Activation of the Xenopus oocyte mitogen-activated protein kinase pathway by Mos is independent of Raf.
Synthesis of the protein kinase Mos is required for progesterone-induced activation of MAP kinase, M-phase promoting factor (MPF), and meiotic maturation of Xenopus oocytes. Mos can function as a MAP kinase kinase kinase, leading to activation of MAP kinase; how Mos causes activation of MPF is not yet known. The protein kinase Raf, which acts as a MAP kinase kinase kinase in somatic cells, also appears to be involved in meiotic maturation, but recent work has suggested that the Raf acts downstream of Mos activity during oocyte maturation. Using an oocyte cell-free system, we report here that a dominant negative Raf, which inhibits Ras-induced MAP kinase activation, does not block Mos-induced activation in vitro. These results indicate that, in contrast to previous conclusions, Mos-induced oocyte MAP kinase activation proceeds independently of Raf. Using a dominant-negative MAP kinase construct, we also show that most of the mitogen-induced hyperphosphorylation and dramatic gel retardation of Raf, which is often taken as a marker for the activation of Raf by upstream components, is actually dependent on, and thus downstream of, MAP kinase activation.
Shibuya
E K
EK
Department of Anatomy and Cell Biology, University of Alberta, Edmonton, Canada.
Morris
J
J
Rapp
U R
UR
Ruderman
J V
JV
D000818:Animals; D002474:Cell-Free System; D004586:Electrophoresis; D005260:Female; D020929:Mitogen-Activated Protein Kinase Kinases; D008970:Molecular Weight; D009865:Oocytes / Q000201:enzymology*; D010766:Phosphorylation; D011494:Protein Kinases / Q000819:agonists / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism* / Q000494:pharmacology; D011518:Proto-Oncogene Proteins / Q000378:metabolism* / Q000494:pharmacology; D017231:Proto-Oncogene Proteins c-mos / Q000378:metabolism* / Q000494:pharmacology; D019908:Proto-Oncogene Proteins c-raf; D015398:Signal Transduction / Q000502:physiology; D014981:Xenopus; D018631:ras Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism
D011518:Proto-Oncogene Proteins; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D017231:Proto-Oncogene Proteins c-mos; D019908:Proto-Oncogene Proteins c-raf; D020929:Mitogen-Activated Protein Kinase Kinases; D018631:ras Proteins