Phosphorylation Regulates the Dynamic Interaction of RCC1 with Chromosomes during Mitosis

Cryo-EM of the Nucleosome Core Particle Bound to Ran-RCC1 Reveals a Dynamic Complex

Ras-related nuclear protein (Ran) is a member of the Ras superfamily of small guanosine triphosphatases (GTPases) and a regulator of multiple cellular processes. In healthy cells, the GTP-bound form of Ran is concentrated at chromatin, creating a Ran•GTP gradient that provides the driving force for nucleocytoplasmic transport, mitotic spindle assembly, and nuclear envelope formation. The Ran•GTP gradient is maintained by the regulator of chromatin condensation 1 (RCC1), a guanine nucleotide exchange factor that accelerates GDP/GTP exchange in Ran. RCC1 interacts with nucleosomes, which are the fundamental repeating units of eukaryotic chromatin. Here, we present a cryo-EM analysis of a trimeric complex composed of the nucleosome core particle (NCP), RCC1, and Ran. While the contacts between RCC1 and Ran in the complex are preserved compared with a previously determined structure of RCC1-Ran, our study reveals that RCC1 and Ran interact dynamically with the NCP and undergo rocking motions on the nucleosome surface. Furthermore, the switch 1 region of Ran, which plays an important role in mediating conformational changes associated with the substitution of GDP and GTP nucleotides in Ras family members, appears to undergo disorder-order transitions and forms transient contacts with the C-terminal helix of histone H2B. Nucleotide exchange assays performed in the presence and absence of NCPs are not consistent with an active role for nucleosomes in nucleotide exchange, at least in vitro. Instead, the nucleosome stabilizes RCC1 and serves as a hub that concentrates RCC1 and Ran to promote efficient Ran•GDP to Ran•GTP conversion.

Genome-wide identification and expression analysis of the regulator of chromosome condensation 1 gene family in wheat (Triticum aestivum L.)

Wheat (Triticum aestivum L., 2n = 6x = 42, AABBDD) is the world's most widely cultivated crop and an important staple food for humans, accounting for one-fifth of calories consumed. Proteins encoded by the regulator of chromosome condensation 1 (RCC1) are highly conserved among eukaryotes and consist of seven repeated domains that fold into a seven-bladed propeller structure. In this study, a total of 76 RCC1 genes of bread wheat were identified via a genome-wide search, and their phylogenetic relationship, gene structure, protein-conserved domain, chromosome localization, conserved motif, and transcription factor binding sites were systematically analyzed using the bioinformatics approach to indicate the evolutionary and functional features of these genes. The expression patterns of 76 TaRCC1 family genes in wheat under various stresses were further analyzed, and RT-PCR verified that RCC1-3A (TraesCS3A02G362800), RCC1-3B (TraesCS3B02G395200), and RCC1-3D (TraesCS3D02G35650) were significantly induced by salt, cold, and drought stresses. Additionally, the co-expression network analysis and binding site prediction suggested that Myb-7B (TraesCS7B02G188000) and Myb-7D (TraesCS7D02G295400) may bind to the promoter of RCC1-3A/3B and upregulate their expression in response to abiotic stresses in wheat. The results have furthered our understanding of the wheat RCC1 family members and will provide important information for subsequent studies and the use of RCC1 genes in wheat.

The intricate roles of RCC1 in normal cells and cancer cells

RCC1 (regulator of chromosome condensation 1) is a highly conserved chromatin-binding protein and the only known guanine-nucleotide exchange factor of Ran (a nuclear Ras homolog). RCC1 plays an essential role in the regulation of cell cycle-related activities such as nuclear envelope formation, nuclear pore complex and spindle assembly, and nucleocytoplasmic transport. Over the last decade, increasing evidence has emerged highlighting the potential relevance of RCC1 to carcinogenesis, especially cervical, lung, and breast cancer. In this review, we briefly discuss the roles of RCC1 in both normal and tumor cells based on articles published in recent years, followed by a brief overview of future perspectives in the field.

Phosphorylation of RCC1 on Serine 11 Facilitates G1/S Transition in HPV E7-Expressing Cells

Persistent infection of high-risk human papillomavirus (HR-HPV) plays a causal role in cervical cancer. Regulator of chromosome condensation 1 (RCC1) is a critical cell cycle regulator, which undergoes a few post-translational modifications including phosphorylation. Here, we showed that serine 11 (S11) of RCC1 was phosphorylated in HPV E7-expressing cells. However, S11 phosphorylation was not up-regulated by CDK1 in E7-expressing cells; instead, the PI3K/AKT/mTOR pathway promoted S11 phosphorylation. Knockdown of AKT or inhibition of the PI3K/AKT/mTOR pathway down-regulated phosphorylation of RCC1 S11. Furthermore, S11 phosphorylation occurred throughout the cell cycle, and reached its peak during the mitosis phase. Our previous data proved that RCC1 was necessary for the G1/S cell cycle progression, and in the present study we showed that the RCC1 mutant, in which S11 was mutated to alanine (S11A) to mimic non-phosphorylation status, lost the ability to facilitate G1/S transition in E7-expressing cells. Moreover, RCC1 S11 was phosphorylated by the PI3K/AKT/mTOR pathway in HPV-positive cervical cancer SiHa and HeLa cells. We conclude that S11 of RCC1 is phosphorylated by the PI3K/AKT/mTOR pathway and phosphorylation of RCC1 S11 facilitates the abrogation of G1 checkpoint in HPV E7-expressing cells. In short, our study explores a new role of RCC1 S11 phosphorylation in cell cycle regulation.

Past, present, and perspectives of protein N-terminal methylation

The posttranslational methylation of the α-N-terminal amino group of proteins was first documented over 40 years ago, but the functional significance of this modification has been underexplored relative to lysine and arginine methylation. Increasing reports implicates α-N-terminal methylation as a widespread and critical regulator of mitosis, chromatin interactions, DNA repair, and translation fidelity. Here, we summarize advances in the current understanding of protein α-N-terminal methylation biological functions and mechanisms across eukaryotic organisms. Also, we describe the recent literature on substrate recognition and the discovery of potent and selective inhibitors for protein N-terminal methyltransferases. Finally, we summarize the emergent crosstalk between α-N-terminal methylation and other N-terminal modifications.

Keep it focused: PRMT6 drives the localization of RCC1 to chromosomes to facilitate mitosis, cell proliferation, and tumorigenesis

Huang et al. (2021) identified a mechanism acting through the arginine methyltransferase PRMT6 that stabilizes the interaction of RCC1 with chromatin, promoting cell proliferation and tumorigenicity. Targeting this mechanism might enhance the treatment of tumors such as glioblastoma.

