Kif2a depletion generates chromosome segregation and pole coalescence defects in animal caps and inhibits gastrulation of the Xenopus embryo.

Kif2a is a member of the kinesin-13 microtubule depolymerases, which tightly regulate microtubule dynamics for many cellular processes. We characterized Kif2a depletion in Xenopus animal caps and embryos. Kif2a depletion generates defects in blastopore closure. These defects are rescued by removing the animal cap, suggesting that Kif2a-depleted animal caps are not compliant enough to allow gastrulation movements. Gastrulation defects are not rescued by a Kif2a mutated in an Aurora kinase phosphorylation site, suggesting that the phenotypes are caused by problems in mitosis. During animal cap mitoses, Kif2a localizes to the spindle poles and centromeres. Depletion of Kif2a generated multipolar spindles in stage 12 embryos. Kif2a-depleted animal caps have anaphase lagging chromosomes in stage 9 and 10 embryos and subsequent cytokinesis failure. Later divisions have greater than two centrosomes, generating extra spindle poles. Kif2a-depleted embryos are also defective at coalescing extra spindle poles into a bipolar spindle. The gastrulation and mitotic phenotypes can be rescued by either human Kif2a or Kif2b, which suggests that the two homologues redundantly regulate mitosis in mammals. These studies demonstrate that defects in mitosis can inhibit large-scale developmental movements in vertebrate tissues.

ICIS and Aurora B coregulate the microtubule depolymerase Kif2a.

Kinesins in the mitotic spindle play major roles in determining spindle shape, size, and bipolarity, although specific regulation of these kinesins at distinct locations on the spindle is poorly understood. So that the forces that are required for spindle bipolarity are balanced, microtubule-depolymerizing kinesins are tightly regulated. Aurora B kinase phosphorylates the neck regions of the kinesin-13 family microtubule depolymerases Kif2a and mitotic centromere-associated kinesin (MCAK) and inhibits their depolymerase activities. How they are reactivated and how this is controlled independently on different kinetochore fibers is unknown. We show that inner centromere Kin-I stimulator (ICIS), which stimulates the related depolymerase MCAK, can reactivate Kif2a after Aurora B inhibition. When antibodies that block the ability of ICIS to activate Kif2a are injected into cells, monopolar spindles are generated. This phenotype is rescued by coinjection of anti-Nuf2 antibodies. We have performed a structure-function analysis of the ICIS protein and find that the N terminus of ICIS binds Aurora B and its regulators INCENP and TD60, whereas a central region binds MCAK, Kif2a, and microtubules, suggesting a scaffold function for ICIS. These data argue that ICIS and the chromosomal passenger complex (CPC) regulate Kif2a depolymerase activity.