Jacalin, a lectin interacting with O-linked sugars and mediating protection of CD4+ cells against HIV-1, binds to the external envelope glycoprotein gp120

Variability in the Glycosylation Patterns of gp120 Proteins from Different Human Immunodeficiency Virus Type 1 Isolates Expressed in Different Host Cells

The mature HIV-1 envelope (Env) glycoprotein is composed of gp120, the exterior subunit, and gp41, the transmembrane subunit assembled as trimer by noncovalent interaction. There is a great body of literature to prove that gp120 binds to CD4 first, then to the coreceptor. Binding experiments and functional assays have demonstrated that CD4 binding induces conformational changes in gp120 that enable or enhance its interaction with a coreceptor. Previous studies provided different glycomic maps for the HIV-1 gp120. Here, we build on previous work to report that the use of LC-MS/MS, in conjunction with hydrophilic interaction liquid chromatography (HILIC) enrichment to glycosylation sites, associated with the assorted neutralizing or binding events of glycosylation targeted antibodies from different clades or strains. In this study, the microheterogeneity of the glycosylation from 4 different clades of gp120s is deeply investigated. Aberrant glycosylation patterns were detected on gp120 that originated from different clades, viral sequences, and host cells. The results of this study may help provide a better understanding of the mechanism of how the glycans participate in the antibody neutralizing process that targets glycosylation sites.

Antiviral lectins: Selective inhibitors of viral entry

Many natural lectins have been reported to have antiviral activity. As some of these have been put forward as potential development candidates for preventing or treating viral infections, we have set out in this review to survey the literature on antiviral lectins. The review groups lectins by structural class and class of source organism we also detail their carbohydrate specificity and their reported antiviral activities. The review concludes with a brief discussion of several of the pertinent hurdles that heterologous proteins must clear to be useful clinical candidates and cites examples where such studies have been reported for antiviral lectins. Though the clearest path currently being followed is the use of antiviral lectins as anti-HIV microbicides via topical mucosal administration, some investigators have also found systemic efficacy against acute infections following subcutaneous administration.

Lectins with anti-HIV activity: a review

Lectins including flowering plant lectins, algal lectins, cyanobacterial lectins, actinomycete lectin, worm lectins, and the nonpeptidic lectin mimics pradimicins and benanomicins, exhibit anti-HIV activity. The anti-HIV plant lectins include Artocarpus heterophyllus (jacalin) lectin, concanavalin A, Galanthus nivalis (snowdrop) agglutinin-related lectins, Musa acuminata (banana) lectin, Myrianthus holstii lectin, Narcissus pseudonarcissus lectin, and Urtica diocia agglutinin. The anti-HIV algal lectins comprise Boodlea coacta lectin, Griffithsin, Oscillatoria agardhii agglutinin. The anti-HIV cyanobacterial lectins are cyanovirin-N, scytovirin, Microcystis viridis lectin, and microvirin. Actinohivin is an anti-HIV actinomycete lectin. The anti-HIV worm lectins include Chaetopterus variopedatus polychaete marine worm lectin, Serpula vermicularis sea worm lectin, and C-type lectin Mermaid from nematode (Laxus oneistus). The anti-HIV nonpeptidic lectin mimics comprise pradimicins and benanomicins. Their anti-HIV mechanisms are discussed.

High-throughput profiling of anti-glycan humoral responses to SIV vaccination and challenge

Recent progress toward an HIV vaccine highlights both the potential of vaccines to end the AIDS pandemic and the need to boost efficacy by incorporating additional vaccine strategies. Although many aspects of the immune response can contribute to vaccine efficacy, the key factors have not been defined fully yet. A particular area that may yield new insights is anti-glycan immune responses, such as those against the glycan shield that HIV uses to evade the immune system. In this study, we used glycan microarray technology to evaluate anti-glycan antibody responses induced by SIV vaccination and infection in a non-human primate model of HIV infection. This comprehensive profiling of circulating anti-glycan antibodies found changes in anti-glycan antibody levels after both vaccination with the Ad5hr-SIV vaccine and SIV infection. Notably, SIV infection produced generalized declines in anti-glycan IgM antibodies in a number of animals. Additionally, some infected animals generated antibodies to the Tn antigen, which is a cryptic tumor-associated antigen exposed by premature termination of O-linked glycans; however, the Ad5hr-SIV vaccine did not induce anti-Tn IgG antibodies. Overall, this study demonstrates the potential contributions that glycan microarrays can make for HIV vaccine development.

