The RING heterodimer BRCA1-BARD1 is a ubiquitin ligase inactivated by a breast cancer-derived mutation

BRCA1-BARD1 constitutes a heterodimeric RING finger complex associated thro ugh its N-terminal regions. Here we demonstrate that the BRCA1-BARD1 hetero dimeric RING finger complex contains significant ubiquitin ligase activity that can be disrupted by a breast cancer-derived RING finger mutation in BR CA1, Whereas individually BRCA1 and BARD1 have very low ubiquitin ligase ac tivities in vitro, BRCA1 combined with BARD1 exhibits dramatically higher a ctivity. Bacterially purified RING finger domains comprising residues 1-304 of BRCA1 and residues 25-189 of BARD1 are capable of polymerizing ubiquiti n, The steady-state level of transfected BRCA1 in vivo was increased by co- transfection of BARD1, and reciprocally that of transfected BARD1 was incre ased by BRCA1 in a dose-dependent manner. The breast cancer-derived BARD1 i nteraction-deficient mutant, BRCA1(C61G), does not exhibit ubiquitin ligase activity in vitro. These results suggest that the BRCA1-BARD1 complex cont ains a ubiquitin ligase activity that is important in prevention of breast and ovarian cancer development.