Conformations of MutS in DNA mismatch repair

Abstract

Prior to cell division, the DNA containing the genetic information of a cell has to be copied. During this process, errors are sometimes incorporated (so-called mismatches), which may cause genetic abnormalities in future cells. To prevent this, cells contain a DNA mismatch repair (MMR) system, which efficiently removes such errors. This is important, since a non-functional MMR system leads to cancer in humans. The activation of MMR requires several sequential steps which are initiated by the evolutionary conserved MutS- and MutL protein homologs. To better understand MMR, the Escherichia coli variants of these proteins were studied in this research. MutS goes through a series of conformational states to execute its role in MMR. This study describes several of these states. A structure of unbound MutS revealed how MutS uses the ability to kink its helical arms to allow embracing of the DNA helix. MutS forms tetramers and dimers, and this study shows that the tetrameric assembly of MutS releases slowly from DNA. Mismatches, however, are recognized by MutS dimers. The binding of the MutS dimer to different mismatches could be determined using a MutS mutant that can not form tetramers. After a mismatch is detected, MutS uses the nucleotide ATP to undergo a large conformational change. The crystal structure of this so-called ‘sliding clamp’ state could be visualized for the first time in this study. The structure explains how MutS needs this state to bind to the MutL protein through two surfaces, and how this loads MutL onto DNA. MutL can then initiate the repair process.