STRUCTURE AND FUNCTION OF CELL CYCLE REGULATORY PROTEINS
"Cell division is studied both for its beauty and for the danger that it represents. When all goes well, new healthy cells are born" (Silke Hauf, Nature 2021). Aberrant cell division, however, causes the transformation of normal growing cells into cancer cells. To maintain genome stability during cell division, each emerging daughter cell needs to receive an identical set of sister chromatids. This requires precision during two key processes: DNA replication in S phase and segregation of sister chromatids during mitosis (M phase).
In early mitosis, the duplicated chromosomes are held together by the ring-shaped cohesin complex. Separation of chromosomes during anaphase is triggered by separase-a large cysteine endopeptidase that cleaves the cohesin subunit SCC1 (also known as RAD21). Separase is activated by degradation of its inhibitors, securin and cyclin B, but the molecular mechanisms of separase regulation are not clear. We show that both, securin and the Cdk1-cyclin B1-Cks1 complex, inhibit separase by pseudosubstrate motifs that block substrate binding at the catalytic site and at nearby docking sites. As in Caenorhabditis elegans (Boland et al., NSMB, 2017) and yeast, human securin contains its own pseudosubstrate motifs. By contrast, CDK1-cyclin B1 inhibits separase by deploying pseudosubstrate motifs from intrinsically disordered loops in separase itself. One autoinhibitory loop is oriented by CDK1-cyclin B1 to block the catalytic sites of both separase and CDK1. Another autoinhibitory loop blocks substrate docking in a cleft adjacent to the separase catalytic site. A third separase loop contains a phosphoserine that promotes complex assembly by binding to a conserved phosphate-binding pocket in cyclin B1. Our study reveals the diverse array of mechanisms by which securin and CDK1-cyclin B1 bind and inhibit separase, providing the molecular basis for the robust control of chromosome segregation.