Introduction of selectin-like binding specificity into a homologous mannose-binding protein

The C-type lectin-like domain superfamily

The superfamily of proteins containing C-type lectin-like domains (CTLDs) is a large group of extracellular Metazoan proteins with diverse functions. The CTLD structure has a characteristic double-loop ('loop-in-a-loop') stabilized by two highly conserved disulfide bridges located at the bases of the loops, as well as a set of conserved hydrophobic and polar interactions. The second loop, called the long loop region, is structurally and evolutionarily flexible, and is involved in Ca2+-dependent carbohydrate binding and interaction with other ligands. This loop is completely absent in a subset of CTLDs, which we refer to as compact CTLDs; these include the Link/PTR domain and bacterial CTLDs. CTLD-containing proteins (CTLDcps) were originally classified into seven groups based on their overall domain structure. Analyses of the superfamily representation in several completely sequenced genomes have added 10 new groups to the classification, and shown that it is applicable only to vertebrate CTLDcps; despite the abundance of CTLDcps in the invertebrate genomes studied, the domain architectures of these proteins do not match those of the vertebrate groups. Ca2+-dependent carbohydrate binding is the most common CTLD function in vertebrates, and apparently the ancestral one, as suggested by the many humoral defense CTLDcps characterized in insects and other invertebrates. However, many CTLDs have evolved to specifically recognize protein, lipid and inorganic ligands, including the vertebrate clade-specific snake venoms, and fish antifreeze and bird egg-shell proteins. Recent studies highlight the functional versatility of this protein superfamily and the CTLD scaffold, and suggest further interesting discoveries have yet to be made.

Sialyl Lewis(x) analogs based on a quinic acid scaffold as the fucose mimic

(-)-Quinic acid was used as a starting material for the preparation of sialyl Lewis(x) mimetics in order to target E-selectin. Spatial orientation of the hydroxyl groups of quinic acid could mimic the l-fucose ones. Introduction of a side chain ending with a carboxylic acid was effected to replace the sialic acid interaction at the carbohydrate recognition domain. A first series of derivatives, incorporating amino acids linked to quinic acid, were tested for their affinity and found to interact with E-selectin with IC(50) within the millimolar range.

Tools for Glycomics: Mapping Interactions of Carbohydrates in Biological Systems

The emerging field of glycomics has been challenged by difficulties associated with studying complex carbohydrates and glycoconjugates. Advances in the development of synthetic tools for glycobiology are poised to overcome some of these challenges and accelerate progress towards our understanding of the roles of carbohydrates in biology. Carbohydrate microarrays, fluorescent neoglycoconjugate probes, and aminoglycoside antibiotic microarrays are among the many new tools becoming available to glycobiologists.

An endogenous Drosophila receptor for glycans bearing alpha 1,3-linked core fucose residues

The genome of Drosophila melanogaster encodes several proteins that are predicted to contain Ca(2+)-dependent, C-type carbohydrate-recognition domains. The CG2958 gene encodes a protein containing 359 amino acid residues. Analysis of the CG2958 sequence suggests that it consists of an N-terminal domain found in other Drosophila proteins, a middle segment that is unique, and a C-terminal C-type carbohydrate-recognition domain. Expression studies show that the full-length protein is a tetramer formed by noncovalent association of disulfide-linked dimers that are linked through cysteine residues in the N-terminal domain. The expressed protein binds to immobilized yeast invertase through the C-terminal carbohydrate-recognition domain. Competition binding studies using monosaccharides demonstrate that CG2958 interacts specifically with fucose and mannose. Fucose binds approximately 5-fold better than mannose. Blotting studies reveal that the best glycoprotein ligands are those that contain N-linked glycans bearing alpha1,3-linked fucose residues. Binding is enhanced by the additional presence of alpha1,6-linked fucose. It has previously been proposed that labeling of the Drosophila neural system by anti-horseradish peroxidase antibodies is a result of the presence of difucosylated N-linked glycans. CG2958 is a potential endogenous receptor for such neural-specific carbohydrate epitopes.

