{"gene":"DIABLO","run_date":"2026-06-09T23:54:42","timeline":{"discoveries":[{"year":2000,"finding":"Crystal structure of Smac/DIABLO at 2.2 Å resolution reveals that it homodimerizes through an extensive hydrophobic interface; missense mutations inactivating the dimeric interface significantly compromise Smac/DIABLO function. The N-terminal amino acids are indispensable for function, and a seven-residue N-terminal peptide promotes procaspase-3 activation in vitro. Smac/DIABLO promotes both proteolytic activation of procaspase-3 and enzymatic activity of mature caspase-3 through physical interaction with IAPs.","method":"X-ray crystallography (2.2 Å), in vitro procaspase-3 activation assay, mutagenesis of dimer interface","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with functional mutagenesis and in vitro reconstitution in a single rigorous study","pmids":["10972280"],"is_preprint":false},{"year":2000,"finding":"Crystal structure of Smac/DIABLO complexed with the BIR3 domain of XIAP at high resolution shows that the N-terminal four residues (Ala-Val-Pro-Ile) of mature Smac bind a surface groove on BIR3, with the first residue Ala making five hydrogen bonds to BIR3 residues and occupying a hydrophobic pocket. Mutation of the very first amino acid abolishes IAP interaction and Smac/DIABLO function.","method":"X-ray crystallography (high-resolution co-crystal structure), mutagenesis, binding assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — two independent crystal structures (PMIDs 11140638 and 11140637) with mutagenesis confirming the binding interface","pmids":["11140638"],"is_preprint":false},{"year":2000,"finding":"NMR solution structure of the XIAP BIR3 domain complexed with a nine-residue Smac N-terminal peptide reveals that the peptide binds across the third beta-strand of BIR3 in an extended conformation; only the first four residues contact the protein, stabilized by four intermolecular hydrogen bonds, an electrostatic interaction at the peptide N-terminus, and hydrophobic contacts. BIR3 and Smac peptide mutants confirmed these contacts.","method":"NMR solution structure, binding assays with BIR3 and peptide mutants","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure with mutagenesis validation; independent of PMID 11140638 X-ray structure, providing orthogonal confirmation","pmids":["11140637"],"is_preprint":false},{"year":2001,"finding":"XIAP binds the active caspase-9–Apaf-1 holoenzyme through the N-terminus of the linker peptide on the caspase-9 small subunit exposed after cleavage at Asp315. The N-terminal four residues of this linker share homology with the Smac N-terminal tetra-peptide, defining a conserved IAP-binding motif. Smac/DIABLO binding to BIR3 is mutually exclusive with caspase-9 linker peptide binding, demonstrating that Smac potentiates caspase-9 activity by competing for the BIR3 binding site.","method":"Co-immunoprecipitation, point mutagenesis of caspase-9 cleavage site, competitive binding assays with BIR3","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — reciprocal binding assays and mutagenesis establishing mutually exclusive competitive mechanism, replicated across multiple experimental approaches in a single rigorous study","pmids":["11242052"],"is_preprint":false},{"year":2000,"finding":"Smac/DIABLO N-terminal sequences are required for interaction with IAP BIR3 (blocking caspase-9 inhibition) but the N-terminus is less critical for interaction with BIR1/BIR2 and promotion of effector caspase activity. Smac functions at both the apoptosome level (via BIR3) and effector caspase level (via BIR1/BIR2), neutralizing IAP inhibition.","method":"In vitro caspase activation assays, deletion/truncation mutants of Smac, co-immunoprecipitation with XIAP domains","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple domain-mapping experiments with in vitro assays defining two distinct functional interfaces","pmids":["10950947"],"is_preprint":false},{"year":2002,"finding":"TRAIL-induced apoptosis in human cancer cells requires Bax-dependent mitochondrial release of Smac/DIABLO to neutralize XIAP and permit complete caspase-3 processing. Bax null cells show incomplete caspase-3 processing due to XIAP inhibition; cytosolic expression of active Smac/DIABLO in Bax-deficient cells reconstitutes TRAIL sensitivity. Caspase-9 inhibition does not affect TRAIL-induced caspase-3 activation, establishing Smac/DIABLO release (not cytochrome c) as the key mitochondrial contribution to death receptor apoptosis.","method":"Bax-null cancer cell lines, cytosolic Smac/DIABLO reconstitution, caspase-9 inhibition, Western blot for caspase processing","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic loss-of-function (Bax null) combined with reconstitution experiment and pharmacological epistasis","pmids":["11782443"],"is_preprint":false},{"year":2001,"finding":"Mitochondrial release of Smac/DIABLO into the cytosol during apoptosis is blocked by Bcl-2 overexpression, showing Bcl-2 regulates Smac export. Unlike cytochrome c release (largely caspase-independent), Smac/DIABLO efflux is blocked by a broad-spectrum caspase inhibitor, demonstrating that Smac release is a caspase-dependent event downstream of cytochrome c release.","method":"Subcellular fractionation, Bcl-2-overexpressing cell lines, broad-spectrum caspase inhibitor (z-VAD-fmk), Western blot","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct fractionation with genetic (Bcl-2 OE) and pharmacological (z-VAD) perturbations, multiple apoptotic stimuli tested","pmids":["11726499"],"is_preprint":false},{"year":2004,"finding":"Apollon (BIRC6) binds to, ubiquitinates, and promotes proteasomal degradation of SMAC/DIABLO (as well as caspase-9), both of which contain IAP-binding motifs. In Apollon-deficient cells, SMAC induces apoptosis; this requires the IAP-binding motif of SMAC, establishing that Apollon prevents SMAC-induced apoptosis through ubiquitin-mediated degradation.","method":"Co-immunoprecipitation, in vitro ubiquitination assay, Apollon knockout mice, cell death assays in Apollon-deficient cells","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro ubiquitination reconstitution, genetic KO, and functional rescue with IAP-binding motif mutant","pmids":["15300255"],"is_preprint":false},{"year":2003,"finding":"cIAP1 and cIAP2 act as E3 ubiquitin ligases for Smac/DIABLO, stimulating its ubiquitination both in vivo and in vitro via their RING domains, leading to Smac degradation. The substrate-dependent E3 activity requires physical interaction between cIAPs and Smac, and cIAP1/2 associate with overlapping but distinct E2 ubiquitin-conjugating enzymes.","method":"In vitro ubiquitination assay, in vivo ubiquitination in cells, RING domain mutants, E2 enzyme identification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution of ubiquitination plus in vivo confirmation; single lab but orthogonal methods","pmids":["12525502"],"is_preprint":false},{"year":2004,"finding":"Smac/DIABLO selectively promotes rapid degradation of cIAP1 and cIAP2 but not XIAP or Livin in HeLa cells via the ubiquitin/proteasome pathway. Smac binding via its N-terminal IAP-binding motif is prerequisite for this effect; Smac N-terminal peptide alone is sufficient to enhance cIAP1 ubiquitination, and mutant cIAP1 lacking all BIR domains is not ubiquitinated by Smac.","method":"Cell-based degradation assays, N-terminal peptide competition, BIR domain deletion mutants, auto-ubiquitination assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal cell-based and mutagenesis experiments, single lab","pmids":["14960576"],"is_preprint":false},{"year":2004,"finding":"Smac/DIABLO binding to IAPs (XIAP, cIAP-1, cIAP2) potently represses their ubiquitin ligase activities. Mutation of the XIAP RING domain reduces its anti-apoptotic efficacy, suggesting that XIAP ubiquitin ligase activity contributes to apoptosis inhibition and Smac antagonizes this in addition to blocking caspase-IAP interactions.","method":"In vitro ubiquitination assay, RING domain mutagenesis, cell-based apoptosis assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1–2 / Weak — in vitro assay with mutagenesis, single lab, single study","pmids":["15078891"],"is_preprint":false},{"year":2005,"finding":"Mature DIABLO/Smac is produced by the inner membrane peptidase (IMP) complex on the mitochondrial inner membrane. The precursor enters mitochondria through a stop-transfer pathway and is cleaved by IMP to generate the active form with the exposed N-terminal IAP-binding motif.","method":"Complementation of yeast IMP mutants with mammalian IMP subunits, cellular processing assays, identification of IMP catalytic subunits by sequence conservation","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional complementation and cellular processing experiments, single lab with multiple approaches","pmids":["15814844"],"is_preprint":false},{"year":2017,"finding":"The rhomboid intramembrane protease PARL in the mitochondrial inner membrane cleaves Smac/DIABLO to generate the N-terminal IAP-binding motif required for apoptotic activity. Loss of PARL impairs Smac proteolytic maturation; PARL-deficient cells show reduced Smac–XIAP binding and impaired apoptosis, which is rescued by Smac peptidomimetics, XIAP downregulation, or cytosolic expression of pre-cleaved Smac.","method":"PARL-based proteomics, PARL knockout cells, co-immunoprecipitation of Smac-XIAP, rescue experiments, N-terminal sequencing of cleaved Smac","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — proteomics substrate identification, genetic KO, biochemical validation, and rescue with multiple orthogonal approaches in a single study","pmids":["28288130"],"is_preprint":false},{"year":2006,"finding":"Livin (an IAP family member) acts as an E3 ubiquitin ligase for Smac/DIABLO; both the BIR and RING domains of Livin are required for Smac degradation in vitro and in vivo. Mutation of the Livin BIR domain abolishes Smac/DIABLO binding and prevents its degradation.","method":"In vitro and in vivo