Breast cancer type-1 susceptibility protein (BRCA1) belongs to the family of tumor suppressor genes that is normally expressed in cells of the breast and other tissues. It helps repair damaged DNA and protects cells from turning into cancer. BRCA1 and its obligatory partner known as BARD1 (BRCA1-associated RING domain protein 1) form a heterodimeric complex (BRCA1/BARD1) which is important for their mutual stability, and correct nuclear localization. BRCA1/BARD1 RING domain, which comprises the largest class of recognized ubiquitin (Ub) ligases (E3), bounds to its cellular target (the nucleosome) with the BRCA1-Ub-conjugating enzyme (E2) UbcH5c, and promotes the Ub transfer reaction. Recently, nucleosomal histone H2A was discovered to act as a substrate for BRCA1/BARD1-dependent E3 ligase activity. BRCA1/BARD1-mediated H2A ubiquitylation is essential for its role as a tumor suppressor and plays a critical role in nearly all DNA-related processes such as DNA replication, transcription, and repair. Although the available data highlight the importance of H2A ubiquitylation by the BRCA1/BARD1 heterodimer, the mechanism of ubiquitylation is not yet fully understood. In this study, we performed molecular dynamics (MD) simulations on the BRCA1/UbcH5c/BARD1/nucleosome complex to provide molecular-level insight into the mechanism of H2A ubiquitylation by BRCA1/BARD1. The interaction between H2A histone protein and the E2 enzyme UbcH5c was studied primarily focusing on the lysine residues in the less studied, flexible, and disordered C-terminal tail. The residues that stand close to the active site of E2~Ub were identified, and their approach mechanism and conjugation to E2 were explained. Furthermore, the effect of mutations on Ub-ligase activity was investigated. The findings of this research will help demonstrate the important interactions between the BRCA1/UbcH5c/BARD1 complex and the nucleosomal H2A at a molecular level and help understand the role of H2A residues in ubiquitylation.

Tumor suppressor BRCA1/BARD1 interactions with nucleosomal histone H2A

ACS Spring 2022

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