Checkpoint activation and recovery: regulation of the 9-1-1 axis by the PP2A phosphatase

Genome integrity is continuously monitored by elaborate cellular networks, collectively referred to as the DNA damage response (DDR), which detect DNA lesions and transmit the information to downstream targets, thereby coordinating a broad range of biological processes. A crucial signal in this response is the generation of single-stranded DNA that, once coated by replication protein A (RPA), serves as a platform for recruiting the apical checkpoint kinase Mec1/ATR. Full activation of Mec1/ATR also requires the 9-1-1 complex, which provides a docking site for additional checkpoint mediators, such as Dpb11/TOPBP1 and Rad9/53BP1. These mediators are important for transducing the checkpoint signal from Mec1/ATR to the effector kinase Rad53/CHK2. The checkpoint signal transduction cascade is tightly regulated by phosphorylation events, which can be counteracted by phosphatases to ensure timely checkpoint inactivation once DNA repair is complete. In this review, we examine the mechanistic aspects of Mec1/ATR activation, with a particular focus on the 9-1-1 checkpoint axis in Saccharomyces cerevisiae. We discuss how phosphorylation and dephosphorylation dynamically regulate the checkpoint pathway, allowing cells to efficiently respond to genotoxic stress while ensuring a timely return to normal cell-cycle progression.