ubiquitination assays, BIR and RING domain mutants, co-immunoprecipitation","journal":"Cell death and differentiation","confidence":"Medium","confidence_rationale":"Tier 1–2 / Weak — in vitro and in vivo ubiquitination with mutagenesis, single lab","pmids":["16729033"],"is_preprint":false},{"year":2003,"finding":"Survivin physically interacts with Smac/DIABLO both in vitro and in vivo (co-immunoprecipitation). A point mutation (D71R) in the Survivin BIR motif and a C-terminal deletion mutant (Surv-BIR) abolish Smac/DIABLO binding and abrogate apoptosis inhibition. The N-terminus of mature Smac is required for Survivin-Smac complex formation. Co-presence of Smac/DIABLO and XIAP is required for Survivin to inhibit caspase cleavage in a cell-free system.","method":"Co-immunoprecipitation, in vitro binding, BIR domain point mutation (D71R), cell-free caspase inhibition assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — reciprocal Co-IP and mutagenesis across multiple experimental systems, single lab","pmids":["12660240"],"is_preprint":false},{"year":2005,"finding":"NMR studies show that Smac/DIABLO N-terminal peptides bind across the third beta-strand of the Survivin BIR domain (near alpha4 and beta3), analogous to Smac binding to XIAP BIR3, as demonstrated by specific chemical shift perturbations in Survivin residues upon Smac peptide addition.","method":"NMR chemical shift perturbation mapping with Smac/DIABLO N-terminal peptides and Survivin","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — NMR structural mapping, single lab, no mutagenesis validation in this study","pmids":["15628841"],"is_preprint":false},{"year":2012,"finding":"Crystal structures of Survivin complexed with Smac/DIABLO N-terminal peptide and with phospho-Thr3 histone H3 peptide establish that Survivin recognizes N-terminal Ala in both ligands at the same binding site. Calorimetric data show Survivin binds the H3T3ph peptide more tightly than the Smac/DIABLO N-terminal peptide; structure-guided mutations increasing hydrophobicity of the phosphate-binding pocket reversed this preference.","method":"X-ray crystallography, isothermal titration calorimetry, structure-guided mutagenesis","journal":"Structure","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structures with calorimetry and mutagenesis, single lab but multiple orthogonal methods","pmids":["22244766"],"is_preprint":false},{"year":2001,"finding":"tBid triggers rapid and essentially complete co-release of Smac/DIABLO and cytochrome c from mitochondria with similar kinetics (onset and completion synchronized), occurring after a ~10 s delay and completing within 50–70 s, coinciding with mitochondrial membrane potential depolarization. Bcl-xL prevents tBid-induced release of both proteins.","method":"GFP/YFP fusion proteins, real-time fluorescence imaging, rapid filtration Western blot, permeabilized HepG2 cells","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — live-cell imaging with rapid kinetics quantification and genetic perturbation (Bcl-xL), single lab","pmids":["11741882"],"is_preprint":false},{"year":2003,"finding":"Real-time confocal imaging of MCF-7 cells expressing Smac/DIABLO-YFP shows that Smac/DIABLO and cytochrome c release onset is simultaneous and coincides with mitochondrial membrane potential depolarization. Smac/DIABLO release kinetics are not affected by caspase inhibition (z-VAD-fmk) or caspase-3 deficiency. DEVDase (caspase-3/7) activation occurs within 10 min of Smac/DIABLO release even without caspase-3.","method":"Stable Smac/DIABLO-YFP expression, real-time confocal microscopy, caspase inhibitor (z-VAD-fmk), caspase-3-deficient MCF-7 cells, DEVDase activity assay","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — live-cell imaging with orthogonal genetic and pharmacological perturbations, single lab","pmids":["12975347"],"is_preprint":false},{"year":2002,"finding":"Smac/DIABLO-deficient (Smac−/−) mice generated by homologous recombination are viable, develop normally, and show no histological abnormalities. Cultured Smac−/− cells respond normally to apoptotic stimuli and show no difference in Fas-mediated apoptosis in vivo, though in vitro procaspase-3 cleavage is inhibited in cell lysates, indicating functional redundancy in vivo.","method":"Homologous recombination knockout mice, in vitro cell death assays, in vivo Fas-mediated apoptosis, cell-free caspase-3 activation assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — rigorously generated knockout mouse with comprehensive phenotypic analysis across multiple assays and cell types","pmids":["11971981"],"is_preprint":false},{"year":2011,"finding":"Combinatorial deletion of Diablo and Casp3 (but neither alone) causes perinatal lethality in mice, demonstrating a physiological role for Smac/DIABLO in regulating programmed cell death. In MEFs, loss of Smac/DIABLO alters both caspase-dependent and caspase-independent intrinsic PCD. siRNA epistasis experiments show Smac/DIABLO modulates inhibitory interactions between specific IAP family members and executioner caspases-3 and -7.","method":"Double-knockout mouse generation, MEF cell death assays, siRNA epistasis (XIAP, cIAP-1, cIAP-2, caspase-6, -7)","journal":"Cell death and differentiation","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with double-KO mice and siRNA knockdown, single lab but multiple orthogonal approaches","pmids":["21597464"],"is_preprint":false},{"year":2003,"finding":"Smac/DIABLO-induced apoptosis in ovarian carcinoma cells proceeds primarily through a caspase-9-mediated pathway; it is inhibited by XIAP overexpression and the caspase-9 inhibitor zLEHD-fmk. Smac-induced apoptosis occurs independently of cytochrome c release from mitochondria and is not inhibited by Bcl-2 overexpression when Smac is expressed ectopically.","method":"Recombinant adenovirus-mediated Smac expression, caspase inhibitors, XIAP overexpression, Bcl-2 overexpression, Western blot for caspase cleavage","journal":"Experimental cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple perturbations (inhibitors, OE) with specific pathway readouts, single lab","pmids":["12749848"],"is_preprint":false},{"year":2011,"finding":"A heterozygous SMAC/DIABLO missense mutation (c.377C>T; p.Ser71Leu in the mature protein) causes dominant progressive non-syndromic hearing loss (DFNA64). The mutant SMAC/DIABLO(S71L) retains proapoptotic function but triggers significant degradation of both wild-type and mutant SMAC/DIABLO and renders host mitochondria susceptible to calcium-induced loss of membrane potential, indicating a gain-of-function mitochondrial dysfunction.","method":"Linkage analysis, mutation identification, in vitro functional study with S71L mutant expression, mitochondrial membrane potential assay, Western blot for Smac protein levels","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional characterization of patient mutation with mitochondrial assays; single lab, moderate methods","pmids":["21722859"],"is_preprint":false},{"year":2006,"finding":"E2F1 directly binds to and activates the SMAC/DIABLO promoter through two E2F1-binding sites (BS2 at −542/−535 bp and BS3 at −200/−193 bp), upregulating Smac/DIABLO mRNA and protein to enhance mitochondria-mediated apoptosis. This activation is E2F1-specific (E2F2 and E2F3 cannot activate these sites). siRNA knockdown of Smac/DIABLO significantly diminishes E2F1-induced apoptosis.","method":"Promoter luciferase reporter assays, ChIP, inducible ER-E2F1 cell line (4-OHT induction), RT-PCR, Western blot, siRNA knockdown","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (reporter, ChIP, inducible expression, siRNA rescue), single lab","pmids":["16617145"],"is_preprint":false},{"year":2007,"finding":"Mitochondrial survivin associates with Smac/DIABLO and delays its release during etoposide-induced apoptosis; cytosolic survivin also stabilizes released Smac/DIABLO levels. This selective retention of Smac/DIABLO (but not other intermembrane space proteins) explains the survivin anti-apoptotic effect in cancer cells.","method":"Co-immunoprecipitation of mitochondrial survivin with Smac/DIABLO, subcellular fractionation, etoposide treatment, Western blot for Smac release kinetics","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2–3 / Weak — single Co-IP with fractionation, mechanistic follow-up limited, single lab","pmids":["17546047"],"is_preprint":false},{"year":2004,"finding":"NAIP (neuronal apoptosis-inhibitory protein) fails to interact with the IAP-binding motif of Smac/DIABLO, and Smac protein does not interact with NAIP BIR domains, demonstrating that NAIP's mechanism of caspase inhibition is distinct from other IAPs and is not countered by Smac. This is a negative mechanistic finding distinguishing NAIP from XIAP.","method":"Recombinant protein binding assays with full-length NAIP and Smac; IAP-binding motif peptide competition assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct negative binding result with recombinant proteins and peptide competition, single lab; negative finding is mechanistically informative","pmids":["15280366"],"is_preprint":false},{"year":2003,"finding":"SMAC/Diablo is essential for NSAID-induced apoptosis in colon cancer cells. Homologous recombination disruption and siRNA knockdown of SMAC/Diablo abrogates NSAID-induced apoptosis; reconstitution restores it. Loss of SMAC decreases caspase activation, cytochrome c release, and mitochondrial membrane potential collapse in this context.","method":"Homologous recombination gene disruption, siRNA knockdown, reconstitution, caspase activity assays, cytochrome c release (Western blot), mitochondrial membrane potential assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic disruption by HR (gold standard) plus siRNA, functional reconstitution, and multiple readouts in single study","pmids":["15557007"],"is_preprint":false},{"year":2007,"finding":"SMAC/Diablo mediates the proapoptotic function of PUMA: in SMAC-deficient cells, PUMA-induced apoptosis is abrogated with decreases in caspase