The First Transmembrane Domain of the Hepatitis B Virus Large Envelope Protein Is Crucial for Infectivity

The early steps of the hepatitis B virus (HBV) life cycle are still poorly understood. Indeed, neither the virus receptor at the cell surface nor the mechanism by which nucleocapsids are delivered to the cytosol of infected cells has been identified. Extensive mutagenesis studies in pre-S1, pre-S2, and most of the S domain of envelope proteins revealed the presence of two regions essential for HBV infectivity: the 77 first residues of the pre-S1 domain and a conformational motif in the antigenic loop of the S domain. In addition, at the N-terminal extremity of the S domain, a putative fusion peptide, partially overlapping the first transmembrane (TM1) domain and preceded by a PEST sequence likely containing several proteolytic cleavage sites, was identified. Since no mutational analysis of these two motifs potentially implicated in the fusion process was performed, we decided to investigate the ability of viruses bearing contiguous deletions or substitutions in the putative fusion peptide and PEST sequence to infect HepaRG cells. By introducing the mutations either in the L and M proteins or in the S protein, we demonstrated the following: (i) that in the TM1 domain of the L protein, three hydrophobic clusters of four residues were necessary for infectivity; (ii) that the same clusters were critical for S protein expression; and, finally, (iii) that the PEST sequence was dispensable for both assembly and infection processes.The hepatitis B virus (HBV) is the main human pathogen responsible for severe hepatic diseases like cirrhosis and hepatocellular carcinoma. Even though infection can be prevented by immunization with an efficient vaccine, about 2 billion people have been infected worldwide, resulting in 350 million chronic carriers that are prone to develop liver diseases (56). Current treatments consist either of the use of interferon α, which modulates antiviral defenses and controls infection in 30 to 40% of cases, or of the use of viral polymerase inhibitors that allow a stronger response to treatment but require long-term utilization and frequently lead to the outcome of resistant viruses (34, 55). A better understanding of the virus life cycle, and particularly of the mechanism by which the virus enters the cell, could provide background for therapeutics that inhibit the early steps of infection, as recently illustrated with the HBV pre-S1-derived entry inhibitor (25, 45).HBV belongs to the Hepadnaviridae family whose members infect different species. All viruses of this family share common properties. The capsid containing a partially double-stranded circular DNA genome is surrounded by a lipid envelope, in which two (in avihepadnaviruses infecting birds) or three (in orthohepadnaviruses infecting mammals) envelope proteins are embedded. A single open reading frame bearing several translation initiation sites encodes these surface proteins. Thus, the HBV envelope contains three proteins: S, M, and L that share the same C-terminal extremity corresponding to the small S protein that is crucial for virus assembly (7, 8, 46) and infectivity (1, 31, 53). These proteins are synthesized in the endoplasmic reticulum (ER), assembled, and secreted as particles through the Golgi apparatus (15, 42). The current model for the transmembrane structure of the S domain implies the luminal exposition of both N- and C-terminal extremities and the presence of four transmembrane (TM) domains: the TM1 and TM2 domains, both necessary for cotranslational protein integration into the ER membrane, and the TM3 and TM4 domains, located in the C-terminal third of the S domain (for a review, see reference 6). Among the four predicted TM domains, only the TM2 domain has a defined position between amino acids 80 and 98 of the S domain. The exact localization of the TM1 domain is still unclear, probably because of the relatively low hydrophobicity of its sequence, which contains polar residues and two prolines. The M protein corresponds to the S protein extended by an N-terminal domain of 55 amino acids called pre-S2. Its presence is dispensable for both assembly and infectivity (20, 21, 37). Finally, the L protein corresponds to the M protein extended by an N-terminal domain of 108 amino acids called pre-S1 (genotype D). The pre-S1 and pre-S2 domains of the L protein can be present either at the inner face of viral particles (on the cytoplasmic side of the ER), playing a crucial role in virus assembly (5, 8, 10, 11, 46), or on the outer face (on the luminal side of the ER), available for the interaction with target cells and necessary for viral infectivity (4, 14, 36). The pre-S translocation is independent from the M and S proteins and is driven by the L protein TM2 domain (33). Finally, HBV surface proteins are not only incorporated into virion envelopes but also spontaneously bud from ER-Golgi intermediate compartment membranes (30, 43) to form empty subviral particles (SVPs) that are released from the cell by secretion (8, 40).One approach to decipher viral entry is to interfere with the function of envelope proteins. Thus, by a mutagenesis approach, two envelope protein domains crucial for HBV infectivity have already been identified: (i) the 77 first amino acids of the pre-S1 domain (4, 36) including the myristic acid at the N-terminal extremity (9, 27) and (ii) possibly a cysteine motif in the luminal loop of the S domain (1, 31). In addition, a putative fusion peptide has been identified at the N-terminal extremity of the S domain due to its sequence homology with other viral fusion peptides (50). This sequence, either N-terminal in the S protein or internal in the L and M proteins, is conserved among the Hepadnaviridae family and shares common structural and functional properties with other fusion peptides (49, 50). Finally, a PEST sequence likely containing several proteolytic cleavage sites has been identified in the L and M proteins upstream of the TM1 domain (39). A cleavage within this sequence could activate the fusion peptide.In this study, we investigated whether the putative fusion peptide and the PEST sequence were necessary for the infection process. For this purpose, we constructed a set of mutant viruses bearing contiguous deletions in these regions and determined their infectivity using an in vitro infection model based on HepaRG cells (28). The introduction of mutations either in the L and M proteins or in only the S protein allowed us to demonstrate that, in the TM1 domain of L protein, three hydrophobic clusters not essential for viral assembly were crucial for HBV infectivity while their presence in the S protein was critical for envelope protein expression. In addition, we showed that the PEST sequence was clearly dispensable for both assembly and infection processes.     

