VISUALIZATION OF A 4-HELIX BUNDLE IN THE HEPATITIS-B VIRUS CAPSID BY CRYOELECTRON MICROSCOPY

Despite the development of vaccines, the hepatitis B virus remains a m ajor cause of human liver disease(1). The virion consists of a lipopro tein envelope surrounding an icosahedral capsid composed of dimers of a 183-residue protein, 'core antigen' (HBcAg)(2). Knowledge of its str ucture is important for the design of antiviral drugs, but it has yet to be determined. Residues 150-183 are known to form a protamine-like domain required for packaging RNA, and residues 1-149 form the 'assemb ly domain' that polymerizes into capsids(2) and, unusually for a capsi d protein, is highly alpha-helical(3). Density maps calculated from cr yo-electron micrographs(4-6) show that the assembly domain dimer is T- shaped: its stem constitutes the dimer interface and the tips of its a rms make the polymerization contacts. By refining the procedures used to calculate the map, we have extended the resolution to 9 Angstrom, r evealing major elements of secondary structure. In particular, the ste m, which protrudes as a spike on the capsid's outer surface, is a 4-he lix bundle, formed by the pairing of alpha-helical hairpins from both subunits.