Expression of processed envelope protein of hepatitis C virus in mammalian and insect cells

Production and immunogenicity of different prophylactic vaccines for hepatitis C virus (Review)

Hepatitis C virus (HCV) infection is a global health challenge, and prophylactic vaccines are the most effective way to eliminate the infection. To date, numerous forms of preventive vaccines have entered the clinical trial stage, including the virus-like particle (VLP) vaccine, recombinant subunit vaccine, peptide vaccine and nucleic acid vaccine. The rational design makes it easier to obtain specific vaccine structures with a broad spectrum and strong immunogenicity. Different vaccine antigens can evoke different immune responses, including humoral and T-cell immune responses, and can be produced using different expression systems, such as bacteria, yeast, mammals, plants, insects or parasites. Intracellular and insoluble production and a narrow immune spectrum are two difficulties that limit the application of vaccines. The present study summarizes the immunogenicity of different preventive vaccines, evaluates the characteristics of different expression systems used for vaccine production, and analyzes the strategies to enhance the secretion and immune spectrum of vaccine proteins.

Molecular Targets in Inhibition of Hepatitis C Virus Replication

Hepatitis C virus (HCV) is the major cause of transfusion-associated hepatitis and accounts for a significant proportion of hepatitis cases worldwide. Most, if not all, infections become persistent and about 60% of cases develop chronic liver disease with various outcomes ranging from an asymptomatic carrier state to chronic active hepatitis and liver cirrhosis, which is strongly associated with the development of hepatocellular carcinoma. Since the initial cloning of the viral genome in 1989, our knowledge of the molecular biology of HCV has increased rapidly and led to the identification of several potential targets for antiviral intervention. In contrast, the low replication of the virus in cell culture, the lack of convenient animal models and the high genome variability present major challenges for drug development. This review will describe candidate drug targets and summarize ‘classical’ and ‘novel’ approaches currently being pursued to develop efficient HCV-specific therapies.

Specific interaction of the envelope glycoproteins E1 and E2 with liver heparan sulfate involved in the tissue tropismatic infection by hepatitis C virus

The first step in the process of infections by the hepatitis C virus (HCV) is attachment to the host cell, which is assumed to be mediated by interaction of the envelope glycoproteins E1 and E2 with cell surface glycosaminoglycans. In this study, a variety of glycosaminoglycans, heparan sulfate (HS) from various bovine tissues as well as chondroitin sulfate (CS)/dermatan sulfate from bovine liver, were used to examine the direct interaction with recombinant E1 and E2 proteins. Intriguingly, among HS preparations from various bovine tissues, only liver HS strongly bound to both E1 and E2. Since HS from liver, which is the target tissue of HCV, contains highly sulfated structures compared to HS from other tissues, the present results suggest that HS-proteoglycan on the liver cell surface appears to be one of the molecules that define the liver-specific tissue tropism of HCV infection. The interaction assay with chemically modified heparin derivatives provided evidence that the binding of the viral proteins to heparin/HS is not only mediated by simple ionic interactions, but that the 6-O-sulfation and N-sulfation are important. Heparin oligosaccharides equal to or larger than 10-mer were required to inhibit the binding. Notably, a highly sulfated CS-E preparation from squid cartilage also strongly interacted with both viral proteins and inhibited the entry of pseudotype HCV into the target cells, suggesting that the highly sulfated CS-E might be useful as an anti-HCV drug.

Factors affecting recombinant Western equine encephalitis virus glycoprotein production in the baculovirus system

In an effort to produce processed, soluble Western equine encephalitis virus (WEEV) glycoproteins for subunit therapeutic vaccine studies, we isolated twelve recombinant baculoviruses designed to express four different WEEV glycoprotein constructs under the transcriptional control of three temporally distinct baculovirus promoters. The WEEV glycoprotein constructs encoded full-length E1, the E1 ectodomain, an E26KE1 polyprotein precursor, and an artificial, secretable E2E1 chimera. The three different promoters induced gene expression during the immediate early (ie1), late (p6.9), and very late (polh) phases of baculovirus infection. Protein expression studies showed that the nature of the WEEV construct and the timing of expression both influenced the quantity and quality of recombinant glycoprotein produced. The full-length E1 product was insoluble, irrespective of the timing of expression. Each of the other three constructs yielded soluble products and, in these cases, the timing of expression was important, as higher protein processing efficiencies were generally obtained at earlier times of infection. However, immediate early expression did not yield detectable levels of every WEEV product, and expression during the late (p6.9) or very late (polh) phases of infection provided equal or higher amounts of processed, soluble product. Thus, while earlier foreign gene expression can provide higher recombinant glycoprotein processing efficiencies in the baculovirus system, in the case of the WEEV glycoproteins, earlier expression did not provide larger amounts of high quality, soluble recombinant glycoprotein product.

Effects of hepatitis C virus envelope glycoprotein unfolded protein response activation on translation and transcription

The hepatitis C virus (HCV) envelope glycoproteins have been shown to cause ER stress and induce the unfolded protein response (UPR). Using a bicistronic reporter, we show that the envelope glycoproteins repressed both cap-dependent and HCV IRES-mediated translation in HeLa cells but displayed a differential repression of cap-dependent translation in Huh-7 cells. In contrast, the envelope glycoproteins repressed E2F transcriptional activity in both HeLa and Huh-7 cells and caused increased accumulation of the underphosphorylated retinoblastoma protein. Expression of the envelope glycoproteins induced eIF2alpha phosphorylation, suggesting a role of the UPR in regulating translation and E2F transcriptional activity. The envelope glycoproteins also enhanced transcriptional activity from the COX-2 promoter and endogenous COX-2 expression in HeLa cells, but not in Huh-7 cells. Together, these results suggest that the envelope glycoproteins may assume more functional roles in viral replication and host cell interactions.

The major form of hepatitis C virus alternate reading frame protein is suppressed by core protein expression

Hepatitis C virus (HCV) is a human RNA virus encoding 10 proteins in a single open reading frame. In the +1 frame, an ‘alternate reading frame’ (ARF) overlaps with the core protein-encoding sequence and encodes the ARF protein (ARFP). Here, we investigated the molecular regulatory mechanisms of ARFP expression in HCV target cells. Chimeric HCV-luciferase reporter constructs derived from the infectious HCV prototype isolate H77 were transfected into hepatocyte-derived cell lines. Translation initiation was most efficient at the internal AUG codon at position 86/88, resulting in the synthesis of a truncated ARFP named _86/88ARFP. Interestingly, _86/88ARFP synthesis was markedly enhanced in constructs containing an inactivated core protein reading frame. This enhancement was reversed by co-expression of core protein in trans, demonstrating suppression of ARFP synthesis by HCV core protein. In conclusion, our results indicate that translation of ARFP occurs mainly by alternative internal initiation at position 86/88 and is regulated by HCV core protein expression. The suppression of ARFP translation by HCV core protein suggests that ARFP expression is inversely linked to the level of viral replication. These findings define key mechanisms regulating ARFP expression and set the stage for further studies addressing the function of ARFP within the viral life cycle.