乙型肝炎病毒前S1抗原检测及其临床意义

目的:探讨乙型肝炎病毒前S1抗原与血清标志物之间的关系,了解其临床意义。方法:采用酶联免疫吸附实验检测乙型肝炎病毒血清标志物及前S1抗原,并采用速率法检测血清丙氨酸氨基转移酶。结果:乙肝表面抗原阳性率为9．2%,乙肝表面抗原阳性人群中前S1抗原阳性率为28．6%;乙肝e抗原阳性及阴性人群中前S1抗原阳性率分别为81．1%、13．9%(p＜0．01);前S1抗原阳性人群ALT(49．5U/L)高于前S1抗原阴性人群(43．2U/L,p＜0．01)。结论:前S1抗原与血清标志物e抗原有较高的一致性,是反映病毒复制的良好指标,能较早发现肝损伤。     

Hepatitis B Virus Replication and Release Are Independent of Core Lysine Ubiquitination

Ubiquitin conjugation to lysine residues regulates a variety of protein functions, including endosomal trafficking and degradation. While ubiquitin plays an important role in the release of many viruses, the requirement for direct ubiquitin conjugation to viral structural proteins is less well understood. Some viral structural proteins require ubiquitin ligase activity, but not ubiquitin conjugation, for efficient release. Recent evidence has shown that, like other viruses, hepatitis B virus (HBV) requires a ubiquitin ligase for release from the infected cell. The HBV core protein contains two lysine residues (K7 and K96), and K96 has been suggested to function as a potential ubiquitin acceptor site based on the fact that previous studies have shown that mutation of this amino acid to alanine blocks HBV release. We therefore reexamined the potential connection between core lysine ubiquitination and HBV replication, protein trafficking, and virion release. In contrast to alanine substitution, we found that mutation of K96 to arginine, which compared to alanine is more conserved but also cannot mediate ubiquitin conjugation, does not affect either virus replication or virion release. We also found that the core lysine mutants display wild-type sensitivity to the antiviral activity of interferon, which demonstrates that ubiquitination of core lysines does not mediate the interferon-induced disruption of HBV capsids. However, mutation of K96 to arginine alters the nuclear-cytoplasmic distribution of core, leading to an accumulation in the nucleolus. In summary, these studies demonstrate that although ubiquitin may regulate the HBV replication cycle, these mechanisms function independently of direct lysine ubiquitination of core protein.The hepatitis B virus (HBV) particle consists of an enveloped nucleocapsid that contains the viral polymerase (Pol) and an incomplete 3.2-kb double-stranded DNA genome (9). In the cytoplasm, the viral core structural proteins interact to form homodimers, which further self-assemble into capsid particles that package Pol and the viral pregenomic RNA. Encapsidated Pol subsequently reverse transcribes pregenomic RNA to give rise to mature double-stranded relaxed circular DNA-containing capsids. HBV DNA-containing capsids are released from the cell as mature virions after acquiring an envelope consisting of cellular membrane lipids and the viral small, middle, and large envelope proteins (4, 9, 41). Due to the directed insertion of the envelope proteins in the endoplasmic reticulum and Golgi membrane, and the requirement of the large envelope protein for virion release, nucleocapsids are hypothesized to bud at intracellular membranes for release through the constitutive secretory pathway (5). Although the mechanism and site of HBV nucleocapsid envelopment and release remain poorly understood, emerging evidence indicates that the cellular ubiquitin pathway may play a role in this process.Structural proteins of some enveloped RNA viruses contain highly conserved sequences [PPXY, P(T/S)AP, and YPXL] termed late (L) domains that mediate interactions with proteins of the endocytic pathway to facilitate virus budding and release (1). The P(T/S)AP motif binds Tsg101 (8, 10, 19, 27, 47), a key ESCRT (for endosomal sorting complex required for transport) component for the recognition and sorting of ubiquitinated proteins to internal vesicles of the multivesicular body (MVB), while the YPXL motif binds Alix, an ESCRT-associated protein (26, 44, 48). The PPXY motif binds proteins of the Nedd4 family ubiquitin ligases, which are responsible for ubiquitination of proteins targeted for endocytosis and sorting to the MVB (20), suggesting a link between ubiquitin and viral budding (3, 16, 17, 22, 43, 55). The observation that proteasome inhibition, which depletes free cellular ubiquitin by interfering with ubiquitin recycling, results in a viral budding defect similar to that seen in virus L domain mutants further supports the implication that ubiquitin plays a role in mediating virion release (15, 31, 40, 43). Furthermore, fusion of ubiquitin to the Rous sarcoma virus (RSV) PPPY-containing Gag protein and the equine infectious anemia virus (EIAV) Gag protein containing a heterologous PTAP or PPPY motif rescues the virus-like particle