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The nuclear transport cycle and spindle microtubule assembly governed by Ran-GTP. ( A ) A schematic of the nuclear import cycle. The importin-cargo complex assembles in the presence of Ran-GDP in the cytoplasm and its interaction with Ran-GTP unloads the cargo in the nucleus. ( B ) A schematic of the nuclear export cycle. Exportin and a cargo associate in the nucleus in the presence of Ran-GTP but dissociate in the cytoplasm. ( C ) In mitosis, the nuclear envelope breaks down (NEBD) and spindle microtubules are assembled in the close vicinity of chromosomes. Ran-GTP molecular gradient centered around the chromosomes dissociates the target molecules (cargos) through an interaction with importin.
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Spindle microtubules capture and segregate chromosomes and, therefore, their assembly is an essential event in mitosis. To carry out their mission, many key players for microtubule formation need to be strictly orchestrated. Particularly, proteins that assemble the spindle need to be translocated at appropriate sites during mitosis. A small GTPase...
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Maintenance of genomic stability during cell division is one of the most important cellular tasks, and it critically depends on the faithful replication of the genetic material and its equal partitioning into daughter cells, gametes, or spores in the case of yeasts. Defective mitotic spindle assembly and disassembly both result in changes in cellul...
Citations
... RanGAP1*SUMO1 plays essential roles in nucleocytoplasmic transport, mitotic spindle formation, checkpoint control and postmitotic nuclear envelope reassembly. [34][35][36] The NPCassociated form of RanGAP1*SUMO1 is targeted to RanBP2 and interacts with RanBP2 and UBC9. The RanBP2/RanGAP1*SUMO1/ UBC9 complex is a multisubunit E3 ligase rather than an E2-E3 complex. ...
Sentrin/small ubiquitin-like modifier (SUMO) has emerged as a powerful mediator regulating biological processes and participating in pathophysiological processes that cause human diseases, such as cancer, myocardial fibrosis and neurological disorders. Sumoylation has been shown to play a positive regulatory role in keloids. However, the sumoylation mechanism in keloids remains understudied. We proposed that sumoylation regulates keloids via a complex. RanGAP1 acted as a synergistic, functional partner of SUMOs in keloids. Nuclear accumulation of Smad4, a TGF-β/Smad pathway member, was associated with RanGAP1 after SUMO1 inhibition. RanGAP1*SUMO1 mediated the nuclear accumulation of Smad4 due to its impact on nuclear export and reduction in the dissociation of Smad4 and CRM1. We clarified a novel mechanism of positive regulation of sumoylation in keloids and demonstrated the function of sumoylation in Smad4 nuclear export. The NPC-associated RanGAP1*SUMO1 complex functions as a disassembly machine for the export receptor CRM1 and Smad4. Our research provides new perspectives for the mechanisms of keloids and nucleocytoplasmic transport.
... The results ( Figure 4B) indicate that the polymerization of β-tubulin was distinctly inhibited in the shRNA3 group. The nuclear envelope serves as an important part of the malignancy maintenance of cancer cells, and the Lamin family is an important component of the nuclear envelope [23]. Lamin B is the type V intermediate filament that is closely associated with cell proliferation [24]. ...
Lymphoid enhancer-binding factor 1 (LEF1) is a key transcription factor mediating the Wnt signaling pathway. LEF1 is a regulator that is closely associated with tumor malignancy and is usually upregulated in cancers, including colonic adenocarcinoma. The underlying molecular mechanisms of LEF1 regulation for colonic adenocarcinoma progression remain unknown. To explore it, the LEF1 expression in caco2 cells was inhibited using an shRNA approach. The results showed that downregulation of LEF1 inhibited the malignancy and motility associated microstructures, such as polymerization of F-actin, β-tubulin, and Lamin B1 in caco2 cells. LEF1 inhibition suppressed the expression of epithelial/endothelial-mesenchymal transition (EMT) relevant genes. Overall, the current results demonstrated that LEF1 plays a pivotal role in maintaining the malignancy of colonic adenocarcinoma by remodeling motility correlated microstructures and suppressing the expression of EMT-relevant genes. Our study provided evidence of the roles LEF1 played in colonic adenocarcinoma progression, and suggest LEF1 as a potential target for colonic adenocarcinoma therapy.
