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

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

Hepatitis B virus uses e antigen (HBe), which is dispensable for virus infectivity, to modulate host immune responses and achieve viral persistence in human hepatocytes. The HBe precursor (p25) is directed to the endoplasmic reticulum (ER), where cleavage of the signal peptide (sp) gives rise to the first processing product, p22. P22 can be retro-translocated back to the cytosol or enter the secretory pathway and undergo a second cleavage event, resulting in secreted p17 (HBe). Here, we report that translocation of p25 to the ER is promoted by translocon-associated protein complex. We have found that p25 is not completely translocated into the ER; a fraction of p25 is phosphorylated and remains in the cytoplasm and nucleus. Within the p25 sp sequence, we have identified three cysteine residues that control the efficiency of sp cleavage and contribute to proper subcellular distribution of the precore pool.

HBV-related hepatocellular carcinoma: the role of integration, viral proteins and miRNA

The development of hepatocellular carcinoma during chronic hepatitis B infection is a multifactorial process thought to be a consequence of several direct and indirect mechanisms. In this review we discuss how viral proteins and cycles of ongoing liver damage and regeneration, coupled with HBV DNA integration and aberrant miRNA expression may enhance the risk for the development of hepatocellular carcinoma.

Proteolytic processing of the hepatitis B virus e antigen precursor. Cleavage at two furin consensus sequences

The Hepatitis B virus P22 protein is a nonstructural protein that is the precursor of the 17-kDa secreted e antigen (HBeAg). The mature HBeAg is obtained after the removal of the C-terminal region of P22, a process which involves a proprotein convertase. Our studies show first that the protease could cleave P22 at the C-terminal side of Arg(167) or Arg(154) and second, that the maturation process can be either done in one step or in two steps with the generation of a processing intermediate (P20). Our data also demonstrate that the removal of the P22 C terminus, which occurs mainly in the trans-Golgi network, can also be achieved after exocytosis. Keeping in mind this characteristic and the amino acid sequence of the cleavage sites, we concluded that furin is involved in the maturation of the HBeAg. In addition, we show that in our experimental system, the HBeAg is a 164-amino acid protein and not a 159-amino acid protein as previously reported.

Overexpression and purification of the hepatitis B e antigen precursor

Circumstantial evidence suggests that the secreted hepatitis B virus (HBV) e antigen (HBeAg) and/or its 22 kDa precursor (P22) have an essential role in the establishment of persistent infection. In order to identify cellular proteins that could interact with P22, large amounts of this protein are required to perform pull-down assays. A plasmid was constructed encoding a recombinant P22 with a Histidine-tag at its N-terminal extremity (P22r). The initial attempts to overexpress P22r in a conventional Escherichia coli strain failed, most likely due to the presence of rare AGA/AGG codon clusters in the 3' part of the gene. To overcome this difficulty, P22r was overexpressed in the Epicurian coli BL21-codonplus (DE3)-RIL strain, which possesses extra copies of the ArgU gene that encodes the tRNA(AGA/AGG). In this strain, P22r was overexpressed successfully and then purified in milligram quantities by metal affinity chromatography on Ni2+-chelated His-Bind resin. The purified recombinant protein P22r was able to interact with a cellular protein (P32), which had previously been shown to co-immunoprecipitate with native P22, indicating that at least some of the P22r molecules were folded correctly.

Demonstration of the presence of protease-cutting site in the spacer of hepatitis B viral Pol protein

Molecular genetic studies have revealed that the human hepatitis B viral (HBV) Pol protein, a polypeptide of about 94 kDa, contains four domains. These are the 5'-terminal protein, spacer, RNA reverse transcriptase/DNA polymerase, and RNase H, respectively, from the amino (N) to carboxy (C) terminus. No evidence indicates as yet the involvement of a specific protease in cleaving the Pol protein or the presence of protease-cutting sites in the Pol protein. An in vitro-translated Pol protein was shown to be cleaved by purified thrombin but not in the presence of its inhibitor, hirudin. Two thrombin-cutting sites, spanning 194 amino acids, were then deduced by thrombin digestion of Pol protein with various lengths of C-terminal deletion. These two putative cutting sites, one located in the spacer region and the other in the beginning of the polymerase region, were found to be conserved at similar positions in the Pol protein of all hepadnaviruses. By using a novel method called the LacZ localization assay (LLA), it was demonstrated that a tripartite fusion protein containing the nucleus localization sequence (NLS) of SV40 large T Ag, the putative thrombin cutting sequence (Ile-Arg-Ile-Pro-Arg320-Thr) of HBV Pol protein and the full length beta-galactosidase of E. coli, exhibited a lower percentage (approximately 53%) of targeting into the nucleus of transfected hepatoma cells when compared with a similar tripartite protein containing a single mutation (Arg320 residue into Trp320) of HBV Pol protein (approximately 78%) or with a bipartite protein of SV40 NLS-beta-galactosidase (approximately 90%). These results indicate that the putative thrombin-cutting site in the spacer region of HBV Pol protein could be cleaved by a cellular protease resulting in the separation of NLS sequence from the beta-galactosidase and rendering a lower frequency of X-gal staining in the nucleus.

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

We have analyzed the translocation of hepatitis B virus (HBV) precore (PC) proteins by using Xenopus oocytes injected with a synthetic PC mRNA. The PC region is a 29-amino-acid sequence that precedes the 21.5-kDa HBV capsid or core (C) protein (p21.5) and directs the secretion of core-related proteins. The first 19 PC amino acids provide a signal peptide that is cleaved with the resultant translocation of a 22.5-kDa species (p22.5), in which the last 10 PC residues precede the complete p21.5 C polypeptide. Most p22.5 is matured to 16-20 kDa species by carboxyl-terminal proteolytic cleavage prior to secretion. Here we show that some four unexpected PC proteins of 24 to 25 kDa are produced in addition to the secretion products described above. Protease protection and membrane cosedimentation experiments reveal that all PC proteins behave as expected for proteins that are translocated into the lumen of the endoplasmic reticulum except for the single largest PC protein (p25), which is not translocated. Like p21.5, p25 is a phosphoprotein that localizes to the oocyte cytosol and nucleus, and protease digestion studies suggest that the two molecules have similar two-domain structures. Radiosequencing of immobilized p25 demonstrates that it contains the intact PC signal peptide and represents the unprocessed translation product of the entire PC/C locus. Thus, while many HBV PC protein molecules are correctly targeted to intracellular membranes and translocated, a significant fraction of these molecules can evade translocation and processing.

How do viral reverse transcriptases recognize their RNA genome?

Reverse transcription is not solely a retroviral mechanism. Hepadnaviruses and caulimoviruses have RNA intermediates that are reverse transcribed into DNA. Moreover non-viral retroelements, retrotransposons, use reverse transcription in their transposition. All these retroelements encode reverse transcriptase but each group developed their own expression modes capable of assuring a specific and efficient replication of their genomes.