Dual recognition of chromatin and microtubules by INCENP is important for mitotic progression.

The chromosomal passenger complex (CPC), composed of inner centromere protein (INCENP), Survivin, Borealin, and the kinase Aurora B, contributes to the activation of the mitotic checkpoint. The regulation of CPC function remains unclear. Here, we reveal that in addition to Survivin and Borealin, the single α-helix (SAH) domain of INCENP supports CPC localization to chromatin and the mitotic checkpoint. The INCENP SAH domain also mediates INCENP's microtubule binding, which is negatively regulated by Cyclin-dependent kinase-mediated phosphorylation of segments flanking the SAH domain. The microtubule-binding capacity of the SAH domain is important for mitotic arrest in conditions of suppressed microtubule dynamics, and the duration of mitotic arrest dictates the probability, but not the timing, of cell death. Although independent targeting of INCENP to microtubules or the kinetochore/centromere promotes the mitotic checkpoint, it is insufficient for a robust mitotic arrest. Altogether, our results demonstrate that dual recognition of chromatin and microtubules by CPC is important for checkpoint maintenance and determination of cell fate in mitosis.

Autoinhibition and Polo-dependent multisite phosphorylation restrict activity of the histone H3 kinase Haspin to mitosis.

The mitosis-specific phosphorylation of histone H3 at Thr3 (H3T3ph) plays an important role in chromosome segregation by recruiting Aurora B. H3T3 phosphorylation is catalyzed by Haspin, an atypical protein kinase whose kinase domain is intrinsically active without phosphorylation at the activation loop. Here, we report the molecular basis for Haspin inhibition during interphase and its reactivation in M phase. We identify a conserved basic segment that autoinhibits Haspin during interphase. This autoinhibition is neutralized when Cdk1 phosphorylates the N terminus of Haspin in order to recruit Polo-like kinase (Plk1/Plx1), which, in turn, further phosphorylates multiple sites at the Haspin N terminus. Although Plx1, and not Aurora B, is critical for H3T3 phosphorylation in Xenopus egg extracts, Plk1 and Aurora B both promote this modification in human cells. Thus, M phase-specific H3T3 phosphorylation is governed by the combinatorial action of mitotic kinases that neutralizes Haspin autoinhibition through a mechanism dependent on multisite phosphorylation.