Calcineurin is required to release Xenopus egg extracts from meiotic M phase.
Fertilization induces a transient increase in cytoplasmic Ca2+ concentration in animal eggs that releases them from cell cycle arrest in the second meiotic metaphase. In frog eggs, Ca2+ activates Ca2+/calmodulin-activated kinase, which inactivates cytostatic factor, allowing the anaphase-promoting factor to turn on and ubiquitinate cyclins and securin, which returns the cell cycle to interphase. Here we show that the calcium-activated protein phosphatase calcineurin is also important in this process. Calcineurin is transiently activated after adding Ca2+ to egg extracts, and inhibitors of calcineurin such as cyclosporin A (ref. 8) delay the destruction of cyclins, the global dephosphorylation of M-phase-specific phosphoproteins and the re-formation of a fully functional nuclear envelope. We found that a second wave of phosphatase activity directed at mitotic phosphoproteins appears after the spike of calcineurin activity. This activity disappeared the next time the extract entered M phase and reappeared at the end of mitosis. We surmise that inhibition of this second phosphatase activity is important in allowing cells to enter mitosis, and, conversely, that its activation is required for a timely return to interphase. Calcineurin is required to break the deep cell cycle arrest imposed by the Mos-MAP (mitogen-activated protein) kinase pathway, and we show that Fizzy/Cdc20, a key regulator of the anaphase-promoting factor, is an excellent substrate for this phosphatase.
Mochida
Satoru
S
Cancer Research UK, London Research Institute, Clare Hall Laboratories, South Mimms, Hertfordshire EN6 3LD, UK.
Hunt
Tim
T
D000818:Animals; D019703:Calcineurin / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D064199:Cdc20 Proteins; D018797:Cell Cycle Proteins / Q000378:metabolism; D002455:Cell Division / Q000187:drug effects; D002457:Cell Extracts; D005260:Female; D005306:Fertilization / Q000187:drug effects / Q000502:physiology*; D008540:Meiosis / Q000187:drug effects; D008938:Mitosis / Q000187:drug effects; D009685:Nuclear Envelope / Q000378:metabolism; D009865:Oocytes / Q000166:cytology* / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D014981:Xenopus; D029867:Xenopus Proteins / Q000378:metabolism
D064199:Cdc20 Proteins; C577501:Cdc20 protein, Xenopus; D018797:Cell Cycle Proteins; D002457:Cell Extracts; D029867:Xenopus Proteins; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D019703:Calcineurin; D010744:Phosphoric Monoester Hydrolases; D002118:Calcium