EVOLUTIONARY CONSERVATION IN THE HEPATITIS-B VIRUS CORE STRUCTURE - COMPARISON OF HUMAN AND DUCK CORES

Background: Hepatitis B virus is a major human pathogen which has been extensively studied, yet its structure is unknown. Cryo-electron micr oscopy of the viral cores expressed in Escherichia roll or isolated fr om infected liver provides a means for determining the structure of th e hepatitis B nucleocapsid. Results: Using cryo-electron microscopy an d three-dimensional image reconstruction, we have determined the struc tures of duck and human hepatitis B virus cores and find that they hav e similar dimer-clustered T=3 and T=4 icosahedral organizations. The d uck virus core protein sequence differs from the human in both length and amino acid content; however, the only significant structural diffe rences observed are the lobes of density on the lateral edges of the p rojecting (distal) domain of the core protein dimer. The different cor es contain varying amounts of nucleic acid, but exhibit similar contac ts between the core protein and the nucleic acid. Immunoelectron micro scopy of intact cores has localized two epitopes on the core surface c orresponding to residues 76-84 and 12.9-132. Conclusions: The bacteria l expression system faithfully reproduces the native hepatitis B virus core structure even in the absence of the complete viral genome. This confirms that proper assembly of the core is independent of genome pa ckaging. Difference imaging and antibody binding map three sequence po sitions in the structure: the C terminus and the regions near amino ac ids 80 and 130. Finally, we suggest that the genome-core interactions and the base (proximal) domain of the core dimer are evolutionarily co nserved whereas the projecting domain, which interacts with the envelo pe proteins, is more variable.