Klp2-mediated Rsp1-Mto1 colocalization inhibits microtubule-dependent microtubule assembly in fission yeast.

Microtubule assembly takes place at the centrosome and noncentrosomal microtubule-organizing centers (MTOCs). However, the mechanisms controlling the activity of noncentrosomal MTOCs are poorly understood. Here, using the fission yeast as a model organism, we demonstrate that the kinesin-14 motor Klp2 interacts with the J-domain Hsp70/Ssa1 cochaperone Rsp1, an inhibitory factor of microtubule assembly, and that Klp2 is required for the proper localization of Rsp1 to microtubules. In addition, we demonstrate that Klp2 is not required for the localization of Mto1, a factor promoting microtubule assembly, to microtubules. Moreover, Rsp1-Ssa1 inhibits the interaction of Mto1-Mto2 with the gamma-tubulin small complex. The absence of Klp2 reduces the colocalization of Rsp1 and Mto1 foci on preexisting microtubules, resulting in an increased microtubule-dependent microtubule assembly. Our results suggest that Klp2 regulates the activity of noncentrosomal MTOCs by targeting Rsp1 to the sites of Mto1 activity and reveal a mechanism for the inhibition of noncentrosomal microtubule assembly by a kinesin-14 motor.