Hepatitis C virus envelope proteins regulate CHOP via induction of the unfolded protein response

Unfolded protein response (UPR) is a cellular adaptive response that functions to reduce stress caused by malfolded proteins in the endoplasmic reticulum (ER). UPR can be induced under physiological or pathological conditions and is responsible for the pathogenesis of many human diseases. Hepatitis C virus (HCV) is a single-stranded, positive-sense RNA virus causing chronic diseases. Its genome encodes two envelope proteins E1 and E2, which mature in the ER to form a noncovalently bound, native complex and disulfide aggregates and have previously been shown to induce expression of the molecular chaperone immunoglobulin heavy chain binding protein. In this study, we show that HCV envelope protein expression regulates another stress indicator CCAAT/enhancer-binding protein-homologous protein (CHOP). The ER-stress element and the activating transcription factor 4 element in the CHOP promoter were activated to a similar extent by HCV envelope protein expression. Using mouse embryonic fibroblasts deficient in the ER stress kinase RNA-activated protein kinase-like ER-resident kinase (PERK), we showed that PERK was necessary and sufficient for activating the CHOP promoter. Expression of HCV E1 and/or E2 also induced splicing of X-box binding protein 1 and transactivation of the unfolded protein response element, leading to the speculation that HCV E1 and E2 not only regulate the UPR but also ER-associated degradation.

Effect of the 5′ non-translated region on self-assembly of hepatitis C virus genotype 1a structural proteins produced in insect cells

The effect of the 5' non-translated region (5'NTR) on hepatitis C virus (HCV) morphogenesis in insect cells is investigated in this study. Expression in baculovirus-infected cells of a sequence encoding the C and E1 structural proteins under the control of the very late promoter P10 (AcSLP10-C-E1) led to the synthesis of C and C-E1 complexes, essentially found in dense reticular material associated with the ER and sedimenting at a density of 1.24-1.26 g ml(-1). Addition of the 5'NTR upstream of the C-E1 sequence (AcSLP10-5'NTR-E1) prevents translation from the initiating codon, probably because of the presence of five AUG codons in this sequence. When cells were co-infected with these two viruses, virus-like particles (VLPs) were found in the cytoplasm. The size and shape of these VLPs were variable. Concomitantly, a shift in the sedimentation profile from 1.24-1.26 to 1.15-1.18 g ml(-1) was observed, suggesting an association of C/E1 with the ER membrane. A unique vector was then constructed bearing a mutated 5'NTR (mutation of the five AUGs) and the sequence encoding all of the structural proteins and part of NS2 (5'NTRm-C-E1-E2-p7-NS2Delta). Translation of structural proteins was restored and electron microscopic observation of a cytoplasmic extract showed the presence of icosahedral particles with a density of 1.15-1.18 g ml(-1).

Mechanisms of hepatitis C virus infection

Hepatitis C virus (HCV) is the major causative agent of chronic non-A, non-B hepatitis. The life cycle of HCV is largely unknown because a reliable culture system has not yet been established. HCV presumably binds to specific receptor(s) and enters cells through endocytosis, as do other members of Flaviviridae. The viral genome is translated into a precursor polyprotein after uncoating, and viral RNA is synthesized by a virus-encoded polymerase complex. Progeny viral particles are released into the luminal side of the endoplasmic reticulum and secreted from the cell after passage through the Golgi apparatus. Understanding the mechanisms of HCV infection is essential to the development of effective new therapies for chronic HCV infection. Several host membrane proteins have been identified as receptor candidates for HCV. Recent advances using pseudotype virus systems have provided information surrounding the initial steps of HCV infection. An HCV RNA replicon system has been useful for elucidating the replication mechanism of HCV. In this review, we summarize our current understanding of the mechanisms of HCV infection and discuss potential antiviral strategies against HCV infection.

Nuclear localization of nucleocapsid-like particles and HCV core protein in hepatocytes of a chronically HCV-infected patient

Little is known about the life cycle of hepatitis C virus. Determination of the subcellular localization of HCV proteins may contribute to our understanding of the in vivo functions of the viral proteins. HCV core protein regulates multiple functions in host cells and it has been detected both in the cytoplasm and in the nucleus using different expression systems. In this study, nucleocapsid-like particles were observed in the nucleus of hepatocytes from a chronically HCV-infected patient. They were similar in size and shape to those of HCV core-like particles purified from recombinant Pichia pastoris cells. In addition the HCV core protein was detected not only in the cytoplasm but also in the nucleus and nucleolus of hepatocytes by immunoelectron microscopy. This is the first report showing nuclear localization of HCV core protein and nucleocapsid-like particles in hepatocytes during in vivo HCV infection.

CD81 engineered with endocytotic signals mediates HCV cell entry: implications for receptor usage by HCV in vivo

Although CD81 has been shown to bind HCV E2 protein, its role as a receptor for HCV remains controversial. In this study, we constructed two CD81 chimeras by linking the cytoplasmic domains of recycling surface receptors, low-density lipoprotein receptor (LDLR), and transferrin receptor (TfR), respectively, to CD81 and compared their internalization properties to wild-type CD81. Binding experiments with anti-hCD81 antibody showed that cell-surface CD81 chimeric receptors were internalized much more efficiently than wild-type CD81. In addition, CD81 chimeras, but not wild-type CD81, could internalize recombinant E2 protein and E2-enveloped viral particles from the serum of HCV-infected patients into Huh7 liver cells. The latter resulted in persistent positive-strand viral RNA and accumulation of replication intermediates, negative-strand viral RNA, in the infected cells, suggesting that the internalized viruses have undergone replication. Therefore, it appeared that CD81, possibly in association with a liver-specific endocytotic protein(s), represents one of the pathways by which HCV can infect hepatocytes.

Glycosylation of hepatitis C virus envelope proteins

Enveloped viruses are surrounded by a membrane derived from the host-cell that contains proteins called "envelope proteins". These proteins play a major role in virus assembly and entry. In most of the enveloped viruses, they are modified by N-linked glycosylation which is supposed to play a role in their stability, antigenicity and biological functions. Glycosylation is also known to play a major role in the biogenesis of proteins by being directly and/or indirectly involved in protein folding. Recent studies on hepatitis C virus (HCV) envelope proteins have revealed a complex interplay between cleavage by signal peptidase, folding and glycosylation. The knowledge that has been accumulated on the early steps of glycosylation of these proteins is presented in this review.

