BRCA1 Phosphorylation by Aurora-A in the Regulation of G₂ to M Transition

Aurora-A/BTAK/STK15 localizes to the centrosome in the G₂-M phase, and its kinase activity regulates the G₂ to M transition of the cell cycle. Previous studies have shown that the BRCA1 breast cancer tumor suppressor also localizes to the centrosome and that BRCA1 inactivation results in loss of the G₂-M checkpoint. We demonstrate here that Aurora-A physically binds to and phosphorylates BRCA1. Biochemical analysis showed that BRCA1 amino acids 1314-1863 binds to Aurora-A. Site-directed mutagenesis indicated that Ser³⁰⁸ of BRCA1 is phosphorylated by Aurora-A in vitro. Antiphospho-specific antibodies against Ser³⁰⁸ of BRCA1 demonstrated that Ser³⁰⁸ is phosphorylated in vivo. Phosphorylation of^Ser308 increased in the early M phase when Aurora-A activity also increases; these effects could be abolished by ionizing radiation. Consistent with these observations, acute loss of Aurora-A by small interfering RNA resulted in reduced phosphorylation of BRCA1 Ser³⁰⁸, and transient infection of adenovirus Aurora-A increased Ser³⁰⁸ phosphorylation. Mutation of a single phosphorylation site of BRCA1 (S308N), when expressed in BRCA1-deficient mouse embryo fibroblasts, decreased the number of cells in the M phase to a degree similar to that with wild type BRCA1-mediated G₂ arrest induced by DNA damage. We propose that BRCA1 phosphorylation by Aurora-A plays a role in G₂ to M transition of cell cycle.