PRMT6 methylation of RCC1 regulates mitosis, tumorigenicity, and radiation response of glioblastoma stem cells

Aberrant cell proliferation is a hallmark of cancer, including glioblastoma (GBM). Here we report that protein arginine methyltransferase (PRMT) 6 activity is required for the proliferation, stem-like properties, and tumorigenicity of glioblastoma stem cells (GSCs), a subpopulation in GBM critical for malignancy. We identified a casein kinase 2 (CK2)-PRMT6-regulator of chromatin condensation 1 (RCC1) signaling axis whose activity is an important contributor to the stem-like properties and tumor biology of GSCs. CK2 phosphorylates and stabilizes PRMT6 through deubiquitylation, which promotes PRMT6 methylation of RCC1, which in turn is required for RCC1 association with chromatin and activation of RAN. Disruption of this pathway results in defects in mitosis. EPZ020411, a specific small-molecule inhibitor for PRMT6, suppresses RCC1 arginine methylation and improves the cytotoxic activity of radiotherapy against GSC brain tumor xenografts. This study identifies a CK2α-PRMT6-RCC1 signaling axis that can be therapeutically targeted in the treatment of GBM.

The Multifaceted Roles of RCC1 in Tumorigenesis

RCC1 (regulator of chromosome condensation 1) is the only known guanine nucleotide exchange factor of Ran, a nuclear Ras-like G protein. RCC1 combines with chromatin and Ran to establish a concentration gradient of RanGTP, thereby participating in a series of cell physiological activities. In this review, we discuss the structure of RCC1 and describe how RCC1 affects the formation and function of the nuclear envelope, spindle formation, and nuclear transport. We mainly focus on the effect of RCC1 on the cell cycle during tumorigenesis and the recent research progress that has been made in relation to different tumor types.

RanBP1 controls the Ran pathway in mammalian cells through regulation of mitotic RCC1 dynamics

The Ran GTPase plays critical roles in multiple cellular processes including interphase nucleocytoplasmic transport and mitotic spindle assembly. During mitosis in mammalian cells, GTP-bound Ran (Ran-GTP) is concentrated near mitotic chromatin while GDP-bound Ran (Ran-GDP) is more abundant distal to chromosomes. This pattern spatially controls spindle formation because Ran-GTP locally releases spindle assembly factors (SAFs), such as Hepatoma Up-Regulated Protein (HURP), from inhibitory interactions near chromosomes. Regulator of Chromatin Condensation 1 (RCC1) is Ran’s chromatin-bound exchange factor, and RanBP1 is a conserved Ran-GTP-binding protein that has been implicated as a mitotic regulator of RCC1 in embryonic systems. Here, we show that RanBP1 controls mitotic RCC1 dynamics in human somatic tissue culture cells. In addition, we observed the re-localization of HURP in metaphase cells after RanBP1 degradation, consistent with the idea that altered RCC1 dynamics functionally modulate SAF activities. Together, our findings reveal an important mitotic role for RanBP1 in human somatic cells, controlling the spatial distribution and magnitude of mitotic Ran-GTP production and thereby ensuring the accurate execution of Ran-dependent mitotic events.

Abbreviations

AID: Auxin-induced degron; FLIP: Fluorescence loss in photobleaching; FRAP: Fluorescence recovery after photobleaching; GDP: guanosine diphosphate; GTP: guanosine triphosphate; HURP: Hepatoma Up-Regulated Protein; NE: nuclear envelope; NEBD: Nuclear Envelope Breakdown; RanBP1: Ran-binding protein 1; RanGAP1: Ran GTPase-Activating Protein 1; RCC1: Regulator of Chromatin Condensation 1; RRR complex: RCC1/Ran/RanBP1 heterotrimeric complex; SAF: Spindle Assembly Factor; TIR1: Transport Inhibitor Response 1 protein; XEE: Xenopus egg extract.

The Myosin Family of Mechanoenzymes: From Mechanisms to Therapeutic Approaches

Myosins are among the most fascinating enzymes in biology. As extremely allosteric chemomechanical molecular machines, myosins are involved in myriad pivotal cellular functions and are frequently sites of mutations leading to disease phenotypes. Human β-cardiac myosin has proved to be an excellent target for small-molecule therapeutics for heart muscle diseases, and, as we describe here, other myosin family members are likely to be potentially unique targets for treating other diseases as well. The first part of this review focuses on how myosins convert the chemical energy of ATP hydrolysis into mechanical movement, followed by a description of existing therapeutic approaches to target human β-cardiac myosin. The next section focuses on the possibility of targeting nonmuscle members of the human myosin family for several diseases. We end the review by describing the roles of myosin in parasites and the therapeutic potential of targeting them to block parasitic invasion of their hosts.

Metabolic rewiring of the hypertensive kidney

Hypertension is a persistent epidemic across the developed world that is closely associated with kidney disease. Here, we applied a metabolomic, phosphoproteomic, and proteomic strategy to analyze the effect of hypertensive insults on kidneys. Our data revealed the metabolic aspects of hypertension-induced glomerular sclerosis, including lipid breakdown at early disease stages and activation of anaplerotic pathways to regenerate energy equivalents to counter stress. For example, branched-chain amino acids and proline, required for collagen synthesis, were depleted in glomeruli at early time points. Furthermore, indicators of metabolic stress were reflected by low amounts of ATP and NADH and an increased abundance of oxidized lipids derived from lipid breakdown. These processes were specific to kidney glomeruli where metabolic signaling occurred through mTOR and AMPK signaling. Quantitative phosphoproteomics combined with computational modeling suggested that these processes controlled key molecules in glomeruli and specifically podocytes, including cytoskeletal components and GTP-binding proteins, which would be expected to compete for decreasing amounts of GTP at early time points. As a result, glomeruli showed increased expression of metabolic enzymes of central carbon metabolism, amino acid degradation, and lipid oxidation, findings observed in previously published studies from other disease models and patients with glomerular damage. Overall, multilayered omics provides an overview of hypertensive kidney damage and suggests that metabolic or dietary interventions could prevent and treat glomerular disease and hypertension-induced nephropathy.

