Treacle-dependent TOPBP1 condensation regulates the nucleolar DNA damage response.

We show that Treacle promotes phase separation of TOPBP1 within the FC to initiate spatially confined n-DDR signaling and to direct rDNA repair pathway choice. Using both a reductionist FKBP-based system for inducible TOPBP1 oligomerization and physiological models of genotoxic rDNA damage, we demonstrate that phosphorylation of Treacle at Ser1191 by CK2 and at Ser1199 by ATR/ATM enables bivalent engagement of the BRCT2 and BRCT5 domains of TOPBP1, thereby nucleating Treacle-dependent TOPBP1 condensation. This condensate is further stabilized by TOPBP1 oligomerization via its BRCT7/8 domains, giving rise to a nested, “Russian doll”-like phase architecture that spatially and temporally compartmentalizes DDR signaling within the nucleolus. Functionally, Treacle-dependent TOPBP1 condensation initiates γH2AX signaling and promotes recruitment of DNA repair factors in a stress-dependent manner. Importantly, disruption of this condensation does not abolish rDNA double-strand break repair but biases repair toward rapid DNA-PK–dependent non-homologous end joining, while impairing ATR/ATM activation and late-phase homologous recombination–associated repair. Consistent with this, Treacle-knockout cells exhibit accelerated early rDNA repair kinetics but incomplete damage resolution at later stages. Together, our findings identify Treacle as a molecular platform that coordinates TOPBP1 condensation, nucleolar DDR signaling, and rDNA repair pathway choice, and establish phase separation as a central organizing principle underlying functional specialization of the nucleolar DNA damage response.