CRL4CDT2 Targets CHK1 for PCNA-Independent Destruction

CDT2 targets proteins involved in replication licensing (CDT1), cell cycle control (p21), and chromatin modification (SET8) for destruction by the CUL4-based E3 ligase (CRL4). CRL4^CDT2 recruits these substrates through interactions with chromatin-bound PCNA and ubiquitinates them exclusively on chromatin. Rereplication and G₂ cell cycle arrest are observed in CDT2-depleted cells. The rereplication phenotype has been attributed to an inability to destroy CDT1, but the molecular target important for G₂ cell cycle arrest in CDT2-depleted cells has not been identified. Here we identify CHK1 as a novel CRL4^CDT2 substrate and demonstrate that CHK1 activity is required for maintaining G₂ arrest in CDT2-depleted cells. We demonstrate that CRL4^CDT2 targets the activated form of CHK1 for destruction in the nucleoplasm rather than on chromatin and that this occurs in a PCNA-independent manner. Although both CRL1 and CRL4 ubiquitinate CHK1, we report that they bind CHK1 in distinct cellular compartments. Our study provides insight into how elevated CDT2 expression levels may provide tumors with a proliferative advantage.