BINDING OF INSERTION DELETION DNA MISMATCHES BY THE HETERODIMER OF YEAST MISMATCH REPAIR PROTEINS MSH2 AND MSH3/

DNA-mismatch repair removes mismatches from the newly replicated DNA s trand. In humans, mutations in the mismatch repair genes hMSH2, hMLH1, hPMS1 and hPMS2 result in hereditary non-polyposis colorectal cancer (HNPCC) [1-8]. The hMSH2 (MSH for MutS homologue) protein forms a comp lex with a 160 kDa protein, and this heterodimer, hMutS alpha, has hig h affinity for a G/T mismatch [9,10]. Cell lines in which the 160 kDa subunit of hMutS alpha is mutated are specifically defective in the re pair of base-base and single-nucleotide insertion/deletion mismatches [9,11]. Genetic studies in S. cerevisiae have suggested that MSH2 func tions 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 y east counterpart of the 160 kDa subunit of hMutS alpha [12,13]. As in humans, yeast MSH6 forms a complex with MSH2, and the MSH2-MSH6 hetero dimer binds a G/T mismatch [14]. Here, we find that MSH2 and MSH3 form another stable heterodimer, and we purify this heterodimer to near ho mogeneity. We show that MSH2-MSH3 has low affinity for a G/T mismatch but binds to insertion/deletion mismatches with high specificity, unli ke MSH2-MSH6.