Insulin stimulation of phosphorylation of the beta subunit of the insulin receptor. Formation of both phosphoserine and phosphotyrosine.

Rat hepatoma cells were labeled with 32Porthophosphate and the insulin receptor subunits were identified by immunoprecipitation and sodium dodecyl sulfate-acrylamide gel electrophoresis. In the basal state, only the Mr = 95,000 (beta) subunit of the insulin receptor was phosphorylated. The covalent labeling with 32P of this subunit was stimulated about 3-fold by insulin (10(-6) M). This stimulation was due to an increase in the content of phosphoserine, the appearance of phosphotyrosine, and a possible increase in phosphothreonine as well. These results suggest phosphorylation of the insulin receptor at multiple sites is an early event in insulin action.