The end to the 30-year debate on the mechanisms behind mismatch DNA repair.
For millions of years, nature has been using a process called mismatch repair to identify and correct genetic errors that arise during DNA replication—similar to the auto-correct feature found in smartphones. However, how this process actually occurs has been a hotly debated topic for decades.
Collaborative research conducted by Professor Jong-Bong Lee’s team from the Department of Physics at Pohang University of Science and Technology, and Professor Richard Fishel’s team from the Department of Cancer Biology & Genetics at The Ohio State University Wexner Medical Center has helped to solve this mystery by analyzing mismatch repair at the single-molecule level. This achievement has been published in the world-renowned Nature.
DNA is a double helix molecule that carries the genetic instructions within all known living organisms. During DNA replication, the helix unwinds and each strand is copied to create a newly synthesized complimentary strand. Errors occur when incorrect bases are incorporated into the complimentary strand, creating defective mismatches that, if not repaired, become the basis of human diseases such as Lynch syndrome or hereditary non-polyposis colorectal cancer.
For decades, it has been thought that three proteins—MutS, MutL, and MutH—recognize a mismatch and introduce a nick in the strand so that it can be excised, but the exact mechanism behind how they work together has been unclear.
The research team added fluorescent tags to the three proteins and utilized single-molecule imaging to analyze their movements along DNA molecules. They found that MutS, acting as a stable ATP-bound sliding clamp, diffuses along the DNA with discontinuous backbone contact and acts as a platform to recruit MutL onto the mismatched DNA. Once a mismatch is located, MutL associates with ATP-bound MutS formed at the mismatch, which results in the MutS-MutL complex that then closely follows the DNA backbone.
Furthermore, they have observed that MutL can detach from the MutS and act as a quicker clamp on its own, taking short excursions away from MutS. In this role, the team showed that MutL can recruit MutH—the cutting protein—to the DNA, thereby creating the MutS-MutL-MutH complex that searches a hemimethylated GATC site in continuous contact with the DNA backbone.
In other words, these observations supersede previous models where the relationship between MutS, MutL, and MutH were unclear, and show that MutS acts as a guiding clamp that sends out energy-efficient scouting clamps to repair mismatches. Additionally, the team has shown that the MutS-MutL-MutH complex can modulate one-dimensional facilitated diffusion mechanics to direct mismatch repair.
In 2011 and 2012, Professor Lee’s research team in collaboration with Professor Changil Ban at POSTECH and Professor Fishel published a study on how MutS identifies mismatches and moves along the DNA. This research builds upon that previous study and identifies the proteins and mechanisms behind the initial steps of mismatch DNA repair. Professor Lee anticipates that this finding will lead not just to a greater understanding of the mysteries of life, but also to real world applications in the form of cures for genetic diseases.
This work was supported by NRF of Korea Grant No.2011-001309 and NIH grant CA67007.