ATR kinase activation mediated by MutSα and MutLα in response to cytotoxic O6-methylguanine adducts

SO-type alkylating agents that produce cytotoxix O-6- methyl-G (O-6-meG) DNA adducts induce cell cycle arrest and apoptosis in a manner requiring the DNA mismatch repair (MMR) proteins MutS alpha and MutL alpha. Here, we show that checkpoint signaling in response to DNA methylation occurs during S phase and requires DNA replication that gives rise to O-6-meG/T mispairs. DNA binding studies reveal that MutSa specifically recognizes O(6_)meG/T mispairs, but not O-6-meG/C. In an in vitro assay, ATR-ATRIP, but not RPA, is preferentially recruited to O-6-meG/T mismatches in a MutS alpha and MutL alpha-dependent manner. Furthermore, ATR kinase is activated to phosphorylate Chk1 in the presence of O-6-meG/T mispairs and MMR proteins. These results suggest that MMR proteins can act as direct sensors of methylation damage and help recruit ATR-ATRIP to sites of cytotoxic O-6-meG adducts to initiate ATR checkpoint signaling.