Minimal requirements for the binding of selectin ligands to a C-type carbohydrate-recognition domain

The C-type carbohydrate-recognition domains of E-selectin and rat serum mannose-binding protein have similar structures. Selectin/mannose-binding protein chimeras created by transfer of key sequences from E-selectin into mannose-binding protein have previously been shown to bind the selectin ligand sialyl-Lewis(X) through a Ca(2+)-dependent subsite, common to many C-type lectins, and an accessory site containing positively charged amino acid residues. Further characterization of these chimeras as well as analysis of novel constructs containing additional regions of E-selectin demonstrate that selectin-like interaction with sialyl-Lewis(X) can be faithfully reproduced even though structural evidence indicates that the mechanisms of binding to E-selectin and the chimeras are different. Selectin-like binding to the nonfucosylated sulfatide and sulfoglucuronyl glycolipids can also be reproduced with selectin/mannose-binding protein chimeras that contain the two subsites involved in sialyl-Lewis(X) binding. These results indicate that binding of structurally distinct anionic glycans to C-type carbohydrate-recognition domains can be mediated by the Ca(2+)-dependent subsite in combination with a positively charged region that forms an ionic strength-sensitive subsite.

Novel carbohydrate binding site recognizing blood group A and B determinants in a hybrid of cholera toxin and Escherichia coli heat-labile enterotoxin B-subunits

The B-subunits of cholera toxin (CTB) and Escherichia coli heat-labile enterotoxin (LTB) are structurally and functionally related. However, the carbohydrate binding specificities of the two proteins differ. While both CTB and LTB bind to the GM1 ganglioside, LTB also binds to N-acetyllactosamine-terminated glycoconjugates. The structural basis of the differences in carbohydrate recognition has been investigated by a systematic exchange of amino acids between LTB and CTB. Thereby, a CTB/LTB hybrid with a gain-of-function mutation resulting in recognition of blood group A and B determinants was obtained. Glycosphingolipid binding assays showed a specific binding of this hybrid B-subunit, but not CTB or LTB, to slowly migrating non-acid glycosphingolipids of human and animal small intestinal epithelium. A binding-active glycosphingolipid isolated from cat intestinal epithelium was characterized by mass spectrometry and proton NMR as GalNAcalpha3(Fucalpha2)Galbeta4(Fucalpha3)Glc NAcbeta3Galbeta4Glc NAcbeta3Galbeta4Glcbeta1Cer. Comparison with reference glycosphingolipids showed that the minimum binding epitope recognized by the CTB/LTB hybrid was Galalpha3(Fucalpha2)Galbeta4(Fucalpha3)GlcNAc beta. The blood group A and B determinants bind to a novel carbohydrate binding site located at the top of the B-subunit interfaces, distinct from the GM1 binding site, as found by docking and molecular dynamics simulations.

The C-type lectin superfamily in the immune system

Protein-carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar-binding proteins) mediate both pathogen recognition and cell-cell interactions using structurally related Ca(2+)-dependent carbohydrate-recognition domains (C-type CRDs). Pathogen recognition by soluble collections such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cell-surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non-self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C-type lectin-like domains (CTLDs) that are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.

Crystal structure of the N-terminal domain of sialoadhesin in complex with 3' sialyllactose at 1.85 A resolution

The structure of the functional N-terminal domain from the extracellular region of the cell surface receptor sialoadhesin has been determined in complex with the oligosaccharide 3' sialyllactose. This provides structural information for the siglec family of proteins. The structure conforms to the V-set immunoglobulin-like fold but contains several distinctive features, including an intra-beta sheet disulphide and a splitting of the standard beta strand G into two shorter strands. These novel features appear important in adapting the V-set fold for sialic acid-mediated recognition. Analysis of the complex with 3'sialyllactose highlights three residues, conserved throughout the siglec family, as key features of the sialic acid-binding template. The complex is representative of the functional recognition interaction with carbohydrate and as such provides detailed information for a heterotypic cell adhesion interaction.

