Cloning and sequencing of winged bean (Psophocarpus tetragonolobus) basic agglutinin (WBA I): presence of second glycosylation site and its implications in quaternary structure

The interaction of N-trifluoroacetylgalactosamine and its derivatives with winged bean (Psophocarpus tetragonolobus) basic agglutinin reveals differential mechanism of their recognition: a fluorine-19 nuclear magnetic resonance study

Here, we show the binding results of a leguminosae lectin, winged bean basic agglutinin (WBA I) to N-trifluoroacetylgalactosamine (NTFAGalN), methyl-α-N-trifluoroacetylgalactosamine (MeαNTFAGalN) and methyl-β-tifluoroacetylgalactosamine (MeβNTFAGalN) using (19) F NMR spectroscopy. No chemical shift difference between the free and bound states for NTFAGalN and MeβNTFAGalN, and 0.01-ppm chemical shift change for MeαNTFAGalN, demonstrate that the MeαNTFAGalN has a sufficiently long residence time on the protein binding site as compared to MeβNTFAGalN and the free anomers of NTFAGalN. The sugar anomers were found in slow exchange with the binding site of agglutinin. Consequently, we obtained their binding parameters to the protein using line shape analyses. Aforementioned analyses of the activation parameters for the interactions of these saccharides indicate that the binding of α and β anomers of NTFAGalN and MeαNTFAGalN is controlled enthalpically, while that of MeβNTFAGalN is controlled entropically. This asserts the sterically constrained nature of the interaction of the MeβNTFAGalN with WBA I. These studies thus highlight a significant role of the conformation of the monosaccharide ligands for their recognition by WBA I.

Isolation of the gene and large-scale expression and purification of recombinant Erythrina cristagalli lectin

Using polymerase chain reaction, the coding sequence for Erythrina cristagalli lectin (ECL) has been cloned and expressed in Escherichia coli. The amplified DNA sequence of ECL is highly homologous to that previously reported for Erythrina corallodendron lectin (ECorL), confirming the absence of introns in the ECL gene. The polypeptide sequences of ECL and ECorL have been compared and five amino acids have been identified that differentiate the two proteins. Recombinant E. cristagalli lectin (recECL) was expressed in E. coli from a genomic clone encoding the mature E. cristagalli lectin gene. Constitutive expression localised recombinant protein in inclusion bodies, which were solubilised, and recECL, subsequently refolded and purified by lactose affinity chromatography. Significant advantages were observed for purification from inclusion bodies rather than from a clone optimised to express soluble protein. A large-scale purification scheme has been developed that can prepare functional recECL from inclusion bodies with a yield of 870 mg/l culture. By the range of characterisation methods employed in this study, it has been demonstrated that recECL is functionally equivalent to native ECL obtained from the E. cristagalli plant. In addition, characterisation of the binding of radiolabelled recECL to cultured dorsal root ganglia demonstrated that recECL binds to a single pool of receptors.

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.

Legume lectin family, the 'natural mutants of the quaternary state', provide insights into the relationship between protein stability and oligomerization

Legume lectins family of proteins, despite having the same 'jelly roll' tertiary structural fold at monomeric level, exhibit considerable variation in their quaternary structure arising out of small changes in their sequence. Nevertheless, their folding behavior and stability correlates very well with their patterns of assembly into dimers and tetramers. A conservation of their fold during evolution, its wide distribution in many protein families together with the availability of structural information on them make them interesting as proteins to explore the effect of inter- versus intra-subunit interactions in the stability of multimeric proteins. Additionally, as 'natural mutants' of quaternary association, proteins of legume lectin family provide interesting paradigms for studies addressing the effect of subunit oligomerization on the stability, folding and function as well as the evolution of multimeric structures.

The primary structure of the acidic lectin from winged bean (Psophocarpus tetragonolobus): insights in carbohydrate recognition, adenine binding and quaternary association

The amino acid sequence of the winged bean acidic lectin (WBA II) was determined by chemical means and by recombinant techniques. From the N- and C-terminal sequence, obtained chemically, primers were designed for PCR amplification of the genomic DNA. The PCR product was cloned and sequenced to get the complete primary structure of WBA II. Peptide fragments for sequencing were also obtained by tryptic cleavages of the native lectin. The WBA II sequence showed a high degree of homology with that of WBA I and Erythrina corallodendron lectin (ECorL), especially in the regions involved in subunit association, where there is a very high conservation of residues. This perhaps implies the importance of this particular region in subunit interactions in this lectin. In addition, many of the residues, involved in carbohydrate binding in legume lectins, appear to be conserved in WBA II. The distinct differences in anomeric specificity observed amongst WBA I, WBA II, ECorL and peanut agglutinin (PNA) may be explained by subtle differences in sequence/structure of their D-loops. WBA II binds adenine quite strongly; a putative adenine binding sequence has been identified.

