Abstract
Mitotic centromere associated kinesin (MCAK) is a kinesin related protein with the ability to stimulate microtubule depolymerization. It is found at spindle poles, where it may be involved in poleward microtubule flux, and at kinetochores and centromeres where it plays a role in correcting chromosome alignment errors. Its microtubule depolymerase activity and recruitment to centromeres is regulated by phosphorylation, but little is known about how MCAK is maintained at appropriate levels. We previously reported that MCAK accumulates during the cell cycle and is then degraded during mitosis. Using proteomic analysis, we have now identified a new phosphorylation site on MCAK that is responsible for its degradation. Mutation of the site to prevent phosphorylation prolonged the stability of the protein beyond the metaphase to anaphase transition and into the subsequent cell cycle whereas a phosphomimetic mutation accelerated degradation. Unexpectedly, the mutation that prevented phosphorylation also inhibited the removal of MCAK from centromeres causing it to remain attached throughout the cell cycle. Even low expression of phosphorylation-resistant MCAK delayed mitosis and interfered with cell division. Mitotic defects were also observed by overexpressing a green fluorescent protein-tagged version of wild-type MCAK that similarly escaped degradation and accumulated to toxic levels, but did not remain associated with kinetochores during interphase. The results demonstrate that degradation is an important mechanism for controlling the activity of MCAK.