Phosphorylated ABRAXAS Promotes BRCA1 Accumulation at DNA Damage Sites

Germline mutations of the tumor suppressor gene BRCA1, which is critical for the maintenance of genomic stability, frequently occur in patients with hereditary breast and ovarian cancers. The tumor suppressor function of BRCA1 is dependent upon its two tandem BRCA1 C-terminus (BRCT) domains, which bind to phosphoproteins containing a phosphorylated serine–proline–x–phenylalanine (pSPxF) motif such as ABRAXAS (encoded byFAM175A), a crucial mediator of the interaction between BRCA1 and other components of the BRCA1-A complex which recruits BRCA1 to DNA double-strand breaks (DSB) and promotes DNA repair. To define the molecular mechanism underlying the interaction between ABRAXAS and BRCA1, Wu, Paul, and colleagues solved crystal structures of BRCT in complex with ABRAXAS phosphopeptides. Ionizing radiation (IR), a major exogenous cause of DSBs, resulted in the ATM-dependent phosphorylation of the serine 404 (S404) residues close to the phosphoserine 406 (S406) containing–pSPxF motif in the C-terminal of ABRAXAS. Crystal structure analysis of BRCT in complex with single- or double-phosphorylated ABRAXAS peptides, representing pS406 and pS406/pS404 respectively, revealed that stable dimerization of the BRCT–ABRAXAS complex was dependent upon pS404. Consistent with these results, the stable dimerization of BRCT in complex with the double-phosphorylated ABRAXAS peptide was observed in solution with recombinant proteins. Similarly, BRCA1–BRCT dimerization occurred in cells in an ABRAXAS-dependent manner, and was disrupted upon mutagenesis of either F1662S or M1663K, two previously identified germline BRCA1 BRCT mutations. Moreover, IR sensitivity assays showed that pS404 is involved in radioresistance and the accumulation of BRCA1 at sites of chromatin damage. Taken together, these results elucidate the molecular mechanism underlying the phosphorylated ABRAXAS–dependent tumor suppressor activity of BRCA1 and provide structural insight into the design of small-molecule compounds that will enhance the formation of the BRCT–ABRAXAS complex.

Wu Q, Paul A, Su D, Mehmood S, Foo TK, Ochi T, et al. Structure of BRCA1-BRCT/Abraxas complex reveals phosphorylation-dependent BRCT dimerization at DNA damage sites. Mol Cell 2016 Jan 14 [Epub ahead of print].