The role of weak protein-protein interactions in multivalent lectin-carbohydrate binding: crystal structure of cross-linked FRIL.

Binding of multivalent glycoconjugates by lectins often leads to the formation of cross-linked complexes. Type I cross-links, which are one-dimensional, are formed by a divalent lectin and a divalent glycoconjugate. Type II cross-links, which are two or three-dimensional, occur when a lectin or glycoconjugate has a valence greater than two. Type II complexes are a source of additional specificity, since homogeneous type II complexes are formed in the presence of mixtures of lectins and glycoconjugates. This additional specificity is thought to become important when a lectin interacts with clusters of glycoconjugates, e.g. as is present on the cell surface. The cryst1al structure of the Glc/Man binding legume lectin FRIL in complex with a trisaccharide provides a molecular snapshot of how weak protein-protein interactions, which are not observed in solution, can become important when a cross-linked complex is formed. In solution, FRIL is a divalent dimer, but in the crystal FRIL forms a tetramer, which allows for the formation of an intricate type II cross-linked complex with the divalent trisaccharide. The dependence on weak protein-protein interactions can ensure that a specific type II cross-linked complex and its associated specificity can occur only under stringent conditions, which explains why lectins are often found forming higher-order oligomers.

The crystal structures of Man(alpha1-3)Man(alpha1-O)Me and Man(alpha1-6)Man(alpha1-O)Me in complex with concanavalin A.

The crystal structures of concanavalin A in complex with Man(alpha1-6)Man(alpha1-O)Me and Man(alpha1-3)Man(alpha1-O)Me were determined at resolutions of 2.0 and 2.8 A, respectively. In both structures, the O-1-linked mannose binds in the conserved monosaccharide-binding site. The O-3-linked mannose of Man(alpha1-3)Man(alpha1-O)Me binds in the hydrophobic subsite formed by Tyr-12, Tyr-100, and Leu-99. The shielding of a hydrophobic surface is consistent with the associated large heat capacity change. The O-6-linked mannose of Man(alpha1-6)Man(alpha1-O)Me binds in the same subsite formed by Tyr-12 and Asp-16 as the reducing mannose of the highly specific trimannose Man(alpha1-3)[Man(alpha1-6)]Man(alpha1-O)Me. However, it is much less tightly bound. Its O-2 hydroxyl makes no hydrogen bond with the conserved water 1. Water 1 is present in all the sugar-containing concanavalin A structures and increases the complementarity between the protein-binding surface and the sugar, but is not necessarily a hydrogen-bonding partner. A water analysis of the carbohydrate-binding site revealed a conserved water molecule replacing O-4 on the alpha1-3-linked arm of the trimannose. No such water is found for the reducing or O-6-linked mannose. Our data indicate that the central mannose of Man(alpha1-3)[Man(alpha1-6)]Man(alpha1-O)Me primarily functions as a hinge between the two outer subsites.

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.

Crystallization of two related lectins from the legume plant Dolichos biflorus.

The seed lectin DBL and the related stem and leaves lectin DB58 of the tropical legume Dolichos biflorus were crystallized, as well as complexes of DBL with adenine and with GalNAc(alpha1-3)[Fuc(alpha1-2)]Gal. The different crystal forms of DBL diffract to about 2.8 A, while DB58 crystals diffract to 3.3 A.

Crystal structure of arcelin-5, a lectin-like defense protein from Phaseolus vulgaris.

In the seeds of the legume plants, a class of sugar-binding proteins with high structural and sequential identity is found, generally called the legume lectins. The seeds of the common bean (Phaseolus vulgaris) contain, besides two such lectins, a lectin-like defense protein called arcelin, in which one sugar binding loop is absent. Here we report the crystal structure of arcelin-5 (Arc5), one of the electrophoretic variants of arcelin, solved at a resolution of 2.7 A. The R factor of the refined structure is 20.6%, and the free R factor is 27.1%. The main difference between Arc5 and the legume lectins is the absence of the metal binding loop. The bound metals are necessary for the sugar binding capabilities of the legume lectins and stabilize an Ala-Asp cis-peptide bond. Surprisingly, despite the absence of the metal binding site in Arc5, this cis-peptide bond found in all legume lectin structures is still present, although the Asp residue has been replaced by a Tyr residue. Despite the high identity between the different legume lectin sequences, they show a broad range of quaternary structures. The structures of three different dimers and three different tetramers have been solved. Arc5 crystallized as a monomer, bringing the number of known quaternary structures to seven.

The crystallographic structure of phytohemagglutinin-L.

The structure of phytohemagglutinin-L (PHA-L), a leucoagglutinating seed lectin from Phaseolus vulgaris, has been solved with molecular replacement using the coordinates of lentil lectin as model, and refined at a resolution of 2.8 A. The final R-factor of the structure is 20.0%. The quaternary structure of the PHA-L tetramer differs from the structures of the concanavalin A and peanut lectin tetramers, but resembles the structure of the soybean agglutinin tetramer. PHA-L consists of two canonical legume lectin dimers that pack together through the formation of a close contact between two beta-strands. Of the two covalently bound oligosaccharides per monomer, only one GlcNAc residue per monomer is visible in the electron density. In this article we describe the structure of PHA-L, and we discuss the putative position of the high affinity adenine-binding site present in a number of legume lectins. A comparison with transthyretin, a protein that shows a remarkable resemblance to PHA-L, gives further ground to our proposal.

Crystallization of glycosylated and nonglycosylated phytohemagglutinin-L.

In the seeds of legume plants a class of sugar-binding proteins can be found, generally called legume lectins. In this paper we present the crystallization of phytohemagglutinin-L (PHA-L), a glycosylated lectin from the seeds of the common bean (Phaseolus vulgaris). Single PHA-L crystals were grown by vapor diffusion, using PEG as precipitant. The protein crystallizes in the monoclinic space group C2, and diffracts to a resolution of 2.7 angstroms. The unit cell parameters are a=106.3 angstroms, 121.2 angstroms, c=90.8 angstroms, and beta=93.7 degrees. The asymmetric unit probably contains one PHA-L tetramer. Crystals of a recombinant nonglycosylated form of PHA-L, grown under identical conditions, and crystals of the native PHA-L, grown in the presence of isopropanol, did not survive the mounting process.