Multiple subfamilies of carbohydrate recognition domains in animal lectins

Calcium ion-dependent carbohydrate recognition domains (CRDs) are found in a range of proteins including receptors for serum glycoproteins and proteoglycans of the extracellular matrix. These C-type CRDs have homologous amino acid sequences characterized by the presence of certain invariant residues. Analysis of the genes for five of the proteins reveals that in each case the CRD-coding sequence is separated from the rest of the gene by an intron. The genes fall into two groups: those in which the coding sequence for the CRD is interrupted by two introns, and those in which the coding sequence is contained in a single exon. The sequences of domains in each category are consistent with the suggestion that the different gene structures reflect early evolutionary divergence of two subfamilies of C-type CRDs in animal lectins. However, carbohydrate-binding specificity does not directly parallel the evolutionary categorization. Comparison of the primary structures of CRDs in each subfamily which have related binding specificities may help to identify residues involved in ligating carbohydrates. This type of analysis is being extended by the use of bacterial expression systems to investigate in greater detail the binding characteristics of the CRDs.

Structural basis for selective recognition of oligosaccharides by DC-SIGN and DC-SIGNR

Dendritic cell specific intracellular adhesion molecule-3 (ICAM-3) grabbing nonintegrin (DC-SIGN), a C-type lectin present on the surface of dendritic cells, mediates the initial interaction of dendritic cells with T cells by binding to ICAM-3. DC-SIGN and DC-SIGNR, a related receptor found on the endothelium of liver sinusoids, placental capillaries, and lymph nodes, bind to oligosaccharides that are present on the envelope of human immunodeficiency virus (HIV), an interaction that strongly promotes viral infection of T cells. Crystal structures of carbohydrate-recognition domains of DC-SIGN and of DC-SIGNR bound to oligosaccharide, in combination with binding studies, reveal that these receptors selectively recognize endogenous high-mannose oligosaccharides and may represent a new avenue for developing HIV prophylactics.

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.

A novel mechanism of carbohydrate recognition by the C-type lectins DC-SIGN and DC-SIGNR. Subunit organization and binding to multivalent ligands

DC-SIGN and DC-SIGNR are cell-surface receptors that mediate cell-cell interactions within the immune system by binding to intercellular adhesion molecule-3. The receptor polypeptides share 77% amino acid sequence identity and are type II transmembrane proteins. The extracellular domain of each comprises seven 23-residue tandem repeats and a C-terminal C-type carbohydrate-recognition domain (CRD). Cross-linking, equilibrium ultracentrifugation, and circular dichroism studies of soluble recombinant fragments of DC-SIGN and DC-SIGNR have been used to show that the extracellular domain of each receptor is a tetramer stabilized by an alpha-helical stalk. Both DC-SIGN and DC-SIGNR bind ligands bearing mannose and related sugars through the CRDs. The CRDs of DC-SIGN and DC-SIGNR bind Man(9)GlcNAc(2) oligosaccharide 130- and 17-fold more tightly than mannose, and affinity for a glycopeptide bearing two such oligosaccharides is increased by a further factor of 5- to 25-fold. These results indicate that the CRDs contain extended or secondary oligosaccharide binding sites that accommodate mammalian-type glycan structures. When the CRDs are clustered in the tetrameric extracellular domain, their arrangement provides a means of amplifying specificity for multiple glycans on host molecules targeted by DC-SIGN and DC-SIGNR. Binding to clustered oligosaccharides may also explain the interaction of these receptors with the gp120 envelope protein of human immunodeficiency virus-1, which contributes to virus infection.

Lectin-like proteins in model organisms: implications for evolution of carbohydrate-binding activity

Classes of intracellular lectins that recognize core-type structures and mediate intracellular glycoprotein trafficking are present in vertebrates, model invertebrates such as Caenorhabditis elegans and Drosophila melanogaster, plants, and yeasts. Lectins that recognize more complex structures at the cell surface, such as C-type lectins and galectins, are also found in invertebrate organisms as well as vertebrates, but the functions of these proteins have evolved differently in different animal lineages.