... To shed some light on how RanBP10 may facilitate L2/vDNA transport along MTs, we focused our attention on MT-dependent motors and their adaptor proteins. MT-dependent motor proteins function as transporters not only during interphase but also during mitosis (reviewed in [54,55]). Importin subunits have been shown to interact with these transporters along with cellular adaptor proteins, such as Targeting Protein for Xklp2 (TPX2), to deliver cellular cargo along MTs and eventually reach mitotic chromatin [56][57][58]. ...
Human papillomaviruses (HPVs) utilize an atypical mode of nuclear import during cell entry. Residing in the Golgi apparatus until mitosis onset, a subviral complex composed of the minor capsid protein L2 and viral DNA (L2/vDNA) is imported into the nucleus after nuclear envelope breakdown by associating with mitotic chromatin. In this complex, L2 plays a crucial role in the interactions with cellular factors that enable delivery and ultimately tethering of the viral genome to mitotic chromatin. To date, the cellular proteins facilitating these steps remain unknown. Here, we addressed which cellular proteins may be required for this process. Using label-free mass spectrometry, biochemical assays, microscopy, and functional virological assays, we discovered that L2 engages a hitherto unknown protein complex of Ran-binding protein 10 (RanBP10), karyopherin alpha2 (KPNA2), and dynein light chain DYNLT3 to facilitate transport towards mitotic chromatin. Thus, our study not only identifies novel cellular interactors and mechanism that facilitate a poorly understood step in HPV entry, but also a novel cellular transport complex.
... On a more exploratory note, it may be worth pointing out that crossings between cytoskeletal filaments are in fact dynamic structures. Indeed, various protein complexes intervene in many ways in the sophisticated assembly, and affect or initiate processes such as filamental growth, branching and disassembly, as well as modify transport along the filaments themselves [49,50,51,52]. As we have seen that crossings affect the dynamics of the collective transport process in non-trivial manners, one may thus wonder to which extent regulating these protein complexes could also be a means of dynamically programming this formidable transport machinery, in order to implement the need of routing and re-routing biological cargos within the cell. ...
The cytoskeleton in eukaryotic cells plays several crucial roles. In terms of intracellular transport, motor proteins use the cytoskeletal filaments as a backbone along which they can actively transport biological cargos such as vesicles carrying biochemical reactants. Crossings between such filaments constitute a key element, as they may serve to alter the destination of such payload. Although motor proteins are known to display a rich behaviour at such crossings, the latter have so far only been modelled as simple branching points. Here we explore a model for a crossing between two microtubules which retains the individual tracks consisting of protofilaments, and we construct a schematic representation of the transport paths. We study collective transport exemplified by the Totally Asymmetric Simple Exclusion Process (TASEP), and provide a full analysis of the transport features and the associated phase diagram, by a generic mean-field approach which we confirm through particle-based stochastic simulations. In particular we show that transport through such a compound crossing cannot be approximated from a coarse-grained structure with a simple branching point. Instead, it gives rise to entirely new and counterintuitive features: the fundamental current-density relation for traffic flow is no longer a single-valued function, and it furthermore differs according to whether it is observed upstream or downstream from the crossing. We argue that these novel features may be directly relevant for interpreting experimental measurements.
... On a more exploratory note, it may be worth pointing out that crossings between cytoskeletal filaments are in fact dynamic structures. Indeed, various protein complexes intervene in many ways in the sophisticated assembly, and affect or initiate processes such as filamental growth, branching and disassembly, as well as modify transport along the filaments themselves [49,50,51,52]. As we have seen that crossings affect the dynamics of the collective transport process in non-trivial manners, one may thus wonder to which extent regulating these protein complexes could also be a means of dynamically programming this formidable transport machinery, in order to implement the need of routing and re-routing biological cargos within the cell. ...
