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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.

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.

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.

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.

Acute myeloid leukemia (AML) represents the most prevalent malignancy within the hematologic system, characterized by refractory relapses and a scarcity of effective treatment options. Karyopherin subunit beta-1 (KPNB1) is a member of karyopherin β family, mediating the nuclear import of its cargoes. In this study, we found that elevated expression levels of KPNB1 are associated with unfavorable outcomes in patients with AML. The knockdown of KPNB1 resulted in growth inhibition and apoptosis in AML cells. Additionally, pharmacological inhibition of KPNB1 using the specific inhibitor importazole (IPZ) significantly reduced tumor burden and prolonged survival in MLL-AF9 -induced AML mice. Notably, the inhibition of KPNB1 by IPZ significantly enhanced the sensitivity of both AML cell lines and patient-derived cells to venetoclax in vitro and in xenograft mice models. At the molecular level, we identified an unrecognized cargo of KPNB1, high mobility group 2 (HMGB2), which plays a crucial role in DNA damage repair. Inhibition of KPNB1 resulted in impaired nuclear import of HMGB2, eventually leading to compromised DNA damage repair in AML cells. Overall, our findings elucidate the essential roles of KPNB1 in AML cells through the HMGB2-DNA damage repair axis and highlight a promising therapeutic target for AML intervention.

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.

Background Androgen deprivation and androgen receptor (AR) antagonists are essential treatments for prostate cancers in the clinic. However, after an initially effective response, most tumors become resistant to androgen deprivation therapies. Resistance often arises from AR mutations, resulting in ligand-independent activation and nuclear translocation of AR. Methods This study investigated the effects of nuclear import inhibition on AR signaling and enzalutamide resistance. We utilized nuclear import inhibitors (importazole and ivermectin) and KPNB1 (importin β) knockdown in prostate cancer cell models. We assessed AR subcellular localization, AR-dependent transactivation (transcriptional activity), cell viability and apoptosis in cells expressing wild-type AR, the AR-F876L mutant, or the constitutively active AR splice variant AR-V7. Results Inhibition of nuclear import suppressed AR-dependent transactivation, increased cytoplasmic AR, and enhanced the action of enzalutamide by promoting the apoptosis of prostate cancer cells. While enzalutamide became an AR agonist in cells harboring the mutated AR (F876L), inhibition of nuclear import inhibited enzalutamide-induced nuclear localization and transactivation of the mutated AR and reduced the viability of AR-F876L-expressing cells. Both importazole and ivermectin attenuated the transcriptional activation induced by a constitutively active AR splice variant (AR-V7), while enzalutamide showed no effect. Conclusion Inhibition of nuclear import can overcome enzalutamide resistance and may serve as a novel strategy for prostate cancer treatment.

The mitogen‐activated protein kinase (MAPK) extracellular signal‐regulated kinase 5 (ERK5) is emerging as a promising target in cancer. Indeed, alterations of the MEK5/ERK5 pathway are present in many types of cancer, including melanoma. One of the key events in MAPK signalling is MAPK nuclear translocation and its subsequent regulation of gene expression. Likewise, the effects of ERK5 in supporting cancer cell proliferation have been linked to its nuclear localization. Despite many processes regulating ERK5 nuclear translocation having been determined, the nuclear transporters involved have not yet been identified. Here, we investigated the role of importin subunit alpha (α importin) and importin subunit beta‐1 (importin β1) in ERK5 nuclear shuttling to identify additional targets for cancer treatment. Either importin β1 knockdown or the α/β1 importin inhibitor ivermectin reduced the nuclear amount of overexpressed and endogenous ERK5 in HEK293T and A375 melanoma cells, respectively. These results were confirmed in single‐molecule microscopy in HeLa cells. Moreover, immunofluorescence analysis showed that ivermectin impairs epidermal growth factor (EGF)‐induced ERK5 nuclear shuttling in HeLa cells. Both co‐immunoprecipitation experiments and proximity ligation assay provided evidence that ERK5 and importin β1 interact and that this interaction is further induced by EGF administration and prevented by ivermectin treatment. The combination of ivermectin and the ERK5 inhibitor AX15836 synergistically reduced cell viability and colony formation ability in A375 and HeLa cells and was more effective than single treatments in preventing the growth of A375 and HeLa spheroids. The increased reduction of cell viability upon the same combination was also observed in patient‐derived metastatic melanoma cells. The combination of ivermectin and ERK5 inhibitors other than AX15836 provided similar effects on cell viability. The identification of importin β1 as the nuclear transporter of ERK5 may be exploited for additional ERK5‐inhibiting strategies for cancer therapy. We identified importin subunit beta‐1 (importin β1) as the nuclear transporter involved in ERK5 nuclear translocation. Combinations of the α/β1 importin inhibitor ivermectin and ERK5 inhibitors were more effective than single treatments in reducing the growth of cancer models. Identification of importin β1 as the nuclear transporter of ERK5 may be exploited for additional ERK5‐inhibiting strategies for cancer therapy.

