Differential roles for the spindle assembly checkpoint in error surveillance and mitotic timing.

Spindle assembly checkpoint (SAC) components have been proposed to act as both a surveillance mechanism monitoring mitotic fidelity and as a timer that ensures sufficient time for chromosome alignment to occur. To evaluate these roles, we utilized an SAC-deficient cell line generated by eliminating the full-length Cdc20 translational isoform (Cdc20 ∆FL), and defined genetic interactions using CRISPR/Cas9-based screening. Cdc20 ∆FL cells display synthetic-lethal relationships with genes required for chromosome segregation, highlighting the role of the SAC in error surveillance. Interestingly, we found that the SAC component Mad2, which has been proposed to act as an essential "timer" to control anaphase onset by preventing premature APC/C activation, becomes dispensable for viability in Cdc20 ∆FL cells. Our targeted analysis indicates that regulation of mitotic timing involves a combination of Mad2 inhibition of APC/C-Cdc20, Cdk1-mediated Cdc20 phosphorylation preventing premature APC/C-Cdc20 activation, and total Cdc20 protein levels. Simultaneously perturbing these pathways results in premature mitotic exit and catastrophic chromosome mis-segregation.