Mismatch Repair Protein

Figure Above: The structure and sequence of MutH.

 Overview of DNA Mismatch Repair System:

During replication, errors are often made in which the wrong base is added on, called a mismatch. Although the replication system is quite accurate, the polymerase adds on an incorrect base once in every ten million nucleotides, thus the need for mismatch proteins that reduce these mutations one-hundredfold (McGraw Hill). These series of proteins in the Mismatch Repair System are responsible for finding mismatches and replacing the incorrect base pair. The steps are:

1) MutS protein follows newly synthesized DNA strand by contact with the phosphate backbone, until it encounters a kink (Brown).

2) MutS clamps down at this mismatch site and uses ATP to recruit MutL, which in turn is responsible for stimulating MutH activity (Journal of Bacteria).

3) MutH binds to the unmethylated daughter strand and is responsible for knicking at the GATC site (Brown).

4) Exonuclease cleaves off bases on unmethylated strand from this knick on past the mutation. DNA polymerase and ligase fill in the resulting gap and seal it.(Brown).

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 Function of Mut H:

As demonstrated above, MutH is one of these Mismatch Repair proteins that works with MutS and MutL to correct mismatches in base pairing that occur during DNA replication or homologous recombination in prokaryotes. MutH is an endonuclease that is sequence specific and methylation specific (Brown). Once activated, it binds to the unmethylated DNA strand of the hemi-methylated duplex, then cuts the phosphodiester bond immediately upstream of G in the methylation sequence of 5'-GATC-3' (DePamphilis). By introducing this knick in the daughter strand, it allows the recruitment of DNA exonuclease, helicase, polymerase, and ligase to remove the nucleotides from the nick to beyond the mismatch, and to fill in this resulting gap with the correct base pair matches (Brown).

Crystal structure of Protein:

The active site of Mut H is located between two arm-like structures that pivot, depending on the position of a protruding C-terminal helix that serves as a lever, facilitating the connection between the other Mut proteins. (The EMBO journal)

How does Mut H relate to Eukarotes?

Because Mut H is the protein that discriminates and recognizes the unmethylated daughter strand in prokaryotic DNA, it is difficult to find its homolog due to the fact that eukaryotic strand differentiation is not methyl-directed (DePamphilis). In eukaryotes, the daughter strand is thought to be recognized by newly synthesized single-stranded DNA gaps that have not yet been connected by the ligase, thus the eukaryotic repair protein most closely related to the Mut H protein is a protein that recognizes those single stranded nicks and gaps (DePamphilis).

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Diseases associated with mutations in Mut H:

• Hereditary Nonpolyposis Colorectal Cancer (HNPCC) is an inherited autosomal dominant syndrome that is mostly caused by mutations in germline mismatch repair genes.
• People with HNPCC inherit one normal and one mutant allele coding for the mismatch repair proteins, thus patients can have either patchy expression or complete loss of expression (Coleman).

• Also known as the lynch syndrome, HNPCC increases the chance of getting colon cancer/ Colorectal Cancer, which thus is the main disease associated with malfunctioning Mut proteins.
• This is a fairly common disease, where about 1 in 15 people develop colon cancer. (More Colon cancer details)
• The eukaryotic genes associated with Lynch syndrome are MLH1, MSH2, MSH6, and PMS2.
• HNPCC also correlates to a high rate of microsatellite instability, which is why defective mismatch repair genes are also thought to be a factor in the tumor development in human prostrate cancer (Chen et al.).
• Overview of correlation between malfunctioning mismatch repair proteins and HNPCC (Dr. Lammert).

How is Mut H being used in today's research?

• Major chemical research companies such as Accelrys are using Mut mismatch repair proteins to study protein-protein interactions which aids in therapeutical research.
• By further exploring and understanding the prokaryotic mismatch proteins, researchers can better understand similar eukaryotic mismatch repair proteins, which could lead to better ways to diagnose and treat HNPCC and related cancers (Fishel).
• Studies are underway for relation of mutant mismatch repair genes to the development of human prostrate cancer (Chen et al.).
• Recently, they are studying these mismatch repair genes to predict treatment response to cancer patients (UT Anderson Cancer Center).