Significance We have shown previously that oxytocin (Oxt), other than regulating lactation and social bonding, is a potent stimulator of bone formation by the osteoblast. Here, we present evidence that this action is exerted through the nuclear localization of the Oxt receptor (Oxtr). Our findings prompt additional studies into the contribution of nuclear Oxtr signaling in regulating lactation and social bonding. We report that oxytocin (Oxt) receptors (Oxtrs), on stimulation by the ligand Oxt, translocate into the nucleus of osteoblasts, implicating this process in the action of Oxt on osteoblast maturation. Sequential immunocytochemistry of intact cells or isolated nucleoplasts stripped of the outer nuclear membrane showed progressive nuclear localization of the Oxtr; this nuclear translocation was confirmed by monitoring the movement of Oxtr–EGFP as well as by immunogold labeling. Nuclear Oxtr localization was conclusively shown by Western immunoblotting and MS of nuclear lysate proteins. We found that the passage of Oxtrs into the nucleus was facilitated by successive interactions with β-arrestins (Arrbs), the small GTPase Rab5, importin-β (Kpnb1), and transportin-1 (Tnpo1). siRNA-mediated knockdown of Arrb1 , Arrb2 , or Tnpo1 abrogated Oxt-induced expression of the osteoblast differentiation genes osterix ( Sp7 ), Atf4 , bone sialoprotein ( Ibsp ), and osteocalcin ( Bglap ) without affecting Erk phosphorylation. Likewise and again, without affecting pErk, inhibiting Arrb recruitment by mutating Ser rich clusters of the nuclear localization signal to Ala abolished nuclear import and Oxtr-induced gene expression. These studies define a previously unidentified mechanism for Oxtr action on bone and open possibilities for direct transcriptional modulation by nuclear G protein-coupled receptors.

The nuclear pore complex (NPC), the sole gateway for nucleocytoplasmic exchange in eukaryotic cells, allows for the passive diffusion of small molecules and transport-receptor-facilitated translocation of signal-dependent cargo molecules. Whether small molecules passively diffuse through a single central channel or through multiple holes of a hydrogel network is a subject of debate. Additionally, whether the passive and facilitated transport systems occupy distinct or overlapping physical regions of the NPC remains unclear. Here, we directly test these models using three-dimensional super-resolution fluorescence microscopy of human cells. This approach reveals that a single viscous central channel in the NPC acts as the sole pathway for passive diffusion of various small molecules; transport receptors and their cargo complexes take distinct transport routes in the periphery, which is occluded by phenylalanine-glycine filaments. Furthermore, the passive and facilitated passageways in the NPC are closely correlated, and their conformations can be simultaneously regulated by Importin β1 (a major transport receptor) and RanGTP (a critical regulator of transport directionality). These results strongly favor a self-regulated viscous channel configuration in native NPCs over the porous hydrogel meshwork model.