Multi-site phosphorylation of the TOPBP1 ATR Activation Domain propels ATR signaling during the response to DNA breaks

TOPBP1 is a BRCT domain-containing scaffold protein that plays important roles in a diverse array of cellular processes, including DNA damage signaling, DNA repair, DNA replication, and mRNA transcription. For DNA damage signaling, TOPBP1 activates the crucial damage response kinase ATR and this occurs during replication stress as well as during a DNA double-strand break response. ATR signaling allows cells to survive genotoxic issues and represents a formidable barrier to transformation to a tumorigenic state. Despite its importance to genome stability, the biochemical mechanism for how TOPBP1 activates ATR is not fully understood. TOPBP1 uses a discrete domain, termed the ATR activation domain, to stimulate ATR kinase. Recent work has shown that the AAD must be in a multimeric state to activate ATR. Other work has shown that phosphorylation of the AAD on serine 1131 (in Xenopus) is important for its function, and some have suggested that this is linked to the formation of TOPBP1 condensates during a DNA damage response. In this study we examine AAD phosphorylation in detail and we report three important new findings. One, S1131 phosphorylation promotes ATR activation in a manner independent of condensate formation and is instead linked to promoting multimerization of the AAD. Two, we identify a novel sight of AAD phosphorylation, on T1098, and show that is required for ATR activation. Three, we identify additional, candidate phosphorylation sites, some of which fit the consensus for casein kinase 2, and we show that casein kinase 2 activity is required for the AAD to perform its function. These studies show that multi-site phosphorylation of the AAD is an important component of the mechanism by which TOPBP1 activates ATR.