activation, cytochrome c release, and mitochondrial membrane potential collapse. Reconstitution of SMAC restores these events, and SMAC participates in a feedback amplification loop promoting cytochrome c release during DNA damage-induced apoptosis.","method":"SMAC-deficient cell lines, SMAC reconstitution, caspase activity assays, cytochrome c release (Western blot), mitochondrial membrane potential","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic loss-of-function with reconstitution, multiple mechanistic readouts, single lab","pmids":["17237824"],"is_preprint":false},{"year":2003,"finding":"A ubiquitin fusion system generates mature, biologically active Smac directly in the cytosol with correct N-terminal Ala-Val-Pro-Ile sequence (without mitochondrial processing). This cytosolic Smac interacts with XIAP and sensitizes cells to apoptotic triggers (etoposide) but is not sufficient alone to trigger apoptosis in healthy cells.","method":"Ubiquitin fusion expression construct, N-terminal sequencing, co-immunoprecipitation with XIAP, cell death assays with etoposide","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — novel expression system with biochemical validation and functional testing, single lab","pmids":["12511567"],"is_preprint":false},{"year":2003,"finding":"Synthetic Smac/DIABLO N-terminal peptides (first 4–8 amino acids) fused to the antennapaedia penetratin carrier enter cells, bind XIAP and cIAP1 in situ, displace caspase-3 from cytoplasmic aggregates, and enhance drug-induced caspase activity and apoptosis when combined with chemotherapeutic agents.","method":"Cell-permeable peptide delivery, co-immunoprecipitation of peptide with XIAP/cIAP1, caspase activity assays, long-term antiproliferative assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct in-cell binding demonstrated by Co-IP plus functional caspase assays, single lab, multiple cell lines","pmids":["12218061"],"is_preprint":false},{"year":2003,"finding":"Cephalostatin 1 selectively triggers mitochondrial release of Smac/DIABLO without releasing cytochrome c, AIF, or activating caspase-8-dependent death receptors, defining a novel apoptosis pathway that uses Smac/DIABLO as the key mitochondrial signaling molecule. Bcl-xL overexpression delays both Smac/DIABLO release and apoptosis onset.","method":"Western blot for selective protein release, Bcl-xL overexpression, CD95/caspase-8-deficient cells, caspase inhibitor, electron microscopy","journal":"Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic and pharmacological perturbations, multiple cell lines, selective release documented, single lab","pmids":["14695204"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structures of BIRC6 in complex with SMAC reveal that BIRC6 forms a megadalton antiparallel dimer with a spacious cavity containing receptor sites for client proteins (caspase-9, HTRA2, LC3B). SMAC binds multivalently to BIRC6 with subnanomolar affinity, obstructing client binding sites and thereby impeding ubiquitination of both apoptotic (caspase-9) and autophagy (LC3B) substrates. BIRC6 directly restricts executioner caspase-3 and -7 and ubiquitinates caspases-3, -7, and -9 exclusively with noncanonical E1 enzyme UBA6.","method":"Cryo-EM structure determination, ubiquitination assays identifying UBA6 as E1, caspase activity assays, competitive displacement assays with SMAC","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 / Strong — two independent cryo-EM studies (PMIDs 36758105 and 36758106) with biochemical reconstitution of ubiquitination and caspase inhibition mechanisms","pmids":["36758105","36758106"],"is_preprint":false},{"year":2004,"finding":"Direct interaction between Smac/DIABLO and NADE (p75NTR-associated cell death executor) was identified by binding screen. The N-terminal region of Smac and C-terminal region of NADE mediate the interaction. Co-expression of NADE and Smac promotes TRAIL-induced apoptosis in MCF-7 cells. Co-presence of Smac and NADE inhibits XIAP-mediated Smac ubiquitination.","method":"Protein interaction screen, co-immunoprecipitation, domain mapping, cell-based apoptosis and ubiquitination assays","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP/pulldown with limited functional validation, single lab","pmids":["15178455"],"is_preprint":false},{"year":2020,"finding":"SMAC/Diablo has a non-apoptotic function in phospholipid synthesis: siRNA silencing of SMAC/Diablo in cancer cells and xenografts reduced cell and tumor growth, altered expression of genes involved in lipid metabolism, and decreased phospholipid (including phosphatidylcholine) levels. This suggests SMAC/Diablo regulates lipid synthesis essential for cancer cell proliferation.","method":"siRNA knockdown in cancer cell lines and subcutaneous xenografts, next-generation sequencing of transcriptome, phospholipid quantification, electron microscopy","journal":"Molecular therapy","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, indirect mechanism (transcriptome + lipidomics), no direct biochemical interaction identified; novel non-apoptotic finding","pmids":["29396267"],"is_preprint":false},{"year":2005,"finding":"During UV-induced apoptosis in HeLa cells, YFP-Smac and GFP-cytochrome c are released from mitochondria in the same time window coinciding with mitochondrial membrane potential depolarization, and endogenous Smac and cytochrome c are always released together within individual cells. Pre-treatment with z-VAD-fmk does not affect Smac release, indicating caspase-independent release in this context (contrasting with the Adrain et al. finding in other apoptotic models).","method":"Live-cell confocal microscopy with dual YFP-Smac / GFP-cytochrome c fusion proteins, immunostaining of endogenous proteins, z-VAD-fmk treatment","journal":"Apoptosis","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — live imaging with dual reporters in same cells, single lab; mechanistically informative contradiction with PMID 11726499 in specific UV context","pmids":["15843890"],"is_preprint":false},{"year":2003,"finding":"A novel Smac/DIABLO splice variant, Smac3, is generated by alternative splicing of exon 4. Smac3 localizes to mitochondria via its N-terminal targeting sequence, is released into cytosol during apoptosis, binds BIR2 and BIR3 of XIAP via its IAP-binding motif, disrupts caspase-9–XIAP interaction, promotes caspase-3 activation, and uniquely accelerates XIAP auto-ubiquitination and destruction (unlike canonical Smac/DIABLO).","method":"Cloning/sequencing, mitochondrial targeting assay, co-immunoprecipitation with XIAP domains, caspase-3 activation assay, XIAP ubiquitination assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple complementary assays defining the unique properties of this isoform; single lab","pmids":["14523016"],"is_preprint":false}],"current_model":"SMAC/DIABLO is a mitochondrial intermembrane space protein whose 55-residue N-terminal targeting sequence is cleaved by the IMP complex (and separately trimmed by PARL) to expose an Ala-Val-Pro-Ile IAP-binding motif; upon apoptotic stimuli, it is released in a Bcl-2/Bax-regulated manner into the cytosol where it competitively displaces caspase-9 from XIAP BIR3, antagonizes cIAP1/cIAP2 ubiquitin-ligase activity toward caspases, promotes cIAP1/2 auto-ubiquitination and proteasomal degradation, and is itself ubiquitinated and degraded by cIAP1/2, Apollon/BIRC6, and Livin; it additionally binds Survivin and NADE, is transcriptionally upregulated by E2F1, and—as shown by the BIRC6 cryo-EM structures—multivalently engages BIRC6 at subnanomolar affinity to competitively block ubiquitination of both apoptotic and autophagy substrates, collectively establishing SMAC/DIABLO as a central relief valve for IAP-mediated suppression of caspase activation."},"narrative":{"mechanistic_narrative":"SMAC/DIABLO is a mitochondrial intermembrane-space protein that acts as a central antagonist of inhibitor-of-apoptosis proteins (IAPs), de-repressing caspase activation during programmed cell death [PMID:10972280, PMID:21597464]. It is synthesized as a precursor whose N-terminal targeting sequence is removed by the inner-membrane peptidase (IMP) complex and further by the rhomboid protease PARL to expose a mature Ala-Val-Pro-Ile IAP-binding motif essential for activity [PMID:15814844, PMID:28288130]. The functional protein homodimerizes through a hydrophobic interface, and this N-terminal tetrapeptide inserts into a surface groove on the XIAP BIR3 domain in a mode that is mutually exclusive with binding of the caspase-9 linker peptide, so SMAC competitively displaces caspase-9 from BIR3 while also neutralizing IAP inhibition of effector caspases through BIR1/BIR2 contacts [PMID:10972280, PMID:11140638, PMID:11140637, PMID:11242052, PMID:10950947]. Beyond steric antagonism, SMAC represses and degrades IAPs: it inhibits the RING-dependent ubiquitin-ligase activity of XIAP, cIAP1 and cIAP2 and drives proteasomal degradation of cIAP1/2, while being reciprocally targeted for ubiquitination and degradation by cIAP1/2, Apollon/BIRC6 and Livin [PMID:15300255, PMID:12525502, PMID:14960576, PMID:15078891, PMID:16729033]. Cryo-EM of the BIRC6 dimer shows SMAC binding multivalently with subnanomolar affinity to obstruct client sites and block BIRC6-mediated ubiquitination of both apoptotic (caspase-9) and autophagy (LC3B) substrates [PMID:36758105, PMID:36758106]. SMAC additionally binds Survivin and is sequestered by it at the same N-terminal-Ala recognition site, linking SMAC to a wider network of BIR-domain partners [PMID:12660240, PMID:22244766, PMID:17546047]. Mitochondrial release of SMAC is gated by Bcl-2/Bcl-xL and accompanies cytochrome c efflux during apoptotic membrane permeabilization, and SMAC is required for full caspase-3 processing in death-receptor (TRAIL), NSAID-, and PUMA-driven death [PMID:11782443, PMID:11726499, PMID:11741882, PMID:15557007, PMID:17237824]. Single knockout mice are viable, but combined loss of Diablo and Casp3 causes perinatal lethality, establishing a physiological role