The N-linked glycan of the V3 region of HIV-1 gp120 and CXCR4-dependent multiplication of a human immunodeficiency virus type 1 lymphocyte-tropic variant

Different HIV-1 env frames: gp120 and ASP (antisense protein) biosynthesis, and theirs co-variation tropic amino acid signatures in X4- and R5-viruses

Antisense protein (ASP) is the new actor of viral life of Human Immunodeficiency Virus type 1 (HIV-1) although proposed above 20 years ago. The asp ORF is into complementary strand of the gp120/gp41 junction of env gene. The ASP biological role remains little known. Knowing the Env markers of viral tropism, a dataset of sequences (660 strains) was used to analyze the hypothetical ASP involvement in CCR5 (R5) and/or CXCR4 (X4) co-receptor interaction. Preliminarily, prevalence of ASP and gp120_V3 mutations was performed; following association among mutations were elaborate. The classical V3 tropic-signatures were confirmed, and 36 R5- and 22 X4-tropic ASP mutations were found. Moreover, by analyzing the ASP sequences, 36 out of 179 amino acid positions significantly associated with different co-receptor usage were found. Several statistically significant associations between gp120_V3 and ASP mutations were observed. The dendrogram showed the existence of a cluster associated with R5-usage and a large cluster associated with X4-usage. These results show that gp120_V3 and specific amino acid changes in ASP are associated together with CXCR4 and/or CCR5-usage. These findings implement previous observations on unclear ASP functions. J. Med. Virol. 89:112-122, 2017. © 2016 Wiley Periodicals, Inc.

Mutation of a Single Envelope N-Linked Glycosylation Site Enhances the Pathogenicity of Bovine Leukemia Virus

Viruses have coevolved with their host to ensure efficient replication and transmission without inducing excessive pathogenicity that would indirectly impair their persistence. This is exemplified by the bovine leukemia virus (BLV) system in which lymphoproliferative disorders develop in ruminants after latency periods of several years. In principle, the equilibrium reached between the virus and its host could be disrupted by emergence of more pathogenic strains. Intriguingly but fortunately, such a hyperpathogenic BLV strain was never observed in the field or designed in vitro. In this study, we sought to understand the role of envelope N-linked glycosylation with the hypothesis that this posttranslational modification could either favor BLV infection by allowing viral entry or allow immune escape by using glycans as a shield. Using reverse genetics of an infectious molecular provirus, we identified a N-linked envelope glycosylation site (N230) that limits viral replication and pathogenicity. Indeed, mutation N230E unexpectedly leads to enhanced fusogenicity and protein stability.

IMPORTANCE

Infection by retroviruses requires the interaction of the viral envelope protein (SU) with a membrane-associated receptor allowing fusion and release of the viral genomic RNA into the cell. We show that N-linked glycosylation of the bovine leukemia virus (BLV) SU protein is, as expected, essential for cell infection in vitro. Consistently, mutation of all glycosylation sites of a BLV provirus destroys infectivity in vivo. However, single mutations do not significantly modify replication in vivo. Instead, a particular mutation at SU codon 230 increases replication and accelerates pathogenesis. This unexpected observation has important consequences in terms of disease control and managing.

The influence of N-linked glycans on the molecular dynamics of the HIV-1 gp120 V3 loop

N-linked glycans attached to specific amino acids of the gp120 envelope trimer of a HIV virion can modulate the binding affinity of gp120 to CD4, influence coreceptor tropism, and play an important role in neutralising antibody responses. Because of the challenges associated with crystallising fully glycosylated proteins, most structural investigations have focused on describing the features of a non-glycosylated HIV-1 gp120 protein. Here, we use a computational approach to determine the influence of N-linked glycans on the dynamics of the HIV-1 gp120 protein and, in particular, the V3 loop. We compare the conformational dynamics of a non-glycosylated gp120 structure to that of two glycosylated gp120 structures, one with a single, and a second with five, covalently linked high-mannose glycans. Our findings provide a clear illustration of the significant effect that N-linked glycosylation has on the temporal and spatial properties of the underlying protein structure. We find that glycans surrounding the V3 loop modulate its dynamics, conferring to the loop a marked propensity towards a more narrow conformation relative to its non-glycosylated counterpart. The conformational effect on the V3 loop provides further support for the suggestion that N-linked glycosylation plays a role in determining HIV-1 coreceptor tropism.

