ATP-DEPENDENT ASSEMBLY OF A TERNARY COMPLEX CONSISTING OF A DNA MISMATCH AND THE YEAST MSH2-MSH6 AND MLH1-PMS1 PROTEIN COMPLEXES

MSH2 and MSH6 proteins exist as a stable complex, as do the MLH1 and P MS1 proteins. To study the mismatch binding properties of the MSH2-MSH 6 complex and to examine its functional interaction with the MLH1-PMS1 complex, these protein complexes were purified to near homogeneity fr om overproducing yeast strains. As has been reported previously, the p urified MSH2-MSH6 complex binds DNA substrates containing a GPT mismat ch and insertion/deletion mismatches, but the binding affinity for the latter decreases as the size of the extrahelical loop increases. Addi tion of ATP or the nonhydrolyzable ATP gamma S reduces binding of the MSH2-MSH6 complex to the DNA substrates markedly. Here, we show that M SH2-MSH6 forms a ternary complex with MLH1-PMS1 on a mismatch containi ng DNA substrate, The formation of this ternary complex requires ATP, which can be substituted by ATP gamma S, suggesting that ATP binding a lone is sufficient for ternary complex formation. Thus, it appears tha t ATP binding by the MSH2-MSH6 complex induces a conformation that is conducive for the interaction with MLH1-PMS1 complex, leading to the f ormation of the ternary complex.