Aurora B and 14-3-3 Coordinately Regulate Clustering of Centralspindlin during Cytokinesis

Centralspindlin is essential for the formation of microtubule bundle structures and the equatorial recruitment of factors critical for cytokinesis [1, 2]. Stable accumulation of centralspindlin at the spindle midzone requires its multimerization into clusters [3] and Aurora B kinase activity [4-10], which peaks at the central spindle during anaphase [11, 12]. Although Aurora B phosphorylates centralspindlin directly [13-17], how this regulates centralspindlin localization is unknown. Here we identify a novel regulatory mechanism by which Aurora B enables centralspindlin to accumulate stably at the spindle midzone. We show that 14-3-3 protein binds centralspindlin when the kinesin-6 component MKLP1 is phosphorylated at S710. 14-3-3 prevents centralspindlin from clustering in vitro, and an MKLP1 mutant that is unable to bind 14-3-3 forms aberrant clusters in vivo. Interestingly, 14-3-3 binding is inhibited by phosphorylation of S708, a known Aurora B target site that lies within the motif bound by 14-3-3. S708 phosphorylation is required for MKLP1 to stably localize to the central spindle, but it is dispensable in an MKLP1 mutant that does not bind 14-3-3. We propose that 14-3-3 serves as a global inhibitor of centralspindlin that allows Aurora B to locally activate clustering and the stable accumulation of centralspindlin between segregating chromosomes.