西亚试剂：Calcineurin is required to release Xenopus egg extracts fro

Calcineurin is required to release Xenopus egg extracts from meiotic M phase

Satoru Mochida1 & Tim Hunt1

Correspondence to: Tim Hunt1 Correspondence and requests for materials should be addressed to T.H. (Email: tim.hunt@cancer.org.uk).

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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