BRCA1 tumor suppressor network: focusing on its tail

Abstract

Germline mutations of the BRCA1 tumor suppressor gene are a major cause of familial breast and ovarian cancer. BRCA1 plays critical roles in the DNA damage response that regulates activities of multiple repair and checkpoint pathways for maintaining genome stability. The BRCT domains of BRCA1 constitute a phospho-peptide binding domain recognizing a phospho-SPxF motif (S, serine; P, proline; × varies; F, phenylalanine). The BRCT domains are frequently targeted by clinically important mutations and most of these mutations disrupt the binding surface of the BRCT domains to phosphorylated peptides. The BRCT domain and its capability to bind phosphorylated protein is required for the tumor suppressor function of BRCA1. Through its BRCT phospho-binding ability BRCA1 forms at least three mutually exclusive complexes by binding to phosphorylated proteins Abraxas, Bach1 and CTIP. The A, B and C complexes, at lease partially undertake BRCA1's role in mechanisms of cell cycle checkpoint and DNA repair that maintain genome stability, thus may play important roles in BRCA1's tumor suppressor function.

Figure 1

BRCA1 domains and interacting proteins. BRCA1 contains a RING domain at its N-terminus, two BRCT domains at the C-terminus and a coiled-coil domain upstream of BRCT domains. The interacting proteins are shown under the region of BRCA1 required for their association. BRCA1 forms an E3 ligase with BARD1 through its RING domain dimerizing with a RING domain containing protein BARD1. A ubiquitin hydrolase BAP1 also interacts with this region. The C-terminal BRCT domains form a phospho-binding module recognizing a phospho-SPxF motif. Abraxas, Bach1 and CtIP have been shown directly bind to BRCT domain through the phospho-SPxF motif. PALB2 binds to the coiled-coil domain of BRCA1 and bridges BRCA1-BRCA2 interaction. A number of other proteins have also been indicated binding to the C-terminal region of BRCA1 or the central region of BRCA1. BRCA1 contains a S/TQ cluster that is phosphorylated by ATM/ATR at multiple sites. In addition, BRCA1 is also a substrate of Chk2

Figure 2

Three proteins Abraxas, Bach1 and CtIP contain p-SPxF motif that binds to the BRCT domains of BRCA1. Schematic diagram showing protein domains of three BRCA1-BRCT associated proteins. The phospho-Ser-Pro-X-Phe (p-SPxF) motif is illustrated on each protein. In addition, Abraxas contains a MPN- domain that binds to Ub and a coiled-coil domain that interacts with BRCC36. Abraxas also contains a ATM/ATR phosphorylation site (T368). Bach1 contains a helicase domain. CtIP contains a coiled-coil domain at its N-terminus that is responsible for dimerization of the protein. T847 of CtIP is phosphorylated by CDK and the phosphorylation is required for its ability to promote DNA end resection. CtIP is also phosphorylated by ATM/ATR at S644, S679 and S745. The 509-557 region of CTIP binds to DNA directly in vitro and is required for recruitment to DSB in cells [34]

Figure 3

A, B and C complex of BRCA1 contributes to BRCA1's role in cell cycle checkpoint regulation and DNA repair. In addition to forming C-terminal associated complexes, BRCA1 contains a coiled-coil domain upstream of BRCT domain, which interacts with a coiled-coil domain at the N-terminus of PALB2. PALB2 associates with BRCA2 thus bridging the interaction of BRCA1 and BRCA2 [117-119]. The N-terminus RING domain, in addition to dimerizing with BARD1 forming an E3 ligase, this region is also reported to interact with a ubiquitin hydrolase BAP1 [120,121]. Previous studies suggested that the central region of BRCA1 is also reported to interact with multiple proteins either directly or indirectly [115,116]. In addition, BRCA1 contains a S/TQ cluster that is phosphorylated by ATM/ATR at multiple sites and the phosphorylation is critical for BRCA1's role in cell cycle checkpoints regulation in the DDR [5]. BRCA1 has also been reported to be a substrate of Chk2 at S988 [122].