A Cascade of Tyrosine Autophosphorylation in the &Subunit Activates the Phosphotransferase of the Insulin Receptor*

We identified the major autophosphorylation sites in the insulin receptor and correlated their phosphoryla- tion with the phosphotransferase activity of the receptor on synthetic peptides. The receptor, purified from Fao hepatoma cells on immobilized wheat germ agglutinin, undergoes autophosphorylation at several tyro- sine residues in its &subunit; however, anti-phospho-tyrosine antibody (a-PY) inhibited most of the phos- phorylation by trapping the initial sites in an inactive complex. Exhaustive trypsin digestion of the inhibited &subunit yielded two peptides derived from the Tyr-1150 domain (Ullrich, A, Bell, J. R., Chen, E. Y., Herrera, R., Gray, A., Coussens, Grunfeld, Rosen, 0. M., and Ra-machandran, J. (1985) Nature 313, 756-761) called pY4 and pY5. Both peptides contained 2 phosphoty- rosy1 residues (ZTyr(P)), one corresponding to Tyr-1146 and the other to Tyr-1150