Membrane Structure of the Hepatitis B Virus Surface

Expression of S protein, an envelope protein of hepatitis B virus, in the absence of other viral proteins, leads to the secretion of hepatitis B virus surface antigen (HBsAg) parti-cles that are formed by budding from the endoplasmic reticulum membranes. The HBsAg particles produced by mouse fibroblast cells show a unique lipid composition, with 1,2-diacyl glycerophosphocholine being the dominant component The lipid organi-zation of the HBsAg particles was studied by measuring electron spin resonance (ESR) using various spin-labeled fatty acids, and the results were compared with a parallel study on HVJ (Sendai virus) and vesicles reconstituted with total lipids of the HBsAg particles (HBs-lipid vesicles). HVJ and the KBs-lipid vesicles showed typical ESR spec-tra of lipids arranged in a lipid bilayer structure. In contrast, the ESR spectra obtained with the HBsAg particles showed that the movement of lipids in the particle is severely restricted and a typical immobilized signal characteristic of tight lipid-protein interac-tions was also evident. Phosphatidylcholine (PC) in the HBsAg particles was not ex-changeable by a PC-specific exchange protein purified from bovine liver, while phospho-lipase Aj from Naja naja vemon was able to hydrolyze all the PC in the particles. Thes