Maintenance of CSF arrest: A role for Cdc2 and PP2A-mediated regulation of Emi2

Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Current Biology (Impact Factor: 9.57). 03/2007; 17(3):213-24. DOI: 10.1016/j.cub.2006.12.045
Source: PubMed


Vertebrate oocytes are arrested in metaphase II of meiosis prior to fertilization by cytostatic factor (CSF). CSF enforces a cell-cycle arrest by inhibiting the anaphase-promoting complex (APC), an E3 ubiquitin ligase that targets Cyclin B for degradation. Although Cyclin B synthesis is ongoing during CSF arrest, constant Cyclin B levels are maintained. To achieve this, oocytes allow continuous slow Cyclin B degradation, without eliminating the bulk of Cyclin B, which would induce release from CSF arrest. However, the mechanism that controls this continuous degradation is not understood.
We report here the molecular details of a negative feedback loop wherein Cyclin B promotes its own destruction through Cdc2/Cyclin B-mediated phosphorylation and inhibition of the APC inhibitor Emi2. Emi2 bound to the core APC, and this binding was disrupted by Cdc2/Cyclin B, without affecting Emi2 protein stability. Cdc2-mediated phosphorylation of Emi2 was antagonized by PP2A, which could bind to Emi2 and promote Emi2-APC interactions.
Constant Cyclin B levels are maintained during a CSF arrest through the regulation of Emi2 activity. A balance between Cdc2 and PP2A controls Emi2 phosphorylation, which in turn controls the ability of Emi2 to bind to and inhibit the APC. This balance allows proper maintenance of Cyclin B levels and Cdc2 kinase activity during CSF arrest.

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    • "In Xenopus, depletion of Cdc25C phosphatase (the only Cdc25 isoform present in CSF extract) results in inhibitory Thr-14/Tyr-15 phosphorylation of Cdk1, as well as in dephosphorylation of MPF substrates , such as Cdc27/Apc3 and XErp1/Emi2 (Lorca et al, 2010). As described above, this relieve of XErp1/Emi2 from Cdk1-mediated inhibitory effects efficiently inhibits the APC/ C (Wu et al, 2007b), leading to loss of the meiotic state in the absence of cyclin B destruction under these conditions. These data suggest that during the MII-arrest in mature Xenopus eggs, Cdc25 activity is continuously required to maintain Cdk1 in its active state (Gautier et al, 1991; Izumi et al, 1992; Smythe and Newport, 1992). "
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    • "The control of meiotic cyclin B translation in the unfertilized egg, however, is different from that of mitotic cycles. Polyadenylation of cyclin B1 mRNA has been reported in the unfertilized egg (Sheets et al., 1994; Ballantyne et al., 1997; Kim and Richter, 2007; Belloc et al., 2008), and we confirmed that meiotic CSF extracts are capable of continuous cyclin translation in the presence of elevated CDK1 activity, in agreement with previously published studies (Hansen et al., 2007; Wu et al., 2007; Isoda et al., 2011). Therefore, although translation of cyclin B in meiotic M phase is continuous despite CDK1 activity, it is attenuated during M phase of mitotic cycles. "
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    • "In the CSF-arrested eggs of vertebrates , a steady level of maturation-promoting factor (MPF, a complex of catalytic subunit CDC2 and regulatory subunit cyclin B) is maintained via a continuous equilibrium between degradation and synthesis of cyclin B (Kubiak 798 Mol Reprod Dev 78:795–807 (2011) Molecular Reproduction & Development et al., 1993; Yamamoto et al., 2005). The ability of EMI2 to bind and inhibit APC/C is controlled via MPF and protein phosphatase 2A (PP2A), recruited through a MOS-mediated pathway (Wu et al., 2007; Tang et al., 2008; Chang et al., 2011). "
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