Effects of mutations in glycosylation sites and disulphide bonds on processing, CD4-binding and fusion activity of human immunodeficiency virus envelope glycoproteins

Hidden Relationships between N-Glycosylation and Disulfide Bonds in Individual Proteins

N-Glycosylation (NG) and disulfide bonds (DBs) are two prevalent co/post-translational modifications (PTMs) that are often conserved and coexist in membrane and secreted proteins involved in a large number of diseases. Both in the past and in recent times, the enzymes and chaperones regulating these PTMs have been constantly discovered to directly interact with each other or colocalize in the ER. However, beyond a few model proteins, how such cooperation affects N-glycan modification and disulfide bonding at selective sites in individual proteins is largely unknown. Here, we reviewed the literature to discover the current status in understanding the relationships between NG and DBs in individual proteins. Our results showed that more than 2700 human proteins carry both PTMs, and fewer than 2% of them have been investigated in the associations between NG and DBs. We summarized both these proteins with the reported relationships in the two PTMs and the tools used to discover the relationships. We hope that, by exposing this largely understudied field, more investigations can be encouraged to unveil the hidden relationships of NG and DBs in the majority of membranes and secreted proteins for pathophysiological understanding and biotherapeutic development.

Increased surface expression of HIV-1 envelope is associated with improved antibody response in vaccinia prime/protein boost immunization

HIV-1 envelope (Env)-based vaccines have so far largely failed to induce antibodies that prevent HIV-1 infection. One factor proposed to limit the immunogenicity of cell-associated Env is its low level of expression on the cell surface, restricting accessibility to antibodies. Using a vaccinia prime/protein boost protocol in mice, we explored the immunologic effects of mutations in the Env cytoplasmic tail (CT) that increased surface expression, including partial truncation and ablation of a tyrosine-dependent endocytosis motif. After vaccinia primes, CT-modified Envs induced up to 7-fold higher gp120-specific IgG, and after gp120 protein boosts, they elicited up to 16-fold greater Tier-1 HIV-1 neutralizing antibody titers, although results were variable between isolates. These data indicate that the immunogenicity of HIV-1 Env in a prime/boost vaccine can be enhanced in a strain-dependent manner by CT mutations that increase Env surface expression, thus highlighting the importance of the prime in this vaccine format.

•

Novel HIV Env cytoplasmic tail (CT) modifications increase surface expression.

•

Vaccinia vector vaccination with CT-modified Envs induces high gp120-specific IgG.

•

gp120 boosts in mice primed with CT-modified Envs induce high Tier-1 Nabs.

Several N-Glycans on the HIV Envelope Glycoprotein gp120 Preferentially Locate Near Disulphide Bridges and Are Required for Efficient Infectivity and Virus Transmission

The HIV envelope glycoprotein gp120 contains nine disulphide bridges and is highly glycosylated, carrying on average 24 N-linked glycans. Using a probability calculation, we here demonstrate that there is a co-localization of disulphide bridges and N-linked glycans in HIV-1 gp120, with a predominance of N-linked glycans in close proximity to disulphide bridges, at the C-terminal side of the involved cysteines. Also, N-glycans are frequently found immediately adjacent to disulphide bridges in gp120 at the N-terminal side of the involved cysteines. In contrast, N-glycans at positions close to, but not immediately neighboring disulphide bridges seem to be disfavored at the N-terminal side of the involved cysteines. Such a pronounced co-localization of disulphide bridges and N-glycans was also found for the N-glycans on glycoprotein E1 of the hepatitis C virus (HCV) but not for other heavily glycosylated proteins such as E2 from HCV and the surface GP from Ebola virus. The potential functional role of the presence of N-glycans near disulphide bridges in HIV-1 gp120 was studied using site-directed mutagenesis, either by deleting conserved N-glycans or by inserting new N-glycosylation sites near disulphide bridges. The generated HIV-1_NL4.3 mutants were subjected to an array of assays, determining the envelope glycoprotein levels in mutant viral particles, their infectivity and the capture and transmission efficiencies of mutant virus particles by DC-SIGN. Three N-glycans located nearby disulphide bridges were found to be crucial for the preservation of several of these functions of gp120. In addition, introduction of new N-glycans upstream of several disulphide bridges, at locations where there was a significant absence of N-glycans in a broad variety of virus strains, was found to result in a complete loss of viral infectivity. It was shown that the N-glycan environment around well-defined disulphide bridges of gp120 is highly critical to allow efficient viral infection and transmission.

Only five of 10 strictly conserved disulfide bonds are essential for folding and eight for function of the HIV-1 envelope glycoprotein

Protein folding in the endoplasmic reticulum goes hand in hand with disulfide bond formation, and disulfide bonds are considered key structural elements for a protein's folding and function. We used the HIV-1 Envelope glycoprotein to examine in detail the importance of its 10 completely conserved disulfide bonds. We systematically mutated the cysteines in its ectodomain, assayed the mutants for oxidative folding, transport, and incorporation into the virus, and tested fitness of mutant viruses. We found that the protein was remarkably tolerant toward manipulation of its disulfide-bonded structure. Five of 10 disulfide bonds were dispensable for folding. Two of these were even expendable for viral replication in cell culture, indicating that the relevance of these disulfide bonds becomes manifest only during natural infection. Our findings refine old paradigms on the importance of disulfide bonds for proteins.