Recombinant jacalin-like plant lectins are produced at high levels in Nicotiana benthamiana and retain agglutination activity and sugar specificity

The plant kingdom is an underexplored source of valuable proteins which, like plant lectins, display unique interacting specificities. Furthermore, plant protein diversity remains under-exploited due to the low availability and heterogeneity of native sources. All these hurdles could be overcome with recombinant production. A narrow phylogenetic gap between the native source and the recombinant platform is likely to facilitate proper protein processing and stability; therefore, the plant cell chassis should be specially suited for the recombinant production of many plant native proteins. This is illustrated herein with the recombinant production of two representatives of the plant jacalin-related lectin (JRLs) protein family in Nicotiana benthamiana using state-of-the-art magnICON technology. Mannose-specific Banlec JRL was produced at very high levels in leaves, reaching 1.0mg of purified protein per gram of fresh weight and showing strong agglutination activity. Galactose-specific jacalin JRL, with its complicated processing requirements, was also successfully produced in N. benthamiana at levels of 0.25 mg of purified protein per gram of fresh weight. Recombinant Jacalin (rJacalin) proved efficient in the purification of human IgA1, and was able to discriminate between plant-made and native IgA1 due to their differential glycosylation status. Together, these results show that the plant cell factory should be considered a primary option in the recombinant production of valuable plant proteins.

Simian immunodeficiency virus from the sooty mangabey and rhesus macaque is modified with O-linked carbohydrate

Although stretches of serine and threonine are sometimes sites for O-linked carbohydrate attachment, specific sequence and structural determinants for O-linked attachment remain ill defined. The gp120 envelope protein of SIVmac239 contains a serine-threonine-rich stretch of amino acids at positions 128 to 139. Here we show that lectin protein from jackfruit seed (jacalin), which binds to non- and monosialylated core 1 O-linked carbohydrate, potently inhibited the replication of SIVmac239. Selection of a jacalin-resistant SIVmac239 variant population resulted in virus with specific substitutions within amino acids 128 to 139. Cloned simian immunodeficiency virus (SIV) variants with substitutions in the 128-to-139 region had infectivities equivalent to, or within 1 log unit of, that of SIVmac239 and were resistant to the inhibitory effects of jacalin. Characterization of the SIVmac239 gp120 O-linked glycome showed the presence of core 1 and core 2 O-linked carbohydrate; a 128-to-139-substituted variant gp120 from jacalin-resistant SIV lacked O-linked carbohydrate. Unlike that of SIVmac239, the replication of HIV-1 strain NL4-3 was resistant to inhibition by jacalin. Purified gp120s from four SIVmac and SIVsm strains bound jacalin strongly in an enzyme-linked immunosorbent assay, while nine different HIV-1 gp120s, two SIVcpz gp120s, and 128-to-139-substituted SIVmac239 gp120 did not bind jacalin. The ability or inability to bind jacalin thus correlated with the presence of the serine-threonine-rich stretch in the SIVmac and SIVsm gp120s and the absence of such stretches in the SIVcpz and HIV-1 gp120s. Consistent with sequence predictions, two HIV-2 gp120s bound jacalin, while one did not. These data demonstrate the presence of non- and monosialylated core 1 O-linked carbohydrate on the gp120s of SIVmac and SIVsm and the lack of these modifications on HIV-1 and SIVcpz gp120s.

