Effect of tyrosine mutations on the kinase activity and transforming potential of an oncogenic human insulin-like growth factor I receptor

The tyrosines in the cytoplasmic domain of an oncogenic human insulin-like growth factor I receptor (gag-IGFR) were systematically mutated to phenylalanines to investigate the role of those tyrosines in the enzymatic and biological function of the gag-IGFR. Our results indicate that tyrosines 1131, 1135, 1136, and 1221 are important for the receptor protein-tyrosine kinase (PTK) activity. However, mutation of Tyr-1136 only slightly affects the kinase activity but dramatically reduces the transforming ability and overall substrate phosphorylation, in particular, annexin II, which is strongly phosphorylated by the gag-IGFR but not by the Phe-1136 mutant. Single mutation of either Tyr-943 or Tyr-950 resulted in significantly reduced phosphorylation of the receptor but not on its PTR activity or transforming ability. Tyr-950 together with its surrounding sequence is involved in mediating the interaction between the gag-IGFR and insulin receptor substrate 1. Our data also suggest that Tyr-1316 is involved in phosphorylation of phospholipase C-gamma, which is, however, not important for cell transforming activity. Overall, our study has identified several tyrosine residues of IGFR important for its PTK activity and substrate interaction. The transforming potential of the gag-IGFR correlates well with its ability to phosphorylate overall cellular substrates and to activate phosphatidylinositol 3-kinase via insulin receptor substrate 1.