S-HDAg-K72ac peptide weakly binds host BAZ2 bromodomains via inverted orientation, revealing HDV replication.
Background
Hepatitis delta virus (HDV) is a satellite RNA virus dependent on hepatitis B virus (HBV) for propagation, yet it replicates its genome independently within the nucleus. The small form of the hepatitis delta antigen (S-HDAg) is essential for this replication, with its activity regulated by post-translational modifications. Specifically, acetylation at lysine 72 (K72ac) enables S-HDAg to interact with the bromodomain (BRD) of the host chromatin remodeler BAZ2B, promoting viral replication. However, the precise structural basis for this crucial interaction has remained elusive, representing a significant gap in understanding viral pathogenesis.
Study Design
Researchers employed isothermal titration calorimetry to quantitatively assess the binding affinities between the bromodomains (BRDs) of host BAZ2B and its close homolog BAZ2A and the viral S-HDAg-K72ac peptide. Subsequently, X-ray crystallography was utilized to determine the high-resolution crystal structure of the BAZ2A-BRD in complex with the S-HDAg-K72ac peptide. To validate the identified interaction sites, targeted mutagenesis studies were conducted, confirming the critical binding interface both in vitro and within cellular environments.
Results
Isothermal titration calorimetryrevealed that the bromodomains ofBAZ2BandBAZ2Abind to the viral S-HDAg-K72ac peptide weakly, withBAZ2A-BRDexhibiting a modestly higher affinity compared toBAZ2B-BRD.
Key Findings
- Host
BAZ2B-BRDandBAZ2A-BRDbind the viral S-HDAg-K72ac peptide weakly. BAZ2A-BRDexhibited a modestly higher affinity for the S-HDAg-K72ac peptide.- The crystal structure of
BAZ2A-BRDwith S-HDAg-K72ac showed an inverted binding orientation relative to canonical histone ligands. - This inverted binding orientation rationalizes the observed weak interaction.
- Mutagenesis studies confirmed the critical binding interface both
in vitroandin cells.
Why It Matters
Understanding the unique, weak-affinity interaction between S-HDAg and host BAZ2 bromodomains provides a novel structural framework for developing antiviral strategies against Hepatitis Delta Virus. This elucidation of viral histone mimicry offers a specific target for small molecule inhibitors designed to disrupt HDV replication by preventing the virus from co-opting host chromatin machinery. While not immediately translatable to a clinical protocol, these structural insights are crucial for rational drug design, potentially leading to new therapeutic avenues for chronic HDV infection, which currently has limited treatment options beyond entry inhibitors like bulevirtide. This mechanism-based approach could complement existing therapies.
hepatitis delta virus
hdv
s-hdag
baz2b
baz2a
viral replication