USP37 protects mammalian cells during DNA replication stress by counteracting CUL2^LRR1 and TRAIP

The USP37 deubiquitylase is essential for mammalian cells to survive DNA replication stress but the underlying mechanisms are unknown. Here we demonstrate that USP37 binds the CDC45-MCM-GINS (CMG) helicase, which forms the stable core of the replisome until DNA replication termination when CMG is ubiquitylated and disassembled. USP37 contacts CDC45, and structure-guided mutations that displace USP37 from CMG cause sensitivity to DNA synthesis defects or topological stress. Binding to CDC45 at replication forks enables USP37 to counteract CMG ubiquitylation by the CUL2LRR1 ligase, which subsequently induces replisome disassembly during termination. Correspondingly, depletion of CUL2LRR1 suppresses the sensitivity of Usp37 mutants to DNA synthesis defects and to ATR checkpoint kinase inhibitors. In contrast, mutation of the TRAIP ubiquitin ligase specifically suppresses the sensitivity of Usp37 mutants to topological stress. We propose that USP37 protects mammalian cells from replication stress by reversing the untimely action of the CUL2LRR1 and TRAIP ubiquitin ligases.