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

ER targeting and retention of the HCV NS4B protein relies on the concerted action of multiple structural features including its transmembrane domains

The Hepatitis C virus (HCV) NS4B protein, a multispanning endoplasmic reticulum (ER) membrane protein, generates intracellular rearrangements of ER-derived membranes, essential for HCV replication. In this study, we characterized NS4B elements involved in the process of targeting, association and retention in the ER membrane. We investigated the localization and membrane association of a number of C- or N-terminal NS4B deletions expressed as GFP chimeras by biochemical and fluorescence microscopy techniques. A second set of GFP-NS4B chimeras containing the plasma membrane ecto-ATPase CD39 at the C-terminus of each NS4B deletion mutant was used to further examine the role of N-terminal NS4B sequences in ER retention. Several structural elements, besides the first two transmembrane domains (TMs), within the NS4B N-terminal half (residues 1-130) were found to mediate association of the NS4B-GFP chimeras with ER membranes. Both TM1 and TM2 are required for ER anchoring and retention but are not sufficient for ER retention. Sequences upstream of TM1 are also required. These include two putative amphipathic alpha-helices and a Leucine Rich Repeat-like motif, a sequence highly conserved in all HCV genotypes. The N-terminal 55peptidic sequence, containing the 1st amphipathic helix, mediates association of the 55N-GFP chimera with cellular membranes including the ER, but is dispensable for ER targeting of the entire NS4B molecule. Importantly, the C-terminal 70peptidic sequence can associate with membranes positive for ER markers in the absence of any predicted TMs. In conclusion, HCV NS4B targeting and retention in the ER results from the concerted action of several NS4B structural elements.

GB virus type C interactions with HIV: the role of envelope glycoproteins

GB virus C/hepatitis G virus (GBV-C/HGV) is the most closely related human virus to hepatitis C virus (HCV). GBV-C is lymphotropic and not associated with any known disease, although it is associated with improved survival in HIV-infected individuals. In peripheral blood mononuclear cells, GBV-C induces the release of soluble ligands for HIV entry receptors (RANTES, MIP-1a, MIP-1b and SDF-1), suggesting that GBV-C may interact with lymphocytes to induce a chemokine and/or cytokine milieu that is inhibitory to HIV infection. Expression of GBV-C envelope glycoprotein E2 in CD4+ T cells or addition of recombinant E2 to CD4 cells recapitulates the HIV inhibition seen with GBV-C infection. Like HCV E2, GBV-C E2 is predicted to be post-translationally processed in the endoplasmic reticulum and is involved with cell binding. The C-termini of GBV-C E1 and E2 proteins contain predicted transmembrane domains sharing features with HCV TM domains. To date, cellular receptor(s) for GBV-C E2 have not been identified. GBV-C E2-mediated HIV inhibition is dose-dependent and HIV replication is blocked at the binding and/or entry step. In addition, a putative GBV-C E2 fusion peptide interferes with HIV gp41 peptide oligomerization required for HIV-1 fusion, further suggesting that GBV-C E2 may inhibit HIV entry. Additional work is needed to identify the GBV-C E2 cellular receptor, characterize GBV-C E2 domains responsible for HIV inhibition, and to examine GBV-C E2-mediated fusion in the context of the entire envelope protein or viral-particles. Understanding the mechanisms of action may identify novel approaches to HIV therapy.

A computational approach identifies two regions of Hepatitis C Virus E1 protein as interacting domains involved in viral fusion process

Background

The E1 protein of Hepatitis C Virus (HCV) can be dissected into two distinct hydrophobic regions: a central domain containing an hypothetical fusion peptide (FP), and a C-terminal domain (CT) comprising two segments, a pre-anchor and a trans-membrane (TM) region. In the currently accepted model of the viral fusion process, the FP and the TM regions are considered to be closely juxtaposed in the post-fusion structure and their physical interaction cannot be excluded. In the present study, we took advantage of the natural sequence variability present among HCV strains to test, by purely sequence-based computational tools, the hypothesis that in this virus the fusion process involves the physical interaction of the FP and CT regions of E1.

Results

Two computational approaches were applied. The first one is based on the co-evolution paradigm of interacting peptides and consequently on the correlation between the distance matrices generated by the sequence alignment method applied to FP and CT primary structures, respectively. In spite of the relatively low random genetic drift between genotypes, co-evolution analysis of sequences from five HCV genotypes revealed a greater correlation between the FP and CT domains than respect to a control HCV sequence from Core protein, so giving a clear, albeit still inconclusive, support to the physical interaction hypothesis.

The second approach relies upon a non-linear signal analysis method widely used in protein science called Recurrence Quantification Analysis (RQA). This method allows for a direct comparison of domains for the presence of common hydrophobicity patterns, on which the physical interaction is based upon. RQA greatly strengthened the reliability of the hypothesis by the scoring of a lot of cross-recurrences between FP and CT peptides hydrophobicity patterning largely outnumbering chance expectations and pointing to putative interaction sites. Intriguingly, mutations in the CT region of E1, reducing the fusion process in vitro, strongly reduced the amount of cross-recurrence further supporting interaction between this region and FP.