Removal of N-linked glycosylation sites in the V1 region of simian immunodeficiency virus gp120 results in redirection of B-cell responses to V3

One mechanism of immune evasion utilized by human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) envelope glycoproteins is the presence of a dense carbohydrate shield. Accumulating evidence from in vitro and in vivo experiments suggests that alterations in N-linked glycosylation of SIV gp120 can enhance host humoral immune responses that may be involved in immune control. The present study was designed to determine the ability of glycosylation mutant viruses to redirect antibody responses to shielded envelope epitopes. The influence of glycosylation on the maturation and specificity of antibody responses elicited by glycosylation mutant viruses containing mutations of specific N-linked sites in and near the V1 and V2 regions of SIVmac239 gp120 was determined. Results from these studies demonstrated a remarkably similar maturation of antibody responses to native, fully glycosylated envelope proteins. However, analyses of antibodies to defined envelope domains revealed that mutation of glycosylation sites in V1 resulted in increased antibody recognition to epitopes in V1. In addition, we demonstrated for the first time that mutation of glycosylation sites in V1 resulted in a redirection of antibody responses to the V3 loop. Taken together, these results demonstrate that N-linked glycosylation is a determinant of SIV envelope B-cell immunogenicity in addition to in vitro antigenicity. In addition, our results demonstrate that the absence of N-linked carbohydrates at specific sites can influence the exposure of epitopes quite distant in the linear sequence.

HIV-1 gag polyprotein rescues HLA-DR intracellular transport in a human CD4+ cell line

Major histocompatibility complex class II HLA-DR molecules are plasma-membrane integral heterodimers, constitutively expressed in antigen-presenting cells. Their expression is known to be upregulated in peripheral T lymphocytes upon cell activation and to be constitutive in T cell lines. In H78-C10.0, a subclone of the human CD4+ T cell line HUT-78, the transport of MHC class II HLA-DR molecules is blocked, resulting in their localization within internal vesicular compartments rather than at the cell surface. In this article, we show that HIV-1(HX10) infection of H78-C10.0 cells induces HLA-DR surface expression. Moreover, the produced infectious viruses harbor the heterodimer molecules in their envelopes. To define which of the viral proteins was involved in this phenomenon, we infected H78-C10.0 cells with recombinant vaccinia vectors containing either the gag-pro coding sequence or the entire env gene. Only gag expression was able to induce HLA-DR cell-surface localization in H78-C10.0 cells. RT-PCR analysis of the infected cells revealed no significant alteration in the amount of HLA-DRalpha-specific RNA compared to untreated cells. This implies that Gag acts on downstream events. When the env viral gene, coding for the precursor glycoprotein gp160, was expressed in H78-C10.0, the Env protein targeted to the cell surface was poorly processed to its final mature forms gp120 and gp41. However, coexpression of the env and gag genes led to restoration of this phenotype. Although the mechanism is unknown, the data compiled in this study strongly suggest that the viral Gag protein can interact with the cellular trafficking apparatus. Moreover, in a specific cell type as H78-C10.0 this interaction can even reverse intracellular transport defects.

Induction of hepatitis C virus E1 envelope protein-specific immune response can be enhanced by mutation of N-glycosylation sites

Deglycosylation of viral glycoproteins has been shown to influence the number of available epitopes and to modulate immune recognition of antigens. We investigated the role played by N-glycans in the immunogenicity of hepatitis C virus (HCV) E1 envelope glycoprotein, a naturally poor immunogen. Eight plasmids were engineered, encoding E1 protein mutants in which the four N-linked glycosylation sites of the protein were mutated separately or in combination. In vitro expression studies showed an influence of N-linked glycosylation on expression efficiency, instability, and/or secretion of the mutated proteins. Immunogenicity of the E1 mutants was studied in BALB/c mice following intramuscular and intraepidermal injection of the plasmids. Whereas some mutations had no or only minor effects on the antibody titers induced, mutation of the fourth glycosylation site (N4) significantly enhanced the anti-E1 humoral response in terms of both seroconversion rates and antibody titers. Moreover, antibody induced by the N4 mutant was able to recognize HCV-like particles with higher titers than those induced by the wild-type construct. Epitope mapping indicated that the E1 mutant antigens induced antibody directed at two major domains: one, located at amino acids (aa) 313 to 332, which is known to be reactive with sera from HCV patients, and a second one, located in the N-terminal domain of E1 (aa 192 to 226). Analysis of the induced immune cellular response confirmed the induction of gamma interferon-producing cells by all mutants, albeit to different levels. These results show that N-linked glycosylation can limit the antibody response to the HCV E1 protein and reveal a potential vaccine candidate with enhanced immunogenicity.

