Raf-1 protein kinase is important for progesterone-induced Xenopus oocyte maturation and acts downstream of mos

In somatic cells, the Raf-I serine/threonine protein kinase is activated by several polypeptide growth factors. We investigated the role of Raf-I in progesterone-induced meiotic maturation ofXenopus laevis oocytes. Raf-l enzymatic activity and phosphorylation (reflected by a mobility shift on sodium dodecyl sulfate gels) were increased in oocytes following progesterone stimulation. The increase in Raf-I activity was concurrent with an elevation in the activity of mitogen-activated protein (MAP) kinase. When RNA encoding an oncogenic form of Raf-1 (v-Raf) was injected into immature oocytes, MAP kinase mobility shift, germinal vesicle breakdown, and histone Hi phosphorylation increased markedly. When RNA encoding a dominant-negative version of Raf-I was injected, progesterone-induced oocyte maturation was blocked. When RNA encoding Xenopus mos (mosxe) was injected into oocytes, Raf-1 and MAP kinase mobility shifts were observed after several hours. Also, when antisense mosxe oligonucleotides were injected into oocytes, progesterone-induced Raf-1 and MAP kinase mobility shifts were blocked. Finally, when antisense mosxe oligonucleotides were coinjected with v-Raf RNA into oocytes, histone Hi kinase activation, germinal vesicle breakdown, and MAP kinase mobility shift occurred. These findings suggest that Raf-I activity is required for progesterone-induced oocyte maturation and that Raf-1 is downstream of mos ' activity. The Raf-1 serine/threonine protein kinase is activated i