in regulating cell death [PMID:11971981, PMID:21597464]. A heterozygous missense mutation (p.Ser71Leu) causes dominant non-syndromic hearing loss DFNA64 through a gain-of-function mitochondrial defect [PMID:21722859].","teleology":[{"year":2000,"claim":"Established the structural basis of SMAC/DIABLO function: how it dimerizes and how its N-terminus engages IAPs to promote caspase activation.","evidence":"X-ray crystallography of SMAC alone and bound to XIAP BIR3, NMR of the BIR3-peptide complex, with dimer-interface and N-terminal mutagenesis","pmids":["10972280","11140638","11140637","10950947"],"confidence":"High","gaps":["Did not resolve how the N-terminal motif is exposed in vivo","Distinct contributions of BIR1/BIR2 versus BIR3 engagement only partially defined"]},{"year":2001,"claim":"Defined the competitive mechanism by showing SMAC and the cleaved caspase-9 linker peptide share an IAP-binding motif and bind BIR3 mutually exclusively, explaining how SMAC potentiates caspase-9.","evidence":"Co-IP, caspase-9 cleavage-site mutagenesis, and competitive BIR3 binding assays","pmids":["11242052"],"confidence":"High","gaps":["Did not address effector-caspase de-repression in cells","In vitro competition not validated by structural snapshot of the ternary state"]},{"year":2001,"claim":"Resolved the cell-biological gating of SMAC release, showing it exits mitochondria during apoptosis under Bcl-2/Bcl-xL control with kinetics coupled to membrane depolarization.","evidence":"Subcellular fractionation and live-cell imaging with Bcl-2/Bcl-xL overexpression and caspase inhibitors","pmids":["11726499","11741882"],"confidence":"Medium","gaps":["Caspase-dependence of release disputed across stimuli (see #18, #34)","Channel/permeabilization mechanism for SMAC efflux not defined"]},{"year":2002,"claim":"Tested the in vivo requirement for SMAC, revealing functional redundancy under basal conditions despite a clear in vitro caspase-3 activation defect.","evidence":"Smac-null knockout mice with phenotypic and cell-free caspase analyses","pmids":["11971981"],"confidence":"High","gaps":["Did not identify the redundant factor","Left open whether combined genetic loss would reveal a phenotype (resolved 2011, #20)"]},{"year":2002,"claim":"Connected SMAC to death-receptor signaling, showing its Bax-dependent release is required to neutralize XIAP for complete caspase-3 processing in TRAIL apoptosis.","evidence":"Bax-null cells with cytosolic SMAC reconstitution and caspase-9 epistasis","pmids":["11782443"],"confidence":"High","gaps":["Did not quantify XIAP threshold setting the Bax requirement","Generalizability beyond the cancer lines tested unaddressed"]},{"year":2003,"claim":"Showed SMAC is reciprocally regulated and itself a ubiquitination substrate, while it both antagonizes and degrades IAPs, expanding its role from steric blocker to regulator of IAP ligase activity.","evidence":"In vitro and in vivo ubiquitination assays with cIAP1/2 and Livin, RING/BIR mutants, and a cytosolic ubiquitin-fusion SMAC expression system","pmids":["12525502","14960576","15078891","16729033","12511567"],"confidence":"Medium","gaps":["E2 enzyme assignments incompletely mapped","Selectivity for cIAP1/2 over XIAP degradation mechanistically unexplained"]},{"year":2003,"claim":"Identified Survivin as a BIR-domain partner that sequesters SMAC, and defined the shared N-terminal-Ala recognition mode, linking SMAC antagonism to Survivin-mediated apoptosis suppression.","evidence":"Co-IP, BIR mutagenesis, NMR and crystallographic mapping of the Survivin-SMAC peptide interface with calorimetry","pmids":["12660240","15628841","22244766","17546047"],"confidence":"Medium","gaps":["Physiological stoichiometry of SMAC-Survivin sequestration unclear","Competition with H3T3ph for the same site not resolved in cells"]},{"year":2004,"claim":"Established Apollon/BIRC6 as an IAP that binds and degrades SMAC via its IAP-binding motif, and distinguished NAIP as an IAP not antagonized by SMAC.","evidence":"In vitro ubiquitination, Apollon knockout mice and rescue with motif mutants; recombinant NAIP binding assays","pmids":["15300255","15280366"],"confidence":"High","gaps":["BIRC6 architecture and binding mode not yet structurally resolved (later #31)","Tissue-specific contribution of BIRC6-SMAC degradation undefined"]},{"year":2005,"claim":"Defined the proteolytic maturation pathway, identifying IMP-mediated cleavage that generates the exposed IAP-binding motif on the inner membrane.","evidence":"Yeast IMP complementation with mammalian subunits and cellular processing assays","pmids":["15814844"],"confidence":"Medium","gaps":["Did not establish whether IMP cleavage alone produces the final mature N-terminus","Relationship to later-identified PARL processing unaddressed"]},{"year":2006,"claim":"Placed SMAC downstream of E2F1, showing direct transcriptional upregulation that contributes to E2F1-induced mitochondrial apoptosis.","evidence":"Promoter reporter assays, ChIP, inducible E2F1, and siRNA rescue","pmids":["16617145"],"confidence":"Medium","gaps":["Other transcriptional inputs not surveyed","In vivo relevance of E2F1-SMAC axis untested"]},{"year":2011,"claim":"Demonstrated a physiological cell-death role through genetic epistasis, with Diablo;Casp3 double loss causing perinatal lethality, and a disease link via the DFNA64 gain-of-function mutation.","evidence":"Double-knockout mice with siRNA epistasis on IAPs and executioner caspases; linkage analysis plus functional study of the S71L mutant","pmids":["21597464","21722859"],"confidence":"High","gaps":["Mechanism connecting SMAC dysfunction to cochlear pathology incomplete","Identity of the IAP-caspase modules dominating each tissue unresolved"]},{"year":2017,"claim":"Completed the maturation model by identifying PARL as the inner-membrane protease that generates the IAP-binding motif required for apoptotic competence.","evidence":"PARL proteomics, knockout cells, SMAC-XIAP Co-IP, N-terminal sequencing, and multi-modal rescue","pmids":["28288130"],"confidence":"High","gaps":["Coordination/order of IMP versus PARL cleavage not fully reconciled","Regulation of PARL processing under apoptotic stimuli unaddressed"]},{"year":2023,"claim":"Provided the structural mechanism for BIRC6 antagonism, showing SMAC multivalently blocks the BIRC6 client cavity to inhibit ubiquitination of both apoptotic and autophagy substrates.","evidence":"Cryo-EM of the BIRC6 dimer with SMAC, ubiquitination assays identifying UBA6, and caspase inhibition assays","pmids":["36758105","36758106"],"confidence":"High","gaps":["Cellular regulation of SMAC-BIRC6 stoichiometry not addressed","Whether SMAC similarly blocks other BIR-megaligases unknown"]},{"year":null,"claim":"Whether SMAC/DIABLO's reported non-apoptotic role in phospholipid synthesis reflects a direct biochemical activity rather than an indirect transcriptional consequence remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No direct interaction or enzymatic activity linking SMAC to lipid metabolism identified","Lipid phenotype rests on a single siRNA/transcriptome study (#33)"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3,4,9,10,31]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[3,31]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[11,12,17,18]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[5,6,28]}],"pathway":[{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[0,5,20]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[8,9,10,31]}],"complexes":[],"partners":["XIAP","BIRC2","BIRC3","BIRC6","BIRC7","BIRC5","CASP9"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NR28","full_name":"Diablo IAP-binding mitochondrial protein","aliases":["Diablo homolog, mitochondrial","Direct IAP-binding protein with low pI","Second mitochondria-derived activator of caspases","SMAC"],"length_aa":239,"mass_kda":27.1,"function":"Promotes apoptosis by activating caspases in the cytochrome c/Apaf-1/caspase-9 pathway. Acts by opposing the inhibitory activity of inhibitor of apoptosis proteins (IAP). Inhibits the activity of BIRC6/BRUCE by inhibiting its binding to caspases (PubMed:15200957, PubMed:36758104, PubMed:36758105, PubMed:36758106) Attenuates the stability and apoptosis-inhibiting activity of XIAP/BIRC4 by promoting XIAP/BIRC4 ubiquitination and degradation through the ubiquitin-proteasome pathway. Also disrupts XIAP/BIRC4 interacting with processed caspase-9 and promotes caspase-3 activation Defective in the capacity to down-regulate the XIAP/BIRC4 abundance","subcellular_location":"Mitochondrion; Cytoplasm, cytosol","url":"https://www.uniprot.org/uniprotkb/Q9NR28/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/DIABLO","classification":"Not Classified","n_dependent_lines":8,"n_total_lines":1208,"dependency_fraction":0.006622516556291391},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"DYNLL1","stoichiometry":0.2},{"gene":"DYNLL2","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/DIABLO","total_profiled":1310},"omim":[{"mim_id":"619205","title":"GROWTH HORMONE-INDUCIBLE TRANSMEMBRANE PROTEIN; GHITM","url":"https://www.omim.org/entry/619205"},{"mim_id":"617679","title":"KELCH-LIKE 20; KLHL20","url":"https://www.omim.org/entry/617679"},{"mim_id":"617081","title":"OMA1 ZINC METALLOPEPTIDASE; OMA1","url":"https://www.omim.org/entry/617081"},{"mim_id":"616254","title":"CASEINOLYTIC PEPTIDASE B; CLPB","url":"https://www.omim.org/entry/616254"},{"mim_id":"615380","title":"APOPTOSIS-RESISTANT E3 UBIQUITIN PROTEIN LIGASE 1; AREL1","url":"https://www.omim.org/entry/615380"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Mitochondria","reliability":"Supported"},{"location":"Cytosol","reliability":"Additional"},{"location":"Flagellar centriole","reliability":"Additional"},{"location":"Mid piece","reliability":"Additional"},{"location":"Principal piece","reliability":"Additional"},{"location":"End piece","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/DIABLO"},"hgnc":{"alias_symbol":["SMAC","DIABLO-S","FLJ25049","FLJ10537","DFNA64"],"prev_symbol":[]},"alphafold":{"accession":"Q9NR28","domains":[{"cath_id":"1.20.58.70","chopping":"73-239","consensus_level":"medium","plddt":95.6139,"start":73,"end":239}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NR28","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NR28-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NR28-F1-predicted_aligned_error_v6.png","plddt_mean":82.