Diablo homolog

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Diablo, IAP-binding mitochondrial protein
Protein DIABLO PDB 1few.png
PDB rendering based on 1few.
Available structures
PDB Ortholog search: PDBe, RCSB
External IDs OMIM605219 MGI1913843 HomoloGene10532 GeneCards: DIABLO Gene
RNA expression pattern
PBB GE DIABLO 219350 s at tn.png
More reference expression data
Species Human Mouse
Entrez 56616 66593
Ensembl ENSG00000184047 ENSMUSG00000029433
UniProt Q9NR28 Q9JIQ3
RefSeq (mRNA) NM_001278302 NM_023232
RefSeq (protein) NP_001265231 NP_075721
Location (UCSC) Chr 12:
122.21 – 122.23 Mb
Chr 5:
123.51 – 123.53 Mb
PubMed search [1] [2]

Diablo homolog (DIABLO) is a mitochondrial protein that in humans is encoded by the DIABLO (direct IAP binding protein with low pI) gene on chromosome 12.[1][2][3] DIABLO is also referred to as second mitochondria-derived activator of caspases or SMAC. This protein binds inhibitor of apoptosis proteins (IAPs), thus freeing caspases to activate apoptosis.[3][4] Due to its proapoptotic function, SMAC is implicated in a broad spectrum of tumors, and small molecule SMAC mimetics have been developed to enhance current cancer treatments.[3][5]



This gene encodes a 130 Å-long, arch-shaped homodimer protein. The full-length protein product spans 239 residues, 55 of which comprise the mitochondrial-targeting sequence (MTS) at its N-terminal. However, once the full-length protein is imported into the mitochondria, this sequence is excised to produce the 184-residue mature protein.[5][6][7] This cleavage also exposes four residues at the N-terminal, Ala-Val-Pro-Ile (AVPI), which is the core of the IAP binding domain and crucial for inhibiting XIAP.[5][6][7] Specifically, the tetrapeptide sequence binds the BIR3 domain of XIAP to form a stable complex between SMAC and XIAP.[5][6][7] The homodimer structure also facilitates SMAC-XIAP binding via the BIR2 domain, though it does not form until the protein is released into the cytoplasm as a result of outer mitochondrial membrane permeabilization.[7] Thus, monomeric SMAC mutants can still bind the BIR3 domain but not the BIR2 domain, which compromises the protein’s inhibitory function.[6] Meanwhile, mutations within the AVPI sequence lead to loss of function, though SMAC may still be able to perform IAP binding-independent functions, such as inducing the ubiquitinylation of XIAP.[6][8]


Several alternatively spliced transcript variants that encode distinct isoforms have been described for this gene, but the validity of some transcripts, and their predicted ORFs, has not been determined conclusively.[3][6] Two known isoforms both lack the MTS and the IAP binding domain, suggesting differential subcellular localization and function.[8]


SMAC is a mitochondrial protein that promotes cytochrome c- and TNF receptor-dependent activation of apoptosis by inhibiting the effect of IAP – a protein that negatively regulates apoptosis, or programmed cell death.[4][9] SMAC is normally a mitochondrial protein localized to the mitochondrial intermembrane space, but it enters the cytosol when cells undergo apoptosis.[3][6][8][10] Through the intrinsic pathway of apoptosis, BCL-2 proteins like BAK and BAX form a pore in the outer mitochondrial membrane, leading to mitochondrial membrane permeabilization and the release of both cytochrome c and SMAC.[5][6] While cytochrome c directly activates APAF1 and caspase 9, SMAC binds IAPs, such as XIAP and cIAP proteins, to inhibit their caspase-binding activity and allow for caspase activation of apoptosis.[3][5][6][8][10] SMAC is ubiquitously expressed in many cell types, implicating it in various biological processes involving apoptosis.[11] Currently, nonapoptotic functions for SMAC remain unclear.[7]

Clinical Significance[edit]