Mechanism of ligand binding to E- and P-selectin analyzed using selectin/mannose-binding protein chimeras

The mechanism of oligosaccharide binding to the selectin cell adhesion molecules has been analyzed by transferring regions of the carbohydrate-recognition domains of E- and P-selectin into corresponding sites in the homologous rat serum mannose-binding protein. Insertion of two basic regions and an adjacent glutamic acid residue leads to efficient binding of HL-60 cells and sialyl-Lewisx-conjugated serum albumin. Substitution of glycine for a histidine residue known to stabilize mannose in the binding site of wild type mannose-binding protein results in dramatic loss of affinity for mannose without decreasing binding to sialyl-Lewisx. The accumulated effect of these changes is to alter the ligand binding selectivity of the domain so that it resembles E- or P-selectin more closely than it resembles the parental mannose-binding domain. Affinity labeling using sialyl-Lewisx in which the sialic acid has been mildly oxidized has been used to verify this switch in specificity and to show that the sialic acid-containing portion of the ligand interacts near the sequence Lys-Lys-Lys corresponding to residues 111-113 of E-selectin. The binding of sialyl-Lewisx-serum albumin is inhibited dramatically at physiological and higher salt concentrations, consistent with a significant electrostatic component to the binding interaction. The binding characteristics of these gain-of-function chimeras suggest that they contain many of the selectin residues responsible for selective ligand binding.

Conserved basic residues in the C-type lectin and short complement repeat domains of the G3 region of proteoglycans

Aggrecan is the major proteoglycan of the extracellular matrix in cartilage. It contains two N-terminal globular regions, G1 and G2, and one C-terminal globular region, G3. G3 is implicated in the intracellular processing of aggrecan and contains a C-type lectin carbohydrate recognition domain (CRD), frequent occurrences of a C-terminal short complement repeat (SCR) domain, and occasionally an N-terminal epidermal growth factor domain. The CRD and SCR domains in 13 G3 sequences were each subjected to structural analysis. Alignment of 131 sequences from all seven groups in the CRD superfamily defined a consensus length of 136 residues, in which 32% of residues were conserved. Although the G3 CRD sequences agreed with this consensus, they also contained five fully conserved basic residues that are atypical of the CRD superfamily. Homology modelling showed that four of these residues are located on a surface region on the CRD that is separate from the Ca2+-binding residues involved in carbohydrate interactions. One conserved basic residue is identical in position with that of a conserved basic residue that mediates hyaluronate binding in the structurally related proteoglycan tandem repeat (PTR) domain in G1 and in link protein. The alignment of 13 G3 SCR sequences with 101 sequences in the SCR superfamily showed good agreement with conserved residues in the SCR superfamily. There are also five conserved basic residues in the G3 SCR that are atypical of the SCR superfamily, and homology modelling showed that all five were located on one surface of the SCR. It is concluded that both the CRD and SCR domains in G3 possess basic residues that are atypical of their superfamilies and might be related to function, and that the G3 CRD domain shows an evolutionary relationship to the PTR domain in G1.

Cell-surface carbohydrate recognition by animal and viral lectins

Many animal and viral lectins are specific for monosaccharides found in particular glycosidic linkages, or for larger oligosaccharide structures. Recent crystal structures of complexes between these proteins and receptor fragments have provided insights into the recognition of linkage isomers and oligosaccharide conformation.

Structure of a selectin-like mutant of mannose-binding protein complexed with sialylated and sulfated Lewis(x) oligosaccharides

