The transmembrane domain of hepatitis C virus glycoprotein E1 is a signal for static retention in the endoplasmic reticulum

Transmembrane domains of hepatitis C virus envelope glycoproteins: residues involved in E1E2 heterodimerization and involvement of these domains in virus entry

The transmembrane (TM) domains of hepatitis C virus (HCV) envelope glycoproteins E1 and E2 have been shown to play multiple roles during the biogenesis of the E1E2 heterodimer. By using alanine scanning insertion mutagenesis within the TM domains of HCV envelope glycoproteins, we have previously shown that the central regions of these domains as well as the N-terminal part of the TM domain of E1 are involved in heterodimerization. Here, we used a tryptophan replacement scan of these regions to identify individual residues that participate in those interactions. Our mutagenesis study identified at least four residues involved in heterodimerization: Gly 354, Gly 358, Lys 370, and Asp 728. Interestingly, Gly 354 and Gly 358 belong to a GXXXG oligomerization motif. Our tryptophan mutants were also used to generate retrovirus-based, HCV-pseudotyped particles (HCVpp) in order to analyze the effects of these mutations on virus entry. Surprisingly, two mutants consistently displayed higher infectivity compared to that of the wild type. In contrast, HCVpp infectivity was strongly affected for many mutants, despite normal E1E2 heterodimerization and normal levels of incorporation of HCV glycoproteins into HCVpp. The characterization of some of these HCVpp mutants in the recently developed in vitro fusion assay using fluorescent-labeled liposomes indicated that mutations reducing HCVpp infectivity without altering E1E2 heterodimerization affected the fusion properties of HCV envelope glycoproteins. In conclusion, this mutational analysis identified residues involved in E1E2 heterodimerization and revealed that the TM domains of HCV envelope glycoproteins play a major role in the fusion properties of these proteins.

Transient and stable expression of the HCV envelope glycoproteins in cell lines and primary hepatocytes transduced with a recombinant baculovirus

A recombinant baculovirus, RecBV-E, encoding the hepatitis C virus (HCV) envelope proteins, E1 and E2, controlled by the cytomegalovirus promoter was constructed. RecBVs can infect mammalian cells, but fail to express proteins or replicate because the viral DNA promoters are not recognised. The RecBV-E transduced 86% of Huh7 cells and 22% of primary marmoset hepatocytes compared with 35% and 0.4%, respectively, after DNA transfection. Several stable cell lines were generated that constitutively expressed E1/E2 in every cell. No evidence of E1/E2-related apoptosis was noted, and the doubling times of cells were similar to that of the parental cells. A proportion of the E1/E2 was expressed on the surface of the stable cells as determined by flow cytometry and was detected by a conformation-dependent monoclonal antibody. It is likely that the continued expression of E1/E2 in the stable cells resulted from integration of the RecBV DNA. Infection of Huh7 cells, in the absence of G418 selection, failed to result in expression of the foreign gene (in this case, eGFP) beyond 14-18 days. RecBVs that express HCV genes from a CMV promoter represent an effective means by which to transduce primary hepatocytes for expression and replication studies.

A conserved Gly436-Trp-Leu-Ala-Gly-Leu-Phe-Tyr motif in hepatitis C virus glycoprotein E2 is a determinant of CD81 binding and viral entry

The hepatitis C virus (HCV) glycoproteins E1 and E2 form a heterodimer that mediates CD81 receptor binding and viral entry. In this study, we used site-directed mutagenesis to examine the functional role of a conserved G436WLAGLFY motif of E2. The mutants could be placed into two groups based on the ability of mature virion-incorporated E1E2 to bind the large extracellular loop (LEL) of CD81 versus the ability to mediate cellular entry of pseudotyped retroviral particles. Group 1 comprised E2 mutants where LEL binding ability largely correlated with viral entry ability, with conservative and nonconservative substitutions (W437 L/A, L438A, L441V/F, and F442A) inhibiting both functions. These data suggest that Trp-437, Leu-438, Leu-441, and Phe-442 directly interact with the LEL. Group 2 comprised E2 glycoproteins with more conservative substitutions that lacked LEL binding but retained between 20% and 60% of wild-type viral entry competence. The viral entry competence displayed by group 2 mutants was explained by residual binding by the E2 receptor binding domain to cellular full-length CD81. A subset of mutants maintained LEL binding ability in the context of intracellular E1E2 forms, but this function was largely lost in virion-incorporated glycoproteins. These data suggest that the CD81 binding site undergoes a conformational transition during glycoprotein maturation through the secretory pathway. The G436P mutant was an outlier, retaining near-wild-type levels of CD81 binding but lacking significant viral entry ability. These findings indicate that the G436WLAGLFY motif of E2 functions in CD81 binding and in pre- or post-CD81-dependent stages of viral entry.

Oligomerization of hepatitis C virus core protein is crucial for interaction with the cytoplasmic domain of E1 envelope protein

Hepatitis C virus (HCV) contains two membrane-associated envelope glycoproteins, E1 and E2, which assemble as a heterodimer in the endoplasmic reticulum (ER). In this study, predictive algorithms and genetic analyses of deletion mutants and glycosylation site variants of the E1 glycoprotein were used to suggest that the glycoprotein can adopt two topologies in the ER membrane: the conventional type I membrane topology and a polytopic topology in which the protein spans the ER membrane twice with an intervening cytoplasmic loop (amino acid residues 288 to 360). We also demonstrate that the E1 glycoprotein is able to associate with the HCV core protein, but only upon oligomerization of the core protein in the presence of tRNA to form capsid-like structures. Yeast two-hybrid and immunoprecipitation analyses reveal that oligomerization of the core protein is promoted by amino acid residues 72 to 91 in the core. Furthermore, the association between the E1 glycoprotein and the assembled core can be recapitulated using a fusion protein containing the putative cytoplasmic loop of the E1 glycoprotein. This fusion protein is also able to compete with the intact E1 glycoprotein for binding to the core. Mutagenesis of the cytoplasmic loop of E1 was used to define a region of four amino acids (residues 312 to 315) that is important for interaction with the assembled HCV core. Taken together, our studies suggest that interaction between the self-oligomerized HCV core and the E1 glycoprotein is mediated through the cytoplasmic loop present in a polytopic form of the E1 glycoprotein.

The membrane-active regions of the hepatitis C virus E1 and E2 envelope glycoproteins

We have identified the membrane-active regions of the full sequences of the HCV E1 and E2 envelope glycoproteins by performing an exhaustive study of membrane leakage, hemifusion, and fusion induced by 18-mer peptide libraries on model membranes having different phospholipid compositions. The data and their comparison have led us to identify different E1 and E2 membrane-active segments which might be implicated in viral membrane fusion, membrane interaction, and/or protein-protein binding. Moreover, it has permitted us to suggest that the fusion peptide might be located in the E1 glycoprotein and, more specifically, the segment comprised by amino acid residues 265-296. The identification of these membrane-active segments from the E1 and E2 envelope glycoproteins, as well as their membranotropic propensity, supports their direct role in HCV-mediated membrane fusion, sustains the notion that different segments provide the driving force for the merging of the viral and target cell membranes, and defines those segments as attractive targets for further development of new antiviral compounds.

