The Kinesin-14 Tail: Dual microtubule binding domains drive spindle morphogenesis through tight microtubule cross-linking and robust sliding.

Proper spindle assembly requires the Kinesin-14 family of motors to organize microtubules (MTs) into the bipolar spindle by cross-linking and sliding anti-parallel and parallel MTs through their motor and tail domains. How they mediate these different activities is unclear. We identified two MT binding domains (MBD1 and MBD2) within the Xenopus Kinesin-14 XCTK2 tail and found that MBD1 MT affinity was weaker than MBD2. Comparable to full-length GFP-XCTK2 wild-type protein (GX-WT), GFP-XCTK2 containing the MBD1 mutations (GX-MBD1mut) stimulated spindle assembly, localized moderately on the spindle, and formed narrow spindles. In contrast, GX-MBD2mut only partially stimulated spindle assembly, localized weakly on the spindle, and formed shorter spindles. Biochemical reconstitution of MT cross-linking and sliding demonstrated that GX-MBD2mut slid anti-parallel MTs faster than GX-WT and GX-MBD1mut. However, GX-WT and GX-MBD1mut statically cross-linked the majority of parallel MTs, whereas GX-MBD2mut equally slid and statically cross-linked parallel MTs without affecting their sliding velocity. These results provide a mechanism by which the two different MT binding domains in the Kinesin-14 tail balance anti-parallel MT sliding velocity (MBD1) and tight parallel MT cross-linking (MBD2), which are important for spindle assembly and localization, and provide a basis for characterizing how molecular motors organize MTs within the spindle.