Binding of insertion/deletion DNA mismatches by the heterodimer of yeast mismatch repair proteins MSH2 and MSH3

Abstract

DNA-mismatch repair removes mismatches from the newly replicated DNA strand. In humans, mutations in the mismatch repair genes hMSH2, hMLH1, hPMS1 and hPMS2 result in hereditary non-polyposis colorectal cancer (HNPCC) [1], [2], [3], [4], [5], [6], [7], [8]. The hMSH2 (MSH for MutS homologue) protein forms a complex with a 160 kDa protein, and this heterodimer, hMutSα, has high affinity for a G/T mismatch [9], [10]. Cell lines in which the 160 kDa subunit of hMutSα is mutated are specifically defective in the repair of base–base and single-nucleotide insertion/deletion mismatches [9], [11]. Genetic studies in S. cerevisiae have suggested that MSH2 functions with either MSH3 or MSH6 in mismatch repair, and, in the absence of the latter two genes, MSH2 is inactive [12], [13]. MSH6 encodes the yeast counterpart of the 160 kDa subunit of hMutSα[12], [13]. As in humans, yeast MSH6 forms a complex with MSH2, and the MSH2–MSH6 heterodimer binds a G/T mismatch [14]. Here, we find that MSH2 and MSH3 form another stable heterodimer, and we purify this heterodimer to near homogeneity. We show that MSH2–MSH3 has low affinity for a G/T mismatch but binds to insertion/deletion mismatches with high specificity, unlike MSH2–MSH6.