Phosphorylation of importin-α1 by CDK1-cyclin B1 controls mitotic spindle assembly

Importin-a serves as an adaptor linking importin-beta to proteins carrying a nuclear localization sequence (NLS). During interphase, this interaction enables nuclear protein import, while in mitosis it regulates spindle assembly factors (SAFs) and controls microtubule nucleation, stabilization and spindle function. Here, we show that human importin-alpha 1 is regulated during the cell cycle and is phosphorylated at two sites (threonine 9 and serine 62) during mitosis by the major mitotic protein kinase CDK1-cyclin B. Mutational analysis indicates that the mitotic phosphorylation of importin-alpha 1 inhibits its binding to importin-beta and promotes the release of TPX2 and KIFC1, which are then targeted like importin-beta to the spindle. Loss of importin-alpha 1 or expression of a non-phosphorylated mutant of importin-alpha 1 results in the formation of shortened spindles with reduced microtubule density and induces a prolonged metaphase, whereas phosphorylation-mimicking mutants are functional in mitosis. We propose that phosphorylation of importin-alpha 1 is a general mechanism for the spatial and temporal control of mitotic spindle assembly by CDK1-cyclin B1 that acts through the release of SAFs such as TPX2 and KIFC1 from inhibitory complexes that restrict spindle assembly.