Abstract
Classical insulin and insulin-like growth factor I (IGF-I) receptors exist as well defined alpha 2 beta 2 heterotetrameric complexes that are assembled from two identical alpha beta heterodimeric half-receptor precursors. Recent evidence suggests that insulin and IGF-I half-receptors can heterologously assemble to form alpha 2 beta 2 insulin/IGF-I hybrid receptor complexes in vivo and in vitro. We have utilized hybrid receptor complexes to examine ligand-stimulated transmembrane signaling of wild-type insulin (alpha beta INS.WT) or IGF-I (alpha beta IGF.WT) half-receptors assembled with a kinase-defective insulin half-receptor mutant (alpha beta INS.A/K). In vitro assembly of either (alpha beta)IGF.WT/(alpha beta)INS.A/K or (alpha beta)INS.WT/(alpha beta)INS.A/K hybrid receptors resulted in decreased substrate protein kinase activity. The degree of protein kinase inactivation directly correlated with the amount of immunologically cross-reactive hybrid receptors formed. In contrast to substrate kinase activity, insulin-stimulated autophosphorylation of the (alpha beta)INS.WT/(alpha beta)INS.A/K hybrid receptor complex was completely unaffected in comparison to the wild-type (alpha beta)INS.WT/(alpha beta)INS.WT receptor. To assess a molecular basis for this difference, autophosphorylation of a hybrid receptor composed of a truncated beta-subunit insulin half-receptor with the kinase-defective half-receptor, (alpha beta)INS. delta CT/(alpha beta)INS.A/K, demonstrated the exclusive autophosphorylation of the (alpha beta)INS.A/K half-receptor beta subunit. These results demonstrate that ligand-dependent substrate phosphorylation by insulin and IGF-I holoreceptors requires interactions between two functional beta subunits within the alpha 2 beta 2 heterotetrameric complex and occurs through an intramolecular trans-phosphorylation reaction.