Securin and B-cyclin/CDK are the only essential targets of the APC

The anaphase-promoting complex/cyclosome (APC) is a highly conserved ubiquitin ligase that controls passage through the cell cycle by targeting many proteins for proteolysis. The complex is composed of at least thirteen core subunits, eight of which are essential, and two activating subunits, Cdc20 (essential) and Cdh1/Hct1 (non-essential). Previously, it was not known which APC targets are sufficient to explain the essential nature of the complex. Here, we show that each of the eight normally essential APC subunits is rendered non-essential ('bypass-suppressed') by the simultaneous removal/inhibition of the APC substrates securin (Pds1) and B-type cyclin/CDK (Clb/CDK). In strains lacking the APC, levels of Clb2 and Clb3 remain constant, but Clb/CDK activity oscillates as cells cycle. This suggests that in the absence of B-type cyclin destruction, oscillation of the Clb/CDK-inhibitor Sic1 is sufficient to trigger the feedback loops necessary for the bi-stable nature of Clb/CDK activity. These results strongly suggest that securin and B-type cyclin/CDK activity are the only obligatory targets of the APC in Saccharomyces cerevisiae.