IDENTIFICATION OF 6 NOVEL AUTOPHOSPHORYLATION SITES ON FIBROBLAST GROWTH-FACTOR RECEPTOR-1 AND ELUCIDATION OF THEIR IMPORTANCE IN RECEPTOR ACTIVATION AND SIGNAL-TRANSDUCTION

Fibroblast growth factor receptor (FGFR) activation leads to receptor autophosphorylation and increased tyrosine phosphorylation of several intracellular proteins. We have previously shown that autophosphorylat ed tyrosine 766 in FGFR1 serves as a binding site for one of the SH2 d omains of phospholipase Cy and couples FGFR1 to phosphatidylinositol h ydrolysis in several cell types. In this report, we describe the ident ification of six additional autophosphorylation sites (Y-463, Y-583, Y -585, Y-653, Y-654, and Y-730) on FGFR1. We demonstrate that autophosp horylation on tyrosines 653 and 654 is important for activation of tyr osine kinase activity of FGFR1 and is therefore essential for FGFR1-me diated biological responses. In contrast, autophosphorylation of the r emaining four tyrosines is dispensable for FGFR1-mediated mitogen-acti vated protein kinase activation and mitogenic signaling in L-6 cells a s well as neuronal differentiation of PC12 cells. Interestingly, both the wild-type and a mutant FGFR1 (FGFR1-4F) are able to phosphorylate She and an unidentified Grb2-associated phosphoprotein of 90 kDa (pp90 ). Binding of the Grb2/Sos complex to phosphorylated She and pp90 may therefore be the key link between FGFR1 and the Ras signaling pathway, mitogenesis, and neuronal differentiation.