TopBP1 assembles nuclear condensates to switch on ATR signaling

•TopBP1 assembles biomolecular condensates•Composition of TopBP1 condensates revealed by proximity proteomics•Phosphorylation induces condensation•TopBP1 condensation switches on ATR/Chk1 signaling and causes fork slowing ATR checkpoint is crucial for cellular responses to DNA replication impediments. Using an optogenetic platform, we show that TopBP1, the main activator ATR, self-assembles extensively yield micrometer-sized condensates. These opto-TopBP1 are functional entities organized in tightly packed clusters spherical nano-particles. reversible, occasionally fuse, co-localize with partner proteins. We provide evidence a molecular switch amplifies activity phosphorylate kinase 1 (Chk1) slow down forks. Single amino acid substitutions key residues intrinsically disordered activation domain disrupt consequently signaling. 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