Phosphorylation of importin-α1 by CDK1-cyclin B1 controls mitotic spindle assembly

Importin-α serves as an adaptor linking importin-β to proteins carrying a nuclear localization sequence (NLS). During interphase, this interaction enables nuclear protein import, while in mitosis it regulates spindle assembly factors (SAFs) and controls microtubule nucleation, stabilization and spindle function. Here, we show that human importin-α1 is regulated during the cell cycle and is phosphorylated at two sites (threonine 9 and serine 62) during mitosis by the major mitotic protein kinase CDK1-cyclin B. Mutational analysis indicates that the mitotic phosphorylation of importin-α1 inhibits its binding to importin-β and promotes the release of TPX2 and KIFC1, which are then targeted like importin-β to the spindle. Loss of importin-α1 or expression of a non-phosphorylated mutant of importin-α1 results in the formation of shortened spindles with reduced microtubule density and induces a prolonged metaphase, whereas phosphorylation-mimicking mutants are functional in mitosis. We propose that phosphorylation of importin-α1 is a general mechanism for the spatial and temporal control of mitotic spindle assembly by CDK1-cyclin B1 that acts through the release of SAFs such as TPX2 and KIFC1 from inhibitory complexes that restrict spindle assembly.

Inhibition of Karyopherin beta 1 suppresses prostate cancer growth

Prostate cancer (PCa) initiation and progression requires activation of numerous oncogenic signaling pathways. Nuclear-cytoplasmic transport of oncogenic factors is mediated by Karyopherin proteins during cell transformation. However, the role of nuclear transporter proteins in PCa progression has not been well defined. Here, we report that the KPNB1, a key member of Karyopherin beta subunits, is highly expressed in advanced prostate cancers. Further study showed that targeting KPNB1 suppressed the proliferation of prostate cancer cells. The knockdown of KPNB1 reduced nuclear translocation of c-Myc, the expression of downstream cell cycle modulators, and phosphorylation of regulator of chromatin condensation 1 (RCC1), a key protein for spindle assembly during mitosis. Meanwhile, CHIP assay demonstrated the binding of c-Myc to KPNB1 promoter region, which indicated a positive feedback regulation of KPNB1 expression mediated by the c-Myc. In addition, NF-κB subunit p50 translocation to nuclei was blocked by KPNB1 inhibition, which led to an increase in apoptosis and a decrease in tumor sphere formation of PCa cells. Furthermore, subcutaneous xenograft tumor models with a stable knockdown of KPNB1 in C42B PCa cells validated that the inhibition of KPNB1 could suppress the growth of prostate tumor in vivo. Moreover, the intravenously administration of importazole, a specific inhibitor for KPNB1, effectively reduced PCa tumor size and weight in mice inoculated with PC3 PCa cells. In summary, our data established the functional link between KPNB1 and PCa prone c-Myc, NF-kB, and cell cycle modulators. More importantly, inhibition of KPNB1 could be a new therapeutic target for PCa treatment.

RCC1 regulates inner centromeric composition in a Ran-independent fashion

RCC1 associates to chromatin dynamically within mitosis and catalyzes Ran-GTP production. Exogenous RCC1 disrupts kinetochore structure in Xenopus egg extracts (XEEs), but the molecular basis of this disruption remains unknown. We have investigated this question, utilizing replicated chromosomes that possess paired sister kinetochores. We find that exogenous RCC1 evicts a specific subset of inner KT proteins including Shugoshin-1 (Sgo1) and the chromosome passenger complex (CPC). We generated RCC1 mutants that separate its enzymatic activity and chromatin binding. Strikingly, Sgo1 and CPC eviction depended only on RCC1's chromatin affinity but not its capacity to produce Ran-GTP. RCC1 similarly released Sgo1 and CPC from synthetic kinetochores assembled on CENP-A nucleosome arrays. Together, our findings indicate RCC1 regulates kinetochores at the metaphase-anaphase transition through Ran-GTP-independent displacement of Sgo1 and CPC.

Three-dimensional context rather than NLS amino acid sequence determines importin α subtype specificity for RCC1

Active nuclear import of Ran exchange factor RCC1 is mediated by importin α3. This pathway is essential to generate a gradient of RanGTP on chromatin that directs nucleocytoplasmic transport, mitotic spindle assembly and nuclear envelope formation. Here we identify the mechanisms of importin α3 selectivity for RCC1. We find this isoform binds RCC1 with one order of magnitude higher affinity than the generic importin α1, although the two isoforms share an identical NLS-binding groove. Importin α3 uses its greater conformational flexibility to wedge the RCC1 β-propeller flanking the NLS against its lateral surface, preventing steric clashes with its Armadillo-core. Removing the β-propeller, or inserting a linker between NLS and β-propeller, disrupts specificity for importin α3, demonstrating the structural context rather than NLS sequence determines selectivity for isoform 3. We propose importin α3 evolved to recognize topologically complex NLSs that lie next to bulky domains or are masked by quaternary structures.Importin α3 facilitates the nuclear transport of the Ran guanine nucleotide exchange factor RCC1. Here the authors reveal the molecular basis for the selectivity of RCC1 for importin α3 vs the generic importin α1 and discuss the evolution of importin α isoforms.

Analyses of PDE-regulated phosphoproteomes reveal unique and specific cAMP-signaling modules in T cells

Specific functions for different cyclic nucleotide phosphodiesterases (PDEs) have not yet been identified in most cell types. Conventional approaches to study PDE function typically rely on measurements of global cAMP, general increases in cAMP-dependent protein kinase (PKA), or the activity of exchange protein activated by cAMP (EPAC). Although newer approaches using subcellularly targeted FRET reporter sensors have helped define more compartmentalized regulation of cAMP, PKA, and EPAC, they have limited ability to link this regulation to downstream effector molecules and biological functions. To address this problem, we have begun to use an unbiased mass spectrometry-based approach coupled with treatment using PDE isozyme-selective inhibitors to characterize the phosphoproteomes of the functional pools of cAMP/PKA/EPAC that are regulated by specific cAMP-PDEs (the PDE-regulated phosphoproteomes). In Jurkat cells we find multiple, distinct PDE-regulated phosphoproteomes that can be defined by their responses to different PDE inhibitors. We also find that little phosphorylation occurs unless at least two different PDEs are concurrently inhibited in these cells. Moreover, bioinformatics analyses of these phosphoproteomes provide insight into the unique functional roles, mechanisms of action, and synergistic relationships among the different PDEs that coordinate cAMP-signaling cascades in these cells. The data strongly suggest that the phosphorylation of many different substrates contributes to cAMP-dependent regulation of these cells. The findings further suggest that the approach of using selective, inhibitor-dependent phosphoproteome analysis can provide a generalized methodology for understanding the roles of different PDEs in the regulation of cyclic nucleotide signaling.

