A ROLE FOR ACTIVATED P21(RAS) IN INHIBITION REGULATION OF PLATELET-DERIVED GROWTH-FACTOR (PDGF) TYPE-BETA RECEPTOR ACTIVATION

Ligand-stimulated Platelet-Derived Growth Factor [PDGF] type-beta rece ptor autophosphorylation, and tyrosine phosphorylation of receptor-ass ociated signalling proteins, is blocked in cells expressing activated Ras genes. A factor present in membrane fractions of v-ras-expressing fibroblasts (Kbalb cells) dominantly inhibits the autophosphorylation of the PDGF type-beta receptor. Purification of this factor, via ion e xchange, reveals that the inhibitor can be physically separated from t he PDGF type-beta receptor, with reconstitution of PDGF type-beta rece ptor kinase activity in response to ligand binding. The inhibitor exhi bited specificity for the PDGF type-beta receptor, and consistently co -purified with activated p21(ras), with Syp/PTP-2, and with Grb2. Neut ralization of the p21(ras) protein from the Kbalb cell membranes by p2 1(ras)-specific monoclonal antibodies, however, completely removed the inhibition of PDGF type-beta receptor, rendering the PDGF type-beta r eceptor molecule capable of autophosphorylation in response to ligand. These results indicate that activated p21(ras) either interacts direc tly with the PDGF type-beta receptor to inhibit autokinase activity, o r complexes with different molecules such as Syp and/or Grb2 at the ce ll membrane to act on another effector which then inhibits PDGF type-b eta receptor function.