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=DIABLO","jax_strain_url":"https://www.jax.org/strain/search?query=DIABLO"},"sequence":{"accession":"Q9NR28","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NR28.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NR28/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NR28"}},"corpus_meta":[{"pmid":"11242052","id":"PMC_11242052","title":"A 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The N-terminal amino acids are indispensable for function, and a seven-residue N-terminal peptide promotes procaspase-3 activation in vitro. Smac/DIABLO promotes both proteolytic activation of procaspase-3 and enzymatic activity of mature caspase-3 through physical interaction with IAPs.\",\n      \"method\": \"X-ray crystallography (2.2 Å), in vitro procaspase-3 activation assay, mutagenesis of dimer interface\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with functional mutagenesis and in vitro reconstitution in a single rigorous study\",\n      \"pmids\": [\"10972280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Crystal structure of Smac/DIABLO complexed with the BIR3 domain of XIAP at high resolution shows that the N-terminal four residues (Ala-Val-Pro-Ile) of mature Smac bind a surface groove on BIR3, with the first residue Ala making five hydrogen bonds to BIR3 residues and occupying a hydrophobic pocket. Mutation of the very first amino acid abolishes IAP interaction and Smac/DIABLO function.\",\n      \"method\": \"X-ray crystallography (high-resolution co-crystal structure), mutagenesis, binding assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — two independent crystal structures (PMIDs 11140638 and 11140637) with mutagenesis confirming the binding interface\",\n      \"pmids\": [\"11140638\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"NMR solution structure of the XIAP BIR3 domain complexed with a nine-residue Smac N-terminal peptide reveals that the peptide binds across the third beta-strand of BIR3 in an extended conformation; only the first four residues contact the protein, stabilized by four intermolecular hydrogen bonds, an electrostatic interaction at the peptide N-terminus, and hydrophobic contacts. BIR3 and Smac peptide mutants confirmed these contacts.\",\n      \"method\": \"NMR solution structure, binding assays with BIR3 and peptide mutants\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure with mutagenesis validation; independent of PMID 11140638 X-ray structure, providing orthogonal confirmation\",\n      \"pmids\": [\"11140637\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"XIAP binds the active caspase-9–Apaf-1 holoenzyme through the N-terminus of the linker peptide on the caspase-9 small subunit exposed after cleavage at Asp315. The N-terminal four residues of this linker share homology with the Smac N-terminal tetra-peptide, defining a conserved IAP-binding motif. Smac/DIABLO binding to BIR3 is mutually exclusive with caspase-9 linker peptide binding, demonstrating that Smac potentiates caspase-9 activity by competing for the BIR3 binding site.\",\n      \"method\": \"Co-immunoprecipitation, point mutagenesis of caspase-9 cleavage site, competitive binding assays with BIR3\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — reciprocal binding assays and mutagenesis establishing mutually exclusive competitive mechanism, replicated across multiple experimental approaches in a single rigorous study\",\n      \"pmids\": [\"11242052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Smac/DIABLO N-terminal sequences are required for interaction with IAP BIR3 (blocking caspase-9 inhibition) but the N-terminus is less critical for interaction with BIR1/BIR2 and promotion of effector caspase activity. Smac functions at both the apoptosome level (via BIR3) and effector caspase level (via BIR1/BIR2), neutralizing IAP inhibition.\",\n      \"method\": \"In vitro caspase activation assays, deletion/truncation mutants of Smac, co-immunoprecipitation with XIAP domains\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple domain-mapping experiments with in vitro assays defining two distinct functional interfaces\",\n      \"pmids\": [\"10950947\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"TRAIL-induced apoptosis in human cancer cells requires Bax-dependent mitochondrial release of Smac/DIABLO to neutralize XIAP and permit complete caspase-3 processing. Bax null cells show incomplete caspase-3 processing due to XIAP inhibition; cytosolic expression of active Smac/DIABLO in Bax-deficient cells reconstitutes TRAIL sensitivity. Caspase-9 inhibition does not affect TRAIL-induced caspase-3 activation, establishing Smac/DIABLO release (not cytochrome c) as the key mitochondrial contribution to death receptor apoptosis.\",\n      \"method\": \"Bax-null cancer cell lines, cytosolic Smac/DIABLO reconstitution, caspase-9 inhibition, Western blot for caspase processing\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic loss-of-function (Bax null) combined with reconstitution experiment and pharmacological epistasis\",\n      \"pmids\": [\"11782443\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Mitochondrial release of Smac/DIABLO into the cytosol during apoptosis is blocked by Bcl-2 overexpression, showing Bcl-2 regulates Smac export. Unlike cytochrome c release (largely caspase-independent), Smac/DIABLO efflux is blocked by a broad-spectrum caspase inhibitor, demonstrating that Smac release is a caspase-dependent event downstream of cytochrome c release.\",\n      \"method\": \"Subcellular fractionation, Bcl-2-overexpressing cell lines, broad-spectrum caspase inhibitor (z-VAD-fmk), Western blot\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct fractionation with genetic (Bcl-2 OE) and pharmacological (z-VAD) perturbations, multiple apoptotic stimuli tested\",\n      \"pmids\": [\"11726499\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Apollon (BIRC6) binds to, ubiquitinates, and promotes proteasomal degradation of SMAC/DIABLO (as well as caspase-9), both of which contain IAP-binding motifs. In Apollon-deficient cells, SMAC induces apoptosis; this requires the IAP-binding motif of SMAC, establishing that Apollon prevents SMAC-induced apoptosis through ubiquitin-mediated degradation.\",\n      \"method\": \"Co-immunoprecipitation, in vitro ubiquitination assay, Apollon knockout mice, cell death assays in Apollon-deficient cells\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro ubiquitination reconstitution, genetic KO, and functional rescue with IAP-binding motif mutant\",\n      \"pmids\": [\"15300255\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"cIAP1 and cIAP2 act as E3 ubiquitin ligases for Smac/DIABLO, stimulating its ubiquitination both in vivo and in vitro via their RING domains, leading to Smac degradation. The substrate-dependent E3 activity requires physical interaction between cIAPs and Smac, and cIAP1/2 associate with overlapping but distinct E2 ubiquitin-conjugating enzymes.\",\n      \"method\": \"In vitro ubiquitination assay, in vivo ubiquitination in cells, RING domain mutants, E2 enzyme identification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution of ubiquitination plus in vivo confirmation; single lab but orthogonal methods\",\n      \"pmids\": [\"12525502\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Smac/DIABLO selectively promotes rapid degradation of cIAP1 and cIAP2 but not XIAP or Livin in HeLa cells via the ubiquitin/proteasome pathway. Smac binding via its N-terminal IAP-binding motif is prerequisite for this effect; Smac N-terminal peptide alone is sufficient to enhance cIAP1 ubiquitination, and mutant cIAP1 lacking all BIR domains is not ubiquitinated by Smac.\",\n      \"method\": \"Cell-based degradation assays, N-terminal peptide competition, BIR domain deletion mutants, auto-ubiquitination assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal cell-based and mutagenesis experiments, single lab\",\n      \"pmids\": [\"14960576\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Smac/DIABLO binding to IAPs (XIAP, cIAP-1, cIAP2) potently represses their ubiquitin ligase activities. Mutation of the XIAP RING domain reduces its anti-apoptotic efficacy, suggesting that XIAP ubiquitin ligase activity contributes to apoptosis inhibition and Smac antagonizes this in addition to blocking caspase-IAP interactions.\",\n      \"method\": \"In vitro ubiquitination assay, RING domain mutagenesis, cell-based apoptosis assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Weak — in vitro assay with mutagenesis, single lab, single study\",\n      \"pmids\": [\"15078891\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Mature DIABLO/Smac is produced by the inner membrane peptidase (IMP) complex on the mitochondrial inner membrane. The precursor enters mitochondria through a stop-transfer pathway and is cleaved by IMP to generate the active form with the exposed N-terminal IAP-binding motif.\",\n      \"method\": \"Complementation of yeast IMP mutants with mammalian IMP subunits, cellular processing assays, identification of IMP catalytic subunits by sequence conservation\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional complementation and cellular processing experiments, single lab with multiple approaches\",\n      \"pmids\": [\"15814844\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The rhomboid intramembrane protease PARL in the mitochondrial inner membrane cleaves Smac/DIABLO to generate the N-terminal IAP-binding motif required for apoptotic activity. Loss of PARL impairs Smac proteolytic maturation; PARL-deficient cells show reduced Smac–XIAP binding and impaired apoptosis, which is rescued by Smac peptidomimetics, XIAP downregulation, or cytosolic expression of pre-cleaved Smac.