Enhanced immunogenicity of a human immunodeficiency virus type 1 env DNA vaccine by manipulating N-glycosylation signals. Effects of elimination of the V3 N306 glycan

DNA encoding HIV-1 env is a poorly efficient B-cell immunogen and one probable explanation is that the numerous gp120 N-linked glycans gp120 may interfere with B-cell epitope presentation. The N306 glycan in gp120 shields HIV-1 from neutralizing antibodies. A DNA immunogen lacking the N306 glycosylation signal (T308A) was constructed to determine whether this glycan affected the immune response. Mice were immunized intranasally twice with DNA containing either the wild type or the mutant env. Two additional groups were primed with wild type or mutant env and boosted with rgp160 protein, containing the complete set of N-linked glycans. Immunization with DNA alone resulted in priming of B-cell clones but was not sufficient to induce a complete antibody response. Animals primed with the N306 mutant and subsequently boosted with rgp160 protein displayed higher serum IgG-binding titers to gp120 than animals primed with wild type env DNA. The manipulation of the glycosylation sites of the env DNA strongly primes antibody responses (but non-neutralizing) as well as T-cell responses to the wild type strain gp160. However, priming with mutant plasmid did not result in higher neutralization titers to wild type or T308A-mutated virus than did the wild type plasmid. With the N306 mutant DNA we thus immunized a non-neutralization epitope, but obtained strong env-binding IgG after rgp160 boosting.

N-linked glycosylation in the V3 region of HIV type 1 surface antigen modulates coreceptor usage in viral infection

The V3 hypervariable region of HIV-1 surface protein has been identified as a major determinant for viral tropism and coreceptor usage. However, the role of the highly conserved N-linked glycan at the V3 loop remains controversial. To further examine its role in viral infection, we introduced a conservative amino acid substitution (asparagine to glutamine) in the V3-proximal glycosylation motif (Asn-X-Ser/Thr) in the surface glycoprotein of a CXCR4-using virus (BRU), a CCR5-using virus (SF162), and a dual-tropic virus (89.6). The effect of the mutation was determined by complementation assays, and by infectivity on CEMx174 and U373-MAGI cells expressing either CXCR4 or CCR5. The mutation resulted in decreased CXCR4 usage by SHIV89.6, but increased usage by BRU. Similarly, it abrogated CCR5 usage by SHIV89.6, but had no effect on SF162. This effect was not dependent on the specific amino acid substitution used, because a threonine-toalanine mutation in the same motif in 89.6 Env yielded identical results as the asparagine-to-glutamine mutation. These findings support the notion that multiple factors, including glycosylation at V3, contribute to coreceptor usage and that the particular effects exerted by the N-linked glycan itself appear to be isolate dependent.

Protection of neutralization epitopes in the V3 loop of oligomeric human immunodeficiency virus type 1 glycoprotein 120 by N-linked oligosaccharides in the V1 region

The V3 region of the human immunodeficiency virus type 1 envelope protein gp120 constitutes a potential neutralization target, but the oligosaccharide of one conserved N-glycosylation site in this region protects it from neutralizing antibodies. Here, we determined whether N-linked glycans of other gp120 domains were also involved in protection of V3 neutralization epitopes. Two molecular clones of HIV-1, one lacking three N-linked glycans of the V1 region (HIV-1(3N/V1)) and another lacking three N-linked glycans of the C2 region (HIV-1(3N/C2)), were created and characterized. gp120 from both mutated viral clones had higher electrophoretic mobilities than gp120 from wild-type virus, confirming loss of N-linked glycans. Wild-type virus and both mutant clones replicated equally well in established T cell lines and all three viruses were able to utilize CXCR4 but not CCR5 as a coreceptor. The induced mutations increased gp120 affinity for CXCR4 but caused no corresponding increase in viral ability to replicate in T cell lines. HIV-1(3N/V1) was neutralized at about 25 times lower concentrations of an antibody to the V3 region than were wild-type virus and HIV-1(3N/C2). Soluble, monomeric gp120 from HIV-1(3N/V1) and wild type virus had identical avidity for the V3 antibody, indicating that the V1 glycans were able to shield V3 only in oligomeric but not monomeric gp120. In conclusion, one or more N-linked glycans of gp120 V1 is engaged in protection of the V3 region from potential neutralizing antibodies, and this effect is dependent on the oligomeric organization of gp120/gp41.

Cyanovirin-N defines a new class of antiviral agent targeting N-linked, high-mannose glycans in an oligosaccharide-specific manner

Herein we report that the novel HIV-inactivating protein cyanovirin-N (CV-N) targets specific, N-linked high-mannose oligosaccharides found on the viral envelope of HIV-1. First, we released the oligosaccharides by PnGase-treatment of HIV-gp120 (containing high-mannose, hybrid-type and complex-type oligosaccharides) or HSV-1 gC (containing only complex-type). Then, in an affinity chromatographic system, we found that CV-N bound to the free oligosaccharides from gp120 but not from gC-1, suggesting that high-mannose oligosaccharides constitute a target structure for CV-N. This was supported by the affinity of CV-N for high-mannose glycans released from gp120 by endo-H as well as high-mannose glycans released from castanospermine-treated HSV-1 gC. Furthermore, free Man-8 or Man-9 oligosaccharides partially inhibited the binding of CV-N to gp120, although neither oligosaccharides smaller than Man-7 nor monosaccharides interfered with CV-N/gp120 interaction, thereby establishing the oligosaccharide-specific affinity of CV-N to high-mannose glycans. This affinity for high-mannose oligosaccharides may explain the broad antiviral activity of CV-N against human and primate immunodeficiency retroviruses as well as certain other viruses that carry these oligosaccharides.

