Villa, F.; Ainsworth, J.; Labib, K.

The USP37 deubiquitylase is important 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 and is regulated by ubiquitylation. The Pleckstrin-Homology Domain of USP37 binds CDC45 and structure-guided mutations that displace USP37 from CMG are sufficient to phenocopy loss of USP37 catalytic activity. Importantly, USP37 counteracts CMG helicase ubiquitylation by the CUL2-LRR1 ligase, which induces helicase disassembly during termination. We show that depletion of CUL2-LRR1 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 during chromosome replication. We propose that USP37 evolved to reverse the untimely action of the two ubiquitin ligases that regulate the CMG helicase during chromosome replication in metazoa.