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Spindle assembly and function require precise control of microtubule nucleation and dynamics. The chromatin-driven spindle assembly pathway exerts such control locally in the vicinity of chromosomes. One of the key targets of this pathway is TPX2. The molecular mechanism of how TPX2 stimulates microtubule nucleation is not understood. Using microscopy-based dynamic in vitro reconstitution assays with purified proteins, we find that human TPX2 directly stabilizes growing microtubule ends and stimulates microtubule nucleation by stabilizing early microtubule nucleation intermediates. Human microtubule polymerase chTOG (XMAP215/Msps/Stu2p/Dis1/Alp14 homologue) only weakly promotes nucleation, but acts synergistically with TPX2. Hence, a combination of distinct and complementary activities is sufficient for efficient microtubule formation in vitro . Importins control the efficiency of the microtubule nucleation by selectively blocking the interaction of TPX2 with microtubule nucleation intermediates. This in vitro reconstitution reveals the molecular mechanism of regulated microtubule formation by a minimal nucleation module essential for chromatin-dependent microtubule nucleation in cells. Using TIRF-based in vitro reconstitution assays Surrey and colleagues characterize how chTOG and TPX2 cooperate in microtubule nucleation and find that importins regulate the process.

Mutations in the RNA-binding protein FUS cause familial amyotropic lateral sclerosis (ALS). Several mutations that affect the proline-tyrosine nuclear localization signal (PY-NLS) of FUS cause severe juvenile ALS. FUS also undergoes liquid–liquid phase separation (LLPS) to accumulate in stress granules when cells are stressed. In unstressed cells, wild type FUS resides predominantly in the nucleus as it is imported by the importin Karyopherin-β2 (Kapβ2), which binds with high affinity to the C-terminal PY-NLS of FUS. Here, we analyze the interactions between two ALS-related variants FUS(P525L) and FUS(R495X) with importins, especially Kapβ2, since they are still partially localized to the nucleus despite their defective/missing PY-NLSs. The crystal structure of the Kapβ2·FUS(P525L) PY-NLS complex shows the mutant peptide making fewer contacts at the mutation site, explaining decreased affinity for Kapβ2. Biochemical analysis revealed that the truncated FUS(R495X) protein, although missing the PY-NLS, can still bind Kapβ2 and suppresses LLPS. FUS(R495X) uses its C-terminal tandem arginine-glycine-glycine regions, RGG2 and RGG3, to bind the PY-NLS binding site of Kapβ2 for nuclear localization in cells when arginine methylation is inhibited. These findings suggest the importance of the C-terminal RGG regions in nuclear import and LLPS regulation of ALS variants of FUS that carry defective PY-NLSs.

Epithelial ovarian cancer (EOC) is a deadly cancer, and its prognosis has not been changed significantly during several decades. To seek new therapeutic targets for EOC, we performed an in vivo dropout screen in human tumor xenografts using a pooled shRNA library targeting thousands of druggable genes. Then, in follow-up studies, we performed a second screen using a genome-wide CRISPR/Cas9 library. These screens identified 10 high-confidence drug targets that included well-known oncogenes such as ERBB2 and RAF1, and novel oncogenes, notably KPNB1, which we investigated further. Genetic and pharmacological inhibition showed that KPNB1 exerts its antitumor effects through multiphase cell cycle arrest and apoptosis induction. Mechanistically, proteomic studies revealed that KPNB1 acts as a master regulator of cell cycle-related proteins, including p21, p27, and APC/C. Clinically, EOC patients with higher expression levels of KPNB1 showed earlier recurrence and worse prognosis than those with lower expression levels of KPNB1. Interestingly, ivermectin, a Food and Drug Administration-approved antiparasitic drug, showed KPNB1-dependent antitumor effects on EOC, serving as an alternative therapeutic toward EOC patients through drug repositioning. Last, we found that the combination of ivermectin and paclitaxel produces a stronger antitumor effect on EOC both in vitro and in vivo than either drug alone. Our studies have thus identified a combinatorial therapy for EOC, in addition to a plethora of potential drug targets.