Mechanism of pH-dependent N-acetylgalactosamine binding by a functional mimic of the hepatocyte asialoglycoprotein receptor

Efficient release of ligands from the Ca(2+)-dependent carbohydrate-recognition domain (CRD) of the hepatic asialoglycoprotein receptor at endosomal pH requires a small set of conserved amino acids that includes a critical histidine residue. When these residues are incorporated at corresponding positions in an homologous galactose-binding derivative of serum mannose-binding protein, the pH dependence of ligand binding becomes more like that of the receptor. The modified CRD displays 40-fold preferential binding to N-acetylgalactosamine compared with galactose, making it a good functional mimic of the asialoglycoprotein receptor. In the crystal structure of the modified CRD bound to N-acetylgalactosamine, the histidine (His(202)) contacts the 2-acetamido methyl group and also participates in a network of interactions involving Asp(212), Arg(216), and Tyr(218) that positions a water molecule in a hydrogen bond with the sugar amide group. These interactions appear to produce the preference for N-acetylgalactosamine over galactose and are also likely to influence the pK(a) of His(202). Protonation of His(202) would disrupt its interaction with an asparagine that serves as a ligand for Ca(2+) and sugar. The structure of the modified CRD without sugar displays several different conformations that may represent structures of intermediates in the release of Ca(2+) and sugar ligands caused by protonation of His(202).

Identification of amino acid residues that determine pH dependence of ligand binding to the asialoglycoprotein receptor during endocytosis

The rat hepatic asialoglycoprotein receptor mediates clearance of galactose- and N-acetylgalactosamine-terminated glycoproteins by endocytosis, binding ligands through a C-type, Ca(2+)-dependent carbohydrate-recognition domain (CRD) at extracellular pH and releasing them at lower pH in endosomes. At physiological Ca(2+) concentrations, the midpoint for ligand release from the CRD of the major subunit of the receptor is pH 7.1. In contrast, the midpoint is pH 5.0 for a galactose-binding derivative of the homologous C-type CRD of serum mannose-binding protein, which would thus not efficiently release ligand at an endosomal pH of 5.4. Site-directed mutagenesis of the CRD from the major subunit of the asialoglycoprotein receptor has been used to identify residues that are essential for efficient release of ligand at endosomal pH. The effects of changes to residues His(256), Asp(266), and Arg(270) singly and in combination indicate that these residues reduce the affinity of the CRD for Ca(2+), so that ligands are released at physiological Ca(2+) concentrations. The proximity of these three residues to the ligand-binding site at Ca(2+) site 2 of the domain suggests that they form a pH-sensitive switch for Ca(2+) and ligand binding. Introduction of histidine and aspartic acid residues into the mannose-binding protein CRD at positions equivalent to His(256) and Asp(266) raises the pH for half-maximal binding of ligand to 6.1. The results, as well as sequence comparisons with other C-type CRDs, confirm the importance of these residues in conferring appropriate pH dependence in this family of domains.

C-Type lectin-like domains in Caenorhabditis elegans: predictions from the complete genome sequence

Protein modules related to the C-type carbohydrate-recognition domains of animal lectins are found in at least 125 proteins encoded in the Caenorhabditis elegans genome. Within these proteins, 183 C-type lectin-like domains (CTLDs) have been identified. The proteins have been classified based on the overall arrangement of modules within the polypeptides and based on sequence similarity between the CTLDs. The C.elegans proteins generally have different domain organization from known mammalian proteins containing CTLDs. Most of the CTLDs are divergent in sequence from those in mammalian proteins. However, 19 show conservation of most of the amino acid residues that ligate Ca(2+)to form a carbohydrate-binding site in vertebrate C-type carbohydrate-recognition domains. Seven of these domains are particularly similar in sequence to mannose- and N-acetylglucosamine-binding domains in the vicinity of this Ca(2+)site.

C-type lectin-like domains

Carbohydrate-recognition domains of C-type (Ca2+-dependent) animal lectins serve as prototypes for an important family of protein modules. Only some domains in this family bind Ca2+ or sugars. A comparison of recent structures of C-type lectin-like domains reveals diversity in the modular fold, particularly in the region associated with Ca2+ and sugar binding. Some of this diversity reflects the changes that occur during normal physiological functioning of the domains. C-type lectin-like domains associate with each other through several different surfaces to form dimers and trimers, from which ligand-binding sites project in a variety of different orientations.

