HEPATITIS-B VIRUS CAPSID - LOCALIZATION OF THE PUTATIVE IMMUNODOMINANT LOOP (RESIDUE-78 TO RESIDUE-83) ON THE CAPSID SURFACE, AND IMPLICATIONS FOR THE DISTINCTION BETWEEN C-ANTIGENS AND E-ANTIGENS

Hepatitis B virus capsid protein comprises a 149 residue ''assembly'' domain that polymerizes into icosahedral particles, and a 34 residue R NA-binding ''protamine'' domain. Recently, the capsid structure has be en studied to resolutions below 10 Angstrom by cryo-electron microscop y, revealing much of its alpha-helical substructure and that it appear s to have a novel fold for a capsid protein; however, the resolution i s still too low for chain-tracing by conventional criteria. Aiming to establish a fiducial marker to aid in the process of chain-tracing, we have used cryomicroscopy to pinpoint the binding site of a monoclonal antibody that recognizes the peptide from residues 78 to 83. This epi tope resides on the outer rim of the 30 Angstrom long spikes that prot rude from the capsid shell. These spikes are four-helix bundles formed by the pairing of helix-turn-helix motifs from two subunits; by means of a tilting experiment, we have determined that this bundle is right -handed. Variants of the same protein present two clinically important and non-crossreactive antigens: core antigen (HBcAg), which appears e arly in infection as assembled capsids; and the sentinel e-antigen (HB eAg), a non-particulate form. Knowledge of the binding site of our ant i-HBcAg antibody bears on the molecular basis of the distinction betwe en the two antigens, which appears to reflect conformational differenc es between the assembled and unassembled states of the capsid protein dimer, in addition to epitope masking in capsids.