|X-linked inhibitor of apoptosis|
PDB rendering based on 1c9q.
|External IDs||ChEMBL: GeneCards:|
|RNA expression pattern|
X-linked inhibitor of apoptosis protein (XIAP), also known as inhibitor of apoptosis protein 3 (IAP3) and baculoviral IAP repeat-containing protein 4 (BIRC), is a protein that stops apoptotic cell death. In human, this protein (XIAP) is produced by a gene named XIAP gene located on the X chromosome.
XIAP is a member of the inhibitor of apoptosis family of proteins (IAP). IAPs were initially identified in baculoviruses, but XIAP is one of the homologous proteins found in mammals. It is so called because it was first discovered by a 273 base pair site on the X chromosome. The protein is also called human IAP-like Protein (hILP), because it is not as well conserved as the human IAPS: hIAP-1 and hIAP-2. XIAP is the most potent human IAP protein currently identified.
Neuronal apoptosis inhibitor protein (NAIP) was the first homolog to baculoviral IAPs that was identified in humans. With the sequencing data of NIAP, the gene sequence for a RING zinc-finger domain was discovered at site Xq24-25. Using PCR and cloning, three BIR domains and a RING finger were found on the protein, which became known as X-linked Inhibitor of Apoptosis Protein. The transcript size of Xiap is 9.0kb, with an open reading frame of 1.8kb. Xiap mRNA has been observed in all human adult and fetal tissues "except peripheral blood leukocytes". The XIAP sequences led to the discovery of other members of the IAP family.
XIAP consists of three major types of structural elements (domains). Firstly, there is the baculoviral IAP repeat (BIR) domain consisting of approximately 70 amino acids, which characterizes all IAP. Secondly, there is a UBA domain, which allows XIAP to bind to ubiquitin. Thirdly, there is a zinc-binding domain, or a “carboxy-terminal RING Finger”. XIAP has been characterized with three amino-terminal BIR domains followed by one UBA domain and finally one RING domain. Between the BIR-1 and BIR-2 domains, there is a linker-BIR-2 region that is thought to contain the only element that comes into contact with the caspase molecule to form the XIAP/Caspase-7 complex.
XIAP stops apoptotic cell death that is induced either by viral infection or by overproduction of caspases. caspases are the enzymes primarily responsible for cell death. XIAP binds to and inhibits caspase 3, 7 and 9. The BIR2 domain of XIAP inhibits caspase 3 and 7, while BIR3 binds to and inhibits caspase 9. The RING domain utilizes E3 ubiquitin ligase activity and enables IAPs to catalyze ubiquination of self, caspase-3, or caspase-7 by degradation via proteasome activity. However, mutations affecting the RING Finger do not significantly affect apoptosis, indicating that the BIR domain is sufficient for the protein’s function. When inhibiting caspase-3 and caspase-7 activity, the BIR2 domain of XIAP binds to the active-site substrate groove, blocking access of the normal protein substrate that would result in apoptosis.
The second BIR domain of XIAP can be shown binding to caspase 3 where a protein substrate would normally bind during aptosis. By blocking this binding, XIAP inhibits apoptosis.
Inhibiting XIAP 
XIAP is inhibited by DIABLO (Smac) and HTRA2 (Omi), two death-signaling proteins released into the cytoplasm by the mitochondria. Smac/DIABLO, a mitochondrial protein and negative regulator of XIAP, can enhance apoptosis by binding to XIAP and preventing it from binding to caspases. This allows normal caspase activity to proceed. The binding process of Smac/DIABLO to XIAP and caspase release requires a conserved tetrapeptide motif.
Clinical significance 
Deregulation of XIAP can result in “cancer, neurodegenerative disorders, and autoimmunity”. High proportions of XIAP may function as a tumor marker. In the development of lung cancer NCI-H460, the overexpression of XIAP not only inhibits caspase, but also stops the activity of cytochrome c (Apoptosis). In developing prostate cancer, XIAP is one of four IAPs overexpressed in the prostatic epithelium, indicating that a molecule that inhibits all IAPs may be necessary for effective treatment. Apoptotic regulation is an extremely important biological function, as evidenced by "the conservation of the IAPs from humans to Drosophila".
