Formation of a DNA mismatch repair complex mediated by ATP.

The mismatch repair proteins, MutS and MutL, interact in a DNA mismatch and ATP-dependent manner to activate downstream events in repair. Here, we assess the role of ATP binding and hydrolysis in mismatch recognition by MutS and the formation of a ternary complex involving MutS and MutL bound to a mismatched DNA. We show that ATP reduces the affinity of MutS for mismatched DNA and that the modulation of DNA binding affinity by nucleotide is even more pronounced for MutS E694A, a protein that binds ATP but is defective for ATP hydrolysis. Despite the ATP hydrolysis defect, E694A, like WT MutS, undergoes rapid, ATP-dependent dissociation from a DNA mismatch. Furthermore, MutS E694A retains the ability to interact with MutL on mismatched DNA. The recruitment of MutL to a mismatched DNA by MutS is also observed for two mutant MutL proteins, E29A, defective for ATP hydrolysis, and R266A, defective for DNA binding. These results suggest that ATP binding in the absence of hydrolysis is sufficient to trigger formation of a MutS sliding clamp. However, recruitment of MutL results in the formation of a dynamic ternary complex that we propose is the intermediate that signals subsequent repair steps requiring ATP hydrolysis.