Expression and purification of E2/NS1 protein of hepatitis C virus and detection of anti-E2/NS1 antibodies in chronic liver disease patients

Glycoproteins on the surface of viral particles present the main target of neutralizing antibodies. The structural proteins of most Flaviviruses are known to elicit neutralizing antibodies and, thus, to help in both the natural resolution of the infection and the protection from challenge with homologous hepatitis C virus (HCV). Because such antigens are associated with the viral clearance in both humans and chimpanzees, we aimed to express the E2/NS1 protein of HCV and to study the role of anti-E2/NS1 antibodies in the natural resolution of HCV infection. The prevalence of anti-E2/NS1 antibodies to recombinant E2/NS1 protein was seen by Western blot in chronic liver disease patients (15 chronic hepatitis and 12 cirrhotic patients), who were positive for anti-HCV and negative for HBV infection. The study also included 2 negative controls (positive for HBV infection and negative for anti-HCV antibodies) and 2 healthy controls (negative for both HBV and HCV infection). Anti-E2/NS1 was present in 20% of the chronic hepatitis and 16% of the cirrhosis patients. None of the controls were positive for anti-E2/NS1 antibodies. Serum samples positive for anti-E2/NS1 antibodies were also positive for HCV RNA by RT/PCR. Accordingly, the presence of anti-E2/NS1 may have very little or no role in the natural resolution of HCV infection.

Characterization of secreted and intracellular forms of a truncated hepatitis C virus E2 protein expressed by a recombinant herpes simplex virus

A replication-defective herpes simplex virus type 1 (HSV-1) recombinant lacking the glycoprotein H (gH)-encoding gene and expressing a truncated form of the hepatitis C (HCV) E2 glycoprotein (E2-661) was constructed and characterized. We show here that cells infected with the HSV/HCV recombinant virus efficiently express the HCV E2-661 protein. Most importantly, cellular and secreted E2-661 protein were both readily detected by the E2-conformational mAb H53 and despite the high expression levels, only limited amounts of misfolded aggregates were detected in either the cellular or secreted fractions. Furthermore, cell-associated and secreted E2-661 protein bound to the major extracellular loop (MEL) of CD81 in a concentration-dependent manner and both were highly reactive with sera from HCV-infected patients. Finally, BALB/c mice immunized intraperitoneally with the recombinant HSV/HCV virus induced high levels of anti-E2 antibodies. Analysis of the induced immunoglobulin G (IgG) isotypes showed high levels of IgG2a while the levels of the IgG1 isotype were significantly lower, suggesting a Th1-type of response. We conclude that the HSV-1 recombinant virus represents a promising tool for production of non-aggregated, immunologically active forms of the E2-661 protein and might have potential applications in vaccine development.

Expression of a full-length hepatitis C virus cDNA up-regulates the expression of CC chemokines MCP-1 and RANTES

We had previously reported the cloning of the complete genome of an isolate of hepatitis C virus (HCV), HCV-S1, of genotype 1b. We have constructed a full-length complementary DNA (cDNA) clone of HCV-S1 using nine overlapping cDNA clones that encompassed its entire genome. HCV core, E1, E2, NS-3, -4B, -5A, and -5B proteins were detected in 293T cells by immunoblot analyses when expression of the full-length HCV-S1 was driven under a CMV promoter. Expression of full-length HCV-S1 led to induction of the CC chemokines RANTES and MCP-1 at both the mRNA and the protein levels in HeLa, Huh7, and HepG2 cells. Reporter gene assays showed that a minimal MCP-1 promoter construct containing 128 nucleotides upstream of its translational start site was sufficient for optimal HCV-mediated activation. HCV induced AP-1 binding activities to this region, as determined from electrophoretic mobility shift assays and supershifts with anti-AP-1 antibodies. Transfection of full-length HCV-S1 up-regulated both AP-1 binding activities as well as c-jun transcripts. A minimal promoter construct containing 181 nucleotides upstream of the RANTES translational start site was sufficient for maximal HCV-mediated induction. Gel mobility shift and supershift assays showed that HCV induced NF-kappaB and other unknown binding activities to the A/B-site within this region. In HeLa cells, HCV core and NS5A could separately augment promoter activities of both MCP-1 and RANTES. In Huh7 cells, only NS5A produced a similar effect, while rather surprisingly, HCV core induced a dramatic reduction in promoter activities of these two genes. This study provides the first direct evidence for the induction of CC chemokines in HCV infection and draws attention to their roles in affecting the progress and outcome of HCV-associated liver diseases.

Genome of human hepatitis C virus (HCV): gene organization, sequence diversity, and variation

Hepatitis C virus (HCV) is the major etiologic agent of non-A, non-B hepatitis. HCV infection frequently causes chronic hepatitis, which progresses to liver cirrhosis and hepatocellular carcinoma. Since the discovery of HCV in 1989, a large number of genetic analyses of HCV have been reported, and the viral genome structure has been elucidated. An enveloped virus, HCV belongs to the family Flaviviridae, whose genome consists of a positive-stranded RNA molecule of about 9.6 kilobases and encodes a large polyprotein precursor (about 3000 amino acids). This precursor protein is cleaved by the host and viral proteinase to generate at least 10 proteins: the core, envelope 1 (E1), E2, p7, nonstructural (NS) 2, NS3, NS4A, NS4B, NS5A, and NS5B. These HCV proteins not only function in viral replication but also affect a variety of cellular functions. HCV has been found to have remarkable genetic heterogeneity. To date, more than 30 HCV genotypes have been identified worldwide. Furthermore, HCV may show quasispecies distribution in an infected individual. These findings may have important implications in diagnosis, pathogenesis, treatment, and vaccine development. The hypervariable region 1 found within the envelope E2 protein was shown to be a major site for the genetic evolution of HCV after the onset of hepatitis, and might be involved in escape from the host immunesurveillance system.

Expression and purification of an active, full-length hepatitis C viral NS4A

The nonstructural protein 3 (NS3) of the hepatitis C virus (HCV) is a bifunctional protein with protease and helicase activities. Nonstructural protein 4A (NS4A) is preceded by NS3 and augments the proteolytic activity of NS3 through protein-protein interaction. The central domain of NS4A has been shown to be sufficient for the enhancement of the NS3 protease activity. However, investigations on the roles of the N-terminal and the C-terminal regions of NS4A have been hampered by the difficulty of purification of full-length NS4A, a polypeptide that contains highly hydrophobic amino acid residues. Here we report a procedure by which one can produce and purify an active, full-length NS4A using maltose-binding protein fusion method. The full-length NS4A fused to the maltose binding protein is soluble and maintains its NS3 protease-enhancing activity.

