Platelet-derived growth factor-dependent association of the GTPase-activating protein of Ras and Src

Here we report that the platelet-derived growth factor beta preceptor (beta PDGFR) is not the only tyrosine kinase able to associate with the GTPase-a ctivating protein of Ras (RasGAP). The interaction of non-beta PDGFR kinase (s) with RasGAP was dependent on stimulation with platelet-derived growth f actor (PDGF) and seemed to require tyrosine phosphorylation of RasGAP. Beca use the tyrosine phosphorylation site of RasGAP is in a sequence context th at is favoured by the Src homology 2 ('SH2') domain of Src family members, we tested the possibility that Src was the kinase that associated with RasG AP. Indeed, Src interacted with phosphorylated RasGAP fusion proteins; immu nodepletion of Src markedly decreased the recovery of the RasGAP-associated kinase activity. Thus PDGF-dependent tyrosine phosphorylation of RasGAP re sults in the formation of a complex between RasGAP and Src. To begin to add ress the relevance of these observations, we focused on the consequences of the interaction of Src and RasGAP. We found that a receptor mutant that di d not activate Src was unable to efficiently mediate the tyrosine phosphory lation of phospholipase C gamma (PLC gamma). Taken together, these observat ions support the following hypothesis. When RasGAP is recruited to the beta PDGFR, it is phosphorylated and associates with Src. Once bound to RasGAP, Src is no longer able to promote the phosphorylation of PLC gamma. This hy pothesis offers a mechanistic explanation for our previously published find ings that the recruitment of RasGAP to the beta PDGFR attenuates the tyrosi ne phosphorylation of PLC gamma. Finally, these findings suggest a novel wa y in which RasGAP negatively regulates signal relay by the beta PDGFR.