Basic residues in hypervariable region 1 of hepatitis C virus envelope glycoprotein e2 contribute to virus entry

The N terminus of hepatitis C virus (HCV) envelope glycoprotein E2 contains a hypervariable region (HVR1) which has been proposed to play a role in viral entry. Despite strong amino acid variability, HVR1 is globally basic, with basic residues located at specific sequence positions. Here we show by analyzing a large number of HVR1 sequences that the frequency of basic residues at each position is genotype dependent. We also used retroviral pseudotyped particles (HCVpp) harboring genotype 1a envelope glycoproteins to study the role of HVR1 basic residues in entry. Interestingly, HCVpp infectivity globally increased with the number of basic residues in HVR1. However, a shift in position of some charged residues also modulated HCVpp infectivity. In the absence of basic residues, infectivity was reduced to the same level as that of a mutant deleted of HVR1. We also analyzed the effect of these mutations on interactions with some potential HCV receptors. Recognition of CD81 was not affected by changes in the number of charged residues, and we did not find a role for heparan sulfates in HCVpp entry. The involvement of the scavenger receptor class B type I (SR-BI) was indirectly analyzed by measuring the enhancement of infectivity of the mutants in the presence of the natural ligand of SR-BI, high-density lipoproteins (HDL). However, no correlation between the number of basic residues within HVR1 and HDL enhancement effect was observed. Despite the lack of evidence of the involvement of known potential receptors, our results demonstrate that the presence of basic residues in HVR1 facilitates virus entry.

Subcellular localization of hepatitis C virus structural proteins in a cell culture system that efficiently replicates the virus

Due to the recent development of a cell culture model, hepatitis C virus (HCV) can be efficiently propagated in cell culture. This allowed us to reinvestigate the subcellular localization of HCV structural proteins in the context of an infectious cycle. In agreement with previous reports, confocal immunofluorescence analysis of the subcellular localization of HCV structural proteins indicated that, in infected cells, the glycoprotein heterodimer is retained in the endoplasmic reticulum. However, in contrast to other studies, the glycoprotein heterodimer did not accumulate in other intracellular compartments or at the plasma membrane. As previously reported, an association between the capsid protein and lipid droplets was also observed. In addition, a fraction of labeling was consistent with the capsid protein being localized in a membranous compartment that is associated with the lipid droplets. However, in contrast to previous reports, the capsid protein was not found in the nucleus or in association with mitochondria or other well-defined intracellular compartments. Surprisingly, no colocalization was observed between the glycoprotein heterodimer and the capsid protein in infected cells. Electron microscopy analyses allowed us to identify a membrane alteration similar to the previously reported "membranous web." However, no virus-like particles were found in this type of structure. In addition, dense elements compatible with the size and shape of a viral particle were seldom observed in infected cells. In conclusion, the cell culture system for HCV allowed us for the first time to characterize the subcellular localization of HCV structural proteins in the context an infectious cycle.

Cell entry of hepatitis C virus

Hepatitis C virus (HCV), an important human pathogen, is an enveloped, positive-stranded RNA virus classified in the hepacivirus genus of the Flaviviridae family. Cell attachment of flaviviruses generally leads to endocytosis of bound virions. Systems that support HCV replication and particle formation in vitro are emerging only now, 16 years after the discovery of the virus. Albeit this limitation, the route of HCV cell entry as well as 'capture' molecules involved in low-affinity interactions for the initial contact of HCV with target cells and potential high-affinity receptor candidates that may mediate HCV trafficking and fusion has been described. The objective of this review is to summarize the contribution of different HCV model systems to our current knowledge about structure of the HCV GPs E1 and E2 and their roles in cell entry comprising cell attachment, interactions with cellular receptors, endocytosis, and fusion.

Assembly of functional hepatitis C virus glycoproteins on infectious pseudoparticles occurs intracellularly and requires concomitant incorporation of E1 and E2 glycoproteins

Hepatitis C virus (HCV) E1 and E2 envelope glycoproteins (GPs) displayed on retroviral cores (HCVpp) are a powerful and highly versatile model system to investigate wild-type HCV entry. To further characterize this model system, the cellular site of HCVpp assembly and the respective roles of the HCV GPs in this process were investigated. By using a combination of biochemical methods with confocal and electron microscopic techniques, it was shown that, in cells producing HCVpp, both E1 and E2 colocalized with retroviral core proteins intracellularly, presumably in multivesicular bodies, but not at the cell surface. When E1 and E2 were expressed individually with retroviral core proteins, only E2 colocalized with and was incorporated on retroviral cores. Conversely, the colocalization of E1 with retroviral core proteins and its efficient incorporation occurred only upon co-expression of E2. Moreover, HCVpp infectivity correlated strictly with the presence of both E1 and E2 on retroviral cores. Altogether, these results confirm that the E1E2 heterodimer constitutes the prebudding form of functional HCV GPs and, more specifically, show that dimerization with E2 is a prerequisite for efficient E1 incorporation onto particles.

Evidence for a polytopic form of the E1 envelope glycoprotein of Hepatitis C virus

The polyprotein precursor of the Hepatitis C virus (HCV) contains multiple membrane-spanning domains that define the membrane topology and subsequent maturation of the viral structural proteins. In order to examine the biogenesis of the E1-E2 heterodimeric complex, we inserted an affinity tag (S-peptide) at specific locations within the envelope glycoproteins. In particular, and based on the prediction that the E1 glycoprotein may be able to assume a polytopic topology containing two membrane-spanning domains, we inserted the affinity tag within a putative cytoplasmic loop of the E1 glycoprotein. The HCV structural polyprotein containing this tag (at amino acids 295/296) was highly expressed and able to form a properly processed and noncovalently associated E1-E2 complex. This complex was bound by murine and conformation-dependent human monoclonal antibodies (MAbs) comparably to the native untagged complex. In addition, MAb recognition was retained upon reconstituting the tagged E1-E2 complex in lipid membrane as topologically constrained proteoliposomes. Our findings are consistent with the model of a topologically flexible E1 glycoprotein that is able to adopt a polytopic form. This form of the E1-E2 complex may be important in the HCV life cycle and in pathogenesis.

Novel insights into hepatitis C virus replication and persistence

Hepatitis C virus (HCV) is a small enveloped RNA virus that belongs to the family Flaviviridae. A hallmark of HCV is its high propensity to establish a persistent infection that in many cases leads to chronic liver disease. Molecular studies of the virus became possible with the first successful cloning of its genome in 1989. Since then, the genomic organization has been delineated, and viral proteins have been studied in some detail. In 1999, an efficient cell culture system became available that recapitulates the intracellular part of the HCV life cycle, thereby allowing detailed molecular studies of various aspects of viral RNA replication and persistence. This chapter attempts to summarize the current state of knowledge in these most actively worked on fields of HCV research.

Efficient formation of vesicular stomatitis virus pseudotypes bearing the native forms of hepatitis C virus envelope proteins detected after sonication

Hepatitis C virus (HCV) causes chronic hepatitis, liver cirrhosis and hepatocellular carcinoma in addition to acute hepatitis. The HCV genome encodes two envelope glycoproteins, E1 and E2. To investigate the role of E1 and E2 in HCV infection, we used a recombinant vesicular stomatitis virus (VSV), VSVdeltaG*, harboring the green fluorescent protein gene instead of the VSV G envelope protein gene. It was complemented with the native form of E1 and E2, or E1 or E2 alone, to make HCV pseudotypes VSVdeltaG*(HCV), VSVdeltaG*(E1), and VSVdeltaG*(E2). Neither E1 nor E2 expression was detected on the cell surface, as reported. Unlike previous reports, infectious activities of VSVdeltaG*(HCV), VSVdeltaG*(E1) and VSVdeltaG*(E2) pseudotypes were detected under conditions where VSV was completely neutralized by anti-VSV. We could enhance the infectious titers 100-fold by sonication upon virus harvest. Bovine lactoferrin efficiently inhibited infection by VSVdeltaG*(HCV) as well as VSVdeltaG*(E2), as the interaction between E2 and lactoferrin has been thought to contribute to the inhibition of HCV infectivity. VSVdeltaG*(HCV) infected many adherent cell lines, including hepatic cell lines, but not most hematopoietic cell lines. Treatment of cells with trypsin, tunicamycin, or sulfated polysaccharides before infection reduced the infectivity of VSVdeltaG*(HCV) by about 90%, suggesting that a cell surface protein(s) with sugar chains plays an important role in HCV infection. The VSV pseudotypes developed here would be useful for analyzing the early stages of HCV infection.

