Lectins: A chemical approach to the cell surface

Abstract

Lectins are a group of plant proteins that bind to cells through the carbohydrate moieties of glycoproteins and glycolipids on the cell surface. These proteins are proving particularly valuable in mapping receptors and studying dynamic processes on the surfaces of animal cells. Some of the uses, chemical properties and biological activities of lectins are discussed in terms of the lectin concanavalin A (Con A), which is obtained from the jack bean. This protein is the first lectin for which the amino acid sequence and three-dimensional structure have been determined. The results of these studies have defined the over-all features of the molecule, the details of the metal-binding sites and the interactions of the subunits. Con A is composed of identical subunits each of which contains 237 amino acid residues, one Mn2+ ion, one Ca2+ ion and one binding site for carbohydrate. Each protomer is dome-shaped, with dimensions of about 42Å × 40Å × 39 Å. At physiological pH the predominant form of the lectin is the tetramer of these subunits, but under acidic conditions, or after chemical modification with certain reagents, dimers are formed. Binding of Con A to the cell surface is necessary for initiation of the biological activities, and binding is inhibited by simple sugars such as glucose or mannose but not by closely related stereo-isomers such as galactose. Con A has been used to study a variety of normal and transformed cells. Studies of the biological effects of Con A on lymphocytes have led to hypotheses on transmembrane regulation of signals which may occur on a variety of cell types. © 1975, Walter de Gruyter. All rights reserved.