Crystal Structure of the BARD1 BRCT Domains,

Abstract

The interaction of the breast tumor suppressor BRCA1 with the protein BARD1 results in the formation of a heterodimeric complex that has ubiquitin ligase activity and plays central roles in cell cycle checkpoint control and DNA repair. Both BRCA1 and BARD1 possess a pair of tandem BRCT domains that interact in a phosphorylation-dependent manner with target proteins. We determined the crystal structure of the human BARD1 BRCT repeats (residues 568−777) at 1.9 Å resolution. The composition and structure of the BARD1 phosphoserine-binding pocket P₁ are strikingly similar to those of the BRCA1 and MDC1 BRCT domains, suggesting a similar mode of interaction with the phosphate group of the ligand. By contrast, the BARD1 BRCT selectivity pocket P₂ exhibits distinct structural features, including two prominent histidine residues, His685 and His686, which may be important for ligand binding. The protonation state of these histidines has a marked effect on the calculated electrostatic potential in the vicinity of P₂, raising the possibility that ligand recognition may be regulated by changes in pH. Importantly, the BARD1 BRCT structure provides insights into the mechanisms by which the cancer-associated missense mutations C645R, V695L, and S761N may adversely affect the structure and function of BARD1.