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

A review of Vicieae lectins studies: End of the book or a story in the writing?

Vicieae tribe, Leguminosae family (Fabaceae), has been extensively studied. In particular, the study of lectins. The purification, physicochemical and structural characterizations of the various purified lectins and the analysis of their relevant biological activities are ongoing. In this review, several works already published about Vicieae lectins are addressed. Initially, we presented the purification protocols and the physicochemical aspects, such as specificity for carbohydrates, optimal activity in the face of variations in temperature and pH, as well metals-dependence. Following, structural characterization studies are highlighted and, finally, various biological activities already reported are summarized. Studies on lectins in almost all genera (Lathyrus, Lens, Pisum and Vicia) are considered, with the exception of Vavilovia which studies of lectins have not yet been reported. Like other leguminous lectins, Vicieae lectins present heterogeneous profiles of agglutination profiles for erythrocytes and other cells of the immune system, and glycoproteins. Most Vicieae lectins consist of two subunits, α and β, products of a single precursor protein derived from a single gene. The differences between the isoforms result from varying degrees of proteolytic processing. Along with the identification of these molecules and their characteristics, biological activities become very relevant and robust for both basic and applied research.

Overview of the Structure⁻Function Relationships of Mannose-Specific Lectins from Plants, Algae and Fungi

To date, a number of mannose-binding lectins have been isolated and characterized from plants and fungi. These proteins are composed of different structural scaffold structures which harbor a single or multiple carbohydrate-binding sites involved in the specific recognition of mannose-containing glycans. Generally, the mannose-binding site consists of a small, central, carbohydrate-binding pocket responsible for the "broad sugar-binding specificity" toward a single mannose molecule, surrounded by a more extended binding area responsible for the specific recognition of larger mannose-containing N-glycan chains. Accordingly, the mannose-binding specificity of the so-called mannose-binding lectins towards complex mannose-containing N-glycans depends largely on the topography of their mannose-binding site(s). This structure⁻function relationship introduces a high degree of specificity in the apparently homogeneous group of mannose-binding lectins, with respect to the specific recognition of high-mannose and complex N-glycans. Because of the high specificity towards mannose these lectins are valuable tools for deciphering and characterizing the complex mannose-containing glycans that decorate both normal and transformed cells, e.g., the altered high-mannose N-glycans that often occur at the surface of various cancer cells.

DC-SIGN increases the affinity of HIV-1 envelope glycoprotein interaction with CD4

Mannose-binding C-type lectin receptors, expressed on Langerhans cells and subepithelial dendritic cells (DCs) of cervico-vaginal tissues, play an important role in HIV-1 capture and subsequent dissemination to lymph nodes. DC-SIGN has been implicated in both productive infection of DCs and the DC-mediated trans infection of CD4⁺ T cells that occurs in the absence of replication. However, the molecular events that underlie this efficient transmission have not been fully defined. In this study, we have examined the effect of the extracellular domains of DC-SIGN and Langerin on the stability of the interaction of the HIV-1 envelope glycoprotein with CD4 and also on replication in permissive cells. Surface plasmon resonance analysis showed that DC-SIGN increases the binding affinity of trimeric gp140 envelope glycoproteins to CD4. In contrast, Langerin had no effect on the stability of the gp140:CD4 complex. In vitro infection experiments to compare DC-SIGN enhancement of CD4-dependent and CD4-independent strains demonstrated significantly lower enhancement of the CD4-independent strain. In addition DC-SIGN increased the relative rate of infection of the CD4-dependent strain but had no effect on the CD4-independent strain. DC-SIGN binding to the HIV envelope protein effectively increases exposure of the CD4 binding site, which in turn contributes to enhancement of infection.

A role of glycosyl moieties in the stabilization of bitter gourd (Momordica charantia) peroxidase

The possible role of carbohydrate moieties in the stabilization of proteins has been investigated by using bitter gourd peroxidase as a model system. A comparative study of glycosylated and non-glycosylated isoenzymes of bitter gourd peroxidase was performed at various temperatures, pH, water-miscible organic solvents, detergents and chaotropic agent like urea. The pH-optima and temperature-optima of both glycosylated and non-glycosylated isoforms of bitter gourd peroxidase remained unchanged. The probes employed were changes in the enzyme activity and fluorescence. The glycosylated form of peroxidase retained greater fraction of enzyme activity against the exposure caused by various physical and chemical denaturants. The unfolding of both forms of enzyme in the presence of high urea concentrations, studied by fluorescence, indicated greater perturbations in the conformation of non-glycosylated preparation. The different properties examined thus indicated that glycosylation plays an important role in the stabilization of native conformation of proteins against the inactivation caused by various types of denaturants.

