Maintenance of G2 arrest in the Xenopus oocyte: a role for 14-3-3-mediated inhibition of Cdc25 nuclear import.
Cdc2-cyclin B1 in the G2-arrested Xenopus oocyte is held inactive by phosphorylation of Cdc2 at two negative regulatory sites, Thr14 and Tyr15. Upon treatment with progesterone, these sites are dephosphorylated by the dual specificity phosphatase, Cdc25, leading to Cdc2-cyclin B1 activation. Whereas maintenance of the G2 arrest depends upon preventing Cdc25-induced Cdc2 dephosphorylation, the mechanisms responsible for keeping Cdc25 in check in these cells have not yet been described. Here we report that Cdc25 in the G2-arrested oocyte is bound to 14-3-3 proteins and that progesterone treatment abrogates this binding. We demonstrate that Cdc25, apparently statically localized in the cytoplasm, is actually capable of shuttling in and out of the oocyte nucleus. Binding of 14-3-3 protein markedly reduces the nuclear import rate of Cdc25, allowing nuclear export mediated by a nuclear export sequence present in the N-terminus of Cdc25 to predominate. If 14-3-3 binding to Cdc25 is prevented while nuclear export is inhibited, the coordinate nuclear accumulation of Cdc25 and Cdc2-cyclin B1 facilitates their mutual activation, thereby promoting oocyte maturation.
Yang
J
J
Department of Pharmacology and Cancer Biology, Duke University Medical Center, C366 LSRC, Research Drive, Box 3686, Durham, NC 27710, USA.
Winkler
K
K
Yoshida
M
M
Kornbluth
S
S
D048948:14-3-3 Proteins; D000818:Animals; D001483:Base Sequence; D001692:Biological Transport; D016203:CDC2 Protein Kinase / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism*; D019926:Cyclin B / Q000378:metabolism; D017931:DNA Primers; D005231:Fatty Acids, Unsaturated / Q000494:pharmacology; D016195:G2 Phase; D016297:Mutagenesis, Site-Directed; D009865:Oocytes / Q000166:cytology* / Q000187:drug effects; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011506:Proteins / Q000378:metabolism / Q000502:physiology*; D014446:Tyrosine 3-Monooxygenase; D014982:Xenopus laevis; D020687:cdc25 Phosphatases
D048948:14-3-3 Proteins; D018797:Cell Cycle Proteins; D019926:Cyclin B; D017931:DNA Primers; D005231:Fatty Acids, Unsaturated; D011506:Proteins; D014446:Tyrosine 3-Monooxygenase; D016203:CDC2 Protein Kinase; D010749:Phosphoprotein Phosphatases; D020687:cdc25 Phosphatases; C038753:leptomycin B