Synchronous chromosome segregation in mouse oocytes is ensured by biphasic securin destruction and cyclin B1-Cdk1

Successful cell division relies on the faithful segregation of chromosomes. If chromosomes segregate prematurely the cell is at risk of aneuploidy. Alternatively, if cell division is attempted in the absence of complete chromosome segregation, non-segregated chromosomes can become trapped within the cleavage furrow and the cell can lose viability. Securin plays a key role in this process, acting as a pseudosubstrate to inhibit the protease separase that functions to cleave the cohesin rings that hold chromosomes together. Consequently, securin must be depleted ahead of anaphase, ensuring chromosome segregation occurs in time with the anaphase trigger. Here we find that MI mouse oocytes contain a large excess of securin over separase and reveal the existence of a novel mechanism that functions to promote the destruction of excess securin in prometaphase. Critically, this mechanism relies on key residues that are only exposed when securin is not bound to separase. We suggest that the majority of non-separase bound securin is removed by this mechanism, allowing for separase activity to be protected until just before anaphase. In addition, we further demonstrate the importance of complementary mechanisms of separase inhibition by directly measuring cleavage activity in live oocytes, confirming that both securin and inhibition by cyclin B1-Cdk1 are independently sufficient to prevent premature separase activation.