Low-level secretion of human hepatitis B virus virions caused by two independent, naturally occurring mutations (P5T and L60V) in the capsid protein

The functional significance of naturally occurring variants of human hepati tis B virus (HEV) remains largely unkown. Previously, we reported an immatu re secretion phenotype caused by a highly frequent mutation at amino acid 9 7 of the HBV core (capsid) protein (HBcAg), This phenotype is characterized by a nonselective and excessive secretion of virions containing an immatur e genome of single-stranded viral DNA, To extend our study of virion secret ion to other naturally occurring variants, we have characterized mutations at HBcAg codons 5, 38, and 60 via site-directed mutagenesis. Although the p henotype of the mutation at codon 38 is nearly identical to that for the wi ld-type virus, our study reveals that a single mutation at codon 5 or 60 ex hibits a new extracellular phenotype,vith significantly reduced virion secr etion get maintains normal intracellular viral DNA replication A complement ation study indicates that the mutant core protein alone is sufficient for the "low-secretion" phenotype. Furthermore, the low-secretion phenotype of the codon 5 mutant appears to be induced by the loss of a parental proline residue, rather than by the gain of a new amino acid. Our study underscores the core protein as another crucial determinant in virion secretion, in ad dition to the known envelope proteins. Our present results suggest that a v ery precise structure of both alpha-helical and nonhelical loop regions of the entire HBcAg molecule is important for virion secretion. The low-secret ion variants may contribute to the phenomenon of gradually decreasing virem ia in chronic carriers during the late phase of persistent infection.