Structural basis of the oligomerization of hepatitis delta antigen

Background: The hepatitis D virus (HDV) is a small satellite virus of hepatitis B virus (HBV). Coinfection with HBV and HDV causes severe liver disease in humans. The small 195 amino-acid form of the hepatitis delta antigen (HDAg) functions as a trans activator of HDV replication. A larger form of the protein containing a 19 amino acid C-terminal extension inhibits viral replication. Both of these functions are mediated in part by a stretch of amino acids predicted to form a coiled coil (residues 13-48) that is common to both forms. It is believed that HDAg forms dimers and higher ordered structures through this coiled-coil region. Results: The high-resolution crystal structure of a synthetic peptide corresponding to residues 12 to 60 of HDAg has been solved. The peptide forms an antiparallel coiled coil, with hydrophobic residues near the termini of each peptide forming an extensive hydrophobic core with residues C-terminal to the coiled-coil domain in the dimer protein. The structure shows how HDAg forms dimers, but also shows the dimers forming an octamer that forms a 50 Angstrom ring lined with basic sidechains. This is confirmed by cross-linking studies of full-length recombinant small HDAg. Conclusions: HDAg dimerizes through an antiparallel coiled coil. Dimers then associate further to form octamers through residues in the coiled-coil domain and residues C-terminal to this region. Our findings suggest that the structure of HDAg represents a previously unseen organization of a nucleocapsid protein and raise the possibility that the N terminus may play a role in binding the viral RNA.