Two different modes of cyclin Clb2 proteolysis during mitosis in Saccharomyces cerevisiae

Sister chromatid separation and mitotic exit are triggered by the anaphase-promoting complex (APC/C) which is a multi-subunit ubiquitin ligase required for proteolytic degradation of various target proteins. Cdc20 and Cdh1 are substrate specific activators of the APC/C. It was previously proposed that Cdh1 is essential for proteolysis of the yeast mitotic cyclin Clb2. We show that Clb2 proteolysis is triggered by two different modes during mitosis. A fraction of Clb2 is degraded during anaphase in the absence of Cdh1. However a second fraction of Clb2 remains stable during anaphase and is degraded in a Cdh1-dependent manner as cells exit from mitosis, Most of cyclin Clb3 is degraded independently of Cdh1. Our data imply that degradation of mitotic cyclins is initiated by a Cdh1-independent mechanism. (C) 2000 Federation of European Biochemical Societies.