Regulation of Kif15 localization and motility by the C-terminus of TPX2 and microtubule dynamics.

Mitotic motor proteins generate force to establish and maintain spindle bipolarity, but how they are temporally and spatially regulated in vivo is unclear. Prior work demonstrated that a microtubule-associated protein, TPX2, targets kinesin-5 and kinesin-12 motors to spindle microtubules. The C-terminal domain of TPX2 contributes to the localization and motility of the kinesin-5, Eg5, but it is not known whether this domain regulates kinesin-12, Kif15. We found that the C-terminal domain of TPX2 contributes to the localization of Kif15 to spindle microtubules in cells and suppresses motor walking in vitro. Kif15 and Eg5 are partially redundant motors, and overexpressed Kif15 can drive spindle formation in the absence of Eg5 activity. Kif15-dependent bipolar spindle formation in vivo requires the C-terminal domain of TPX2. In the spindle, fluorescent puncta of GFP-Kif15 move toward the equatorial region at a rate equivalent to microtubule growth. Reduction of microtubule growth with paclitaxel suppresses GFP-Kif15 motility, demonstrating that dynamic microtubules contribute to Kif15 behavior. Our results show that the C-terminal region of TPX2 regulates Kif15 in vitro, contributes to motor localization in cells, and is required for Kif15 force generation in vivo and further reveal that dynamic microtubules contribute to Kif15 behavior in vivo.