RAS MEDIATES THE ACTIVATION OF PHOSPHOLIPASE-D BY V-SRC

We demonstrated previously that v-Src activates a phospholipase D (PLD) activity (Song, J., Pfeffer, L. M., and Foster, D. A. (1991) Mol. Cell. Biol. 11, 4903-4908) and that this activation is dependent upon a G protein(s) (Jiang, H., Alexandropoulos, K., Song, J., and Foster, D. A, (1994) Mol. Cell. Biol. 14, 3676-3682). An in vitro PLD assay was developed to study G protein involvement in v-Src-induced PLD activity. Maximal PLD activity in membranes isolated from v-Src-transformed cells was dependent upon both GTP and cytosol. In this report, we present three lines of evidence demonstrating that v-Src-induced PLD activity is mediated by Ras. First, a neutralizing Ras monoclonal antibody (Y13-259) inhibits PLD activity in membranes isolated from v-Src-transformed Balb/c 3T3 cells. Second, immobilized Ras protein depleted cytosol of the ability to stimulate PLD activity. This effect was dependent upon preloading immobilized Ras with GTP. Last, expression of a dominant negative Ras mutant in v-Src-transformed cells reduced PLD activity to the level observed in the nontransformed parental cells. These data establish a novel role for Ras in the regulation of PLD activity.