Epstein-Barr virus nuclear antigen 1 interacts with regulator of chromosome condensation 1 dynamically throughout the cell cycle

The Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA1) is a sequence-specific DNA-binding protein that plays an essential role in viral episome replication and segregation, by recruiting the cellular complex of DNA replication onto the origin (oriP) and by tethering the viral DNA onto the mitotic chromosomes. Whereas the mechanisms of viral DNA replication are well documented, those involved in tethering EBNA1 to the cellular chromatin are far from being understood. Here, we have identified regulator of chromosome condensation 1 (RCC1) as a novel cellular partner for EBNA1. RCC1 is the major nuclear guanine nucleotide exchange factor for the small GTPase Ran enzyme. RCC1, associated with chromatin, is involved in the formation of RanGTP gradients critical for nucleo-cytoplasmic transport, mitotic spindle formation and nuclear envelope reassembly following mitosis. Using several approaches, we have demonstrated a direct interaction between these two proteins and found that the EBNA1 domains responsible for EBNA1 tethering to the mitotic chromosomes are also involved in the interaction with RCC1. The use of an EBNA1 peptide array confirmed the interaction of RCC1 with these regions and also the importance of the N-terminal region of RCC1 in this interaction. Finally, using confocal microscopy and Förster resonance energy transfer analysis to follow the dynamics of interaction between the two proteins throughout the cell cycle, we have demonstrated that EBNA1 and RCC1 closely associate on the chromosomes during metaphase, suggesting an essential role for the interaction during this phase, perhaps in tethering EBNA1 to mitotic chromosomes.

Targeting nuclear transporters in cancer: Diagnostic, prognostic and therapeutic potential

The Karyopherin superfamily is a major class of soluble transport receptors consisting of both import and export proteins. The trafficking of proteins involved in transcription, cell signalling and cell cycle regulation among other functions across the nuclear membrane is essential for normal cellular functioning. However, in cancer cells, the altered expression or localization of nuclear transporters as well as the disruption of endogenous nuclear transport inhibitors are some ways in which the Karyopherin proteins are dysregulated. The value of nuclear transporters in the diagnosis, prognosis and treatment of cancer is currently being elucidated with recent studies highlighting their potential as biomarkers and therapeutic targets.

Revisiting chromatin binding of the Arabidopsis UV-B photoreceptor UVR8

BACKGROUND

Plants perceive UV-B through the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor and UVR8 activation leads to changes in gene expression such as those associated with UV-B acclimation and stress tolerance. Albeit functionally unrelated, UVR8 shows some homology with RCC1 (Regulator of Chromatin Condensation 1) proteins from non-plant organisms at the sequence level. These proteins act as guanine nucleotide exchange factors for Ran GTPases and bind chromatin via histones. Subsequent to the revelation of this sequence homology, evidence was presented showing that UVR8 activity involves interaction with chromatin at the loci of some target genes through histone binding. This suggested a UVR8 mode-of-action intimately and directly linked with gene transcription. However, several aspects of UVR8 chromatin association remained undefined, namely the impact of UV-B on the process and how UVR8 chromatin association related to the transcription factor ELONGATED HYPOCOTYL 5 (HY5), which is important for UV-B signalling and has overlapping chromatin targets. Therefore, we have investigated UVR8 chromatin association in further detail.

RESULTS

Unlike the claims of previous studies, our chromatin immunoprecipitation (ChIP) experiments do not confirm UVR8 chromatin association. In contrast to human RCC1, recombinant UVR8 also does not bind nucleosomes in vitro. Moreover, fusion of a VP16 activation domain to UVR8 did not alter expression of proposed UVR8 target genes in transient gene expression assays. Finally, comparison of the Drosophila DmRCC1 and the Arabidopsis UVR8 crystal structures revealed that critical histone- and DNA-interaction residues apparent in DmRCC1 are not conserved in UVR8.

CONCLUSION

This has led us to conclude that the cellular activity of UVR8 likely does not involve its specific binding to chromatin at target genes.

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.

Identification of the ISWI Chromatin Remodeling Complex of the Early Branching Eukaryote Trypanosoma brucei

ISWI chromatin remodelers are highly conserved in eukaryotes and are important for the assembly and spacing of nucleosomes, thereby controlling transcription initiation and elongation. ISWI is typically associated with different subunits, forming specialized complexes with discrete functions. In the unicellular parasite Trypanosoma brucei, which causes African sleeping sickness, TbISWI down-regulates RNA polymerase I (Pol I)-transcribed variant surface glycoprotein (VSG) gene expression sites (ESs), which are monoallelically expressed. Here, we use tandem affinity purification to determine the interacting partners of TbISWI. We identify three proteins that do not show significant homology with known ISWI-associated partners. Surprisingly, one of these is nucleoplasmin-like protein (NLP), which we had previously shown to play a role in ES control. In addition, we identify two novel ISWI partners, regulator of chromosome condensation 1-like protein (RCCP) and phenylalanine/tyrosine-rich protein (FYRP), both containing protein motifs typically found on chromatin proteins. Knockdown of RCCP or FYRP in bloodstream form T. brucei results in derepression of silent variant surface glycoprotein ESs, as had previously been shown for TbISWI and NLP. All four proteins are expressed and interact with each other in both major life cycle stages and show similar distributions at Pol I-transcribed loci. They are also found at Pol II strand switch regions as determined with ChIP. ISWI, NLP, RCCP, and FYRP therefore appear to form a single major ISWI complex in T. brucei (TbIC). This reduced complexity of ISWI regulation and the presence of novel ISWI partners highlights the early divergence of trypanosomes in evolution.

Reprint of "Nuclear transport factors: global regulation of mitosis"

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator – the γ-TuRC complex – and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

Nuclear transport factors: global regulation of mitosis

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator - the γ-TuRC complex - and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

Human cells contain natural double-stranded RNAs with potential regulatory functions

Recent evidence has suggested the existence of sense-antisense transcription in mammals, but the existence of double-stranded RNAs endowed with biological function has remained elusive. Herein we show that hundreds of putative natural double-stranded RNAs (ndsRNAs) are expressed from interspersed genomic locations and respond to cellular cues. We demonstrate that a subset of ndsRNAs localize in the nucleus and, in their double-stranded form, interact with nuclear proteins. Detailed characterization of an ndsRNA (nds-2a) revealed that this molecule displays differential localization throughout the cell cycle and directly interacts with RCC1 and RAN and, through the latter, with the mitotic RANGAP1-SUMO1-RANBP2 complex. Notably, altering nds-2a levels led to postmitotic abnormalities, mitotic catastrophe and cell death, thus supporting a mitosis-related role. Altogether, our study reveals a hitherto-unrecognized class of RNAs that potentially participate in major biological processes in human cells.

