The Evi5 Oncogene Regulates Cyclin AccumulationbyStabilizingtheAnaphase- Promoting Complex Inhibitor Emi1

The anaphase-promoting complex/cyclosome (APC/C) inhibitor Emi1 controls progression to S phase and mitosis by stabilizing key APC/C ubiquitination substrates, including cyclin A. Examining Emi1 binding proteins, we identified the Evi5 oncogene as a regulator of Emi1 accumulation. Evi5 antagonizes SCFβTrCP-dependent Emi1 ubiquitination and destruction by binding to a site adjacent to Emi1's DSGxxS degron and blocking both degron phosphorylation by Polo-like kinases and subsequent βTrCP binding. Thus, Evi5 functions as a stabilizing factor maintaining Emi1 levels in S/G2 phase. Evi5 protein accumulates in early G1 following Plk1 destruction and is degraded in a Plk1- and ubiquitin-dependent manner in early mitosis. Ablation of Evi5 induces precocious degradation of Emi1 by the Plk/SCFβTrCP pathway, causing premature APC/C activation; cyclin destruction; cell-cycle arrest; centrosome overduplication; and, finally, mitotic catastrophe. We propose that the balance of Evi5 and Polo-like kinase activities determines the timely accumulation of Emi1 and cyclin, ensuring mitotic fidelity.