BRD3 belongs to the BET subclass of proteins, which are distinguished by two N-terminal bromodomains and one ET (Extra Terminal) domain. BRDs have been found to be associated with chromatin. The poorly characterized ET domain functions as a protein-protein interaction motif and may be part of an atypical serine-kinase activity. The subclass consists of at least four members in mouse and human, Brd2 (also referred to as Fsrg1, RING3), Brd3(Fsrg2, ORFX), Brd4 (Fsrg4, MCAP/HUNK1), and Brdt (Fsrg3, BRD6). BRD proteins are related to the female sterile homeotic protein gene in Drosophila, a gene required maternally for proper expression of other homeotic genes, such as Ubx, which is involved in pattern formation. BRD3 mRNA is ubiquitously expressed in human adult and fetal tissues with highest expression in testis, ovary, placenta, uterus, and brain. BRD3 expression is induced in activated lymphocytes and it is highly expressed in undifferentiated ES cells whereas expression levels are reduced upon endothelial differentiation.
Down regulated expression or loss of BRD3 has been detected in biopsies of nasopharyngeal carcinomas and altered expression levels have been found in bladder cancer. In addition, the BRD3 gene is located to chromosome 9q34, a region susceptible to genomic rearrangement in tumors . BRD3 has also been proposed as a marker for hormone dependent cancer. BRD3 also interacts with LANA-1, the Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen 1, which is required for the replication of episomal viral genomes. Here we report the structure of the second individual bromodomain of BRD3.