Regulation of the BRCA1 gene by an SRC3/53BP1 complex

We have used protein purification and mass spectrometry to identify the 53BP1 tumour suppressor as a novel SRC3-associated protein. Copurification was demonstrated using multiple antibodies, and was not dependent on DNA damage suggesting that SRC3 is not directly involved in the DNA damage response. However using chromatin immunoprecipitation(ChIP) and siRNA knockdown, we have demonstrated that both SRC3 and 53BP1 co-occupy the same region of the BRCA1 promoter and both are required for BRCA1 expression in HeLa cells.Our results suggest that both 53BP1 and SRC3 have a common function that converge at the BRCA1 promoter and possibly other genes important for DNA repair and genomic stability.The steroid receptor coactivator 3 (SRC3) (also known as p/CIP/AIB1/ACTR/NCoA3) is a member of the SRC family of proteins which bind to nuclear hormone receptors, and other transcription factors, to promote coactivator complex assembly at target genes [1-6]. This is accomplished through direct protein interactions mediated by several structural domains conserved among all of the SRC family members. These domains include a basic helix-loop-helix Per/ARNT/SIM (bHLH-PAS) domain, a nuclear receptor interacting domain which consists of three leucine-rich motifs with the consensus amino acid sequence LxxLL (X = any amino acid and L = leucine), and two transcriptional activation domains (AD1 and AD2) within the carboxy terminus of SRC3. The AD1 domain interacts directly with various protein acetyltransferases such as CBP/p300, or p/CAF/GCN5 and is absolutely essential for SRC3-mediated transcriptional activation [1,2,7-10]. A second transactivation domain (AD2) serves as an interaction surface for several members of the protein arginine methyltransferase (PRMT) class of enzymes such as the coactivator associated arginine methyltransferase 1 (CARM1, also known as PRMT4) and PRMT1 [11,12]. CARM1 interacts directly with the AD2 domain of all the SRC proteins and co-transfection assays using