Fucose-binding lectin from opportunistic pathogen Burkholderia ambifaria binds to both plant and human oligosaccharidic epitopes

Dynamic Cooperative Glycan Assembly Blocks the Binding of Bacterial Lectins to Epithelial Cells

Pathogens frequently rely on lectins for adhesion and cellular entry into the host. Since these interactions typically result from multimeric binding of lectins to cell-surface glycans, novel therapeutic strategies are being developed with the use of glycomimetics as competitors of such interactions. Herein we study the benefit of nucleic acid based oligomeric assemblies with PNA-fucose conjugates. We demonstrate that the interactions of a lectin with epithelial cells can be inhibited with conjugates that do not form stable assemblies in solution but benefit from cooperativity between ligand-protein interactions and PNA hybridization to achieve high affinity. A dynamic dimeric assembly fully blocked the binding of the fucose-binding lectin BambL of Burkholderia ambifaria, a pathogenic bacterium, to epithelial cells with an efficiency of more than 700-fold compared to l-fucose.

A Novel Fucose-binding Lectin from Photorhabdus luminescens (PLL) with an Unusual Heptabladed β-Propeller Tetrameric Structure

Photorhabdus luminescens is known for its symbiosis with the entomopathogenic nematode Heterorhabditis bacteriophora and its pathogenicity toward insect larvae. A hypothetical protein from P. luminescens was identified, purified from the native source, and characterized as an l-fucose-binding lectin, named P. luminescens lectin (PLL). Glycan array and biochemical characterization data revealed PLL to be specific toward l-fucose and the disaccharide glycan 3,6-O-Me₂-Glcβ1-4(2,3-O-Me₂)Rhaα-O-(p-C₆H₄)-OCH₂CH₂NH₂ PLL was discovered to be a homotetramer with an intersubunit disulfide bridge. The crystal structures of native and recombinant PLL revealed a seven-bladed β-propeller fold creating seven putative fucose-binding sites per monomer. The crystal structure of the recombinant PLL·l-fucose complex confirmed that at least three sites were fucose-binding. Moreover, the crystal structures indicated that some of the other sites are masked either by the tetrameric nature of the lectin or by incorporation of the C terminus of the lectin into one of these sites. PLL exhibited an ability to bind to insect hemocytes and the cuticular surface of a nematode, H. bacteriophora.

Profiling of core fucosylated N-glycans using a novel bacterial lectin that specifically recognizes α1,6 fucosylated chitobiose

A novel fucose-binding lectin (SL2-1) from the bacterium Streptomyces rapamycinicus was identified by analysis of metagenomic DNA sequences. SL2-1 belongs to a new group of bacterial fucose-specific lectins that have no similarity to known bacterial fucose-binding proteins, but are related to certain eukaryotic fucose-binding lectins. The 17 kDa protein was expressed recombinantly in E. coli and purified by affinity chromatography. Glycan microarray analysis with fluorescently labeled recombinant SL2-1 demonstrated its ability to bind to core α1-6 fucosylated N-glycans, but not to core α1-3 fucosylated N-glycans, or other α1-2, α1-3 and α1-4 fucosylated oligosaccharides. The minimal high affinity binding epitope of SL2-1 was α1-6 fucosylated di-n-acetylchitobiose. The recombinant lectin was efficient in detection of N-glycan core fucosylation using lectin blotting and lectin ELISA assays. Finally, a workflow using SL2-1 for selective and quantitative profiling of core fucosylated N-glycans using UPLC-HILIC-FLR analysis was established. The approach was validated for selective capture and analysis of core fucosylated N-glycans present in complex glycan mixtures derived from mammalian serum IgG.

