Phosphorylation Regulates the p31Comet-Mitotic Arrest-deficient 2 (Mad2) Interaction to Promote Spindle Assembly Checkpoint (SAC) Activity

The spindle assembly checkpoint (SAC) ensures the faithful segregation of the genome during mitosis by ensuring that sister chromosomes form bipolar attachments with microtubules of the mitotic spindle. p31^Comet is an antagonist of the SAC effector Mad2 and promotes silencing of the SAC and mitotic progression. However, p31^Comet interacts with Mad2 throughout the cell cycle. We show that p31^Comet binds Mad2 solely in an inhibitory manner. We demonstrate that attenuating the affinity of p31^Comet for Mad2 by phosphorylation promotes SAC activity in mitosis. Specifically, phosphorylation of Ser-102 weakens p31^Comet-Mad2 binding and enhances p31^Comet-mediated bypass of the SAC. Our results provide the first evidence for regulation of p31^Comet and demonstrate a previously unknown event controlling SAC activity.