Transmembrane signaling by the human insulin receptor kinase: Relationship between intramolecular β subunit trans- and cis-autophosphorylation and substrate kinase activation

To examine the role of intramolecular β subunit trans- and cis-autophosphorylation in signal transduction, the vaccinia virus/bacteriophage T7 expression system was used to generate insulin holoreceptors composed of a kinase-defective half-receptor precursor (αβ_A/K or αβ_A/K.ΔCT) and a kinase-active half-receptor precursor (αβ_ΔCT or αβ_WT). In the αβ_WT-αβ_ΔCT hybrid insulin receptor, insulin stimulated a 20-fold increase in intramolecular β subunit trans-phosphorylation, whereas cis-phosphorylation increased only 3-fold over the basal state. Similarly, in the αβ_WT-αβ_A/K.ΔCT hybrid insulin receptor, insulin stimulated trans-phosphorylation approximately 30-fold and cis-phosphorylation only 3-fold over the basal state. Although cis-phosphorylation of the kinase-functional αβ half-receptor was observed within these hybrid receptor species, this was not sufficient to stimulate exogenous substrate kinase activity. These data demonstrate that insulin primarily activates an intramolecular β subunit trans-phosphorylation reaction within the insulin holoreceptor and suggest that this reaction is necessary for activation of the holoreceptor. Furthermore, our results suggest a molecular basis for the dominant-negative phenotype observed in insulin-resistant patients possessing one kinase-defective insulin receptor allele.