Protein disulfide isomerase-mediated disulfide bonds regulate the gelatinolytic activity and secretion of matrix metalloproteinase-9

Matrix metalloproteinase-9 (MMP-9) is one of the major MMPs that can degrade extracellular matrix. Besides normal physiological functions, MMP-9 is involved in metastasis and tumor angiogenesis. Although several inhibitors of MMP-9 have been identified, in vivo regulators of MMP-9 activation are unknown. In the present study we intended to investigate novel therapeutic target protein(s) that regulate MMP-9 activation and/or secretion. We have identified protein disulfide isomerase as a novel upstream regulator of MMP-9. Mass spectrometric analysis of post-translational modification in MMP-9 confirmed six disulfide bonds in the catalytic domain and one disulfide bond in the hemopexin domain of MMP-9. Establishment of cells that overexpressed wild-type and mutant forms of MMP-9 revealed that 'cysteine-switch' and disulfide bonds within the catalytic domain are necessary for the secretion and intracellular trafficking of MMP-9. However, the disulfide bond of the hemopexin domain and other cysteines have no significant role in secretion. These insights into the secretion of MMP-9 constitute the basis for the development of potential drugs against metastasis.     

Regions required for CD4 binding in the external glycoprotein gp120 of simian immunodeficiency virus.

The external domain of the envelope glycoprotein, gp120, of simian immunodeficiency virus (SIV) has been expressed as a mature secreted product using recombinant baculoviruses and the expressed protein, which has an observed molecular mass of 110 kDa, was purified by monoclonal antibody (MAb) affinity chromatography. N-terminal sequence analysis showed a signal sequence cleavage identity similar to that of the gp120s of both human immunodeficiency virus type 1 (HIV-1) and HIV type 2. The expressed molecule bound to soluble CD4 with an affinity that was approximately 10-fold lower than that of gp120 from HIV-1. A screening of the ability of SIV envelope MAbs to inhibit CD4 binding revealed two groups of inhibitory MAbs. One group is dependent on conformation, while the second group maps to a discrete epitope near the amino terminus. The particular role of the V3 loop region of the molecule in CD4 binding was investigated by the construction of an SIV-HIV hybrid in which the V3 loop of SIV was precisely replaced with the equivalent domain from HIV-1 MN. The hybrid glycoprotein bound HIV-1 V3 loop MAbs and not SIV V3 MAbs but continued to bind conformational SIV MAbs and soluble CD4 as well as the parent molecule.     

Sindbis virus membrane fusion is mediated by reduction of glycoprotein disulfide bridges at the cell surface.

We have examined the role of thiol-disulfide exchange reactions during the penetration of cells by Sindbis virus. The protein-protein association that form the rigid icosahedral lattice of the Sindbis virus envelope have been shown to be stabilized by disulfide bridges, and reduction of these critical disulfide bridges during cell penetration may be the mechanism by which the rigid protein lattice is disrupted prior to fusion (R. Anthony and D. T. Brown, J. Virol. 65:1187-1194, 1991; R. Anthony, A. Paredes, and D. T. Brown, Virology 190:330-336, 1992). Reduction of disulfide bridges occurs at near neutral pHs via thiol-disulfide exchange reactions, and these reactions can be blocked by covalent modification of the thiol involved. In this study, the effects of the reducing agent 2-mercaptoethanol on Sindbis virus-mediated cell-cell fusion from without and the effects of the membrane-impermeable thiol-alkylating reagent 5,5'-dithiobis(2-nitrobenzoic acid) on Sindbis virus penetration were determined. The presence of exogenous reducing agent was found to induce fusion from without under conditions unfavorable to both typical Sindbis virus-mediated fusion from without and cysteine-mediated thiol-disulfide exchange reactions. In addition, the thiol-alkylating reagent was found to inhibit Sindbis virus entry when present during infection. These results are consistent with a model for Sindbis virus entry in which reduction of critical disulfide bridges at the cell surface disrupts the rigid protein-protein associations of the envelope, allowing membrane fusion and release of the viral genome into the cell.     

