Multiple functions for the N-terminal region of Msh6

The eukaryotic mismatch repair protein Msh6 shares five domains in common with other MutS members. However, it also contains several hundred additional residues at its N-terminus. A few of these residues bind to PCNA, but the functions of the other amino acids in the N-terminal region (NTR) are unknown. Here we demonstrate that the Msh6 NTR binds to duplex DNA in a salt-sensitive, mismatch-independent manner. Partial proteolysis, DNA affinity chromatography and mass spectrometry identified a fragment comprised of residues 228-299 of yeast Msh6 that binds to DNA and is rich in positively charged residues. Deleting these residues, or replacing lysines and arginines with glutamate, reduces DNA binding in vitro and elevates spontaneous mutation rates and resistance to MNNG treatment in vivo. Similar in vivo defects are conferred by alanine substitutions in a highly conserved motif in the NTR that immediately precedes domain I of MutS proteins, the domain that interacts with mismatched DNA. These data suggest that, in addition to PCNA binding, DNA binding and possibly other functions in the amino terminal region of Msh6 are important for eukaryotic DNA mismatch repair and cellular response to alkylation damage.