In the Xenopus oocyte, progesterone triggers M phase Promoting Factor (MPF) activation in a protein synthesis dependent manner. Although the synthesis of the p42 MAPK activator Mos appears to be required for MPF activation, p42 MAPK activity has been shown to be dispensable. To clarify this paradox, we attempted to activate the p42 MAPK pathway independently of Mos synthesis by cloning and using Xenopus H-Ras in the oocyte. We demonstrate that the injection of the constitutively active Xe H-RasV12 mutant induces p42 MAPK and MPF activation through two independent pathways. Xe H-RasV12 induces only a partial activation of p42 MAPK when protein synthesis and MPF activation are prevented. A full level of p42 MAPK activation is reached when MPF is activated and Mos is present. In contrast, MPF activation induced by Xe HRasV12 is achieved independently of Mos synthesis and p42 MAPK activation but still depends on protein synthesis. Therefore, the amphibian oocyte represents a new model system to analyse an original H-Ras pathway ending to MPF activation and distinct from the p42 MAPK pathway. The identification of the proteins synthesized in response to Xe H-RasV12 and required for MPF activation, represents an important clue in understanding the mechanism of progesterone action.

Xenopus oocytes are arrested in prophase of the first meiotic division. In response to progesterone, they re-enter meiosis and arrest again in metaphase of the second meiotic division. This process, called meiotic maturation, is under the control of the Cyclin B-Cdc2 complex, M phase promoting factor (MPF). Injection of a constitutively active Xenopus H-Ras protein activates MPF, suggesting that Ras proteins could be implicated in the progesterone transduction pathway. The aim of this study was (1) to elucidate the pathway triggered by H-Ras leading to MPF activation in Xenopus oocytes and (2) to investigate whether endogenous H-Ras is involved in the physiological process of meiotic maturation. We generated three constitutively active double mutants, each of them recruiting a single effector in mammalian cells, mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K) or RalGDS. Our results show that the activation of a PI3K-related enzyme is crucial for H-Ras-induced MPF activation, whereas the recruitment of either MAPK or RalGDS is not. However, although the H-Ras/PI3K pathway is functional in Xenopus oocytes, it is not the physiological transducer of progesterone responsible for meiotic resumption.