SRC PHOSPHORYLATES THE INSULIN-LIKE GROWTH-FACTOR TYPE-I RECEPTOR ON THE AUTOPHOSPHORYLATION SITES - REQUIREMENT FOR TRANSFORMATION BY SRC

The insulin-like growth factor type I (IGF-I) receptor can become tyro sine phosphorylated and enzymatically activated either in response to ligand or because of the activity of the Src tyrosine kinase (Peterson , J. E., Jelinek, T., Kaleko, M., Siddle, K., and Weber, M. J. (1994) J. Biol. Chem. 269, 27315-27321). The goal of the present study was to analyze the mechanistic basis and functional significance of the Src- induced phosphorylation and activation of the IGF-I receptor, 1) We ma pped the sites of IGF-I receptor autophosphorylation to peptides repre senting three different receptor domains: tyrosines 943 and 950 in the juxtamembrane region; tyrosines 1131, 1135, and 1136 within the kinas e domain; and tyrosine 1316 in the carboxyl-terminal domain. The juxta membrane and kinase-domain peptides were phosphorylated both in vivo a nd in vitro. The carboxyl-terminal site, although phosphorylated in vi tro and in src-transformed cells, was not a major site of ligand-induc ed phosphorylation in vivo. 2) We determined that the sites of Src-ind uced phosphorylation of the IGF-I receptor are the same as the Ligand- induced autophosphorylation sites and that the Src kinase can catalyze these phosphorylations directly. 3) We showed that cells cultured fro m mice in which the IGF-I receptor has been knocked out by homologous recombination are defective for morphological transformation by src. T hus, the Src kinase can substitute for the receptor kinase in phosphor ylating and activating the IGF-I receptor, and this receptor phosphory lation and activation are essential for transformation by src.