Abstract
Vertebrate eggs awaiting fertilization are arrested at metaphase of meiosis II by a biochemical activity termed cytostatic factor (CSF)1,2. This activity inhibits the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase that triggers anaphase onset and mitotic/meiotic exit by targeting securin and M-phase cyclins for destruction3,4,5. On fertilization a transient rise in free intracellular calcium6 causes release from CSF arrest and thus APC/C activation. Although it has previously been shown that calcium induces the release of APC/C from CSF inhibition through calmodulin-dependent protein kinase II (CaMKII)7,8, the relevant substrates of this kinase have not been identified. Recently, we characterized XErp1 (Emi2), an inhibitor of the APC/C and key component of CSF activity in Xenopus egg extract9. Here we show that calcium-activated CaMKII triggers exit from meiosis II by sensitizing the APC/C inhibitor XErp1 for polo-like kinase 1 (Plx1)-dependent degradation. Phosphorylation of XErp1 by CaMKII leads to the recruitment of Plx1 that in turn triggers the destruction of XErp1 by phosphorylating a site known to serve as a phosphorylation-dependent degradation signal. These results provide a molecular explanation for how the fertilization-induced calcium increase triggers exit from meiosis II.
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Acknowledgements
We thank O. Stemmann and F. Barr for reagents and discussions during this work. We are grateful to I. Gorr and D. Boos for advice on Δ90 experiments, S. Hümmer for continuous support and R. Neef for assistance with the far-western analyses.
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Supplementary information
Supplementary Figure S1
Excess IVT-XErp1WT or -XErp1T195A blocks calcium-induced CSF release demonstrating that both proteins are active and folded properly. The stability of 35S-labelled IVT-securin and the morphology of the chromatin was used to monitor the cell cycle stage of the extract. (PDF 547 kb)
Supplementary Figure S2
XErp1 is degraded in Xenopus δ90 extracts possesing CaMKII activity. (PDF 436 kb)
Supplementary Figure S3
CaMKII triggers Plx1-dependent degradation of XErp1. Endogenous XErp1 is rapidly degraded in CSF extract supplemented with constitutively active CaMKII or calcium. CaMKII induced degradation of XErp1 depends on Plx1 activity as indicated by the fact that a dominant negative acting form of Plx1 (PBD) prevents CaMKII triggered XErp1 destruction. (PDF 327 kb)
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Rauh, N., Schmidt, A., Bormann, J. et al. Calcium triggers exit from meiosis II by targeting the APC/C inhibitor XErp1 for degradation. Nature 437, 1048–1052 (2005). https://doi.org/10.1038/nature04093
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DOI: https://doi.org/10.1038/nature04093
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