release defect induced by proteasome inhibition (18, 31). While the role of L domains in mediating virion release is relatively well established, it remains unclear whether direct ubiquitination of viral structural proteins is generally required for virion release. Mutation of ubiquitin acceptor lysine residues in the RSV Gag protein inhibits virus budding, but such mutations in human immunodeficiency virus type 1 (HIV-1) or murine leukemia virus Gag protein exert no effect on virus release (29, 42). Recently, a retroviral (i.e., prototypic foamy virus) Gag protein engineered to lack ubiquitin acceptor lysines and encoding either the PSAP or PPXY motif of the L domain displayed no defect in viruslike particle release (58). Altogether, these results suggest that recruitment of host proteins to the L domain and ubiquitination of interacting proteins, but not the viral structural proteins, is required for ubiquitin-dependent virion release, at least for some viruses.The HBV core structural protein contains two potential ubiquitin acceptor lysine residues (K7 and K96) and an L-domain-like PPAY motif (Fig. ​(Fig.1A).1A). Structural studies indicate that residue K96 and the PPAY motif may be exposed on the surface of HBV capsid particles, at least transiently (4, 32, 37). Studies aimed at identifying interaction factors important for HBV particle release demonstrated a number of interesting findings. First, γ2-adaptin, a cellular trafficking adaptor that contains a ubiquitin-interacting motif (UIM), interacts with both the viral large envelope protein and HBV core, and disruption of the HBV/γ2-adaptin interaction inhibits virus secretion (14, 39). Second, core protein interacts with the Nedd4 ubiquitin ligase through the PPAY motif in core (39). Mutation of the tyrosine in the PPAY motif results in disrupted binding of Nedd4, and overexpression of a catalytically inactive Nedd4 mutant inhibits HBV particle secretion (39). Third, mutation of core K96, but not K7, to alanine results in a defective release phenotype, suggesting that K96 may serve as a ubiquitin conjugation site that aids virion release (32, 39). Recently, overexpression of dominant-negative proteins of the MVB machinery, such as the Vps4 ATPases and the ESCRT-III complex-forming CHMP proteins, were also shown to disrupt HBV budding and virion release, while subviral particles comprised only of envelope proteins were released efficiently (21, 24, 49). This suggests that nucleocapsids may release from the cell by a mechanism distinct from constitutive secretion. These studies show that similar to RNA viruses, HBV utilizes components of the cellular protein trafficking machinery to mediate virion release.Open in a separate windowFIG. 1.Generation of core lysine mutants. (A) The 21-kDa HBV core structural protein contains two lysine residues at positions 7 and 96 that serve as potential ubiquitin conjugation sites. These residues are highly conserved among the four major HBV genotypes (6). Core contains a late-domain-like PPXY motif that serves as a binding site for the Nedd4 E3 ubiquitin ligase. Core additionally contains a potential noncanonical SUMOylation motif at position 96. (B) Lysine mutations were generated by site-directed mutagenesis in the core gene contained within the HBV genome under the control of a CMV promoter. K7R contains a lysine-to-arginine mutation at position 7, K96R contains a lysine-to-arginine mutation at position 96, K96A contains a lysine-to-alanine mutation at position 96, and K7R/K96R contains arginine substituted at position 7 and position 96.Although these findings imply that core ubiquitination may be necessary for HBV particle release, direct evidence of core ubiquitination has been elusive (33, 39; unpublished results). As suggested by previous Gag lysine mutagenesis studies, however, ubiquitin may instead indirectly be required through conjugation to an interacting protein that is essential for mediating HBV release (29, 58). Although core K7 and K96 have been previously assayed in the context of virion release by mutation of the lysine residues to alanine (32, 39), we expanded these studies by assaying core mutants with an arginine substitution at position K7 (K7R) and K96 (K96R), as well as a double lysine-to-arginine mutation (K7R/K96R). Compared to alanine, arginine serves as a more conserved mutation for lysine while still abolishing the potential ubiquitin conjugation site. In the present study, we utilized these mutants to comprehensively examine the role of the core lysines in HBV virus release, the formation of replication intermediates, intracellular localization of core, and the interferon (IFN)-mediated antiviral response.     