Characterisation of a novel integrative and conjugative element ICESsD9 carrying erm(B) and tet(O) resistance determinants in Streptococcus suis, and the distribution of ICESsD9-like elements in clinical isolates

This study identified a novel integrative and conjugative element (ICESsD9) carrying erm(B) and tet(O) resistance determinants in Streptococcus suis D9 and determined its prevalence in clinical isolates. Comparative genome analysis was performed using Mauve and Artemis Comparison Tool visualisation programs. Inverse PCR was utilised to detect its circular intermediate. The transfer capacity of ICESsD9 was evaluated by mating assays using S. suis A7 and Enterococcus faecalis JH2-2 as recipients. A genome walking approach was employed to analyse the characteristics of integration sites in transconjugants. A total of 118 clinical S. suis isolates were tested by PCR mapping assays to detect ICESsD9-like elements. MLST was performed on isolates containing ICESsD9 variants to determine their clonal relatedness. This 55 683-bp element can actively excise from the chromosome. Additionally, it was capable of transferring both into S. suis and E. faecalis with frequencies of 1.2×10^-4 and 5.8×10^-6 per donor, respectively. When investigating integration site features, it was found that ICESsD9 can enter S. suis and E. faecalis chromosomes by different sites, generating 15-bp and 3-bp direct repeat sequences, respectively. Twelve isolates mainly belonging to sequence types ST1, ST7 and ST28 were confirmed to harbour ICESsD9-like elements. In conclusion, this study provides the first description of an ICE in S. suis that is capable of transferring both into S. suis and E. faecalis. The presence of different ICESsD9 variants in clinical isolates suggests already wide dissemination of this family element in S. suis in China.

The Hidden Conformation of Lewis x, a Human Histo-Blood Group Antigen, Is a Determinant for Recognition by Pathogen Lectins

Histo-blood group epitopes are fucosylated branched oligosaccharides with well-defined conformations in solution that are recognized by receptors, such as lectins from pathogens. We report here the results of a series of experimental and computational endeavors revealing the unusual distortion of histo-blood group antigens by bacterial and fungal lectins. The Lewis x trisaccharide adopts a rigid closed conformation in solution, while crystallography and molecular dynamics reveal several higher energy open conformations when bound to the Ralstonia solanacearum lectin, which is in agreement with thermodynamic and kinetic measurements. Extensive molecular dynamics simulations confirm rare transient Le(x) openings in solution, frequently assisted by distortion of the central N-acetyl-glucosamine ring. Additional directed molecular dynamic trajectories revealed the role of a conserved tryptophan residue in guiding the fucose into the binding site. Our findings show that conformational adaptation of oligosaccharides is of paramount importance in cell recognition and should be considered when designing anti-infective glyco-compounds.

Combining 3D structure with glycan array data provides insight into the origin of glycan specificity

Defining how a glycan-binding protein (GBP) specifically selects its cognate glycan from among the ensemble of glycans within the cellular glycome is an area of intense study. Powerful insight into recognition mechanisms can be gained from 3D structures of GBPs complexed to glycans; however, such structures remain difficult to obtain experimentally. Here an automated 3D structure generation technique, called computational carbohydrate grafting, is combined with the wealth of specificity information available from glycan array screening. Integration of the array data with modeling and crystallography allows generation of putative co-complex structures that can be objectively assessed and iteratively altered until a high level of agreement with experiment is achieved. Given an accurate model of the co-complexes, grafting is also able to discern which binding determinants are active when multiple potential determinants are present within a glycan. In some cases, induced fit in the protein or glycan was necessary to explain the observed specificity, while in other examples a revised definition of the minimal binding determinants was required. When applied to a collection of 10 GBP-glycan complexes, for which crystallographic and array data have been reported, grafting provided a structural rationalization for the binding specificity of >90% of 1223 arrayed glycans. A webtool that enables researchers to perform computational carbohydrate grafting is available at www.glycam.org/gr (accessed 03 March 2016).

