The DNA mismatch repair system is critical for accurate DNA replication. This system is highly conserved across organisms ranging from bacteria to humans reflecting the importance of minimizing genomic defects during cell division. The mismatch repair protein MutS has been identified the key factor that detects base-base mismatches and insertion-deletion mismatches in double stranded DNA and signals for their repair. Despite intense study, a temporally resolved understanding of the molecular details of the MutS:DNA interactions during mismatch repair initiation has been difficult to obtain because these transient interactions occur within an overwhelming background of properly matched DNA basepairs.
We used single molecule fluorescence resonance energy transfer (smFRET) to characterize conformational changes in MutS as it scans homoduplex DNA, recognizes mismatches, activates to a sliding clamp, and interacts with downstream proteins in the repair signaling pathway. We found a series of sequential conformational changes that provide a mechanistic picture of: i) dynamic DNA bending by MutS, ii) concomitant conformational changes within MutS itself, iii) motion of MutS scanning along DNA, iv) ATP binding states that commit MutS:mismatch DNA complexes to convert to sliding states used in signaling, and v) the modulation of these MutS behaviors by other regulatory factors.