Role for casein kinase 1 in the phosphorylation of Claspin on critical residues necessary for the activation of Chk1

The mediator protein Claspin is critical for the activation of the checkpoint kinase Chk1 during checkpoint responses to stalled replication forks. This function involves the Chk1-activating domain (CKAD) of Claspin, which undergoes phosphorylation on multiple conserved sites. These phosphorylations promote binding of Chk1 to Claspin and ensuing activation of Chk1 by ATR. However, despite the importance of this regulatory process, the kinase responsible for these phosphorylations has remained unknown. By using a multifaceted approach, we have found that casein kinase 1 gamma 1 (CK1 gamma 1) carries out this function. CK1 gamma 1 phosphorylates the CKAD of Claspin efficiently in vitro, and depletion of CK1 gamma 1 from human cells by small interfering RNA (siRNA) results in dramatically diminished phosphorylation of Claspin. Consequently, the siRNA-treated cells display impaired activation of Chk1 and resultant checkpoint defects. These results indicate that CK1 gamma 1 is a novel component of checkpoint responses that controls the interaction of a key checkpoint effector kinase with its cognate mediator protein.