The nuclear transport receptor importin-β/karyopherin-β1 is overexpressed in cancers that display genomic instability. It is regarded as a promising cancer target and inhibitors are being developed. In addition to its role in nucleo-cytoplasmic transport, importin-β regulates mitosis, but the programmes and pathways in which it operates are defined only in part. To unravel importin-β’s mitotic functions we have developed cell lines expressing either wild-type or a mutant importin-β form in characterised residues required for nucleoporin binding. Both forms similarly disrupted spindle pole organisation, while only wild-type importin-β affected microtubule plus-end function and microtubule stability. A proteome-wide search for differential interactors identified a set of spindle regulators sensitive to mutations in the nucleoporin-binding region. Among those, HURP (hepatoma up-regulated protein) is an importin-β interactor and a microtubule-stabilising factor. We found that induction of wild type, but not mutant importin-β, under the same conditions that destabilise mitotic microtubules, delocalised HURP, indicating that the spatial distribution of HURP along the spindle requires importin-β’s nucleoporin-binding residues. Concomitantly, importin-β overexpression sensitises cells to taxanes and synergistically increases mitotic cell death. Thus, the nucleoporin-binding domain is dispensable for importin-β function in spindle pole organisation, but regulates microtubule stability, at least in part via HURP, and renders cells vulnerable to certain microtubule-targeting drugs.
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This work was supported by grants from AIRC (Associazione Italiana per la Ricerca sul Cancro IG14534) and the CNR Flagship programme InterOmics (project SYNLETHER). MD was supported by a PhD fellowship from MIUR. We are grateful to our colleagues Alessandro Rosa (Department of Biology and Biotechnology, University La Sapienza, Rome) for the design and engineering of inducible expression vectors, to Enrico Cundari and Daniela Trisciuoglio (IBPM-CNR, Rome) for help with FACS experiments, to Maria Giubettini (Joint Lab CrestOptics/IIT CLNS@Sapienza, Rome) for contributing to early stages of this work, and to Veronica Morea and Andrea Ilari (IBPM-CNR and Biocrystal Facility, Rome) for insightful discussions and suggestions. We wish to dedicate this paper to our late Director and founder, Prof. Emilia Chiancone.
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Verrico, A., Rovella, P., Di Francesco, L. et al. Importin-β/karyopherin-β1 modulates mitotic microtubule function and taxane sensitivity in cancer cells via its nucleoporin-binding region. Oncogene 39, 454–468 (2020). https://doi.org/10.1038/s41388-019-0989-x