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Verhagen AM, Vaux DL. Cell death regulation by the mammalian IAP antagonist Diablo/Smac. Apoptosis 2002. [PMID: 11865200 DOI: 10.1023/a: 1014318615955] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
In Drosophila, the genetic locus 75CI1,2 is essential for all developmental cell death. Within this region are the genes for three pro-death proteins, Grim, Reaper and HID. These proteins are transcriptionally regulated and their expression tightly associated with cell death in the developing fly embryo. When ectopically expressed in the retina, Grim, Reaper and HID cause apoptosis and eye ablation. They have a short region of similarity at their N-termini through which they can interact with inhibitor of apoptosis (IAP) proteins, and it is by antagonising IAP inhibition of caspases that Grim, Reaper and HID promote cell death. The observation that Grim, Reaper and HID can interact with mammalian IAPs and induce apoptosis in mammalian cells suggested that mammalian IAP antagonists might also exist. Diablo/Smac, identified six years after the first description of a Drosophila IAP antagonist, is the only mammalian protein identified to date that is clearly functionally related to the Drosophila proteins. Since its discovery, there have been numerous studies investigating how Diablo/Smac interacts with IAPs and promotes cell death. Here we review what is currently known about Diablo/Smac and speculate on other mammalian IAP antagonists.
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O'Reilly LA, Ekert P, Harvey N, Marsden V, Cullen L, Vaux DL, Hacker G, Magnusson C, Pakusch M, Cecconi F, Kuida K, Strasser A, Huang DCS, Kumar S. Caspase-2 is not required for thymocyte or neuronal apoptosis even though cleavage of caspase-2 is dependent on both Apaf-1 and caspase-9. Cell Death Differ 2002; 9:832-41. [PMID: 12107826 DOI: 10.1038/sj.cdd.4401033] [Citation(s) in RCA: 157] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2001] [Revised: 01/28/2002] [Accepted: 02/08/2002] [Indexed: 11/09/2022] Open
Abstract
We have generated rat monoclonal antibodies that specifically recognise caspase-2 from many species, including mouse, rat and humans. Using these antibodies, we have investigated caspase-2 expression, subcellular localisation and processing. We demonstrate that caspase-2 is expressed in most tissues and cell types. Cell fractionation and immunohistochemistry experiments show that caspase-2 is found in the nuclear and cytosolic fractions, including a significant portion present in the Golgi complex. We found that caspase-2 is processed in response to many apoptotic stimuli but experiments with caspase-2 deficient mice demonstrated that it is not required for apoptosis of thymocytes or dorsal root ganglia (DRG) neurons in response to a variety of cytotoxic stimuli. Caspase-2 processing does not occur in thymocytes lacking Apaf-1 or caspase-9, suggesting that in this cell type, activation of caspase-2 occurs downstream of apoptosome formation.
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Tikoo A, O'Reilly L, Day CL, Verhagen AM, Pakusch M, Vaux DL. Tissue distribution of Diablo/Smac revealed by monoclonal antibodies. Cell Death Differ 2002; 9:710-6. [PMID: 12058276 DOI: 10.1038/sj.cdd.4401031] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2001] [Revised: 01/22/2002] [Accepted: 01/23/2002] [Indexed: 11/08/2022] Open
Abstract
Diablo/Smac is a mammalian pro-apoptotic protein that can antagonize the inhibitor of apoptosis proteins (IAPs). We have produced monoclonal antibodies specific for Diablo and have used these to examine its tissue distribution and subcellular localization in healthy and apoptotic cells. Diablo could be detected in a wide range of mouse tissues including liver, kidney, lung, intestine, pancreas and testes by Western blot analysis. Immunohistochemical analysis found Diablo to be most abundant in the germinal cells of the testes, the parenchymal cells of the liver and the tubule cells of the kidney. In support of previous subcellular localization analysis, Diablo was present within the mitochondria of healthy cells, but released into the cytosol following the induction of apoptosis by UV.
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Silke J, Hawkins CJ, Ekert PG, Chew J, Day CL, Pakusch M, Verhagen AM, Vaux DL. The anti-apoptotic activity of XIAP is retained upon mutation of both the caspase 3- and caspase 9-interacting sites. J Cell Biol 2002; 157:115-24. [PMID: 11927604 PMCID: PMC2173256 DOI: 10.1083/jcb.200108085] [Citation(s) in RCA: 105] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The X-linked mammalian inhibitor of apoptosis protein (XIAP) has been shown to bind several partners. These partners include caspase 3, caspase 9, DIABLO/Smac, HtrA2/Omi, TAB1, the bone morphogenetic protein receptor, and a presumptive E2 ubiquitin-conjugating enzyme. In addition, we show here that XIAP can bind to itself. To determine which of these interactions are required for it to inhibit apoptosis, we generated point mutant XIAP proteins and correlated their ability to bind other proteins with their ability to inhibit apoptosis. partial differential RING point mutants of XIAP were as competent as their full-length counterparts in inhibiting apoptosis, although impaired in their ability to oligomerize with full-length XIAP. Triple point mutants, unable to bind caspase 9, caspase 3, and DIABLO/HtrA2/Omi, were completely ineffectual in inhibiting apoptosis. However, point mutants that had lost the ability to inhibit caspase 9 and caspase 3 but retained the ability to inhibit DIABLO were still able to inhibit apoptosis, demonstrating that IAP antagonism is required for apoptosis to proceed following UV irradiation.