Coexpression of hepatitis C virus E1 and E2 chimeric envelope glycoproteins displays separable ligand sensitivity and increases pseudotype infectious titer

We have previously reported that a pseudotype virus generated by reconstitution of hepatitis C virus (HCV) chimeric envelope glycoprotein E1-G or E2-G on the surface of a temperature-sensitive mutant of vesicular stomatitis virus (VSVts045) interacts independently with mammalian cells to initiate infection. Here, we examined whether coexpression of both of the envelope glycoproteins on pseudotype particles would augment virus infectivity and/or alter the functional properties of the individual subunits. Stable transfectants of baby hamster kidney (BHK) epithelial cells expressing either one or both of the chimeric envelope glycoproteins of HCV on the cell surface were generated. The infectious titer of the VSV pseudotype, derived from a stable cell line incorporating both of the chimeric glycoproteins of HCV, was approximately 4- to 5-fold higher than that of a pseudotype bearing E1-G alone or approximately 25- to 30-fold higher than that of E2-G alone when assayed with a number of mammalian cell lines. Further studies suggested that that the E1-G/E2-G or E2-G pseudotype was more sensitive to the inhibitory effect of heparin than the E1-G pseudotype. Treatment of the E1-G/E2-G pseudotype with a negatively charged sulfated sialyl lipid (NMSO3) displayed a approximately 4-fold-higher sensitivity to neutralization than pseudotypes with either of the two individual glycoproteins. In contrast, VSVts045, used as a backbone for the generation of pseudotypes, displayed at least 20-fold-higher sensitivity to NMSO3-mediated inhibition of virus plaque formation. The effect of low-density lipoprotein on the E1-G pseudotype was greater than that apparent for the E1-G/E2-G pseudotype. The treatment of cells with monoclonal antibodies to CD81 displayed an inhibitory effect upon the pseudotype with E1-G/E2-G or with E2-G alone. Taken together, our results indicate that the HCV E1 and E2 glycoproteins have separable functional properties and that the presence of these two envelope glycoproteins on VSV/HCV pseudotype particles increases infectious titer.

The transmembrane domains of the prM and E proteins of yellow fever virus are endoplasmic reticulum localization signals

The immature flavivirus particle contains two envelope proteins, prM and E, that are associated as a heterodimer. Virion morphogenesis of the flaviviruses occurs in association with endoplasmic reticulum (ER) membranes, suggesting that there should be accumulation of the virion components in this compartment. This also implies that ER localization signals must be present in the flavivirus envelope proteins. In this work, we looked for potential subcellular localization signals in the yellow fever virus envelope proteins. Confocal immunofluorescence analysis of the subcellular localization of the E protein in yellow fever virus-infected cells indicated that this protein accumulates in the ER. Similar results were obtained with cells expressing only prM and E. Chimeric proteins containing the ectodomain of CD4 or CD8 fused to the transmembrane domains of prM or E were constructed, and their subcellular localization was studied by confocal immunofluorescence and by analyzing the maturation of their associated glycans. Although a small fraction was detected in the ER-to-Golgi intermediate and Golgi compartments, these chimeric proteins were located mainly in the ER. The C termini of prM and E form two antiparallel transmembrane alpha-helices. Interestingly, the first transmembrane passage contains enough information for ER localization. Taken altogether, these data indicate that, besides their role as membrane anchors, the transmembrane domains of yellow fever virus envelope proteins are ER retention signals. In addition, our data show that the mechanisms of ER retention of the flavivirus and hepacivirus envelope proteins are different.

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.

The transmembrane domain of hepatitis C virus E1 glycoprotein induces cell death

The E1 protein of hepatitis C virus (HCV) shows the ability to induce cell lysis by the alteration of membrane permeability when expressed in Escherichia coli cells. This function seems to be an intrinsic property of a C-terminal hydrophobic region of E1 as permeability changes and cell lysis can be blocked by mutagenesis of specific amino acids in this domain. To establish whether the expression of E1 protein and its C-terminal domain was able to induce cell death also in eukaryotic cell, we cloned HCV sequences expressing the full-length E1 (E383), the C-terminal domain (SVP) and a mutant lacking the C-terminal region (E340) in the pRC/CMV expression vector. HepG2 cell line was co-transfected with empty vector or HCV expression plasmids and a reporter vector that expressed beta-galactosidase (beta-gal) to visualize co-transfected blue cells. At 60 h after transfection, the loss of blue cells, considered as a measure of cell death, was 31.5 and 64.3% for the E1 and SVP clones. On the contrary, the number of blue cells after transfection with E340 plasmid was similar to that observed with the control vector. The analysis by the terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling (TUNEL) assay revealed an increased number of apoptotic cells at 48 h after transfection with E1 and SVP clones. Furthermore, cells transfected with SVP revealed a typical internucleosomal DNA fragmentation and the activation of caspase-3-like proteases as the specific inhibitor Ac-DEVD-CHO peptide partially blocked SVP apoptosis. These data indicate that the intracellular expression of HCV E1 protein and its C-terminal domain induces an apoptotic response in human hepatoma cell line.

Influence of N-linked glycans on intracellular transport of hepatitis C virus E1 chimeric glycoprotein and its role in pseudotype virus infectivity

We have previously reported a functional role associated with hepatitis C virus (HCV) E1 glycoprotein using vesicular stomatitis virus (VSV)/HCV pseudotype. In this study, we have investigated the role of glycosylation upon intracellular transport of chimeric E1-G, and in infectivity of the pseudotyped virus. Interestingly, surface expressed E1-G exhibited sensitivity to Endoglycosidase H (Endo H) treatment, which was similar to full-length E1, suggesting that additional complex oligosaccharides were not added while E1-G was in transit from the endoplasmic reticulum (ER) to the mammalian cell surface. As a next step, each of the four potential N-linked glycosylation sites located at amino acid position 196, 209, 234, or 305 of the E1 ectodomain were mutated separately (asparagine --> glutamine), or in some combination. FACS analysis suggested that mutation(s) of the glycosylation sites affect the translocation of E1-G to the cell surface to different extents, with no single site being particularly essential. VSV pseudotype virus generated from glycosylation mutants exhibited a decrease in titer with an increasing number of mutations at the glycosylation sites on chimeric E1-G. In a separate experiment, N-glycosidase F treatment of pseudotype generated from the already synthesized E1-G or its mutants decreased virus titer by approximately 35%, and the neutralization activity of patient sera was not significantly altered with N-glycosidase F-treated pseudotype virus. Taken together, our results suggested that E1-G does not add complex sugar moieties during transport to the cell surface and retain the glycosylation profile of its parental E1 sequence. Additionally, the removal of glycans from the E1-G reduced, but does not completely impair, virus infectivity.

Characterization of infectious retroviral pseudotype particles bearing hepatitis C virus glycoproteins

The recent development of infectious retroviral pseudotypes bearing hepatitis C virus (HCV) glycoproteins represents an opportunity to study the functionally active form of the HCV E1 and E2 glycoproteins. In the culture supernatant of cells producing HCV retroviral pseudotypes, the majority of E2 was not associated with infectious particles and failed to sediment on sucrose gradients. The E2 that was incorporated into infectious particles appeared as a triplet of diffuse bands at 60, 70, and 90 kDa. Similarly, three major forms of E1 were incorporated into the pseudotype particles, migrating at 33, 31, and 25 kDa. Endoglycosidase H (endo-H) treatment of particles demonstrated that the incorporated E1 was partially or completely sensitive to digestion. In contrast, the majority of the incorporated E2 was endo-H resistant. Purified pseudotype particles were found to contain both disulfide-linked aggregates and nonaggregated E1 and E2. HCV pseudotypes generated from cells expressing E1E2p7 showed similar heterogeneity in the incorporated glycoproteins and were of comparable infectivity to those generated by expression of E1E2. Our results demonstrate the heterogeneous nature of E1 and E2 incorporated into retroviral pseudotypes and highlight the difficulty in identifying forms of the HCV glycoproteins that initiate infection.