SMAC is involved in cancer, and its overexpression is linked to increased sensitivity in tumor cells to apoptosis.[3][9] So far, SMAC overexpression has been observed to oppose cancer progression in head and neck squamous cell carcinoma, hepatocellular carcinoma, Hodgkin lymphoma, breast cancer, glioblastoma, thyroid cancer, renal cell carcinoma, testicular germ cell tumors, colorectal cancer, lung cancer, bladder cancer, endometrioid endometrial cancer, and other sarcomas.[9][12][11] However, the exact relationship between SMAC and leukemia and hematological diseases remains controversial. Interestingly, SMAC mimetics monotherapy displays improved cytotoxic effects on leukemic cell lines compared to combined therapy with other drugs, which is commonly more effective in other types of cancers.[13]

Following experimental elucidation of SMAC structure, small-molecule SMAC mimetics have been developed to mimic the tetrapeptide AVPI in the IAP binding domain of SMAC, which is responsible for binding the BIR3 domains in IAPs like XIAP, cIAP1, and cIAP2 to induce cell death, and sometimes, necroptosis.[5][12] Several of the numerous SMAC mimetics designed within the last decade or so are now undergoing clinical trials, including SM-406 by Bai and colleagues and two mimetics by Genentech. These mimetics are also designed to target tumor cells directly through interacting with inflammatory proteins, such as IL-1β, which are commonly produced by solid tumor lesions.[5] Notably, preclinical studies indicate that the use of SMAC mimetics in conjunction with chemotherapy, death receptor ligands and agonists, as well as small molecule targeted drugs enhance the sensitivity of tumor cells to these treatments.[5][9][12] In addition to improving the success of tumor elimination, this increased sensitivity can permit smaller doses, thus minimizing side effects while maintaining efficacy.[12] Nonetheless, there still exists the potential for side effects, such as elevated levels of cytokines and chemokines in normal tissues, depending on the cellular environment.[5]

In addition to cancers, mutations in DIABLO is associated with young-adult onset of nonsyndromic deafness-64.[3]


Diablo homolog has been shown to interact with:


