The GTPase-Activating Protein of Ras Suppresses Platelet-Derived Growth Factor β Receptor Signaling by Silencing Phospholipase C-γ1

Abstract

The β receptor for platelet-derived growth factor (βPDGFR) is activated by binding of PDGF and undergoes phosphorylation at multiple tyrosine residues. The tyrosine-phosphorylated receptor associates with numerous SH2-domain-containing proteins which include phospholipase C-γ1 (PLCγ), the GTPase-activating protein of Ras (GAP), the p85 subunit of phosphatidylinositol 3 kinase (PI3K), the phosphotyrosine phosphatase Syp, and several other proteins. Our previous studies indicated that PI3K and PLCγ were required for relay of the mitogenic signal of βPDGFR, whereas GAP and Syp did not appear to be required for this response. In this study, we further investigated the role of GAP and Syp in mitogenic signaling by βPDGFR. Focusing on the PLCγ-dependent branch of βPDGFR signaling, we constructed a series of mutant βPDGFRs that contained the binding sites for pairs of the receptor-associated proteins: PLCγ and PI3K, PLCγ and GAP, or PLCγ and Syp. Characterization of these mutants showed that while all receptors were catalytically active and bound similar amounts of PLCγ, they differed dramatically in their ability to initiate DNA synthesis. This signaling deficiency related to an inability to efficiently tyrosine phosphorylate and activate PLCγ. Surprisingly, the crippled receptor was the one that recruited PLCγ and GAP. Thus, GAP functions to suppress signal relay by the βPDGFR, and it does so by silencing PLCγ. These findings demonstrate that the biological response to PDGF depends not only on the ability of the βPDGFR to recruit signal relay enzymes but also on the blend of these receptor-associated proteins.