Calcium elevation at fertilization coordinates phosphorylation of XErp1/Emi2 by Plx1 and CaMK II to release metaphase arrest by cytostatic factor.
BACKGROUND
Vertebrate oocytes are arrested at second meiotic metaphase by cytostatic factor (CSF) while awaiting fertilization. Accumulating evidence has suggested that inhibition of the anaphase-promoting complex/cyclosome (APC/C) is responsible for this arrest. Xenopus polo-like kinase 1 (Plx1) is required for activation of the APC/C at the metaphase-anaphase transition, and calcium elevation, upon fertilization/activation of eggs, acting through calmodulin-dependent kinase II (CaMKII) is sufficient to activate the APC/C and terminate CSF arrest. However, connections between the Plx1 pathway and the CaMKII pathway have not been identified.


RESULTS
Overexpression of Plx1 causes CSF release in the absence of calcium, and depletion of Plx1 from egg extracts blocks induction of CSF release by calcium and CaMKII. Prior phosphorylation of the APC/C inhibitor XErp1/Emi2 by CaMK II renders it a good substrate for Plx1, and phosphorylation by both kinases together promotes its degradation in egg extracts. The pathway is enhanced by the ability of Plx1 to cause calcium-independent activation of CaMKII. The results identify the targets of CaMKII and Plx1 that promote egg activation and define the first known pathway of CSF release in which an APC/C inhibitor is targeted for degradation only when both CaMKII and Plx1 are active after calcium elevation at fertilization.


CONCLUSIONS
Plx1 with an intact polo-box domain is necessary for release of CSF arrest and sufficient when overexpressed. It acts at the same level as CaMKII in the pathway of calcium-induced CSF release by cooperating with CaMKII to regulate APC/C regulator(s), such as XErp1/Emi2, rather than by directly activating the APC/C itself.
Liu
Junjun
J
Howard Hughes Medical Institute and Department of Pharmacology, University of Colorado School of Medicine, Denver, Colorado 80262, USA.
Maller
James L
JL
D064173:Anaphase-Promoting Complex-Cyclosome; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D002467:Cell Nucleus / Q000648:ultrastructure; D003001:Cloning, Molecular; D017931:DNA Primers; D044783:F-Box Proteins / Q000378:metabolism*; D005306:Fertilization / Q000502:physiology*; D015151:Immunoblotting; D008297:Male; D008677:Metaphase / Q000502:physiology*; D009865:Oocytes / Q000378:metabolism / Q000502:physiology*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D017231:Proto-Oncogene Proteins c-mos / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D013094:Spermatozoa / Q000648:ultrastructure; D043743:Ubiquitin-Protein Ligase Complexes / Q000378:metabolism; D014981:Xenopus; D029867:Xenopus Proteins / Q000235:genetics / Q000378:metabolism*; D000097869:Polo-Like Kinase 1
D064173:Anaphase-Promoting Complex-Cyclosome; D002118:Calcium; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D018797:Cell Cycle Proteins; D017931:DNA Primers; D044783:F-Box Proteins; D017346:Protein Serine-Threonine Kinases; D017231:Proto-Oncogene Proteins c-mos; D043743:Ubiquitin-Protein Ligase Complexes; D029867:Xenopus Proteins; C498166:FBXO43 protein, Xenopus; C101085:Plk1 protein, Xenopus; D000097869:Polo-Like Kinase 1