Lipid effects on the binding properties of a reconstituted insulin receptor

The turkey erythrocyte membrane insulin receptor was solubilized and reconstituted into vesicles composed of either soy or dimyristoyl phosphatidylcholine. Reconstitution with soy phosphatidylcholine provided a lipid environment containing 43% unsaturated fatty acids, as compared with 82% saturated fatty acids in the dimyristoyl phosphatidylcholine preparation. After reconstitution, both species of vesicles were isolated from a 2 to 30% continuous sucrose gradient at a density of 1.071 g/ml. Scatchard analysis of binding data obtained at 15 degrees C revealed that the reconstituted receptor had a greater affinity for [125I]iodoinsulin in the saturated lipid environment (Ke = 0.167 nM-1; K1 = 2.18 nm-1) than in the unsaturated lipid environment (Ke = 0.0162 nM-1; K1 = 0.479 nm-1). Low affinity binding also was increased in the saturated vesicles. These increases were paralleled by a reduction in the number of available insulin binding sites in the saturated lipid environment. There was no difference, however, in the relative affinity of the reconstituted receptor preparations for insulin or proinsulin. Electron microscopy and gel filtration indicated that the binding differences are not due to differences in vesicle size. They also are not due to differences in the orientation of the receptor within the lipid bilayer, for its sensitivity to trypsin digestion was similar in both types of vesicles. Solubilization studies with 1% beta-octylglucoside indicated, however, that the dimyristoyl phosphatidylcholine vesicles incorporated a slightly lesser amount of insulin receptor. Similar results were also observed at 37 degrees C. These results suggest that the membrane lipid environment, especially the degree of unsaturation of the phospholipid fatty acyl chains, can influence the binding properties of the insulin receptor.