Molecular analysis of sialoside binding to sialoadhesin by NMR and site-directed mutagenesis

The molecular interactions between sialoadhesin and sialylated ligands have been investigated by using proton NMR. Addition of ligands to the 12 kDa N-terminal immunoglobulin-like domain of sialoadhesin result in resonance shifts in the protein spectrum that have been used to determine the affinities of sialoadhesin for several sialosides. The results indicate that alpha2, 3-sialyl-lactose and alpha2,6-sialyl-lactose bind respectively 2- and 1.5-fold more strongly than does alpha-methyl-N-acetylneuraminic acid (alpha-Me-NeuAc). The resonances corresponding to the methyl protons within the N-acetyl moiety of sialic acid undergo upfield shifting and broadening during titrations, reflecting an interaction of this group with Trp2 in sialoadhesin as observed in co-crystals of the terminal domain with bound ligand. This resonance shift was used to measure the affinities of mutant and wild-type forms of sialoadhesin in which the first three domains are fused to the Fc region of human IgG1. Substitution of Arg97 by alanine completely abrogated measurable interaction with alpha-Me-NeuAc, whereas a conservative substitution with lysine resulted in a 10-fold decrease in affinity. These results provide the first direct measurement of the affinity of sialoadhesin for sialosides and confirm the critical importance of the conserved arginine in interactions between sialosides and members of the siglec family of sialic acid-binding, immunoglobulin-like lectins.

Molecular determinants of oligomer formation and complement fixation in mannose-binding proteins

Rat serum mannose-binding protein (MBP-A) functions as part of the innate immune system by targetting complement toward potentially pathogenic microorganisms. In order to examine the molecular basis for complement activation, rat MBP-A has been overproduced in Chinese hamster ovary cells. Recombinant protein is post-translationally modified in the same way as the native lectin. Hydrodynamic studies indicate that MBP-A consists predominantly of covalent oligomers containing one to four copies of a subunit that comprises a trimer of polypeptides. These oligomers are non-interconverting and do not assemble into higher order structures at concentrations in excess of those normally found in serum. Disulfide bonds formed between cysteine residues at the N-terminal end of the collagen-like domain link polypeptides to form covalent oligomers. Analysis of wild-type MBP-A and MBP-A containing the substitution Cys6 --> Ser suggests that polypeptides within each trimeric structural unit are mostly linked by disulfide bonds between cysteine residues at positions 13 and 18 arranged in an asymmetrical configuration. Disulfide bonds involving Cys6 connect polypeptides within separate trimers. Analysis of chimeras between MBP-A and rat liver MBP (MBP-C) indicates that residues within the N-terminal region of the collagenous domain and the cysteine-rich domain of MBP-A enable assembly of trimers into higher order oligomers. The activity of MBP-A in a hemolytic complement fixation assay using mannan-coated sheep erythrocytes was approximately 20-fold greater than the activity of MBP-C. Analysis of the MBP chimeras and isolated oligomers of MBP-A reveals that the larger oligomers are more efficient at complement activation. These data indicate that the overall complement fixing activity of MBP-A is a function of the individual molecular activities of oligomers and their relative abundance within the serum.

Evolving views of protein glycosylation

The composition, biosynthesis and known roles of oligosaccharides that are attached to glycoproteins suggest that multiple forces have driven the evolution of proteins that create and recognize these structures. The evolution of glycoprotein biosynthesis and recognition mechanisms can be best understood as a sequential development of functions associated with oligosaccharides.

Mechanism of N-acetylgalactosamine binding to a C-type animal lectin carbohydrate-recognition domain

