Common and Distinct Capsid and Surface Protein Requirements for Secretion of Complete and Genome-Free Hepatitis B Virions

During the morphogenesis of hepatitis B virus (HBV), an enveloped virus, two types of virions are secreted: (i) a minor population of complete virions containing a mature nucleocapsid with the characteristic, partially double-stranded, relaxed circular DNA genome and (ii) a major population containing an empty capsid with no DNA or RNA (empty virions). Secretion of both types of virions requires interactions between the HBV capsid or core protein (HBc) and the viral surface or envelope proteins. We have studied the requirements from both HBc and envelope proteins for empty virion secretion in comparison with those for secretion of complete virions. Substitutions within the N-terminal domain of HBc that block secretion of DNA-containing virions reduced but did not prevent secretion of empty virions. The HBc C-terminal domain was not essential for empty virion secretion. Among the three viral envelope proteins, the smallest, S, alone was sufficient for empty virion secretion at a basal level. The largest protein, L, essential for complete virion secretion, was not required but could stimulate empty virion secretion. Also, substitutions in L that eliminated secretion of complete virions reduced but did not eliminate empty virion secretion. S mutations that blocked secretion of the hepatitis D virus (HDV), an HBV satellite, did not block secretion of either empty or complete HBV virions. Together, these results indicate that both common and distinct signals on empty capsids and mature nucleocapsids interact with the S and L proteins during the formation of complete and empty virions. IMPORTANCE Hepatitis B virus (HBV) is a major cause of severe liver diseases, including cirrhosis and cancer. In addition to the complete infectious virion particle, which contains an outer envelope layer and an interior capsid that, in turn, encloses a DNA genome, HBV-infected cells also secrete noninfectious, incomplete viral particles in large excess over the number of complete virions. In particular, the empty (or genome-free) virion shares with the complete virion the outer envelope and interior capsid but contains no genome. We have carried out a comparative study on the capsid and envelope requirements for the secretion of these two types of virion particles and uncovered both shared and distinct determinants on the capsid and envelope for their secretion. These results provide new information on HBV morphogenesis and have implications for efforts to develop empty HBV virions as novel biomarkers and a new generation of HBV vaccine.