CD4 binding site antibodies inhibit human immunodeficiency virus gp120 envelope glycoprotein interaction with CCR5

The human immunodeficiency virus type 1 (HIV-1) gp120 exterior glycoprotein is conformationally flexible. Upon binding the host cell receptor, CD4, gp120 assumes a conformation that is able to bind the chemokine receptors CCR5 or CXCR4, which act as coreceptors for the virus. CD4-binding-site (CD4BS) antibodies are neutralizing antibodies elicited during natural infection that are directed against gp120 epitopes that overlap the binding site for CD4. Recent studies (S. H. Xiang et al., J. Virol. 76:9888-9899, 2002) suggest that CD4BS antibodies recognize conformations of gp120 distinct from the CD4-bound conformation. This predicts that the binding of CD4BS antibodies will inhibit chemokine receptor binding. Here, we show that Fab fragments and complete immunoglobulin molecules of CD4BS antibodies inhibit CD4-independent gp120 binding to CCR5 and cell-cell fusion mediated by CD4-independent HIV-1 envelope glycoproteins. These results are consistent with a model in which the binding of CD4BS antibodies limits the ability of gp120 to assume a conformation required for coreceptor binding.     

Enhanced dynamics of HIV gp120 glycoprotein by small molecule binding

HIV cell entry and infection are driven by binding events to the CD4 and chemokine receptors with associated conformational change of the viral glycoprotein, gp120. Scyllatoxin miniprotein CD4 mimetics and a small molecule inhibitor of CD4 binding, NBD-556, also effectively induce gp120 conformational change. In this study we examine the fluctuation profile of gp120 in context of CD4, a miniprotein mimetic, and NBD-556 with the aim of understanding the effect of ligand binding on gp120 conformational dynamics. Analysis of molecular dynamics trajectories indicate that NBD-556 binding in the Phe 43 cavity enhances the overall mobility of gp120, especially in the outer domain in comparison to CD4 or miniprotein bound complex. Interactions with the more flexible bridging sheet strengthen upon NBD-556 binding and may contribute to gp120 restructuring. The enhanced mobility of D368, E370, and I371 with NBD-556 bound in the Phe 43 cavity suggests that interactions with α3-helix in the outer domain are not optimal, providing further insights into gp120--small molecule interactions that may impact small molecule designs.     

Localization of the intrachain disulfide bonds of the envelope glycoprotein 71 from Friend murine leukemia virus.

Envelope glycoprotein 71 from Friend murine leukemia virus was purified to homogeneity by reversed-phase HPLC. It could be shown that all 20 cysteine residues of the molecule are linked by disulfide bonds. After complete tryptic digestion, peptides containing cystine were identified by comparison of the reversed-phase HPLC profile of the digest with that of a reduced aliquot which had been subjected to affinity chromatography on thiol-Sepharose. The locations of the 10 disulfide bonds were determined by isolation, further digestion and analysis of peptides containing cystine. The first cysteine residue of the sequence (Cys46) was shown to be coupled to the sixth (Cys98), leading to a large loop containing four additional cysteine residues. Computer model building and energy calculations led to the assignment of Cys72 to Cys87 and Cys73 to Cys83. The following four cysteine residues of the sequence also constitute a structural unit, with Cys121 bonded to Cys141 and Cys133 to Cys146, and the last two cysteine residues in the amino-terminal domain of glycoprotein 71 form a small loop (Cys178 to Cys184). The first two cysteine residues of the carboxy-terminal domain produce a very small hydrophobic loop (Cys312-Cys315). Cys361 is bound to Cys373, Cys342 to Cys396 and Cys403 to Cys416. A model for the folding pattern of the viral glycoprotein is proposed.     

Inhibitors of protein-disulfide isomerase prevent cleavage of disulfide bonds in receptor-bound glycoprotein 120 and prevent HIV-1 entry

We previously reported that monoclonal antibodies to protein-disulfide isomerase (PDI) and other membrane-impermeant PDI inhibitors prevented HIV-1 infection. PDI is present at the surface of HIV-1 target cells and reduces disulfide bonds in a model peptide attached to the cell membrane. Here we show that soluble PDI cleaves disulfide bonds in recombinant envelope glycoprotein gp120 and that gp120 bound to the surface receptor CD4 undergoes a disulfide reduction that is prevented by PDI inhibitors. Concentrations of inhibitors that prevent this reduction and inhibit the cleavage of surface-bound disulfide conjugate prevent infection at the level of HIV-1 entry. The entry of HIV-1 strains differing in their coreceptor specificities is similarly inhibited, and so is the reduction of gp120 bound to CD4 of coreceptor-negative cells. PDI inhibitors also prevent HIV envelope-mediated cell-cell fusion but have no effect on the entry of HIV-1 pseudo-typed with murine leukemia virus envelope. Importantly, PDI coprecipitates with both soluble and cellular CD4. We propose that a PDI.CD4 association at the cell surface enables PDI to reach CD4-bound virus and to reduce disulfide bonds present in the domain of gp120 that binds to CD4. Conformational changes resulting from the opening of gp120-disulfide loops may drive the processes of virus-cell and cell-cell fusion. The biochemical events described identify new potential targets for anti-HIV agents.     