The cytoskeleton in eukaryotic cells plays several crucial roles. In terms of intracellular transport, motor proteins use the cytoskeletal filaments as a backbone along which they can actively transport biological cargos such as vesicles carrying biochemical reactants. Crossings between such filaments constitute a key element, as they may serve to alter the destination of such payload. Although motor proteins are known to display a rich behaviour at such crossings, the latter have so far only been modelled as simple branching points. Here we explore a model for a crossing between two microtubules which retains the individual tracks consisting of protofilaments, and we construct a schematic representation of the transport paths. We study collective transport exemplified by the Totally Asymmetric Simple Exclusion Process (TASEP), and provide a full analysis of the transport features and the associated phase diagram, by a generic mean-field approach which we confirm through particle-based stochastic simulations. In particular we show that transport through such a compound crossing cannot be approximated from a coarse-grained structure with a simple branching point. Instead, it gives rise to entirely new and counterintuitive features: the fundamental current-density relation for traffic flow is no longer a single-valued function, and it furthermore differs according to whether it is observed upstream or downstream from the crossing. We argue that these novel features may be directly relevant for interpreting experimental measurements.
... The structures are represented as in Fig. 1 SPB insertion requires changes in NE-lipid composition, and the recruitment of membrane spanning and bending proteins such as Cut11, Brr6, or Tts1 [25][26][27]. In closed mitosis, factors required for spindle assembly, including tubulin, spindle associated proteins, and motor proteins, which are operating during interphase in shaping cytoplasmic MT organization, must access the nucleus by active transport across NPCs [28]. Nuclear import is driven by specialized transport adaptors known as importins, and depends on the nucleocytoplasmic gradient generated by the small GTPase Ran. ...
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Maintenance of genomic stability during cell division is one of the most important cellular tasks, and it critically depends on the faithful replication of the genetic material and its equal partitioning into daughter cells, gametes, or spores in the case of yeasts. Defective mitotic spindle assembly and disassembly both result in changes in cellular ploidy that ultimately impinge proliferation fitness and might increase tumor malignancy. Although a great progress has been made in understanding how spindles are assembled to orchestrate chromosome segregation, much less is known about how they are disassembled once completed their function. Here, we review two recently uncovered mechanisms of spindle disassembly that operate at different stages of the fission yeast life cycle.
... Complex karyotypes and aneuploidy are common characteristics, suggesting that strategies that specifically target aneuploidy might be successful. The inventors, who have built a large biobank of ovarian cancer tissue samples, have reported earlier that overexpression of Ran, a small Ras-type GTPase that is specialized in nuclear trafficking and regulates mitotic spindle formation [40], is associated with a poor prognosis and that its downregulation induces caspase-3 associated apoptosis [41]. Because of the natural high affinity of GTP toward Ran, the binding-pocket itself is widely considered difficult to target effectively with small-molecule inhibitors. ...
... The best known is the nucleocytoplasmic transport of proteins and ribonucleoprotein molecules (called "cargo") [31]: RAN alternates between two different states, GTP-(mainly in the cytoplasm) and GDP-bound (mainly in the nucleus), resulting in a gradient across the nuclear envelope. By coupling cargo-transport receptor interactions to this cycle, RAN drives the shuttling of different cargoes through the nuclear pore complex in both import and export directions [32][33][34]. A similar mechanism regulates the nuclear envelope formation and mitotic spindle assembly. ...