Mutations in the XIAP gene can result in a severe and rare type of inflammatory bowel disease. Defects in the XIAP gene can also result in an extremely rare condition called X-linked lymphoproliferative disease.
XIAP has been shown to interact with:
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- Suzuki Y, Nakabayashi Y, Nakata K, Reed JC, Takahashi R (July 2001). "X-linked inhibitor of apoptosis protein (XIAP) inhibits caspase-3 and -7 in distinct modes". J. Biol. Chem. 276 (29): 27058–63. doi:10.1074/jbc.M102415200. PMID 11359776.
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- Deveraux QL, Roy N, Stennicke HR, Van Arsdale T, Zhou Q, Srinivasula SM, Alnemri ES, Salvesen GS, Reed JC (April 1998). "IAPs block apoptotic events induced by caspase-8 and cytochrome c by direct inhibition of distinct caspases". EMBO J. 17 (8): 2215–23. doi:10.1093/emboj/17.8.2215. PMC 1170566. PMID 9545235.
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- Song Z, Yao X, Wu M (June 2003). "Direct interaction between survivin and Smac/DIABLO is essential for the anti-apoptotic activity of survivin during taxol-induced apoptosis". J. Biol. Chem. 278 (25): 23130–40. doi:10.1074/jbc.M300957200. PMID 12660240.
- Verhagen AM, Ekert PG, Pakusch M, Silke J, Connolly LM, Reid GE, Moritz RL, Simpson RJ, Vaux DL (July 2000). "Identification of DIABLO, a mammalian protein that promotes apoptosis by binding to and antagonizing IAP proteins". Cell 102 (1): 43–53. doi:10.1016/S0092-8674(00)00009-X. PMID 10929712.
- Hegde R, Srinivasula SM, Datta P, Madesh M, Wassell R, Zhang Z, Cheong N, Nejmeh J, Fernandes-Alnemri T, Hoshino S, Alnemri ES (October 2003). "The polypeptide chain-releasing factor GSPT1/eRF3 is proteolytically processed into an IAP-binding protein". J. Biol. Chem. 278 (40): 38699–706. doi:10.1074/jbc.M303179200. PMID 12865429.
- Jordan BW, Dinev D, LeMellay V, Troppmair J, Gotz R, Wixler L, Sendtner M, Ludwig S, Rapp UR (October 2001). "Neurotrophin receptor-interacting mage homologue is an inducible inhibitor of apoptosis protein-interacting protein that augments cell death". J. Biol. Chem. 276 (43): 39985–9. doi:10.1074/jbc.C100171200. PMID 11546791.
- Winsauer G, Resch U, Hofer-Warbinek R, Schichl YM, de Martin R (November 2008). "XIAP regulates bi-phasic NF-kappaB induction involving physical interaction and ubiquitination of MEKK2". Cell. Signal. 20 (11): 2107–12. doi:10.1016/j.cellsig.2008.08.004. PMID 18761086.
- Yamaguchi K, Nagai S, Ninomiya-Tsuji J, Nishita M, Tamai K, Irie K, Ueno N, Nishida E, Shibuya H, Matsumoto K (January 1999). "XIAP, a cellular member of the inhibitor of apoptosis protein family, links the receptors to TAB1-TAK1 in the BMP signaling pathway". EMBO J. 18 (1): 179–87. doi:10.1093/emboj/18.1.179. PMC 1171113. PMID 9878061.
- Liston P, Fong WG, Kelly NL, Toji S, Miyazaki T, Conte D, Tamai K, Craig CG, McBurney MW, Korneluk RG (February 2001). "Identification of XAF1 as an antagonist of XIAP anti-Caspase activity". Nat. Cell Biol. 3 (2): 128–33. doi:10.1038/35055027. PMID 11175744.
Further reading 
- Lacasse EC, Kandimalla ER, Winocour P, et al. (2006). "Application of XIAP antisense to cancer and other proliferative disorders: development of AEG35156/ GEM640". Ann. N. Y. Acad. Sci. 1058: 215–34. doi:10.1196/annals.1359.032. PMID 16394139.
- Eckelman BP, Salvesen GS, Scott FL (2006). "Human inhibitor of apoptosis proteins: why XIAP is the black sheep of the family". EMBO Rep. 7 (10): 988–94. doi:10.1038/sj.embor.7400795. PMC 1618369. PMID 17016456.