Radiation-mediated proteolysis of CDT1 by CUL4–ROC1 and CSN complexes constitutes a new checkpoint

Genomic integrity is maintained by checkpoints that guard against undesired replication after DNA damage. Here, we show that CDT1, a licensing factor of the pre-replication complex (preRC)1,2,3, is rapidly proteolysed after UV- or γ-irradiation. The preRC assembles on replication origins at the end of mitosis and during G1 to license DNA for replication in S phase. Once the origin recognition complex (ORC) binds to origins, CDC6 and CDT1 associate with ORC and promote loading of the MCM2-7 proteins onto chromatin, generating the preRC. We show that radiation-mediated CDT1 proteolysis is independent of ATM and CHK2 and can occur in G1-phase cells. Loss of the COP9–signalosome (CSN) or CUL4–ROC1 complexes completely suppresses CDT1 proteolysis. CDT1 is specifically polyubiquitinated by CUL4 complexes and the interaction between CDT1 and CUL4 is regulated in part by γ-irradiation. Our study reveals an evolutionarily conserved and uncharacterized G1 checkpoint that induces CDT1 proteolysis by the CUL4–ROC1 ubiquitin E3 ligase and CSN complexes in response to DNA damage.