DNA mismatch repair (MMR) is one of several DNA repair processes that are vital for maintaining genome stability. In DNA synthesis, a mismatch (non-Watson-Crick base pairing) occurs once in every 106-107 base pairs. The MMR system has been shown to improve the fidelity of DNA synthesis (by post-replicative methods) by 100-1000 fold. The MMR machinery is able to correct not only mismatches but also insertion-deletion loops (IDLs) which may occur during replication and recombination of DNA.

MMR is also able to correct DNA damages caused by internal or external sources. Inactivation or deterioration of this pathway has been linked to a variety of cancers, most notably Hereditary Non-Polyposis Colorectal Cancer. The process of MMR is best understood in Escherichia coli (E. coli), where it is carried out by the mismatch repair proteins MutS, MutL and MutH along side the replication machinery. Hemi-methylation of a GATC site provides discrimination between the parental and daughter strand of DNA.
 
Figure 1: E. coli mismatch repair. MutS binds to the mismatched DNA and then MutL in an ATP dependant manner. MutL then interacts with MutH which is bound at a hemi-methylated GATC site. MutH cuts the un-methylated strand of DNA, and then UvrD (E.coli helicase II) enters to unwind the DNA. Exonucleases (dependant on the direction of the nick from the mismatch) then chew up the DNA before it is resynthesized by DNA polymerase III and ligated by DNA ligase.
 
The two initiation proteins in MMR, MutS and MutL are conserved from prokaryotes to eukaryotes. Unlike the homodimeric prokaryotic proteins, there is an array of heterodimers that function not only in mismatch repair but also in meiosis and crossing over. Since there is no known MutH homolog and no methylation occurs in eukaryotes, the mechanism of strand discrimination is unknown. However, recent research has reconstituted yeast MMR in vitro.
 
Figure 2: Simplified version of DNA MMR initiation in S. cerevisiae. Msh2-Msh6 binds to a mismatch in an ATP-dependant fashion. Mlh1-Pms1 is then recruited in an ATP-dependant fashion. PCNA, RFC and ExoI all take part in the excision of the DNA containing the mismatched base.
 
Our lab has been focusing on elucidating the interactions of both prokaryotic and eukaryotic MMR proteins with DNA and with each other. We use the technique of atomic force microscopy to study MMR at a single molecule level in order to visualize transient states that wouldn’t be observed via bulk solution methods.
 
 
We additionally are looking at other protein-protein and protein-DNA interactions related to DNA repair, including interactions of RFC and PCNA and MutS mutants with mismatched DNA.
 
 
 
 

The Erie Lab
UNC Chapel Hill
Department of Chemistry
CB #3290
Kenan Labs B825
Chapel Hill, NC 27599
lab phone (919) 962-6371

 
 
Updated: Wednesday, June 7, 2006 8:34 AM