A cellular gradient of the GTPase Ran orchestrates the movement of import and export complexes through the Nuclear Pore Complex (NPC). Ran-GTP modulates two essential activities of importin β during nuclear import. On the one hand, it reduces the avidity of importin β for phenylalanine-glycine-rich nucleoporins (FG-nups), thereby facilitating the passage of import complexes through the permeability barrier. On the other hand, it disassembles import complexes, releasing the import cargo into the nucleus. The precise mechanisms by which Ran-GTP modulates importin β activities have remained hypothetical. Leveraging cryogenic electron microscopy (cryo-EM) single-particle analysis, in this paper, we describe five distinct conformational states of importin β in complex with various effectors encountered during an import reaction, specifically IBB-cargos, FG-repeats, Ran-GTP, Ran-GTP:RanBP1, and Ran-GDP:RanBP1. Comparing these states allows us to decipher the conformational landscape of importin β without interference from crystallization agents and lattice forces. By correlating structural data with biochemical activities, we find that Ran-GTP, but not Ran-GDP, constrains the solenoid structure of importin β, closing high-affinity FG-binding pockets and displacing import cargos through allosteric crosstalk between the concave and convex surfaces. We propose that this allosteric mechanism is relevant to other β-karyopherins involved in nuclear import. Importin β, the prototypical eukaryotic nuclear import receptor, transports a wide variety of cargos into the nucleus. Ko et al. used cryogenic electron microscopy to reveal an unexpected allosteric regulation of Importin β’s affinity for FG-nucleoporins, triggered by Ran-GTP.

Nuclear envelope (NE) rupture is a hallmark of cancer cells, and persistent NE damage drives genome instability and inflammation. NE repair relies on activation of the endosomal sorting complex required for transport (ESCRT)-III repair machinery by the LEMD2-CHMP7 compartmentalization sensor, but little is known beyond these core factors. Here, we use convergent proximity proteomics to inventorise proteins mobilized to the NE upon assembly of LEMD2-CHMP7 and activation of ESCRT-III. Within this NE repairome, we identify LRRC59 as a critical regulator of LEMD2 accumulation at NE ruptures. We find that LRRC59, together with the nuclear transporters KPNB1 and XPO1, restricts the assembly of LEMD2-CHMP7 complexes to the site of rupture. Disruption of this regulatory axis escalates LEMD2-CHMP7 spreading across the NE, driving torsional DNA damage in ruptured nuclei and micronuclei. Thus, our work identifies a central regulatory layer of NE repair centered on LRRC59 and KPNB1. We propose that altered LRRC59 levels and deregulated nuclear transport coordinately compromise NE repair, driving genome instability and cancer development. Nuclear envelope (NE) rupture triggers cancer genome instability and inflammation, but mechanisms of NE repair are unclear. Here, the authors map the NE repairome and identify LRRC59 and KPNB1 as a key regulatory axis to restrain NE repair activity.

Background Hepatocellular carcinoma (HCC) is one of the most intractable tumors in the world due to its high rate of recurrence and heterogeneity. Liver cancer initiating cells also called cancer stem cells (CSCs) play a critical role in resistance against typical therapy and high tumor-initiating potential. However, the role of the novel circular RNA (circRNA) circIPO11 in the maintenance of liver cancer initiating cells remains elusive. Methods CircRNAs highly conserved in humans and mice were identified from 3 primary HCC samples by circRNA array. The expression and function of circIPO11 were further evaluated by Northern blot, limiting dilution xenograft analysis, chromatin isolation by RNA purification-PCR assay (ChIRP) and HCC patient-derived tumor cells (PDC) models. CircIpo11 knockout (KO) mice were generated by a CRISPR/Cas9 technology. Results CircIPO11 is highly expressed in HCC tumor tissues and liver CSCs. CircIPO11 is required for the self-renewal maintenance of liver CSCs to initiate HCC development. Mechanistically, circIPO11 recruits TOP1 to GLI1 promoter to trigger its transcription, leading to the activation of Hedgehog signaling. Moreover, GLI1 is also highly expressed in HCC tumor tissues and liver CSCs, and TOP1 expression levels positively correlate with the metastasis, recurrence and survival of HCC patients. Additionally, circIPO11 knockout in mice suppresses the progression of chemically induced liver cancer development. Conclusion Our findings reveal that circIPO11 drives the self-renewal of liver CSCs and promotes the propagation of HCC via activating Hedgehog signaling pathway. Antisense oligonucleotides (ASOs) against circIPO11 combined with TOP1 inhibitor camptothecin (CPT) exert synergistic antitumor effect. Therefore, circIPO11 and the Hedgehog signaling pathway may provide new potential targets for the treatment of HCC patients.

