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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 metaphase1. In frog eggs, Ca2+ activates Ca2+/calmodulin-activated kinase, which inactivates cytostatic factor2,3,4,5, allowing the anaphase-promoting factor to turn on and ubiquitinate cyclins and securin, which returns the cell cycle to interphase6. Here we show that the calcium-activated protein phosphatase calcineurin7 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 pathway4,5,9, and we show that Fizzy/Cdc20, a key regulator of the anaphase-promoting factor6, is an excellent substrate for this phosphatase.

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Figure 1: Calcineurin is responsible for the dephosphorylation of Apc3 and Fzy induced by Ca2+.
Figure 2: Ca 2+ activates calcineurin in Xenopus egg extracts.
Figure 3: Calcineurin and CaMKII activities together allow nuclear envelope formation.
Figure 4: Calcineurin is not required for mitotic cell cycles, but a second phosphatase activity appears after the calcineurin transient.


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We thank H. Yamano and M. Dasso for gifts of the Xenopus Erp1 and GFP–NLS clones, respectively; C. Klee for advice about calcineurin biochemistry; T. Nishiyama and K. Ohsumi for anti-calcineurin A serum, anti-Erp1 antibody and sharing details of their parallel findings with us; S. Shenolikar for clones for inhibitor-1; H. Mahbubani and J. Kirk for care of frogs; and other members of the laboratory for advice and reagents. S.M. is supported by an EMBO long-term fellowship.

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Mochida, S., Hunt, T. Calcineurin is required to release Xenopus egg extracts from meiotic M phase. Nature 449, 336–340 (2007).

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