Glycosylation of the hepatitis C virus envelope protein E1 is dependent on the presence of a downstream sequence on the viral polyprotein

The addition of N-linked oligosaccharides to Asn-X-(Ser/Thr) sites is catalyzed by the oligosaccharyltransferase, an enzyme closely associated with the translocon and generally thought to have access only to nascent chains as they emerge from the ribosome. However, the presence of the sequon does not automatically ensure core glycosylation because many proteins contain sequons that remain either nonglycosylated or glycosylated to a variable extent. In this study, hepatitis C virus (HCV) envelope protein E1 was used as a model to study the efficiency of N-glycosylation. HCV envelope proteins, E1 and E2, were released from a polyprotein precursor after cleavage by host signal peptidase(s). When expressed alone, E1 was not efficiently glycosylated. However, E1 glycosylation was improved when expressed as a polyprotein including full-length or truncated forms of E2. These data indicate that glycosylation of E1 is dependent on the presence of polypeptide sequences located downstream of E1 on HCV polyprotein.

Hepatitis C virus glycoprotein E2 binding to CD81: the role of E1E2 cleavage and protein glycosylation in bioactivity

The hepatitis C virus glycoproteins E1 and 2 have been expressed using recombinant baculoviruses following fusion to the carrier protein glutathione S-transferase (GST). Proteins were expressed singly and as an E1E2 polyprotein with and without an N-terminal affinity tag. Expression of the E1E2 polyprotein, even when preceded by GST, led to processing in insect cells and detection of an E1E2 complex that could be specifically purified by glutathione affinity chromatography. Baculovirus expressed E2 and a purified GST-E1E2 protein bound to the second extracellular loop of CD81 (EC2), a reported ligand for the molecule, but not to a truncated derivative of CD81 consisting of only the central domain of the loop. Purified GST-E2, however, failed to bind to CD81 suggesting a requirement for a free E2 amino terminus for biological activity. The binding to CD81 by baculovirus expressed E2 protein was comparable to that observed for E2 derived from mammalian cells when detected by a monoclonal antibody sensitive to protein conformation. Furthermore, E2 protein expressed in insect cells in the presence of N-butyldeoxynojirimycin, an inhibitor of terminal glucose residue processing, formed complexes with E1 and bound to CD81-EC2 similarly to untreated protein. Together these data suggest that although hyperglucosylation of E2 does not have a major effect on bioactivity, polyprotein processing to reveal the free amino terminus is required.

Molecular virology of the hepatitis C virus

Infection with the hepatitis C virus (HCV) is the major cause of nonA-nonB hepatitis worldwide. Although this virus cannot be cultivated in vitro, several of its key features have been elucidated in the past few years. The viral genome is a positive-sense, single-stranded, 9.6 kb long RNA molecule. The viral genome is translated into a single polyprotein of about 3000 amino acids. The viral polyprotein is proteolytically processed by the combination of cellular and viral proteinases in order to yield all the mature viral gene products. The genomic order of HCV has been shown to be C-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B. C, E1 and E2 are the virion.structural proteins. The function of p7 is currently unknown. These proteins have been shown to arise from the viral polyprotein via proteolytic processing by the host signal peptidases. Generation of the mature nonstructural proteins, NS2 to NS5B, relies on the activity of viral proteinases. Cleavage at the NS2/NS3 junction is accomplished by a metal-dependent autocatalytic proteinase encoded within NS2 and the N-terminus of NS3. The remaining cleavages downstream from this site are effected by a serine proteinase also contained within the N-terminal region of NS3. NS3 also contains an RNA helicase domain at its C-terminus. NS3 forms a heterodimeric complex with NS4A. The latter is a membrane protein that has been shown to act as a cofactor of the proteinase. While no function has yet been attributed to NS4B, it has recently been suggested that NS5A is involved in mediating the resistance of the hepatitis C virus to the action of interferon. Finally, the NS5B protein has been shown to be the viral RNA-dependent RNA polymerase.

Expression of noncovalent hepatitis C virus envelope E1-E2 complexes is not required for the induction of antibodies with neutralizing properties following DNA immunization

Interactive glycoproteins present on the surface of viral particles represent the main target of neutralizing antibodies. The ability of DNA vaccination to induce antibodies directed at such structures was investigated by using eight different expression plasmids engineered either to favor or to prevent interaction between the hepatitis C virus (HCV) envelope glycoproteins E1 and E2. Independently of the injection route (intramuscular or intraepidermal), plasmids expressing antigens capable of forming heterodimers presumed to be the prebudding form of the HCV envelope protein complex failed to induce any significant, stable antibodies following injection in mice. In sharp contrast, high titers of antibodies directed at both conformational and linear determinants were induced by using plasmids expressing severely truncated antigens that have lost the ability to form native complexes. In addition, only a truncated form of E2 induced antibodies reacting against the hypervariable region 1 of E2 (specifically with the C-terminal part of it) known to contain a neutralization site. When injected intraepidermally into small primates, the truncated E2-encoding plasmid induced antibodies able to neutralize in vitro the binding of a purified E2 protein onto susceptible cells. Because such antibodies have been associated with viral clearance in both humans and chimpanzees, these findings may have important implications for the development of protective immunity against HCV.

The NS3/4A proteinase of the hepatitis C virus: unravelling structure and function of an unusual enzyme and a prime target for antiviral therapy

The hepatitis C virus (HCV) is a major causative agent of transfusion-acquired and sporadic non-A, non-B hepatitis worldwide. Infections most often persist and lead, in approximately 50% of all patients, to chronic liver disease. As is characteristic for a member of the family Flaviviridae, HCV has a plus-strand RNA genome encoding a polyprotein, which is cleaved co- and post-translationally into at least 10 different products. These cleavages are mediated, among others, by a virally encoded chymotrypsin-like serine proteinase located in the N-terminal domain of non-structural protein 3 (NS3). Activity of this enzyme requires NS4A, a 54-residue polyprotein cleavage product, to form a stable complex with the NS3 domain. This review will describe the biochemical properties of the NS3/4A proteinase, its X-ray crystal structure and current attempts towards development of efficient inhibitors.

