Purification and characterization of the receptor for insulin-like growth factor I.

The receptor for insulin-like growth factor I (IGF-I) was purified from the rat liver cell line BRL-3A by a combination monoclonal anti-receptor antibody column and a wheat germ agglutinin column. Analyses of these receptor preparations on reduced sodium dodecyl sulfate-polyacrylamide gels yielded protein bands of Mr 136K (alpha subunit) and Mr 85K and 94K (beta subunit). These receptor preparations bound 5 times more IGF-I than insulin, and the binding of both labeled ligands was more potently inhibited by unlabeled IGF-I than by insulin. These results indicate that these receptor preparations contained predominantly the IGF-I receptor. This highly purified receptor preparation was found to possess an intrinsic kinase activity; autophosphorylation of the receptor beta subunit was stimulated by low concentrations of IGF-I (half-maximal stimulation at 0.4 nM IGF-I). Twentyfold higher concentrations of insulin were required to give comparable levels of stimulation. A monoclonal antibody that inhibits the insulin receptor kinase was found to inhibit the IGF-I receptor kinase with the same potency with which it inhibits the insulin receptor. In contrast, monoclonal antibodies to other parts of the insulin receptor only poorly recognized the IGF-I receptor. A comparison of V8 protease digests of the insulin and IGF-I receptors again revealed some similarities and also some differences in the structures of these two receptors. Thus, the IGF-I receptor is structurally, antigenically, and functionally similar to but not identical with the insulin receptor.