Hepatitis B virus core antigen: enhancement of its production in Escherichia coli, and interaction of the core particles with the viral surface antigen.

The core antigen (HBcAg) of hepatitis B Virus (HBV) can be expressed in Escherichia coil where it assembles into icosahedral particles containing 240 or 180 subunits. Analysis of the two kinds of particles by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) showed that a substantial proportion of their subunits were smaller than the full-length HBcAg monomer and of variable size, but all had the same N-terminal sequence showing that the smaller species were heterogeneous in their arginine-rich C-terminal regions. Around 50% of these arginine residues are encoded by the triplet AGA which is rare in E. coli. Supplementation of the level of AGA tRNA in the cell by transformation with plasmids expressing the T4 AGA tRNA gene significantly enhanced the yield of HBcAg. Fusion phage carrying a ligand specific for HBcAg showed no significant difference in the affinity for the two sizes of HBcAg particles, but in similar reactions in solution HBV surface antigen exhibited differential affinities for the same two HBcAg preparations.