Identification of a domain containing B-cell epitopes in hepatitis C virus E2 glycoprotein by using mouse monoclonal antibodies

Evidence from clinical and experimental studies of human and chimpanzees suggests that hepatitis C virus (HCV) envelope glycoprotein E2 is a key antigen for developing a vaccine against HCV infection. To identify B-cell epitopes in HCV E2, six murine monoclonal antibodies (MAbs), CET-1 to -6, specific for HCV E2 protein were generated by using recombinant proteins containing E2t (a C-terminally truncated domain of HCV E2 [amino acids 386 to 693] fused to human growth hormone and glycoprotein D). We tested whether HCV-infected sera were able to inhibit the binding of CET MAbs to the former fusion protein. Inhibitory activity was observed in most sera tested, which indicated that CET-1 to -6 were similar to anti-E2 antibodies in human sera with respect to the epitope specificity. The spacial relationship of epitopes on E2 recognized by CET MAbs was determined by surface plasmon resonance analysis and competitive enzyme-linked immunosorbent assay. The data indicated that three overlapping epitopes were recognized by CET-1 to -6. For mapping the epitopes recognized by CET MAbs, we analyzed the reactivities of CET MAbs to six truncated forms and two chimeric forms of recombinant E2 proteins. The data suggest that the epitopes recognized by CET-1 to -6 are located in a small domain of E2 spanning amino acid residues 528 to 546.

A human liver cell line exhibits efficient translation of HCV RNAs produced by a recombinant adenovirus expressing T7 RNA polymerase

An in vitro system that supports the efficient growth of hepatitis C virus (HCV) and reflects its complete in vitro replication cycle has not yet been established. The establishment of a minigene RNA of HCV in mammalian cells could facilitate the study of virus-cell interactions and the molecular pathogenesis of this virus. We constructed a replication-deficient recombinant adenovirus expressing bacteriophage T7 RNA polymerase under the control of CAG promoter (AdexCAT7). A high level of T7 RNA polymerase was detectable for at least 11 days after inoculation. Cells infected with AdexCAT7 were then transfected with plasmids carrying the authentic T7 promoter, the 5' untranslated region (UTR) of encephalomyocarditis virus, a luciferase gene, and a T7 terminator (pT7EMCVLuc) or carrying the modified T7 promoter, the 5'UTR of HCV, a luciferase gene, the coding region of C-terminal of NS5B and the 3'UTR of HCV, a ribozyme of hepatitis D virus and a T7 terminator (pT7HCVLuc). Most of the cell lines examined supported a higher expression of luciferase by transfection with pT7EMCVLuc than with pT7HCVLuc. However, one cell line, FLC4, derived from a human hepatocellular carcinoma, exhibited very high reporter gene expression with pT7HCVLuc. In this cell line, transfection with RNA synthesized in vitro from pT7HCVLuc induced a higher level of reporter gene expression than RNA from pT7EMCVLuc. The T7-adenovirus system for the synthesis of HCV minigenes in vivo provides useful information on the molecular mechanisms of HCV translation in human liver cells.

High titers of antibodies inhibiting the binding of envelope to human cells correlate with natural resolution of chronic hepatitis C

Most cases of hepatitis C virus (HCV) infection result in chronic disease; however, a very small fraction of patients naturally clear the virus and resolve chronic hepatitis. In an attempt to correlate immune response with chronic disease resolution, we compared the antibody response in patients with different outcomes of the infection. Antibody responses to HCV structural proteins were assessed in 34 patients originally diagnosed with acute hepatitis. Five cases resolved acute infection, 22 developed chronic hepatitis, and 7 naturally resolved chronic hepatitis C. To estimate HCV neutralizing antibodies we used the neutralization of binding (NOB) assay, which evaluates inhibition of the envelope-2 protein binding to human cells. Enzyme-linked immunosorbent assay was used for the quantitative assessment of serum antibodies. The presence of HCV RNA was ascertained by reverse transcription-polymerase chain reaction. In 6 of 7 patients naturally recovered from chronic hepatitis C, the emergence and the persistence (for more than 3 months) of high serum titers (>1/600) of NOB antibodies coincided with virus clearance and clinical resolution of hepatitis. NOB antibody activity was observed in only 2 of 5 patients recovered from acute hepatitis C. Chronic patients who did not show any resolution during the course of the study developed low or no NOB antibodies. Because of the correlation between prolonged high NOB titers and natural resolution of chronic hepatitis C, vaccination or passive immunization aimed at high titers of NOB antibodies may be valuable new therapeutic approaches for chronic hepatitis C.

Characterization of the structural proteins of hepatitis C virus expressed by an adenovirus recombinant

Human adenoviruses have been used for mammalian expression vectors and recombinant vaccines for heterologous antigens. We constructed and characterized an infectious adenovirus recombinant containing core-E1-E2 genes of hepatitis C virus (HCV). The core protein was produced mainly during the early phase of viral infection. Expression of HCV E1 and E2 envelope proteins was detected by an immunoprecipitation with HCV-positive patient's sera. The purified E1 and E2 proteins appeared to be composed of mainly a heterodimeric form via noncovalent interaction, as previously observed in other mammalian expression systems. A small portion of E1 and E2 monomers as well as E1E2 aggregates by interdisulfide linkage were detected. Apparently heterodimeric E1E2 complexes were serologically reactive. The results suggest that adenovirus is an useful HCV antigen-expression vector.

Hepatitis C virus structural proteins assemble into viruslike particles in insect cells

Hepatitis C virus (HCV) is a leading cause of chronic hepatitis in the world. The study of HCV has been hampered by the low level of viral particles in infected individuals, the inability to propagate efficiently the virus in cultured cells, and the lack of a convenient animal model. Due to these obstacles, neither the structure of the virus nor the prerequisites for its assembly have been clearly defined. In this report, we describe a model for the production and purification of HCV-like particles in insect cells using a recombinant baculovirus containing the cDNA of the HCV structural proteins. In insect cells, expressed HCV structural proteins assembled into enveloped viruslike particles (40 to 60 nm in diameter) in large cytoplasmic cisternae, presumably derived from the endoplasmic reticulum. Biophysical characterization of viruslike particles by CsCl and sucrose gradient centrifugation revealed biophysical properties similar to those of putative virions isolated from infected humans. The results suggested that HCV core and envelope proteins without p7 were sufficient for viral particle formation. Analysis of particle-associated nucleic acids demonstrated that HCV RNAs were selectively incorporated into the particles over non-HCV transcripts. The synthesis of HCV-like particles in insect cells may provide an important tool to determine the structural requirements for HCV particle assembly as well as to study viral genome encapsidation and virus-host interactions. The described system may also represent a potential approach toward vaccine development.

