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Microinjection of Xenopus oocytes with ras protein (p21) was used to investigate the role of phospholipid metabolism in ras-induced meiotic maturation. Induction of meiosis by ras was compared with induction by progesterone, insulin, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Neomycin, which specifically binds to phosphatidylinositides and inhibits their metabolism, blocked meiotic maturation induced by ras or insulin but not by progesterone or TPA. In addition, p21 and TPA, but not insulin or progesterone, stimulated the incorporation of 32Pi into oocyte lipids. ras protein specifically stimulated 32P incorporation into phosphatidylinositides, whereas both ras and TPA stimulated 32P incorporation into phosphatidylcholine and phosphatidylethanolamine. The stimulatory effect of p21 on phosphatidylinositide metabolism correlated with the dose response and kinetics of ras-induced meiotic maturation. In addition, the ras oncogene protein was more potent than the proto-oncogene protein both in inducing meiotic maturation and in stimulating phosphatidylinositide metabolism. These results indicate that phosphatidylinositide turnover is required for ras-induced meiosis and suggest that phosphatidylinositide-derived second messengers mediate the biological activity of ras in Xenopus oocytes.

Single amino acid substitutions were introduced into a region of the rasH protein (residues 116, 117, and 119) homologous to a variety of diverse GTP-binding proteins. Each of the mutant p21 proteins displayed a significant reduction (10- to 5,000-fold) in GTP binding affinity. Activated rasH proteins deficient in GTP binding were unaltered in their ability to morphologically transform NIH 3T3 cells.