Histone Tail Electrostatics Modulate E2-E3 Enzyme Dynamics: A Gateway to Regulate Ubiquitination Machinery

Abstract

BRCA1 (BReast Cancer-Associated protein 1), a human tumor suppressor, plays a key role in genome stability and DNA repair. Heterodimerization of BRCA1 with BARD1 is important for its stability, maximal Ub ligase (E3) activity and cooperative activation of UbcH5c (E2). Recent studies demonstrate the importance of ubiquitination of the nucleosomal H2A C-terminal tail by BRCA1/BARD1-UbcH5c (E3-E2) in which its mutations inhibit ubiquitination, predispose cells to chromosomal instability and greatly increase the likelihood of breast and ovarian cancer development. Due to the lack of molecular-level insight on the flexible and disordered H2A C-tail, its ubiquitination mechanism by BRCA1/BARD1-UbcH5c and its function and relationship to cancer susceptibility remain elusive. Here, we use molecular dynamics simulations to provide molecular-level insights into the dynamics of the less-studied H2A C-tail and BRCA1/BARD1-UbcH5c on the nucleosome surface. Our results precisely identify the key interactions and residues that trigger conformational transitions of BRCA1/BARD1-UbcH5c, and characterize the important role of histone electrostatics in their dynamics. We show that the dynamics of the H2A C-tail, combined with the highly mobile UbcH5c, define the ubiquitination capacity. Furthermore, our data demonstrate a mechanistic basis for the probability of ubiquitination of C-tail lysines in the ordered and disordered regions. Altogether, the findings of this study will provide unrevealed insights into the mechanism of H2A C-tail ubiquitination and help us understand the communication between E2-E3 enzymes and nucleosome to regulate ubiquitination machinery, paving the way for the development of effective treatments for cancer and chronic pain.