Srs2 binding to proliferating cell nuclear antigen (PCNA) and its sumoylation contribute to replication protein A (RPA) antagonism during the DNA damage response.

Activation of the DNA damage checkpoint upon genotoxin treatment induces a multitude of cellular changes to cope with genome stress. After prolonged genotoxin treatment, the checkpoint can be downregulated to allow cell cycle and growth resumption. In yeast, downregulation of the DNA damage checkpoint requires the Srs2 DNA helicase, which removes the ssDNA binding complex replication protein A (RPA) and the associated Mec1 checkpoint kinase from DNA, thus dampening Mec1-mediated checkpoint. However, it is unclear whether the 'anti-checkpoint' role of Srs2 is temporally and spatially regulated to allow timely checkpoint termination while preventing superfluous RPA removal. Here we address this question by examining regulatory elements of Srs2, such as its phosphorylation, sumoylation, and protein-interaction sites. Our genetic analyses and checkpoint level assessment suggest that the RPA countering role of Srs2 is promoted by Srs2 binding to proliferating cell nuclear antigen (PCNA), which recruits Srs2 to a subset of ssDNA containing regions. RPA antagonism is further fostered by Srs2 sumoylation, which we found depends on the Srs2-PCNA interaction and Mec1, and peaks after Mec1 activity reaches maximal levels. These data support a model in which Srs2 recruitment to PCNA adjacent to ssDNA-RPA filaments, followed by Mec1-dependent sumoylation, modulates RPA-mediated checkpoint signaling, while Srs2 action is limited at ssDNA regions lacking proximal PCNA, thereby favoring RPA-mediated ssDNA protection and repair.