Early steps in O-linked glycosylation and clustered O-linked glycans of herpes simplex virus type 1 glycoprotein C: effects on glycoprotein properties

The pathogenesis of herpes simplex virus type 1 (HSV-1) implies the sequential infection of many cell types from mucosal cells to neurons, each having a unique pattern of protein glycosylation. The HSV-1 glycoprotein gC-1 is highly glycosylated and contains not only N-linked glycans but also a large number of O-linked glycans, some of which are clustered into two pronase-resistant arrays in the vicinity of the HSV-1 receptor-binding domain of gC-1. The aim of the present study was to characterize gC-1 signals for addition of clustered glycans, to determine the efficacy of synthetic peptides, representing putative O-glycosylation signals, as substrates for a panel of GalNAc transferases, and to identify possible effects of early O-linked glycosylation on the biological functions of gC-1. Gel filtration analysis of the pronase-resistant gC-1 O-glycan clusters from a glycoprotein mutant, lacking a site for N-linked glycosylation at Asn 73 in the vicinity of the O-glycosylation signal, suggested that one function of this N-linked glycan was to modulate the access for GalNAc transferases to one particular O-glycosylation peptide signal (aa 80-104). The ability of four GalNAc-transferase isoenzymes with different cell type expression patterns to initialize O-glycosylation of synthetic gC-1 derived peptides was analyzed. Two synthetic gC-1 peptides (aa 55-69 and aa 80-104) were excellent substrates for all four GalNAc-transferases, suggesting that cell types expressing less frequent GalNAc transferase species with unusual acceptor peptide sequence specificities may also produce a highly O-glycosylated gC-1 after HSV-1 infection. The O-linked glycans were not essential for cell surface expression of gC-1, but monoclonal antibody-assisted epitope analysis of N-acetylgalactosaminidase-treated gC-1 showed that the O-linked monosaccharide GalNAc contributed to expression of a three-dimensional epitope overlapping the heparan sulfate-binding domain of gC-1.

Processing and routage of HIV glycoproteins by furin to the cell surface

Proteolytic activation of HIV-1 and HIV-2 envelope glycoprotein precursors (gp160 and gp140, respectively) occurs at the carboxyl side of a consensus motif consisting of the highly basic amino acid sequence. We have shown previously (Hallenberger et al., 1997) and confirmed in this report, that furin and PC7 can be considered as the putative physiological enzymes involved in the proteolytic activation of the HIV-1 and HIV-2 envelope precursors. In this study, we show by cell surface biotinylation and immunoprecipitation of the cell surface associated viral glycoproteins with antibodies that the mature viral envelope glycoproteins are correctly transported to the cell. membrane. Furthermore, we show that the uncleaved forms of the glycoproteins (gp160HIV-1 and gp140HIV-2) are also highly represented at the cell surface. First, transient expression of gp160 and gp140 into CV1, a cell line known to be inefficient in the proteolytic processing of the env gene, results in the expression of gp160 and gp140 at the cell surface. Moreover, HIV-1 infection of T cells also showed that gp160 is directed to the cell surface. In addition, we show that the precursor is not incorporated in the virus particle following the budding from the cell surface. Furthermore, a gp160 mutant (deficient for three carbohydrate sites on the gp41), shown to be poorly processed with the coexpressed endoproteases, is found to be transported as an uncleaved precursor to the cell surface. In contrast to HIV envelope glycoproteins, the influenza hemagglutinin precursor (HA0), that is thought to be matured by the furin-like enzymes as well, is found to be retained within the cell and is not able to reach the cell surface. Taken together, these results show that the proteolytic maturation of the viral envelope precursors of human immunodeficiency viruses type 1 and type 2 is not a prerequisite for cell surface targeting of the HIV glycoproteins. Implications of these results for antiviral immune response are discussed.

Limited breadth of the protective immunity elicited by simian immunodeficiency virus SIVmne gp160 vaccines in a combination immunization regimen

We previously reported that immunization with recombinant simian immunodeficiency virus SIVmne envelope (gp160) vaccines protected macaques against an intravenous challenge by the cloned homologous virus, E11S. In this study, we confirmed this observation and found that the vaccines were effective not only against virus grown on human T-cell lines but also against virus grown on macaque peripheral blood mononuclear cells (PBMC). The breadth of protection, however, was limited. In three experiments, 3 of 10 animals challenged with the parental uncloned SIVmne were completely protected. Of the remaining animals, three were transiently virus positive and four were persistently positive after challenge, as were 10 nonimmunized control animals. Protection was not correlated with levels of serum-neutralizing antibodies against the homologous SIVmne or a related virus, SIVmac251. To gain further insight into the protective mechanism, we analyzed nucleotide sequences in the envelope region of the uncloned challenge virus and compared them with those present in the PBMC of infected animals. The majority (85%) of the uncloned challenge virus was homologous to the molecular clone from which the vaccines were made (E11S type). The remaining 15% contained conserved changes in the V1 region (variant types). Control animals infected with this uncloned virus had different proportions of the two genotypes, whereas three of four immunized but persistently infected animals had >99% of the variant types early after infection. These results indicate that the protective immunity elicited by recombinant gp160 vaccines is restricted primarily to the homologous virus and suggest the possibility that immune responses directed to the V1 region of the envelope protein play a role in protection.

