DIMORPHISM OF HEPATITIS-B VIRUS CAPSIDS IS STRONGLY INFLUENCED BY THEC-TERMINUS OF THE CAPSID PROTEIN

Hepatitis B virus (HBV) is an enveloped virus with an icosahedral caps id. Its homodimeric capsid protein (''core antigen'') assembles into p articles of two sizes, one with T = 3 icosahedral symmetry (90 dimers) and the other with T = 4 symmetry (120 dimers). We have investigated this assembly process in vitro, using a variety of purified, bacterial ly expressed, capsid proteins. All of our constructs lacked the predom inantly basic C-terminal 34 amino acids of the full-length capsid prot ein (183 amino acids) and were further truncated to terminate at speci fic points between residues 138 and 149. While the smallest construct (138 residues) did not assemble into capsids, those terminating at res idue 140, and beyond, assembled into mixtures of T = 3 and T = 4 parti cles. The two kinds of capsids could be separated on sucrose gradients and did not interconvert upon protracted storage. The proportion of T = 3 capsids, assayed by sucrose gradient fractionation, analytical ul tracentrifugation and cryoelectron microscopy, was found to increase s ystematically with larger deletions from the C-terminus. The variant t erminating at residue 149 formed similar to 5% of T = 3 capsids, while the 140-residue protein produced similar to 85% of this isomorph. For the 147-residue capsid protein, the structures of both capsids were d etermined to 17 Angstrom resolution by three-dimensional reconstructio n of cryoelectron micrographs. In these density maps, the boundaries o f the constituent dimers can be clearly seen and the quaternary struct ures of the two capsids compared. The arrangement of dimers around the ir icosahedral five-fold axes is almost identical, whereas the quasi-s ix-fold arrangements of dimers are distinctly different.