A CONFORMATIONAL CHANGE IN THE BETA-SUBUNIT OF THE INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR IDENTIFIED BY ANTIPEPTIDE ANTIBODIES

Insulin-like growth factor I (IGF-I) binding to its receptor results in receptor autophosphorylation and phosphorylation of several cellular substrates. The mechanism by which binding of the ligand to the extracellular receptor domain activates the intracellular kinase remains to be defined. Using polyclonal antibodies against four regions of the IGF-I receptor, we searched for putative conformational changes occurring in purified receptors. We studied the ability of the antipeptide antibodies to immunoprecipitate the native, ligand-occupied, or autophosphorylated IGF-I receptor. We found that the antipeptide antibody directed to the sequence 985-998 of the kinase domain immunoprecipitated the phosphorylated receptor, but not the native or the ligand-occupied receptor. By contrast, the antibody against the sequence 950-957 of the juxtamembrane domain immunoprecipitated the three receptor forms. The difference between phosphorylated receptors and unphosphorylated receptors was not observed in Western blot experiments, indicating that the conformational modification of the receptors is not detected upon unfolding. These data demonstrate that the IGF-I receptor undergoes an autophosphorylation-induced conformational change detectable in the kinase domain. Our work provides evidence that conformational changes induced by autophosphorylation may be a common activation mechanism for tyrosine kinase receptors.