The microtubule-destabilizing kinesin XKCM1 is required for chromosome positioning during spindle assembly.

Xenopus kinesin catastrophe modulator-1 (XKCM1) is a Kin I kinesin family member that uses the energy of ATP hydrolysis to depolymerize microtubules. We demonstrated previously that XKCM1 is essential for mitotic-spindle assembly in vitro and acts by regulating microtubule dynamics as a pure protein, in extracts and in cells. A portion of the XKCM1 pool is specifically localized to centromeres during mitosis and may be important in chromosome movement. To selectively analyze the function of centromere-bound XKCM1, we generated glutathione-S-transferase (GST) fusion proteins containing the N-terminal globular domain (GST-NT), the centrally located catalytic domain (GST-CD), and the C-terminal alpha-helical tail (GST-CT) of XKCM1. The GST-NT protein targeted to centromeres during spindle assembly, suggesting that the N-terminal domain of XKCM1 is sufficient for centromere localization. Addition of GST-NT prior to or after spindle assembly replaced endogenous XKCM1, indicating that centromere targeting is a dynamic process. Loss of endogenous XKCM1 from centromeres caused a misalignment of chromosomes on the metaphase plate without affecting global spindle structure. These results suggest that centromere bound XKCM1 has an important role in chromosome positioning on the spindle.