Six independent fucose-binding sites in the crystal structure of Aspergillus oryzae lectin

The crystal structure of AOL (a fucose-specific lectin of Aspergillus oryzae) has been solved by SAD (single-wavelength anomalous diffraction) and MAD (multi-wavelength anomalous diffraction) phasing of seleno-fucosides. The overall structure is a six-bladed β-propeller similar to that of other fucose-specific lectins. The fucose moieties of the seleno-fucosides are located in six fucose-binding sites. Although the Arg and Glu/Gln residues bound to the fucose moiety are common to all fucose-binding sites, the amino-acid residues involved in fucose binding at each site are not identical. The varying peak heights of the seleniums in the electron density map suggest that each fucose-binding site has a different carbohydrate binding affinity.

Anti-viral Effect of Bifidobacterium adolescentis against Noroviruses

This study aims to investigate the effect of Bifidobacterium adolescentis against noroviruses (NoVs). Murine norovirus-1 (MNV-1) used as a surrogate was detected by plaque assay and RT-qPCR. Human NoV virus like particles (VLPs) were detected by cell-binding assay. It was shown that the presence of B. adolescentis could inhibit the multiplication of MNV-1 on RAW 264.7 cells within 48 h of co-incubation period at 37°C. This inhibition did not occur at the viral binding stage, as no difference was observed in MNV-1 genomic copies collected from washed RAW 264.7 cells without and with B. adolescentis after co-incubation for 1 h at room temperature. Meanwhile, the presence of B. adolescentis decreased the binding of human NoV GI.1 VLPs to both Caco-2 cells and HT-29 cells, while no reduction was induced for the binding of human NoV GII.4 VLPs to Caco-2 cells.

FleA Expression in Aspergillus fumigatus Is Recognized by Fucosylated Structures on Mucins and Macrophages to Prevent Lung Infection

The immune mechanisms that recognize inhaled Aspergillus fumigatus conidia to promote their elimination from the lungs are incompletely understood. FleA is a lectin expressed by Aspergillus fumigatus that has twelve binding sites for fucosylated structures that are abundant in the glycan coats of multiple plant and animal proteins. The role of FleA is unknown: it could bind fucose in decomposed plant matter to allow Aspergillus fumigatus to thrive in soil, or it may be a virulence factor that binds fucose in lung glycoproteins to cause Aspergillus fumigatus pneumonia. Our studies show that FleA protein and Aspergillus fumigatus conidia bind avidly to purified lung mucin glycoproteins in a fucose-dependent manner. In addition, FleA binds strongly to macrophage cell surface proteins, and macrophages bind and phagocytose fleA-deficient (∆fleA) conidia much less efficiently than wild type (WT) conidia. Furthermore, a potent fucopyranoside glycomimetic inhibitor of FleA inhibits binding and phagocytosis of WT conidia by macrophages, confirming the specific role of fucose binding in macrophage recognition of WT conidia. Finally, mice infected with ΔfleA conidia had more severe pneumonia and invasive aspergillosis than mice infected with WT conidia. These findings demonstrate that FleA is not a virulence factor for Aspergillus fumigatus. Instead, host recognition of FleA is a critical step in mechanisms of mucin binding, mucociliary clearance, and macrophage killing that prevent Aspergillus fumigatus pneumonia.

Pentavalent pillar[5]arene-based glycoclusters and their multivalent binding to pathogenic bacterial lectins

Anti-adhesive glycoclusters offer potential as therapeutic alternatives to classical antibiotics in treating infections. Pillar[5]arenes functionalised with either five galactose or five fucose residues were readily prepared using CuAAC reactions and evaluated for their binding to three therapeutically relevant bacterial lectins: LecA and Lec B from Pseudomonas aeuruginosa and BambL from Burkholderia ambifaria. Steric interactions were demonstrated to be a key factor in achieving good binding to LecA with more flexible galactose glycoclusters showing enhanced activity. In contrast binding to the fucose-selective lectins confirmed the importance of topology of the glycoclusters for activity with the pillar[5]arene ligand proving a selective ligand for BambL.

