ABSTRACT
v-Src-induced increases in diglyceride are derived from phosphatidylcholine via a type D phospholipase (PLD) and a phosphatidic acid phosphatase. v-Src-induced PLD activity, as measured by PLD-catalyzed transphosphatidylation of phosphatidylcholine to phosphatidylethanol, is inhibited by GDP beta S, which inhibits G-protein-mediated intracellular signals. Similarly, v-Src-induced increases in diglyceride are also blocked by GDP beta S. In contrast to the PLD activity induced by v-Src, PLD activity induced by the protein kinase C agonist, 12-O-tetradecanoylphorbol-13-acetate (TPA), was insensitive to GDP beta S. Consistent with the involvement of a G protein in the activation of PLD activity by v-Src, GTP gamma S, a nonhydrolyzable analog of GTP that potentiates G-protein-mediated signals, strongly enhanced PLD activity in v-Src-transformed cells relative to that in parental BALB/c 3T3 cells. The effect of GTP gamma S on PLD activity in v-Src-transformed cells was observed only when cells were prelabeled with [3H]myristate, which is incorporated exclusively into phosphatidylcholine, the substrate for the v-Src-induced PLD. There was no difference in the effect of GTP gamma S-induced PLD activity on v-Src-transformed and BALB/c 3T3 cells when the cells were prelabeled with [3H]arachidonate, which is not incorporated into phospholipids that are substrates for the v-Src-induced PLD. Similarly, GDP beta S inhibited PLD activity in v-Src-transformed cells much more strongly than in BALB/c 3T3 cells when [3H]myristate was used to prelabel the cells. The GTP-dependent activation of PLD by v-Src was dependent upon the presence of ATP but was unaffected by either cholera or pertussis toxin. These data suggest that v-Src induces PLD activity through a phosphorylation event and is mediated by a cholera and pertussis toxin-insensitive G protein.