Structural Proteins of Mammalian Oncogenic RNA Viruses: Multiple Antigenic Determinants of the Major Internal Protein and Envelope Glycoprotein

The antigenic determinants of two purified protein constituents of mammalian C-type RNA viruses, the major structural protein of about 30,000 daltons, and the membrane glycopeptides of about 70,000 daltons were examined by competition radioimmunoassay. By the appropriate choice of antiserum and competing proteins, it was possible to distinguish type-specific, group-specific, and interspecies determinants. Both of the viral constituents were found to contain each of these three classes of antigens. The results suggested that the majority of the determinants of the major structural protein were group specific, 5% to 30% were interspecies, and a small fraction were type specific. In the case of the envelope glycopeptides, the chief determinants were type and group specific, and a small fraction were interspecies.     

Infection Process of the Hepatitis B Virus Depends on the Presence of a Defined Sequence in the Pre-S1 Domain

During the life cycle of hepatitis B virus (HBV), the large envelope protein (L) plays a pivotal role. Indeed, this polypeptide is essential for viral assembly and probably for the infection process. By performing mutagenesis experiments, we have previously excluded a putative involvement of the pre-S2 domain of the L protein in viral infectivity. In the present study, we have evaluated the role of the pre-S1 region in HBV infection. For this purpose, 21 mutants of the L protein were created. The entire pre-S1 domain was covered by contiguous deletions of 5 amino acids. First, after transfection into HepG2 cells, the efficient expression of both glycosylated and unglycosylated L mutant proteins was verified. The secretion rate of envelope proteins was modified positively or negatively by deletions, indicating that the pre-S1 domain contains several regulating sequences able to influence the surface protein secretion. The ability of mutant proteins to support the production of virions was then studied. Only the four C-terminal deletions, covering the 17 amino acids suspected to interact with the cytoplasmic nucleocapsids, inhibited virion release. Finally, the presence of the modified pre-S1 domain at the external side of all secreted virions was confirmed, and their infectivity was assayed on normal human hepatocytes in primary culture. Only a short sequence including amino acids 78 to 87 tolerates internal deletions without affecting viral infectivity. These results confirm the involvement of the L protein in the infection step and demonstrate that the sequence between amino acids 3 and 77 is involved in this process.     

Evolutionary Changes of Hepatitis B Virus Pre-S Mutations Prior to Development of Hepatocellular Carcinoma

Background and Aims

Deletions/mutations in the hepatitis B virus (HBV) pre-S region have been associated with hepatocellular carcinoma (HCC). We aimed to study the evolutionary changes of pre-S mutations prior to HCC development.

