Insulin stimulates the tyrosine phosphorylation of a Mr = 160,000 glycoprotein in rat adipocyte plasma membranes.

The action of insulin on tyrosine phosphorylation of plasma membrane-associated proteins in rat adipocytes was investigated. Incubation of plasma membranes from insulin-treated adipocytes with [gamma-32P] ATP results in a marked increase in tyrosine phosphorylation of Mr = 160,000 (P160) and Mr = 92,000 proteins when compared to controls. Based on the immunoreactivities of these two proteins with anti-insulin receptor antibodies, the Mr = 92,000 species is identified as the insulin receptor beta subunit while P160 is unrelated to the receptor structure. P160 appears to be a glycoprotein as evidenced by its adsorption to wheat germ agglutinin-agarose. The tyrosine phosphorylation of P160 exhibits a rapid response to insulin (maximal within 2 min at 37 degrees C) and is readily reversed following removal of the free hormone by anti-insulin serum. The time courses of insulin-stimulated phosphorylation as well as the dephosphorylation of P160 coincide with those of the activation and deactivation of the insulin receptor kinase in the same plasma membrane preparation. Concanavalin A and hydrogen peroxide mimic insulin stimulation of the insulin receptor kinase and enhance the tyrosine phosphorylation of P160. Isoproterenol, epidermal growth factor, and phorbol diester are without effects. Analysis of the insulin dose-response relationship between P160 tyrosine phosphorylation and insulin receptor kinase activity reveals that maximal phosphorylation of P160 occurs when only a fraction (25%) of the receptor kinase is activated by the hormone. A similar relationship between these two parameters is observed for the insulinomimetic agent hydrogen peroxide. The close correlation between the level of P160 phosphorylation and insulin receptor kinase activity suggests that P160 may be tyrosine phosphorylated by the receptor kinase following receptor kinase activation by the hormone or insulin-like agents. This hypothesis is further supported by the finding that the insulin receptor kinase is the only insulin-sensitive tyrosine kinase detectable in adipocyte plasma membranes under the conditions of our experiments.