A human 200-kDa protein binds selectively to DNA fragments containing G.T mismatches.

G.T mispairs, the sole mismatch type that can arise in "resting" mammalian DNA (through spontaneous hydrolytic deamination of 5-methylcytosine), are corrected in vivo with high efficiency and mostly to a G.C. We identified a protein factor, present in HeLa cell extracts, that binds selectively to DNA substrates containing this mismatch. The partially purified protein was shown by gel-filtration chromatography and UV cross-linking experiments to have an apparent molecular mass of 200 kDa. Its binding to G.T mispairs was not influenced by sequences flanking the mismatch, but methylation of guanines either within the mismatch itself or in its immediate vicinity abolished the formation of the protein-DNA complex. The protein appears to lack both endo- and exonuclease activities and requires neither magnesium nor zinc nor ATP for binding. We discuss the possible role of this protein in a repair pathway, which helps mammalian cells counter the mutagenic effect of the hydrolytic deamination of 5-methylcytosine.