PPI Found to Have Role in DNA Double-Strand Break Repair

Researchers from Osaka University have discovered that protein phosphatase 1 (PP1) binds to double-strand DNA breaks early on to promote repair via nonhomologous end joining instead of by homologous recombination. The decision on which of these two pathways is used to repair double-strand breaks, they report, is carefully regulated by the cell in a number of different ways. One method involves RIF1 binding to broken DNA ends, where it prevents other proteins from further degrading the break site to repair by homologous recombination.

"Double-strand breaks that are not protected by the RIF1 protein complex are susceptible to digestion by other proteins, which creates a section of single-stranded DNA for repair by homologous recombination," explained Shin-Ya Isobe, lead author of the study published today in Cell Reports. "A protein called Shieldin can bind to this single-stranded DNA tail through RIF1 to prevent further digestion, but we suspected that other factors may also play a role in this process."

To identify other factors that could help protect newly broken DNA ends, the researchers used proteomic mass spectrometry to find out which proteins interact with RIF1. "We found that PP1 binds specifically to RIF1 at the broken DNA ends, and that the physical interaction between these two proteins is necessary to block proteins that create single-stranded DNA from binding at double-strand break sites," added Chikashi Obuse, senior author.

Importantly, the interaction between PP1 and RIF1 helps keep double-strand DNA breaks from developing a single-stranded "tail," which is what Shieldin binds to. This means that PP1 acts earlier in the process than Shieldin to help push the cell toward the non-homologous end joining repair pathway.

"Our findings reveal a novel mechanism for selecting a double-strand DNA break repair pathway that acts early on in the repair process," noted Isobe.