INVOLVEMENT OF RAL GTPASE IN V-SRC-INDUCED PHOSPHOLIPASE-D ACTIVATION

AN early response to the tyrosine kinase activity of v-Src is an increase in phospholipase D (PLD) activity(1), which leads to the generation of biologically active lipid second messengers, including phosphatidic acid, lysophosphatidic acid and diacylglycerol(2). We have recently demonstrated that v-Src-induced PLD activity is mediated by Ras(3), although Ras involvement was indirect, requiring a cytosolic factor for PLD activation(3). Ras interacts with(4-6) and activates Ral-GDS(13), the exchange factor responsible for the activation of Ral GTPases. Here we report that this newly identified Ras/Ral signalling pathway mediates PLD activation by v-Src. PLD activity could be precipitated from v-Src-transformed cell lysates with immobilized RalA protein and with an anti-Ral antibody. A mutation to the region of RalA analogous to the 'effector domain' of Ras did not reduce the ability of RalA to complex with PLD, although deletion of a Ral-specific amino-terminal region did. Overexpression of RalA potentiated PLD activation by v-Src, and expression of dominant negative RalA mutants inhibited both v-Src- and v-Ras-induced PLD activity. Thus RalA is involved in the tyrosine kinase activation of PLD through its unique N terminus, and that PLD is a downstream target of a Ras/Ral GTPase cascade.