Pin1 stabilizes Emi1 during G2 phase by preventing its association with SCFβtrcp

The anaphase-promoting complex (APC) early mitotic inhibitor 1 (Emi1) is required to induce S- and M-phase entries by stimulating the accumulation of cyclin A and cyclin B through APCCdh1/cdc20 inhibition. In this report, we show that Emi1 proteolysis can be induced by cyclin A/cdk (cdk for cyclin-dependent kinase). Paradoxically, Emi1 is stable during G2 phase, when cyclin A/cdk, Plx1 and SCFbtrcp (SCF for Skp1-Cul1-Fbox protein) which play a role in its degradation - are active. Here, we identify Pin1 as a new regulator of Emi1 that induces Emi1 stabilization by preventing its association with SCFbtrcp. We show that Pin1 binds to Emi1 and prevents its association with beta-trcp in an isomerization-dependent pathway. We also show that Emi1-Pin1 binding is present in vivo in XL2 cells during G2 phase and that this association protects Emi1 from being degraded during this phase of the cell cycle. We propose that S- and M-phase entries are mediated by the accumulation of cyclin A and cyclin B through a Pin1-dependent stabilization of Emi1 during G2.