The role of the newly identified early mitotic inhibitor, Emi1, in cell cycle regulation and the early embryo

Ubiquitin-mediated proteolysis plays a crucial role in cell cycle progression. In particular the SCF (Skp1, Cullin, F-box protein) and the Anaphase-Promoting Complex (APC) ubiquitin ligases target key cell cycle regulators for destruction at specific points in the cell cycle. SCF activity is required for the G1-S transition, whereas APC activation by Cdc20 is required for anaphase entry and APC activation by Cdh1 for mitotic exit and G1. A screen to isolate novel F-box proteins has lead to the identification of the E&barbelow;arly M&barbelow;itotic I&barbelow;nhibitor, Emi1, a protein that regulates the cell cycle by inhibiting the APC.; Emi1 is a conserved F-box protein containing a zinc-binding region essential for its ability to inhibit APC activity. Emi1's ability to inhibit APC activity is evolutionarily conserved. Emi1 protein accumulates before mitosis and is ubiquitylated and destroyed in mitosis, independent of the APC. Emi1 immunodepletion from cycling Xenopus egg extracts strongly delays cyclin B accumulation and mitotic entry, whereas nondestructable Emi1 stabilizes APC substrates and causes a mitotic block. Emi1 binds the APC activators Cdc20 and Cdh1, and prevents substrate binding to the APC. Collectively, the data suggest that Emi1 regulates progression through early mitosis by preventing premature APCCdc20 activation, and that Emi1 promotes S phase entry in somatic cells by inactivating APCCdh1 at the G1-S transition.; Vertebrate eggs are arrested at metaphase of meiosis II (MII) with an inactive APC through the activity of Cytostatic factor (CSF), whose molecular composition has remained elusive. Fertilization causes a transient cytoplasmic calcium leading to APC activation and mitotic exit. The APC activator Cdc20 is required for APC activation following fertilization. Studies of Emi1 function in CSF-arrested Xenopus egg extracts demonstrate that Emi1 is required and sufficient to inhibit the APC and prevent mitotic exit in metaphase of Meiosis II-arrested eggs. CSF extracts immunodepleted of Emi1 degrade cyclin B and prematurely exit mitosis in the absence of calcium. Addition of Emi1 to these Emi1-depleted extracts blocks premature inactivation of the CSF-arrested state. Thus, Emi1 is required to arrest unfertilized eggs at metaphase of meiosis II and appears to be the long sought inhibitory mediator of CSF.