Biosynthesis of hepatitis B virus e antigen: directed mutagenesis of the putative aspartyl protease site

Abstract

The C gene products of all mammalian hepadnaviruses contain a region with sequence similarities to the catalytic center of the aspartyl proteases. This region could have the capacity to cleave precore proteins, leading to the synthesis of e antigen. By site-directed mutagenesis on a plasmid containing the hepatitis B virus C gene, we have replaced either the Asp residue of the putative aspartyl protease catalytic center or an Asp residue located 3 amino acids upstream. Transient expression of the mutated hepatitis B virus C gene in human and mouse cells showed that none of these mutations prevented the secretion of an accurately processed HBe antigen. Thus, we demonstrated that the aspartyl protease responsible for e antigen precursor processing is not C gene encoded but is more likely to be a cellular enzyme. From these results, we suggest a model for the mechanism of e antigen synthesis.

Hepatitis B virus p25 precore protein accumulates in Xenopus oocytes as an untranslocated phosphoprotein with an uncleaved signal peptide

“…Although cleavage of the PC signal peptide is usually efficient in transfected animal cells, there have been several reports of uncharacterized -25-kDa PC species (11,12,13,32,33). We speculate that high-level PC protein expression may again be responsible for the phenomenon of p25 accumulation in these cases.…”

“…This simple HBeAg secretion model does not explain the fact that uncharacterized PC species of -25 kDa have been detected sporadically in oocytes (34) and in mammalian cells (12,13,32,33), nor does it explain why PC proteins have been detected in such unexpected locations as the cytosol (9), the nucleus (24), stuck in ER membranes (1), or in the plasma membrane (4,14,31). Such mislocation of PC * Corresponding author.…”

Biogenesis of hepatitis B virus e antigen is driven by translocon‐associated protein complex and regulated by conserved cysteine residues within its signal peptide sequence

“…The majority of p22 is further processed at its C-terminus by furin protease in the trans-Golgi network, giving rise to mature HBe antigen (p17). The mature antigen is secreted [11][12][13][14] and performs an immunomodulatory function in the establishment of persistent infection [15][16][17][18]. Interestingly, a certain portion of the precore protein is secreted with sp still attached as PreC protein, but the biogenesis and function of this protein have yet to be elucidated [19][20][21].…”

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“…However, the analogies stop here as the hepadnaviruses have neither zinc finger-like protein, nor integrase nor aspartic protease [27,28]. Moreover, it has been demonstrated that the synthesis and the encapsidation of hepadnavirus reverse transcriptase do not require formation of capsid-polymerase fusion proteins [29,30].…”