Regulation of B-type cyclin proteolysis by Cdc28-associated kinases in budding yeast

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Abstract
In budding yeast, stability of the mitotic B‐type cyclin Clb2 is tightly cell cycle‐regulated. B‐type cyclin proteolysis is initiated during anaphase and persists throughout the G1 phase. Cln‐Cdc28 kinase activity at START is required to repress B‐type cyclin‐specific proteolysis. Here, we show that Clb‐dependent kinases, when expressed during G1, are also capable of repressing the B‐type cyclin proteolysis machinery. Furthermore, we find that inactivation of Cln‐ and Clb‐Cdc28 kinases is sufficient to trigger Clb2 proteolysis and sister‐chromatid separation in G2/M phase‐arrested cells, where the B‐type cyclin‐specific proteolysis machinery is normally inactive. Our results suggest that Cln‐ and Clb‐dependent kinases are both capable of repressing B‐type cyclin‐specific proteolysis and that they are required to maintain the proteolysis machinery in an inactive state in S and G2/M phase‐arrested cells. We propose that in yeast, as cells pass through START, Cln‐Cdc28‐dependent kinases inactivate B‐type cyclin proteolysis. As Cln‐Cdc28‐dependent kinases decline during G2, Clb‐Cdc28‐dependent kinases take over this role, ensuring that B‐type cyclin proteolysis is not activated during S phase and early mitosis.