  1. ^ McNeish IA, Bell S, McKay T, Tenev T, Marani M, Lemoine NR (Jun 2003). "Expression of Smac/DIABLO in ovarian carcinoma cells induces apoptosis via a caspase-9-mediated pathway". Experimental Cell Research 286 (2): 186–98. doi:10.1016/S0014-4827(03)00073-9. PMID 12749848. 
  2. ^ Yu J, Wang P, Ming L, Wood MA, Zhang L (Jun 2007). "SMAC/Diablo mediates the proapoptotic function of PUMA by regulating PUMA-induced mitochondrial events". Oncogene 26 (29): 4189–98. doi:10.1038/sj.onc.1210196. PMID 17237824. 
  3. ^ a b c d e f g h "Entrez Gene: DIABLO diablo homolog (Drosophila)". 
  4. ^ a b Vucic D, Deshayes K, Ackerly H, Pisabarro MT, Kadkhodayan S, Fairbrother WJ, et al. (Apr 2002). "SMAC negatively regulates the anti-apoptotic activity of melanoma inhibitor of apoptosis (ML-IAP)". The Journal of Biological Chemistry 277 (14): 12275–9. doi:10.1074/jbc.M112045200. PMID 11801603. 
  5. ^ a b c d e f g h i j k l m Bai L, Smith DC, Wang S (Oct 2014). "Small-molecule SMAC mimetics as new cancer therapeutics". Pharmacology & Therapeutics 144 (1): 82–95. doi:10.1016/j.pharmthera.2014.05.007. PMID 24841289. 
  6. ^ a b c d e f g h i j k l m Shi Y (May 2001). "A structural view of mitochondria-mediated apoptosis". Nature Structural Biology 8 (5): 394–401. doi:10.1038/87548. PMID 11323712. 
  7. ^ a b c d e Galluzzi L, Joza N, Tasdemir E, Maiuri MC, Hengartner M, Abrams JM, et al. (Jul 2008). "No death without life: vital functions of apoptotic effectors". Cell Death and Differentiation 15 (7): 1113–23. doi:10.1038/cdd.2008.28. PMID 18309324. 
  8. ^ a b c d Martinez-Ruiz GU, Victoria-Acosta G, Vazquez-Santillan KI, Jimenez-Hernandez L, Muñoz-Galindo L, Ceballos-Cancino G, et al. (2014). "Ectopic expression of new alternative splice variant of Smac/DIABLO increases mammospheres formation". International Journal of Clinical and Experimental Pathology 7 (9): 5515–26. PMID 25337193. 
  9. ^ a b c d Zeng H, Zhang S, Yang KY, Wang T, Hu JL, Huang LL, et al. (Dec 2010). "Knockdown of second mitochondria-derived activator of caspase expression by RNAi enhances growth and cisplatin resistance of human lung cancer cells". Cancer Biotherapy & Radiopharmaceuticals 25 (6): 705–12. doi:10.1089/cbr.2010.0786. PMID 21204765. 
  10. ^ a b Anguiano-Hernandez YM, Chartier A, Huerta S (Jul 2007). "Smac/DIABLO and colon cancer". Anti-Cancer Agents in Medicinal Chemistry 7 (4): 467–73. PMID 17630921. 
  11. ^ a b Dobrzycka B, Terlikowski SJ, Bernaczyk PS, Garbowicz M, Niklinski J, Chyczewski L, et al. (Dec 2010). "Prognostic significance of smac/DIABLO in endometrioid endometrial cancer". Folia Histochemica Et Cytobiologica / Polish Academy of Sciences, Polish Histochemical and Cytochemical Society 48 (4): 678–81. doi:10.2478/v10042-010-0091-2. PMID 21478115. 
  12. ^ a b c d Sun Q, Zheng X, Zhang L, Yu J (Apr 2011). "Smac modulates chemosensitivity in head and neck cancer cells through the mitochondrial apoptotic pathway". Clinical Cancer Research 17 (8): 2361–72. doi:10.1158/1078-0432.CCR-10-2262. PMID 21242120. 
  13. ^ Scavullo C, Servida F, Lecis D, Onida F, Drago C, Ferrante L, et al. (Jul 2013). "Single-agent Smac-mimetic compounds induce apoptosis in B chronic lymphocytic leukaemia (B-CLL)". Leukemia Research 37 (7): 809–15. doi:10.1016/j.leukres.2013.03.016. PMID 23618690. 
  14. ^ Hegde R, Srinivasula SM, Datta P, Madesh M, Wassell R, Zhang Z, et al. (Oct 2003). "The polypeptide chain-releasing factor GSPT1/eRF3 is proteolytically processed into an IAP-binding protein". The Journal of Biological Chemistry 278 (40): 38699–706. doi:10.1074/jbc.M303179200. PMID 12865429. 
  15. ^ a b Song Z, Yao X, Wu M (Jun 2003). "Direct interaction between survivin and Smac/DIABLO is essential for the anti-apoptotic activity of survivin during taxol-induced apoptosis". The Journal of Biological Chemistry 278 (25): 23130–40. doi:10.1074/jbc.M300957200. PMID 12660240. 
  16. ^ Kuai J, Nickbarg E, Wooters J, Qiu Y, Wang J, Lin LL (Apr 2003). "Endogenous association of TRAF2, TRAF3, cIAP1, and Smac with lymphotoxin beta receptor reveals a novel mechanism of apoptosis". The Journal of Biological Chemistry 278 (16): 14363–9. doi:10.1074/jbc.M208672200. PMID 12571250. 
  17. ^ Verhagen AM, Ekert PG, Pakusch M, Silke J, Connolly LM, Reid GE, et al. (Jul 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. 
  18. ^ Hunter AM, Kottachchi D, Lewis J, Duckett CS, Korneluk RG, Liston P (Feb 2003). "A novel ubiquitin fusion system bypasses the mitochondria and generates biologically active Smac/DIABLO". The Journal of Biological Chemistry 278 (9): 7494–9. doi:10.1074/jbc.C200695200. PMID 12511567. 
  19. ^ Davoodi J, Lin L, Kelly J, Liston P, MacKenzie AE (Sep 2004). "Neuronal apoptosis-inhibitory protein does not interact with Smac and requires ATP to bind caspase-9". The Journal of Biological Chemistry 279 (39): 40622–8. doi:10.1074/jbc.M405963200. PMID 15280366. 
  20. ^ Verhagen AM, Silke J, Ekert PG, Pakusch M, Kaufmann H, Connolly LM, et al. (Jan 2002). "HtrA2 promotes cell death through its serine protease activity and its ability to antagonize inhibitor of apoptosis proteins". The Journal of Biological Chemistry 277 (1): 445–54. doi:10.1074/jbc.M109891200. PMID 11604410. 

Further reading[edit]