Autoantibodies Against the Insulin Receptor Recognize the Insulin Binding Subunits of an Oligomeric Receptor

¹²⁵I-insulin was specifically cross-linked to membranes of human cultured lymphocytes (IM-9 line) using disuccinimidyl suberate. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions and autoradiography of this preparation revealed a major ¹²⁵I-labeled band with an apparent mol wt of 130,000 and minor bands of about 300,000 and 95,000. Labeling of these bands was inhibited by incubation of membranes with either unlabeled insulin or autoantibodies to the insulin receptor. The bands were also observed after the ¹²⁵I-insulin cross-linked preparation was solubilized and immunoprecipitated with a panel of autoantibodies to the insulin receptor. However, immunoprecipitation of the ¹²⁵I-insulin-receptor complex was inhibited by preincubation with excess unlabeled insulin. Finally, ¹²⁵I-Fab fragments of mol wt 50,000 prepared from anti-receptor antibodies and cross-linked to membranes were resolved into a major complex of mol wt 180,000 and a minor band of 125,000. Neither band was observed when ¹²⁵I-Fab fragments were cross-linked to membranes in the presence of an excess of unlabeled insulin. These findings indicate that autoantibodies to the insulin receptor are directed against the insulin binding subunits of an oligomeric receptor.