Emi1-mediated M-phase arrest in Xenopus eggs is distinct from cytostatic factor arrest.
Oocytes of most vertebrates arrest at metaphase of the second meiosis (meta-II) to await fertilization, thus preventing parthenogenetic activation. This arrest is caused by a cytoplasmic activity called cytostatic factor (CSF), which was first identified in the frog Rana pipiens oocyte >30 years ago. CSF arrest is executed by maintaining the activity of cyclin B-Cdc2 at elevated levels largely through prevention of cyclin B destruction. Although CSF arrest is established by the Mos-mitogen-activated protein kinase pathway and is released by the Ca-calmodulin kinase II pathway, it remains unclear precisely how cyclin B destruction is regulated. Recently, an early mitotic inhibitor, Emi1, was reported to be a critical component of CSF. This report has been expected to provide a final resolution to the CSF problem because Emi1 inhibits the anaphase-promoting complex/cyclosome, a ubiquitin ligase for cyclin B destruction, through sequestration of Cdc20, an activator for the anaphase-promoting complex/cyclosome. In mitotic cycles, however, Emi1 is destroyed in every pro-metaphase, and accordingly, it is unclear why Emi1 should be required for CSF activity, which is seen only in meta-II. Here, we show that Emi1 is absent in unfertilized mature Xenopus eggs and that exogenous Emi1 is destroyed in meta-II and mitotic metaphase. The expression of Emi1 in oocytes hinders meiotic progression. Although both Emi1 and Mos can inhibit progression through M phase, the Emi1-mediated arrest does not require mitogen-activated protein kinase activity and is not released by Ca. Together, our results indicate that Emi1 is unlikely to be a component of CSF.
Laboratory of Cell and Developmental Biology, Graduate School of Bioscience, Tokyo Institute of Technology, Nagatsuta, Midoriku, Yokohama 226-8501, Japan. kohsumi@bio.titech.ac.jp
Keita
K
Ohsumi
Ayako
A
Koyanagi
Tomomi M
TM
Yamamoto
Tetsuya
T
Gotoh
Takeo
T
Kishimoto
D000818:Animals; D002118:Calcium / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D004625:Embryo, Nonmammalian / Q000502:physiology; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D008938:Mitosis / Q000502:physiology*; D009865:Oocytes / Q000502:physiology*; D017231:Proto-Oncogene Proteins c-mos / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D029867:Xenopus Proteins / Q000378:metabolism*; D014982:Xenopus laevis
D018797:Cell Cycle Proteins; C429107:FBXO5 protein, Xenopus; D011993:Recombinant Fusion Proteins; D029867:Xenopus Proteins; D017231:Proto-Oncogene Proteins c-mos; D020928:Mitogen-Activated Protein Kinases; D002118:Calcium