C-type Lectins L-SIGN and DC-SIGN Capture and Transmit Infectious Hepatitis C Virus Pseudotype Particles

The molecular mechanisms involved in the hepatic tropism of hepatitis C virus (HCV) have not been identified. We have shown previously that liver-expressed C-type lectins L-SIGN and DC-SIGN bind the HCV E2 glycoprotein with high affinity (Lozach, P. Y., Lortat-Jacob, H., de Lacroix de Lavalette, A., Staropoli, I., Foung, S., Amara, A., Houles, C., Fieschi, F., Schwartz, O., Virelizier, J. L., Arenzana-Seisdedos, F., and Altmeyer, R. (2003) J. Biol. Chem. 278, 20358-20366). To analyze the functional relevance of this interaction, we generated pseudotyped lentivirus particles presenting HCV glycoproteins E1 and E2 at the virion surface (HCV-pp). High mannose N-glycans are present on E1 and, to a lesser extent, on E2 proteins of mature infectious HCV-pp. Such particles bind to both L-SIGN and DC-SIGN, but they cannot use these receptors for entry into cells. However, infectious virus is transmitted efficiently when permissive Huh-7 cells are cocultured with HCV-pp bound to L-SIGN or to DC-SIGN-positive cell lines. HCV-pp transmission via L-SIGN or DC-SIGN is inhibited by characteristic inhibitors such as the calcium chelator EGTA and monoclonal antibodies directed against lectin carbohydrate recognition domains of both lectins. In support of the biological relevance of this phenomenon, dendritic cells expressing endogenous DC-SIGN transmitted HCV-pp with high efficiency in a DC-SIGN-dependent manner. Our results support the hypothesis that C-type lectins such as the liver sinusoidal endothelial cell-expressed L-SIGN could act as a capture receptor for HCV in the liver and transmit infectious virions to neighboring hepatocytes.

Characterization of functional hepatitis C virus envelope glycoproteins

Hepatitis C virus (HCV) encodes two envelope glycoproteins, E1 and E2, that assemble as a noncovalent heterodimer which is mainly retained in the endoplasmic reticulum. Because assembly into particles and secretion from the cell lead to structural changes in viral envelope proteins, characterization of the proteins associated with the virion is necessary in order to better understand how they mature to be functional in virus entry. There is currently no efficient and reliable cell culture system to amplify HCV, and the envelope glycoproteins associated with the virion have therefore not been characterized yet. Recently, infectious pseudotype particles that are assembled by displaying unmodified HCV envelope glycoproteins on retroviral core particles have been successfully generated. Because HCV pseudotype particles contain fully functional envelope glycoproteins, these envelope proteins, or at least a fraction of them, should be in a mature conformation similar to that on the native HCV particles. In this study, we used conformation-dependent monoclonal antibodies to characterize the envelope glycoproteins associated with HCV pseudotype particles. We showed that the functional unit is a noncovalent E1E2 heterodimer containing complex or hybrid type glycans. We did not observe any evidence of maturation by a cellular endoprotease during the transport of these envelope glycoproteins through the secretory pathway. These envelope glycoproteins were recognized by a panel of conformation-dependent monoclonal antibodies as well as by CD81, a molecule involved in HCV entry. The functional envelope glycoproteins associated with HCV pseudotype particles were also shown to be sensitive to low-pH treatment. Such conformational changes are likely necessary to initiate fusion.

HCV E2 glycoprotein: mutagenesis of N-linked glycosylation sites and its effects on E2 expression and processing

An expression system for analysis of the synthesis and processing of the E2 glycoprotein of a hepatitis C virus (HCV) genotype 1a strain was developed in transiently transfected cells. E2 proteins representing the entire length of the protein, including the transmembrane segment (E2) as well as two truncated versions (E2(660) and E2(715)), were characterized for acquisition of N-linked glycans and transport to the media of transfected cells. To investigate the utilization of the 10 potential N-linked glycosylation sites on this E2 protein, a series of mutations consisting of single or multiple (two, three, four or eight) ablations of asparagine residues in the background of the E2(660) construct were analyzed. E2(660) proteins harboring single or multiple site mutations were produced at levels similar to that of wild-type protein, but secretion of the single mutants was mildly diminished, and elimination of two or more sites dramatically reduced delivery of the protein to the media. Similar results were obtained in Huh-7 cells with respect to intracellular synthesis and secretion of the mutant proteins. Analysis of oligosaccharide composition using endoglycosidase digestion revealed that all of the glycan residues on the intracellular forms of E2(660), E2(715), and E2 contained N-linked glycans modified into high-mannose carbohydrates, in contrast to the secreted forms, which were endo H resistant. The parental E2(660) protein could be readily detected in Huh-7 cells using anti-polyhistidine or antibody to recombinant E2. In contrast, E2(660) lacking the eight N-linked glycans was expressed but not detectable with anti-E2 antibody, and proteins lacking four glycans exhibited reduced reactivity. These experiments provide direct evidence that the presence of multiple N-linked glycans is required for the proper folding of the E2 protein in the ER and secretory pathway as well as for formation of its antigenic structure.

Expression of unmodified hepatitis C virus envelope glycoprotein-coding sequences leads to cryptic intron excision and cell surface expression of E1/E2 heterodimers comprising full-length and partially deleted E1

Hepatitis C virus (HCV) is a positive-strand RNA virus that replicates exclusively in the cytoplasm of infected cells. The viral envelope glycoproteins, E1 and E2, appear to be retained in the endoplasmic reticulum, where viral budding is thought to occur. Surprisingly, we found that the expression system used to generate HCV envelope glycoproteins influences their subcellular localization and processing. These findings have important implications for optimizing novel HCV fusion and entry assays as well as for budding and virus particle formation.

Structural biology of hepatitis C virus

Hepatitis C virus (HCV) causes acute and chronic liver disease in humans, including chronic hepatitis, cirrhosis, and hepatocellular carcinoma. Studies of this virus have been hampered by the lack of a productive cell culture system; most information thus has been obtained from analysis of the HCV genome, heterologous expression systems, in vitro and in vivo models, and structural analyses. Structural analyses of HCV components provide an essential framework for understanding of the molecular mechanisms of HCV polyprotein processing, RNA replication, and virion assembly and may contribute to a better understanding of the pathogenesis of hepatitis C. Moreover, these analyses should allow the identification of novel targets for antiviral intervention and development of new strategies to prevent and combat viral hepatitis. This article reviews the current knowledge of HCV structural biology.

Dengue virus M protein contains a proapoptotic sequence referred to as ApoptoM

The induction of apoptotic cell death is a prominent cytopathic effect of dengue (DEN) viruses. One of the key questions to be addressed is which viral components induce apoptosis in DEN virus-infected cells. This study investigated whether the small membrane (M) protein was involved in the induction of apoptosis by DEN virus. This was addressed by using a series of enhanced green fluorescent protein-fused DEN proteins. Evidence is provided that intracellular production of the M ectodomains (residues M-1 to M-40) of all four DEN serotypes triggered apoptosis in host cells such as mouse neuroblastoma Neuro 2a and human hepatoma HepG2 cells. The M ectodomains of the wild-type strains of Japanese encephalitis, West Nile and yellow fever viruses also had proapoptotic properties. The export of the M ectodomain from the Golgi apparatus to the plasma membrane appeared to be essential for the initiation of apoptosis. The study found that anti-apoptosis protein Bcl-2 protected HepG2 cells against the death-promoting activity of the DEN M ectodomain. This suggests that the M ectodomain exerts its cytotoxic effects by activating a mitochondrial apoptotic pathway. The cytotoxicity of the DEN M ectodomain reflected the intrinsic proapoptotic properties of the nine carboxy-terminal amino acids (residues M-32 to M-40) designated ApoptoM: Residue M-36 was unique in that it modulated the death-promoting activity of the M ectodomain. Defining the ApoptoM-activated signalling pathways leading to apoptosis will provide the basis for studying how the M protein might play a key role in the fate of the flavivirus-infected cells.

