Destruction of Claspin by SCFβTrCP Restrains Chk1 Activation and Facilitates Recovery from Genotoxic Stress

We show that Claspin, an adaptor protein required for Chk1 activation, becomes degraded at the onset of mitosis. Claspin degradation was triggered by its interaction with, and ubiquitylation by, the SCFβTrCP ubiquitin ligase. This interaction was phosphorylation dependent and required the activity of the Plk1 kinase and the integrity of a βTrCP recognition motif (phosphodegron) in the N terminus of Claspin. Uncoupling of Claspin from βTrCP by mutating the conserved serines in Claspin's phosphodegron or by knocking down βTrCP stabilized Claspin in mitosis, impaired Chk1 dephosphorylation, and delayed G2/M transition during recovery from cell cycle arrest imposed by DNA damage or replication stress. Moreover, the inability to degrade Claspin allowed partial reactivation of Chk1 in cells exposed to DNA damage after passing the G2/M transition. Our data suggest that degradation of Claspin facilitates timely reversal of the checkpoint response and delineates the period permissive for Chk1 activation during cell cycle progression.