Insulin-stimulated phosphorylation of the insulin receptor precursor

The alpha and beta subunits of the insulin receptor, Mr = 135K and 95K, appear to be synthesized via a single polypeptide precursor of Mr = 190K. We have investigated whether insulin stimulates the phosphorylation of this proreceptor, as is the case with mature receptor. Rat liver endoplasmic reticulum membranes were solubilized in Triton X-100 and chromatographed sequentially on wheat-germ agglutinin-agarose and lentil lectin-agarose columns. Phosphorylation of the lentil eluate with [gamma 32P]ATP revealed an insulin-stimulated phosphoprotein of Mr = 192K, which was recognized by antireceptor antibody, compatible with the receptor precursor. This suggests that further processing of the Mr = 190K insulin receptor precursor is not necessary for insulin binding, kinase activation, and receptor phosphorylation.