Analysis of yeast MSH2-MSH6 suggests that the initiation of mismatch repair can be separated into discrete steps

The yeast MSH2-MSH6 complex is required to repair both base-pair and single base insertion/deletion mismatches. MSH2-MSH6 binds to mismatch substrates and displays an ATPase activity that is modulated by mispairs that are rep aired in vivo. To understand early steps in mismatch repair, we analyzed mi smatch repair (MMR) defective MSH2-msh6-F337A and MSH2-msh6-340 complexes t hat contained amino acid substitutions in the MSH6 mismatch recognition dom ain. While both heterodimers were defective in forming stable complexes wit h mismatch substrates, only MSH2-msh6-340 bound to homoduplex DNA with an a ffinity that was similar to that observed for MSH2-MSH6. Additional analyse s suggested that stable binding to a mispair is not sufficient to initiate recruitment of downstream repair factors. Previously, we observed that MSH2 -MSH6 forms a stable complex with a palindromic insertion mismatch that esc apes correction by MMR in vivo. Here we show that this binding is not accom panied by either a modulation in MSH2-MSH6 ATPase activity or an ATP-depend ent recruitment of the MLH1-PMS1 complex. Together, these observations sugg est that early stages in MMR can be divided into distinct recognition, stab le binding, and downstream factor recruitment steps. (C) 2000 Academic Pres s.