Legume lectins--a large family of homologous proteins

More than 70 lectins from leguminous plants belonging to different suborders and tribes have been isolated, mostly from seeds, and characterized to varying degrees. Although they differ in their carbohydrate specificities, they resemble each other in their physicochemical properties. They usually consist of two or four subunits (25-30 kDa), each with one carbohydrate binding site. Interaction with carbohydrates requires tightly bound Ca2+ and Mn2+ (or another transition metal). The primary sequences of more than 15 legume lectins have been established by chemical or molecular genetic techniques. They exhibit remarkable homologies, with a significant number of invariant amino acid residues, among them most of those involved in metal binding. The 3-dimensional structures of the legume lectins are similar, too, and are characterized by a high content of beta-sheets and a lack of alpha-helix. The location of the metal and carbohydrate binding sites, established unequivocally in concanavalin A by high resolution X-ray crystallography, appears to be the same in the other legume lectins. Several of the lectin genes have been cloned and expressed in heterologous systems. This opens the way for the application of molecular genetics to the investigation of the atomic structure of the carbohydrate binding sites of the lectins, and of the relationship between their structure and biological activity. The new approaches may also provide information on the mechanisms that control gene expression in plants and on the role of lectins in nature.