Full-length complementary DNA of hepatitis C virus genome from an infectious blood sample

We constructed a full-length complementary DNA (cDNA) clone of hepatitis C virus (HCV) from a blood sample of an HCV carrier. The blood from the carrier was eventually transfused to a patient who later developed typical posttransfusion hepatitis C. It was also shown to be infectious to chimpanzees. We obtained 12 overlapping cDNA fragments altogether, covering the entire HCV genome. By subcloning and sequencing, clones considered to constitute the major population were selected. We could also detect 98 base pairs of extra sequences at the 3' end of the genome. After confirming the overlapping sequences, we combined the fragments to make a full-length cDNA. The HCV population in the donor was heterogeneous, as determined by their nucleotide sequences of the hypervariable region in envelope protein, but a few virus clones were selected in the recipient after transmission. The similar convergence of the virus population was previously observed when the same blood sample was injected into a chimpanzee. Interestingly, virus clones isolated during the acute phase in the recipient and the chimpanzee had sequences in the hypervariable region identical to that of the full-length cDNA clone. The full-length cDNA clone of HCV constructed in this study may originate from infectious virus clones.

Comparison of the rate of sequence variation in the hypervariable region of E2/NS1 region of hepatitis C virus in normal and hypogammaglobulinemic patients

The hypervariable region (HVR) of the E2/NS1 region of hepatitis C virus (HCV) varies greatly between viral isolates with high rates of genomic change reported during the course of chronic infection. The HVR is thought to encode a structurally unconstrained envelope protein containing several linear B cell epitopes recognized by neutralizing antibody. It has been postulated that amino acid changes in the HVR could result from humoral immune pressure leading to the selection of escape mutants. The aim of this study was to compare the rates of nucleotide and amino acid variation in the HVR of control patients to patients with common variable immunodeficiency (CVID) where the effect of the humoral immune system is reduced. Five controls and four patients with CVID were studied. Serum samples were taken over periods of between 1 and 6 years. HCV was detected by polymerase chain reaction (PCR) with primers derived from conserved flanking regions of the HVR. PCR products were cloned into a plasmid vector and recombinant clones identified by restriction enzyme digestion. Purified DNA from at least three individual clones from each time point was sequenced by the dideoxynucleotide chain-termination method. Consensus sequences were extracted from the three clones, and the DNA and deduced protein sequences were compared. Control patients had a mean rate of nucleotide change of 6.954 nucleotide substitutions per year, compared with patients with CVID with a rate of 0.415 nucleotide substitutions per year (P < .02). The corresponding rates for amino acid variation were 3.868 amino acid substitutions per year for the control patients compared with 0.185 amino acid substitutions per year for the patients with CVID. These findings suggest that in the absence of humoral immune selective pressure, the frequency of occurrence of genetic variation in the major viral species is reduced. The mutations occur, but in the absence of immune selection remain as minor species. The evolution of viral mutants capable of evading the host's immune system may contribute to the ability of HCV to establish chronic infection.

Determinants of substrate specificity in the NS3 serine proteinase of the hepatitis C virus

Processing of the nonstructural polyprotein of the hepatitis C virus (HCV) requires the serine-type proteinase located in the amino-terminal domain of NS3. To identify residues within NS3 determining substrate specificity, a mutation analysis was performed. Using sequence alignments and three-dimensional structure predictions, amino acids assumed to be important for specificity were replaced and the enzymes were tested in an intracellular trans-processing assay for their effects on cleavage of an NS4B-5B substrate. For some of the substitutions at positions 133, 134, 135, 136, 138, 152, 155, 157, and 169, slightly reduced processing efficiencies were observed but in no case was the substrate specificity altered. In contrast, substitutions of the phenylalanine at position 154 resulted in a modified cleavage pattern, suggesting an important role for this residue in substrate specificity. To substantiate this assumption, a panel of NS4B-5B substrates carrying different P1 residues at the NS4B/5A site were tested for cleavage by these altered proteinases. We found that substitution of Phe-154 by alanine, by valine, and particularly by threonine generated enzymes with the following affinities for aliphatic P1 residues: C > L > I > V for 154 F --> A, C = L > I > V for 154 F --> V and L > C > I > V for 154 F --> T. Neither leucine nor isoleucine nor valine was accepted by the parental NS3 proteinase, showing that Phe-154 is an important determinant for substrate specificity. Furthermore, we present evidence that Ala-157 plays an additional but minor role for this property.

Sialadenitis histologically resembling Sjogren syndrome in mice transgenic for hepatitis C virus envelope genes

Hepatitis C virus (HCV), a major causative agent of non-A, non-B chronic hepatitis, is also suggested to be associated with extrahepatic manifestations such as mixed cryoglobulinemia and glomerulonephritis. Two independent lines of transgenic mice carrying the HCV envelope genes have been shown previously to express the HCV envelope proteins in organs, including the liver and salivary glands, which results in no pathological changes in the liver. Further analysis of these animals now has revealed that they develop an exocrinopathy involving the salivary and lachrymal glands. This pathology resembles Sjogren syndrome, which also is suggested to have a possible association with chronic hepatitis C. These observations suggest that HCV might be involved in the pathogenesis of sialadenitis in humans and that this transgenic mouse system would be a good animal model for the study of HCV infection.

Purification and in vitro-phospholabeling of secretory envelope proteins E1 and E2 of hepatitis C virus expressed in insect cells

The putative envelope glycoproteins of hepatitis C virus (HCV), E1 and E2, were expressed as recombinant, secretory proteins in Sf9 insect cells through infection with recombinant baculoviruses. The influenza virus hemagglutinin signal sequence (HASS) was inserted upstream of the HCV-cDNAs in order to effect secretion. Furthermore, a hexa-histidine tag for purification on a Ni(2+)-nitrilotriacetic acid (Ni(2+)-NTA) column and a protein kinase A (PKA) recognition sequence for in vitro-phospholabeling were fused upstream of the HCV-cDNA. E1- and E2 proteins lacking their carboxy-terminal, hydrophobic sequence were produced by baculovirus-infected insect cells in bioreactors of 23 1. The medium was concentrated and proteins were purified under native conditions on Ni(2+)-NTA columns. Purified proteins could be phospholabeled in vitro using the catalytic subunit of protein kinase. A isolated from bovine heart and gamma-[32P]ATP. Labeled E1 and E2 proteins expressed in insect cells could be immunoprecipitated with sera from HCV-infected patients. Co-expression of these E1 and E2 proteins led to the formation of E1-E2 complexes within the insect cell and to secretion of these complexes into the medium.

Overexpression and simple purification of a truncated, immunologically reactive GST-HCV core (1-123) fusion protein

A full-length and a truncated gene for the core protein of hepatitis C virus (HCV) were linked to the gene for glutathione S-transferase (GST), and the expression of each GST-HCV core fusion protein was analyzed. The truncated GST-HCV core (1-123) fusion protein was expressed as a mostly soluble and partly insoluble form comprising more than 50% of the total protein in Escherichia coli after induction by isopropylthio-beta-D-galactoside (IPTG), while the full length GST-HCV core (1-191) fusion protein was not expressed, suggesting that the hydrophobic carboxy terminal region in the core protein affects its expression. In addition, the GST-HCV core (1-123) fusion protein purified by GST-agarose chromatography reacted specifically with an anti-HCV serum from a patient.