The comparison of genetic variation in the envelope protein between various immunodeficiency viruses and equine infectious anemia virus

The envelope protein (Env) of lentiviruses such as HIV, SIV, FIV and EIAV is larger than that of other retroviruses. The Chinese EIAV attenuated vaccine is based on Env and has helped to successfully control this virus, demonstrating that envelope is crucial for vaccine. We compared Env variation of the four kinds of lentiviruses. Phylogenetic analysis showed that the evolutionary relationship of Env between HIV and SIV was the closest and they appeared to descend from a common ancestor, and the relationship of HIV and EIAV was the furthest. EIAV had the shortest Env length and the least number of potential N-linked glycosylation sites (PNGS) as well as glycosylation density compared to various immunodeficiency viruses. However, HIV had the longest Env length and the most PNGS. Moreover, the alignment of HIV and SIV showed that PNGS were primarily distributed within extracellular membrane protein gp120 rather than transmembrane gp41. It implies that the size difference among these viruses is associated with a lentivirus specific function and also the diversity of env. There are low levels of modification of glycosylation sites of Env and selection of optimal protective epitopes might be useful for development of an effective vaccine against HIV/AIDS.

Selected amino acid changes in HIV-1 subtype-C gp41 are associated with specific gp120(V3) signatures in the regulation of co-receptor usage

The majority of studies have characterized the tropism of HIV-1 subtype-B isolates, but little is known about the determinants of tropism in other subtypes. So, the goal of the present study was to genetically characterize the envelope of viral proteins in terms of co-receptor usage by analyzing 356 full-length env sequences derived from HIV-1 subtype-C infected individuals. The co-receptor usage of V3 sequences was inferred by using the Geno2Pheno and PSSM algorithms, and also analyzed to the "11/25 rule". All reported env sequences were also analyzed with regard to N-linked glycosylation sites, net charge and hydrophilicity, as well as the binomial correlation phi coefficient to assess covariation among gp120(V3) and gp41 signatures and the average linkage hierarchical agglomerative clustering were also performed. Among env sequences present in Los Alamos Database, 255 and 101 sequences predicted as CCR5 and CXCR4 were selected, respectively. The classical V3 signatures at positions 11 and 25, and other specific V3 and gp41 amino acid changes were found statistically associated with different co-receptor usage. Furthermore, several statistically significant associations between V3 and gp41 signatures were also observed. The dendrogram topology showed a cluster associated with CCR5-usage composed by five gp41 mutated positions, A22V, R133M, E136G, N140L, and N166Q that clustered with T2V(V3) and G24T(V3) (bootstrap=1). Conversely, a heterogeneous cluster with CXCR4-usage, involving S11GR(V3), 13-14insIG/LG(V3), P16RQ(V3), Q18KR(V3), F20ILV(V3), D25KRQ(V3), Q32KR(V3) along with A30T(gp41), S107N(gp41), D148E(gp41), A189S(gp41) was identified (bootstrap=0.86). Our results show that as observed for HIV-1 subtype-B, also in subtype-C specific and different gp41 and gp120V3 amino acid changes are associated individually or together with CXCR4 and/or CCR5 usage. These findings strengthen previous observations that determinants of tropism may also reside in the gp41 protein.

Partial enzymatic deglycosylation preserves the structure of cleaved recombinant HIV-1 envelope glycoprotein trimers

The trimeric envelope glycoprotein complex (Env) is the focus of vaccine development programs aimed at generating protective humoral responses to human immunodeficiency virus type 1 (HIV-1). N-Linked glycans, which constitute almost half of the molecular mass of the external Env domains, produce considerable structural heterogeneity and are a major impediment to crystallization studies. Moreover, by shielding the peptide backbone, glycans can block attempts to generate neutralizing antibodies against a substantial subset of potential epitopes when Env proteins are used as immunogens. Here, we describe the partial deglycosylation of soluble, cleaved recombinant Env trimers by inhibition of the synthesis of complex N-glycans during Env production, followed by treatment with glycosidases under conditions that preserve Env trimer integrity. The partially deglycosylated trimers are stable, and neither abnormally sensitive to proteolytic digestion nor prone to aggregation. Moreover, the deglycosylated trimers retain or increase their ability to bind CD4 and antibodies that are directed to conformational epitopes, including the CD4-binding site and the V3 region. However, as expected, they do not react with glycan-dependent antibodies 2G12 and PGT123, or the C-type lectin receptor DC-SIGN. Electron microscopic analysis shows that partially deglycosylated trimers have a structure similar to fully glycosylated trimers, indicating that removal of glycans does not substantially perturb the structural integrity of the trimer. The glycan-depleted Env trimers should be useful for structural and immunogenicity studies.