Fusion proteins of HIV-1 envelope glycoprotein gp120 with CD4-induced antibodies showed enhanced binding to CD4 and CD4 binding site antibodies

Development of successful AIDS vaccine immunogens continues to be a major challenge. One of the mechanisms by which HIV-1 evades antibody-mediated neutralizing responses is the remarkable conformational flexibility of its envelope glycoprotein (Env) gp120. Some recombinant gp120s do not preserve their conformations on gp140s and functional viral spikes, and exhibit decreased recognition by CD4 and neutralizing antibodies. CD4 binding induces conformational changes in gp120 leading to exposure of the coreceptor-binding site (CoRbs). In this study, we test our hypothesis that CD4-induced (CD4i) antibodies, which target the CoRbs, could also induce conformational changes in gp120 leading to better exposed conserved neutralizing antibody epitopes including the CD4-binding site (CD4bs). We found that a mixture of CD4i antibodies with gp120 only weakly enhanced CD4 binding. However, such interactions in single-chain fusion proteins resulted in gp120 conformations which bound to CD4 and CD4bs antibodies better than the original or mutagenically stabilized gp120s. Moreover, the two molecules in the fusion proteins synergized with each other in neutralizing HIV-1. Therefore, fusion proteins of gp120 with CD4i antibodies could have potential as components of HIV-1 vaccines and inhibitors of HIV-1 entry, and could be used as reagents to explore the conformational flexibility of gp120 and mechanisms of entry and immune evasion.     

The stylar 120 kDa glycoprotein is required for S-specific pollen rejection in Nicotiana

S-RNase participates in at least three mechanisms of pollen rejection. It functions in S-specific pollen rejection (self-incompatibility) and in at least two distinct interspecific mechanisms of pollen rejection in Nicotiana. S-specific pollen rejection and rejection of pollen from Nicotiana plumbaginifolia also require additional stylar proteins. Transmitting-tract-specific (TTS) protein, 120 kDa glycoprotein (120K) and pistil extensin-like protein III (PELP III) are stylar glycoproteins that bind S-RNase in vitro and are also known to interact with pollen. Here we tested whether these glycoproteins have a direct role in pollen rejection. 120K shows the most polymorphism in size between Nicotiana species. Larger 120K-like proteins are often correlated with S-specific pollen rejection. Sequencing results suggest that the polymorphism primarily reflects differences in glycosylation, although indels also occur in the predicted polypeptides. Using RNA interference (RNAi), we suppressed expression of 120K to determine if it is required for S-specific pollen rejection. Transgenic SC N. plumbaginifolia x SI Nicotiana alata (S105S105 or SC10SC10) hybrids with no detectable 120K were unable to perform S-specific pollen rejection. Thus, 120K has a direct role in S-specific pollen rejection. However, suppression of 120K had no effect on rejection of N. plumbaginifolia pollen. In contrast, suppression of HT-B, a factor previously implicated in S-specific pollen rejection, disrupts rejection of N. plumbaginifolia pollen. Thus, S-specific pollen rejection and rejection of N. plumbaginifolia pollen are mechanistically distinct, because they require different non-S-RNase factors.     

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.     

