Chk1-mediated phosphorylation of Cdh1 promotes the SCF(betaTRCP)-dependent degradation of Cdh1 during S-phase and efficient cell-cycle progression.

APC/C(Cdh1) is a ubiquitin ligase with roles in numerous diverse processes, including control of cellular proliferation and multiple aspects of the DNA damage response. Precise regulation of APC/C(Cdh1) activity is central to efficient cell-cycle progression and cellular homeostasis. Here, we have identified Cdh1 as a direct substrate of the replication stress checkpoint effector kinase Chk1 and demonstrate that Chk1-mediated phosphorylation of Cdh1 contributes to its recognition by the SCF(betaTRCP) ubiquitin ligase, promotes efficient S-phase entry, and is important for cellular proliferation during otherwise unperturbed cell cycles. We also find that prolonged Chk1 activity in late S/G2 inhibits Cdh1 accumulation. In addition to promoting control of APC/C(Cdh1) activity by facilitating Cdh1 destruction, we find that Chk1 also antagonizes activity of the ligase by perturbing the interaction between Cdh1 and the APC/C. Overall, these data suggest that the rise and fall of Chk1 activity contributes to the regulation of APC/C(Cdh1) activity that enhances the replication process.