New light has been shed on a DNA repair mechanism that could lead to ways of enhancing chemotherapy drugs that destroy cancer cells by damaging their DNA.
Researchers at Washington University School of Medicine in St. Louis used yeast cells to study protein molecules that have an important role in homologous recombination, which is one way that cells repair breaks in the DNA double helix.
Earlier it was found that a protein molecule named Srs2 regulates homologous recombination by counteracting the work of another protein, Rad51.
In the new study, researchers detailed the mechanism of how Srs2 removes Rad51 from DNA and thereby prevents it from making repairs to broken strands.
"Our findings may make it possible to uncover ways to augment the effect of DNA-damaging agents that are used for cancer chemotherapy. Many chemotherapeutic agents work by causing DNA damage in cancer cells, leading to their death, and tumors can become resistant to chemotherapy by using DNA repair mechanisms to keep the cells alive. Drugs that inhibit the DNA repair process could help increase the efficiency of chemotherapeutic agents," said senior author Dr. Tom Ellenberger.
Srs2 is a helicase molecule - a motor protein that's able to walk or slide along a strand of DNA and remove other proteins from DNA or separate the two strands of the twisted double helix.
Rad51's job in the cell is to promote the exchange of sequences between two related DNA molecules, which can be used to repair breaks in DNA where both strands of the double helix are compromised.
Rad51 acts as a DNA matchmaker and forms long filaments on DNA. Srs2 can remove these to prevent unwanted exchanges of DNA sequences.
Without Srs2, cells lose their ability to maintain the normal structure of chromosomes, and DNA sequences become shuffled.
It was found that Srs2 possesses a small arm that interacts with Rad51 and triggers a chemical reaction within the Rad51 protein causing it to fall off the DNA.
"Scientists had assumed that as Srs2 moved along the DNA strand, it just pushed off everything in its path. This isn't the case - we showed that Srs2 has a specialized structure that allows it to interact specifically with Rad51," said lead author Dr. Edwin Antony.
The finding has shown how a motor protein like Srs2 can perform the specialized task of remodelling a protein-DNA complex without interference by other similar helicases.
And now, the researchers can narrow their search for drugs that will block DNA repair by Rad51.
The study has been published in the latest issue of the journal Molecular Cell.