Transportin 3 (TNPO3) is a nuclear import receptor known for its broad substrate specificity, often recognizing arginine-serine (SR/RS) repeat-rich nuclear localization signals (NLS) in SRSF proteins. While serine phosphorylation or glutamate presence has been associated with these NLSs, recent proteomic studies identified TNPO3 cargoes lacking SR/RS repeats. One such example is the cold-inducible RNA-binding protein (CIRBP), which contains a non-classical RSY-NLS. Using X-ray crystallography, here we investigate the TNPO3-CIRBP interaction and find that tyrosines within the RSY-NLS play a key role in binding, independent of phosphorylation. Surprisingly, serine and tyrosine phosphorylation in CIRBP’s NLS inhibits TNPO3 binding, suggesting a regulatory mechanism for nuclear import. Our study reveals a non-conventional nuclear import mechanism mediated by TNPO3, which may extend to other known or yet undiscovered TNPO3 cargoes. Here, the authors present the crystal structure of Transportin 3 (TNPO3) bound to its cargo cold-inducible RNA-binding protein (CIRBP), uncovering a distinct mechanism of protein nuclear import regulation independent of phosphorylation.

Importin-(Imp)β family nucleocytoplasmic transport receptors (NTRs) are supposed to bind to their cargoes through interaction between a confined interface on an NTR and a nuclear localization or export signal (NLS/NES) on a cargo. Although consensus NLS/NES sequence motifs have been defined for cargoes of some NTRs, many experimentally identified cargoes of those NTRs lack those motifs, and consensus NLSs/NESs have been reported for only a few NTRs. Crystal structures of NTR–cargo complexes have exemplified 3D structure-dependent binding of cargoes lacking a consensus NLS/NES to different sites on an NTR. Since only a limited number of NTR–cargo interactions have been studied, whether most cargoes lacking a consensus NLS/NES bind to the same confined interface or to various sites on an NTR is still unclear. Addressing this issue, we generated four mutants of transportin-(Trn)SR, of which many cargoes lack a consensus NLS, and eight mutants of Imp13, where no consensus NLS has been defined, and we analyzed their binding to as many as 40 cargo candidates that we previously identified by a nuclear import reaction-based method. The cargoes bind differently to the NTR mutants, suggesting that positions on an NTR contribute differently to the binding of respective cargoes.

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.

CENP-F is a large protein acting in fundamental cell cycle processes, including nuclear envelope breakdown, mitotic microtubule function and chromosome segregation. These activities are mediated by specific CENP-F protein elements that interact with microtubules, motor proteins, centrosomes and kinetochores. CENP-F is then ubiquitinated and degraded in late mitosis. The C-terminal region of CENP-F contains regulatory elements, including a region required for nuclear localisation in interphase and a KEN box driving proteolysis in late mitosis. Here we show that CENP-F generates proximity ligation products with importin beta during mitosis. Furthermore, induction of importin beta overexpression influences CENP-F at two levels: it alters CENP-F mitotic localisation, promoting its accumulation at spindle poles and decreasing its association with kinetochores, and also causes its persistence in the late mitotic window in which CENP-F normally disappears, in a process that requires microtubule integrity and dynamics. These data implicate therefore importin beta in spatial and temporal control of CENP-F during mitosis, and uncover a functional interplay between CENP-F’s ability to regulate mitotic microtubules and, in turn, a protective role of microtubules against CENP-F premature ubiquitination.