Interaction of aromatic imino glycoconjugates with jacalin: experimental and computational docking studies

Altering the lectin properties by chemically modified glycoconjugates can have profound effect on their biological applications. In the present case, jacalin has been chosen to study the binding aspects toward glycoconjugates modified by connecting aromatic moieties through imine conjugation at their C-1- or C-2-positions. Out of 10 glycoconjugates, the galactosyl-naphthyl imine (1c) was found to be most effective toward agglutination inhibition (260 times better than galactose), quenching fluorescence intensity, and exhibiting greater binding (K(a), 1.3 × 10(4)M(-1)) with jacalin. The specific binding of galactose conjugates and the nonspecific binding of other conjugates have been demonstrated based on ITC. Changes in the secondary structures have been addressed by far- and near-UV CD spectroscopy. The present studies demonstrated that galactose-based conjugates bind at carbohydrate recognition domain (CRD) mainly through polar interactions in addition to exhibiting some nonpolar/hydrophobic interactions, whereas the conjugates other than galactose primarily interact through hydrophobic interactions. Binding of galactosyl conjugates at CRD has been further demonstrated by rigid docking.

Domain-swapped structure of the potent antiviral protein griffithsin and its mode of carbohydrate binding

The crystal structure of griffithsin, an antiviral lectin from the red alga Griffithsia sp., was solved and refined at 1.3 A resolution for the free protein and 0.94 A for a complex with mannose. Griffithsin molecules form a domain-swapped dimer, in which two beta strands of one molecule complete a beta prism consisting of three four-stranded sheets, with an approximate 3-fold axis, of another molecule. The structure of each monomer bears close resemblance to jacalin-related lectins, but its dimeric structure is unique. The structures of complexes of griffithsin with mannose and N-acetylglucosamine defined the locations of three almost identical carbohydrate binding sites on each monomer. We have also shown that griffithsin is a potent inhibitor of the coronavirus responsible for severe acute respiratory syndrome (SARS). Antiviral potency of griffithsin is likely due to the presence of multiple, similar sugar binding sites that provide redundant attachment points for complex carbohydrate molecules present on viral envelopes.

A beta-galactose-specific lectin isolated from the marine worm Chaetopterus variopedatus possesses anti-HIV-1 activity

A 30 kDa beta-galactose-specific lectin named CVL was isolated from the polychaete marine worm Chaetopterus variopedatus (Annelida) and its anti-HIV-1 activity in vitro was determined. Results showed that CVL inhibited cytopathic effect induced by HIV-1 and the production of viral p24 antigen. The EC(50) values were 0.0043 and 0.057 microM, respectively. Time-of-addition analysis of anti-HIV-1 activity indicated its action was at the early stage of virus replication. CVL could blocked the cell-to-cell fusion process of HIV infected and uninfected cells with an EC(50) of 0.073 microM. The inhibition of HIV-1 entry into host cells was demonstrated by using fluorescence-based real-time quantify PCR. At CVL concentration of 0.33 microM and 0.07 microM, 86% and 21% virus attachment were blocked, respectively. The anti-HIV-1 action of CVL might relate to blockade of HIV-1 entry into cells.

Proteins that bind high-mannose sugars of the HIV envelope

A broad range of proteins bind high-mannose carbohydrates found on the surface of the envelope protein gp120 of the human immunodeficiency virus and thus interfere with the viral life cycle, providing a potential new way of controlling HIV infection. These proteins interact with the carbohydrate moieties in different ways. A group of them interacts as typical C-type lectins via a Ca2+ ion. Another group interacts with specific single, terminal sugars, without the help of a metal cation. A third group is involved in more intimate interactions, with multiple carbohydrate rings and no metal ion. Finally, there is a group of lectins for which the interaction mode has not yet been elucidated. This review summarizes, principally from a structural point of view, the current state of knowledge about these high-mannose binding proteins and their mode of sugar binding.

Structural basis for the energetics of jacalin-sugar interactions: promiscuity versus specificity