Simplified procedure for fractionation and structural characterisation of complex mixtures of N-linked glycans, released from HIV-1 gp120 and other highly glycosylated viral proteins

HIV-1 gp120 is heavily glycosylated containing 24 N-glycosylation sites, and this makes elucidation of the significance of glycans at individual glycosylation sites a difficult task. A procedure is described where a complex mixture of biologically radiolabelled glycans of gp120, derived from a relatively small number of virus-infected cells may be characterized by a combination of N-glycanase release, single lectin separation, and normal phase HPLC (NP-HPLC). The method was applied in analysis of three N-linked glycosylation sites essential for the in vivo priming of T-cells, specific for an epitope in their vicinity (Sjölander, S., Bolmstedt, A., Akerblom, 1996. Virology 215, 124-133.). The carbohydrate compositions of wild type gp120 and of mutant variants gp120 lacking one, two, or all of these three active N-linked glycans were analysed. Cells were infected with r-vaccinia virus expressing wild-type gp120 or mutated gp120, or were infected with HIV-1BRU (wild type) or mutant virus variants. HIV-1 glycoproteins were purified by immunosorbent affinity chromatography and released glycans were separated on lectins, then analysed with NP-HPLC. Our data showed that the structural composition of glycans occupying two of the three glycosylation sites was heterogeneous but the site located adjacent to the T-cell epitope was equipped with one large, high mannose-type structure (> 11 units) with the capacity to cover a substantial part of the gp120 surface.

Dissociation of the CD4 and CXCR4 binding properties of human immunodeficiency virus type 1 gp120 by deletion of the first putative alpha-helical conserved structure

To evaluate conserved structures of the surface gp120 subunit (SU) of the human immunodeficiency virus type 1 (HIV-1) envelope in gp120-cell interactions, we designed and produced an HIV-1 IIIB (HXB2R) gp120 carrying a deletion of amino acids E61 to S85. This sequence corresponds to a highly conserved predicted amphipathic alpha-helical structure located in the gp120 C1 region. The resultant soluble mutant with a deleted alpha helix 1 (gp120 DeltaalphaHX1) exhibited a strong interaction with CXCR4, although CD4 binding was undetectable. The former interaction was specific since it inhibited the binding of the anti-CXCR4 monoclonal antibody (12G5), as well as SDF1alpha, the natural ligand of CXCR4. Additionally, the mutant gp120 was able to bind to CXCR4(+)/CD4(-) cells but not to CXCR4(-)/CD4(-) cells. Although efficiently expressed on cell surface, HIV envelope harboring the deleted gp120 DeltaalphaHX1 associated with wild-type transmembrane gp41 was unable to induce cell-to-cell fusion with HeLa CD4(+) cells. Nevertheless, the soluble gp120 DeltaalphaHX1 efficiently inhibited a single round of HIV-1 LAI infection in HeLa P4 cells, with a 50% inhibitory concentration of 100 nM. Our data demonstrate that interaction with the CXCR4 coreceptor was maintained in a SUgp120 HIV envelope lacking alphaHX1. Moreover, in the absence of CD4 binding, the interaction of gp120 DeltaalphaHX1 with CXCR4 was sufficient to inhibit HIV-1 infection.

Identification of replication-competent strains of simian immunodeficiency virus lacking multiple attachment sites for N-linked carbohydrates in variable regions 1 and 2 of the surface envelope protein

Carbohydrates comprise about 50% of the mass of gp120, the external envelope glycoprotein of simian immunodeficiency virus (SIV) and human immunodeficiency virus. We identified 11 replication-competent derivatives of SIVmac239 lacking two, three, four, or five potential sites for N-linked glycosylation. These sites were located within and around variable regions 1 and 2 of the surface envelope protein of the virus. Asn (AAT) of the canonical N-linked glycosylation recognition sequence (Asn X Ser/Thr) was changed in each case to the structurally similar Gln (CAG or CAA) such that two nucleotide changes in the codon would be required for reversion. Replication of one triple mutant (g456), however, was severely impaired. A revertant of the g456 mutant was recovered from CEMx174 cells with a Met-to-Val compensatory substitution at position 144, 2 amino acids upstream of attachment site 5. Thus, a debilitating loss of sites for N-linked glycosylation can be compensated for by amino acid changes not involving the Asn-X-Ser/Thr consensus motif. These results provide a framework to begin testing the hypothesis that carbohydrates form a barrier that can limit the humoral immune responses to the virus.