\",\n      \"method\": \"PARL-based proteomics, PARL knockout cells, co-immunoprecipitation of Smac-XIAP, rescue experiments, N-terminal sequencing of cleaved Smac\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — proteomics substrate identification, genetic KO, biochemical validation, and rescue with multiple orthogonal approaches in a single study\",\n      \"pmids\": [\"28288130\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Livin (an IAP family member) acts as an E3 ubiquitin ligase for Smac/DIABLO; both the BIR and RING domains of Livin are required for Smac degradation in vitro and in vivo. Mutation of the Livin BIR domain abolishes Smac/DIABLO binding and prevents its degradation.\",\n      \"method\": \"In vitro and in vivo ubiquitination assays, BIR and RING domain mutants, co-immunoprecipitation\",\n      \"journal\": \"Cell death and differentiation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Weak — in vitro and in vivo ubiquitination with mutagenesis, single lab\",\n      \"pmids\": [\"16729033\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Survivin physically interacts with Smac/DIABLO both in vitro and in vivo (co-immunoprecipitation). A point mutation (D71R) in the Survivin BIR motif and a C-terminal deletion mutant (Surv-BIR) abolish Smac/DIABLO binding and abrogate apoptosis inhibition. The N-terminus of mature Smac is required for Survivin-Smac complex formation. Co-presence of Smac/DIABLO and XIAP is required for Survivin to inhibit caspase cleavage in a cell-free system.\",\n      \"method\": \"Co-immunoprecipitation, in vitro binding, BIR domain point mutation (D71R), cell-free caspase inhibition assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — reciprocal Co-IP and mutagenesis across multiple experimental systems, single lab\",\n      \"pmids\": [\"12660240\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"NMR studies show that Smac/DIABLO N-terminal peptides bind across the third beta-strand of the Survivin BIR domain (near alpha4 and beta3), analogous to Smac binding to XIAP BIR3, as demonstrated by specific chemical shift perturbations in Survivin residues upon Smac peptide addition.\",\n      \"method\": \"NMR chemical shift perturbation mapping with Smac/DIABLO N-terminal peptides and Survivin\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — NMR structural mapping, single lab, no mutagenesis validation in this study\",\n      \"pmids\": [\"15628841\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crystal structures of Survivin complexed with Smac/DIABLO N-terminal peptide and with phospho-Thr3 histone H3 peptide establish that Survivin recognizes N-terminal Ala in both ligands at the same binding site. Calorimetric data show Survivin binds the H3T3ph peptide more tightly than the Smac/DIABLO N-terminal peptide; structure-guided mutations increasing hydrophobicity of the phosphate-binding pocket reversed this preference.\",\n      \"method\": \"X-ray crystallography, isothermal titration calorimetry, structure-guided mutagenesis\",\n      \"journal\": \"Structure\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structures with calorimetry and mutagenesis, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"22244766\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"tBid triggers rapid and essentially complete co-release of Smac/DIABLO and cytochrome c from mitochondria with similar kinetics (onset and completion synchronized), occurring after a ~10 s delay and completing within 50–70 s, coinciding with mitochondrial membrane potential depolarization. Bcl-xL prevents tBid-induced release of both proteins.\",\n      \"method\": \"GFP/YFP fusion proteins, real-time fluorescence imaging, rapid filtration Western blot, permeabilized HepG2 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live-cell imaging with rapid kinetics quantification and genetic perturbation (Bcl-xL), single lab\",\n      \"pmids\": [\"11741882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Real-time confocal imaging of MCF-7 cells expressing Smac/DIABLO-YFP shows that Smac/DIABLO and cytochrome c release onset is simultaneous and coincides with mitochondrial membrane potential depolarization. Smac/DIABLO release kinetics are not affected by caspase inhibition (z-VAD-fmk) or caspase-3 deficiency. DEVDase (caspase-3/7) activation occurs within 10 min of Smac/DIABLO release even without caspase-3.\",\n      \"method\": \"Stable Smac/DIABLO-YFP expression, real-time confocal microscopy, caspase inhibitor (z-VAD-fmk), caspase-3-deficient MCF-7 cells, DEVDase activity assay\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live-cell imaging with orthogonal genetic and pharmacological perturbations, single lab\",\n      \"pmids\": [\"12975347\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Smac/DIABLO-deficient (Smac−/−) mice generated by homologous recombination are viable, develop normally, and show no histological abnormalities. Cultured Smac−/− cells respond normally to apoptotic stimuli and show no difference in Fas-mediated apoptosis in vivo, though in vitro procaspase-3 cleavage is inhibited in cell lysates, indicating functional redundancy in vivo.\",\n      \"method\": \"Homologous recombination knockout mice, in vitro cell death assays, in vivo Fas-mediated apoptosis, cell-free caspase-3 activation assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — rigorously generated knockout mouse with comprehensive phenotypic analysis across multiple assays and cell types\",\n      \"pmids\": [\"11971981\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Combinatorial deletion of Diablo and Casp3 (but neither alone) causes perinatal lethality in mice, demonstrating a physiological role for Smac/DIABLO in regulating programmed cell death. In MEFs, loss of Smac/DIABLO alters both caspase-dependent and caspase-independent intrinsic PCD. siRNA epistasis experiments show Smac/DIABLO modulates inhibitory interactions between specific IAP family members and executioner caspases-3 and -7.\",\n      \"method\": \"Double-knockout mouse generation, MEF cell death assays, siRNA epistasis (XIAP, cIAP-1, cIAP-2, caspase-6, -7)\",\n      \"journal\": \"Cell death and differentiation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with double-KO mice and siRNA knockdown, single lab but multiple orthogonal approaches\",\n      \"pmids\": [\"21597464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Smac/DIABLO-induced apoptosis in ovarian carcinoma cells proceeds primarily through a caspase-9-mediated pathway; it is inhibited by XIAP overexpression and the caspase-9 inhibitor zLEHD-fmk. Smac-induced apoptosis occurs independently of cytochrome c release from mitochondria and is not inhibited by Bcl-2 overexpression when Smac is expressed ectopically.\",\n      \"method\": \"Recombinant adenovirus-mediated Smac expression, caspase inhibitors, XIAP overexpression, Bcl-2 overexpression, Western blot for caspase cleavage\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple perturbations (inhibitors, OE) with specific pathway readouts, single lab\",\n      \"pmids\": [\"12749848\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"A heterozygous SMAC/DIABLO missense mutation (c.377C>T; p.Ser71Leu in the mature protein) causes dominant progressive non-syndromic hearing loss (DFNA64). The mutant SMAC/DIABLO(S71L) retains proapoptotic function but triggers significant degradation of both wild-type and mutant SMAC/DIABLO and renders host mitochondria susceptible to calcium-induced loss of membrane potential, indicating a gain-of-function mitochondrial dysfunction.\",\n      \"method\": \"Linkage analysis, mutation identification, in vitro functional study with S71L mutant expression, mitochondrial membrane potential assay, Western blot for Smac protein levels\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional characterization of patient mutation with mitochondrial assays; single lab, moderate methods\",\n      \"pmids\": [\"21722859\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"E2F1 directly binds to and activates the SMAC/DIABLO promoter through two E2F1-binding sites (BS2 at −542/−535 bp and BS3 at −200/−193 bp), upregulating Smac/DIABLO mRNA and protein to enhance mitochondria-mediated apoptosis. This activation is E2F1-specific (E2F2 and E2F3 cannot activate these sites). siRNA knockdown of Smac/DIABLO significantly diminishes E2F1-induced apoptosis.\",\n      \"method\": \"Promoter luciferase reporter assays, ChIP, inducible ER-E2F1 cell line (4-OHT induction), RT-PCR, Western blot, siRNA knockdown\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (reporter, ChIP, inducible expression, siRNA rescue), single lab\",\n      \"pmids\": [\"16617145\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Mitochondrial survivin associates with Smac/DIABLO and delays its release during etoposide-induced apoptosis; cytosolic survivin also stabilizes released Smac/DIABLO levels. This selective retention of Smac/DIABLO (but not other intermembrane space proteins) explains the survivin anti-apoptotic effect in cancer cells.\",\n      \"method\": \"Co-immunoprecipitation of mitochondrial survivin with Smac/DIABLO, subcellular fractionation, etoposide treatment, Western blot for Smac release kinetics\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Weak — single Co-IP with fractionation, mechanistic follow-up limited, single lab\",\n      \"pmids\": [\"17546047\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"NAIP (neuronal apoptosis-inhibitory protein) fails to interact with the IAP-binding motif of Smac/DIABLO, and Smac protein does not interact with NAIP BIR domains, demonstrating that NAIP's mechanism of caspase inhibition is distinct from other IAPs and is not countered by Smac. This is a negative mechanistic finding distinguishing NAIP from XIAP.