N-linked glycosylation of Gn (but not Gc) is important for Crimean Congo hemorrhagic fever virus glycoprotein localization and transport

The mature Gn glycoprotein of Crimean Congo hemorrhagic fever (CCHF) virus contains two predicted glycosylation sites (557N and 755N). Of these, N-glycans are added only at 557N, as evidenced by abrogation of Gn-glycosylation by mutation of 557N but not 755N site. Mutational block of Gn-glycosylation at 557N did not significantly affect Gn proteolytic processing but did result in mislocalization and retention of Gn and other proteins synthesized from the virus M segment ORF (GP160, GP85, GP38 and Gc) in the endoplasmic reticulum. In contrast to Gn, similar mutational analysis demonstrated that, while N-glycosylation occurs at the two predicted sites in Gc, abrogation of their glycosylation did not alter localization of any of the CCHF virus glycoproteins. Studies of Gn expressed in the absence of Gc demonstrate that, while Gn processing and localization are independent of Gc, all the CCHF virus glycoproteins appear dependent on N-glycosylation of Gn for correct folding, localization and transport.

CCR5 use by human immunodeficiency virus type 1 is associated closely with the gp120 V3 loop N-linked glycosylation site

Human immunodeficiency virus type 1 (HIV-1) enters cells through the chemokine receptors CCR5 (R5 virus) and/or CXCR4 (X4 virus). Loss of N-linked glycans and increased net charge of the third variable loop (V3) of the gp120 envelope glycoprotein have been observed to be important steps towards CXCR4 use. All reported sequences using CCR5 or CXCR4 exclusively, or using both, were gathered from the Los Alamos HIV Database and analysed with regard to the V3 N-linked glycosylation motifs (sequons) and charge. The V3 loop glycan had a sensitivity of 0.98 and a 0.92 positive predictive value in the context of CCR5 use. The difference from X4 was remarkable (P<10(-12)). Especially, the sequon motif NNT within the V3 loop was conserved in 99.2 % of the major clades. The results suggest a close association between the V3 loop glycan and CCR5 use and may provide new insight into HIV-1 tropism and help to improve phenotype-prediction models.

N-linked glycosylation of the V3 loop and the immunologically silent face of gp120 protects human immunodeficiency virus type 1 SF162 from neutralization by anti-gp120 and anti-gp41 antibodies

We examined how asparagine-linked glycans within and adjacent to the V3 loop (C2 and C3 regions) and within the immunologically silent face (V4, C4, and V5 regions) of the human immunodeficiency virus (HIV) SF612 envelope affect the viral phenotype. Five of seven potential glycosylation sites are utilized when the virus is grown in human peripheral blood mononuclear cells, with the nonutilized sites lying within the V4 loop. Elimination of glycans within and adjacent to the V3 loop renders SF162 more susceptible to neutralization by polyclonal HIV(+)-positive and simian/human immunodeficiency virus-positive sera and by monoclonal antibodies (MAbs) recognizing the V3 loop, the CD4- and CCR5-binding sites, and the extracellular region of gp41. Importantly, our studies also indicate that glycans located within the immunologically silent face of gp120, specifically the C4 and V5 regions, also conferred on SF162 resistance to neutralization by anti-V3 loop, anti-CD4 binding site, and anti-gp41 MAbs but not by antibodies targeting the coreceptor binding site. We also observed that the amino acid composition of the V4 region contributes to the neutralization phenotype of SF162 by anti-V3 loop and anti-CD4 binding site MAbs. Collectively, our data support the proposal that the glycosylation and structure of the immunologically silent face of the HIV envelope plays an important role in defining the neutralization phenotype of HIV type 1.

