The In Vivo Dynamic Organization of BRCA1-A Complex Proteins at DNA Damage-Induced Nuclear Foci

The breast cancer associated gene 1 (BRCA1)-A protein complex assembles at DNA damage-induced nuclear foci to facilitate repair of double-stranded breaks. Here, we describe the first systematic comparison of the dynamics, copy number and organization of its core components at foci. We show that the protein pools at individual foci generally comprise a small immobile fraction (similar to 20%) and larger mobile fraction (similar to 80%), which together occupy the same focal space but exist at different densities. In the mobile fraction, Abraxas (CCDC98) and the heterodimer BARD1BRCA1 share similar rates of dynamic exchange (complete turnover in similar to 500?seconds). In contrast, RAP80, which is required for initial foci assembly, was more dynamic with 25-fold faster turnover at mature foci. In addition, Abraxas, BARD1, BRCA1 and Merit40 (NBA1) were stably retained in the immobile fraction of foci under conditions causing loss of BRCC36 and RAP80, suggesting a shift to RAP80-independent localization after foci formation. These results, combined with our finding that RAP80 (similar to 1200 copies per focus) is twofold more abundant than Abraxas/BARD1/BRCA1 at foci, suggest new models defining the dynamic organization of BRCA1-A complex at mature foci, wherein the unusually fast turnover of RAP80 may contribute to its regulation of BRCA1-dependent DNA repair.