Physicochemical analysis of the hepatitis B virus core antigen produced by a baculovirus expression vector

The hepatitis B virus particle consists of an envelope carrying the surface antigen of the virus and an internal capsid consisting of the core antigen (HBcAg). The internal capsid contains the circular, partially dsDNA genome and the viral polymerase. Empty core particles have been produced in Spodoptera frugiperda cells using a recombinant baculovirus vector, YM1KTc, that expresses a 21.4K derivative of the HBcAg gene. The particles have been purified to homogeneity by caesium chloride density gradient centrifugation followed by glycerol gradient centrifugation. Physicochemical analysis of the core particles showed that they exhibited a sedimentation coefficient (s20,(0)w) of 82.5S and a diffusion coefficient (D) of 1.28 x 10(-7) cm2/s. The Mr obtained by substitution of these values in the Svedberg equation was 5.8 x 10(6), using a partial specific volume of 0.73 ml/g for the viral protein as estimated from the amino acid composition. The Mr determined from sedimentation equilibrium analyses was 6.3 x 10(6). Spectrophotometric and metabolic labelling analyses failed to detect nucleic acids in the core preparations. The data are at variance with the prediction that cores exhibit a T = 3 symmetry and contain some 180 subunits. The results suggest that the baculovirus-expressed cores may contain up to 300 subunits of HBcAg protein.