Emi1 and Emi2 regulate meiotic maturation and cytostatic factor arrest of vertebrate eggs

Emi1 is an Anaphase Promoting Complex (APC) inhibitor that plays a critical role in the unfertilized egg. An activity called cytostatic factor (CSF) arrests unfertilized vertebrate eggs in metaphase of meiosis II with high cyclin B/cdc2 activity to prevent parthenogenesis. The Mos/MEK/MAPK/Rsk pathway establishes CSF arrest and Emi1 maintains it by protecting cyclin B from APC-mediated proteolysis. Calcium signaling downstream of fertilization inactivates Mos and Emi1, initiating embryonic divisions.; To investigate the unknown role of Emi1 earlier in meiosis, Xenopus laevis oocytes were injected with Emi1 neutralizing antibodies. G2 oocytes require Emi1 function for meiotic entry in response to progesterone, independent of the APC. During the meiosis I to meiosis II transition, Emi proteins prevent the complete destruction of cyclin B by inhibiting the APC, thus suppressing chromosome decondensation and DNA synthesis between the coupled rounds of meiotic division. Furthermore, Emi proteins sustain an active MAPK pathway to establish CSF arrest.; In mitotic cells, Emi1 is phosphorylated by cyclin B/cdc2 and Plk1, triggering its recognition by SCFbetaTrCP ubiquitin ligase and its subsequent destruction. Since cyclin B/cdc2 and Plk1, activity are high during metaphase arrest in unfertilized eggs, how does Emi1 avoid proteolysis and maintain CSF activity? Using Xenopus CSF egg extract, we show that endogenous Emi1 is an exceedingly stable protein, whereas exogenous Emi1 is rapidly destroyed with similar kinetics as in mitosis. Therefore, a stabilizing factor protects Emi1 from SCFbetaTrCP, allowing maintenance of CSF arrest.; Emi2, a homolog of Emi1 and a conserved APC inhibitor, is (1) sufficient for CSF arrest, (2) stable in CSF-arrested eggs, and (3) rapidly destroyed during CSF release in response to calcium signaling in a Plx1 and SCFbetaTrCP-dependent process.