Redistribution of BRCA1 among four different protein complexes following replication blockage

The BRCA1 protein is known to participate in multiple cellular processes. I n these experiments, we resolved four distinct BRCA1-containing complexes. We found BRCA1 associated with the RNA polymerase II holoenzyme (holo-pol), a large mass complex called the fraction 5 complex, the Rad50-Mre11-Nbs1 c omplex, and a complex that has not been described previously. We observed t his new complex after treating cells with hydroxyurea, suggesting that the hydroxyurea-induced complex (HUIC) is involved with the response to DNA rep lication blockage. After hydroxyurea treatment of cells, BRCA1 content decr eased in the holo-pol and the fraction 5 complex, and BRCA1 was redistribut ed to the HUIC. The HUIC was shown not to contain a number of holo-pol comp onents or the Rad50-Mre11-Nbs1 complex but was associated with the BRCA1-as sociated RING domain protein BARD1. These data suggest that BRCA1 participa tes in multiple cellular processes by multiple protein complexes and that t he BRCA1 content of these complexes is dynamically altered after DNA replic ation blockage.