Disruption of the ran system by cysteine oxidation of the nucleotide exchange factor RCC1

Transport regulation by the Ran GTPase requires its nuclear localization and GTP loading by the chromatin-associated exchange factor RCC1. These reactions generate Ran protein and Ran nucleotide gradients between the nucleus and the cytoplasm. Cellular stress disrupts the Ran gradients, but the specific mechanisms underlying this disruption have not been elucidated. We used biochemical approaches to determine how oxidative stress disrupts the Ran system. RCC1 exchange activity was reduced by diamide-induced oxidative stress and restored with dithiothreitol. Using mass spectrometry, we found that multiple solvent-exposed cysteines in RCC1 are oxidized in cells treated with diamide. The cysteines oxidized in RCC1 included Cys93, which is solvent exposed and unique because it becomes buried upon contact with Ran. A Cys93Ser substitution dramatically reduced exchange activity through an effect on RCC1 binding to RanGDP. Diamide treatment reduced the size of the mobile fraction of RCC1-green fluorescent protein in cells and inhibited nuclear import in digitonin-permeabilized cell assays. The Ran protein gradient was also disrupted by UV-induced stress but without affecting RCC1 exchange activity. Our data suggest that stress can disrupt the Ran gradients through RCC1-dependent and RCC1-independent mechanisms, possibly dependent on the particular stress condition.

RanBP1 governs spindle assembly by defining mitotic Ran-GTP production

Accurate control of the Ras-related nuclear protein (Ran) GTPase cycle depends on the regulated activity of regulator of chromosome condensation 1 (RCC1), Ran's nucleotide exchange factor. RanBP1 has been characterized as a coactivator of the Ran GTPase-activating protein RanGAP1. RanBP1 can also form a stable complex with Ran and RCC1, although the dynamics and function of this complex remain poorly understood. Here, we show that formation of the heterotrimeric RCC1/Ran/RanBP1 complex in M phase Xenopus egg extracts controls both RCC1's enzymatic activity and partitioning between the chromatin-bound and soluble pools of RCC1. This mechanism is critical for spatial control of Ran-guanosine triphosphate (GTP) gradients that guide mitotic spindle assembly. Moreover, phosphorylation of RanBP1 drives changes in the dynamics of chromatin-bound RCC1 pools at the metaphase-anaphase transition. Our findings reveal an important mitotic role for RanBP1, controlling the spatial distribution and magnitude of mitotic Ran-GTP production and thereby ensuring accurate execution of Ran-dependent mitotic events.

Phosphorylation of Crm1 by CDK1-cyclin-B promotes Ran-dependent mitotic spindle assembly

Mitotic spindle assembly in animal cells is orchestrated by a chromosome-dependent pathway that directs microtubule stabilization. RanGTP generated at chromosomes releases spindle assembly factors from inhibitory complexes with importins, the nuclear transport factors that facilitate protein import into the nucleus during interphase. In addition, the nuclear export factor Crm1 has been proposed to act as a mitotic effector of RanGTP through the localized assembly of protein complexes on the mitotic spindle, notably at centrosomes and kinetochores. It has been unclear, however, how the functions of nuclear transport factors are controlled during mitosis. Here, we report that human Crm1 is phosphorylated at serine 391 in mitosis by CDK1-cyclin-B (i.e. the CDK1 and cyclin B complex). Expression of Crm1 with serine 391 mutated to either non-phosphorylated or phosphorylation-mimicking residues indicates that phosphorylation directs the localization of Crm1 to the mitotic spindle and facilitates spindle assembly, microtubule stabilization and chromosome alignment. We find that phosphorylation of Crm1 at serine 391 enhances its RanGTP-dependent interaction with RanGAP1-RanBP2 and promotes their recruitment to the mitotic spindle. These results show that phosphorylation of Crm1 controls its molecular interactions, localization and function during mitosis, uncovering a new mechanism for the control of mitotic spindle assembly by CDK1-cyclin-B. We propose that nuclear transport factors are controlled during mitosis through the selection of specific molecular interactions by protein phosphorylation.

Cell cycle-dependent binding modes of the ran exchange factor RCC1 to chromatin

The formation of an activity gradient of the small G-protein Ran around chromatin depends on the differential partitioning of the opposing enzyme activities of the Ran guanine nucleotide exchange factor RCC1 that resides on chromatin, and the cytoplasmic Ran GTPase activating protein RanGAP. We studied the time-dependent interaction kinetics between RCC1 and chromatin and the mobility of the Ran-RCC1 complex in living cells by fluorescence correlation spectroscopy to investigate whether binding of RCC1 to chromatin regulates the exchange activity of RCC1, and whether the stability of the RCC1-chromatin interaction is regulated during the cell cycle. We found that RCC1 mobility is dominated by two states: a highly mobile state that is trapped within chromatin, and a transiently immobilized state that is stabilized during mitosis. We show that only the immobilized state of RCC1 interacts with Ran and conclude that its guanine nucleotide exchange activity is restricted to specific sites on chromatin.

Determinants of human cyclin B1 association with mitotic chromosomes

Cyclin B1–CDK1 activity is essential for mitotic entry, but questions remain regarding how the activity of this kinase is spatially regulated. Previous studies showed that the cyclin B1 subunit localizes to several compartments of a mitotic cell, including the centrosomes, mitotic spindle, kinetochores and chromosomes via distinct sequence elements. Mitotic chromosome association occurs through the unstructured N-terminal domain of cyclin B1 and is independent of CDK1 binding. Here, we use live cell imaging of human cyclin B1 fused to GFP to precisely define the sequence elements within cyclin B1 that mediate its association with condensed mitotic chromosomes. We find that a short, evolutionarily conserved N-terminal motif is required for cyclin B1 to localize to mitotic chromosomes. We further reveal a role for arginine residues within and near the destruction box sequence in the chromosome association of cyclin B1. Additionally, our data suggest that sequences further downstream in cyclin B1, such as the cytoplasmic retention sequence and the cyclin box, may negatively modulate chromosome association. Because multiple basic residues are required for cyclin B1 association with mitotic chromosomes, electrostatic interactions with DNA may facilitate cyclin B1 localization to chromosomes.

