Importin-beta/karyopherin-beta1 modulates mitotic microtubule function and taxane sensitivity in cancer cells via its nucleoporin-binding region

The nuclear transport receptor importin-beta/karyopherin-beta1 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-beta regulates mitosis, but the programmes and pathways in which it operates are defined only in part. To unravel importin-beta's mitotic functions we have developed cell lines expressing either wild-type or a mutant importin-beta form in characterised residues required for nucleoporin binding. Both forms similarly disrupted spindle pole organisation, while only wild-type importin-beta 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-beta interactor and a microtubule-stabilising factor. We found that induction of wild type, but not mutant importin-beta, under the same conditions that destabilise mitotic microtubules, delocalised HURP, indicating that the spatial distribution of HURP along the spindle requires importin-beta's nucleoporin-binding residues. Concomitantly, importin-beta overexpression sensitises cells to taxanes and synergistically increases mitotic cell death. Thus, the nucleoporin-binding domain is dispensable for importin-beta function in spindle pole organisation, but regulates microtubule stability, at least in part via HURP, and renders cells vulnerable to certain microtubule-targeting drugs.