Retroviral envelope glycoprotein processing: structural investigation of the cleavage site

Proteolytic activation of retroviral envelope glycoprotein precursors occurs at the carboxyl side of a consensus motif consisting of the amino acid sequence (Arg/Lys)-Xaa-(Arg/Lys)-Arg. Synthetic peptides spanning the processing sites of HIV-1/2 and SIV glycoprotein precursors were examined for their ability to be cleaved by the subtilisin-like endoproteases kexin and furin. To determine the potential role of secondary structure on proteolytic activation, we examined the secondary structure of synthetic peptides by circular dichroism and NMR spectroscopy. The results indicate that (i) the peptides were correctly cleaved by kexin and furin and therefore could be used as specific substrates for the purification and characterization of the lymphocyte endoprotease(s) responsible for proteolytic processing of precursors; (ii) the regions surrounding the cleavage sites could be characterized by their flexibility in aqueous solutions. However, a loop has been shown to be a determinant for the specificity of the interaction between the enzyme and its substrate as determined by molecular modeling. Furthermore, we determine and propose a possible structure of the cleavage site which fits to the active site of the modeled furin.

Conserved N-linked oligosaccharides of the C-terminal portion of human immunodeficiency virus type 1 gp120 and viral susceptibility to neutralizing antibodies

We have constructed a mutated infectious HIV variant lacking the signals for addition of three N-linked glycans situated in the V4, C4 and V5 regions of HIV gp120. When comparing mutated virus with wildtype virus we found essentially no differences in the phenotypic characteristics of the two viruses except for the expected electrophoretic mobility shift of radioimmuno-precipitated mutated gp120, resulting from the missing N-glycans. Thus, the infectivity titer and the capacity to induce syncytia were similar for the two viruses. The sensitivity of mutant and wildtype virus to a number of neutralizing agents was determined. As expected, the mutant virus was significantly less sensitive to neutralization by Con A, with affinity for the N-glycans eliminated. We found, however, that antibodies to the V3 loop and sCD4 neutralized wild-type virus as efficiently as mutant virus, whereas 2G12, a monoclonal antibody, binding to a discontinuous neutralization epitope, and GP13, binding to the CD4-binding domain, neutralized wildtype virus better than mutant virus. Altogether the data suggest that the three conserved N-linked glycans, despite their location in immediate association with the CD4-binding domain, which is an important neutralization epitope, are not essential for virus replication in cell culture and they are not engaged in shielding neutralization epitopes of gp120 from neutralizing antibodies. However, the glycans evidently influence the three-dimensional conformation of gp120, since their presence increases the availability of the neutralization epitope of 2G12.

Influence of N-linked glycans in V4-V5 region of human immunodeficiency virus type 1 glycoprotein gp160 on induction of a virus-neutralizing humoral response

One of the functions of N-linked glycans of viral glycoproteins is protecting otherwise accessible neutralization epitopes of the viral envelope from neutralizing antibodies. The aim of the present study was to explore the possibility to obtain a more broadly neutralizing immune response by immunizing guinea pigs with gp160 depleted of three N-linked glycans in the CD4-binding domain by site-directed mutagenesis. Mutant and wild type gp160 were formulated into immunostimulating complexes and injected s.c. into guinea pigs. Both preparations induced high serum antibody response to native gp120 and V3 peptides. Both preparations also induced antibodies that bound equally well to the V3 loop or the CD4-binding region, as determined by a competitive enzyme-linked immunosorbent assay (ELISA). The sera from animals, immunized with mutated glycoprotein, did not neutralize nonrelated HIV strains better than did sera from animals, immunized with wild type glycoprotein. Instead, a pattern of preferred homologous neutralization was observed, i.e., sera from animals, immunized with mutant gp160, neutralized mutant virus better than wild type virus, and vice versa. These data indicated that elimination of the three N-linked glycans from gp160 resulted in an altered local antigenic conformation but did not uncover hidden neutralization epitopes, broadening the immune response.

Cooperative effects of the human immunodeficiency virus type 1 envelope variable loops V1 and V3 in mediating infectivity for T cells

Insertion of T-cell line-tropic V3 and V4 loops from the HXB2 strain into the macrophage-tropic YU-2 envelope resulted in a virus with delayed infectivity for HUT78 and Jurkat cells compared with HXB2. Sequence analysis of viral DNA derived from long-term cultures of Jurkat cells revealed a specific mutation that changed a highly conserved Asn residue in the V1 loop of Env to an Asp residue (N-136-->D). Introduction of this mutation into clones containing a T-cell line-tropic V3 loop, either with or without a T-cell line-tropic V4 loop, resulted in viruses that replicated to high levels in Jurkat cells and peripheral blood lymphocytes. The Env proteins from these constructs were expressed with the vaccinia virus/T7 hybrid system and were found to be translated, processed, and cleaved and to bind to soluble CD4 similar to the wild-type HXB2 and YU-2 Env proteins. Env-mediated fusion with HeLa T4+ cells, however, was regulated by both the altered V1 loop and T-cell line-tropic V3 loop. These results suggest that subsequent to the initial gp120-CD4 binding event, a functional interaction can occur between the altered V1 loop and T-cell line-tropic V3 loop that results in infection of Jurkat cells and peripheral blood lymphocytes.