Poc1A and Poc1B act together in human cells to ensure centriole integrity

Proteomic studies in unicellular eukaryotes identified a set of centriolar proteins that included proteome of centriole 1 (Poc1). Functional studies in these organisms implicated Poc1 in centriole duplication and length control, as well as ciliogenesis. Using isoform-specific antibodies and RNAi depletion, we have examined the function of the two related human proteins, Poc1A and Poc1B. We find that Poc1A and Poc1B each localize to centrioles and spindle poles, but do so independently and with different dynamics. However, although loss of one or other Poc1 protein does not obviously disrupt mitosis, depletion of both proteins leads to defects in spindle organization with the generation of unequal or monopolar spindles. Our data indicate that, once incorporated, a fraction of Poc1A and Poc1B remains stably associated with parental centrioles, but that depletion prevents incorporation into nascent centrioles. Nascent centrioles lacking both Poc1A and Poc1B exhibit loss of integrity and maturation, and fail to undergo duplication. Thus, when Poc1A and Poc1B are co-depleted, new centrosomes capable of maturation cannot assemble and unequal spindles result. Interestingly, Poc1B, but not Poc1A, is phosphorylated in mitosis, and depletion of Poc1B alone was sufficient to perturb cell proliferation. Hence, Poc1A and Poc1B play redundant, but essential, roles in generation of stable centrioles, but Poc1B may have additional independent functions during cell cycle progression.

Phosphorylation of Ran-binding protein-1 by Polo-like kinase-1 is required for interaction with Ran and early mitotic progression

Polo-like kinase-1 (Plk1) is essential for progression of mitosis and localizes to centrosomes, central spindles, midbody, and kinetochore. Ran, a small GTPase of the Ras superfamily, plays a role in microtubule dynamics and chromosome segregation during mitosis. Although Ran-binding protein-1 (RanBP1) has been reported as a regulator of RanGTPase for its mitotic functions, the action mechanism between Ran and RanBP1 during mitosis is still unknown. Here, we demonstrated in vitro and in vivo phosphorylation of RanBP1 by Plk1 as well as the importance of phosphorylation of RanBP1 in the interaction between Plk1 and Ran during early mitosis. Both phosphorylation-defective and N-terminal deletion mutant constructs of RanBP1 disrupted the interaction with Ran, and depletion of Plk1 also disrupted the formation of a complex between Ran and RanBP1. In addition, the results from both ectopic expression of phosphorylation-defective mutant construct and a functional complementation on RanBP1 deficiency with this mutant indicated that phosphorylation of RanBP1 by Plk1 might be crucial to microtubule nucleation and spindle assembly during mitosis.

Ran control of mitosis in human cells: gradients and local signals

Roles of the GTPase Ran in cell life and division rely on a largely conserved mechanism, i.e. Ran's ability to interact with transport vectors. Modes of control of downstream factors, however, are diversified at particular times of the cell cycle. Specificity and fine-tuning emerge most clearly during mitosis. In the present article, we focus on the distinction between global mitotic control by the chromosomal Ran gradient and specific spatial and temporal control operated by localized Ran network members at sites of the mitotic apparatus in human cells.

A TACC3/ch-TOG/clathrin complex stabilises kinetochore fibres by inter-microtubule bridging

Kinetochore fibres (K-fibres) of the spindle apparatus move chromosomes during mitosis. These fibres are discrete bundles of parallel microtubules (MTs) that are crosslinked by inter-MT 'bridges' that are thought to improve fibre stability during chromosomal movement. The identity of these bridges is unknown. Clathrin is a multimeric protein that has been shown to stabilise K-fibres during early mitosis by a mechanism independent of its role in membrane trafficking. In this study, we show that clathrin at the mitotic spindle is in a transforming acidic colied-coil protein 3 (TACC3)/colonic, hepatic tumour overexpressed gene (ch-TOG)/clathrin complex. The complex is anchored to the spindle by TACC3 and ch-TOG. Ultrastructural analysis of clathrin-depleted K-fibres revealed a selective loss of a population of short inter-MT bridges and a general loss of MTs. A similar loss of short inter-MT bridges was observed in TACC3-depleted K-fibres. Finally, immunogold labelling confirmed that inter-MT bridges in K-fibres contain clathrin. Our results suggest that the TACC3/ch-TOG/clathrin complex is an inter-MT bridge that stabilises K-fibres by physical crosslinking and by reducing rates of MT catastrophe.

Substrate-specific regulation of ubiquitination by the anaphase-promoting complex

By orchestrating the sequential degradation of a large number of cell cycle regulators, the ubiquitin ligase anaphase-promoting complex (APC/C) is essential for proliferation in all eukaryotes. The correct timing of APC/C-dependent substrate degradation, a critical feature of progression through mitosis, was long known to be controlled by mechanisms targeting the core APC/C-machinery. Recent experiments, however, have revealed an important contribution of substrate-specific regulation of the APC/C to achieve accurate cell division. In this perspective, we describe different mechanisms of substrate-specific APC/C-regulation and discuss their importance for cell division.

The role of RanGTP gradient in vertebrate oocyte maturation

The maturation of vertebrate oocyte into haploid gamete, the egg, consists of two specialized asymmetric cell divisions with no intervening S-phase. Ran GTPase has an essential role in relaying the active role of chromosomes in their own segregation by the meiotic process. In addition to its conserved role as a key regulator of macromolecular transport between nucleus and cytoplasm, Ran has important functions during cell division, including in mitotic spindle assembly and in the assembly of nuclear envelope at the exit from mitosis. The cellular functions of Ran are mediated by RanGTP interactions with nuclear transport receptors (NTRs) related to importin β and depend on the existence of chromosome-centered RanGTP gradient. Live imaging with FRET biosensors indeed revealed the existence of RanGTP gradient throughout mouse oocyte maturation. NTR-dependent transport of cell cycle regulators including cyclin B1, Wee2, and Cdc25B between the oocyte cytoplasm and germinal vesicle (GV) is required for normal resumption of meiosis. After GVBD in mouse oocytes, RanGTP gradient is required for timely meiosis I (MI) spindle assembly and provides long-range signal directing egg cortex differentiation. However, RanGTP gradient is not required for MI spindle migration and may be dispensable for MI spindle function in chromosome segregation. In contrast, MII spindle assembly and function in maturing mouse and Xenopus laevis eggs depend on RanGTP gradient, similar to X. laevis MII-derived egg extracts.

