Interaction of Concanavalin A and Wheat Germ Agglutinin with the Insulin Receptor of Fat Cells and Liver

Abstract

Abstract Wheat germ agglutinin enhances the specific binding of insulin to isolated fat cells and to liver cell membranes at a concentration of about 1 µg per ml. Wheat germ agglutinin increases insulin binding by increasing the rate of insulin-receptor complex formation; the protein does not alter the rate of dissociation of the insulin-membrane complex or the total number of binding sites for insulin. Higher concentrations of wheat germ agglutinin, as well as concanavalin A, block the binding of insulin to fat cells when the plant lectins are added to the cells before insulin. If the insulin-receptor complex is formed before addition of the plant lectin, only an increase in insulin binding with wheat germ agglutinin and virtually no effect of concanavalin A are demonstrable. Studies using combinations of both plant lectins suggest that these proteins are binding to different regions of the insulin receptor and that some lectin molecules can bind to the cell in a way which is without effect on insulin binding unless the other lectin is also present. Wheat germ agglutinin and concanavalin A modify insulin binding in membrane preparations in a manner similar to that described for intact fat cells. Similar alterations of insulin binding occur with soluble preparations of the insulin receptor of liver and fat cell membranes. These results indicate that the changes in the binding of insulin occur by direct binding of the plant lectins to the insulin receptor macromolecule. All of the effects of the plant lectins are reversed rapidly by adding simple sugars having selective specificity for the proteins, N-acetyl-d-glucosamine (for wheat germ agglutinin) or α-methyl-d-mannopyranoside (for concanavalin A). Digestion of fat cells with trypsin, which results in a marked fall in the affinity of the membrane receptor for insulin, similarly reduces the apparent affinity of wheat germ agglutinin for the membrane sites with which the lectin interacts to enhance glucose oxidation in the fat cells. These studies suggest that the insulin-like biological activity of wheat germ agglutinin may result from direct interactions with the insulin receptor in a way which closely resembles the interaction with insulin. If concanavalin A stimulates glucose transport by interacting with the insulin receptor, such an interaction must be somewhat dissimilar from that of insulin since tryptic digestion of cells does not markedly affect the biological response to this protein. These studies indicate that the insulin-binding macromolecules of liver and fat cell membranes are proteins of complex carbohydrate composition which have several chemically distinct sites capable of binding plant lectins in a manner which perturbs the insulin-receptor interaction.