Dev Biol 2005 Mar 15;2792:345-55. doi: 10.1016/j.ydbio.2004.12.025.

APC/C-Cdc20-mediated degradation of cyclin B participates in CSF arrest in unfertilized Xenopus eggs.

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In vertebrates, unfertilized eggs are arrested at meiotic metaphase II (meta-II) by cytostatic factor (CSF), with Cdc2 activity maintained at a constant, high level. CSF is thought to suppress cyclin B degradation through the inhibition of the anaphase-promoting complex/cyclosome (APC/C)-Cdc20 while cyclin B synthesis continues in unfertilized eggs. Thus, it is a mystery how Cdc2 activity is kept constant during CSF arrest. Here, we show that the APC/C-Cdc20 can mediate cyclin B degradation in CSF-arrested Xenopus eggs and extracts, in such a way that when Cdc2 activity is elevated beyond a critical level, APC/C-Cdc20-dependent cyclin B degradation is activated and Cdc2 activity consequently declines to the critical level. This feedback control of Cdc2 activity is shown to be required for keeping Cdc2 activity constant during meta-II arrest. We have also shown that Mos/MAPK pathway is essential for preventing the cyclin B degradation from inactivating Cdc2 below the critical level required to sustain meta-II arrest. Our results indicate that under CSF arrest, Mos/MAPK activity suppresses cyclin B degradation, preventing Cdc2 activity from falling below normal meta-II levels, whereas activation of APC/C-Cdc20-mediated cyclin B degradation at elevated levels of Cdc2 activity prevents Cdc2 activity from reaching excessively high levels.

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Species referenced: Xenopus laevis
Genes referenced: ccnb1.2 cdc20 cdk1