Efficient generation of recombinant adenoviruses using adenovirus DNA-terminal protein complex and a cosmid bearing the full-length virus genome

An efficient method of constructing recombinant adenoviruses (Ads) has been established. The expression unit to be introduced into recombinant Ad was first inserted into the unique Swa I site of the full-length Ad genome cloned in a cassette cosmid. The cassette bearing the expression unit was then cotransfected into human embryonic kidney 293 cells together with the Ad DNA-terminal protein complex digested at several sites with Eco T22I or Ase I/EcoRI. The use of the parent Ad DNA-terminal protein complex instead of the deproteinized Ad genome DNA allowed very efficient recovery of the desired recombinant Ad, and the above restriction digestion drastically reduced regeneration of the parent virus. Several hundred virus clones were readily obtained in each experiment, and about 70% of the clones were the desired recombinant viruses. Furthermore, because the cassette contained the full-length Ad genome, any position of the genome could be easily modified to develop a new vector design. We established construction systems for two types of Ad vectors, the E1-substitution type and the E4-insertion type. This method may greatly facilitate the application of recombinant Ads and should be useful for further improvement of Ad vectors.

Human monoclonal antibodies for the immunological characterization of a highly conserved protein domain of the hepatitis C virus glycoprotein E1

Although both envelope glycoproteins of the hepatitis C virus, E1 and E2/NS1, show a high degree of sequence variation, the E1 protein includes a well conserved domain, which may be functionally important. We have analysed the human B cell response to a peptide fragment from amino acid residues 314-330 (EP3) covering the central conserved sequence of this domain. Anti-hepatitis C virus-positive blood donors were screened for anti-EP3 antibodies with an ELISA based on immobilized peptide. Thirty out of 92 (32%) RIBA-confirmed donors displayed a significant antibody response to EP3. From three of these blood donors we established four anti-EP3-producing heterohybridoma cell lines: Ul/F30 and Ul/F31 produced IgM-kappa, whereas Ul/F32 and Ul/F33 secreted the isotypes IgG1-lambda and IgG1-kappa, respectively. Epitope analysis with overlapping nonapeptides suggests the existence of different antigenic determinants within the EP3 fragment. Although both IgG antibodies Ul/F32 and Ul/F33 have dissociation constants to the peptide of approximately 10(-9) M, binding to recombinant E1 protein expressed in COS-7 cells was different. Only Ul/F33 detected envelope protein of approximately 24-35 kD in Western blot. This human MoAb will be useful for further investigations on the hepatitis C virus glycoprotein E1.

NS3-4A of hepatitis C virus is a chymotrypsin-like protease

The polyprotein encoded by a single open reading frame of hepatitis C virus (HCV) is processed by host- and virus-encoded proteases. The viral protease NS3 is responsible for the cleavage of at least four sites (NS3/4A, NS4A/4B, NS4B/5A, and NS5A/5B junctions) in the nonstructural protein region. To characterize the protease function of NS3 and NS4 on various target sites, efficient cis- and trans-cleavage assay systems were developed by using in vitro transcription and translation. Deletion of the C-terminal two-thirds from NS3 in an NS3-NS4A-4B polypeptide (NS3 delta C-4A-4B) hampered cleavage of the NS3/4A junction but not that of the NS4A/4B junction. As a consequence, expression of NS3 delta C-4A-4B containing an internal deletion of NS3 results in an NS3 delta C-4A fusion protein. NS3 delta C-4A shows very efficient and specific trans-cleavage activity at NS4A/4B, NS4B/5A, and NS5A/5B junctions. In addition, the biochemical properties of HCV NS3 delta C-4A were further elucidated by adding known protease inhibitors in trans-cleavage reactions. The HCV protease NS3-4A is inhibited by chymotrypsin-specific inhibitors N-tosyl-L-phenylalanine chloromethyl ketone (TPCK), chymostatin, and Pefabloc SC but not by trypsin-like protease inhibitors antipain, leupeptin, and N-alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) or by the protease inhibitors E-64, bestatin, pepstatin, and phosphoramidon. This finding strongly suggests that HCV protease NS3-4A is a chymotrypsin-like serine protease.

Density analysis of hepatitis C virus particle population in the circulation of infected hosts: implications for virus neutralization or persistence

Hepatitis C virus has a low buoyant density in sucrose, but high-density particles are often observed in hepatitis C virus infection. To investigate the characteristics of circulating hepatitis C virus particles and their association with liver disease progression, we examined sera from two histologically normal hepatitis C virus carriers, 20 chronic hepatitis patients and five acute hepatitis C patients. The supernatants obtained after immunoprecipitation with anti-immunoglobulins antibody were subjected to sucrose equilibrium centrifugation. HCV-RNA positive fractions separated after the treatments were further examined for immunoprecipitation with anti-core hepatitis C virus antibody. We separated hepatitis C virus particle populations according to the density difference on 35% sucrose with centrifugation. The proportions of high and low density particles in hepatitis C virus populations were determined by means of competitive reverse transcription and polymerase chain reaction. Circulating hepatitis C virus particles in chronically infected patients could be separated into two populations: those immunoglobulin-bound with high densities and -unbound with low densities. Patients with severe liver inflammation had high-density hepatitis C virus that did not precipitate with anti-immunoglobulins but with anti-core hepatitis C virus antibodies. Thus, hepatitis C virus particle populations consist of low-density virions and high-density immune complexes and/or nucleocapsids. Among the chronic hepatitis patients, the dominant population shifted from low-density to high-density particles with the progression of liver disease. In acute hepatitis patients, this density shift was observed with alanine aminotransferase normalizations. Therefore, the major hepatitis C virus populations change from virion to immune complex and/or nucleocapsid with the progression of liver disease or inflammation.