Methods

We studied the HBV pre-S sequences at 1 to 10 years preceding diagnosis of HCC in 74 patients with HBV-related HCC (HCC group). 148 chronic hepatitis B patients matched for sex and age in 2:1 ratio, who had been followed up for at least 3 years without HCC (HCC-free group) were recruited as controls. 56 and 47 patients of HCC and HCC-free groups respectively had serially stored sera for longitudinally examination at 1–3 years, 4–6 years, 7–9 years and ≥10 years prior to the recruitment of the study.

Results

Compared to the HCC-free group, higher frequencies of pre-S deletions and point mutations (at 11 codons) were observed in the HCC group (p<0.05). Multiple logistic regression analysis showed that pre-S deletions, point mutations at codon 51 and 167 were independent factors associated with HCC. Longitudinal observation showed that pre-S deletions and most of the 11 HCC-associated pre-S point mutations existed at least 10 years before HCC development, and were more prevalent preceding HCC development in patients from HCC groups than HCC-free group. The number of HCC-associated pre-S point mutations increased over time preceding HCC development, and correlated positively with the time to HCC diagnosis (r = 0.220, p = 0.005).

Conclusions

High prevalence and cumulative evolution of pre-S mutations preceding HCC development suggested a possible carcinogenic role of pre-S mutations and their potential application in HCC risk prediction.     

HIV-1 Envelope Glycoprotein Variable Loops Are Indispensable for Envelope Structural Integrity and Virus Entry

HIV-1 envelope (Env) glycoprotein is a trimer of heterodimer of gp120 and gp41, and derives from a trimeric glycoprotein precursor, gp160. Gp120 contains five conserved regions that are interspersed with 5 variable loop regions (V1–V5). Env variations in variable loop length and amino acid composition may associate with virus pathogenesis, virus sensitivity to neutralizing antibodies (nAbs) and disease progression. To investigate the role of each variable loop in Env function, we generated a panel of JRFL gp160 loop deletion mutants and examined the effects of each loop deletion on Env expression, Env cell surface display and Env-mediated virus entry into permissive cells. We found that deletion of V1 and V2 (ΔV1V2), or loop D (ΔlpD) abolished virus entry, the same effect as deletion of V3 (ΔV3), while deletion of V3 crown (ΔV3C) significantly enhanced virus assembly and entry. We further found that deletion of V4 (ΔV4) or V5 (ΔV5), or replacement of V4 or V5 with flexible linkers of the same lengths knocked out the receptor and coreceptor binding sites in gp120, but significantly enhanced the exposure of the N-trimer structure and the membrane proximal external region (MPER) in gp41. Although deletion of V4 or V5 did not affect Env expression, they negatively affected Env cell surface display, leading to the failure in virus assembly and subsequent entry. Taken together, we found that Env variable loops were indispensable for Env structural integrity and virus entry. Our findings may have implications for development of HIV-1 vaccine immunogens and therapeutics.     

Impaired Virion Secretion by Hepatitis B Virus Immune Escape Mutants and Its Rescue by Wild-Type Envelope Proteins or a Second-Site Mutation

Hepatitis B virus immune escape mutants have been associated with vaccine failure and reinfection of grafted liver despite immune prophylaxis, but their biological properties remain largely unknown. Transfection of 20 such mutants in a human hepatoma cell line identified many with severe impairment in virion secretion, which can be rescued to various extents by coexpression of wild-type envelope proteins or introduction of a novel glycosylation site. Consistent with their role in maintaining intra- or intermolecular disulfide bonds, cysteine residues within the “a” determinant are critical for virion secretion.     

Biological Expression of Antigenic Determinants of Murine Leukemia Virus Proteins gp69/71 and p30

Antisera to purified structural proteins of Rauscher murine leukemia virus, the major envelope glycoprotein, gp69/71, and the major internal protein, p30, were studied by immunofluorescence of viable and fixed virus-infected cells and by virus neutralization. Group-specific and type-specific determinants of gp69/71 were demonstrated by immunofluorescence and virus neutralization tests, indicating that these determinants are located in the cytoplasm and probably on the cell surface as well as on virus envelope. Antisera against p30 showed anti-group and anti-interspecies activities by immunofluorescence with no virus-neutralizing activity. Both antigenic determinants of gp69/71 were sensitive to guanidine-hydrochloride and to a lesser degree to ether treatment, whereas the group-specific determinants of p30 were relatively stable to these treatments.     

