Early Expressed Clb Proteins Allow Accumulation of Mitotic Cyclin by Inactivating Proteolytic Machinery during S Phase

Abstract

Periodic accumulation and destruction of mitotic cyclins are important for the initiation and termination of M phase. It is known that both APC^Cdc20 and APC^Hct1 collaborate to destroy mitotic cyclins during M phase. Here we show that this relationship between anaphase-promoting complex (APC) and Clb proteins is reversed in S phase such that the early Clb kinases (Clb3, Clb4, and Clb5 kinases) inactivate APC^Hct1 to allow Clb2 accumulation. This alternating antagonism between APC and Clb proteins during S and M phases constitutes an oscillatory system that generates undulations in the levels of mitotic cyclins.