Anti-apoptotic protein which can regulate cell death by controlling caspases and by acting as an E3 ubiquitin-protein ligase. Has an unusual ubiquitin conjugation system in that it could combine in a single polypeptide, ubiquitin conjugating (E2) with ubiquitin ligase (E3) activity, forming a chimeric E2/E3 ubiquitin ligase. Its targets include CASP9 and DIABLO/SMAC. Acts as an inhibitor of CASP3, CASP7 and CASP9. Important regulator for the final stages of cytokinesis. Crucial for normal vesicle targeting to the site of abscission, but also for the integrity of the midbody and the midbody ring, and its striking ubiquitin modification. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Inhibited by DIABLO/SMAC, HTRA2, CASP3, CASP6, CASP7 and CASP9. Homodimer. Binds the activated, processed forms of CASP3, CASP6, CASP7 and CASP9. Interacts with RNF41, DIABLO/SMAC, HTRA2, KIF23/MKLP1, USP8/UBPY, BIRC5/survivin, MAP2K1/MEK1, RAB8A/RAB8, RAB11A/RAB11, PLK1, EXOC3/SEC6 and EXOC4/SEC8. Exhibits cell cycle-dependent localization. Concentrates in a pericentriolar compartment in interphase, moves partially to spindle poles in metaphase, and finally localizes to the spindle midzone and the midbody in telophase and during cytokinesis. On the midbody, localizes to the midbody ring, also called Flemming body (PubMed:18329369). In interphase cells, localizes to the trans-Golgi network membrane and endosomes. During cytokinesis, a fraction moves to the midzone where it specifically arrives at the midbody ring. After abscission completion, travels with the midbody remnant into one daughter cell, and remains bound to it until a new midbody ring is formed during the next cell division (PubMed:18329369). Expressed in brain cancer cells. The BIR domain is essential for its antiapoptotic function and is important for binding to DIABLO/SMAC and CASP9. Ubiquitinated. Ubiquitination is mediated by the RNF41 E3 ligase and leads to proteasomal degradation, impairing inhibition of apoptosis. Deubiquitinated by USP8/UBPY. In the C-terminal section; belongs to the ubiquitin-conjugating enzyme family.