Recognition of Lewis X by Anti-Lex Monoclonal Antibody IG5F6

mAbs directed toward the Lewis X (Le^x) determinant have been shown to display different specificities, depending on the presentation of Le^x to the immune system. Of interest is the murine anti-Le^x mAb IG5F6, generated against the O chain polysaccharide of Helicobacter pylori that contains polymeric Le^x structures. The mAb was found to have a higher affinity for polymeric Le^x over monomeric Le^x In this study, we explore the recognition of monomeric Le^x by IG5F6 using a panel of Le^x analogues in which N-acetyl-d-glucosamine, l-fucose, or d-galactose (D-Gal) are replaced with d-glucose and/or l-rhamnose. Our studies show that all analogues were weaker inhibitors than the Le^x Ag, indicating that all three residues are essential in the recognition of Le^x by mAb IG5F6. We explored the involvement of 4″-OH of d-Gal in the binding with IG5F6 using a panel of 4″-modified Le^x analogues. Although the 4″-OH is only involved in a weak polar interaction, we conclude that the D-Gal residue in Le^x is primarily involved in aromatic stacking interactions with the Ab binding site. We compared these results to our work with mAb SH1. Although stacking interactions between D-Gal and an aromatic residue was also suggested for SH1, an H-bond involving the 4″-OH was identified that is not found in the binding of IG5F6 to Le^x Thus, anti-Le^x mAbs SH1 and IG5F6 bind to Le^x in different manners, even though the hydrophobic patch displayed by the β-galactoside in Le^x is essential in both cases for their binding to Le^x.

Differential affinities of Erythrina cristagalli lectin (ECL) toward monosaccharides and polyvalent mammalian structural units

Previous studies on the carbohydrate specificities of Erythrina cristagalli lectin (ECL) were mainly limited to analyzing the binding of oligo-antennary Galbeta1-->4GlcNAc (II). In this report, a wider range of recognition factors of ECL toward known mammalian ligands and glycans were examined by enzyme-linked lectinosorbent and inhibition assays, using natural polyvalent glycotopes, and a glycan array assay. From the results, it is shown that GalNAc was an active ligand, but its polyvalent structural units, in contrast to those of Gal, were poor inhibitors. Among soluble natural glycans tested for 50% molecular mass inhibition, Streptococcus pneumoniae type 14 capsular polysaccharide of polyvalent II was the most potent inhibitor; it was 2.1 x 10(4), 3.9 x 10(3) and 2.4 x 10(3) more active than Gal, tri-antennary II and monomeric II, respectively. Most type II-containing glycoproteins were also potent inhibitors, indicating that special polyvalent II and Galbeta1-related structures play critically important roles in lectin binding. Mapping all information available, it can be concluded that: [a] Galbeta1-->4GlcNAc (II) and some Galbeta1-related oligosaccharides, rather than GalNAc-related oligosaccharides, are the core structures for lectin binding; [b] their polyvalent II forms within macromolecules are a potent recognition force for ECL, while II monomer and oligo-antennary II forms play only a limited role in binding; [c] the shape of the lectin binding domains may correspond to a cavity type with Galbeta1-->4GlcNAc as the core binding site with additional one to four sugars subsites, and is most complementary to a linear trisaccharide, Galbeta1-->4GlcNAcbeta1-->6Gal. These analyses should facilitate the understanding of the binding function of ECL.

Isolating and characterising a lectin from Galactia lindenii seeds that recognises blood group H determinants

A lectin was isolated from Galactia lindenii seeds and characterised. The lectin, purified by affinity chromatography, readily agglutinated O(H) human erythrocytes and interacted weakly with rabbit and rat erythrocytes. Specificity towards blood group H-type determinants was established; among them H-type 2 (alpha-L-Fuc (1-2)-beta-D-Gal (1-4)-beta-D-GlcNAc-O-R) was recognised by the lectin. The binding to the glycoconjugate was partially inhibited by GalNAc and Me-beta-Gal. The protein is an M=104,256 tetramer which dissociates into identical M=26,064 subunits under non-reducing conditions. Its amino acid composition, pI, A(1%), and N-terminal sequence (23 residues) were determined. The N-terminal region showed a unique sequence found hitherto only in some lectins (designated type-II) from the Dioclea genus. This work presents the evidence concerning a distinct type of lectin found in the Diocleinae tribe able to recognise the H-type 2 human blood group determinant and clearly different from the Glc/Man-specific lectins. The protein is a potential tool in cellular and histochemical studies.

Probing genetic variation and glycoform distribution in lectins of the Erythrina genus by mass spectrometry

Six leguminous lectins from the seeds of plants of the Erythrina genus, namely E. caffra (ECafL), E. cristagalli (ECL), E. flabelliformis (EFL), E. lysistemon (ELysL), E. rubrinerva (ERL), and E. vespertilio (EVL), were examined to establish their sequence homology and to determine the structure and sites of attachment of their glycans. Tryptic digests of these lectins were analyzed by capillary electrophoresis coupled to electrospray mass spectrometry (CE-ESMS). Assignments were made by comparing the molecular masses of the observed tryptic peptides with those of Erythrina corallodendron lectin (ECorL), the sequence of which had been established previously. Glycan structure and genetic variations in the amino acid sequence were probed by tandem mass spectrometry. Small differences were found between the sequences of the various lectins examined and all of them exhibited C-terminal processing resulting in proteins with a C-terminal Asn residue. The major glycan of these glycoproteins was shown to be the heptasaccharide Man(3)XylFucGlcNAc(2), consistent with previous investigations on ECL and ECorL. A minor glycan heterogeneity was observed for most lectins examined except for that of ECafL and ECorL where an extra hexose residue was observed on the reducing GlcNAc residue of the heptasaccharide.

