REGULATION OF B-TYPE CYCLIN PROTEOLYSIS BY CDC28-ASSOCIATED KINASES IN BUDDING YEAST

In budding yeast, stability of the mitotic B-type cyclin Clb2 is tight ly cell cycle-regulated. B-type cyclin proteolysis is initiated during anaphase and persists throughout the G(1) phase. Cln-Cdc28 kinase act ivity at START is required to repress B-type cyclin-specific proteolys is. Here, we show that Gib-dependent kinases, when expressed during G( 1), are also capable of repressing the B-type cyclin proteolysis machi nery. Furthermore, we find that inactivation of Cln- and Clb-Cdc28 kin ases is sufficient to trigger Clb2 proteolysis and sister-chromatid se paration in G(2)/M phase-arrested cells, where the B-type cyclin-speci fic proteolysis machinery is normally inactive. Our results suggest th at Cln- and Gib-dependent kinases are both capable of repressing B-typ e cyclin-specific proteolysis and that they are required to maintain t he proteolysis machinery in an inactive state in S and G(2)/M phase-ar rested cells, We propose that in yeast, as cells pass through START, C ln-Cdc28-dependent kinases inactivate B-type cyclin proteolysis. As Cl n-Cdc28-dependent kinases decline during G(2), Clb-Cdc28-dependent kin ases take over this role, ensuring that B-type cyclin proteolysis is n ot activated during S phase and early mitosis.