A mitotic kinase TOPK enhances Cdk1/cyclin B1-dependent phosphorylation of PRC1 and promotes cytokinesis

A MAPKK-like mitotic kinase, TOPK, implies the formation of mitotic spindles and spindle midzone and accomplishing cytokinesis, however, its underlying mechanism remains unclear. A microtubule bundling protein, PRC1, plays a pivotal role in the formation of mitotic spindles and spindle midzone. Because of their functional resemblance, we attempted to clarify the links between these two molecules. TOPK supported mitotic advance via the cdk1/cyclin B1-dependent phosphorylation of PRC1. TOPK induced the phosphorylation of PRC1 at T481 in vivo, however, TOPK did not phosphorylate PRC1 in vitro. TOPK induced the phosphorylation of PRC1 at T481 only when the cdk1/cyclin B1 existed simultaneously in vitro. Both the enzymatic activity of TOPK and association competence of TOPK with PRC1 were mandatory for this phosphorylation. TOPK binds to cdk1/cyclin B1, microtubules and PRC1 via its unique region near the C terminus. TOPK co-localized closely with cdk1 throughout the cell cycle in vivo. Collectively, these data indicate that TOPK, which makes a kinase-substrate complex with cdk1/cyclin B1 and PRC1 on microtubules during mitosis, enhances the cdk1/cyclin B1-dependent phosphorylation of PRC1 and thereby strongly promotes cytokinesis.