A structural comparison of the subunits of the Dolichos biflorus Seed lectin by peptide mapping and carboxyl terminal amino acid analysis

Structure predictions of two Bauhinia variegata lectins reveal patterns of C-terminal properties in single chain legume lectins

Bauhinia variegata lectins (BVL-I and BVL-II) are single chain lectins isolated from the plant Bauhinia variegata. Single chain lectins undergo post-translational processing on its N-terminal and C-terminal regions, which determines their physiological targeting, carbohydrate binding activity and pattern of quaternary association. These two lectins are isoforms, BVL-I being highly glycosylated, and thus far, it has not been possible to determine their structures. The present study used prediction and validation algorithms to elucidate the likely structures of BVL-I and -II. The program Bhageerath-H was chosen from among three different structure prediction programs due to its better overall reliability. In order to predict the C-terminal region cleavage sites, other lectins known to have this modification were analysed and three rules were created: (1) the first amino acid of the excised peptide is small or hydrophobic; (2) the cleavage occurs after an acid, polar, or hydrophobic residue, but not after a basic one; and (3) the cleavage spot is located 5-8 residues after a conserved Leu amino acid. These rules predicted that BVL-I and -II would have fifteen C-terminal residues cleaved, and this was confirmed experimentally by Edman degradation sequencing of BVL-I. Furthermore, the C-terminal analyses predicted that only BVL-II underwent α-helical folding in this region, similar to that seen in SBA and DBL. Conversely, BVL-I and -II contained four conserved regions of a GS-I association, providing evidence of a previously undescribed X4+unusual oligomerisation between the truncated BVL-I and the intact BVL-II. This is the first report on the structural analysis of lectins from Bauhinia spp. and therefore is important for the characterisation C-terminal cleavage and patterns of quaternary association of single chain lectins.

Carbohydrate binding, quaternary structure and a novel hydrophobic binding site in two legume lectin oligomers from Dolichos biflorus

The seed lectin (DBL) from the leguminous plant Dolichos biflorus has a unique specificity among the members of the legume lectin family because of its high preference for GalNAc over Gal. In addition, precipitation of blood group A+H substance by DBL is slightly better inhibited by a blood group A trisaccharide (GalNAc(alpha1-3)[Fuc(alpha1-2)]Gal) containing pentasaccharide, and about 40 times better by the Forssman disaccharide (GalNAc(alpha1-3)GalNAc) than by GalNAc. We report the crystal structures of the DBL-blood group A trisaccharide complex and the DBL-Forssman disaccharide complex.A comparison with the binding sites of Gal-binding legume lectins indicates that the low affinity of DBL for Gal is due to the substitution of a conserved aromatic residue by an aliphatic residue (Leu127). Binding studies with a Leu127Phe mutant corroborate these conclusions. DBL has a higher affinity for GalNAc because the N-acetyl group compensates for the loss of aromatic stacking in DBL by making a hydrogen bond with the backbone amide group of Gly103 and a hydrophobic contact with the side-chains of Trp132 and Tyr104. Some legume lectins possess a hydrophobic binding site that binds adenine and adenine-derived plant hormones, i.e. cytokinins. The exact function of this binding site is unknown, but adenine/cytokinin-binding legume lectins might be involved in storage of plant hormones or plant growth regulation. The structures of DBL in complex with adenine and of the dimeric stem and leaf lectin (DB58) from the same plant provide the first structural data on these binding sites. Both oligomers possess an unusual architecture, featuring an alpha-helix sandwiched between two monomers. In both oligomers, this alpha-helix is directly involved in the formation of the hydrophobic binding site. DB58 adopts a novel quaternary structure, related to the quaternary structure of the DBL heterotetramer, and brings the number of know legume lectin dimer types to four.

C-terminal post-translational proteolysis of plant lectins and their recombinant forms expressed in Escherichia coli. Characterization of "ragged ends" by mass spectrometry

Electrospray mass spectrometry was used to accurately measure the molecular masses of single chain lectins from legume seeds and also of three recombinant lectins, expressed in Escherichia coli. The five single chain lectins, Erythrina corallodendron lectin, soybean and peanut agglutinins, Dolichos biflorus lectin, and Phaseolus vulgaris hemagglutinin E, all showed evidence of C-terminal proteolytic processing, in some cases to "ragged" ends, when their masses were compared to those expected from their cDNA sequences and their known carbohydrate chains. Recombinant forms of the lectins from E. corallodendron, soybean, and peanut also showed C-terminal trimming, but not to the same points as the natural forms. Discrepancies between the protein and cDNA sequences of the E. corallodendron lectin were resolved by combined liquid chromatography-mass spectrometry peptide mapping and protein sequencing experiments, and the presence of a second glycosylation site was demonstrated. Our data show that all of these lectins undergo C-terminal proteolytic processing of a readily attacked peptide segment. This trimming is frequently imprecise, and the resulting heterogeneity may be a major contributor to the appearance of isolectin forms of these proteins.

