Figure 2.

Regulation of Emi2 and the APC during the MI–MII transition. Phosphorylation controls Emi2 stability during oocyte maturation. At MI, CDK1 phosphorylates four amino-terminal sites (S213, T239, T252, and T267) on Emi2; this triggers Emi2 degradation, required for MI exit. At MI anaphase, cyclin B is degraded, leading to a drop in CDK1 activity. Emi2 is stabilized by dephosphorylation triggered by Mos signaling. Emi accumulates, resulting in APC inhibition, critical for S phase block and MII entry. CDK1 activity is low in MII relative to MI. Emi2 is stable in MII, as required for CSF arrest. At fertilization, Emi2 is quickly degraded through a CaMKII-mediated pathway, allowing activation of the APC and exit from MII. At the onset of MI anaphase, APC-mediated cyclin B degradation results in decreased CDK1 activity. With the Mos-PP2A pathway predominant, dephosphorylated and stabilized Emi2 protein prevents complete ubiquitylation of cyclin B by the APC. This is essential for the inhibition of S phase between MI and MII. (From Tang et al. 2008; adapted, with permission.)