FUNCTION AND REGULATION OF P31COMET IN THE SPINDLE ASSEMBLY CHECKPOINT

The correct partitioning of the genomic content during cell division in eukaryotes is required for proper cell physiology. The presence of an abnormal number of chromosomes, a condition known as aneuploidy, impinges on cell fitness and eventually leads to tumorigenesis. To avoid aneuploidy, eukaryotic cells have developed a molecular safety device known as the Spindle Assembly Checkpoint (SAC). By delaying anaphase onset until all chromosomes are bioriented, the SAC ensures the fidelity of chromosome segregation. To do this, the SAC inhibits the Anaphase Promoting Complex/Cyclosome (APC/C), a protein complex responsible for mitotic exit. The Mad2 protein is a crucial component of the SAC. Mad2 exists as two different and interacting conformers (open, O, and closed, C). Once biorientation is achieved, the SAC is promptly switched off and anaphase occurs. The timely silencing of the SAC requires the protein p31comet. p31comet selectively binds to C-Mad2 and prevents the O-C dimerization. Moreover, p31comet has been shown to activate the APC/C in vitro. How p31comet is regulated in mitosis and how it ensures SAC silencing are poorly investigated issues. Here, we report a characterization of p31comet in cells. We show that, upon SAC activation, p31comet is phosphorylated by the mitotic kinases Mps1 and Aurora B, and in a Mad2-dependent manner. The function of this phosphorylation still remains elusive. We also report new p31comet interactors, and demonstrate that p31comet physiologically regulates APC/C activity, by regulating its association to the inhibitory Mitotic Checkpoint Complex. We also find that p31comet regulates the checkpoint slippage and the mitotic turnover of the protein Cdc20. Finally, we prove a previously unreported role for protein synthesis in the maintenance of the SAC.