Phosphorylation of a Borealin Dimerization Domain Is Required for Proper Chromosome Segregation

The chromosomal passenger complex (CPC) has been identified is a master regulator of mitosis. In particular, proper chromosome segregation and cytokinesis depend oil the correct localization and function of the CPC. Within the complex, the kinase Aurora B associates with Incenp, Survivin, and Borealin. The stoichiometry or the complex as well as a complete understanding of how these four components interact with each other remains to be elucidated. Here, we identify a new domain of Borealin. We determined its structure using NMR spectroscopy and discovered a novel dimerization motif. Interestingly, we found that substitutions at Borealin T230, recently identified as an Mpsl phosphorylation site, call modulate the dimerization state of Borealin. Mutation of this single residue to alanine or valine impairs Aurora B activity during mitosis and causes chromosome segregation defects. This study reveals that Mpsl regulates the CPC through a novel Borealin domain.