Regulates progression through early mitosis by inhibiting the anaphase promoting complex/cyclosome (APC) (PubMed:15314241). Binds to the APC activator cdc20 to prevent APC activation (PubMed:11389834). Can also bind directly to the APC to inhibit substrate-binding (PubMed:11751633). 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 (PubMed:11976684). Protein modification; protein ubiquitination. Part of a SCF (SKP1-cullin-F-box) protein ligase complex (By similarity). Interacts with btrc (PubMed:17159919). Interacts with skp1 (PubMed:11389834). Interacts with cdc20 (PubMed:11976684, PubMed:11389834). 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 (PubMed:17159919). Interacts with plk1 (PubMed:17159919). In interphase, localizes in a punctate manner in the nucleus and cytoplasm with some perinuclear concentration (PubMed:11389834, PubMed:17159919). In mitotic cells, localizes throughout the cell, particularly at the spindle (PubMed:11389834). At metaphase, localized at mitotic centrosomes. Decreases centrosome localization as cells progressed through telophase (PubMed:17159919). Accumulates before mitosis with levels increasing in S phase and decreasing in M phase (PubMed:11389834). Weakly detected in cycling egg at the stage just before entry into M phase, but not detected in immature (stage VI), metaphase-I and metaphase-II oocytes, and cycling egg that has entered in M phase. Destroyed at every M phase during both meiotic cycles and during cleavage cycles (PubMed:15314241). Decreases at metaphase and completely disappears as cells exited mitosis. First observed at 4 h (S phase) reaching a peak at 7 h (G2 phase), and then a subsequent decrease from 9 to 10 h corresponding to early and late M phases, respectively (PubMed:17159919). Expression is first activated after the gastrula stage of development. 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.