Activation of the Ras/MAPK signaling cascade is essential for growth factor-induced cell proliferation and differentiation. In this report, we describe the purification, cloning, and characterization of a novel protein, designated FRS2, that is tyrosine phosphorylated and binds to Grb2/Sos in response to FGF or NGF stimulation. We find that FRS2 is myristylated and that this modification is essential for membrane localization, tyrosine phosphorylation, Grb2/Sos recruitment, and MAPK activation. FRS2 functions as a lipid-anchored docking protein that targets signaling molecules to the plasma membrane in response to FGF stimulation to link receptor activation with the MAPK and other signaling pathways essential for cell growth and differentiation. Finally, we demonstrate that FRS2 is closely related and probably indentical to SNT, the long-sought target of FGF and NGF receptors.

The family of fibroblast growth factors (FGFs) consists of at least 10 different growth factors that control cellular processes such as growth, differentiation, and cell migration (reviewed in reference 2). FGFs induce their biological responses by binding to and activating a family of cell surface receptors with intrinsic protein tyrosine kinase activity (reviewed in reference 12). By contrast to other growth factors such as platelet-derived growth factor (PDGF) or epidermal growth factor, acidic FGF (aFGF; also called FGF1) binds to the FGF receptor (FGFR1) monovalently, and FGFR dimerization and activation are mediated by multivalent interactions between heparin sulfate proteoglycans and FGF (reviewed in reference 26).Upon activation, receptor tyrosine kinases undergo rapid autophosphorylation on numerous tyrosine residues. Autophosphorylation sites located within the catalytic domain are crucial for stimulation of kinase activity, while autophosphorylation sites located in other regions are usually involved in the recruitment of cellular target proteins (21). FGFR1 (encoded by flg) contains at least seven autophosphorylation sites. Two are located in the catalytic domain (Y653 and Y654) and are essential for kinase activation (17). One phosphorylation site in the C-terminal tail (Y766) functions as a high-affinity binding site for the SH2 domain of phospholipase C-␥ (19). Phosphorylation of Y766 is essential for phosphatidylinositol hydrolysis but not for FGF-induced DNA synthesis in myoblasts or differentiation of PC12 cells, indicating that these biological responses are mediated by different FGF-dependent signaling pathways (18,22). Interestingly, elimination of all known tyrosine autophosphorylation sites on FGFR1 by site-directed mutagenesis (except the two sites in the catalytic domain) does not impair FGF-induced mitogen-activated protein (MAP) kinase activation, mitogenesis, or PC12 cell differentiation (17).The Ras/MAP kinase signaling pathway plays an important role in signaling via FGF receptors (1,20). It is well established that the adapter protein Grb2 (6, 16) links receptor tyrosine kinases with the Ras signaling pathway by binding to the guanine nucleotide-releasing factor Sos through its SH3 domains and to tyrosine-phosphorylated receptors or docking molecules via its SH2 domain (25). We have recently identified a lipid-anchored docking protein, termed FRS2, that links FGFR molecules with the Ras/MAP kinase signaling pathway (14). We demonstrated that FRS2 is tyrosine phosphorylated and forms a complex with Grb2 and Sos in response to FGF stimulation (14). In this report, we demonstrate that in addition to the direct interactions with Grb2, tyrosine-phosphorylated FRS2 forms a complex with the SH2 domain-containing protein tyrosine phosphatase Shp2. This interaction results in tyrosine phosphorylation of Shp2 and complex formation between Shp2 and Grb2. Moreover, an FRS2 mutant impaired in Grb2 and Shp2 binding induces weak and transient MAP kinase response and fails to induce neuronal diffe...

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