REQUIREMENT FOR RAF AND MAP KINASE FUNCTION DURING THE MEIOTIC MATURATION OF XENOPUS-OOCYTES

The role of Raf and MAPK (mitogen-activated 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 co incident with the activation of MAPK. Expression of a kinase-defective mutant of the human Raf-1 protein (KD-RAF) inhibited progesterone-med iated 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 re ceptor tyrosine kinase (EGFr), and Ha-Ras(V12). KD-RAF completely inhi bited GVBD induced by the RTK pathway. In contrast, KD-RAF did not inh ibit GVBD and the progression to Meiosis II in progesterone-treated oo cytes. Injection of Mos-specific antisense oligodeoxy-ribonucleotides inhibited MAPK activation in response to progesterone and Tpr-Met, but failed to inhibit these events in oocytes expressing an oncogenic del etion mutant of Raf-1 (DELTAN'Raf). Injection of antisense oligodeoxyr ibonucleotides to Mos also reduced the progesterone- and Tpr-Met-induc ed electrophoretic mobility shift of Xenopus Raf. These results demons trate that RTKs and progesterone participate in distinct yet overlappi ng signaling pathways resulting in the activation of maturation or M-p hase promoting factor (MPF). Maturation induced by the RTK pathway req uires activation of Raf and MAPK, while progesterone-induced maturatio n does not. Furthermore, the activation of MAPK in oocytes appears to require the expression of Mos.