SCFβTrCP-Mediated Degradation of Claspin Regulates Recovery from the DNA Replication Checkpoint Response

During replicative stress, Claspin mediates the phos- phorylation and consequent activation of Chk1 by ATR. We found that during recovery from the DNA rep- lication checkpoint response, Claspin is degraded in a bTrCP-dependent manner. In vivo, Claspin is phos- phorylated in a canonical DSGxxS degron sequence, which is typical of bTrCP substrates. Phosphorylation of Claspin is mediated by Plk1 and is essential for binding to bTrCP. In vitro ubiquitylation of Claspin requires bTrCP, Plk1, and an intact DSGxxS degron. Significantly, expression of a stable Claspin mutant unable to bind bTrCP prolongs the activation of Chk1, thereby attenuating the recovery from the DNA replication stress response and significantly delaying entry into mitosis. Thus, the SCFbTrCP-dependent deg- radation of Claspin is necessary for the efficient and timely termination of the DNA replication checkpoint. Importantly, in response to DNA damage in G2, Clas- pin proteolysis is inhibited to allow the prompt rees- tablishment of the checkpoint.