Oligomerization and interacellular localization of the glycoprotein receptor ERGIC-53 is independent of disulfide bonds

ERGIC-53 is a type I transmembrane lectin facilitating the efficient export of a subset of secretory glycoproteins from the endoplasmic reticulum. Previous results have shown that ERGIC-53 is present as reduction-sensitive homo-oligomers, i.e. as a balanced mixture of disulfide-linked hexamers and dimers, with the two cysteine residues located close to the transmembrane domain playing a crucial role in oligomerization. Here, we demonstrate, using sucrose gradient sedimentation, cross-linking analyses, and non-denaturing gel electrophoresis, that ERGIC-53 is present exclusively as a hexameric complex in cells. However, the hexamers exist in two forms, one as a disulfide-linked, Triton X-100, perfluoro-octanic acid, and SDS-resistant complex, and the other as a non-covalent, Triton X-100, perfluoro-octanoic acid-resistant, but SDS-sensitive, complex made up of three disulfide-linked dimers that are likely to interact through the coiled-coil domains present in the luminal part of the protein. In contrast to what was previously believed, neither of the membrane-proximal cysteine residues plays an essential role in the formation, or maintenance, of the latter form of hexamers. Subcellular fractionation revealed that the double-cysteine mutant was present in the endoplasmic reticulum-Golgi-intermediate compartment, indicating that the two cysteine residues are not essential for the intracellular distribution of ERGIC-53. Based on these results, we present a model for the formation of the two hexameric forms.     

A cellular function is required for pseudorabies virus envelope glycoprotein processing and virus egress.

The mouse L-cell mutant gro29 is defective for egress of herpes simplex virus type 1 (HSV-1) virions and is significantly reduced in HSV-1 glycoprotein export (B. W. Banfield and F. Tufaro, J. Virol. 64:5716-5729, 1990). In this report, we demonstrate that pseudorabies virus (PRV), a distantly related alphaherpesvirus, shows a distinctive set of defects after infection of gro29 cells. Specifically, we identify defects in the rate and extent of viral glycoprotein export, infectious particle formation, plaque formation, and virus egress. The initial rate of viral glycoprotein synthesis was unaffected in gro29 cells, but the extent of export from the endoplasmic reticulum to the Golgi apparatus was impaired and export through the Golgi apparatus became essentially blocked late in infection. Moreover, by using a secreted variant of a viral membrane protein, we found that export from the Golgi apparatus out of the cell was also defective in gro29 cells. PRV does not form plaques on gro29 monolayers. A low level of infectious virus is formed and released early after infection, but further virus egress is blocked. Taken together, these observations suggest that the gro29 phenotype involves either multiple proteins or a single protein used at multiple steps in viral glycoprotein export and virus egress from cells. Moreover, this host cell protein is required by both HSV and PRV for efficient propagation in infected cells.     

Calcium ions are required for cell fusion mediated by the CD4-human immunodeficiency virus type 1 envelope glycoprotein interaction.

Calcium ions are required for fusion of a wide variety of artificial and biological membranes. To examine the role of calcium ions for cell fusion mediated by interactions between CD4 and the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (gp120-gp41), we used two experimental systems: (i) cells expressing gp120-gp41 and its receptor CD4, both encoded by recombinant vaccinia viruses, and (ii) chronically infected cells producing low levels of HIV-1. Fusion was measured by counting the number of syncytia and by monitoring the redistribution of fluorescence dyes by video microscopy. Syncytia did not form in solutions without calcium ions. Addition of calcium ions partially restored the formation of syncytia. EDTA and EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] blocked syncytium formation in culture media containing calcium ions. Membrane fusion as monitored by fluorescence dye redistribution also required calcium ions. Cell fusion increased with an increase in calcium ion concentration from 100 microM to 10 mM but was not affected by magnesium ions in the concentration range from 0 to 30 mM. Fibrinogen and fibronectin did not promote fusion in the absence or presence of Ca2+. Binding of soluble CD4 to gp120-gp41-expressing cells was not affected by Ca2+ and Mg2+. We conclude that Ca2+ is involved in postbinding steps in cell fusion mediated by the CD4-HIV-1 envelope glycoprotein interaction.     

GTP hydrolysis is required for vesicle fusion during nuclear envelope assembly in vitro