Analysis of sequence variation among legume lectins. A ring of hypervariable residues forms the perimeter of the carbohydrate-binding site

Twelve plant lectins from the Papilionoideae subfamily were selected to represent a range of carbohydrate specificities, and their sequences were aligned. Two variability indices were applied to the aligned sequences and the results were analysed using the three-dimensional structures of concanavalin A and the pea lectin. The areas of greatest variability were located in the carbohydrate-binding site region, forming a perimeter around a well-conserved core. These residues are inferred to be specificity determining, in the manner of antibodies, and the most variable position corresponded to Tyr100 in concanavalin A, a known ligand contact residue. In addition to the five peptide loops known to form the binding site from crystallographic studies, a sixth segment with variable residues was located in the binding-site region, and this may contribute to oligosaccharide specificity. In their overall composition, the lectin sites resemble those of the sugar-transport proteins rather than antibodies. The prospects for modelling lectin binding sites by the methods used for antibodies were also assessed.

Isolation and characterization of a lectin from the roots of Dolichos biflorus

A lectin has been isolated from the roots of 7-day-old Dolichos biflorus plants and has been compared with the D. biflorus seed lectin. The root lectin differs from the seed lectin in molecular weight, subunit stoichiometry, amino acid composition, amino terminal amino acid sequence, and isoelectric focusing pattern. However, the root lectin has in common with the seed lectin a specificity for N-acetyl-D-galactosamine, and upon denaturation the root lectin will react weakly with antiserum made to denatured seed lectin. Distribution studies of this lectin in germinating seedlings show that the highest levels of lectin are found in 1-day-old roots. Upon dissection and analysis of 7-day-old roots, the highest levels of the lectin are in the uppermost segment. In addition, isoforms of this lectin also exist in the stems and leaves of the plant.

cDNA cloning and in vitro synthesis of the Dolichos biflorus seed lectin

The Dolichos biflorus seed lectin contains two structurally related subunits. A cDNA library was constructed using RNA isolated from D. biflorus seeds actively synthesizing the seed lectin. The library was expressed in Escherichia coli using a lambda Charon 16 vector, and lectin-specific antiserum was used to isolate a seed lectin cDNA. Hybridization of the D. biflorus seed lectin cDNA to RNA isolated from seeds actively producing both lectin subunits identifies a single-size RNA of 1100 bases. An oligodeoxyribonucleotide probe, constructed from an amino acid sequence common to both lectin subunits, detects the same size RNA. Translation of seed mRNA in vitro and immunoprecipitation of translation products using a lectin-specific antiserum yields a single polypeptide of slightly higher molecular mass than the largest seed lectin subunit. This seed lectin precursor is indistinguishable from a polypeptide synthesized from mRNA hybrid selected by the seed lectin cDNA. These data support the existence of a single polypeptide precursor for both subunit types of the D. biflorus seed lectin and suggest that differences between the subunit types arise by posttranslational processing.

A structural comparison of the A and B subunits of Griffonia simplicifolia I isolectins

A structural comparison between the A and B subunits of the five tetrameric Griffonia simplicifolia I isolectins (A4, A3B, A2B2, AB3, B4) was undertaken to determine the extent of homology between the subunits. The first 25 N-terminal amino acids of both A and B subunits were determined following the enzymatic removal of N-terminal pyroglutamate blocking groups with pyroglutamate aminopeptidase. Although 21 amino acids were common to both subunits, there were four unique amino acids in the N-terminal sequence of A and B. Residues 8, 9, 17, and 19 were asparagine, leucine, lysine, and asparagine in subunit A and threonine, phenylalanine, glutamic acid, and serine in subunit B. The last six C-terminal amino acids, released by digestion with carboxypeptidase Y, were the same for both subunits: Arg-(Phe, Val)-Leu-Thr-Ser-COOH. Subunit B, which contains one methionyl residue, was cleaved by cyanogen bromide into two fragments, a large (Mr = 31,000) and a small (Mr = 2700) polypeptide. Failure of the small fragment to undergo manual Edman degradation indicated an N-terminal blocking group, presumably pyroglutamate. Both subunits were digested with trypsin and the tryptic peptides were analyzed using reverse-phase HPLC. Tryptic glycopeptides were identified by labeling the carbohydrate moiety of the A and B subunit using sodium [3H] borohydride. Cysteine-containing tryptic peptides were similarly identified by using [1-14C]iodoacetamide. Approximately 30% of the tryptic peptides were common to both subunits. Thus, although the N- and C-terminal regions of A and B are similar, the subunits each possess unique sequences.