The mammalian hepatic asialoglycoprotein receptor, a member of the C-type animal lectin family, displays preferential binding to N-acetylgalactosamine compared with galactose. The structural basis for selective binding to N-acetylgalactosamine has been investigated. Regions of the carbohydrate-recognition domain of the receptor believed to be important in preferential binding to N-acetylgalactosamine have been inserted into the homologous carbohydrate-recognition domain of a mannose-binding protein mutant that was previously altered to bind galactose. Introduction of a single histidine residue corresponding to residue 256 of the hepatic asialoglycoprotein receptor was found to cause a 14-fold increase in the relative affinity for N-acetylgalactosamine compared with galactose. The relative ability of various acyl derivatives of galactosamine to compete for binding to this modified carbohydrate-recognition domain suggest that it is a good model for the natural N-acetylgalactosamine binding site of the asialoglycoprotein receptor. Crystallographic analysis of this mutant carbohydrate-recognition domain in complex with N-acetylgalactosamine reveals a direct interaction between the inserted histidine residue and the methyl group of the N-acetyl substituent of the sugar. Evidence for the role of the side chain at position 208 of the receptor in positioning this key histidine residue was obtained from structural analysis and mutagenesis experiments. The corresponding serine residue in the modified carbohydrate-recognition domain of mannose-binding protein forms a hydrogen bond to the imidazole side chain. When this serine residue is changed to valine, loss in selectivity for N-acetylgalactosamine is observed. The structure of this mutant reveals that the beta-branched valine side chain interacts directly with the histidine side chain, resulting in an altered imidazole ring orientation.

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.

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.

Asymmetry adjacent to the collagen-like domain in rat liver mannose-binding protein

Rat liver mannose-binding protein (MBP-C) is the smallest known member of the collectin family of animal lectins, many of which are involved in defence against microbial pathogens. It consists of an N-terminal collagen-like domain linked to C-terminal carbohydrate-recognition domains. MBP-C, overproduced in Chinese-hamster ovary cells, is post-translationally modified and processed in a manner similar to the native lectin. Analytical ultracentrifugation experiments indicate that MBP-C is trimeric, with a weight-averaged molecular mass of approx. 77 kDa. The rate of sedimentation of MBP-C and its mobility on gel filtration suggest a highly elongated molecule. Anomalous behaviour on gel filtration due to this extended conformation may explain previous suggestions that MBP-C forms a higher oligomer. The polypeptide chains of the MBP-C trimer are linked by disulphide bonds between two cysteine residues at the N-terminal junction of the collagen-like domain. Analysis of an N-terminal tryptic fragment reveals that the disulphide bonding in MBP-C is heterogeneous and asymmetrical. These results indicate that assembly of MBP-C oligomers probably proceeds in a C- to N-terminal direction: trimerization at the C-terminus is followed by assembly of the collagenous domain and finally formation of N-terminal disulphide bonds. The relatively simple organization of MBP-C provides a template for understanding larger, more complex collectins.

Selective binding of N-acetylglucosamine to the chicken hepatic lectin

Among Ca2+-dependent (C-type) animal lectins, the chicken hepatic lectin (CHL) is unique in displaying almost complete selectivity for N-acetylglucosamine over other monosaccharide ligands. The crystal structures of the carbohydrate-recognition domain (CRD) from serum mannose-binding protein (MBP) and of a complex between the CRD from liver MBP and the methyl glycoside of N-acetylglucosamine were used to model the binding site in CHL. Substitution of portions of CHL into the MBP framework did not substantially increase selectivity. A bacterial expression system for the CRD of CHL was developed so that specific residues predicted to be near the 2-acetamido substituent of N-acetylglucosamine could be altered by site-directed mutagenesis. The results indicate that the ligand is bound to CHL in the same orientation as it binds to liver MBP. A tyrosine and a valine residue that probably contact the the N-acetyl group have been identified. These results, together with studies of ligand-binding selectivity, suggest that these residues form part of a binding pocket for the N-acetyl group, which confers selective binding of N-acetylglucosamine.

Making a fitting choice: common aspects of sugar-binding sites in plant and animal lectins

Comparing sequences of plant and animal lectins reveals that the ability to bind any one type of sugar has evolved several times independently in diverse protein frameworks. Conversely, families of lectins that share common structural features often contain members that recognize different groups of sugars. In the context of this combination of convergent and divergent evolution, our knowledge of the structures of lectin-sugar complexes provides valuable insights into the principles that underlie specific sugar binding.