Jacalin, a tetrameric lectin, is one of the two lectins present in jackfruit (Artocarpus integrifolia) seeds. Its crystal structure revealed, for the first time, the occurrence of the beta-prism I fold in lectins. The structure led to the elucidation of the crucial role of a new N terminus generated by post-translational proteolysis for the lectin's specificity for galactose. Subsequent X-ray studies on other carbohydrate complexes showed that the extended binding site of jacalin consisted of, in addition to the primary binding site, a hydrophobic secondary site A composed of aromatic residues and a secondary site B involved mainly in water-bridges. A recent investigation involving surface plasmon resonance and the X-ray analysis of a methyl-alpha-mannose complex, had led to a suggestion of promiscuity in the lectin's sugar specificity. To explore this suggestion further, detailed isothermal titration calorimetric studies on the interaction of galactose (Gal), mannose (Man), glucose (Glc), Me-alpha-Gal, Me-alpha-Man, Me-alpha-Glc and other mono- and oligosaccharides of biological relevance and crystallographic studies on the jacalin-Me-alpha-Glc complex and a new form of the jacalin-Me-alpha-Man complex, have been carried out. The binding affinity of Me-alpha-Man is 20 times weaker than that of Me-alpha-Gal. The corresponding number is 27, when the binding affinities of Gal and Me-alpha-Gal, and those of Man and Me-alpha-Man are compared. Glucose (Glc) shows no measurable binding, while the binding affinity of Me-alpha-Glc is slightly less than that of Me-alpha-Man. The available crystal structures of jacalin-sugar complexes provide a convincing explanation for the energetics of binding in terms of interactions at the primary binding site and secondary site A. The other sugars used in calorimetric studies show no detectable binding to jacalin. These results and other available evidence suggest that jacalin is specific to O-glycans and its affinity to N-glycans is extremely weak or non-existent and therefore of limited value in processes involving biological recognition.

Structural basis of the carbohydrate specificities of jacalin: an X-ray and modeling study

The structures of the complexes of tetrameric jacalin with Gal, Me-alpha-GalNAc, Me-alpha-T-antigen, GalNAcbeta1-3Gal-alpha-O-Me and Galalpha1-6Glc (mellibiose) show that the sugar-binding site of jacalin has three components: the primary site, secondary site A, and secondary site B. In these structures and in the two structures reported earlier, Gal or GalNAc occupy the primary site with the anomeric carbon pointing towards secondary site A. The alpha-substituents, when present, interact, primarily hydrophobically, with secondary site A which has variable geometry. O-H..., centered pi and C-H...pi hydrogen bonds involving this site also exist. On the other hand, beta-substitution leads to severe steric clashes. Therefore, in complexes involving beta-linked disaccharides, the reducing sugar binds at the primary site with the non-reducing end located at secondary site B. The interactions at secondary site B are primarily through water bridges. Thus, the nature of the linkage determines the mode of the association of the sugar with jacalin. The interactions observed in the crystal structures and modeling based on them provide a satisfactory qualitative explanation of the available thermodynamic data on jacalin-carbohydrate interactions. They also lead to fresh insights into the nature of the binding of glycoproteins by jacalin.

Crystal structure of the jacalin-T-antigen complex and a comparative study of lectin-T-antigen complexes

Thomsen-Friedenreich antigen (Galbeta1-3GalNAc), generally known as T-antigen, is expressed in more than 85% of human carcinomas. Therefore, proteins which specifically bind T-antigen have potential diagnostic value. Jacalin, a lectin from jack fruit (Artocarpus integrifolia) seeds, is a tetramer of molecular mass 66kDa. It is one of the very few proteins which are known to bind T-antigen. The crystal structure of the jacalin-T-antigen complex has been determined at 1.62A resolution. The interactions of the disaccharide at the binding site are predominantly through the GalNAc moiety, with Gal interacting only through water molecules. They include a hydrogen bond between the anomeric oxygen of GalNAc and the pi electrons of an aromatic side-chain. Several intermolecular interactions involving the bound carbohydrate contribute to the stability of the crystal structure. The present structure, along with that of the Me-alpha-Gal complex, provides a reasonable qualitative explanation for the known affinities of jacalin to different carbohydrate ligands and a plausible model of the binding of the lectin to T-antigen O-linked to seryl or threonyl residues. Including the present one, the structures of five lectin-T-antigen complexes are available. GalNAc occupies the primary binding site in three of them, while Gal occupies the site in two. The choice appears to be related to the ability of the lectin to bind sialylated sugars. In either case, most of the lectin-disaccharide interactions are at the primary binding site. The conformation of T-antigen in the five complexes is nearly the same.