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Abstract
In Drosophila, the genetic locus 75CI1,2 is essential for all developmental cell death. Within this region are the genes for three pro-death proteins, Grim, Reaper and HID. These proteins are transcriptionally regulated and their expression tightly associated with cell death in the developing fly embryo. When ectopically expressed in the retina, Grim, Reaper and HID cause apoptosis and eye ablation. They have a short region of similarity at their N-termini through which they can interact with inhibitor of apoptosis (IAP) proteins, and it is by antagonising IAP inhibition of caspases that Grim, Reaper and HID promote cell death. The observation that Grim, Reaper and HID can interact with mammalian IAPs and induce apoptosis in mammalian cells suggested that mammalian IAP antagonists might also exist. Diablo/Smac, identified six years after the first description of a Drosophila IAP antagonist, is the only mammalian protein identified to date that is clearly functionally related to the Drosophila proteins. Since its discovery, there have been numerous studies investigating how Diablo/Smac interacts with IAPs and promotes cell death. Here we review what is currently known about Diablo/Smac and speculate on other mammalian IAP antagonists.
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Verhagen AM, Silke J, Ekert PG, Pakusch M, Kaufmann H, Connolly LM, Day CL, Tikoo A, Burke R, Wrobel C, Moritz RL, Simpson RJ, Vaux DL. HtrA2 promotes cell death through its serine protease activity and its ability to antagonize inhibitor of apoptosis proteins. J Biol Chem 2002; 277:445-54. [PMID: 11604410 DOI: 10.1074/jbc.m109891200] [Citation(s) in RCA: 430] [Impact Index Per Article: 19.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Inhibitor of apoptosis (IAP) proteins inhibit caspases, a function counteracted by IAP antagonists, insect Grim, HID, and Reaper and mammalian DIABLO/Smac. We now demonstrate that HtrA2, a mammalian homologue of the Escherichia coli heat shock-inducible protein HtrA, can bind to MIHA/XIAP, MIHB, and baculoviral OpIAP but not survivin. Although produced as a 50-kDa protein, HtrA2 is processed to yield an active serine protease with an N terminus similar to that of Grim, Reaper, HID, and DIABLO/Smac that mediates its interaction with XIAP. HtrA2 is largely membrane-associated in healthy cells, with a significant proportion observed within the mitochondria, but in response to UV irradiation, HtrA2 shifts into the cytosol, where it can interact with IAPs. HtrA2 can, like DIABLO/Smac, prevent XIAP inhibition of active caspase 3 in vitro and is able to counteract XIAP protection of mammalian NT2 cells against UV-induced cell death. The proapoptotic activity of HtrA2 in vivo involves both IAP binding and serine protease activity. Mutations of either the N-terminal alanine of mature HtrA2 essential for IAP interaction or the catalytic serine residue reduces the ability of HtrA2 to promote cell death, whereas a complete loss in proapoptotic activity is observed when both sites are mutated.
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Verhagen AM, Pakusch M, Silke J, Vaux DL. TNF and CD95 promote IL-8 gene transactivation via independent elements in colon carcinoma cells. Cytokine 2001; 15:108-12. [PMID: 11500086 DOI: 10.1006/cyto.2001.0915] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The pro-inflammatory cytokine interleukin-8 (IL-8) is produced by HT29 colon epithelial cells following engagement of either CD95 or tumour necrosis factor (TNF) receptors. While the IL-8 promotor elements activated by TNF are well characterised, those responsible for induction of IL-8 by CD95 are unknown. We examined the pathway for CD95 induced IL-8 secretion using two luciferase reporter constructs; the first comprising approximately 500 bp of the IL-8 promotor that includes the nuclear factor kappa B (NFkappaB), C/EBP and AP-1 sites known to be involved in TNF mediated IL-8 induction; the second that encompasses these elements but extends approximately 1.1 kb further upstream. Although IL-8 mRNA and protein were produced in response to either TNF or CD95 ligation, only TNF induced an increase in the reporter activity of the promoter constructs. Nevertheless, IL-8 induction by CD95 resulted primarily from increased transcription and not from an increase in IL-8 mRNA stability. These results suggest that promoter elements/enhancers involved in CD95 mediated IL-8 induction are distinct from those used by TNF and not contained within the 1.6 kb region immediately upstream of the initiation codon.
