Biochemical analysis of the human mismatch repair proteins hMutSα MSH2(G674A)-MSH6 and MSH2-MSH6(T1219D).

The heterodimeric human MSH2-MSH6 protein initiates DNA mismatch repair (MMR) by recognizing mismatched bases that result from replication errors. Msh2G674A or Msh6T1217D mice that have mutations in or near the ATP binding site of MSH2 or ATP hydrolysis catalytic site of MSH6 develop cancer and have a reduced lifespan due to loss of the MMR pathway (Lin, D. P., Wang, Y., Scherer, S. J., Clark, A. B., Yang, K., Avdievich, E., Jin, B., Werling, U., Parris, T., Kurihara, N., Umar, A., Kucherlapati, R., Lipkin, M., Kunkel, T. A., and Edelmann, W. (2004) Cancer Res. 64, 517–522; Yang, G., Scherer, S. J., Shell, S. S., Yang, K., Kim, M., Lipkin, M., Kucherlapati, R., Kolodner, R. D., and Edelmann, W. (2004) Cancer Cell 6, 139–150). Mouse embryonic fibroblasts from these mice retain an apoptotic response to DNA damage. Mutant human MutSα proteins MSH2G674A-MSH6wt and MSH2wt-MSH6T1219D are profiled in a variety of functional assays and as expected fail to support MMR in vitro, although they retain mismatch recognition activity. Kinetic analyses of DNA binding and ATPase activities and examination of the excision step of MMR reveal that the two mutants differ in their underlying molecular defects. MSH2wt-MSH6T1219D fails to couple nucleotide binding and mismatch recognition, whereas MSH2G674A-MSH6wt has a partial defect in nucleotide binding. Nevertheless, both mutant proteins remain bound to the mismatch and fail to promote efficient excision thereby inhibiting MMR in vitro in a dominant manner. Implications of these findings for MMR and DNA damage signaling by MMR proteins are discussed.