Mitogen-activated protein kinase (MAPK) and MAPK kinase (MAPKK) are activated during Xenopus oocyte maturation concomitant with the activation of maturation promoting factor (MPF). We reported previously that an anti-MAPKK neutralizing antibody inhibited progesterone- or Mos- induced initiation of oocyte maturation. Here, we show that injection of CL100 (also called MAPK phosphatase-1) into immature oocytes inhibited progesterone-induced oocyte maturation as well as MAPK activation and that injection of mRNA encoding a constitutively active MAPKK induced activation of histone H1 kinase and germinal vesicle breakdown in the absence of progesterone. Injection of recombinant STE11 protein (a yeast MAPKK kinase) also induced initiation of oocyte maturation. These data support the idea that the MAPKK/MAPK cascade plays an important role in oocyte maturation. Interestingly, injection of the active MAPKK mRNA or the STE11 protein resulted in induction and accumulation of Mos protein. Furthermore, in the presence of cycloheximide, the STE11-induced activation of MPF as well as the induction and accumulation of Mos was blocked, and the activation of MAPK was greatly reduced. The increase in Mos protein and the activation of MAPK by injecting cyclin A protein into immature oocytes were both blocked also by cycloheximide treatment. These results are consistent with an idea that there may exist a positive feedback loop consisting of Mos, the MAPKK/MAPK cascade, and MPF, which may be important for the initiation of oocyte maturation induced by progesterone.
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