Mapping the functional domains of BRCA1 - Interaction of the ring finger domains of BRCA1 and BARD1

Breast cancer I (BRCA1) and BRCA1-associated RING domain 1 (BARD1) are mult idomain proteins that interact in vivo via their N-terminal RING finger mot if regions. To characterize functional aspects of the BRCA1/BARD1 interacti on, we have defined the structural domains required for the interaction, as well as their oligomerization state, relative stability, and possible nucl eic acid binding activity. We have found that the RING finger motifs do not themselves constitute stable structural domains but are instead part of la rger domains comprising residues 1-109 of BRCA1 and residues 26-119 of BARD 1, These domains exist as homodimers and preferentially form a stable heter odimer. Shorter BRCA1 RING finger constructs do not interact with BARD1 or with longer BRCA1 constructs, indicating that the heterodimeric and homodim er interactions are mediated by regions outside the canonical RING finger m otif, Nucleic acid binding is a generally proposed function of RING finger domains. We show that neither the homodimers nor the heterodimer displays a ffinity for nucleic acids, indicating that the proposed roles of BRCA1 and BARD1 in DNA repair and/or transcriptional activation must be mediated eith er by other regions of the proteins or by additional cofactors.