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Updated: Aug 10, 2026
A Cell Free Assay to Study Chromatin Decondensation at the End of Mitosis
Published on: December 19, 2015
Fizzy is required for activation of the APC/cyclosome in Xenopus egg extracts
T Lorca1, A Castro, A M Martinez
1Centre de Recherches de Biochimie Macromoléculaire, CNRS UPR 1086, 1919 route de Mende, 34293 Montpellier Cedex 5, France.
Abstract:
The Xenopus homologue of Drosophila Fizzy and budding yeast CDC20 has been characterized. The encoded protein (X-FZY) is a component of a high molecular weight complex distinct from the APC/cyclosome. Antibodies directed against FZY were produced and shown to prevent calmodulin-dependent protein kinase II (CaMKII) from inducing the metaphase to anaphase transition of spindles assembled in vitro in Xenopus egg extracts, and this was associated with suppression of the degradation of mitotic cyclins. The same antibodies suppressed M phase-promoting factor (MPF)-dependent activation of the APC/cyclosome in interphase egg extracts, although they did not appear to alter the pattern or extent of MPF-dependent phosphorylation of APC/cyclosome subunits. As these phosphorylations are thought to be essential for APC/cyclosome activation in eggs and early embryos, we conclude that at least two events are required for MPF to activate the APC/cyclosome, allowing both chromatid segregation and full degradation of mitotic cyclins. The first one, which does not require FZY function, is the phosphorylation of APC/cyclosome subunits. The second one, that requires FZY function (even in the absence of MAD2 protein and when the spindle assembly checkpoint is not activated) is not yet understood at its molecular level.
Insights
Xenopus Fizzy (X-FZY) is crucial for activating the Anaphase-Promoting Complex/Cyclosome (APC/cyclosome), enabling mitotic cyclin degradation and chromatid segregation. This process requires X-FZY after initial APC/cyclosome subunit phosphorylation.
Area of Science:
- Cell Biology
- Molecular Biology
- Developmental Biology
Background:
- The Anaphase-Promoting Complex/Cyclosome (APC/cyclosome) regulates cell cycle progression.
- Fizzy/CDC20 proteins are key activators of the APC/cyclosome.
- Understanding APC/cyclosome regulation is vital for cell division control.
Purpose of the Study:
- To characterize the Xenopus homologue of Fizzy/CDC20 (X-FZY).
- To investigate the role of X-FZY in APC/cyclosome activation and mitotic progression in Xenopus egg extracts.
Main Methods:
- Production of antibodies against X-FZY.
- In vitro spindle assembly assays using Xenopus egg extracts.
- Analysis of mitotic cyclin degradation and APC/cyclosome activity.
Main Results:
- X-FZY is a component of a distinct high molecular weight complex.
- Anti-FZY antibodies inhibited CaMKII-induced metaphase-anaphase transition and cyclin degradation.
- Anti-FZY antibodies suppressed MPF-dependent APC/cyclosome activation without affecting subunit phosphorylation.
Conclusions:
- APC/cyclosome activation by MPF requires at least two distinct events.
- The first event involves phosphorylation of APC/cyclosome subunits.
- The second, FZY-dependent event is essential for chromatid segregation and full cyclin degradation, even without spindle assembly checkpoint activation.
