Low-Level Secretion of Human Hepatitis B Virus Virions Caused by Two Independent, Naturally Occurring Mutations (P5T and L60V) in the Capsid Protein

Abstract

The functional significance of naturally occurring variants of human hepatitis B virus (HBV) remains largely unknown. Previously, we reported an immature secretion phenotype caused by a highly frequent mutation at amino acid 97 of the HBV core (capsid) protein (HBcAg). This phenotype is characterized by a nonselective and excessive secretion of virions containing an immature genome of single-stranded viral DNA. To extend our study of virion secretion to other naturally occurring variants, we have characterized mutations at HBcAg codons 5, 38, and 60 via site-directed mutagenesis. Although the phenotype of the mutation at codon 38 is nearly identical to that for the wild-type virus, our study reveals that a single mutation at codon 5 or 60 exhibits a new extracellular phenotype with significantly reduced virion secretion yet maintains normal intracellular viral DNA replication. A complementation study indicates that the mutant core protein alone is sufficient for the "lowsecretion" 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 addition to the known envelope proteins. Our present results suggest that a very precise structure of both ␣-helical and nonhelical loop regions of the entire HBcAg molecule is important for virion secretion. The low-secretion variants may contribute to the phenomenon of gradually decreasing viremia in chronic carriers during the late phase of persistent infection.Hepatitis B virus (HBV) is an important human infectious agent. Worldwide, at least 300 million individuals are chronic HBV carriers. Chronic infection with HBV leads to the development of cirrhosis and liver cancer (31, 32). Naturally occurring variants have been found frequently in chronic HBV carriers. The potential role of these variants in chronicity and pathogenesis remains unclear, since no universal assay is available to evaluate these diverse HBV variants. For example, naturally occurring variants of HBV core antigen (HBcAg) have been frequently reported. This 183-amino-acid protein has multiple functions, including interaction with the pregenomic RNA and polymerase during encapsidation, polymerization with itself to form the nucleocapsid, import of relaxed circular DNA to the nucleus, and targeting to the endoplasmic reticulum for envelope formation (3,7,16,25,26,37). In addition, HBcAg is structurally related to the secreted e antigen (27), and thus certain mutations in HBcAg should also be present in the e antigen. Because of the highly versatile nature of the HBV core protein, it is difficult to predict which functional assays should be used for the study of HBcAg variants in order to yield more informative results. Therefore, the biological significance of these mutations remains unclear.Previously, we described several missense mutations in HBcAg which coincide with major histocompatibili...

Thermodynamic origins of protein folding, allostery, and capsid formation in the human hepatitis B virus core protein

“…Two of the mutants reported here (L60V and F97L) are reported phenotypic mutants (54). L60V stabilized N greatly but is capsid assembly-competent (Table S1), suggesting HBc Ass and HBc Inc may be stabilized by similar amounts.…”

“…F97L enhances HBV capsid assembly and causes premature virion secretion (25,(51)(52)(53)). L60V causes low-level virion secretion in vivo (53,54). S87G eliminates a phosphorylation site and is associated with lower levels of capsid formation than phosphorylated WT HBc (53,55).…”

A Small Molecule Inhibits and Misdirects Assembly of Hepatitis B Virus Capsids

“…In HBV, the dominant form of capsid (25) is composed of 120 copies of the capsid protein dimer (6,10,29). Though assembly is robust in vitro (3,23,29,35), even modest mutations of the capsid protein can have dramatic effects on the viability of progeny virus (12,18,19,30,31). This suggests that capsid assembly could be an effective therapeutic target for chronic HBV.…”

Reverse transcription-associated dephosphorylation of hepadnavirus nucleocapsids

“…Its N-terminal domain is sufficient for assembly of the capsid shell (10-13), whereas its extremely basic C-terminal domain (CTD) can bind nucleic acid and is essential for packaging pgRNA into NCs and for progression of reverse transcription within these NCs (6,12,[14][15][16][17]. Mutations in the HBV core protein can, indeed, affect NC envelopment and secretion (18)(19)(20)(21)(22). Also, the CTD contains three S-P motifs (plus one T-P motif for DHBV) that have so far been identified as major phosphorylation sites (23, 24), mutations to which affect various stages of the viral lifecycle, including pgRNA packaging and DNA synthesis (16,17,23,(25)(26)(27)(28).…”