Anomer-Specific Recognition and Dynamics in a Fucose-Binding Lectin

Sugar binding by a cell surface ∼29 kDa lectin (RSL) from the bacterium Ralstonia solanacearum was characterized by NMR spectroscopy. The complexes formed with four monosaccharides and four fucosides were studied. Complete resonance assignments and backbone dynamics were determined for RSL in the sugar-free form and when bound to l-fucose or d-mannose. RSL was found to interact with both the α- and the β-anomer of l-fucose and the "fucose like" sugars d-arabinose and l-galactose. Peak splitting was observed for some resonances of the binding site residues. The assignment of the split signals to the α- or β-anomer was confirmed by comparison with the spectra of RSL bound to methyl-α-l-fucoside or methyl-β-l-fucoside. The backbone dynamics of RSL were sensitive to the presence of ligand, with the protein adopting a more compact structure upon binding to l-fucose. Taking advantage of tryptophan residues in the binding sites, we show that the indole resonance is an excellent reporter on ligand binding. Each sugar resulted in a distinct signature of chemical shift perturbations, suggesting that tryptophan signals are a sufficient probe of sugar binding.

Pillar[5]arene-Based Glycoclusters: Synthesis and Multivalent Binding to Pathogenic Bacterial Lectins

The synthesis of pillar[5]arene-based glycoclusters has been readily achieved by CuAAC conjugations of azido- and alkyne-functionalized precursors. The lectin binding properties of the resulting glycosylated multivalent ligands have been studied by at least two complementary techniques to provide a good understanding. Three lectins were selected from bacterial pathogens based on their potential therapeutic applications as anti-adhesives, namely LecA and LecB from Pseudomonas aeruginosa and BambL from Burkholderia ambifaria. As a general trend, multivalency improved the binding to lectins and a higher affinity can be obtained by increasing to a certain limit the length of the spacer arm between the carbohydrate subunits and the central macrocyclic core.

Importance of topology for glycocluster binding to Pseudomonas aeruginosa and Burkholderia ambifaria bacterial lectins

Pseudomonas aeruginosa (PA) and Burkholderia ambifaria (BA) are two opportunistic Gram negative bacteria and major infectious agents involved in lung infection of cystic fibrosis patients. Both bacteria can develop resistance to conventional antibiotherapies. An alternative strategy consists of targeting virulence factors in particular lectins with high affinity ligands such as multivalent glycoclusters. LecA (PA-IL) and LecB (PA-IIL) are two tetravalent lectins from PA that recognise galactose and fucose respectively. BambL lectin from BA is trimeric with 2 binding sites per monomer and is also specific for fucose. These three lectins are potential therapeutic targets in an anti-adhesive anti-bacterial approach. Herein, we report the synthesis of 18 oligonucleotide pentofuranose-centered or mannitol-centered glycoclusters leading to tri-, penta- or decavalent clusters with different topologies. The linker arm length between the core and the carbohydrate epitope was also varied leading to 9 galactoclusters targeting LecA and 9 fucoclusters targeting both LecB and BambL. Their dissociation constants (Kd) were determined using a DNA-based carbohydrate microarray technology. The trivalent xylo-centered galactocluster and the ribo-centered fucocluster exhibited the best affinity for LecA and LecB respectively while the mannitol-centered decafucocluster displayed the best affinity to BambL. These data demonstrated that the topology and nature of linkers were the predominant factors for achieving high affinity rather than valency.