Association of Hepatitis B Virus Pre-S Deletions with the Development of Hepatocellular Carcinoma in Qidong,China

Background/Aim

To investigate the roles of mutations in pre-S and S regions of hepatitis B virus (HBV) on the progression of hepatocellular carcinoma (HCC) in Qidong, China.

Methods

We conducted an age matched case-control study within a cohort of 2387 male HBV carriers who were recruited from August, 1996. The HBV DNA sequence in pre-S/S regions was successfully determined in 96 HCC cases and 97 control subjects. In addition, a consecutive series of samples from 11 HCC cases were employed to evaluate the pre-S deletion patterns before and after the occurrence of HCC.

Results

After adjustment for age, history of cigarette smoking and alcohol consumption, HBeAg positivity, pre-S deletions, pre-S2 start codon mutations, and T53C mutation were significantly associated with HCC, showing adjusted odds ratios (ORs) from 1.914 to 3.199. HCC patients also had a lower frequency of T31C mutation in pre-S2 gene, compared with control subjects (0.524; 95% CI 0.280-0.982). HBV pre-S deletions were clustered mainly in the 5′ end of pre-S2 region. Multivariate analysis showed that pre-S deletions and pre-S2 start codon mutations were independent risk factors for HCC. The OR (95% CI) were 2.434 (1.063–5.573) and 3.065 (1.099–8.547), respectively. The longitudinal observation indicated that the pre-S deletion mutations were not acquired at the beginning of HBV infection, but that the mutations occurred during the long course of liver disease.

Conclusion

Pre-S deletions and pre-S2 start codon mutations were independently associated with the development of HCC. The results also provided direct evidence that pre-S deletion mutations were not acquired from the beginning of infection but arose de novo during the progression of liver disease.     

丙型肝炎病毒蛋白的分子生物学研究进展

丙型肝炎病毒(Hepatitis C viruS,HCV)是造成慢性肝炎,肝硬化及肝癌的重要原因之一.目前全球发现约有1亿7千万人口感染HCV.HCV的急性感染通常是没有症状的,但50%～80%的病人会转变成为慢性感染患者,而且大约20%的患者会在20年内转变成为肝硬化.     

Role of N-Linked Glycans in the Functions of Hepatitis C Virus Envelope Proteins Incorporated into Infectious Virions