CD81-dependent binding of hepatitis C virus E1E2 heterodimers

Hepatitis C virus (HCV) is the leading cause of chronic liver disease worldwide. HCV is also the major cause of mixed cryoglobulinemia, a B-lymphocyte proliferative disorder. Direct experimentation with native viral proteins is not feasible. Truncated versions of recombinant E2 envelope proteins, used as surrogates for viral particles, were shown to bind specifically to human CD81. However, truncated E2 may not fully mimic the surface of HCV virions because the virus encodes two envelope glycoproteins that associate with each other as E1E2 heterodimers. Here we show that E1E2 complexes efficiently bind to CD81 whereas truncated E2 is a weak binder, suggesting that truncated E2 is probably not the best tool with which to study cellular interactions. To gain better insight into virus-cell interactions, we developed a method by which to isolate E1E2 complexes that are properly folded. We demonstrate that purified E1E2 heterodimers bind to cells in a CD81-dependent manner. Furthermore, engagement of B cells by purified E1E2 heterodimers results in their aggregation and in protein tyrosine phosphorylation, a hallmark of B-cell activation. These studies provide a possible clue to the etiology of HCV-associated B-cell lymphoproliferative diseases. They also delineate a method by which to isolate biologically functional E1E2 complexes for the study of virus-host cell interaction in other cell types.

Molecular viral oncology of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer, but the third leading cause of cancer death, in the world, with more than 500,000 fatalities annually. The major etiology of HCC/liver cancer in people is hepatitis B virus (HBV), followed by hepatitis C virus infection (HCV), although nonviral causes also play a role in a minority of cases. Recent molecular studies confirm what was suspected: that HCC tissue from different individuals have many phenotypic differences. However, there are clearly features that unify HCC occurring in a background of viral hepatitis B and C. HCC due to HBV and HCV may be an indirect result of enhanced hepatocyte turnover that occurs in an effort to replace infected cells that have been immunologically attacked. Viral functions may also play a more direct role in mediating oncogenesis. This review considers the molecular and cellular mechanisms involved in primary hepatocellular carcinoma, using a viral perspective.

Cell surface expression of functional hepatitis C virus E1 and E2 glycoproteins

Hepatitis C virus (HCV) glycoproteins E1 and E2 are believed to be retained in the endoplasmic reticulum (ER) or cis-Golgi compartment via retention signals located in their transmembrane domains. Here we describe the detection of E1 and E2 at the surface of transiently transfected HEK 293T and Huh7 cells. Surface-localized E1E2 heterodimers presented exclusively as non-covalently associated complexes. Surface-expressed E2 contained trans-Golgi modified complex/hybrid type carbohydrate and migrated diffusely between 70 and 90 kDa while intracellular E1 and E2 existed as high mannose 35 kDa and 70 kDa precursors, respectively. In addition, surface-localized E1E2 heterodimers were incorporated into E1E2-pseudotyped HIV-1 particles that were competent for entry into Huh7 cells. These studies suggest that functional HCV glycoproteins are not retained exclusively in the ER and transit through the secretory pathway.

Topology of hepatitis C virus envelope glycoproteins

Hepatitis C virus encodes two envelope glycoproteins, E1 and E2, that are released from a polyprotein precursor after cleavage by host signal peptidase(s). These proteins contain a large N-terminal ectodomain and a C-terminal transmembrane domain, and they assemble as a noncovalent heterodimer. The transmembrane domains of hepatitis C virus envelope glycoproteins have been shown to be multifunctional: (1) they are membrane anchors, (2) they bear ER retention signals, (3) they contain a signal sequence function, and (4) they are involved in E1-E2 heterodimerisation. Due to these multiple functions, the topology adopted by these transmembrane domains has given rise to much controversy. They are less than 30 amino acid residues long and are composed of two stretches of hydrophobic residues separated by a short segment containing one or two fully conserved positively charged residues. The presence of a signal sequence function in the C-terminal half of the transmembrane domains of E1 and E2 had suggested that these domains are composed of two membrane spanning segments. However, the two hydrophobic stretches are too short to make two membrane spanning alpha-helices. These discrepancies can now be explained by a dynamic model, based on experimental data, describing the early steps of the biogenesis of hepatitis C virus envelope glycoproteins. In this model, the transmembrane domains of E1 and E2 form a hairpin structure before cleavage by a signal peptidase, and a reorientation of the second hydrophobic stretch occurs after cleavage to produce a single membrane spanning domain.

Nonstructural protein precursor NS4A/B from hepatitis C virus alters function and ultrastructure of host secretory apparatus

The nonstructural proteins of hepatitis C virus (HCV) have been shown previously to localize to the endoplasmic reticulum (ER) when expressed singly or in the context of other HCV proteins. To determine whether the expression of HCV nonstructural proteins alters ER function, we tested the effect of expression of NS2/3/4A, NS4A, NS4B, NS4A/B, NS4B/5A, NS5A, and NS5B from genotype 1b HCV on anterograde traffic from the ER to the Golgi apparatus. Only the nominal precursor protein NS4A/B affected the rate of ER-to-Golgi traffic, slowing the rate of Golgi-specific modification of the vesicular stomatitis virus G protein expressed by transfection by approximately threefold. This inhibition of ER-to-Golgi traffic was not observed upon expression of the processed proteins NS4A and NS4B, singly or in combination. To determine whether secretion of other cargo proteins was inhibited by NS4A/B expression, we monitored the appearance of newly synthesized proteins on the cell surface in the presence and absence of NS4A/B expression; levels of all were reduced in the presence of NS4A/B. This reduction is also seen in cells that contain genome length HCV replicons: the rate of appearance of major histocompatibility complex class I (MHC-I) on the cell surface was reduced by three- to fivefold compared to that for a cured cell line. The inhibition of protein secretion caused by NS4A/B does not correlate with the ultrastructural changes leading to the formation a "membranous web" (D. Egger et al., J. Virol. 76:5974-5984, 2002), which can be caused by expression of NS4B alone. Inhibition of global ER-to-Golgi traffic could, by reducing cytokine secretion, MHC-I presentation, and transport of labile membrane proteins to the cell surface, have significant effects on the host immune response to HCV infection.

DC-SIGN and L-SIGN are high affinity binding receptors for hepatitis C virus glycoprotein E2

The hepatitis C virus (HCV) genome codes for highly mannosylated envelope proteins, which are naturally retained in the endoplasmic reticulum. We found that the HCV envelope glycoprotein E2 binds the dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) and the related liver endothelial cell lectin L-SIGN through high-mannose N-glycans. Competing ligands such as mannan and an antibody directed against the carbohydrate recognition domains (CRD) abrogated binding. While no E2 interaction with distant monomeric CRDs on biosensor chips could be detected, binding is observed if CRDs are closely seeded (Kd = 48 nm) and if the CRD is part of the oligomeric-soluble extracellular domain of DC-SIGN (Kd = 30 nm). The highest affinity is seen for plasma membrane-expressed DC-SIGN and L-SIGN (Kd = 3 and 6 nm, respectively). These results indicate that several high-mannose N-glycans in a structurally defined cluster on E2 bind to several subunits of the oligomeric lectin CRD. High affinity interaction of viral glycoproteins with oligomeric lectins might represent a strategy by which HCV targets to and concentrates in the liver and infects dendritic cells.

