CLOSING THE CELL-CYCLE CIRCLE IN YEAST - G2 CYCLIN PROTEOLYSIS INITIATED AT MITOSIS PERSISTS UNTIL THE ACTIVATION OF G1 CYCLINS IN THE NEXTCYCLE

It is thought that DNA replication and mitosis in yeast are triggered by oscillations in the level of G1-specific (CLN1 and CLN2) and G2-spe cific (CLB1-CLB4) cyclins, which determine the substrate specificity o f the CDC28 protein kinase. It is not understood how the time and orde r of appearance of different cyclin types are determined. We show here that CLB2 proteolysis, which is important for transition from mitosis to G1, is not confined to a narrow window at the end of mitosis as pr eviously thought but continues until reactivation of CDC28 by CLN cycl ins toward the end of the subsequent G1 period. Thus, cell cycle-regul ated proteolysis prevents accumulation of G2-specific CLB cyclins duri ng G1 and thereby ensures that the CLN-associated forms of the CDC28 k inase are activated without interference from CLB cylins. Accumulation of CLN cyclins leads to inactivation of CLB cyclin proteolysis, which is a precondition for subsequent activation of G2-specific B-type cyc lins.