Carbohydrate-protein interactions: molecular modeling insights

The article reviews the significant contributions to, and the present status of, applications of computational methods for the characterization and prediction of protein-carbohydrate interactions. After a presentation of the specific features of carbohydrate modeling, along with a brief description of the experimental data and general features of carbohydrate-protein interactions, the survey provides a thorough coverage of the available computational methods and tools. At the quantum-mechanical level, the use of both molecular orbitals and density-functional theory is critically assessed. These are followed by a presentation and critical evaluation of the applications of semiempirical and empirical methods: QM/MM, molecular dynamics, free-energy calculations, metadynamics, molecular robotics, and others. The usefulness of molecular docking in structural glycobiology is evaluated by considering recent docking- validation studies on a range of protein targets. The range of applications of these theoretical methods provides insights into the structural, energetic, and mechanistic facets that occur in the course of the recognition processes. Selected examples are provided to exemplify the usefulness and the present limitations of these computational methods in their ability to assist in elucidation of the structural basis underlying the diverse function and biological roles of carbohydrates in their dialogue with proteins. These test cases cover the field of both carbohydrate biosynthesis and glycosyltransferases, as well as glycoside hydrolases. The phenomenon of (macro)molecular recognition is illustrated for the interactions of carbohydrates with such proteins as lectins, monoclonal antibodies, GAG-binding proteins, porins, and viruses.

A recombinant fungal lectin for labeling truncated glycans on human cancer cells

Cell surface glycoconjugates present alterations of their structures in chronic diseases and distinct oligosaccharide epitopes have been associated with cancer. Among them, truncated glycans present terminal non-reducing β-N-acetylglucosamine (GlcNAc) residues that are rare on healthy tissues. Lectins from unconventional sources such as fungi or algi provide novel markers that bind specifically to such epitopes, but their availability may be challenging. A GlcNAc-binding lectin from the fruiting body of the fungus Psathyrella velutina (PVL) has been produced in good yield in bacterial culture. A strong specificity for terminal GlcNAc residues was evidenced by glycan array. Affinity values obtained by microcalorimetry and surface plasmon resonance demonstrated a micromolar affinity for GlcNAcβ1-3Gal epitopes and for biantennary N-glycans with GlcNAcβ1-2Man capped branches. Crystal structure of PVL complexed with GlcNAcβ1-3Gal established the structural basis of the specificity. Labeling of several types of cancer cells and use of inhibitors of glycan metabolism indicated that rPVL binds to terminal GlcNAc but also to sialic acid (Neu5Ac). Analysis of glycosyltransferase expression confirmed the higher amount of GlcNAc present on cancer cells. rPVL binding is specific to cancer tissue and weak or no labeling is observed for healthy ones, except for stomach glands that present unique αGlcNAc-presenting mucins. In lung, breast and colon carcinomas, a clear delineation could be observed between cancer regions and surrounding healthy tissues. PVL is therefore a useful tool for labeling agalacto-glycans in cancer or other diseases.

Three-dimensional structure and ligand-binding site of carp fishelectin (FEL)

Carp FEL (fishelectin or fish-egg lectin) is a 238-amino-acid lectin that can be purified from fish eggs by exploiting its selective binding to Sepharose followed by elution withN-acetylglucosamine. Its amino-acid sequence and other biochemical properties have previously been reported. The glycoprotein has four disulfide bridges and the structure of the oligosaccharides linked to Asn27 has been described. Here, the three-dimensional structures of apo carp FEL (cFEL) and of its complex withN-acetylglucosamine determined by X-ray crystallography at resolutions of 1.35 and 1.70 Å, respectively, are reported. The molecule folds as a six-bladed β-propeller and internal short consensus amino-acid sequences have been identified in all of the blades. A calcium atom binds at the bottom of the funnel-shaped tunnel located in the centre of the propeller. Two ligand-binding sites, α and β, are present in each of the two protomers in the dimer. The first site, α, is closer to the N-terminus of the chain and is located in the crevice between the second and the third blades, while the second site, β, is located between the fourth and the fifth blades. The amino acids that participate in the contacts have been identified, as well as the conserved water molecules in all of the sites. Both sites can bind the two anomers, α and β, ofN-acetylglucosamine, as is clearly recognizable in the electron-density maps. The lectin presents sequence homology to members of the tachylectin family, which are known to have a function in the innate immune system of arthropods, and homologous genes are present in the genomes of other fish and amphibians. This structure is the first of a protein of this group and, given the degree of homology with other members of the family, it is expected that it will be useful to experimentally determine other crystal structures using the coordinates of cFEL as a search probe in molecular replacement.