Mannose-specific plant lectins from the Amaryllidaceae family qualify as efficient microbicides for prevention of human immunodeficiency virus infection

The plant lectins derived from Galanthus nivalis (Snowdrop) (GNA) and Hippeastrum hybrid (Amaryllis) (HHA) selectively inhibited a wide variety of human immunodeficiency virus type 1 (HIV-1) and HIV-2 strains and clinical (CXCR4- and CCR5-using) isolates in different cell types. They also efficiently inhibited infection of T lymphocytes by a variety of mutant virus strains. GNA and HHA markedly prevented syncytium formation between persistently infected HUT-78/HIV cells and uninfected T lymphocytes. The plant lectins did not measurably affect the antiviral activity of other clinically approved anti-HIV drugs used in the clinic when combined with these drugs. Short exposure of the lectins to cell-free virus particles or persistently HIV-infected HUT-78 cells markedly decreased HIV infectivity and increased the protective (microbicidal) activity of the plant lectins. Flow cytometric analysis and monoclonal antibody binding studies and a PCR-based assay revealed that GNA and HHA do not interfere with CD4, CXCR4, CCR5, and DC-SIGN and do not specifically bind with the membrane of uninfected cells. Instead, GNA and HHA likely interrupt the virus entry process by interfering with the virus envelope glycoprotein. HHA and GNA are odorless, colorless, and tasteless, and they are not cytotoxic, antimetabolically active, or mitogenic to human primary T lymphocytes at concentrations that exceed their antivirally active concentrations by 2 to 3 orders of magnitude. GNA and HHA proved stable at high temperature (50 degrees C) and low pH (5.0) for prolonged time periods and can be easily formulated in gel preparations for microbicidal use; they did not agglutinate human erythrocytes and were not toxic to mice when administered intravenously.

Actinohivin, a novel anti-human immunodeficiency virus protein from an actinomycete, inhibits viral entry to cells by binding high-mannose type sugar chains of gp120

We searched human immunodeficiency virus (HIV) entry inhibitors and found a novel anti-HIV protein, actinohivin (AH), in a culture filtrate of the newly discovered genus actinomycete Longispora albida gen. nov., sp. nov. This paper deals with the mechanism of action of the anti-HIV activity of AH. AH exhibited potent anti-HIV activities against various strains of HIV-1 and HIV-2. AH bound to the glycoprotein gp120 of various strains of HIV-1 and gp130 of simian immunodeficiency virus (SIV), but did not bind to non-glycosylated gp120 nor to cells having CD4 and coreceptors, suggesting that AH inhibits viral entry to cells by binding to the envelope glycoprotein. The investigation of the effects of various sugars on AH-gp120 binding by ELISA revealed that yeast mannan alone strongly inhibited the binding (IC50 = 3.0 microg/ml). Experiments investigating the binding of AH to other glycoproteins revealed that AH binds to ribonuclease B and thyroglobulin that have a high-mannose type saccharide chain, but not to other glycoproteins having a N-glycoside type saccharide chain. The above results indicate that high-mannose type saccharide chains of gp120 are molecular targets of AH in its anti-HIV activity.

DC-SIGN binds to HIV-1 glycoprotein 120 in a distinct but overlapping fashion compared with ICAM-2 and ICAM-3

DC-SIGN is a C-type lectin that binds to endogenous adhesion molecules ICAM-2 and ICAM-3 as well as the viral envelope glycoprotein human immunodeficiency virus, type 1, glycoprotein (gp) 120. We wished to determine whether DC-SIGN binds differently to its endogenous ligands ICAM-2 and ICAM-3 versus HIV-1 gp120. We found that recombinant soluble DC-SIGN bound to gp120-Fc more than 100- and 50-fold better than ICAM-2-Fc and ICAM-3-Fc, respectively. This relative difference was maintained using DC-SIGN expressed on three different CD4-negative cell lines. Although the cell surface affinity for gp120 varied by up to 4-fold on the cell lines examined, the affinity for gp120 was not a correlate of the ability of the cell line to transfer virus. Monosaccharides with equatorial 4-OH groups competed as well as D-mannose for gp120 binding to DC-SIGN, regardless of how the other hydroxyl groups were positioned. Disaccharide competitors and glycan chip analysis showed that DC-SIGN has a preference for oligosaccharides linked in an alpha-anomeric configuration. Alanine-scanning mutagenesis of DC-SIGN revealed that highly conserved residues that coordinate calcium (Asp-366) and/or are involved in both calcium and specific carbohydrate interactions (Glu-347, Asn-349, Glu-354, and Asp-355) significantly compromised binding to all three ligands. Mutating non-conserved residues (Asn-311, Arg-345, Val-351, Gly-352, Glu-353, Ser-360, Gly-361, and Asn-362) minimally affected binding except for the Asp-367 mutant, which enhanced gp120 binding but diminished ICAM-2 and ICAM-3 binding. Conversely, mutating the moderately conserved residue (Gly-346) abrogated gp120 binding but enhanced ICAM-2 and ICAM-3 binding. Thus, DC-SIGN appears to bind in a distinct but overlapping manner to gp120 when compared with ICAM-2 and ICAM-3.

