The BRCT domain was originally identified in the tumor suppressor protein, Breast cancer associated 1 (BRCA1). Truncation and missense mutations in this region correlate with an increased risk for breast and ovarian cancers, underscoring its importance in the tumor suppressor function of BRCA1. Subsequent bioinformatics analysis revealed that BRCT domains exist in a myriad of proteins, most of which have roles in DNA metabolism and repair. The conserved phospho‐peptide binding function was later identified in BRCA1 and other tandem BRCT domains, as well as various single BRCT domains. For example, the tandem BRCT domain pair in BRCA1 recognizes the phosphorylated peptide motif pSer-Pro-Thr-Phe in various protein partners such as BRCA1‐associated C‐terminal helicase/Fanconi anemia group J protein (BACH1/FANCJ), CtIP and Abraxas/Coiled‐coil domain‐ containing protein 98 (CCDC98), and the ability of BRCA1 to recognize different binding partners in DDR signaling regulates BRCA1 recruitment and function.
It is now evident that BRCT domains possess an array of activities other than phosphorylated protein interactions. Besides the conserved phospho- peptide recognition exhibited by tandem BRCT domains, BRCT domains have also been implicated in phosphorylation‐independent protein interactions, DNA binding and poly(ADP‐ribose) (PAR) binding. This diversity can be credited not only to variations in sequence and structure within a single BRCT domain, but also to the unique ability of BRCT domains to assemble as multi‐domain complexes with other BRCT domains or even other functional domains, adding another level of complexity, specificity and regulation.
The BRCT domain fold was first revealed in the crystal structure of the X-ray repair cross‐complementing protein 1 (XRCC1) N‐terminal BRCT domain and comprises of a central four‐stranded b‐sheet flanked by a single a‐helix (a2) on one side and 2 a‐helices (a1 and a3) on the opposite side. Comparison of various BRCT domain structures illustrate that deviations in structure and sequence are mainly localized to the connecting loops, whereas conserved residues are situated in the hydrophobic core and in residues involved in recognizing the phosphorylated amino acid in phospho-peptide targets.
Interestingly, the domain architecture of BRCT domains is remarkably diverse. BRCT domains range from isolated individual domains to multiple tandem BRCT repeats, or even as fusions with other functional domains. Single BRCT domains represent a large class of BRCT domains that exist as single copies in proteins such as Poly(ADP‐ribose) polymerase 1 (PARP‐1) and DNA Ligase III (Lig3). They can also be found in multiple but isolated copies, such as in XRCC1, where two distinct single BRCT domains are separated by 135 amino acids in sequence. In tandem BRCT repeats, multiple BRCT domains are separated by a variable linker region. The initial crystal structure of the tandem BRCT domains in BRCA1 shed light on the canonical BRCT‐BRCT domain packing, which occurs through a hydrophobic interface consisting of the a2 helix of the N‐ terminal BRCT and a1¢ and a3¢ helices of the C‐terminal BRCT. Mutations at this hydrophobic interface in BRCA1 (M1775R and A1708E) are linked to breast and ovarian cancer, highlighting the requirement for an intact tandem BRCT interface for normal function. This conserved BRCT‐BRCT domain packing has since been observed in numerous BRCT repeat structures such as in Mediator of DNA damage checkpoint protein 1 (MDC1), BRCA1‐associated RING domain protein 1 (BARD1), S. pombe Crb2 and S. pombe Brc1, and their functional role in phospho‐ peptide binding is well established. However, variations in tandem BRCT domain structure also exist. For example, the tandem BRCT domains of DNA Ligase IV (LigIV) do not pack together and are separated by a significantly longer inter‐BRCT linker, which ultimately ensures its unique mode of recognition for X‐ray repair cross‐complementing protein 4 (XRCC4). A triple BRCT domain module also exists in TopBP1 BRCT0/1/2, suggesting even higher order tandem BRCT domain architecture. Interestingly, the packing of BRCT0/BRCT1 and BRCT1/BRCT2 at the interface are significantly different from canonical tandem BRCT domains. This unconventional BRCT domain interface is likely a consequence of the shorter inter‐BRCT linker regions, suggesting a role for the inter‐BRCT linker in driving the packing of tandem BRCT domains. Whereas longer linker lengths ranging from 30‐60 amino acids permit the parallel juxtaposition characteristic of canonical tandem BRCT domain packing, shorter linkers restrict the packing of adjacent BRCT repeats such that they interact in a twisted orientation.
The diversity in BRCT domain architecture extends to other functional domains, although the significance of these hetero‐domain modules is still unclear. A single BRCT domain is packed alongside an N‐terminal fibronectin type III (FN3) domain in S. cerevisiae Chs5p, a component of the exomes coat complex. Studies in yeast suggest that both the FN3 and BRCT domain act as a single module and are both required for localization and cargo delivery function. The tandem BRCT repeats in the DNA DSB repair protein Nijmegen breakage syndrome protein 1 (NBS1) are coupled to an N‐ terminal FHA domain, another phospho‐peptide binding domain. This unique FHA‐BRCT‐BRCT domain setup may present an intriguing platform for cross‐talk between the two adjacent functional phospho‐peptide interacting modules.