How proteins bind carbohydrates: lessons from legume lectins

The pioneering studies of Irvin Liener on soybean agglutinin (SBA) in the early 1950s served as the starting point of our involvement in lectin research during the past four decades. Initially we characterized SBA extensively as a glycoprotein and showed that its covalently linked glycan is an oligomannoside commonly present in animal glycoproteins. We have also introduced the use of the lectin to the study of normal and malignant cells and to the purging of bone marrow for transplantation. Our recent work focuses on the combining site of Erythrina corallodendron lectin, closely related to SBA. In this legume lectin, as in essentially all other members of the same protein family, irrespective of their sugar specificity, interactions with a constellation of three invariant residues (aspartic acid, asparagine, and an aromatic residue) are essential for ligand binding. Lectins from other families, whether of plants or animals, also combine with carbohydrates by H-bonds and hydrophobic interactions, but the amino acids involved may differ even if the specificity of the lectins is the same. Therefore, nature finds diverse solutions for the design of binding sites for structurally similar ligands, such as mono- or oligosaccharides. This diversity strongly suggests that lectins are products of convergent evolution.

High-resolution crystal structures of Erythrina cristagalli lectin in complex with lactose and 2'-alpha-L-fucosyllactose and correlation with thermodynamic binding data

The primary sequence of Erythrina cristagalli lectin (ECL) was mapped by mass spectrometry, and the crystal structures of the lectin in complex with lactose and 2'-alpha-L-fucosyllactose were determined at 1.6A and 1.7A resolution, respectively. The two complexes were compared with the crystal structure of the closely related Erythrina corallodendron lectin (ECorL) in complex with lactose, with the crystal structure of the Ulex europaeus lectin II in complex with 2'-alpha-L-fucosyllactose, and with two modeled complexes of ECorL with 2'-alpha-L-fucosyl-N-acetyllactosamine. The molecular models are very similar to the crystal structure of ECL in complex with 2'-alpha-L-fucosyllactose with respect to the overall mode of binding, with the L-fucose fitting snugly into the cavity surrounded by Tyr106, Tyr108, Trp135 and Pro134 adjoining the primary combining site of the lectin. Marked differences were however noted between the models and the experimental structure in the network of hydrogen bonds and hydrophobic interactions holding the L-fucose in the combining site of the lectin, pointing to limitations of the modeling approach. In addition to the structural characterization of the ECL complexes, an effort was undertaken to correlate the structural data with thermodynamic data obtained from microcalorimetry, revealing the importance of the water network in the lectin combining site for carbohydrate binding.

KD, Toone EJ. Towards high affinity carbohydrate-binding proteins: Directed evolution of murine galectin-3

Towards a better understanding of the molecular basis of affinity, a directed evolution of murine galectin-3 (G3) was initiated to produce mutants with improved affinity for lactose and N-acetyllactosamine relative to the wild-type protein. A series of N-terminal truncations were developed to facilitate incorporation of the 35 kDa protein into a phage-display construct. Analysis of the various assemblies revealed that all such deletions produced protein unsuitable for use in directed evolution studies. Following fusion of the full-length galectin to p3 of filamentous phage, three libraries were constructed and biopanned for increased affinity for lactose. The first two libraries, of 1 × 105and 1 × 106members, respectively, were assembled through a combination of error-prone PCR and DNA shuffling. A third library was constructed using a modified staggered extension protocol (StEP), but contained only 10 members. Mutants were also engineered site-specifically to test the role of key residues in or near the binding pocket. Analysis of the mutants by ITC identified one mutation (R158G) that produces a twofold increase in affinity for lactose and another that results in a sixfold increase in affinity for N-acetyllactosamine. Solid-phase binding analysis of phage for nonexpressing proteins indicated that two other mutants demonstrated increased binding to beta-methyllactose relative to the wild-type protein. Together these studies validate the evolutionary approach and set the stage for the development of novel carbohydrate-binding proteins.Key words: phage display, directed evolution, galectin, thermodynamics, carbohydrates.

Synthesis of disaccharide congeners of the Trichinella spiralis glycan and binding site mapping of two monoclonal antibodies

The tetrasaccharide epitope, β-D-Tyvp(1[Formula: see text]3)β-D-GalNAcp(1[Formula: see text]4)[α-L-Fucp(1[Formula: see text]3)]β-D-GlcNAcp (1) is the major constituent of the N-glycan expressed on the cell surface of the parasite Trichinella spiralis. Two monoclonal antibodies (Mabs 9D4 and 18H1) that protect rats against infection by T. spiralis bind the terminal disaccharide epitope β-D-Tyvp(1[Formula: see text]3)β-D-GalNAcp conjugated to BSA. The syntheses of disaccharide congeners containing mono-deoxy, mono-methyl, as well as modifications to replace the acetamido group are reported. These target disaccharides were assayed for binding to the protective MAbs. For each antibody different clusters of three hydroxyl groups, that include C-2 and C-4 of tyvelose and for 18H1, the GalNAc acetamido group, provide the key polar interactions with the antibody binding sites. Mapping of the sites by functional group replacement revealed a similar pattern of recognition for the dideoxyhexose by the two MAbs while each recognizes distinct surfaces of the GalNAc residue. Consequently although both antibodies bury the 4-OH of tyvelose, the principal contact surface occurs on opposite sides of the 3,6-dideoxyhexose.Key words: β-tyveloside, 3,6-dideoxy-D-arabino-hexose, Trichinella carbohydrate antigen, antibody mapping, Trichinella spiralis, N-glycans, molecular recognition of carbohydrates, antigen topology, functional group replacement.