The nuclear envelope

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

Nuclear reformation after mitosis requires downregulation of the Ran GTPase effector RanBP1 in mammalian cells

The GTPase Ran regulates nucleocytoplasmic transport in interphase and spindle organisation in mitosis via effectors of the importin beta superfamily. Ran-binding protein 1 (RanBP1) regulates guanine nucleotide turnover on Ran, as well as its interactions with effectors. Unlike other Ran network members that are steadily expressed, RanBP1 abundance is modulated during the mammalian cell cycle, peaking in mitosis and declining at mitotic exit. Here, we show that RanBP1 downregulation takes place in mid to late telophase, concomitant with the reformation of nuclei. Mild RanBP1 overexpression in murine cells causes RanBP1 to persist in late mitosis and hinders a set of events underlying the telophase to interphase transition, including chromatin decondensation, nuclear expansion and nuclear lamina reorganisation. Moreover, the reorganisation of nuclear pores fails associated with defective nuclear relocalisation of NLS cargoes. Co-expression of importin beta, together with RanBP1, however mitigates these defects. Thus, RanBP1 downregulation is required for nuclear reorganisation pathways operated by importin beta after mitosis.

The methylated N-terminal tail of RCC1 is required for stabilisation of its interaction with chromatin by Ran in live cells

Background

Regulator of chromosome condensation 1 (RCC1) is the guanine nucleotide exchange factor for Ran GTPase. Localised generation of Ran-GTP by RCC1 on chromatin is critical for nucleocytoplasmic transport, mitotic spindle assembly and nuclear envelope formation. Both the N-terminal tail of RCC1 and its association with Ran are important for its interaction with chromatin in cells. In vitro, the association of Ran with RCC1 induces a conformational change in the N-terminal tail that promotes its interaction with DNA.

Results

We have investigated the mechanism of the dynamic interaction of the α isoform of human RCC1 (RCC1α) with chromatin in live cells using fluorescence recovery after photobleaching (FRAP) of green fluorescent protein (GFP) fusions. We show that the N-terminal tail stabilises the interaction of RCC1α with chromatin and this function can be partially replaced by another lysine-rich nuclear localisation signal. Removal of the tail prevents the interaction of RCC1α with chromatin from being stabilised by Ran^T24N, a mutant that binds stably to RCC1α. The interaction of RCC1α with chromatin is destabilised by mutation of lysine 4 (K4Q), which abolishes α-N-terminal methylation, and this interaction is no longer stabilised by Ran^T24N. However, α-N-terminal methylation of RCC1α is not regulated by the binding of Ran^T24N. Conversely, the association of Ran with precipitated RCC1α does not require the N-terminal tail of RCC1α or its methylation. The mobility of RCC1α on chromatin is increased by mutation of aspartate 182 (D182A), which inhibits guanine-nucleotide exchange activity, but RCC1α^D182A can still bind nucleotide-free Ran and its interaction with chromatin is stabilised by Ran^T24N.

Conclusions

These results show that the stabilisation of the dynamic interaction of RCC1α with chromatin by Ran in live cells requires the N-terminal tail of RCC1α. α-N-methylation is not regulated by formation of the binary complex with Ran, but it promotes chromatin binding through the tail. This work supports a model in which the association of RCC1α with chromatin is promoted by a conformational change in the α-N-terminal methylated tail that is induced allosterically in the binary complex with Ran.

Phosphorylation of Mcl-1 by CDK1-cyclin B1 initiates its Cdc20-dependent destruction during mitotic arrest

The balance between cell cycle progression and apoptosis is important for both surveillance against genomic defects and responses to drugs that arrest the cell cycle. In this report, we show that the level of the human anti-apoptotic protein Mcl-1 is regulated during the cell cycle and peaks at mitosis. Mcl-1 is phosphorylated at two sites in mitosis, Ser64 and Thr92. Phosphorylation of Thr92 by cyclin-dependent kinase 1 (CDK1)-cyclin B1 initiates degradation of Mcl-1 in cells arrested in mitosis by microtubule poisons. Mcl-1 destruction during mitotic arrest requires proteasome activity and is dependent on Cdc20/Fizzy, which mediates recognition of mitotic substrates by the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. Stabilisation of Mcl-1 during mitotic arrest by mutation of either Thr92 or a D-box destruction motif inhibits the induction of apoptosis by microtubule poisons. Thus, phosphorylation of Mcl-1 by CDK1-cyclin B1 and its APC/C(Cdc20)-mediated destruction initiates apoptosis if a cell fails to resolve mitosis. Regulation of apoptosis, therefore, is linked intrinsically to progression through mitosis and is governed by a temporal mechanism that distinguishes between normal mitosis and prolonged mitotic arrest.

Microtubule assembly by the Apc protein is regulated by importin-β—RanGTP

Mutations in the tumour suppressor Adenomatous polyposis coli (Apc) initiate most sporadic colorectal cancers. Apc is implicated in regulating microtubule (MT) dynamics in interphase and mitosis. However, little is known about the underlying mechanism or regulation of this Apc function. We identified importin-β as a binding partner of Apc that regulates its effect on MTs. Apc binds importin-β in vitro and in Xenopus egg extracts, and RanGTP inhibits this interaction. The armadillo-like repeat domain of importin-β binds to the middle of Apc, where it can compete with β-catenin. In addition, two independent sites in the C terminus of Apc bind the N-terminal region of importin-β. Binding to importin-β reduces the ability of Apc to assemble and bundle MTs in vitro and to promote assembly of microtubule asters in Xenopus egg extracts, but does not affect the binding of Apc to MTs or to EB1. Depletion of Apc decreases the formation of cold-stable spindles in Xenopus egg extracts. Importantly, the ability of purified Apc to rescue this phenotype was reduced when it was constitutively bound to importin-β. Thus, importin-β binds to Apc and negatively regulates the MT-assembly and spindle-promoting activity of Apc in a Ran-regulatable manner.

Self-organization of intracellular gradients during mitosis

Gradients are used in a number of biological systems to transmit spatial information over a range of distances. The best studied are morphogen gradients where information is transmitted over many cell lengths. Smaller mitotic gradients reflect the need to organize several distinct events along the length of the mitotic spindle. The intracellular gradients that characterize mitosis are emerging as important regulatory paradigms. Intracellular gradients utilize intrinsic auto-regulatory feedback loops and diffusion to establish stable regions of activity within the mitotic cytosol. We review three recently described intracellular mitotic gradients. The Ran GTP gradient with its elaborate cascade of nuclear transport receptors and cargoes is the best characterized, yet the dynamics underlying the robust gradient of Ran-GTP have received little attention. Gradients of phosphorylation have been observed on Aurora B kinase substrates both before and after anaphase onset. In both instances the phosphorylation gradient appears to result from a soluble gradient of Aurora B kinase activity. Regulatory properties that support gradient formation are highlighted. Intracellular activity gradients that regulate localized mitotic events bare several hallmarks of self-organizing biologic systems that designate spatial information during pattern formation. Intracellular pattern formation represents a new paradigm in mitotic regulation.

