Isolation of MutSβ from Human Cells and Comparison of the Mismatch Repair Specificities of MutSβ and MutSα

A human MSH2-human MSH3 (hMSH2·hMSH3) complex of approximately 1:1 stoichiometry (human MutSβ (hMutSβ)) has been demonstrated in several human tumor cell lines and purified to near homogeneity. In vitro, hMutSβ supports the efficient repair of insertion/deletion (I/D) heterologies of 2–8 nucleotides, is weakly active on a single-nucleotide I/D mispair, and is not detectably active on the eight base-base mismatches. Human MutSα (hMutSα), a heterodimer of hMSH2 and hMSH6, efficiently supports the repair of single-nucleotide I/D mismatches, base-base mispairs, and all substrates tested that were repaired by hMutSβ. Thus, the repair specificities of hMutSα and hMutSβ are redundant with respect to the repair of I/D heterologies of 2–8 nucleotides. The hMutSα level in repair-proficient HeLa cells (1.5 μg/mg nuclear extract) is approximately 10 times that of hMutSβ. In HCT-15 colorectal tumor cells, which do not contain hMSH6 and consequently lack hMutSα, the hMutSβ level is elevated severalfold relative to that in HeLa cells and is responsible for the repair of I/D mismatches that has been observed in this cell line. LoVo tumor cells, which are genetically deficient in hMSH2, lack both hMutSα and hMutSβ, and hMSH3 and hMSH6 levels are less than 4% of those found in repair-proficient cells. Coupled with previous findings (J. T. Drummond, J. Genschel, E. Wolf, and P. Modrich (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 10144–10149), these results suggest that hMSH2 partitions between available pools of hMSH3 and hMSH6 and indicate that hMSH2 positively modulates hMSH6 and hMSH3 levels, perhaps by stabilization of the polypeptides upon heterodimer formation.