The effect of O-6-methylguanine DNA adducts on the adenosine nucleotide switch functions of hMSH2-hMSH6 and hMSH2-hMSH3

The human homologs of prokaryotic mismatch repair have been shown to mediat e the toxicity of certain DNA damaging agents; cells deficient in the misma tch repair pathway exhibit resistance to the killing effects of several of these agents. Although previous studies have suggested that the human MutS homologs, hMSH2-hMSH6, bind to DNA containing a variety of DNA adducts, as well as mispaired nucleotides, a number of studies have suggested that DNA binding does not correlate with repair activity. In contrast, the ability t o process adenosine nucleotides by MutS homologs appears to be fundamentall y linked to repair activity. In this study oligonucleotides containing a si ngle well defined O-6-methylguanine adduct were used to examine the extent of lesion-provoked DNA binding, single-step ADP --> ATP exchange, and stead y-state ATPase activity by hMSH2-hMSH3 and hMSH2-hMSH6 heterodimers. Intere stingly, O-6-methylguanine lesions when paired with either a C or T were fo und to stimulate ADP --> ATP exchange, as well as the ATPase activity of pu rified hMSH2-hMSH6, whereas there was no significant stimulation of hMSH2-h MSH3. These results suggest that O-6-methylguanine uniquely activates the m olecular switch functions of hMSH2-hMSH6.