LEGUME LECTIN STRUCTURE

The legume lectins are a large family of homologous carbohydrate bindi ng proteins that are found mainly in the seeds of most legume plants. Despite their strong similarity on the level of their amino acid seque nces and tertiary structures, their carbohydrate specificities and qua ternary structures vary widely. In this. review we will focus on the s tructural features of legume lectins and their complexes with carbohyd rates. These will be discussed in the light of recent mutagenesis resu lts when appropriate. Monosaccharide specificity seems to be achieved by the use of a conserved core of residues that hydrogen bond to the s ugar, and a variable loop that determines the exact shape of the monos accharide binding site. The higher affinity for particular oligosaccha rides and monosaccharides containing a hydrophobic aglycon results mai nly from a few distinct subsites next to the monosaccharide binding si te. These subsites consist of a small number of variable residues and are found in both the mannose and galactose specificity groups. The qu aternary structures of these proteins form the basis of a higher level of specificity, where the spacing between individual epitopes of mult ivalent carbohydrates becomes important. This results in homogeneous c ross-linked lattices even in mixed precipitation systems, and is of re levance for their effects on the biological activities of cells such a s mitogenic responses. Quaternary structure is also thought to play an important role in the high affinity interaction between some legume l ectins and adenine and a series of adenine-derived plant hormones. The molecular basis of the variation in quarternary structure in this gro up of proteins is poorly understood. (C) 1997 Elsevier Science B.V.