Hepatitis C virus (HCV) envelope glycoproteins are highly glycosylated, with generally 4 and 11 N-linked glycans on E1 and E2, respectively. Studies using mutated recombinant HCV envelope glycoproteins incorporated into retroviral pseudoparticles (HCVpp) suggest that some glycans play a role in protein folding, virus entry, and protection against neutralization. The development of a cell culture system producing infectious particles (HCVcc) in hepatoma cells provides an opportunity to characterize the role of these glycans in the context of authentic infectious virions. Here, we used HCVcc in which point mutations were engineered at N-linked glycosylation sites to determine the role of these glycans in the functions of HCV envelope proteins. The mutants were characterized for their effects on virus replication and envelope protein expression as well as on viral particle secretion, infectivity, and sensitivity to neutralizing antibodies. Our results indicate that several glycans play an important role in HCVcc assembly and/or infectivity. Furthermore, our data demonstrate that at least five glycans on E2 (denoted E2N1, E2N2, E2N4, E2N6, and E2N11) strongly reduce the sensitivity of HCVcc to antibody neutralization, with four of them surrounding the CD81 binding site. Altogether, these data indicate that the glycans associated with HCV envelope glycoproteins play roles at different steps of the viral life cycle. They also highlight differences in the effects of glycosylation mutations between the HCVpp and HCVcc systems. Furthermore, these carbohydrates form a “glycan shield” at the surface of the virion, which contributes to the evasion of HCV from the humoral immune response.Hepatitis C virus (HCV) is a single-stranded positive-sense RNA virus that causes serious liver diseases in humans (31). More than 170 million people worldwide are seropositive for HCV and at risk for developing cirrhosis and hepatocellular carcinoma (50). HCV is a small, enveloped virus that belongs to the Hepacivirus genus in the Flaviviridae family (31). Its genome encodes a single polyprotein precursor of about 3,000-amino-acid residues that is cleaved co- and posttranslationally by cellular and viral proteases to yield at least 10 mature products (31). The two envelope glycoproteins, E1 and E2, are released from the polyprotein by signal peptidase cleavages. These two proteins assemble as noncovalent heterodimers, which are retained mainly in the endoplasmic reticulum (ER) (36), and they are found as large disulfide-linked oligomers on the surfaces of HCV particles (46). HCV glycoproteins are involved in the entry process, and since they are present on the surfaces of viral particles, these proteins are the targets of neutralizing antibodies (4, 21).E1 and E2 generally contain 4 and 11 N-glycosylation sites, respectively, all of which have been shown to be modified by glycans (19). Despite variability in HCV envelope glycoprotein sequences, the four glycosylation sites of E1 and nine of E2 are highly conserved, suggesting that the glycans associated with these proteins play an essential role in the HCV life cycle (22). Using retroviral particles pseudotyped with genotype 1a (H strain) HCV envelope glycoproteins (HCVpp), recent studies have determined the potential roles played by these glycans in protein folding, HCV entry, and protection against neutralization (14, 19, 22). Indeed, the lack of glycan E1N1, E1N4, E2N8, or E2N10 strongly affects the incorporation of HCV glycoproteins into HCVpp, suggesting that these glycans are important for correct protein folding (19). Furthermore, mutation of glycosylation sites E2N2 or E2N4 alters HCVpp infectivity despite normal incorporation into pseudotyped particles, suggesting a role for the corresponding glycans in viral entry, at least in this model system (19). Finally, glycans at positions E2N1, E2N6, and E2N11 were shown to reduce the sensitivity of HCVpp to antibody neutralization as well as access of the CD81 coreceptor to its binding site on E2, suggesting that glycans also contribute to HCV evasion of the humoral immune response (14, 22).It has recently been proposed that targeting glycans could be a promising approach to inhibiting viral infection (1). Indeed, HCV, as well as several other viruses with highly glycosylated envelope proteins, can be inhibited by carbohydrate binding agents such as cyanovirin-N and pradimicin A (1, 7, 23). Furthermore, resistance against drugs that target glycans is likely to develop and will probably result in mutations at some glycosylation sites (3, 52). However, since glycans associated with viral envelope proteins play an important role in the viral life cycle, adaptation of viruses to the selective pressure of carbohydrate-binding agents will most likely come at a replicative cost to the virus (2).Although the role of HCV glycans has been studied using mutant recombinant HCV envelope glycoproteins incorporated into HCVpp, these particles do not recapitulate all the functions of HCV envelope proteins. Cell culture-derived virus (HCVcc) (32, 49, 55) assembles in an ER-derived compartment in association with very low density lipoproteins (17, 26), whereas HCVpp are assembled in a post-Golgi compartment and are not associated with lipoproteins (44). Importantly, this leads to differences between HCVpp and HCVcc in the oligomerization of the envelope glycoproteins (46). It is also important to note that the carbohydrate composition of viral glycoproteins can differ when the same virus is grown in different cell lines (13). Thus, HCVpp that are produced in 293T cells are not the most appropriate model for glycosylation studies, since HCV tropism is restricted to the liver. Furthermore, differences in envelope protein glycosylation have been observed between HCVpp and HCVcc particles (46). Differences in some HCV envelope protein functions were also observed when the HCVpp and HCVcc systems were compared (28, 29, 42, 43). The development of the HCVcc system provides, therefore, the opportunity to characterize the role of E1/E2-associated glycans in the context of authentic infectious virions. Here, we analyzed the role of E1/E2 glycans by introducing point mutations at N-linked glycosylation sites in the context of the HCVcc system. The effects of these mutations on virus replication, particle secretion, infectivity, and sensitivity to neutralizing antibodies were investigated. Our results demonstrate that several glycans play an important role in HCVcc assembly and/or infectivity and reduce access of neutralizing antibodies to their epitopes.     

Contributions of Herpes Simplex Virus 1 Envelope Proteins to Entry by Endocytosis

Herpes simplex virus (HSV) proteins specifically required for endocytic entry but not direct penetration have not been identified. HSVs deleted of gE, gG, gI, gJ, gM, UL45, or Us9 entered cells via either pH-dependent or pH-independent endocytosis and were inactivated by mildly acidic pH. Thus, the required HSV glycoproteins, gB, gD, and gH-gL, may be sufficient for entry regardless of entry route taken. This may be distinct from entry mechanisms employed by other human herpesviruses.