Regulates progression through early mitosis by inhibiting the anaphase promoting complex/cyclosome (APC). Binds to the APC activators cdc20 to prevent APC activation. Can also bind directly to the APC to inhibit substrate-binding. Required to arrest unfertilized eggs at metaphase of meiosis II, by preventing their release from metaphase of meiosis II, through inhibition of APC-dependent cyclin B destruction leading to stabilization of cyclin B-cdk1 complex activity. Protein modification; protein ubiquitination. Part of a SCF (SKP1-cullin-F-box) protein ligase complex (By similarity). Interacts with btrc. Interacts with skp1. Interacts with cdc20. Interacts with pin1; stabilizes fbxo5 by preventing its association with btrc in an isomerization-dependent pathway; this interaction is present during G2 phase and prevents fbxo5 degradation. Interacts with plk1 (By similarity). In interphase, localizes in a punctate manner in the nucleus and cytoplasm with some perinuclear concentration. In mitotic cells, localizes throughout the cell, particularly at the spindle. At metaphase, localized at mitotic centrosomes. Decreases centrosome localization as cells progressed through telophase. The C-terminal region is required for APC inhibition. Proteolysed; proteolysis is induced by both cyclin B-cdk1 and cyclin A-cdk1/2 complex through probable phosphorylation. Proteolysis is inhibited by pin1 during G2.