A cytoplasmic region of the NSP4 enterotoxin of rotavirus is involved in retention in the endoplasmic reticulum

The rotavirus genome encodes two glycoproteins, one structural (VP7) and one non-structural (NSP4), both of which mature and remain in the endoplasmic reticulum (ER). While three amino acids in the N terminus have been proposed to function as a retention signal for VP7, no information is yet available on how NSP4 remains associated with the ER. In this study, we have investigated the ER retention motif of NSP4 by producing various C-terminal truncations. Deleting the C terminus by 52 amino acids did not change the intracellular distribution of NSP4, but an additional deletion of 38 amino acids diminished the ER retention and resulted in the expression of NSP4 on the cell surface. Brefeldin A treatment prevented NSP4 from reaching the cell surface, suggesting that C-terminal truncated plasma membrane NSP4 is transported through the normal secretory pathway. On the basis of these results, we propose that the region between amino acids 85 and 123 in the cytoplasmic region of NSP4 are involved in ER retention.

The hepatitis C virus persistence: how to evade the immune system?

Hepatitis C virus (HCV) is an emerging virus of medical importance. A majority of HCV infections become chronic and lead to chronic hepatitis, liver cirrhosis and hepatocellular carcinoma. HCV usually induces robust immune responses, but it frequently escapes the immune defense to establish persistent infection. The fact that HCV exists as an evolving quasispecies plays an important role in the selection of escape mutants. Furthermore, several viral proteins interfere with cellular functions, in particular, those involved in the immune response of the host. Several HCV proteins also modulate cell signalling through interaction with different effectors involved in cell proliferation and apoptosis, or in the interferon-signalling pathway. In addition, HCV infects immune cells such as B and T cells, and thus affects their normal functions. These various strategies used by HCV to counter the immune response of the host are reviewed here. A better understanding of these mechanisms would help design new therapeutic targets.

Analysis of the subcellular localization of hepatitis C virus E2 glycoprotein in live cells using EGFP fusion proteins

Hepatitis C virus (HCV) E1 and E2 glycoproteins assemble intracellularly to form a non-covalently linked heterodimer, which is retained in the endoplasmic reticulum (ER). To study the subcellular localization of E2 in live cells, the enhanced green fluorescent protein (EGFP) was fused to the N terminus of E2. Using fluorescence and confocal microscopy, we have confirmed that E2 is located in the ER, where budding of HCV virions is thought to occur. Immunoprecipitation experiments using a conformation-sensitive antibody and a GST pull-down assay showed that fusion of EGFP to E2 interferes neither with its heterodimeric assembly with E1, nor with proper folding of the ectodomain, nor with the capacity of E2 to interact with human CD81, indicating that the EGFP-E2 fusion protein is functional. As a tool to study binding of E2 to target cells, we also described the expression of an EGFP-E2 fusion protein at the cell surface.

Yellow fever virus replicons as an expression system for hepatitis C virus structural proteins

Chimeric yellow fever virus (YF) RNAs were constructed in which the YF structural genes were replaced by the hepatitis C virus (HCV) structural genes or fusions between the YF and HCV structural genes. Interestingly, RNA replication required nucleotide complementarity between the 3'-located conserved sequence 1 and an RNA sequence located in the 5' end of the YF capsid sequence. The (chimeric-)HCV structural proteins were efficiently expressed and processed, and the native E1/E2 heterodimer was formed. However, no indication for the production of HCV-like particles was obtained.

Recognition of native hepatitis C virus E1E2 heterodimers by a human monoclonal antibody

The majority of hepatitis C virus (HCV)-infected individuals progress from acute to chronic disease, despite the presence of a strong humoral immune response to the envelope glycoproteins E1 and E2. When expressed in mammalian cells, E1 and E2 form both noncovalently linked E1E2 heterodimers, believed to be properly folded, and disulfide-linked, high-molecular-weight aggregates that are misfolded. Previously, we identified 10 human monoclonal antibodies (HMAbs) that bind E2 glycoproteins from different genotypes. Here we demonstrate that one of these HMAbs, CBH-2, is unique in its ability to distinguish between properly folded and misfolded envelope proteins. This HMAb recognizes HCV-E2 only when complexed with E1. The E1E2 complexes recognized by CBH-2 are noncovalently linked heterodimers and not misfolded disulfide-linked, high-molecular-weight aggregates. The E1E2 heterodimers seen by CBH-2 no longer associate with the endoplasmic reticulum chaperone calnexin and are likely to represent the prebudding form of the HCV virion.

The first hydrophobic domain of the hepatitis C virus E1 protein is important for interaction with the capsid protein

The interaction between the hepatitis C virus capsid protein and the envelope protein E1 has been demonstrated previously in vivo. To determine the binding region of the E1 protein with the capsid protein, this interaction was characterized in vitro. This study shows that the interaction between these proteins should occur in the endoplasmic reticulum membrane rather than in the cytosol and that the first hydrophobic domain of the E1 protein (aa 261-291) is important for the interaction with the capsid protein.

The hepatitis C virus RNA-dependent RNA polymerase membrane insertion sequence is a transmembrane segment

The hepatitis C virus (HCV) RNA-dependent RNA polymerase (RdRp) belongs to a class of membrane proteins termed tail-anchored proteins. Here, we show that the HCV RdRp C-terminal membrane insertion sequence traverses the phospholipid bilayer as a transmembrane segment. Moreover, the HCV RdRp was found to be retained in the endoplasmic reticulum (ER) or an ER-derived modified compartment both following transient transfection and in the context of a subgenomic replicon. An absolutely conserved GVG motif was not essential for membrane insertion but possibly provides a docking site for transmembrane protein-protein interactions. These findings have important implications for the functional architecture of the HCV replication complex.

Replication of a cytopathic strain of bovine viral diarrhea virus activates PERK and induces endoplasmic reticulum stress-mediated apoptosis of MDBK cells

Endoplasmic reticulum (ER) stress signaling is an adaptive cellular response to the loss of ER Ca(2+) homeostasis and/or the accumulation of misfolded, unassembled, or aggregated proteins in the ER lumen. ER stress-activated signaling pathways regulate protein synthesis initiation and can also trigger apoptosis through the ER-associated caspase 12. Viruses that utilize the host cell ER as an integral part of their life cycle would be predicted to cause some level of ER stress. Bovine viral diarrhea virus (BVDV) is a positive-stranded RNA virus of the Flaviviridae family. BVDV and related flaviviruses use the host ER as the primary site of envelope glycoprotein biogenesis, genomic replication, and particle assembly. We are using a cytopathic strain of BVDV (cpBVDV) that causes cellular apoptosis as a model system to determine how virus-induced ER stress contributes to pathogenesis. We show that, in a natural infection of MDBK cells, cpBVDV activates the ER transmembrane kinase PERK (PKR-like ER kinase) and causes hyperphosphorylation of the translation initiation factor eIF2 alpha, consistent with the induction of an ER stress response. Additionally, we show that initiation of cellular apoptosis correlates with downregulation of the antiapoptotic Bcl-2 protein, induced expression of caspase 12, and a decrease in intracellular glutathione levels. Defining the molecular stress pathways leading to cpBVDV-induced apoptosis provides the basis to study how other ER-tropic viruses, such as hepatitis C and B viruses, modulate the host cell ER stress response during the course of persistent infection.