Conclusion

Our results support a fusion model for HCV in which the FP and the C-terminal region of E1 are juxtaposed and interact in the post-fusion structure. These findings have general implications for viruses, as any visualization of the post-fusion FP-TM complex has been precluded by the impossibility to obtain crystallised viral fusion proteins containing the trans-membrane region. This limitation gives to sequence based modelling efforts a crucial role in the sketching of a molecular interpretation of the fusion process. Moreover, our data also have a more general relevance for cell biology as the mechanism of intracellular fusion showed remarkable similarities with viral fusion

Structural characterisation of the hepatitis C envelope glycoprotein E1 ectodomain derived from a mammalian and a yeast expression system

The structure of the ectodomain of the hepatitis C envelope glycoprotein E1 (E1s) was characterised by spectroscopic methods. Monomeric E1s was purified from a mammalian and from a Hansenula polymorpha cell lysate, and cysteine-blocked monomers were reconstituted into stable particles. Particles from yeast E1s and mammalian E1s showed a comparable reactivity in ELISA with sera from human chronic HCV carriers, similar antibody titers in the sera of immunised mice as well as a comparable structure as analyzed by spectroscopic methods (tryptophan fluorescence, circular dichroism, and Fourier transform infrared spectroscopy). The overall secondary structure of E1s was neither influenced by the degree of glycosylation nor by the nature of cysteine modification used during purification. The structural comparability of mammalian- and H. polymorpha-expressed E1s opens new perspectives for further development of E1s-based therapeutics as yeast systems generally allow a more easy scaling up.

Preferential association of Hepatitis C virus with apolipoprotein B48-containing lipoproteins

Hepatitis C virus (HCV) in cell culture has a density comparable to that of other members of the family Flaviviridae, whereas in vivo infectious particles are found partially in low-density fractions, associated with triacylglycerol (TG)-rich lipoproteins (TRLs). In the blood of infected patients, HCV circulates as heterogeneous particles, among which are lipo-viroparticles (LVPs), globular particles rich in TG and containing viral capsid and RNA. The dual viral and lipoprotein nature of LVPs was addressed further with respect to apolipoprotein composition and post-prandial dynamic lipid changes. The TRLs exchangeable apoE, -CII and -CIII, but not the high-density lipoprotein apoA-II, were present on LVPs, as well as the viral envelope proteins. apoB100 and -B48, the two isoforms of the non-exchangeable apoB, were represented equally on LVPs, despite the fact that apoB48 was barely detectable in the plasma of these fasting patients. This indicates that a significant fraction of plasma HCV was associated with apoB48-containing LVPs. Furthermore, LVPs were enriched dramatically and rapidly in triglycerides after a fat meal. As apoB48 is synthesized exclusively by the intestine, these data highlight the preferential association of HCV with chylomicrons, the intestine-derived TRLs. These data raise the question of the contribution of the intestine to the viral load and suggest that the virus could take advantage of TRL assembly and secretion for its own production and of TRL fate to be delivered to the liver.

Monoclonal antibody AP33 defines a broadly neutralizing epitope on the hepatitis C virus E2 envelope glycoprotein

Hepatitis C virus (HCV) remains a significant threat to the general health of the world's population, and there is a pressing need for the development of new treatments and preventative vaccines. Here, we describe the generation of retrovirus-based pseudoparticles (HCVpp) incorporating a panel of full-length E1E2 clones representative of the major genotypes 1 through 6, and their application to assess the reactivity and neutralizing capability of antisera and monoclonal antibodies raised against portions of the HCV E2 envelope protein. Rabbit antisera raised against either the first hypervariable region or ectodomain of E2 showed limited and strain specific neutralization. By contrast, the monoclonal antibody (MAb) AP33 demonstrated potent neutralization of infectivity against HCVpp carrying E1E2 representative of all genotypes tested. The concentration of AP33 required to achieve 50% inhibition of infection by HCVpp of diverse genotypes ranged from 0.6 to 32 mug/ml. The epitope recognized by MAb AP33 is linear and highly conserved across different genotypes of HCV. Thus, identification of a broadly neutralizing antibody that recognizes a linear epitope is likely to be of significant benefit to future vaccine and therapeutic antibody development.

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.

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.

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.

Proteomics computational analyses suggest that hepatitis C virus E1 and pestivirus E2 envelope glycoproteins are truncated class II fusion proteins

Class II fusion proteins encoded by tick-borne encephalitis virus (TBEV), dengue virus, and Semliki Forest virus have a fusion peptide located at the end of a rod-like molecule comprised of three antiparallel beta sheet domains. Proteomics computational analyses suggest that hepatitis C virus (HCV) envelope glycoprotein E1 and pestivirus envelope glycoprotein E2 are truncated class II fusion proteins. Similarities were also detected between the receptor-binding portion of TBEV E and HCV E2, and between TBEV small membrane protein precursor prM and pestivirus E1. The proposed models of Flaviviridae envelope proteins can facilitate drug and vaccine development.

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.

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.