Interactions of Bowringia mildbraedii agglutinin with complex- and hybrid-type glycans

Structural basis of carbohydrate recognition by a Man(alpha1-2)Man-specific lectin from Bowringia milbraedii

The crystal structure of the seed lectin from the tropical legume Bowringia milbraedii was determined in complex with the disaccharide ligand Man(alpha1-2)Man. In solution, the protein exhibits a dynamic dimer-tetramer equilibrium, consistent with the concanavalin A-type tetramer observed in the crystal. Contacts between the tetramers are mediated almost exclusively through the carbohydrate ligand, resulting in a crystal lattice virtually identical to that of the concanavalin-A:Man(alpha1-2)Man complex, even though both proteins have less than 50% sequence identity. The disaccharide binds exclusively in a "downstream" binding mode, with the non-reducing mannose occupying the monosaccharide-binding site. The reducing mannose is bound in a predominantly polar subsite involving Tyr131, Gln218, and Tyr219.

Crystallization and preliminary X-ray analysis of the Man(alpha1-2)Man-specific lectin from Bowringia mildbraedii in complex with its carbohydrate ligand

The lectin from Bowringia mildbraedii seeds crystallizes in the presence of the disaccharide Man(alpha1-2)Man. The best crystals grow at 293 K within four weeks after a pre-incubation at 277 K to induce nucleation. A complete data set was collected to a resolution of 1.90 A using synchrotron radiation. The crystals belong to space group I222, with unit-cell parameters a = 66.06, b = 86.35, c = 91.76 A, and contain one lectin monomer in the asymmetric unit.

Legume lectin structure

The legume lectins are a large family of homologous carbohydrate binding proteins that are found mainly in the seeds of most legume plants. Despite their strong similarity on the level of their amino acid sequences and tertiary structures, their carbohydrate specificities and quaternary structures vary widely. In this review we will focus on the structural features of legume lectins and their complexes with carbohydrates. These will be discussed in the light of recent mutagenesis results when appropriate. Monosaccharide specificity seems to be achieved by the use of a conserved core of residues that hydrogen bond to the sugar, and a variable loop that determines the exact shape of the monosaccharide binding site. The higher affinity for particular oligosaccharides and monosaccharides containing a hydrophobic aglycon results mainly from a few distinct subsites next to the monosaccharide binding site. These subsites consist of a small number of variable residues and are found in both the mannose and galactose specificity groups. The quaternary structures of these proteins form the basis of a higher level of specificity, where the spacing between individual epitopes of multivalent carbohydrates becomes important. This results in homogeneous cross-linked lattices even in mixed precipitation systems, and is of relevance for their effects on the biological activities of cells such as mitogenic responses. Quaternary structure is also thought to play an important role in the high affinity interaction between some legume lectins and adenine and a series of adenine-derived plant hormones. The molecular basis of the variation in quaternary structure in this group of proteins is poorly understood.

Bowringia mildbraedii agglutinin: polypeptide composition, primary structure and homologies with other legume lectins

The amino-acid sequences of the subunits of the lectin BMA from seeds of Bowringia mildbraedii have been determined. The data indicate that the lectin consists of a precursor polypeptide of approx. 29 kDa that is cleaved almost completely into two fragments of approx. 13.3 kDa (alpha subunit) and approx. 11.9 kDa (beta subunit), respectively. The beta subunit represents the N-terminal half of precursor polypeptides and is disulphide-linked in a beta beta dimer in the native (alpha beta)2 protein. BMA shows extensive amino-acid sequence homologies with known legume lectins. The site of post-translational proteolysis of the putative precursor occurs at a position similar to that identified in lectins obtained from other Sophoreae plants such as Sophora japonica and in Diocleae lectins such as Concanavalin A, but different from that of two chain lectins obtained from other tribes of the Papilionaceae.