CDT1 acts with the replisome to remodel replication forks.

Replication forks encounter problems during genome duplication that trigger replication stress. Fork reversal is a key stress tolerance pathway that helps mitigate replication challenges to facilitate DNA synthesis. Here, we report the function of origin licensing factor CDT1 in fork remodeling by replication fork reversal. Characterization of replication dynamics during early stages of origin reactivation revealed that CDT1 blocks fork progression without inducing DNA breaks. Notably, CDT1 mediates replication fork reversal under conditions of re-replication and genotoxic stress exposure, and this function is dependent on the interaction with the CMG helicase. Although proliferating cell nuclear antigen (PCNA) sequesters CDT1 for proteolytic degradation under unperturbed conditions, ATAD5-mediated PCNA unloading at stressed forks releases CDT1 to interact with the CMG helicase and promote fork remodeling. Thus, contrary to the notion that CDT1 function must be inactivated in the S phase, our findings uncover a regulatory mechanism that facilitates fork remodeling function of CDT1 in response to replication stress.