Nucleotide-promoted release of hMutSα from heteroduplex DNA is consistent with an ATP-dependent translocation Mechanism

ATP hydrolysis by bacterial and eukaryotic MutS activities is required for their function in mismatch correction, and two different models for the role of ATP in MutS function have been proposed. In the translocation model, based on study of bacterial MutS, ATP binding reduces affinity of the protein for a mismatch and activates secondary DNA binding sites that are subsequently used for movement of the protein along the helix contour in a reaction dependent on nucleotide hydrolysis (Allen, D. J., Makhov, A., Grilley, M., Taylor, J., Thresher, R., Modrich, P., and Griffith, J. D. (1997) EMBO J. 16, 4467-4476). The molecular switch model, based on study of human MutS alpha, invokes mismatch recognition by the MutS alpha ADP complex. After recruitment of downstream repair activities to the MutS alpha mismatch complex, ATP binding results in release of MutS alpha from the heteroduplex (Gradia, S., Acharya, S., and Fishel, R.(1997) Cell 91, 995-1005). To further clarify the function of ATP binding and hydrolysis in human MutSa action, we evaluated the effects of ATP, ADP, and nonhydrolyzable ATP analogs on the lifetime of protein DNA complexes. All of these nucleotides were found to increase the rate of dissociation of MutS alpha from oligonucleotide heteroduplexes. These experiments also showed that ADP is not required for mismatch recognition by MutS alpha, but that the nucleotide alters the dynamics of formation and dissociation of specific complexes. Analysis of the mechanism of ATP-promoted dissociation of MutS alpha from a 200-base pair heteroduplex demonstrated that dissociation occurs at DNA ends in a reaction dependent on ATP hydrolysis, implying that release from this molecule involves movement of the protein along the helix contour as predicted for a translocation mechanism. In order to reconcile the relatively large rate of movement of MutS homologs along the helix with their modest rate of ATP hydrolysis, we propose a novel mechanism for protein translocation along DNA that supports directional movement over long distances with minimal energy input.