Structural insights into Aspergillus fumigatus lectin specificity: AFL binding sites are functionally non-equivalent

The Aspergillus fumigatus lectin AFL was recently described as a new member of the AAL lectin family. As a lectin from an opportunistic pathogen, it might play an important role in the interaction of the pathogen with the human host. A detailed study of structures of AFL complexed with several monosaccharides and oligosaccharides, including blood-group epitopes, was combined with affinity data from SPR and discussed in the context of previous findings. Its six binding sites are non-equivalent, and owing to minor differences in amino-acid composition they exhibit a marked difference in specific ligand recognition. AFL displays a high affinity in the micromolar range towards oligosaccharides which were detected in plants and also those bound on the human epithelia. All of these results indicate AFL to be a complex member of the lectin family and a challenging target for future medical research and, owing to its binding properties, a potentially useful tool in specific biotechnological applications.

Fucofullerenes as tight ligands of RSL and LecB, two bacterial lectins

A series of water-soluble glycofullerenes containing up to 24 fucose residues were tested against the two bacterial lectins LecB and RSL, and C60(E)12 bearing 24 fucose residues appeared to be the best known inhibitor of both lectins to date.

The carbohydrate-binding promiscuity of Euonymus europaeus lectin is predicted to involve a single binding site

Euonymus europaeus lectin (EEL) is a carbohydrate-binding protein derived from the fruit of the European spindle tree. EEL was first identified for its erythrocyte agglutinating properties and specificity for B and H blood groups. However, a detailed molecular picture of the structural basis of carbohydrate recognition by EEL remains to be developed. In this study, we performed fluorescence titrations of a range of carbohydrates against EEL. Binding of EEL to a wide range of carbohydrates was observed, including a series of blood group-related carbohydrates, mannosides, chitotriose and sialic acid. Affinity was strongest for carbohydrates with H-related structures and the B trisaccharide. A homology model of EEL was produced from templates identified using the HHPred server, which employs hidden Markov models (HMMs) to identify templates. The HMM approach identified that the best templates for EEL were proteins featuring a ricin B-like (R-type) fold. Separate templates were used to model the core and binding site regions of the lectin. Through the use of constrained docking and spatial comparison with a template ligand, binding modes for the carbohydrate ligands were predicted. A relationship between the experimental binding energies and the computed binding energies of the selected docked poses was determined and optimized. Collectively, our results suggest that EEL utilizes a single site for recognition of carbohydrates terminating in a variety of monosaccharides.

PNA-encoded synthesis (PES) of a 10 000-member hetero-glycoconjugate library and microarray analysis of diverse lectins

Identification of selective and synthetically tractable ligands to glycan-binding proteins is important in glycoscience. Carbohydrate arrays have had a tremendous impact on profiling glycan-binding proteins and as analytical tools. We report a highly miniaturized synthetic format to access nucleic-acid-encoded hetero-glycoconjugate libraries with an unprecedented diversity in the combinations of glycans, linkers, and capping groups. Novel information about plant and bacterial lectin specificity was obtained by microarray profiling, and we show that a ligand identified on the array can be converted to a high-affinity soluble ligand by straightforward chemistry.

Fucosylation of triethyleneglycol-based acceptors into 'clickable' α-fucosides

Design of multivalent glycoconjugates can find applications such as in anti-adhesive therapy against bacterial infections. Nevertheless, the access to such macromolecules requires functionalized building blocks prepared in a minimum number of steps and on a multi-gram scale at least for the laboratory. Fucose is a representative epitope used by several bacteria for adhesion to their host cells. The stereoselective, rapid, and efficient access to two 'clickable' α-fucosides was re-investigated using PPh3/CBr4-promoted glycosylation of chloro- (as precursors of azido-) and alkyne-functionalized triethyleneglycols with fully unprotected l-fucose. The convenient access to such building blocks paves the way to the design of new multivalent glycoconjugates functionalized with fucose epitopes and their applications.