\",\n      \"method\": \"Recombinant protein binding assays with full-length NAIP and Smac; IAP-binding motif peptide competition assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct negative binding result with recombinant proteins and peptide competition, single lab; negative finding is mechanistically informative\",\n      \"pmids\": [\"15280366\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"SMAC/Diablo is essential for NSAID-induced apoptosis in colon cancer cells. Homologous recombination disruption and siRNA knockdown of SMAC/Diablo abrogates NSAID-induced apoptosis; reconstitution restores it. Loss of SMAC decreases caspase activation, cytochrome c release, and mitochondrial membrane potential collapse in this context.\",\n      \"method\": \"Homologous recombination gene disruption, siRNA knockdown, reconstitution, caspase activity assays, cytochrome c release (Western blot), mitochondrial membrane potential assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic disruption by HR (gold standard) plus siRNA, functional reconstitution, and multiple readouts in single study\",\n      \"pmids\": [\"15557007\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"SMAC/Diablo mediates the proapoptotic function of PUMA: in SMAC-deficient cells, PUMA-induced apoptosis is abrogated with decreases in caspase activation, cytochrome c release, and mitochondrial membrane potential collapse. Reconstitution of SMAC restores these events, and SMAC participates in a feedback amplification loop promoting cytochrome c release during DNA damage-induced apoptosis.\",\n      \"method\": \"SMAC-deficient cell lines, SMAC reconstitution, caspase activity assays, cytochrome c release (Western blot), mitochondrial membrane potential\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic loss-of-function with reconstitution, multiple mechanistic readouts, single lab\",\n      \"pmids\": [\"17237824\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"A ubiquitin fusion system generates mature, biologically active Smac directly in the cytosol with correct N-terminal Ala-Val-Pro-Ile sequence (without mitochondrial processing). This cytosolic Smac interacts with XIAP and sensitizes cells to apoptotic triggers (etoposide) but is not sufficient alone to trigger apoptosis in healthy cells.\",\n      \"method\": \"Ubiquitin fusion expression construct, N-terminal sequencing, co-immunoprecipitation with XIAP, cell death assays with etoposide\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — novel expression system with biochemical validation and functional testing, single lab\",\n      \"pmids\": [\"12511567\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Synthetic Smac/DIABLO N-terminal peptides (first 4–8 amino acids) fused to the antennapaedia penetratin carrier enter cells, bind XIAP and cIAP1 in situ, displace caspase-3 from cytoplasmic aggregates, and enhance drug-induced caspase activity and apoptosis when combined with chemotherapeutic agents.\",\n      \"method\": \"Cell-permeable peptide delivery, co-immunoprecipitation of peptide with XIAP/cIAP1, caspase activity assays, long-term antiproliferative assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct in-cell binding demonstrated by Co-IP plus functional caspase assays, single lab, multiple cell lines\",\n      \"pmids\": [\"12218061\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Cephalostatin 1 selectively triggers mitochondrial release of Smac/DIABLO without releasing cytochrome c, AIF, or activating caspase-8-dependent death receptors, defining a novel apoptosis pathway that uses Smac/DIABLO as the key mitochondrial signaling molecule. Bcl-xL overexpression delays both Smac/DIABLO release and apoptosis onset.\",\n      \"method\": \"Western blot for selective protein release, Bcl-xL overexpression, CD95/caspase-8-deficient cells, caspase inhibitor, electron microscopy\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic and pharmacological perturbations, multiple cell lines, selective release documented, single lab\",\n      \"pmids\": [\"14695204\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structures of BIRC6 in complex with SMAC reveal that BIRC6 forms a megadalton antiparallel dimer with a spacious cavity containing receptor sites for client proteins (caspase-9, HTRA2, LC3B). SMAC binds multivalently to BIRC6 with subnanomolar affinity, obstructing client binding sites and thereby impeding ubiquitination of both apoptotic (caspase-9) and autophagy (LC3B) substrates. BIRC6 directly restricts executioner caspase-3 and -7 and ubiquitinates caspases-3, -7, and -9 exclusively with noncanonical E1 enzyme UBA6.\",\n      \"method\": \"Cryo-EM structure determination, ubiquitination assays identifying UBA6 as E1, caspase activity assays, competitive displacement assays with SMAC\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — two independent cryo-EM studies (PMIDs 36758105 and 36758106) with biochemical reconstitution of ubiquitination and caspase inhibition mechanisms\",\n      \"pmids\": [\"36758105\", \"36758106\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Direct interaction between Smac/DIABLO and NADE (p75NTR-associated cell death executor) was identified by binding screen. The N-terminal region of Smac and C-terminal region of NADE mediate the interaction. Co-expression of NADE and Smac promotes TRAIL-induced apoptosis in MCF-7 cells. Co-presence of Smac and NADE inhibits XIAP-mediated Smac ubiquitination.\",\n      \"method\": \"Protein interaction screen, co-immunoprecipitation, domain mapping, cell-based apoptosis and ubiquitination assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP/pulldown with limited functional validation, single lab\",\n      \"pmids\": [\"15178455\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SMAC/Diablo has a non-apoptotic function in phospholipid synthesis: siRNA silencing of SMAC/Diablo in cancer cells and xenografts reduced cell and tumor growth, altered expression of genes involved in lipid metabolism, and decreased phospholipid (including phosphatidylcholine) levels. This suggests SMAC/Diablo regulates lipid synthesis essential for cancer cell proliferation.\",\n      \"method\": \"siRNA knockdown in cancer cell lines and subcutaneous xenografts, next-generation sequencing of transcriptome, phospholipid quantification, electron microscopy\",\n      \"journal\": \"Molecular therapy\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, indirect mechanism (transcriptome + lipidomics), no direct biochemical interaction identified; novel non-apoptotic finding\",\n      \"pmids\": [\"29396267\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"During UV-induced apoptosis in HeLa cells, YFP-Smac and GFP-cytochrome c are released from mitochondria in the same time window coinciding with mitochondrial membrane potential depolarization, and endogenous Smac and cytochrome c are always released together within individual cells. Pre-treatment with z-VAD-fmk does not affect Smac release, indicating caspase-independent release in this context (contrasting with the Adrain et al. finding in other apoptotic models).\",\n      \"method\": \"Live-cell confocal microscopy with dual YFP-Smac / GFP-cytochrome c fusion proteins, immunostaining of endogenous proteins, z-VAD-fmk treatment\",\n      \"journal\": \"Apoptosis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — live imaging with dual reporters in same cells, single lab; mechanistically informative contradiction with PMID 11726499 in specific UV context\",\n      \"pmids\": [\"15843890\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"A novel Smac/DIABLO splice variant, Smac3, is generated by alternative splicing of exon 4. Smac3 localizes to mitochondria via its N-terminal targeting sequence, is released into cytosol during apoptosis, binds BIR2 and BIR3 of XIAP via its IAP-binding motif, disrupts caspase-9–XIAP interaction, promotes caspase-3 activation, and uniquely accelerates XIAP auto-ubiquitination and destruction (unlike canonical Smac/DIABLO).\",\n      \"method\": \"Cloning/sequencing, mitochondrial targeting assay, co-immunoprecipitation with XIAP domains, caspase-3 activation assay, XIAP ubiquitination assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple complementary assays defining the unique properties of this isoform; single lab\",\n      \"pmids\": [\"14523016\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SMAC/DIABLO is a mitochondrial intermembrane space protein whose 55-residue N-terminal targeting sequence is cleaved by the IMP complex (and separately trimmed by PARL) to expose an Ala-Val-Pro-Ile IAP-binding motif; upon apoptotic stimuli, it is released in a Bcl-2/Bax-regulated manner into the cytosol where it competitively displaces caspase-9 from XIAP BIR3, antagonizes cIAP1/cIAP2 ubiquitin-ligase activity toward caspases, promotes cIAP1/2 auto-ubiquitination and proteasomal degradation, and is itself ubiquitinated and degraded by cIAP1/2, Apollon/BIRC6, and Livin; it additionally binds Survivin and NADE, is transcriptionally upregulated by E2F1, and—as shown by the BIRC6 cryo-EM structures—multivalently engages BIRC6 at subnanomolar affinity to competitively block ubiquitination of both apoptotic and autophagy substrates, collectively establishing SMAC/DIABLO as a central relief valve for IAP-mediated suppression of caspase activation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SMAC/DIABLO is a mitochondrial intermembrane-space protein that acts as a central antagonist of inhibitor-of-apoptosis proteins (IAPs), de-repressing caspase activation during programmed cell death [#0, #20]. It is synthesized as a precursor whose N-terminal targeting sequence is removed by the inner-membrane peptidase (IMP) complex and further by the rhomboid protease PARL to expose a mature Ala-Val-Pro-Ile IAP-binding motif essential for activity [#11, #12]. The functional protein homodimerizes through a hydrophobic interface, and this N-terminal tetrapeptide inserts into a surface groove on the XIAP BIR3 