Chromosome alignment relies on spindle-localized control of Cdk1 activity

Abstract During mitosis, chromosome alignment at the mitotic spindle equator grants correct chromosome segregation and proper nuclei formation in daughter cells. A central role for chromosome alignment is exerted by the kinesin 8 family member Kif18A that localizes at the kinetochore-microtubule (K-MT) plus ends where it dampens MT dynamics stabilizing K-MT attachments. Kif18A action is directly antagonized by the master mitotic kinase cyclin B-dependent kinase 1 (Cdk1) and promoted by protein phosphatase 1 (PP1). Since chromosome alignment precedes Cdk1 inactivation by cyclin B proteolysis it is unclear how Kif18A evicts Cdk1 inhibition. We show here that chromosome alignment in human cells relies on a recently identified fraction of Cdk1 that is inhibited by phosphorylation in mitosis (i-Cdk1, for inhibited/inactive-Cdk1), localized at spindle structures and required for proper spindle assembly. Indeed, lowering i-Cdk1 induced several spindle defects including spindles with misaligned, bipolarly attached, chromosomes that showed poor Kif18A localization at K-MT plus ends. Both alignment defects and Kif18A localization were reversed by restoring i-Cdk1. In i-Cdk1-lowered cells, alignment defects were also significantly rescued by expressing a phosphorylation-resistant Kif18A version at Cdk1-dependent sites. Mechanistically, our evidence indicates that i-Cdk1 and active PP1 promoted spindle-localized Kif18A dephosphorylation. Given the relevance of Kif18A for survival of aneuploid cancer cells, these observations may also have relevance for cancer therapy.