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Domain map of BRCA1; RING, serine containing domain (SCD), and BRCT domains are indicated. Horizontal black lines indicate protein binding domains for the listed partners. Red circles mark phosphorylation sites3

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Protein structure

Domain map of BRCA1; RING, serine containing domain (SCD), and BRCT domains are indicated. Horizontal black lines indicate protein-binding domains for the listed partners. Red circles mark phosphorylation sites.

The BRCA1 protein contains the following domains:

• Zinc finger, C3HC4 type (RING finger)
• BRCA1 C Terminus (BRCT) domain

This protein also contains nuclear localization signal and nuclear export signal motifs.

The human BRCA1 protein consists of four major protein domains; the Znf C3HC4- RING domain, the BRCA1 serine domain and two BRCT domains. These domains encode approximately 27% of BRCA1 protein. There are six known isoforms of P38398 BRCA1, with isoforms 1 and 2 comprised of 1863 amino acids each.

Zinc ring finger domain

The RING motif, a Zn finger found in eukaryotic peptides, is 40-60 amino acids long and consists of eight conserved metal-binding residues, two quartets of cysteine or histidine residues that coordinate two zinc atoms. The BARD1/BRCA1 interaction is disrupted by tumorigenic amino acid substitutions in BRCA1, implying that the formation of a stable complex between these proteins may be an essential aspect of BRCA1 tumor suppression. This motif contains a short anti-parallel beta-sheet, two zinc binding loops and a central alpha helix in a small domain. This RING domain interacts with associated proteins including BARD1, which also contains a RING motif, to form a heterodimer. The BRCA1 RING motif is flanked by alpha helices formed by residues 8-22 and 81-96 of the BRCA1 protein. It interacts with a homologous region in BARD1 also consisting of a RING finger is flanked by two alpha-helices formed from residues 36-48 and 101-116. These four helices combine to form a heterodimerization interface and stabilise the BRCA1-BARD1 heterodimer complex. Additional stabilisation is achieved by interactions between adjacent residues in the flanking region and hydrophobic interactions. The ring domain is an important element of ubiquitin E3 ligases which catalyse protein ubiquitination2. Ubiquitin is a small regulatory protein found in all tissues which directs proteins to compartments within the cell. BRCA1 polypeptides, in particular Lys-48-linked polyubiquitin chains, are dispersed throughout within the resting cell nucleus but when DNA synthesis begins they gather in restrained groups that also contain BRCA2 and BARD1. BARD1 is thought to be involved in the recognition and binding of protein targets for ubiquitination. It attaches to proteins and labels them for destruction. Ubiquitination occurs via the BRCA1 fusion protein and is abolished by zinc chelation. The enzyme activity of the fusion protein is dependent on the proper folding of the ring domain.

Serine cluster domain

The BRCA1 serine cluster domain (SCD) spans amino acids 1280-1524. A portion of the domain is located in exons 11-13. High rates of mutation occur in exons 11-13. Reported phosphorylation sites of BRCA1 are concentrated in the SCD where they are phosphorylated by ataxia telangiectasia mutated/ataxia telangiectasia and Rad3-related protein (ATM/ATR)kinases both in vitro and in vivo. ATM/ATR are kinases activated by DNA damage. Mutation of serine residues may affect localization of BRCA1 to sites of DNA damage and DNA damage response function.

BRCT domains

The dual repeat BRCT domain of the BRCA1 protein is an elongated structure approximately 70 Å long and 30-35 Å wide. The 85-95 amino acid domains in BRCT can be found as single modules or as multiple tandem repeats containing two domains. Both of these possibilities can occur in a single protein in a variety of different conformations. The C-terminal BRCT region of the BRCA1 protein is essential for repair of DNA, transcription regulation and tumor suppressor function. In BRCA1 the dual tandem repeat BRCT domains are arranged in a head-to-tail-fashion in the three-dimensional structure, burying 1600Å of hydrophobic, solvent accessible surface area in the interface. These all contribute to the tightly packed knob-in-hole structure that comprises the interface. These homologous domains interact to control cellular responses to DNA damage. It is therefore no surprise, that a missense mutation at the interface of these two proteins can have devastating consequences on the cell cycle, resulting in protein dysfunction and a greater risk of developing cancer. The linker that joins these two homologs need also be considered, as its poorly defined electron density eludes to a possible complex function; the ability to flex.