Abstract. The role of Raf and MAPK (mitogenactivated protein kinase) during the maturation of Xenopus oocytes was investigated. Treatment of oocytes with progesterone resulted in a shift in the electrophoretic mobility of Raf at the onset of germinal vesicle breakdown (GVBD), which was coincident with the activation of MAPK. Expression of a kinasedefective mutant of the human Raf-1 protein (KD-RAF) inhibited progesterone-mediated MAPK activation. MAPK activation was also inhibited by KD-Raf in oocytes expressing signal transducers of the receptor tyrosine kinase (RTK) pathway, including an activated tyrosine kinase (Tpr-Met), a receptor tyrosine kinase (EGFr), and Ha-Ras w2. KD-RAF completely inhibited GVBD induced by the RTK pathway. In contrast, KD-RAF did not inhibit GVBD and the progression to Meiosis II in progesterone-treated oocytes. Injection of Mos-specific antisense oligodeoxyribonucleotides inhibited MAPK activation in response to progesterone and Tpr-Met, but failed to inhibit these events in oocytes expressing an oncogenic deletion mutant of Raf-1 (AN'Raf). Injection of antisense oligodeoxyribonucleotides to Mos also reduced the progesterone-and Tpr-Met-induced electrophoretic mobility shift of Xenopus Raf. These results demonstrate that RTKs and progesterone participate in distinct yet overlapping signaling pathways resulting in the activation of maturation or M-phase promoting factor (MPF). Maturation induced by the RTK pathway requires activation of Raf and MAPK, while progesterone-induced maturation does not. Furthermore, the activation of MAPK in oocytes appears to require the expression of Mos.T I-IE c-raf-1 gene is the normal cellular counterpart of the v-raf transforming gene of the murine sarcoma virus 3611 (59). The proto-oncogene product encoded by the c-raf-1 gene, Raf-1, is a 70-74-kD phosphoprotein with intrinsic kinase activity towards serine and threonine residues (for review see Morrison [45]). The Raf-1 protein consists of a carboxyl-terminal kinase domain and an aminoterminal regulatory region that is deleted in v-raf, generating a constitutively active kinase (29,68,69). Raf-1 is ubiquitously expressed and is hyperphosphorylated, primarily on serine residues, in many cell lines in response to mitogen treatment (30, 46). A close correlation has been established between Raf-1 hyperphosphorylation and activation of its kinase activity in cells stimulated by growth factors (30,40). Moreover, Raf-1 activity is required for growth factorinduced proliferation of NIH/3T3 cells and certain erythroid cells (11,34).Recently, genetic and biochemical evidence has placed Raf-1 in a signal transduction cascade downstream of both receptor tyrosine kinases (RTKs) 1 and ras, and upstream of 1. Abbreviations used in this paper: EGFr, EGF receptor; GVBD, germinal vesicle breakdown; IGF, insulin-like growth factor; KD-Raf, kinasemitogen-activated protein kinase (MAPK) (for review see Roberts [60]). Furthermore, recent reports suggest that the protein which activates MAPK (also known as MAPK kinase [M...