Structure-based, targeted deglycosylation of HIV-1 gp120 and effects on neutralization sensitivity and antibody recognition

The human immunodeficiency virus (HIV-1) exterior envelope glycoprotein, gp120, mediates receptor binding and is the major target for neutralizing antibodies. Primary HIV-1 isolates are characteristically more resistant to broadly neutralizing antibodies, although the structural basis for this resistance remains obscure. Most broadly neutralizing antibodies are directed against functionally conserved gp120 regions involved in binding to either the primary virus receptor, CD4, or the viral coreceptor molecules that normally function as chemokine receptors. These antibodies are known as CD4 binding site (CD4BS) and CD4-induced (CD4i) antibodies, respectively. Inspection of the gp120 crystal structure reveals that although the receptor-binding regions lack glycosylation, sugar moieties lie proximal to both receptor-binding sites on gp120 and thus in proximity to both the CD4BS and the CD4i epitopes. In this study, guided by the X-ray crystal structure of gp120, we deleted four N-linked glycosylation sites that flank the receptor-binding regions. We examined the effects of selected changes on the sensitivity of two prototypic HIV-1 primary isolates to neutralization by antibodies. Surprisingly, removal of a single N-linked glycosylation site at the base of the gp120 third variable region (V3 loop) increased the sensitivity of the primary viruses to neutralization by CD4BS antibodies. Envelope glycoprotein oligomers on the cell surface derived from the V3 glycan-deficient virus were better recognized by a CD4BS antibody and a V3 loop antibody than were the wild-type glycoproteins. Absence of all four glycosylation sites rendered a primary isolate sensitive to CD4i antibody-mediated neutralization. Thus, carbohydrates that flank receptor-binding regions on gp120 protect primary HIV-1 isolates from antibody-mediated neutralization.

Molecular characteristics of human immunodeficiency virus type 1 subtype C viruses from KwaZulu-Natal, South Africa: implications for vaccine and antiretroviral control strategies

The KwaZulu-Natal region of South Africa is experiencing an explosive outbreak of human immunodeficiency virus type 1 (HIV-1) subtype C infections. Understanding the genetic diversity of C viruses and the biological consequences of this diversity is important for the design of effective control strategies. We analyzed the protease gene, the first 935 nucleotides of reverse transcriptase, and the C2V5 envelope region of a representative set of 72 treatment-naïve patients from KwaZulu-Natal and correlated the results with amino acid signature and resistance patterns. Phylogenetic analysis revealed multiple clusters or "lineages" of HIV-1 subtype C that segregated with other C viruses from southern Africa. The same pattern was observed for both black and Indian subgroups and for retrospective specimens collected prior to 1990, indicating that multiple sublineages of HIV-1 C have been present in KwaZulu-Natal since the early stages of the epidemic. With the exception of three nonnucleoside reverse transcriptase inhibitor mutations, no primary resistance mutations were identified. Numerous accessory polymorphisms were present in the protease, but none were located at drug-binding or active sites of the enzyme. One frequent polymorphism, I93L, was located near the protease/reverse transcriptase cleavage site. In the envelope, disruption of the glycosylation motif at the beginning of V3 was associated with the presence of an extra protein kinase C phosphorylation site at codon 11. Many polymorphisms were embedded within cytotoxic T lymphocyte or overlapping cytotoxic T-lymphocyte/T-helper epitopes, as defined for subtype B. This work forms a baseline for future studies aimed at understanding the impact of genetic diversity on vaccine efficacy and on natural susceptibility to antiretroviral drugs.

Infection of cells expressing CXCR4 mutants lacking N-glycosylation at the N-terminal extracellular domain is enhanced for R5X4-dualtropic human immunodeficiency virus type-1

BACKGROUND

Infection with human immunodeficiency virus type-1 (HIV-1) requires binding of the viral envelope gp120 to CD4 and to the CXCR4 coreceptor. Both, gp120 and CXCR4 are subject to N-glycosylation. Here we investigated the influence of the N-linked glycans g1 and g2 present on CXCR4 for HIV-1 infection.

METHODS

The two CXCR4 N-glycosylation sites g1 (NYT) and g2 (NVS) were mutated by changing the first or third amino acids N or T/S to Q and A respectively (g1; N11Q or T13A; g2, N176Q or S178A). Human osteosarcoma cells (GHOST) expressing human CD4 and the various CXCR4 glycosylation mutants were tested for infection using NL4-3-based viruses with X4, R5 or R5X4-tropism differing only in the V3 loop region.