Glycan arrays as tools for infectious disease research

Infectious diseases cause millions of deaths worldwide each year and are a major burden for economies, especially in underdeveloped countries. Glycans and their interactions with other biomolecules are involved in all major steps of infection. Glycan arrays enable the rapid and sensitive detection of those interactions and are among the most powerful techniques to study the molecular biology of infectious diseases. This review will focus on recent developments and discuss the applications of glycan arrays to the elucidation of host-pathogen and pathogen-pathogen interactions, the development of tools for infection diagnosis and the use of glycan arrays in modern vaccine design.

A soluble fucose-specific lectin from Aspergillus fumigatus conidia--structure, specificity and possible role in fungal pathogenicity

Aspergillus fumigatus is an important allergen and opportunistic pathogen. Similarly to many other pathogens, it is able to produce lectins that may be involved in the host-pathogen interaction. We focused on the lectin AFL, which was prepared in recombinant form and characterized. Its binding properties were studied using hemagglutination and glycan array analysis. We determined the specificity of the lectin towards l-fucose and fucosylated oligosaccharides, including α1-6 linked core-fucose, which is an important marker for cancerogenesis. Other biologically relevant saccharides such as sialic acid, d-mannose or d-galactose were not bound. Blood group epitopes of the ABH and Lewis systems were recognized, Le(Y) being the preferred ligand among others. To provide a correlation between the observed functional characteristics and structural basis, AFL was crystallized in a complex with methyl-α,L-selenofucoside and its structure was solved using the SAD method. Six binding sites, each with different compositions, were identified per monomer and significant differences from the homologous AAL lectin were found. Structure-derived peptides were utilized to prepare anti-AFL polyclonal antibodies, which suggested the presence of AFL on the Aspergillus' conidia, confirming its expression in vivo. Stimulation of human bronchial cells by AFL led to IL-8 production in a dose-dependent manner. AFL thus probably contributes to the inflammatory response observed upon the exposure of a patient to A. fumigatus. The combination of affinity to human epithelial epitopes, production by conidia and pro-inflammatory activity is remarkable and shows that AFL might be an important virulence factor involved in an early stage of A. fumigatus infection.

Reduction of lectin valency drastically changes glycolipid dynamics in membranes but not surface avidity

Multivalency is proposed to play a role in the strong avidity of lectins for glycosylated cell surfaces and also in their ability to affect membrane dynamics by clustering glycosphingolipids. Lectins with modified valency were designed from the β-propeller fold of Ralstonia solanacearum lectin (RSL) that presents six fucose binding sites. After identification of key amino acids by molecular dynamics calculations, two mutants with reduced valency were produced. Isothermal titration calorimetry confirmed the loss of three high affinity binding sites for both mutants. Crystal structures indicated that residual low affinity binding occurred in W76A but not in R17A. The trivalent R17A mutant presented unchanged avidity toward fucosylated surfaces, when compared to hexavalent RSL. However, R17A is not able anymore to induce formation of membrane invaginations on giant unilamellar vesicules, indicating the crucial role of number of binding sites for clustering of glycolipids. In the human lung epithelial cell line H1299, wt-RSL is internalized within seconds whereas the kinetics of R17A uptake is largely delayed. Neolectins with tailored valency are promising tools to study membrane dynamics.

