A NEW FUNCTION FOR A PHOSPHOTYROSINE PHOSPHATASE - LINKING GRB2-SOS TO A RECEPTOR TYROSINE KINASE

Autophosphorylated growth factor receptors provide binding sites for t he src homology 2 domains of intracellular signaling molecules. In res ponse to epidermal growth factor (EGF), the activated EGF receptor bin ds to a complex containing the signaling protein GRB2 and the Ras guan ine nucleotide-releasing factor Sos, leading to activation of the Ras signaling pathway. We have investigated whether the platelet-derived g rowth factor (PDGF) receptor binds GRB2-Sos. In contrast with the EGF receptor, the GRB2 does not bind to the PDGF receptor directly. Instea d, PDGF stimulation induces the formation of a complex containing GRB2 ; 70-, 80-, and 110-kDa tyrosine-phosphorylated proteins; and the PDGF receptor. Moreover, GRB2 binds directly to the 70-kDa protein but not to the PDGF receptor. Using a panel of PDGF beta-receptor mutants wit h altered tyrosine phosphorylation sites, we identified Tyr-1009 in th e PDGF receptor as required for GRB2 binding. Binding is inhibited by a phosphopeptide containing a YXNX motif. The protein tyrosine phospha tase Syp/PTP1D/SHPTP2/PTP2C is approximately 70 kDa, binds to the PDGF receptor via Tyr-1009, and contains several YXNX sequences. We found that the 70-kDa protein that binds to the PDGF receptor and to GRB2 co migrates with Syp and is recognized by anti-Syp antibodies. Furthermor e, both GRB2 and Sos coimmunoprecipitate with Syp from lysates of PDGF -stimulated cells, and GRB2 binds directly to tyrosine-phosphorylated Syp in vitro. These results indicate that GRB2 interacts with differen t growth factor receptors by different mechanisms and the cytoplasmic phosphotyrosine phosphatase Syp acts as an adapter between the PDGF re ceptor and the GRB2-Sos complex.