Dynamic localisation of Ran GTPase during the cell cycle

Background

Ran GTPase has multiple functions during the cell division cycle, including nucleocytoplasmic transport, mitotic spindle assembly and nuclear envelope formation. The activity of Ran is determined by both its guanine nucleotide-bound state and its subcellular localization.

Results

Here, we have characterised the localisation and mobility of Ran coupled to green fluorescent protein (GFP) during the cell cycle in live human cells. Ran-GFP is nuclear during interphase and is dispersed throughout the cell during mitosis. GFP-Ran^Q69L, a mutant locked in the GTP-bound state, is less highly concentrated in the nucleus and associates with nuclear pore complexes within the nuclear envelope. During mitosis, GFP-Ran^Q69L is excluded from chromosomes and localizes to the spindle. By contrast, GFP-Ran^T24N, a mutant with low affinity for nucleotides, interacts relatively stably with chromatin throughout the cell cycle and is highly concentrated on mitotic chromosomes.

Conclusion

These results show that Ran interacts dynamically with chromatin, nuclear pore complexes and the mitotic spindle during the cell cycle. These interactions are dependent on the nucleotide-bound state of the protein. Our data indicate that Ran-GTP generated at chromatin is highly mobile and interacts dynamically with distal structures that are involved in nuclear transport and mitotic spindle assembly.

Regulation of chromatin binding by a conformational switch in the tail of the Ran exchange factor RCC1

RCC1 is the only known exchange factor for the Ran guanosine triphosphatase and performs essential roles in nuclear transport, spindle organization, and nuclear envelope formation. RCC1 binds to chromatin through a bimodal attachment to DNA and histones, and defects in binding cause chromosome missegregation. Chromatin binding is enhanced by apo-Ran. However, the mechanism underlying this regulation has been unclear. We now demonstrate that the N-terminal tail of RCC1 is essential for association with DNA but inhibits histone binding. Apo-Ran significantly promotes RCC1 binding to both DNA and histones, and these effects are tail mediated. Using a fluorescence resonance energy transfer biosensor, we detect conformational changes in the tail of RCC1 coupled to the two binding modes and in response to interactions with Ran and importin-alpha. The biosensor also reports changes accompanying mitosis in living cells. We propose that Ran induces an allosteric conformational switch in the tail that exposes the histone-binding surface on RCC1 and facilitates association of the positively charged tail with DNA.

Proteomics identification of nuclear Ran GTPase as an inhibitor of human VRK1 and VRK2 (vaccinia-related kinase) activities

Human vaccinia-related kinase (VRK) 1 is a novel serine-threonine kinase that regulates several transcription factors, nuclear envelope assembly, and chromatin condensation and is also required for cell cycle progression. The regulation of this kinase family is unknown. Mass spectrometry has permitted the identification of Ran as an interacting and regulatory protein of the VRK serine-threonine kinase activities. The stable interaction has been validated by pulldown of endogenous proteins as well as by reciprocal immunoprecipitations. The three members of the VRK family stably interact with Ran, and the interaction was not affected by the bound nucleotide, GDP or GTP. The interaction was stronger with the RanT24N that is locked in its inactive conformation and cannot bind nucleotides. None of the kinases phosphorylated Ran or RCC1. VRK1 does not directly interact with RCC1, but if Ran is present they can be isolated as a complex. The main effect of the interaction of inactive RanGDP with VRK1 is the inhibition of its kinase activity, which was detected by a reduction in VRK1 autophosphorylation and a reduction in phosphorylation of histone H3 in residues Thr-3 and Ser-10. The kinase activity inhibition can be relieved by the interaction with the constitutively active RanGTP or RanL43E, which locks Ran in its GTP-bound active conformation. In this complex, the interaction with VRK proteins does not alter the effect of its guanine exchange factor, RCC1. Ran is a novel negative regulator of nuclear VRK1 and VRK2 kinase activity, which may vary in different subcellular localizations generating an asymmetric intracellular distribution of kinase activity depending on local protein interactions.

Ran GTPase guanine nucleotide exchange factor RCC1 is phosphorylated on serine 11 by cdc2 kinase in vitro

RCC1, a guanine nucleotide exchange factor for Ran GTPase, plays essential roles in the growth and viability of mammalian cells. Here, we examined the phosphorylation of specific serine and threonine residues of RCC1 in vivo and showed that RCC1 is indeed phosphorylated. Analysis by two-dimensional (2D) gel electrophoresis suggested that serine 11 (S11) of hamster RCC1 is phosphorylated in vivo. A point mutation of S11 of hamster RCC1 resulted in a decrease in the number of 2D gel spots, indicating a lack of phosphorylation at the mutant residue. S11 phosphorylation in vitro depended on cyclin B-cdc2 kinase. An RCC1 mutant in which all N-terminal serine and threonine residues were substituted with glutamate residues to mimic phosphorylation at these residues showed decreased binding to the karyopherin, KPNA4, compared with wild type RCC1. We conclude that RCC1 undergoes post-translational phosphorylation.

Protein kinase C delta (PKCdelta) interacts with microtubule organizing center (MTOC)-associated proteins and participates in meiotic spindle organization

Defects in meiotic spindle structure can lead to chromosome segregation errors and genomic instability. In this study the potential role of protein kinase C delta (PKCdelta) on meiotic spindle organization was evaluated in mouse oocytes. PKCdelta was previously shown to be phosphorylated during meiotic maturation and concentrate on the meiotic spindle during metaphases I and II. Currently we show that when phosphorylated on Threonine 505 (pPKCdelta(Thr505)), within the activation loop of its C4 domain, PKCdelta expression was restricted to the meiotic spindle poles and a few specific cytoplasmic foci. In addition, pPKCdelta(Thr505) co-localized with two key microtubule organizing center (MTOC)-associated proteins, pericentrin and gamma-tubulin. An interaction between pPKCdelta(Thr505) and pericentrin as well as gamma-tubulin was confirmed by co-immunoprecipitation analysis using both fetal fibroblast cells and oocytes. Notably, targeted knockdown of PKCdelta expression in oocytes using short interfering RNAs effectively reduced pPKCdelta(Thr505) protein expression at MTOCs and leads to a significant (P < 0.05) disruption of meiotic spindle organization and chromosome alignment during MI and MII. Moreover, both gamma-tubulin and pericentrin expression at MTOCs were decreased in pPKCdelta(Thr505)-depleted oocytes. In sum, these results indicate that pPKCdelta(Thr505) interacts with MTOC-associated proteins and plays a role in meiotic spindle organization in mammalian oocytes.