Rtt107 cooperates with Rad55 to limit specific types of genome instability in Saccharomyces cerevisiae.

A range of genome maintenance factors respond to endogenous and exogenous DNA damage to prevent mutations and cell death. The scaffold protein, Rtt107, is important for the growth of cells exposed to DNA-damaging agents in the budding yeast Saccharomyces cerevisiae. Rtt107 binds to a diverse array of partner proteins and responds to DNA damage by localizing to phosphorylated histone H2A. Rad55-Rad57, a heterodimer involved in DNA repair, also binds to Rtt107, but the function of the Rtt107-Rad55-Rad57 complex remains unclear. In addition to their sensitivity to DNA-damaging agents, rtt107∆ mutants exhibit spontaneous genome instability phenotypes, including spontaneous loss of heterozygosity (LOH) caused by crossovers and other genetic events. However, the binding partners with which Rtt107 interacts to prevent spontaneous genome instability have yet to be elucidated. Here, we showed that Rtt107 acts in the same pathway as Rad55 to limit LOH, specifically by preventing crossover events. A rad55-S404A mutation largely disrupted the interaction between Rtt107 and Rad55-Rad57, resulting in increased LOH and crossover rates, consistent with the contribution of Rtt107-Rad55-Rad57 interaction to genome stability. Strikingly, an rtt107-K887M mutation that reduces Rtt107 recruitment to H2A did not result in an LOH phenotype, suggesting that the role of Rtt107 in preventing LOH is distinct from its function as an H2A-binding scaffold. Taken together, our observations suggested that Rtt107 limits spontaneous LOH and crossover events in part by binding to Rad55 in a manner dependent on Rad55-S404.