A comparison of BRCA1 nuclear localization with 14 DNA damage response proteins and domains: identification of specific differences between BRCA1 and 53BP1 at DNA damage-induced foci.

BRCA1 is an important mediator of the DNA damage response pathway. Previous studies have identified a number of proteins that associate with BRCA1 at nuclear foci after ionizing radiation (IR)-induced DNA damage. However, the co-localization patterns of BRCA1 and various DNA damage response proteins have not yet been systematically quantified and compared within the same experimental system. In this study, a new inducible human cell line was established to allow unambiguous detection of YFP-BRCA1 at nuclear foci. Quantitative 2-D microscopic analysis was performed to compare the intranuclear co-localization of YFP-BRCA1 with 10 cellular proteins and 4 cellular domains before and after IR. Intriguingly, YFP-BRCA1 displayed significantly better focal co-localization with BARD1, RAP80 and Abraxas than with the upstream foci-initiating proteins gamma H2AX and MDC1. In contrast to previous reports, we found that the co-localization between YFP-BRCA1 and 53BP1 foci was surprisingly weak. Quantitative analyses of 3-D confocal images showed that approximately 60% of 53BP1 foci were unrelated to YFP-BRCA1 foci, approximately 35% of foci were abutting and only approximately 5% of foci co-localized. The YFP-BRCA1 and 53BP1 nuclear foci were distinctively separated within the first 3h after IR. In addition, in situ nuclear retention analysis revealed YFP-BRCA1 and BARD1 are less mobile than 53BP1 at IR-induced nuclear foci. Our findings indicate that BRCA1-BARD1 and 53BP1 are proximal but not overlapping at DNA break sites and are consistent with recent evidence for distinct roles of these proteins in the DNA damage response pathway.