A retrospective look at the cationic peanut peroxidase structure

The cationic peanut peroxidase has been studied in detail, not only with regard to its peptide structure, but also to the sites and role of the three moieties linked to it. Peanut peroxidase lends itself well to a close examination as a potential example for other plant peroxidase studies. It was the first plant peroxidase for which a 3-D structure was derived from crystals, with the glycans intact. Subsequent analysis of peroxidases structures from other plants have not shown great differences to that of the peanut peroxidase. As the period of proteomics follows on the era of genomics, the study of glycans has been brought back into focus. With the potential use of peroxidase as a polymerization agent for industry, there are some aspects of the overall structure that should be kept in mind for successful use of this enzyme. A variety of techniques are now available to assay for these structures/moieties and their roles. Peanut peroxidase data are reviewed in that light, as well as defining some true terms for isozymes. Because a high return of the enzyme in a pure form has been obtained from cultured cells in suspension culture, a brief review of this is also offered.

Spying on HIV with SPR

The application of surface plasmon resonance (SPR)-based optical biosensors has contributed extensively to our understanding of functional aspects of HIV. SPR biosensors allow the analysis of real-time interactions of any biomolecule, be it protein, nucleic acid, lipid, carbohydrate or small molecule, without the need for intrinsic or extrinsic probes. As such, the technology has been used to analyze molecular interactions associated with every aspect of the viral life cycle, from basic studies of binding events occurring during docking, replication, budding and maturation to applied research related to vaccine and inhibitory drug development. Along the way, SPR biosensors have provided a unique and detailed view into the inner workings of HIV.

Identification of different binding sites in the dendritic cell-specific receptor DC-SIGN for intercellular adhesion molecule 3 and HIV-1

The novel dendritic cell (DC)-specific human immunodeficiency virus type 1 (HIV-1) receptor DC-SIGN plays a key role in the dissemination of HIV-1 by DC. DC-SIGN is thought to capture HIV-1 at mucosal sites of entry, facilitating transport to lymphoid tissues, where DC-SIGN efficiently transmits HIV-1 to T cells. DC-SIGN is also important in the initiation of immune responses by regulating DC-T cell interactions through intercellular adhesion molecule 3 (ICAM-3). We have characterized the mechanism of ligand binding by DC-SIGN and identified the crucial amino acids involved in this process. Strikingly, the HIV-1 gp120 binding site in DC-SIGN is different from that of ICAM-3, consistent with the observation that glycosylation of gp120, in contrast to ICAM-3, is not crucial to the interaction with DC-SIGN. A specific mutation in DC-SIGN abrogated ICAM-3 binding, whereas the HIV-1 gp120 interaction was unaffected. This DC-SIGN mutant captured HIV-1 and infected T cells in trans as efficiently as wild-type DC-SIGN, demonstrating that ICAM-3 binding is not necessary for HIV-1 transmission. This study provides a basis for the design of drugs that inhibit or alter interactions of DC-SIGN with gp120 but not with ICAM-3 or vice versa and that have a therapeutic value in immunological diseases and/or HIV-1 infections.

Biologically active proteins from natural product extracts

The term "biologically active proteins" is almost redundant. All proteins produced by living creatures are, by their very nature, biologically active to some extent in their homologous species. In this review, a subset of these proteins will be discussed that are biologically active in heterologous systems. The isolation and characterization of novel proteins from natural product extracts including those derived from microorganisms, plants, insects, terrestrial vertebrates, and marine organisms will be reviewed and grouped into several distinct classes based on their biological activity and their structure.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

The effects of the site-directed removal of N-glycosylation from cationic peanut peroxidase on its function

Peanut peroxidase has been diffracted. The location of its heme and calcium moieties have been shown and their role demonstrated. However, the structure and role of its glycans is only now being elucidated. The role of three N-linked complex glycans on cationic peroxidase (cPrx) of peanut (Arachis hypogaea L cv. Valencia), as expressed by prxPNC1 in transgenic tobacco, was analyzed by site-directed replacement of each of the three glycosylation sites, N-60, N-144, and N-185 with Q, individually. The mutant prxPNC1 cDNAs with a 3' histidine-tag were expressed in transgenic tobacco. The effect on the catalytic ability, thermal stability, and unfolding properties of the mutant peroxidases, isolated from the medium of transgenic tobacco cell suspension cultures were compared with those of the wild cPrx from peanut. It was found that the ablation of the glycans at N-60 and N-144 influences the full expression of the cPrx catalytic ability. The glycan at N-185 is important for the thermostability, as is the removal of the carbohydrate chain at N-185, resulting in rapid enzymatic decrease at temperatures of 50 degrees C. All three glycans appeared to influence the folding of the protein.