Characterization of hepatitis C virus quasispecies by matrix-assisted laser desorption ionization-time of flight (mass spectrometry) mutation detection

Hepatitis C virus (HCV), the causative agent of hepatitis C, frequently causes chronic infection. The mechanisms of viral persistence continue to be the object of investigation. An important aspect of HCV chronic infection is the quasispecies nature of the viral population, which has been particularly well documented in the hypervariable region 1 of the E2 glycoprotein. Recent studies show that characterization of the quasispecies diversity at the amino acid level can help to predict the outcome of HCV infection. Currently the accurate characterization of HCV quasispecies requires the cloning of PCR products, followed by the sequencing of many clones. In this study we present a new method to characterize HCV quasispecies, based on in vitro translation of the amplicons, followed by mass spectrometry analysis of the resulting peptide mix. The assay was used on reference HCV samples and on clinical samples. In principle, this method could be applied to other chronic viral infections in which quasispecies play a role.

Intramembrane proteolysis promotes trafficking of hepatitis C virus core protein to lipid droplets

Hepatitis C virus (HCV) is the major causative pathogen associated with liver cirrhosis and hepatocellular carcinoma. The virus has a positive-sense RNA genome encoding a single polyprotein with the virion components located in the N-terminal portion. During biosynthesis of the polyprotein, an internal signal sequence between the core protein and the envelope protein E1 targets the nascent polypeptide to the endoplasmic reticulum (ER) membrane for translocation of E1 into the ER. Following membrane insertion, the signal sequence is cleaved from E1 by signal peptidase. Here we provide evidence that after cleavage by signal peptidase, the signal peptide is further processed by the intramembrane-cleaving protease SPP that promotes the release of core protein from the ER membrane. Core protein is then free for subsequent trafficking to lipid droplets. This study represents an example of a potential role for intramembrane proteolysis in the maturation of a viral protein.

Analysis of antigenicity and topology of E2 glycoprotein present on recombinant hepatitis C virus-like particles

Purification of hepatitis C virus (HCV) from sera of infected patients has proven elusive, hampering efforts to perform structure-function analysis of the viral components. Recombinant forms of the viral glycoproteins have been used instead for functional studies, but uncertainty exists as to whether they closely mimic the virion proteins. Here, we used HCV virus-like particles (VLPs) generated in insect cells infected with a recombinant baculovirus expressing viral structural proteins. Electron microscopic analysis revealed a population of pleomorphic VLPs that were at least partially enveloped with bilayer membranes and had viral glycoprotein spikes protruding from the surface. Immunogold labeling using specific monoclonal antibodies (MAbs) demonstrated these protrusions to be the E1 and E2 glycoproteins. A panel of anti-E2 MAbs was used to probe the surface topology of E2 on the VLPs and to compare the antigenicity of the VLPs with that of truncated E2 (E2(660)) or the full-length (FL) E1E2 complex expressed in mammalian cells. While most MAbs bound to all forms of antigen, a number of others showed striking differences in their abilities to recognize the various E2 forms. All MAbs directed against hypervariable region 1 (HVR-1) recognized both native and denatured E2(660) with comparable affinities, but most bound either weakly or not at all to the FL E1E2 complex or to VLPs. HVR-1 on VLPs was accessible to these MAbs only after denaturation. Importantly, a subset of MAbs specific for amino acids 464 to 475 and 524 to 535 recognized E2(660) but not VLPs or FL E1E2 complex. The antigenic differences between E2(660,) FL E1E2, and VLPs strongly point to the existence of structural differences, which may have functional relevance. Trypsin treatment of VLPs removed the N-terminal part of E2, resulting in a 42-kDa fragment. In the presence of detergent, this was further reduced to a trypsin-resistant 25-kDa fragment, which could be useful for structural studies.

Characterization of vesicular stomatitis virus recombinants that express and incorporate high levels of hepatitis C virus glycoproteins

We generated recombinant vesicular stomatitis viruses (VSV) expressing genes encoding hybrid proteins consisting of the extracellular domains of hepatitis C virus (HCV) glycoproteins fused at different positions to the transmembrane and cytoplasmic domains of the VSV G glycoprotein (E1G and E2G). We show that these chimeric proteins are transported to the cell surface and incorporated into VSV virions efficiently. We also generated VSV recombinants in which the gene encoding the VSV G protein was deleted and replaced by one or both of the E1G and E2G genes, together with a green fluorescent protein gene. These DeltaG viruses incorporated E1G and E2G proteins at levels approximately equivalent to the normal level of VSV G itself, or about 1,200 molecules of each protein per virion. Given the potency of VSV recombinants as vaccines in other studies, this high-level expression and incorporation of HCV proteins into virions could be very important for development of an HCV vaccine. Despite the presence of E1G and E2G proteins at high levels in the virions, these virions did not infect cell lines that have been reported to support at least a low level of HCV infection and replication.

Topological changes in the transmembrane domains of hepatitis C virus envelope glycoproteins

Hepatitis C virus proteins are synthesized as a polyprotein cleaved by a signal peptidase and viral proteases. The behaviour of internal signal sequences at the C-terminus of the transmembrane domains of hepatitis C virus envelope proteins E1 and E2 is essential for the topology of downstream polypeptides. We determined the topology of these transmembrane domains before and after signal sequence cleavage by tagging E1 and E2 with epitopes and by analysing their accessibility in selectively permeabilized cells. We showed that, after cleavage by signal peptidase in the endoplasmic reticulum, the C-terminal orientation of these transmembrane domains changed from luminal to cytosolic. The dynamic behaviour of these transmembrane domains is unique and it is linked to their multifunctionality. By reorienting their C-terminus toward the cytosol and being part of a transmembrane domain, the signal sequences at the C-terminus of E1 and E2 contribute to new functions: (i) membrane anchoring; (ii) E1E2 heterodimerization; and (iii) endoplasmic reticulum retention.

Full-length core sequence dependent complex-type glycosylation of hepatitis C virus E2 glycoprotein

AIM

To study HCV polyprotein processing is important for the understanding of the natural history of HCV and the design of vaccines against HCV. The purpose of this study is to investigate the affection of context sequences on hepatitis C virus (HCV) E2 processing.

METHODS

HCV genes of different lengths were expressed and compared in vaccinia virus/T7 system with homologous patient serum S94 and mouse anti-serum M( E2116) raised against E.coli -derived E2 peptide, respectively. Deglycosylation analysis and GNA ( Galanthus nivalus ) lectin binding assay were performed to study the post-translational processing of the expressed products.

RESULTS

E2 glycoproteins with different molecular weights (-75 kDa and -60 kDa) were detected using S94 and M( E2116), respectively. Deglycosylation analysis showed that this difference was mainly due to different glycosylation. Endo H resistance and its failure to bind to GNA lectin demonstrated that the higher molecular weight form (75 kDa) of E2 was complex-type glycosylated, which was readily recognized by homologous patient serum S94. Expression of complex-type glycosylated E2 could not be detected in all of the core-truncated constructs tested, but readily detected in constructs encoding full-length core sequences.

CONCLUSION

The upstream conserved full-length core coding sequence was required for the production of E2 glycoproteins carrying complex-type N-glycans which reacted strongly with homologous patient serum and therefore possibly represented more mature forms of E2. As complex-type N-glycans indicated modification by Golgi enzymes, the results suggest that the presence of full-length core might be critical for E1/E2 complex to leave ER. Our data may contribute to a better understanding of the processing of HCV structural proteins as well as HCV morphogenesis.

