Cell cycle-dependent translocation of PRC1 on the spindle by Kif4 is essential for midzone formation and cytokinesis.

The spindle midzone, a conspicuous network of antiparallel interdigitating nonkinetochore microtubules between separating chromosomes, plays a crucial role in regulating the initiation and completion of cytokinesis. In this study, we report the use of time-lapse microscopy and a human kinesin endoribonucleases RNase III-prepared short interfering RNA (esiRNA) library to identify Kif4 as a motor protein that translocates PRC1, a spindle midzone-associated cyclin-dependent kinase substrate protein, to the plus ends of interdigitating spindle microtubules during the metaphase-to-anaphase transition. We show that Kif4 binds to PRC1 through its "stalk plus tail" domains and Kif4 and PRC1 colocalize on the spindle midzone/midbody during anaphase and cytokinesis. Suppression of Kif4 expression by Kif4 esiRNA results in the inhibition of PRC1 translocation, a block of the midzone formation, and a failure of cytokinesis. PRC1 translocation and midzone formation can be restored, and the cytokinetic defects can be rescued in Kif4 esiRNA-treated cells by coexpression of Kif4 but not its motor dead mutant Kif4md. Furthermore, we show that cyclin-dependent kinase phosphorylation of PRC1 controls the timing of PRC1 translocation by Kif4. These results, in light of the crucial role of PRC1 in midzone formation, indicate that cell cycle-dependent translocation of PRC1 by Kif4 is essential for midzone formation and cytokinesis.