Multi-functional protein which regulates not only caspases and apoptosis, but also modulates inflammatory signaling and immunity, copper homeostasis, mitogenic kinase signaling, cell proliferation, as well as cell invasion and metastasis. Acts as a direct caspase inhibitor. Directly bind to the active site pocket of CASP3 and CASP7 and obstructs substrate entry. Inactivates CASP9 by keeping it in a monomeric, inactive state. Acts as an E3 ubiquitin-protein ligase regulating NF-kappa-B signaling and the target proteins for its E3 ubiquitin-protein ligase activity include: RIPK1, CASP3, CASP7, CASP8, CASP9, MAP3K2/MEKK2, DIABLO/SMAC, AIFM1, CCS and BIRC5/survivin. Ubiquitinion of CCS leads to enhancement of its chaperone activity toward its physiologic target, SOD1, rather than proteasomal degradation. Ubiquitinion of MAP3K2/MEKK2 and AIFM1 does not lead to proteasomal degradation. Plays a role in copper homeostasis by ubiquitinating COMMD1 and promoting its proteasomal degradation. Can also function as E3 ubiquitin-protein ligase of the NEDD8 conjugation pathway, targeting effector caspases for neddylation and inactivation. Ubiquitinates and therefore mediates the proteosomal degradation of BCL2 in response to apoptosis (PubMed:29020630). Regulates the BMP signaling pathway and the SMAD and MAP3K7/TAK1 dependent pathways leading to NF-kappa-B and JNK activation. Acts as an important regulator of innate immune signaling via regulation of Nodlike receptors (NLRs). Protects cells from spontaneous formation of the ripoptosome, a large multi-protein complex that has the capability to kill cancer cells in a caspase-dependent and caspase-independent manner. Suppresses ripoptosome formation by ubiquitinating RIPK1 and CASP8. Acts as a positive regulator of Wnt signaling and ubiquitinates TLE1, TLE2, TLE3, TLE4 and AES. Ubiquitination of TLE3 results in inhibition of its interaction with TCF7L2/TCF4 thereby allowing efficient recruitment and binding of the transcriptional coactivator beta-catenin to TCF7L2/TCF4 that is required to initiate a Wnt-specific transcriptional program. 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. Monomer, and homodimer. Part of a complex composed of SEPTIN4 isoform ARTS, XIAP and BCL2, within the complex interacts with SEPTIN4 isoform ARTS and BCL2, SEPTIN4 isoform ARTS acts as a scaffold protein and stabilizes the complex (PubMed:29020630). Interacts (via BIR3 domain) with DIABLO/SMAC; the interaction inhibits apoptotic suppressor activity (PubMed:21695558, PubMed:11140637). Interacts with HTRA2/PRSS25; the interaction inhibits apoptotic suppressor activity (PubMed:11604410). Interacts with TAB1/MAP3K7IP1 and AIFM1. Interaction with DIABLO/SMAC hinders binding of TAB1/MAP3K7IP1 and AIFM1. Interacts with TCF25 and COMMD1. Interacts (via BIR3 domain) with SEPTIN4 isoform ARTS (PubMed:21695558, PubMed:15029247). Interacts (via BIR3 domain) with SEPTIN4 (By similarity). Interacts with RIP1, RIP2, RIP3, RIP4, CCS and USP19. Interacts (via BIR 2 domain and BIR 3 domain) with HAX1 (via C-terminus) and this interaction blocks ubiquitination of XIAP/BIRC4. Interacts with the monomeric form of BIRC5/survivin. Interacts with TLE3 and TCF7L2/TCF4. Interacts (via BIR 3 and RING domains) with PDCL3 (PubMed:19012568). TLE3 promotes its nuclear localization. Expressed in colonic crypts (at protein level) (PubMed:30389919). Ubiquitous, except peripheral blood leukocytes (PubMed:8654366). The first BIR domain is involved in interaction with TAB1/MAP3K7IP1 and is important for dimerization. The second BIR domain is sufficient to inhibit CASP3 and CASP7, while the third BIR is involved in CASP9 inhibition. The interactions with DIABLO/SMAC and HTRA2/PRSS25 are mediated by the second and third BIR domains. S-Nitrosylation down-regulates its E3 ubiquitin-protein ligase activity. Autoubiquitinated. The disease is caused by variants affecting the gene represented in this entry. Belongs to the IAP family. Was originally shown to be phosphorylated at Ser-87 by PKB, protecting the protein from ubiquitination and proteasomal degradation (PubMed:14645242). However, this work was later retracted (PubMed:27825084). XIAP mutation db