Persistent and transient replication of full-length hepatitis C virus genomes in cell culture

The recently developed subgenomic hepatitis C virus (HCV) replicons were limited by the fact that the sequence encoding the structural proteins was missing. Therefore, important information about a possible influence of these proteins on replication and pathogenesis and about the mechanism of virus formation could not be obtained. Taking advantage of three cell culture-adaptive mutations that enhance RNA replication synergistically, we generated selectable full-length HCV genomes that amplify to high levels in the human hepatoma cell line Huh-7 and can be stably propagated for more than 6 months. The structural proteins are efficiently expressed, with the viral glycoproteins E1 and E2 forming heterodimers which are stable under nondenaturing conditions. No disulfide-linked glycoprotein aggregates were observed, suggesting that the envelope proteins fold productively. Electron microscopy studies indicate that cell lines harboring these full-length HCV RNAs contain lipid droplets. The majority of the core protein was found on the surfaces of these structures, whereas the glycoproteins appear to localize to the endoplasmic reticulum and cis-Golgi compartments. In agreement with this distribution, no endoglycosidase H-resistant forms of these proteins were detectable. In a search for the production of viral particles, we noticed that these cells release substantial amounts of nuclease-resistant HCV RNA-containing structures with a buoyant density of 1.04 to 1.1 g/ml in iodixanol gradients. The same observation was made in transient-replication assays using an authentic highly adapted full-length HCV genome that lacks heterologous sequences. However, the fact that comparable amounts of such RNA-containing structures were found in the supernatant of cells carrying subgenomic replicons demonstrates a nonspecific release independent of the presence of the structural proteins. These results suggest that Huh-7 cells lack host cell factors that are important for virus particle assembly and/or release.

Subcellular localization and topology of the p7 polypeptide of hepatitis C virus

Although biological and biochemical data have been accumulated on most hepatitis C virus proteins, the structure and function of the 63-amino-acid p7 polypeptide of this virus have never been investigated. In this work, sequence analyses predicted that p7 contains two transmembrane passages connected by a short hydrophilic segment. The C-terminal transmembrane domain of p7 was predicted to function as a signal sequence, which was confirmed experimentally by analyzing the translocation of a reporter glycoprotein fused at its C terminus. The p7 polypeptide was tagged either with the ectodomain of CD4 or with a Myc epitope to study its membrane integration, its subcellular localization, and its topology. Alkaline extraction studies confirmed that p7 is an integral membrane polypeptide. The CD4-p7 chimera was detected by immunofluorescence on the surface of nonpermeabilized cells, indicating that it is exported to the plasma membrane. However, pulse-chase analyses showed that only approximately 20% of endoglycosidase H-resistant CD4-p7 was detected after long chase times, suggesting that a large proportion of p7 stays in an early compartment of the secretory pathway. Finally, by inserting a Myc epitope in several positions of p7 and analyzing the accessibility of this epitope on the plasma membrane of HepG2 cells, we showed that p7 has a double membrane-spanning topology, with both its N and C termini oriented toward the extracellular environment. Altogether, these data indicate that p7 is a polytopic membrane protein that could have a functional role in several compartments of the secretory pathway.

Inhibition of host ER glucosidase activity prevents Golgi processing of virion-associated bovine viral diarrhea virus E2 glycoproteins and reduces infectivity of secreted virions

Recently, it was shown that replication of bovine viral diarrhea virus (BVDV) is sensitive to inhibitors of host ER glucosidases. Consistent with these findings, we report that incubation of BVDV-infected MDBK cells with the glucosidase inhibitor n-butyl-deoxynojirimycin (nB-DNJ) reduced BVDV yields by 70- to 100-fold (n = 27), while having no effect on MDBK cell viability. However, the 70- to 100-fold reduction in infectious virus was associated with only a 2-fold reduction in genomic RNA synthesis and secretion of enveloped virus particles. Analysis of secreted virions showed that in the absence of glucosidase inhibitor, approximately 50% of the virion-associated BVDV E2 glycoprotein was resistant to endoglycosidase H (endo H) digestion, whereas intracellular E2 was completely sensitive to endo H digestion. In the presence of glucosidase inhibitor, virion-associated E2 and intracellular E2 were completely sensitive to endo H digestion. Taken together, these results suggest that BVDV is secreted through a Golgi-mediated pathway and that host ER glucosidase activity is required for production of infectious virions and Golgi processing of envelope E2 protein during virus egress.

Expression of hepatitis C virus envelope glycoproteins by herpes simplex virus type 1-based amplicon vectors

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors expressing hepatitis C virus (HCV) E1 and E2 glycoproteins were investigated. HSV-1 amplicon vectors carrying the E1E2p7- or E2p7-coding sequences of HCV type 1a under the control of the HSV-1 IE4 (alpha22/alpha47) promoter were constructed. Studies of infected HepG2, WRL 68 or Vero cells indicated that HSV-1-based amplicon vectors express high levels of HCV glycoproteins that are processed correctly. Immunofluorescence microscopy combined with immunoprecipitation and endoglycosidase treatment of cells infected with the HSV-1-based vectors expressing E1 and E2 showed that the two glycoproteins were retained in the endoplasmic reticulum and had the expected glycosylation patterns. Furthermore, although most of the E1 and E2 proteins formed disulfide-linked aggregates, significant amounts of monomeric forms of the two proteins were detected by SDS-PAGE under non-reducing conditions, suggesting the presence of non-covalently associated E1 and E2. Similar results were produced by a replication-competent recombinant HSV-1 vector expressing HCV E1 and E2. These results indicated that HSV-1-based amplicon vectors represent a useful expression system for the study of HCV glycoproteins.

Analysis of the C-terminal membrane anchor domains of hepatitis C virus glycoproteins E1 and E2: toward a topological model

The hepatitis C virus (HCV) glycoproteins E1 and E2 should be anchored in the viral membrane by their C-terminal domains. During synthesis, they are translocated to the endoplasmic reticulum (ER) lumen where they remain. The 31 C-terminal residues of the E1 protein and the 29 C-terminal residues of the E2 protein are implicated in the ER retention. Moreover, the E1 and E2 C termini are implicated in E1-E2 heterodimerization. We studied the E1 and E2 C-terminal sequences of 25 HCV strains in silico using molecular modeling techniques. We conclude that both C-terminal domains should adopt a similar and peculiar configuration: one amphipathic alpha-helix followed by a pair of transmembrane beta-strands. Several three-dimensional (3-D) models were generated. After energy minimization, their ability to interact with membranes was studied using the molecular hydrophobicity potentials calculation and the IMPALA procedure. The latter simulates interactions with a membrane by a Monte Carlo minimization of energy. These methods suggest that the beta-hairpins could anchor the glycoproteins in the ER membrane at least transiently. Anchoring could be stabilized by the adsorption of the nearby amphipathic alpha-helices at the membrane surface. The 3-D models correlate with experimental results which indicate that the E1-E2 transmembrane domains are involved in the heterodimerization and have ER retention properties.

Glycosylation of the hepatitis C virus envelope protein E1 occurs posttranslationally in a mannosylphosphoryldolichol-deficient CHO mutant cell line

The addition of N-linked glycans to a protein is catalyzed by oligosaccharyltransferase, an enzyme closely associated with the translocon. N-glycans are believed to be transferred as the protein is being synthesized and cotranslationally translocated in the lumen of the endoplasmic reticulum. We used a mannosylphosphoryldolichol-deficient Chinese hamster ovary mutant cell line (B3F7 cells) to study the temporal regulation of N-linked core glycosylation of hepatitis C virus envelope protein E1. In this cell line, truncated Glc(3)Man(5)GlcNAc(2) oligosaccharides are transferred onto nascent proteins. Pulse-chase analyses of E1 expressed in B3F7 cells show that the N-glycosylation sites of E1 are slowly occupied until up to 1 h after protein translation is completed. This posttranslational glycosylation of E1 indicates that the oligosaccharyltransferase has access to this protein in the lumen of the endoplasmic reticulum for at least 1 h after translation is completed. Comparisons with the N-glycosylation of other proteins expressed in B3F7 cells indicate that the posttranslational glycosylation of E1 is likely due to specific folding features of this acceptor protein.