New insight into the molecular mechanisms that establish the cytostatic factor activity in mouse oocytes

Vertebrate eggs arrest at metaphase of the second meiotic division before fertilization under the effect of a cytostatic factor (CSF). This arrest is established during oocyte maturation by the MAPK kinase module, comprised by Mos, MEK, MAPKs and p90Rsk. Maintenance of CSF arrest at metaphase requires inhibitors of the anaphase promoting complex (APC) like Emi1, which sequesters the APC activator Cdc20. Although it was proposed that the Mos pathway and Emi1 act independently, neither one alone is sufficient to entirely reproduce CSF arrest. Herein we demonstrate that p90Rsk2 associates with and phosphorylates Emi1 upstream of the binding region for Cdc20. This phosphorylation stabilizes the interaction between Emi1 and Cdc20, sugegsting that it can potentiate the cytostatic activity of Emi1. Indeed, experiments in transfected cells and 2-cell embryos indicate that Emi1 and p90Rsk2 cooperate to induce the metaphase arrest. Moreover, oocyte maturation was impaired by interfering with the interaction between p90Rsk2 and Emi1 or by RNA interference of Emi1. In addition, mutation of two residues in Emi1 phosphorylated by p90Rsk2 impairs the cooperation between the two proteins in vivo. Our results indicate that p90Rsk2 and Emi1 functionally interact during oocyte maturation and that the Mos pathway establishes CSF activity through stabilization of an APC-inhibitory complex composed by Emi1 and Cdc20 before fertilization.