Quantitative phosphoproteomics identifies ETAA1- and TOPBP1-dependent phosphorylation sites. (A) Schematic of SILAC-phosphoproteomics approach. (B) Scatter plot of phosphoproteomic data. Each point represents a phosphosite, and the x and y value corresponds to the log₂ transformed ratios of ETAA1- and TOPBP1-dependent phosphorylation events. Higher ratio values correspond to more phosphorylation in WT cells than in ETAA1ΔAAD or TOPBP1-AID cells. (C and D) Number of phosphosites (C) and phosphoproteins (D) dependent on ETAA1, TOPBP1, or both ATR activators. (E) Diagram of ATR signaling pathway with preferred phosphorylation motifs of ATR and CHK1. (F and G) Abundance of ATR and CHK1 motifs in ETAA1- and TOPBP1-dependent phosphorylation sites.