosome movement during mitosis. The spindle plays a vital role in accurate chromosome segregation fidelity and is a therapeutic target in cancer. Nevertheless, the molecular mechanisms by which many spindle-associated proteins function remains unknown. The nucleolar and spindle-associated protein NUSAP1 is a microtubule-binding protein implicated in spindle stability and chromosome segregation. We show here that NUSAP1 localizes to dynamic spindle microtubules in a unique chromosome-centric pattern, in the vicinity of overlapping microtubules, during metaphase and anaphase of mitosis. Mass spectrometry-based analysis of endogenous NUSAP1 interacting proteins uncovered a cell cycle-regulated interaction between the RanBP2-RanGAP1-UBC9 SUMO E3 ligase complex and NUSAP1. Like NUSAP1 depletion, RanBP2 depletion impaired the response of cells to the microtubule poison Taxol. NUSAP1 contains a conserved SAP domain (SAF-A/B, Acinus, and PIAS). SAP domains are common among many other SUMO E3s, and are implicated in substrate recognition and ligase activity. We speculate that NUSAP1 contributes to accurate chromosome segregation by acting as a co-factor for RanBP2-RanGAP1-UBC9 during cell division.

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.