Deciphering the glycan preference of bacterial lectins by glycan array and molecular docking with validation by microcalorimetry and crystallography

Recent advances in glycobiology revealed the essential role of lectins for deciphering the glycocode by specific recognition of carbohydrates. Integrated multiscale approaches are needed for characterizing lectin specificity: combining on one hand high-throughput analysis by glycan array experiments and systematic molecular docking of oligosaccharide libraries and on the other hand detailed analysis of the lectin/oligosaccharide interaction by x-ray crystallography, microcalorimetry and free energy calculations. The lectins LecB from Pseudomonas aeruginosa and BambL from Burkholderia ambifaria are part of the virulence factors used by the pathogenic bacteria to invade the targeted host. These two lectins are not related but both recognize fucosylated oligosaccharides such as the histo-blood group oligosaccharides of the ABH(O) and Lewis epitopes. The specificities were characterized using semi-quantitative data from glycan array and analyzed by molecular docking with the Glide software. Reliable prediction of protein/oligosaccharide structures could be obtained as validated by existing crystal structures of complexes. Additionally, the crystal structure of BambL/Lewis x was determined at 1.6 Å resolution, which confirms that Lewis x has to adopt a high-energy conformation so as to bind to this lectin. Free energies of binding were calculated using a procedure combining the Glide docking protocol followed by free energy rescoring with the Prime/Molecular Mechanics Generalized Born Surface Area (MM-GBSA) method. The calculated data were in reasonable agreement with experimental free energies of binding obtained by titration microcalorimetry. The established predictive protocol is proposed to rationalize large sets of data such as glycan arrays and to help in lead discovery projects based on such high throughput technology.

Antibacterial activity of a lectin-like Burkholderia cenocepacia protein

Bacteriocins of the LlpA family have previously been characterized in the γ-proteobacteria Pseudomonas and Xanthomonas. These proteins are composed of two MMBL (monocot mannose-binding lectin) domains, a module predominantly and abundantly found in lectins from monocot plants. Genes encoding four different types of LlpA-like proteins were identified in genomes from strains belonging to the Burkholderia cepacia complex (Bcc) and the Burkholderia pseudomallei group. A selected recombinant LlpA-like protein from the human isolate Burkholderia cenocepacia AU1054 displayed narrow-spectrum genus-specific antibacterial activity, thus representing the first functionally characterized bacteriocin within this β-proteobacterial genus. Strain-specific killing was confined to other members of the Bcc, with mostly Burkholderia ambifaria strains being susceptible. In addition to killing planktonic cells, this bacteriocin also acted as an antibiofilm agent.

Endolysins of Bacillus anthracis bacteriophages recognize unique carbohydrate epitopes of vegetative cell wall polysaccharides with high affinity and selectivity

Bacteriophages express endolysins which are the enzymes that hydrolyze peptidoglycan resulting in cell lysis and release of bacteriophages. Endolysins have acquired stringent substrate specificities, which have been attributed to cell wall binding domains (CBD). Although it has been realized that CBDs of bacteriophages that infect Gram-positive bacteria target cell wall carbohydrate structures, molecular mechanisms that confer selectivity are not understood. A range of oligosaccharides, derived from the secondary cell wall polysaccharides of Bacillus anthracis, has been chemically synthesized. The compounds contain an α-d-GlcNAc-(1→4)-β-d-ManNAc-(1→4)-β-d-GlcNAc backbone that is modified by various patterns of α-d-Gal and β-d-Gal branching points. The library of compounds could readily be prepared by employing a core trisaccharide modified by the orthogonal protecting groups N(α)-9-fluorenylmethyloxycarbonate (Fmoc), 2-methylnaphthyl ether (Nap), levulinoyl ester (Lev) and dimethylthexylsilyl ether (TDS) at key branching points. Dissociation constants for the binding the cell wall binding domains of the endolysins PlyL and PlyG were determined by surface plasmon resonance (SPR). It was found that the pattern of galactosylation greatly influenced binding affinities, and in particular a compound having a galactosyl moiety at C-4 of the nonreducing GlcNAc moiety bound in the low micromolar range. It is known that secondary cell wall polysaccharides of various bacilli may have both common and variable structural features and in particular differences in the pattern of galactosylation have been noted. Therefore, it is proposed that specificity of endolysins for specific bacilli is achieved by selective binding to a uniquely galactosylated core structure.