Alteration in hepatitis B virus (HBV) secretion efficiency may have pathological consequences. Naturally occurring mutations that regulate virion secretion have not been defined. We recently identified HBV genomes displaying high (4B), substantially reduced (3.4), or negative (4C) virion secretion. In the present study, the underlying mutations were mapped. A T552C point mutation in the 4B genome was responsible for its enhanced virion secretion, whereas a G510A mutation in 3.4 and G660C in 4C impaired virus secretion. The three point mutations generate M133T, G119E, and R169P substitutions in the S domains of viral envelope proteins, respectively, without modifying the coding capacity of the overlapping polymerase gene. The mutated residues are predicted to lie in the luminal side of the endoplasmic reticulum (ER) or to be embedded in the ER membrane and thus are not involved in contact with core particles during envelopment. Of the two mutations inhibitory of virion secretion, G510A greatly reduced small envelope protein (hepatitis B surface antigen [HBsAg]) levels both inside cells and in culture medium, whereas G660C specifically abolished HBsAg secretion. Surprisingly, a T484G mutation in the 4B genome, generating an I110M substitution in the S domain, could also reduce HBsAg secretion and block virion secretion. However, its inhibitory effect was suppressed in the 4B genome by the T552C mutation, the enhancer of virion secretion. T552C can also override the inhibitory G510A mutation, but not the G660C mutation. These findings suggest a hierarchy in the regulation of virion secretion and a close link between defective virion secretion and impaired HBsAg formation or secretion.

The pregenomic RNA directs replication of the hepatitis B virus (HBV) genome by serving both as the messenger for core protein and polymerase and as the genome precursor following its packaging into the core particle. RNA packaging is mediated by a stem-loop structure present at its 5 end designated the signal, which includes the core gene initiator AUG. The precore RNA has a slightly extended 5 end to cover the entire precore region and, consequently, directs the translation of a precore/core protein, which is secreted as e antigen (HBeAg) following removal of precore-derived signal peptide and the carboxyl terminus. A naturally occurring G1862T mutation upstream of the core AUG affects the bulge of the signal and generates a "forbidden" residue at the ؊3 position of the signal peptide cleavage site. Transfection of this and other mutants into human hepatoma cells failed to prove their inhibition of HBeAg secretion but rather revealed great impairment of genome replication. This replication defect was associated with reduced expression of core protein and could be overcome by a G1899A covariation, or by nonsense or frameshift mutation in the precore region. All these mutations antagonized the G1862T mutation on core protein expression. Cotransfection of the G1862T mutant with a replication-deficient HBV genome that provides core protein in trans also restored genome replication. Consistent with our findings in cell culture, HBV genotype A found in African/Asian patients has T1862 and is associated with much lower viremia titers than the European subgroup of genotype A.The hepatitis B virus (HBV) primarily infects the liver and causes chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma worldwide. It is an enveloped DNA virus with a small double-stranded genome of 3.2 kb. Inside hepatocytes, viral genomic and subgenomic RNAs are transcribed from a covalently closed circular DNA template in the nucleus and exported to the cytoplasm for translation into viral proteins. The core protein assembles into the core particle, packaging both the pregenomic RNA and DNA polymerase. Subsequently, the DNA polymerase synthesizes the negative-strand DNA via reverse transcription from the RNA template, followed by RNA degradation and synthesis of positive-strand DNA. The core particle with the double-stranded DNA genome is enveloped by host-derived lipids and viral envelope proteins and secreted as an infectious virus particle (for a review, see reference 7). Since the 3.5-kb pregenomic RNA is also the mRNA for the expression of both core protein and DNA polymerase, it is the sole component required for genome replication. Consequently, increased transcription of pregenomic RNA will lead to enhanced HBV replication, as exemplified by the naturally occurring core promoter mutants (3,20).The AUG initiator of the core gene (position 1901 to 1903) is located approximately 80 nucleotides (nt) downstream of the 5Ј end of its mRNA, while the initiation codon of polymerase is 400 nt further downstream. Therefore, core pro...

Hepatitis B virus (HBV) variants with impaired expression of e antigen (HBeAg) frequently arise at the chronic stage of infection, as exemplified by precore and core promoter mutants. Since an intramolecular disulfide bond maintains the secondary structure of HBeAg, we explored effect of missense mutations of either cysteine codon. Consistent with earlier reports, substitution of each cysteine rendered HBeAg nearly undetectable. With underlying nucleotide changes at the loop of pregenome encapsidation signal, the C-7 mutants were severely impaired in pregenomic RNA packaging and hence DNA replication. Although none of the missense mutations at C61 reduced DNA replication, replacement with arginine, but not alanine, aspartic acid, phenylalanine, or serine, blocked virion secretion. Consistent with the detection of C61R genome from a patient serum, secretion block of the C61R mutant could be overcome by co-expression of wild-type core protein. In conclusion, point mutations of the C61 codon may generate viable HBeAg-negative variants.