The stability of the Cdc6 protein is regulated by cyclin-dependent kinase/cyclin B complexes in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Cdc6 protein is necessary for the formation of prereplicative complexes that are a prerequisite for firing origins during DNA replication in the S phase. In budding yeast, the presence of Cdc6 protein is normally restricted to the G(1) phase of the cell cycle, at least partly because of its proteolytic degradation in the late G(1)/early S phase. Here we show that a Cdc28-dependent mechanism targets p57(CDC6) for degradation in mitotic arrested budding yeast cells. Consistent with this observation, Cdc6-7 and Cdc6-8 proteins, mutants lacking Cdc28 phosphorylation sites, are stabilized relative to wild-type Cdc6, Our data also suggest a correlation between the absence of Cdc28/Clb kinase activity and Cdc6 protein stabilization, because a drop in Cdc28/Clb-associated kinase activity allows mitotic-arrested cells to accumulate Cdc6 protein. Finally, we also show that cdc28 temperature-sensitive G(1) mutants accumulate Cdc6 protein because of a post-transcriptional mechanism. Our data suggest that budding yeast cells target Cdc6 for degradation through a Cdc28-dependent mechanism in each cell cycle.