Dissociation of MAP kinase activation and MPF activation in hormone-stimulated maturation of Xenopus oocytes

MAP kinase activation occurs during meiotic maturation of oocytes from all animals, but the requirement for MAP kinase activation in reinitiation of m eiosis appears to vary between different classes. In particular, it has bec ome accepted that MAP kinase activation is necessary for progesterone-stimu lated meiotic maturation of Xenopus oocytes, while this is clearly not the case in other systems. In this paper, we demonstrate that MAP kinase activa tion in Xenopus oocytes is an early response to progesterone and can be tem porally dissociated from MPF activation. We show that MAP kinase activation can be suppressed by treatment with geldanamycin or by overexpression of t he MAP kinase phosphatase Pyst1. A transient and low-level early activation of MAP kinase increases the efficiency of cell cycle activation later on, when MAP kinase activity is no longer essential. Many oocytes can still und ergo reinitiation of meiosis in the absence of active MAP kinase, Suppressi on of MAP kinase activation does not affect the formation or activation of Cdc2-cyclin B complexes, but reduces the level of active Cdc2 kinase, We di scuss these findings in the context of a universal mechanism for meiotic ma turation in oocytes throughout the animal kingdom.