Carbohydrate specificity and quaternary association in basic winged bean lectin: X-ray analysis of the lectin at 2.5 A resolution

The structure of basic Winged Bean Agglutinin (WBAI) with two dimeric molecules complexed with methyl-alpha-D-galactopyranoside in the asymmetric unit, has been determined by the molecular replacement method and refined with 2.5 A X-ray intensity data. The polypeptide chain of each monomer has the characteristic legume lectin tertiary fold. The structure clearly defines the lectin-carbohydrate interactions. It reveals how the unusually long variable loop in the binding region endows the lectin with its characteristic sugar specificity. The lectin forms non-canonical dimers of the type found in Erythrina corallodendron lectin (EcorL) even though glycosylation, unlike in EcorL, does not prevent the formation of canonical dimers. The structure thus further demonstrates that the mode of dimerisation of legume lectins is not necessarily determined by the covalently bound carbohydrate but is governed by features intrinsic to the protein. The present analysis and our earlier work on peanut lectin (PNA), show that legume lectins are a family of proteins in which small alterations in essentially the same tertiary structure lead to wide variations in quaternary association. A relationship among the non-canonical modes of dimeric association in legume lectins is presented.

Effect of substituents on the thermodynamics of D-galactopyranoside binding to winged bean (Psophocarpus tetragonolobus) basic lectin

Isothermal titration calorimetric measurements of the binding of deoxy, fluorodeoxy, and methoxy derivatives of D-galactopyranoside (alpha-D-Gal) to the basic lectin from winged bean Psophocarpus tetragonolobus, WBA I, have been carried out. Each of the ligands binding to WBA I displayed the same stoichiometry of one per subunit (29 kDa) of WBA I. The binding enthalpies for various derivatives are essentially independent of temperature and show complementary changes with respect to binding entropies. Replacement of the hydroxyl group by fluorine or hydrogen on C3 and C4 of the galactopyranoside eliminates binding to the lectin, consistent with C3-OH and C4-OH acting as hydrogen bond donors. The affinity for C2 derivatives of galactose decreases in the order GalNAc > 2MeOGal > 2FGal congruent with Gal > 2HGal, which suggests that both polar and nonpolar residues surround the C2 locus of galactose, consistent with the observed high affinity of WBA I toward GalNAc where the acetamido group at C2 position is probably stabilized by both nonpolar interactions with the methyl group and polar interactions with the carbonyl group. The binding of C6 derivatives follows the order Gal > 6FGal > D-Fuc >> 6MeOGal congruent with L-Ara, indicating the presence of favourable polar interactions with a hydrogen bond donor in the vicinity. On the basis of these results the hydrogen bond donor-acceptor relationship of the complexation of methyl-alpha-D-galactopyranoside with the primary combining site of WBA I is proposed.

Analyses of carbohydrate recognition by legume lectins: size of the combining site loops and their primary specificity

Recognition of cell-surface carbohydrates by lectins has wide implications in important biological processes. The ability of plant lectins to detect subtle variations in carbohydrate structures found on molecules, cells and organisms have made them a paradigm for protein-carbohydrate recognition. Legume lectins, one of the most well studied family of plant proteins, display a considerable repertoire of carbohydrate specificities owing perhaps to the sequence hypervariability in the loops constituting their combining site. However, lack of a rigorous framework to explain their carbohydrate binding specificities has precluded a rational approach to alter their ligand binding activity in a meaningful manner. This study reports an extensive analysis of sequences and structures of several legume lectins and shows that despite the hypervariability of their combining regions they exhibit within a significant pattern of uniformity. The results show that the size of the binding site loop D is invariant in the Man/Glc specific lectins and is possibly a primary determinant of the monosaccharide specificities of the legume lectins. Analyses of size and sequence variability of loops reveal the existence of a common theme that subserves to define their binding specificities. These results thus provide not only a framework for understanding the molecular basis of carbohydrate recognition by legume lectins but also a rationale for redesign of their ligand binding propensities.