Activity of the ApC(Cdh1) form of the anaphase-promoting complex persists until S phase and prevents the premature expression of Cdc20p

Cell cycle progression is driven by waves of cyclin expression coupled with regulated protein degradation. An essential step for initiating mitosis is the inactivation of proteolysis mediated by the anaphase-promoting complex /cyclosome (APC/C) bound to its regulator Cdh1p/Hct1p. Yeast APC(Cdh1) was Proposed previously to be inactivated at Start by G1 cyclin/cyclin-dependen t kinase (CDK). Here, we demonstrate that in a normal cell cycle APC(Cdh1) is inactivated in a graded manner and is not extinguished until S phase. Co mplete inactivation of APC(Cdh1) requires S phase cyclins. Further, persist ent APC(Cdh1) activity throughout G1 helps to ensure the proper timing of C dc20p expression. This suggests that S phase cyclins have an important role in allowing the accumulation of mitotic cyclins and further suggests a reg ulatory loop among S phase cyclins, APC(Cdh1), and APC(Cdh1).