Here we summarize the most recent update of mesothelioma research in basic science presented at the 14th iMig2018 international conference. The symposium of basic science track mainly focused on the drivers of mesothelioma initiation and progression, molecular pathogenesis, and perspectives on potential therapeutic approaches. This review covers several promising fields including strategies efficiently inhibiting YAP/TAZ functions or their critical downstream targets, heparanase inhibitors, RAN depletion, and MIF/CD74 inhibitors that may be developed as novel therapeutic approaches. In addition, targeting mesothelioma stem cells by depleting M2-polarized macrophages in tumor microenvironment or blocking Tnfsf18 (GITRL)-GITR signalling might be translated into therapeutic modalities in mesothelioma treatment.
... This unusual binding mode may enable competitive sequestration of TPX2 even in the presence of many free NLS-containing proteins and may ensure proper spatial regulation of TPX2 in response to RanGTP [69], Figure 4. Beyond TPX2 and NuMA, numerous proteins have been identified in Xenopus egg extracts, which are under the control of RanGTP and importins, including the microtubule associated proteins HURP (hepatoma upregulated protein) and NuSAP (nucleolar spindle associated protein), the chromatin remodelling proteins ISWI and KANSL1/3, the RNA binding protein Rae1 and the nuclear pore complex (NPC) component Mel-28/ELYS. Consistently, all these proteins remain nuclear in interphase but perform key functions in M-phase in microtubule nucleation, microtubule organisation, spindle pole formation and anaphase spindle stability [70][71][72][73]. ...
Sexual reproduction requires the generation of gametes, which are highly specialised for fertilisation. Female reproductive cells, oocytes, grow up to large sizes when they accumulate energy stocks and store proteins as well as mRNAs to enable rapid cell divisions after fertilisation. At the same time, metazoan oocytes eliminate their centrosomes, i.e., major microtubule-organizing centres (MTOCs), during or right after the long growth phases. Centrosome elimination poses two key questions: first, how can the centrosome be re-established after fertilisation? In general, metazoan oocytes exploit sperm components, i.e., the basal body of the sperm flagellum, as a platform to reinitiate centrosome production. Second, how do most metazoan oocytes manage to build up meiotic spindles without centrosomes? Oocytes have evolved mechanisms to assemble bipolar spindles solely around their chromosomes without the guidance of pre-formed MTOCs. Female animal meiosis involves microtubule nucleation and organisation into bipolar microtubule arrays in regulated self-assembly under the control of the Ran system and nuclear transport receptors. This review summarises our current understanding of the molecular mechanism underlying self-assembly of meiotic spindles, its spatio-temporal regulation, and the key players governing this process in animal oocytes.
... The GTP-bound form of Ran (Ran-GTP) is predominantly nuclear, while the hydrolyzed Ran-GDP form is enriched in the cytoplasm. Cargo protein binds importin b in the cytoplasm (directly or through importin a adaptors) in the presence of Ran-GDP and is released in the nucleoplasm when bound to Ran-GTP (Lange et al., 2007;Okada and Sato, 2015). ...
In metazoans, the nuclear envelope (NE) breakdown (NEBD) occurs during “open” mitosis and meiosis. In the fission yeast Schizosaccharomyces pombe, the mitosis and the first meiotic division (MI) are “closed,” during which the NE is maintained. Intriguingly, during the second meiotic division (MII), the NE is also maintained, but nuclear and cytoplasmic molecules are mixed similarly to open mitosis, a phenomenon of unknown biological significance called “virtual” NEBD (vNEBD). Here, we show that importin-α-dependent nucleocytoplasmic transport regulates spindle disassembly late in anaphase B at MI, as previously reported for mitosis. At MII, however, spindle dissolution is triggered by vNEBD early in anaphase B, a mechanism that short-circuits the nucleocytoplasmic transport system. We demonstrate that the sequential action of these two spindle disassembly systems regulates the spatiotemporal order and ploidy of the meiotic products. vNEBD is a phenomenon described in the fission yeast second meiotic division (MII), which resembles the nuclear envelope breakdown that characterizes the mitotic phase in eukaryotes. Flor-Parra et al. describe a biological role of vNEBD in spindle disassembly.
