An Arabidopsis Kinesin-14D motor is associated with midzone microtubules for spindle morphogenesis.

The acentrosomal spindle apparatus has kinetochore fibers organized and converged toward opposite poles; however, mechanisms underlying the organization of these microtubule fibers into an orchestrated bipolar array were largely unknown. Kinesin-14D is one of the four classes of Kinesin-14 motors that are conserved from green algae to flowering plants. In Arabidopsis thaliana, three Kinesin-14D members displayed distinct cell cycle-dependent localization patterns on spindle microtubules in mitosis. Notably, Kinesin-14D1 was enriched on the midzone microtubules of prophase and mitotic spindles and later persisted in the spindle and phragmoplast midzones. The kinesin-14d1 mutant had kinetochore fibers disengaged from each other during mitosis and exhibited hypersensitivity to the microtubule-depolymerizing herbicide oryzalin. Oryzalin-treated kinesin-14d1 mutant cells had kinetochore fibers tangled together in collapsed spindle microtubule arrays. Kinesin-14D1, unlike other Kinesin-14 motors, showed slow microtubule plus end-directed motility, and its localization and function were dependent on its motor activity and the novel malectin-like domain. Our findings revealed a Kinesin-14D1-dependent mechanism that employs interpolar microtubules to regulate the organization of kinetochore fibers for acentrosomal spindle morphogenesis.