Identification of three N-linked glycans in the V4-V5 region of HIV-1 gp 120, dispensable for CD4-binding and fusion activity of gp 120

Site-directed mutagenesis was used to study the biological significance of three N-linked glycans (linked to Asn406, Asn448, and Asn463), situated in the CD4-binding region of gp120. Mutagenesis was carried out in a phage M13 system, and the mutated env genes were inserted into recombinant vaccinia virus (r-vaccinia virus). To evaluate if the level of expression affected the biological phenotype of mutant gp120, we expressed the envelope glycoproteins using either a weak (7.5 K) or a strong (11 K) promoter of vaccinia virus. The expression of mutated env proteins was analyzed after infecting CD4-expressing HeLa cells with the r-vaccinia virus, by monitoring the ability of the infected cells to generate CD4-dependent syncytia. Env gene products lacking all three glycans as well as env gene products lacking different permutations of one or two glycans were analyzed. All mutated gp120 species had the expected electrophoretical mobility as anticipated from elimination of one, two, and three N-linked glycans, respectively. Moreover, all mutant env gene products demonstrated the same capacity to induce formation of syncytia, irrespective of using the weak or strong promoter for expression. These data indicate that the three N-linked glycans studied are dispensable for HIV env gene products to function in CD4-binding and the subsequent fusion step.

Cysteine residues in the Vif protein of human immunodeficiency virus type 1 are essential for viral infectivity

The infectivity factor of human immunodeficiency virus type 1 (HIV-1), Vif, contains two cysteine residues which are highly conserved among animal lentiviruses. We introduced substitutions of leucine for cysteine residues in the vif gene of a full-length HIV-1 clone to analyze their roles in viral infection. Mutant viruses containing substitutions in either Cys-114, Cys-133, or both displayed a vif-negative infection phenotype similar to that of an isogeneic vif deletion mutant, namely, a cell-dependent complete to partial loss of infectivity. The vif defect could be complemented by cotransfection of mutant viral DNA with a Vif expression vector, and there was no evidence that recombination contributed to the repair of the vif deficiency. The viral protein profile, as determined by immunoblotting, in cells infected with cysteine substitution mutants and that in wild-type virus were similar, including the presence of the 23-kDa Vif polypeptide. In addition, immunoblotting with an antiserum directed against the carboxyl terminus of gp41 revealed that gp41 was intact in cells infected with either wild-type or vif mutant HIV-1, excluding that Vif cleaves the C terminus of gp41. Our results indicate that the cysteines in HIV-1 Vif are critical for Vif function in viral infectivity.

Functions of HIV envelope glycans

The unusually highly glycosylated state of the major envelope glycoprotein (gp160) of the human immunodeficiency virus has offered a challenge to both glycobiologists and virologists. What is the functional significance of such a mass of glycans and how might they be manipulated to disadvantage virus pathogenesis? Some answers to each of these questions have already been obtained: N-linked glycans are necessary for the creation, but not the maintenance, of a bioactive conformation, and drug-induced alteration of the glycosylation pattern can lead to impaired virus infectivity. As a model for studying glycan function and as a target for antiviral therapy, gp160 represents a unique candidate.

trans-dominant interference with virus infection at two different stages by a mutant envelope protein of Friend murine leukemia virus

A dominant negative mutant Friend murine leukemia virus (FMLV) env gene was cloned from an immunoselected Friend erythroleukemia cell. The mutant env had a point mutation which resulted in a Cys-to-Arg substitution at the 361st amino acid in the FMLV envelope protein (Env). The mutant Env was retained in the endoplasmic reticulum (ER) and accumulated because of its slow degradation. The NIH 3T3 cells expressing the mutant env were resistant to ecotropic Moloney MLV (MoMLV) penetration, suggesting that the mutant Env traps the ecotropic MLV receptors in the ER. When the mutant env gene was transfected into and expressed in the cells persistently infected with MoMLV, the wild-type Env was trapped in the ER, and the MoMLV production was suppressed. Thus, the mutant Env accumulating in the ER trans-dominantly and efficiently interfered with the ecotropic MLV infection at both the early and the late stages.

Mechanism of action of N-butyl deoxynojirimycin in inhibiting HIV-1 infection and activity in combination with nucleoside analogs

The effects on HIV-1 infection of a glucosidase inhibitor, N-butyl deoxynojirimycin (N-buDNJ), were examined. The combinations of N-buDNJ and nucleoside analogs dideoxyinosine (DDI), dideoxycytidine (DDC), or azidothymidine (AZT) were examined in an acute infection assay. The combination of N-buDNJ and nucleoside analog reduced the yield of reverse transcriptase activity more than did either agent alone, and the effects on the number of infectious virus particles were additive or synergistic. In studies of the mechanism whereby N-buDNJ alters HIV-1 envelope fusion activity, no effects on CD4 binding were detected. However, cleavage within the V3 loop of gp120 was reduced by N-buDNJ treatment, possibly reflecting an altered conformation of this region of the envelope protein.