Rat serum mannose-binding protein in which residues 211-213 have been changed to the Lys-Lys-Lys sequence found in E-selectin binds HL-60 cells and the oligosaccharide 3'-NeuAc-Le(x). To understand how this mutant, designated K3, mimics the carbohydrate-binding properties of E-selectin, structures of K3 alone and in complexes with 3'-NeuAc-Le(x), 3'-sulfo-Le(x) and 4'-sulfo-Le(x) have been determined at 1.95-2.1 A resolution by X-ray crystallography. The region of K3 that interacts with bound oligosaccharides superimposes closely with the corresponding region of unliganded E-selectin. In each of the oligosaccharide-protein complexes, the 2- and 3-OH of Fuc coordinate Ca2+ and form a network of cooperative hydrogen bonds with amino acid side chains that also coordinate the Ca2+. Lys211 of the K3 mutant, which corresponds to Lys111 of E-selectin, interacts with each of the three bound ligands: the N zeta atom donates a hydrogen bond to the 4-OH of Gal in 3'-NeuAc-Le(x), forms a water-mediated hydrogen bond with the 4-OH of Gal in 3'-sulfo-Le(x), and forms a salt bridge with the sulfate group of 4'-sulfo-Le(x). Lys213 packs against an otherwise exposed aromatic residue and forms a water-mediated hydrogen bond with Lys211 which may help to position that residue for interactions with bound oligosaccharides. These structures are consistent with previous mutagenesis and chemical modification studies which demonstrate the importance of the Ca2+ ligands as well as Lys111 and Lys113 for carbohydrate binding in the selectins, and they provide a structural basis for understanding the selective recognition of negatively charged Le(x) derivatives by the selectins.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

Selectin-ligand interactions revealed by molecular dynamics simulation in solution

Through a computer modeling and simulation technique, we investigated the binding mode of a complex of E-selectin-GSC-150, which is a novel selectin blocker. GSC-150 is the 3'-sulfated Lewis X derivative with a long, branched alkyl chain. Initial attempts to construct a model for E-selectin-GSC-150 complex were performed based on a previously reported model of E-selectin-sialyl Lewis X (sLex) complex [Kogan, T.P.; Revelle, B.M.; Tapp, S.; Scott, D.; Beck, P.J.J. Biol. Chem. 1995, 270, 14047-14055]. In our model, the carbohydrate portion of GSC-150 interacted with the protein in a similar manner as that of sLex reported previously. Interestingly, each of the branched alkyl chains extended on the surface of E-selectin and interacted with two different hydrophobic portions. One of these hydrophobic portions consists of Tyr44, Pro46, and Tyr48. Another portion forms a shallow cavity, and it consists of Ala9, Leu114, and the alkyl moieties of the side chains of Lys111, Lys112, and Lys113. A subsequent 200-ps molecular dynamics simulation in solution revealed that the interactions involved in the sugar portion of the ligand were relatively weak, whereas the hydrophobic interactions involved in the branched alkyl chains were fairly stable in solution. These results suggest that the branched alkyl chain serves as an "anchor" for the tight binding of GSC-150 on the surface of E-Selectin. This is the first attempt to evaluate the dynamics of E-Selectin-ligand interactions in solution, and it sheds light on the nature of ligand recognition by selectins.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.

Carbohydrate recognition systems: functional triads in cell-cell interactions

Considerable progress is being made in our understanding of the molecular basis for mammalian carbohydrate recognition systems. Selectins, related proteins and sialoadhesins are carbohydrate-binding proteins which serve as receptors in the orchestration of innate and acquired immune responses, inflammation and other forms of cell-cell communication. Protein structural studies and gain-of-function and loss-of-function mutations are providing clues to ways in which the receptors interact with monosaccharide elements of the oligosaccharide ligands. Binding experiments using oligosaccharides on lipid or protein carriers indicate that modes of presentation such as the clustered state and the manner of display on proteins are crucial factors determining whether a functional triad of receptor and ligand + carrier (counter-receptor) is formed.

A molecular model of the carbohydrate recognition domain of a rat macrophage lectin and analysis of its binding site

A three-dimensional model of the carbohydrate recognition domain of a rat macrophage C-type lectin has been constructed by comparative modeling and assessed by inverse folding analysis. Comparative modeling in the presence of low sequence similarity was based on information provided by comparison of X-ray structures and sequence-structure alignments. The sequence-structure compatibility of the model was sound. Its binding site was analyzed in comparison to the X-ray structure of a galactose-specific mutant of the mannose-binding protein. The specificity of the macrophage lectin was discussed in light of mutagenesis data on asialoglycoprotein receptors.

Three-dimensional structure of cell adhesion molecules

Recent structural data have provided insights into various forms of specific cell adhesion interactions, including both protein-protein and protein-glycoconjugate recognition events. The major advances have been made in the structural characterization of cadherin-cadherin and integrin-ligand mediated adhesion.