The effects of wheat germ agglutinin on the adipocyte insulin receptor

Abstract

Treatment of rat adipocytes with wheat germ agglutinin markedly enhances insulin binding by increasing the affinity of the insulin receptor. The structure of the adipocyte receptor following wheat germ agglutinin treatment was studied by column chromatography and polyacrylamide gel electrophoresis to determine if aggregates of the insulin receptor are present. Solubilization of the receptor by Triton X-100 and passage of this material over Sepharose 6B revealed two insulin binding activities: a major peak which has a Stokes radius of 87 Å and a minor peak with a Stokes radius of 47 Å. Wheat germ agglutinin treatment produced an increase in the binding activity of both peaks, but their molecular weights did not change. In addition, 125I-labeled insulin was covalently attached to the adipocyte insulin binding sites by a cross-linking reagent. Column chromatography of the insulin-receptor complex again indicated the presence of two species with Stokes radii of 87 Å and 47 Å. Heterogeneity in these complexes was also demonstrated by polyacrylamide gel electrophoresis in the presence of Triton X-100. Ferguson plots indicated that the peak of radioactivity had a molecular radius of 60 Å, a size found both in the presence and absence of wheat germ treatment of adipocytes. These findings suggest that the increase in receptor affinity produced by wheat germ agglutinin treatment is not caused by the formation of receptor clusters. Instead, it appears that a simple interaction between the plant lectin and the receptor is sufficient to induce the changes in the insulin binding properties of adipocytes.