Breast cancer-associated gene 1 (BRCA1) regulates the duplication and the function of centrosomes in breast cells. We have previously shown that BRCA1 ubiquitin ligase activity directly inhibits centrosome-dependent microtubule nucleation. However, there is a paradox because centrosome microtubule nucleation potential is highest during mitosis, a phase when BRCA1 is most abundant at the centrosome. In this study, we resolve this conundrum by testing whether centrosomes from cells in M phase are regulated differently by BRCA1 when compared with other phases of the cell cycle. We observed that BRCA1-dependent inhibition of centrosome microtubule nucleation was high in S phase but was significantly lower during M phase. The cell cycle-specific effects of BRCA1 on centrosome-dependent microtubule nucleation were detected in living cells and in cell-free experiments using centrosomes purified from cells at specific stages of the cell cycle. We show that Aurora-A kinase modulates the BRCA1 inhibition of centrosome function by decreasing the E3 ubiquitin ligase activity of BRCA1. In addition, dephosphorylation of BRCA1 by protein phosphatase 1α enhances the E3 ubiquitin ligase activity of BRCA1. These observations reveal that the inhibition of centrosome microtubule nucleation potential by the BRCA1 E3 ubiquitin ligase is controlled by Aurora-A kinase and protein phosphatase 1α-mediated phosphoregulation through the different phases of the cell cycle. ©2007 American Association for Cancer Research.