RESULTS

All constructed cell lines expressing the various CXCR4 glycomutants showed similar permissiveness for the X4-monotropic virus and no change in the coreceptor specificity that allows infection of a CCR5-dependent R5-monotropic virus. Interestingly, the removal of glycan g1 significantly enhanced the permissiveness of GHOST cells for the R5X4 dualtropic virus. GHOST cells expressing the CXCR4-g1 or CXCR4-g1/2 mutants were infected at higher rates by the R5X4-dualtropic virus compared to cells expressing CXCR4-wt or CXCR4-g2 coreceptors.

CONCLUSION

Our present observations underscore a role for glycans present on the CXCR4 coreceptor in the entry process of HIV-1. The data will help to better understand the multifaceted mechanism of HIV infection and the selective forces which drive HIV-1 evolution from mono- to dual-tropism.

The N-linked glycan g15 within the V3 loop of the HIV-1 external glycoprotein gp120 affects coreceptor usage, cellular tropism, and neutralization

We have studied infectivity and neutralization of X4, R5, and R5X4 tropic HIV-1 mutants, which are lacking N-linked glycosylation sites for glycans g13, g14, g15, and g17 in the V3 loop region of gp120. X4-tropic NL4-3 mutants lacking combinations of g14/15 or g15/17 showed markedly higher infectivity in CXCR4-specific infection. The role of g15 in CCR5-specific infection was investigated using viruses with high (NL-918, R5-monotropic), medium (NL-991, R5-monotropic), and low (NL-952, R5X4-dualtropic) CCR5-specific infectivity. For NL-991, a reduction of infectivity on GHOST-CCR5 cells was observed for a mutant lacking g15. For NL-952 mutants all lacking g15, a complete loss of CCR5-specificity was observed and NL-952 was shifted from R5X4 to X4 tropism. For all mutants of NL4-3, NL-991, and NL-952, where the lack of g15 markedly influenced infectivity or coreceptor usage, neutralization was enhanced. In contrast, NL-918 mutants with or without g15 showed no difference in neutralization and no difference in GHOST-CCR5 infection rates. Thus, for viruses with a low or medium CCR5-specificity the role of g15 for changing CCR5-usage and sensitivity to neutralization was more significant than for viruses with high infection rates on GHOST-CCR5 cells. Our data demonstrate that V3 glycans play an important role in the usage of CXCR4 and CCR5. The lack of g15 was relevant for a more efficient use of CXCR4, whereas interaction with CCR5 was facilitated in the presence of g15. This study also demonstrates that glycan g15 is involved in blocking of neutralizing antibodies and shifting HIV tropism from R5X4 to X4.

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.

N-linked glycosylation sites adjacent to and within the V1/V2 and the V3 loops of dualtropic human immunodeficiency virus type 1 isolate DH12 gp120 affect coreceptor usage and cellular tropism

The envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) is extensively glycosylated, containing approximately 23 asparagine (N)-linked glycosylation sites on its gp120 subunit. In this study, specific glycosylation sites on gp120 of a dualtropic primary HIV-1 isolate, DH12, were eliminated by site-directed mutagenesis and the properties of the resulting mutant envelopes were evaluated using a recombinant vaccinia virus-based cell-to-cell fusion assay alone or in the context of viral infections. Of the glycosylation sites that were evaluated, those proximal to the V1/V2 loops (N135, N141, N156, N160) and the V3 loops (N301) of gp120 were functionally critical. The glycosylation site mutations near the V1/V2 loop compromised the use of CCR5 and CXCR4 equally. In contrast, a mutation within the V3 loop preferentially inhibited the usage of CCR5; although this mutant protein completely lost its CCR5-dependent fusion activity, it retained 50% of the wild-type fusion activity with CXCR4. The replication of a virus containing this mutation was severely compromised in peripheral blood mononuclear cells, MT-4 cells, and primary monocyte-derived macrophages. A revertant virus, which acquired second site changes in the V3 loop that resulted in an increase in net positive charge, was isolated. The revertant virus fully recovered the usage of CXCR4 but not of CCR5, thereby altering the tropism of the parental virus from dualtropic to T-tropic. These results suggest that carbohydrate moieties near the V1/V2 and the V3 loops play critical roles in maintaining proper conformation of the variable loops for optimal interaction with receptors. Our results, combined with those of previously reported studies, further demonstrate that the function of individual glycans may be virus isolate dependent.