MutSβ facilitates DSB repair by binding RPA–ssDNA hairpin loops and activating ATR signaling.
Major finding: MutSβ facilitates DSB repair by binding RPA–ssDNA hairpin loops and activating ATR signaling.
Mechanism: MutSβ interacts with ATR and ATRIP and promotes the recruitment of ATRIP to RPA-coated ssDNA.
Impact: MutSβ-mediated activation of ATR during DSB repair may underlie its tumor-suppressive function.
Activation of the ataxia telangiectasia-mutated and RAD3-related (ATR) DNA damage response (DDR) kinase is mediated by RPA-coated single-stranded DNA (ssDNA), which forms at stalled replication forks and DNA double-strand breaks (DSB). Within the DDR, the heterodimeric complexes MutS homolog 2 (MSH2)–MSH6 (MutSα) and MSH2–MSH3 (MutSβ) function to correct DNA mismatches generated during DNA replication. However, recent work suggests that MutSβ is also recruited to damaged DNA and is required for proper DSB repair, prompting Burdova, Mihaljevic, and colleagues to examine the role of MutSβ in the DDR more closely. Depletion of MSH2 or MSH3, but not MSH6, inhibited homologous recombination (HR)–mediated DNA repair and augmented genotoxic stress–induced DNA breaks. In line with this finding, DNA damage–induced recruitment of MSH2 to chromatin was dependent on the DSB end–processing enzymes CtBP-interacting protein (CtIP) and meiotic recombination 11 (MRE11), further supporting a role for MutSβ in the early steps of HR. Mechanistically, MSH2 and MSH3 activated ATR-mediated phosphorylation of RPA and CHK1 by binding to the ATR–ATRIP complex and promoting its recruitment to DSB sites. MutSβ-mediated recruitment of ATR–ATRIP to chromatin required binding of MutSβ to persistent hairpin loop structures in RPA-coated ssDNA. Of note, binding of ATP by MutSβ or mutation of the MSH3 subunit at residues Y245 and K246, which mediate interactions of MutSβ with DNA hairpin loops, inhibited the ability of MutSβ to bind RPA-coated ssDNA and activate ATR following genotoxic stress. Together, these results suggest that ssDNA hairpin loop formation may serve as a molecular switch that promotes activation of ATR signaling in response to DNA damage and suggest that this role of MutSβ may contribute to its tumor-suppressive function.