Substrate determinants for cleavage in cis and in trans by the hepatitis C virus NS3 proteinase

Processing of the hepatitis C virus polyprotein is accomplished by a series of cotranslational and posttranslational cleavages mediated by host cell signalases and two virally encoded proteinases. Of these the NS3 proteinase is essential for processing at the NS3/4A, NS4A/4B, NS4B/5A, and NS5A/5B junctions. Processing between NS3 and NS4A occurs in cis, implying an intramolecular reaction mechanism, whereas cleavage at the other sites can also be mediated in trans. Sequence analysis of the amino termini of mature cleavage products and comparisons of amino acid residues around the scissile bonds of various hepatitis C virus isolates identified amino acid residues which might contribute to substrate specificity and processing efficiency: an acidic amino acid at the P6 position, a Thr or Cys at the P1 position, and a Ser or Ala at the P1' position. To study the importance of these residues for NS3-mediated cleavage we have undertaken a mutational analysis using an NS3'-5B polyprotein expressed by recombinant vaccinia viruses in mammalian cells. For all NS3-dependent cleavage sites P1 substitutions had the most drastic effects on cleavage efficiency, showing that amino acid residues at this position are the most critical substrate determinants. Since less drastic effects were found for substitutions at the P1' position, these residues appear to be less important for proper cleavage. For all cleavage sites the P6 acidic residue was dispensable, suggesting that it is not essential for substrate recognition and subsequent cleavage. Analysis of a series of mutations at the NS3/4A site revealed great flexibility for substitutions compared with more stringent requirements at the trans cleavage sites. On the basis of these results we propose a model in which processing in cis is determined primarily by polyprotein folding, whereas cleavage in trans is governed not only by the structure of the polyprotein but also by specific interactions between the proteinase and the polyprotein substrate at or around the scissile bond.

Antigenic structure of the hepatitis C virus envelope 2 protein

The antigenic structure of the envelope 2 (e2) protein of the hepatitis C virus (HCV) was characterized by the use of 70 synthetic peptides and 131 human sera from persons with antibodies to HCV. Among 34 overlapping peptides spanning the e2 protein of HCV, two major antigenic regions were located to residues 484-499 and residues 554-569. The sequence of the two major antigenic regions of the e2 protein are partly well conserved within the described types of HCV. Both regions contain two Cys residues in close proximity, and the region at residues 554-569 contains a putative N-glycosylation site, which are factors that previously have been suggested to affect the immune recognition of the e2 protein. Using substitution peptide analogues where each position within residues 484-499 and 554-569 were sequentially substituted by Ala or Gly, the most essential residues for antibody binding were found to be the Pro-498, Ala-499, Ala-566, Pro-567, and Pro-568. All of these, except for the Pro-498 and Ala-566, are conserved among different HCV strains. Also, according to previous studies, position 496 often shows variations, which could be explained by position 496 being contained within the antigenic region at residues 484-499. Interestingly, none of the Cys residues at positions 486, 494, 564 and 569 were found to be essential for antibody binding, indicating that these are not essential in maintaining the e2 antigenicity of the peptides. In a material of 114 confirmed anti-HCV positive sera, derived from patients during the acute or the chronic phase of HCV infection, the prevalence of antibodies to the two major linear antigenic regions of the e2 protein was found to be 55% among HCV RNA-positive sera, and 53% among HCV RNA-negative sera. In conclusion, we have identified and characterized two major linear antigenic regions outside the two hypervariable regions of the e2 protein. Since these regions are accessible to the B cells of the infected host, these two regions are likely to be surface exposed either on the precursor polyprotein or the native e2 protein. Also, we could confirm that antibodies to the e2 protein co-exist with HCV viraemia.

Truncating the putative membrane association region circumvents the difficulty of expressing hepatitis C virus protein E1 in Escherichia coli

The putative E1 of hepatitis C virus (HCV) was expressed in Escherichia coli using a glutathione-S-transferase (GST) fusion protein system. The full length E1 protein is difficult to express. A series of E1 DNA fragments was generated and used for expression vector construction. Fusion proteins containing the E1 C-terminal region could not be expressed. When this region was truncated, the fusion proteins were synthesized to high levels. The possibility of this C-terminal region hampering the production of fusion protein was further explored. A construct with this segment directly fused to the C-terminus of GST indeed generated no detectable recombinant protein. According to the predicted structure of E1, this region may have membrane-associating properties. The expression results suggest a general approach to facilitate the production of viral membrane proteins in prokaryotes. Furthermore, these recombinant E1 proteins generated as antigens were used for Western blotting with sera from HCV-infected individuals. It was found that E1 is antigenic during HCV natural infection.

Formation and intracellular localization of hepatitis C virus envelope glycoprotein complexes expressed by recombinant vaccinia and Sindbis viruses

Hepatitis C virus (HCV) encodes two putative virion glycoproteins (E1 and E2) which are released from the polyprotein by signal peptidase cleavage. In this report, we have characterized the complexes formed between E1 and E2 (called E1E2) for two different HCV strains (H and BK) and studied their intracellular localization. Vaccinia virus and Sindbis virus vectors were used to express the HCV structural proteins in three different cell lines (HepG2, BHK-21, and PK-15). The kinetics of association between E1 and E2, as studied by pulse-chase analysis and coprecipitation of E2 with an anti-E1 monoclonal antibody, indicated that formation of stable E1E2 complexes is slow. The times required for half-maximal association between E1 and E2 were 60 to 85 min for the H strain and more than 165 min for the BK strain. In the presence of nonionic detergents, two forms of E1E2 complexes were detected. The predominant form was a heterodimer of E1 and E2 stabilized by noncovalent interactions. A minor fraction consisted of heterogeneous disulfide-linked aggregates, which most likely represent misfolded complexes. Posttranslational processing and localization of the HCV glycoproteins were examined by acquisition of endoglycosidase H resistance, subcellular fractionation, immunofluorescence, cell surface immunostaining, and immunoelectron microscopy. HCV glycoproteins containing complex N-linked glycans were not observed, and the proteins were not detected at the cell surface. Rather, the proteins localized predominantly to the endoplasmic reticular network, suggesting that some mechanism exists for their retention in this compartment.

Processing in the hepatitis C virus E2-NS2 region: identification of p7 and two distinct E2-specific products with different C termini

The hepatitis C virus (HCV) H strain polyprotein is cleaved to produce at least nine distinct products: NH2-C-E1-E2-NS2-NS3-NS4A-NS4B-NS5A-NS5B-CO OH. In this report, a series of C-terminal truncations and fusion with a human c-myc epitope tag allowed identification of a tenth HCV-encoded cleavage product, p7, which is located between the E2 and NS2 proteins. As determined by N-terminal sequence analysis, p7 begins with position 747 of the HCV H strain polyprotein. p7 is preceded by a hydrophobic sequence at the C terminus of E2 which may direct its translocation into the endoplasmic reticulum, allowing cleavage at the E2/p7 site by host signal peptidase. This hypothesis is supported by the observation that cleavage at the E2/p7 and p7/NS2 sites in cell-free translation studies was dependent upon the addition of microsomal membranes. However, unlike typical cotranslational signal peptidase cleavages, pulse-chase experiments indicate that cleavage at the E2/p7 site is incomplete, leading to the production of two E2-specific species, E2 and E2-p7. Possible roles of p7 and E2-p7 in the HCV life cycle are discussed.