Insulin stimulates the tyrosine phosphorylation of a 165 kDa protein that is associated with microsomal membranes of rat adipocytes.

The beta-subunit of the insulin receptor possesses an insulin-stimulatable protein tyrosine kinase activity. It has been widely postulated that this activity may mediate the transduction of the insulin signal by phosphorylation of cellular substrates involved in the mechanism of insulin action. We have identified, by immunoblotting with antiphosphotyrosine antibodies, a 165 kDa protein in rat adipocytes that is rapidly phosphorylated in response to insulin. Phosphorylation of this protein (pp165) occurs within 5-10 s of exposure to 10 nM insulin, suggesting that it may be a direct substrate for the insulin receptor. This protein was recovered in an intracellular membrane that fractionates with the low-density microsomes. Using discontinuous sucrose density-gradient centrifugation, pp165-containing vesicles were separated from other vesicles of the low-density microsomes including the glucose transporter-containing vesicles, indicating that pp165 is probably not a regulatory component of the vesicles that translocate glucose transporters in response to insulin. However, pp165 may be involved in conveying receptor activation at the cell surface to an intracellular site of insulin action.