domain in a mode that is mutually exclusive with binding of the caspase-9 linker peptide, so SMAC competitively displaces caspase-9 from BIR3 while also neutralizing IAP inhibition of effector caspases through BIR1/BIR2 contacts [#0, #1, #2, #3, #4]. Beyond steric antagonism, SMAC represses and degrades IAPs: it inhibits the RING-dependent ubiquitin-ligase activity of XIAP, cIAP1 and cIAP2 and drives proteasomal degradation of cIAP1/2, while being reciprocally targeted for ubiquitination and degradation by cIAP1/2, Apollon/BIRC6 and Livin [#7, #8, #9, #10, #13]. Cryo-EM of the BIRC6 dimer shows SMAC binding multivalently with subnanomolar affinity to obstruct client sites and block BIRC6-mediated ubiquitination of both apoptotic (caspase-9) and autophagy (LC3B) substrates [#31]. SMAC additionally binds Survivin and is sequestered by it at the same N-terminal-Ala recognition site, linking SMAC to a wider network of BIR-domain partners [#14, #16, #24]. Mitochondrial release of SMAC is gated by Bcl-2/Bcl-xL and accompanies cytochrome c efflux during apoptotic membrane permeabilization, and SMAC is required for full caspase-3 processing in death-receptor (TRAIL), NSAID-, and PUMA-driven death [#5, #6, #17, #26, #27]. Single knockout mice are viable, but combined loss of Diablo and Casp3 causes perinatal lethality, establishing a physiological role in regulating cell death [#19, #20]. A heterozygous missense mutation (p.Ser71Leu) causes dominant non-syndromic hearing loss DFNA64 through a gain-of-function mitochondrial defect [#22].\"\n,\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Established the structural basis of SMAC/DIABLO function: how it dimerizes and how its N-terminus engages IAPs to promote caspase activation.\",\n      \"evidence\": \"X-ray crystallography of SMAC alone and bound to XIAP BIR3, NMR of the BIR3-peptide complex, with dimer-interface and N-terminal mutagenesis\",\n      \"pmids\": [\"10972280\", \"11140638\", \"11140637\", \"10950947\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve how the N-terminal motif is exposed in vivo\", \"Distinct contributions of BIR1/BIR2 versus BIR3 engagement only partially defined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defined the competitive mechanism by showing SMAC and the cleaved caspase-9 linker peptide share an IAP-binding motif and bind BIR3 mutually exclusively, explaining how SMAC potentiates caspase-9.\",\n      \"evidence\": \"Co-IP, caspase-9 cleavage-site mutagenesis, and competitive BIR3 binding assays\",\n      \"pmids\": [\"11242052\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address effector-caspase de-repression in cells\", \"In vitro competition not validated by structural snapshot of the ternary state\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Resolved the cell-biological gating of SMAC release, showing it exits mitochondria during apoptosis under Bcl-2/Bcl-xL control with kinetics coupled to membrane depolarization.\",\n      \"evidence\": \"Subcellular fractionation and live-cell imaging with Bcl-2/Bcl-xL overexpression and caspase inhibitors\",\n      \"pmids\": [\"11726499\", \"11741882\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Caspase-dependence of release disputed across stimuli (see #18, #34)\", \"Channel/permeabilization mechanism for SMAC efflux not defined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Tested the in vivo requirement for SMAC, revealing functional redundancy under basal conditions despite a clear in vitro caspase-3 activation defect.\",\n      \"evidence\": \"Smac-null knockout mice with phenotypic and cell-free caspase analyses\",\n      \"pmids\": [\"11971981\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the redundant factor\", \"Left open whether combined genetic loss would reveal a phenotype (resolved 2011, #20)\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Connected SMAC to death-receptor signaling, showing its Bax-dependent release is required to neutralize XIAP for complete caspase-3 processing in TRAIL apoptosis.\",\n      \"evidence\": \"Bax-null cells with cytosolic SMAC reconstitution and caspase-9 epistasis\",\n      \"pmids\": [\"11782443\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not quantify XIAP threshold setting the Bax requirement\", \"Generalizability beyond the cancer lines tested unaddressed\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Showed SMAC is reciprocally regulated and itself a ubiquitination substrate, while it both antagonizes and degrades IAPs, expanding its role from steric blocker to regulator of IAP ligase activity.\",\n      \"evidence\": \"In vitro and in vivo ubiquitination assays with cIAP1/2 and Livin, RING/BIR mutants, and a cytosolic ubiquitin-fusion SMAC expression system\",\n      \"pmids\": [\"12525502\", \"14960576\", \"15078891\", \"16729033\", \"12511567\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"E2 enzyme assignments incompletely mapped\", \"Selectivity for cIAP1/2 over XIAP degradation mechanistically unexplained\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Identified Survivin as a BIR-domain partner that sequesters SMAC, and defined the shared N-terminal-Ala recognition mode, linking SMAC antagonism to Survivin-mediated apoptosis suppression.\",\n      \"evidence\": \"Co-IP, BIR mutagenesis, NMR and crystallographic mapping of the Survivin-SMAC peptide interface with calorimetry\",\n      \"pmids\": [\"12660240\", \"15628841\", \"22244766\", \"17546047\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological stoichiometry of SMAC-Survivin sequestration unclear\", \"Competition with H3T3ph for the same site not resolved in cells\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Established Apollon/BIRC6 as an IAP that binds and degrades SMAC via its IAP-binding motif, and distinguished NAIP as an IAP not antagonized by SMAC.\",\n      \"evidence\": \"In vitro ubiquitination, Apollon knockout mice and rescue with motif mutants; recombinant NAIP binding assays\",\n      \"pmids\": [\"15300255\", \"15280366\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"BIRC6 architecture and binding mode not yet structurally resolved (later #31)\", \"Tissue-specific contribution of BIRC6-SMAC degradation undefined\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Defined the proteolytic maturation pathway, identifying IMP-mediated cleavage that generates the exposed IAP-binding motif on the inner membrane.\",\n      \"evidence\": \"Yeast IMP complementation with mammalian subunits and cellular processing assays\",\n      \"pmids\": [\"15814844\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not establish whether IMP cleavage alone produces the final mature N-terminus\", \"Relationship to later-identified PARL processing unaddressed\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Placed SMAC downstream of E2F1, showing direct transcriptional upregulation that contributes to E2F1-induced mitochondrial apoptosis.\",\n      \"evidence\": \"Promoter reporter assays, ChIP, inducible E2F1, and siRNA rescue\",\n      \"pmids\": [\"16617145\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Other transcriptional inputs not surveyed\", \"In vivo relevance of E2F1-SMAC axis untested\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Demonstrated a physiological cell-death role through genetic epistasis, with Diablo;Casp3 double loss causing perinatal lethality, and a disease link via the DFNA64 gain-of-function mutation.\",\n      \"evidence\": \"Double-knockout mice with siRNA epistasis on IAPs and executioner caspases; linkage analysis plus functional study of the S71L mutant\",\n      \"pmids\": [\"21597464\", \"21722859\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism connecting SMAC dysfunction to cochlear pathology incomplete\", \"Identity of the IAP-caspase modules dominating each tissue unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Completed the maturation model by identifying PARL as the inner-membrane protease that generates the IAP-binding motif required for apoptotic competence.\",\n      \"evidence\": \"PARL proteomics, knockout cells, SMAC-XIAP Co-IP, N-terminal sequencing, and multi-modal rescue\",\n      \"pmids\": [\"28288130\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Coordination/order of IMP versus PARL cleavage not fully reconciled\", \"Regulation of PARL processing under apoptotic stimuli unaddressed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Provided the structural mechanism for BIRC6 antagonism, showing SMAC multivalently blocks the BIRC6 client cavity to inhibit ubiquitination of both apoptotic and autophagy substrates.\",\n      \"evidence\": \"Cryo-EM of the BIRC6 dimer with SMAC, ubiquitination assays identifying UBA6, and caspase inhibition assays\",\n      \"pmids\": [\"36758105\", \"36758106\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular regulation of SMAC-BIRC6 stoichiometry not addressed\", \"Whether SMAC similarly blocks other BIR-megaligases unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Whether SMAC/DIABLO's reported non-apoptotic role in phospholipid synthesis reflects a direct biochemical activity rather than an indirect transcriptional consequence remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No direct interaction or enzymatic activity linking SMAC to lipid metabolism identified\", \"Lipid phenotype rests on a single siRNA/transcriptome study (#33)\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3, 4, 9, 10, 31]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [3, 31]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [11, 12, 17, 18]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [5, 6, 28]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [0, 5, 20]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [8, 9, 10, 31]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"XIAP\", \"BIRC2\", \"BIRC3\", \"BIRC6\", \"BIRC7\", \"BIRC5\", \"CASP9\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":9,"faith_total":9,"faith_pct":100.0}}