LONDON - During cell replication, the DNA of the cell must be copied so that each daughter cell has its own set of genes. When an error occurs, a team of specialized proteins detects the mistake, chops out the nonsense section, and corrects the sequence of bases.
This process is so important that people who have defects in the genes encoding these proteins are at higher risk of developing cancer at an early age than the rest of the population. Hereditary non-polyposis colon cancer is an example of a disease caused in this way.
The proteins are known as the mismatch repair proteins, and scientists have been working on how they carry out these functions. Two groups have reported in the current issue of Nature, dated Oct. 12, 2000, on the crystal structure of one of the mismatch repair proteins and examined how it binds with DNA.
Galina Obmolova and colleagues at the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Md., describe their study in a paper titled "Crystal structures of mismatch repair protein MutS and its complex with a substrate DNA." Melndert Lamers, of the Division of Molecular Carcinogenesis at the Netherlands Cancer Institute in Amsterdam, together with colleagues at the same institute and at the European Molecular Biology Laboratory in Grenoble, France, report their experiments in a paper titled "The crystal structure of DNA mismatch repair protein MutS binding to a G-T mismatch."
Commenting on the two studies, Richard Kolodner at the Ludwig Institute for Cancer Research in La Jolla, Calif., said the signaling cascades that seem to be important in mismatch repair appear to rely on interactions between proteins and conformational changes in proteins. In a News & Views article in the same issue of Nature, he said this is why it is so important that the structure of the first protein in the process has now been revealed. Such findings, he noted, may allow us "to gain greater insight into a fundamental biological process [as well as] learn more about stumbling blocks to the effectiveness of chemotherapy."
Lamers and his colleagues decided to work with the MutS protein from Escherichia coli because at the time they began their study, the human equivalent had not been well characterized. The MutS protein from E. coli is a homodimer - it is made up of two identical parts - whereas that from humans is a heterodimer, having two different protein components.
Titia Sixma, senior author of the second paper, said the protein from E. coli does the same repair in a similar manner. She told BioWorld International: "We wanted to know how this protein recognizes errors and how it initiates repair. So we solved the crystal structure of MutS in complex with a mismatched DNA."
The results were "quite spectacular," Sixma said. "We can see that the complex is built up from a lot of different domains - it is very modular." It includes a DNA-recognition domain and a domain containing adenine triphosphatase (ATPase). "But these are quite far apart from each other," she added.
While human MutS recognizes the DNA molecule, only one of the two proteins in the heterodimer is specific for the mismatch; the other helps to hold the DNA in place. Sixma said, "One of our most striking findings is that even though the E. coli protein is composed of two identical proteins, it acts as a structural heterodimer, with only one of the two proteins recognizing the mismatch." In addition, Obmolova and her colleagues showed that, when DNA is not present, the DNA binding domains of MutS are flexible.
Sixma's group found that the ATPase domains of MutS were not identical. "We found that one has adenosine diphosphate bound and one does not. This is new. No one knew before this that these sites did not act simultaneously - there has been a big controversy about the role of the ATP hydrolysis." Now, she said, "it seems likely that they act alternately rather than in concert; this will help us understand how the process of repair takes place."
Sixma said the ATPases found in the family of proteins known as the ABC transporters show a similar alternating pattern. These transmembrane proteins include CFTR - the protein responsible for cystic fibrosis - as well as multidrug-resistance proteins and some involved in antigen presentation.
For mismatch repair to take place, the proteins have to recognize not only where a mismatch exists, but also which strand of the DNA the error is on. This involves searching for a "strand discrimination signal" that will tell the molecule which strand is the new one, as this is the one that is most likely to be wrong. Researchers know that both MutS and another protein called MutL are involved in strand discrimination. Sixma's next goal is to crystallize a complex of mismatched DNA with both MutS and MutL and study this structure - but she predicts that it will take some time to make such a complex.