Nuclear envelope assembly was studied in vitro using extracts from Xenopus eggs. Nuclear-specific vesicles bound to demembranated sperm chromatin but did not fuse in the absence of cytosol. Addition of cytosol stimulated vesicle fusion, pore complex assembly, and eventual nuclear envelope growth. Vesicle binding and fusion were assayed by light and electron microscopy. Addition of ATP and GTP to bound vesicles caused limited vesicle fusion, but enclosure of the chromatin was not observed. This result suggested that nondialyzable soluble components were required for nuclear vesicle fusion. GTP gamma S and guanylyl imidodiphosphate significantly inhibited vesicle fusion but had no effect on vesicle binding to chromatin. Preincubation of membranes with 1 mM GTP gamma S or GTP did not impair vesicle binding or fusion when assayed with fresh cytosol. However, preincubation of membranes with GTP gamma S plus cytosol caused irreversible inhibition of fusion. The soluble factor mediating the inhibition by GTP gamma S, which we named GTP-dependent soluble factor (GSF), was titratable and was depleted from cytosol by incubation with excess membranes plus GTP gamma S, suggesting a stoichiometric interaction between GSF and a membrane component in the presence of GTP gamma S. In preliminary experiments, cytosol depleted of GSF remained active for fusion of chromatin-bound vesicles, suggesting that GSF may not be required for the fusion reaction itself. We propose that GTP hydrolysis is required at a step before the fusion of nuclear vesicles.     

Structural features of CD4 required for binding to HIV

A soluble form of the human CD4 glycoprotein (sCD4), the cellular receptor for human HIV, was treated with various physical, chemical, and enzymic regimens and tested over a range of concentrations for its capacity to inhibit the binding of HIV to CD4+ T cells. Reduction of disulfide bonds and alkylation in denaturing buffer (8 M urea) destroyed the inhibitory activity of sCD4, whereas reduction and alkylation in PBS had no effect. Derivatization or digestion of carbohydrate groups by periodate oxidation or by glycolytic enzyme digestion did not affect sCD4 inhibitory capacity. Digestion with trypsin or endoproteinase Glu-C destroyed activity. A limited digestion of sCD4 with endoproteinase Glu-C resulted in a mixture of fragments, however, and the mixture had inhibitory activity equivalent to that of intact sCD4. Within this mixture, a fragment of 23 kDa was identified that binds to HIV. Although sCD4 can be digested to yield fully active fragments, the requirement for intrachain disulfide bonding indicates that the minimum sized portion of CD4 that will retain full affinity for HIV will have to be formulated with a proper tertiary structure.     

Thiol/disulfide exchange is required for membrane fusion directed by the Newcastle disease virus fusion protein

Newcastle disease virus (NDV), an avian paramyxovirus, initiates infection with attachment of the viral hemagglutinin-neuraminidase (HN) protein to sialic acid-containing receptors, followed by fusion of viral and cell membranes, which is mediated by the fusion (F) protein. Like all class 1 viral fusion proteins, the paramyxovirus F protein is thought to undergo dramatic conformational changes upon activation. How the F protein accomplishes extensive conformational rearrangements is unclear. Since several viral fusion proteins undergo disulfide bond rearrangement during entry, we asked if similar rearrangements occur in NDV proteins during entry. We found that inhibitors of cell surface thiol/disulfide isomerase activity--5'5-dithio-bis(2-nitrobenzoic acid) (DTNB), bacitracin, and anti-protein disulfide isomerase antibody--inhibited cell-cell fusion and virus entry but had no effect on cell viability, glycoprotein surface expression, or HN protein attachment or neuraminidase activities. These inhibitors altered the conformation of surface-expressed F protein, as detected by conformation-sensitive antibodies. Using biotin maleimide (MPB), a reagent that binds to free thiols, free thiols were detected on surface-expressed F protein, but not HN protein. The inhibitors DTNB and bacitracin blocked the detection of these free thiols. Furthermore, MPB binding inhibited cell-cell fusion. Taken together, our results suggest that one or several disulfide bonds in cell surface F protein are reduced by the protein disulfide isomerase family of isomerases and that F protein exists as a mixture of oxidized and reduced forms. In the presence of HN protein, only the reduced form may proceed to refold into additional intermediates, leading to the fusion of membranes.     

Thioredoxin-1 and protein disulfide isomerase catalyze the reduction of similar disulfides in HIV gp120