Structural basis for the unusual carbohydrate-binding specificity of jacalin towards galactose and mannose

Evidence is presented that the specificity of jacalin, the seed lectin from jack fruit (Artocarpus integrifolia), is not directed exclusively against the T-antigen disaccharide Galbeta1,3GalNAc, lactose and galactose, but also against mannose and oligomannosides. Biochemical analyses based on surface-plasmon-resonance measurements, combined with the X-ray-crystallographic determination of the structure of a jacalin-alpha-methyl-mannose complex at 2 A resolution, demonstrated clearly that jacalin is fully capable of binding mannose. Besides mannose, jacalin also interacts readily with glucose, N-acetylneuraminic acid and N-acetylmuramic acid. Structural analyses demonstrated that the relatively large size of the carbohydrate-binding site enables jacalin to accommodate monosaccharides with different hydroxyl conformations and provided unambiguous evidence that the beta-prism structure of jacalin is a sufficiently flexible structural scaffold to confer different carbohydrate-binding specificities to a single lectin.

Accessibility of the high-mannose glycans of glycoprotein gp120 from human immunodeficiency virus type 1 probed by in vitro interaction with mannose-binding lectins

The direct interaction of mannose-specific plant lectins with gp120 of HIV-1 was studied by surface plasmon resonance. Inhibition experiments indicated that exposed high mannose type glycans play a key role in the interaction. Most of the lectins specifically accommodate outer alpha1,2-, alpha1,3-, or alpha1,6-linked di- or trimannosides, and especially legume lectins, also interact with the trimannoside core of the complex type glycans. The unexpected affinity of some lectins towards gp120 presumably results from conformational differences in their binding sites. These results demonstrate that mannose-specific plant lectins are powerful tools to study the accessibility and elucidate the function of the gp120 glycans in the recognition and infection of the host cells by HIV-1.

Jacalin: a jackfruit (Artocarpus heterophyllus) seed-derived lectin of versatile applications in immunobiological research

Jacalin, the major protein from the jackfruit (Artocarpus heterophyllus) seeds, is a tetrameric two-chain lectin (molecular mass 65 kDa) combining a heavy alpha chain of 133 amino acid residues with a light beta chain of 20-21 amino acid residues. It is highly specific for the alpha-O-glycoside of the disaccharide Thomsen-Friedenreich antigen (Gal beta1-3GalNAc), even in its sialylated form. This property has made jacalin suitable for studying various O-linked glycoproteins, particularly human IgA1. Jacalin's uniqueness in being strongly mitogenic for human CD4+ T lymphocytes has made it a useful tool for the evaluation of the immune status of patients infected with human immunodeficiency virus (HIV)-1. The abundance of source material for the production of jacalin, its ease of purification, yield and stability have made it an attractive cost-effective lectin. It has found applications in diverse areas such as the isolation of human plasma glycoproteins (IgA1, C1-inhibitor, hemopexin, alpha2-HSG), the investigation of IgA-nephropathy, the analysis of O-linked glycoproteins and the detection of tumours.

Involvement of the HIV-1 external envelope glycoprotein 120 (gp120) C2 region in gp120 oligomerization

A synthetic peptide resembling the C2 region of human immunodeficiency virus type 1 (HIV-1) gp120 (C2-Lai: amino acids (aa) 273-288), inhibited (C50 = 200 microM) gp120 calcium-dependent binding of N-acetyl-beta-D-glucosaminyl and mannosyl residues exposed on natural glycoprotein bovine fetuin whereas a peptide derived from an aa sequence downstream of C2-Lai (C2-SC19) had no such effect (C50 > 1000 microM). No calcium-carbohydrate-specific binding of C2-Lai to fetuin was detected. In addition, C2-Lai was also found to inhibit the calcium-dependent oligomerization of gp120: while recombinant gp120 (rgp120) was recovered mainly as oligomers (78%) in 10 mM CaCl2, in contrast to 100% monomers in 1mM CaCl2, mostly monomers (67%) were found in 10 mM CaCl2 in the presence of C2-Lai. Peptide C2-SC19 and carbohydrate structures such as fetuin, fucoidin, dextran or mannan did not significantly affect gp120 oligomerization. Electrophoresis and gel filtration analysis also showed that C2-Lai aggregated in the form of 20 kDa compounds, which is compatible with association of 10 molecules. Taken together, these results demonstrate that the C2 domain is involved in gp120 oligomerization and suggest that gp120 oligomers but not monomers have specific carbohydrate binding properties.