Differential ligand binding by two subunits of the rat liver asialoglycoprotein receptor

The rat liver asialoglycoprotein receptor consists of two types of subunits, a predominant polypeptide designated rat hepatic lectin 1 (RHL-1) and a minor polypeptide, RHL-2/3, that comes in two differentially glycosylated forms. The exact stoichiometry and arrangement of the subunits in the RHL oligomer are not known. The carbohydrate-recognition domain of RHL-2/3 has been prepared by limited proteolysis of the liver receptor so that its properties can be compared with those of the corresponding domain of RHL-1 previously produced in a bacterial expression system. Binding studies indicate that while RHL-1 binds N-acetylgalactosamine with approximately 60-fold higher affinity than it binds galactose, RHL-2/3 has only 2-fold selectivity for N-acetylgalactosamine. In general, the pattern of monosaccharide-binding specificity for RHL-2/3 is similar to RHL-1, but the discrimination of various sugars relative to galactose is reduced substantially. Limited proteolysis and crosslinking studies demonstrate that RHL-2/3 is easily removed from the RHL oligomer in detergent solution and that RHL-1 remains at least trimeric following removal of RHL-2/3. These studies suggest that RHL-1 forms a ligand-binding core while RHL-2/3 acts more as an accessory subunit contributing to selective binding of certain oligosaccharide structures.

Structural basis of lectin-carbohydrate recognition

Lectins are responsible for cell surface sugar recognition in bacteria, animals, and plants. Examples include bacterial toxins; animal receptors that mediate cell-cell interactions, uptake of glycoconjugates, and pathogen neutralization; and plant toxins and mitogens. The structural basis for selective sugar recognition by members of all of these groups has been investigated by x-ray crystallography. Mechanisms for sugar recognition have evolved independently in diverse protein structural frameworks, but share some key features. Relatively low affinity binding sites for monosaccharides are formed at shallow indentations on protein surfaces. Selectivity is achieved through a combination of hydrogen bonding to the sugar hydroxyl groups with van der Waals packing, often including packing of a hydrophobic sugar face against aromatic amino acid side chains. Higher selectivity of binding is achieved by extending binding sites through additional direct and water-mediated contacts between oligosaccharides and the protein surface. Dramatically increased affinity for oligosaccharides results from clustering of simple binding sites in oligomers of the lectin polypeptides. The geometry of such oligomers helps to establish the ability of the lectins to distinguish surface arrays of polysaccharides in some instances and to crosslink glycoconjugates in others.

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

The structures of the ligand-binding C-type carbohydrate-recognition domains of selectin cell adhesion molecules and of mannose-binding proteins (MBPs) are similar to each other even though these proteins bind very different carbohydrate ligands. Our current understanding of ligand binding by E-selectin is based on structural studies of unliganded E-selectin and of MBP-carbohydrate complexes, combined with results from mutagenesis of E-selectin. Five regions of E-selectin that differ in sequence from the corresponding regions of MBP have been introduced into the carbohydrate-recognition domain of MBP. Four of the changes have little effect on ligand binding. Insertion of one stretch of positively charged amino acids alters the sugar binding selectivity of the domain so that it now binds HL-60 cells and serum albumin derivatized with sialyl-Lewis X tetrasaccharide, thus mimicking the properties of E-selectin.

Selective sugar binding to the carbohydrate recognition domains of the rat hepatic and macrophage asialoglycoprotein receptors

Asialoglycoprotein receptors on the surfaces of both hepatocytes and peritoneal macrophages bind terminal galactose residues of desialylated glycoproteins and mediate endocytosis and eventual degradation of these ligands. The hepatic receptor binds oligosaccharides with terminal N-acetylgalactosamine residues more tightly than ligands with terminal galactose residues, but the macrophage receptor shows no such differential binding affinity. Carbohydrate recognition domains from the macrophage receptor and the major subunit of the hepatic receptor have been expressed in a bacterial system and have been shown to retain the distinct binding selectivities of the receptors from which they derive. Binding of a series of N-acyl derivatives of galactosamine suggests that the 2-substituent of these sugars interacts with the surface of the hepatic receptor with highest affinity binding observed for the N-propionyl derivative. Chimeric sugar-binding domains have been used to identify three regions of the hepatic receptor that are essential for establishing selectivity for N-acetylgalactosamine over galactose. Based on these results and the orientation of N-acetylgalactosamine when bound to an homologous galactose-binding mutant of rat serum mannose-binding protein, a fourth region likely to interact with N-acetylgalactosamine has been identified and probed by site-directed mutagenesis. The results of these studies define a binding pocket for the 2-substituent of N-acetylgalactosamine in the hepatic asialoglycoprotein receptor.