Synthetic peptides and anti-peptide antibodies as probes to study interdomain interactions involved in virus assembly: the envelope of the human immunodeficiency virus (HIV-1)

Synthetic peptides and anti-peptide antibodies have been widely used as probes to map B- and T-cell epitopes on proteins. Such probes also have the potential to delineate contact sites involved generally in protein-protein interactions or in association of domains within a protein. We applied peptide/anti-peptide probes to define: (1) regions on the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins gp120 and gp41 involved in the association between these two glycoproteins; and (2) sites on gp120/gp41, essential for the association of HIV-1 with the CD4 cell receptor. Results of this examination suggested the following: (1) two segments on gp120, encompassing residues (102-126) and (425-452), contribute to the binding site for CD4 and are expected to be juxtaposed in the folded gp120 chain; (2) portions of immunodominant gp120 and gp41 epitopes, encompassing residues (303-338) and (579-611), respectively, appeared to be involved in the gp120-gp41 association, as suggested by direct binding studies and by the limited accessibility of these epitopes on HIV-1 virions: other portions of gp120 also appeared to contribute to the association between these two glycoproteins; (3) there is a partial overlap between gp41 and CD4 binding sites on gp120; (4) the fusion domain and a segment (637-666) of gp41 are not accessible to antibodies after oligomerization of gp41; and 5) the gp120-gp41 association was blocked by aurintricarboxylic acid, suggesting the possibility of developing antiviral compounds interfering with HIV-1 assembly.

Nonrandom distribution of gp120 N-linked glycosylation sites important for infectivity of human immunodeficiency virus type 1

More than 20 consensus N-linked glycosylation sites occur in the gp120 coding sequence of most isolates of human immunodeficiency virus type 1. Based on the N-linked glycosylation pattern of a well-characterized recombinant gp120, it is likely that N-linked sugars are present at most, if not all, of the consensus glycosylation sites of the heavily glycosylated gp120. In this study, we evaluated the relative importance of each of the 24 N-linked glycosylation sites of gp120 in the molecular clone HXB2 to viral infectivity. The ability of HXB2-derived mutants, each having 1 of the 24 N-linked glycosylation sites mutated by site-directed mutagenesis, to infect CD4-positive SupT1 cells was compared with that of the wild-type virus. We found that most of the individual consensus N-linked glycosylation sites are dispensable for viral infectivity. The five consensus N-linked glycosylation sites that are likely to have important roles in infectivity are all located in the amino-terminal half of gp120, indicating that the N-linked glycosylation sites that are important for infectivity of human immunodeficiency virus type 1 are not randomly distributed in gp120. We predict that a partially glycosylated gp120 with most of the dispensable N-linked glycosylation sites removed may be a better vaccine candidate than the fully glycosylated gp120.

An O-linked carbohydrate neutralization epitope of HIV-1 gp 120 is expressed by HIV-1 env gene recombinant vaccinia virus

Previous studies have disagreed about the presence of O-linked carbohydrate epitopes on gp 120 of HIV, although antibodies against short-chain O-linked glycans neutralize HIV infection and block syncytium formation in vitro. To settle this question, we analysed the O-linked glycans of gp 120 by chemical methods using purified HIV-1 gp 120 from cells infected with recombinant vaccinia virus solely expressing gp 160 or gp 120. Alkaline borohydride degradation of recombinant gp 120 released monosaccharides and also slightly larger structures (di/trisaccharides) by a beta-elimination, confirming the presence of simple O-linked oligosaccharides. The functional activity as neutralisation epitopes of the O-linked oligosaccharides expressed on recombinant gp 120 was preserved, since fusion between uninfected CD4+ cells and cells infected with recombinant vaccinia was blocked by monoclonal antibodies to the O-linked oligosaccharides of gp 120. Although the mechanism for HIV induction of O-linked oligosaccharide neoantigens is unknown, these results indicate that the O-linked neutralization epitopes are inherent to the glycoprotein itself, and that the unusual appearance of simple O-linked oligosaccharides on gp 120 is independent of any interaction between the host cell and retroviral genes other than env.

Role of asparagine-linked glycosylation in human immunodeficiency virus type 1 transmembrane envelope function

Transmembrane envelope protein (TM) residues 100, 105, and 128 of human immunodeficiency virus type 1 (HIV-1) strain HXB2 are potential sites for asparagine-linked oligosaccharide additions which are conserved among HIV-1 isolates, and all other lentivirus TM proteins. Site-specific mutants of each of the asparagine residues did not eliminate the ability of the virus to infect and replicate in CD4+ cells, but infectivity was reduced with all of these mutants, and syncytia induction was attenuated with two of these mutants. Studies of envelope expression of the mutant with the most severe defect demonstrated no significant effects on envelope protein synthesis, conformation, processing, multimerization, or release into the culture medium, suggesting that N-linked oligosaccharides are important in the specific fusion activity of TM.

Glucosidase inhibitors for treatment of HIV-1 infection

The HIV-1 envelope protein is a glycoprotein composed of 120 kD and 41 kD subunits. It contains 30-38 potential asparagine-linked glycosylation sites which have been shown to play a role in CD4 binding, virus uptake, and cytopathogenicity. Several inhibitors of oligosaccharide attachment or modification have been tested. An agent which inhibits glucosidases, N-butyl deoxynojirimycin was found to inhibit HIV-1 and SIVmac infectivity, and is currently in clinical trials.