Specialized mismatch repair function of Glu339 in the Phe-X-Glu motif of yeast Msh6.

The major eukaryotic mismatch repair (MMR) pathway requires Msh2-Msh6, which, like Escherichia coli MutS, binds to and participates in repair of the two most common replication errors, single base-base and single base insertion-deletion mismatches. For both types of mismatches, the side chain of E. coli Glu38 in a conserved Phe-X-Glu motif interacts with a mismatched base. The Ovarepsilon of Glu38 forms a hydrogen bond with either the N7 of purines or the N3 of pyrimidines. We show here that changing E. coli Glu38 to alanine results in nearly complete loss of repair of both single base-base and single base deletion mismatches. In contrast, a yeast strain with alanine replacing homologous Glu339 in Msh6 has nearly normal repair for insertion-deletion and most base-base mismatches, but is defective in repairing base-base mismatches characteristic of oxidative stress, e.g. 8-oxo-G.A mismatches. The results suggest that bacterial MutS and yeast Msh2-Msh6 differ in how they recognize and/or process replication errors involving undamaged bases, and that Glu339 in Msh6 may have a specialized role in repairing mismatches containing oxidized bases.