HIV-1 enters cells via interaction of the viral glycoprotein gp120, the host cell surface receptor CD4 and the co-receptors CCR5 or CXCR4. For entry, gp120 undergoes conformational changes that depend on the reduction of one or more disulfides. Previous studies indicate that protein disulfide isomerase (PDI), thioredoxin-1 (Trx1), and glutaredoxin-1 (Grx1) catalyze gp120 reduction, but their specific disulfide targets are not known. Here, it was demonstrated that PDI and Trx1 have similar gp120 disulfide targets as determined by labeling after reduction, but with some pattern differences, including overall stronger labeling with Trx1 than with PDI. Furthermore, uneven labeling of the residues of a disulfide may reflect altered accessibility by conformational changes upon the reduction process. Since both PDI and Trx1 may be involved in viral entry, compounds that target the host redox system or the viral gp120 were tested in vitro to investigate whether redox regulation is a target for anti-HIV therapy. Carbohydrate binding agents (CBAs), previously shown to bind gp120 and inhibit HIV entry, were now demonstrated to inhibit gp120 disulfide reduction. Auranofin, an inhibitor of thioredoxin reductase 1 (TrxR1), also showed inhibitory activity towards HIV infection, although close to its cytotoxic concentration. Our results demonstrate that both the host redox system and the viral surface glycoproteins are of interest for the development of new generations of anti-HIV therapeutics.     

Binding of soluble recombinant HIV envelope glycoprotein, rgp120, induces conformational changes in the cellular membrane-anchored CD4 molecule

During HIV entry or resulting cell to cell fusion, the envelope glycoprotein gp120 binds first to the CD4 membrane distal domain and second to a chemokine receptor as coreceptor. Taking into consideration the relative length of these two molecules' extracellular parts, structural modulations of CD4 would be required to make the second interaction possible. In this work, we assessed the effect of gp120 binding on the conformation of CD4 expressed on cell surface. We demonstrated that following gp120 binding the avidity of some, but not all, monoclonal antibodies specific to epitopes, outside of the gp120-binding site, in D1, D3 and D4 domains of CD4 was decreased dramatically. This finding demonstrates that the gp120-CD4 interaction induces local and specific conformational changes of CD4 and constitutes functional evidence for hinge regions that could confer to this molecule the flexibility required for its various functions.     

The naive repertoire of human T helper cells specific for gp120, the envelope glycoprotein of HIV.

The envelope glycoprotein of HIV gp120 is a T cell Ag in experimental animals and in humans infected with HIV or deliberately immunized with gp120 in various forms. Inasmuch as T cell responses result from the interaction of Ag processed and presented by APC with the unprimed T cell repertoire, we have investigated the human T cell repertoire specific for gp120 in seronegative, normal individuals. T cell lines and clones specific for HIV gp120 were generated by repeated in vitro stimulation of peripheral blood T lymphocytes with gp120-pulsed APC, followed by IL-2 expansion. We observed that the T cell response to whole gp120 involved single restricted immunodominant epitopes in gp120 that differ between responding individuals. Focusing of the response to limited regions of gp120 when the whole Ag is used for priming suggests that one or more adjacent epitopes are immunodominant and mask responses to "immunorecessive" epitopes. We have been able to generate primary in vitro responses to recessive epitopes by stimulation in vitro with synthetic peptides of gp120. The results indicate that a much broader T repertoire can be detected when individual peptides are used for priming in vitro rather than gp120. This information has important implications for the development of vaccination protocols aimed at eliciting diverse immune responses to "immunorecessive" regions of envelope glycoprotein.     

N-linked glycosylation of the HIV type-1 gp120 envelope glycoprotein as a major determinant of CCR5 and CXCR4 coreceptor utilization

The variable V1V2 and V3 regions of the human immunodeficiency virus type-1 (HIV-1) envelope glycoprotein (gp120) can influence viral coreceptor usage. To substantiate this we generated isogenic HIV-1 molecularly cloned viruses that were composed of the HxB2 envelope backbone containing the V1V2 and V3 regions from viruses isolated from a patient progressing to disease. We show that the V3 amino acid charge per se had little influence on altering the virus coreceptor phenotype. The V1V2 region and its N-linked glycosylation degree were shown to confer CXCR4 usage and provide the virus with rapid replication kinetics. Loss of an N-linked glycosylation site within the V3 region had a major influence on the virus switching from the R5 to X4 phenotype in a V3 charge-dependent manner. The loss of this V3 N-linked glycosylation site was also linked with the broadening of the coreceptor repertoire to incorporate CCR3. By comparing the amino acid sequences of primary HIV-1 isolates, we identified a strong association between high V3 charge and the loss of this V3 N-linked glycosylation site. These results demonstrate that the N-linked glycosylation pattern of the HIV-1 envelope can strongly influence viral coreceptor utilization and the R5 to X4 switch.