The microtubule-binding and coiled-coil domains of Kid are required to turn off the polar ejection force at anaphase.

Mitotic chromosomes move dynamically along the spindle microtubules using the forces generated by motor proteins such as chromokinesin Kid (also known as KIF22). Kid generates a polar ejection force and contributes to alignment of the chromosome arms during prometaphase and metaphase, whereas during anaphase, Kid contributes to chromosome compaction. How Kid is regulated and how this regulation is important for chromosome dynamics remains unclear. Here, we address these questions by expressing mutant forms of Kid in Kid-deficient cells. We demonstrate that Cdk1-mediated phosphorylation of Thr463 is required to generate the polar ejection force on Kid-binding chromosomes, whereas dephosphorylation of Thr463 prevents generation of the ejection force on such chromosomes. In addition to activation of the second microtubule-binding domain through dephosphorylation of Thr463, the coiled-coil domain is essential in suspending generation of the polar ejection force, preventing separated chromosomes from becoming recongressed during anaphase. We propose that phosphorylation of Thr463 switches the mitotic chromosome movement from an anti-poleward direction to a poleward direction by converting the Kid functional mode from polar-ejection-force-ON to -OFF during the metaphase-anaphase transition, and that both the second microtubule-binding domain and the coiled-coil domain are involved in this switching process.