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Silke J, Ekert PG, Day CL, Hawkins CJ, Baca M, Chew J, Pakusch M, Verhagen AM, Vaux DL. Direct inhibition of caspase 3 is dispensable for the anti-apoptotic activity of XIAP. EMBO J 2001; 20:3114-23. [PMID: 11406588 PMCID: PMC150202 DOI: 10.1093/emboj/20.12.3114] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
XIAP is a mammalian inhibitor of apoptosis protein (IAP). To determine residues within the second baculoviral IAP repeat (BIR2) required for inhibition of caspase 3, we screened a library of BIR2 mutants for loss of the ability to inhibit caspase 3 toxicity in the yeast Schizosaccharomyces pombe. Four of the mutations, not predicted to affect the structure of the BIR fold, clustered together on the N-terminal region that flanks BIR2, suggesting that this is a site of interaction with caspase 3. Introduction of these mutations into full-length XIAP reduced caspase 3 inhibitory activity up to 500-fold, but did not affect its ability to inhibit caspase 9 or interact with the IAP antagonist DIABLO. Furthermore, these mutants retained full ability to inhibit apoptosis in transfected cells, demonstrating that although XIAP is able to inhibit caspase 3, this activity is dispensable for inhibition of apoptosis by XIAP in vivo.
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Silke J, Vaux DL. Two kinds of BIR-containing protein - inhibitors of apoptosis, or required for mitosis. J Cell Sci 2001; 114:1821-7. [PMID: 11329368 DOI: 10.1242/jcs.114.10.1821] [Citation(s) in RCA: 84] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
The baculoviral IAP repeat (BIR) is a zinc-binding fold. Some BIR-containing proteins (BIRCs), including several from insect viruses, insects and vertebrates, are inhibitors of cell death and act by binding to active caspases. Their ability to do so can be antagonized by pro-apoptotic insect proteins such as Grim, HID and Reaper, or the mammalian protein Diablo/Smac. Members of one structurally distinct subgroup of BIR-containing proteins, which are present in yeasts and Caenorhabditis elegans as well as insects and vertebrates, do not act as caspase inhibitors; instead, they are required for chromosome segregation and cytokinesis, and act in concert with inner centromere protein (INCENP) homologues and aurora kinase homologues.
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Ekert PG, Silke J, Hawkins CJ, Verhagen AM, Vaux DL. DIABLO promotes apoptosis by removing MIHA/XIAP from processed caspase 9. J Cell Biol 2001; 152:483-90. [PMID: 11157976 PMCID: PMC2195997 DOI: 10.1083/jcb.152.3.483] [Citation(s) in RCA: 146] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
MIHA is an inhibitor of apoptosis protein (IAP) that can inhibit cell death by direct interaction with caspases, the effector proteases of apoptosis. DIABLO is a mammalian protein that can bind to IAPs and antagonize their antiapoptotic effect, a function analogous to that of the proapoptotic Drosophila molecules, Grim, Reaper, and HID. Here, we show that after UV radiation, MIHA prevented apoptosis by inhibiting caspase 9 and caspase 3 activation. Unlike Bcl-2, MIHA functioned after release of cytochrome c and DIABLO from the mitochondria and was able to bind to both processed caspase 9 and processed caspase 3 to prevent feedback activation of their zymogen forms. Once released into the cytosol, DIABLO bound to MIHA and disrupted its association with processed caspase 9, thereby allowing caspase 9 to activate caspase 3, resulting in apoptosis.
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Abstract
SUMMARY Apoptosis is a physiological cell death process important for development, homeostasis and the immune defence of multicellular animals. The key effectors of apoptosis are caspases, cysteine proteases that cleave after aspartate residues. The inhibitor of apoptosis (IAP) family of proteins prevent cell death by binding to and inhibiting active caspases and are negatively regulated by IAP-binding proteins, such as the mammalian protein DIABLO/Smac. IAPs are characterized by the presence of one to three domains known as baculoviral IAP repeat (BIR) domains and many also have a RING-finger domain at their carboxyl terminus. More recently, a second group of BIR-domain-containing proteins (BIRPs) have been identified that includes the mammalian proteins Bruce and Survivin as well as BIR-containing proteins in yeasts and Caenorhabditis elegans. These Survivin-like BIRPs regulate cytokinesis and mitotic spindle formation. In this review, we describe the IAPs and other BIRPs, their evolutionary relationships and their subcellular and tissue localizations.
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Abstract
To try to understand autoimmunity, attention has often fallen on the process of cell death. After all, apoptosis is used during selection of immunocytes, cells in the target organs end up dying and mutations to cell death genes have been found in some autoimmune diseases. Furthermore, some autoimmune-prone mice fail to develop disease when certain cell death genes are deleted, and transgenic mice expressing other cell death genes develop autoimmunity. However, only a tiny proportion of human autoimmune disease is associated with mutations to individual genes and even in these rare cases the genetic background has a major influence on the severity of disease. An understanding of the pathophysiology of common autoimmune diseases will require elucidation of many different systems that interact in complex ways, of which the process of apoptosis is just one.
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Silke J, Verhagen AM, Ekert PG, Vaux DL. Sequence as well as functional similarity for DIABLO/Smac and Grim, Reaper and Hid? Cell Death Differ 2000; 7:1275. [PMID: 11270364 DOI: 10.1038/sj.cdd.4400790] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
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Uren AG, Wong L, Pakusch M, Fowler KJ, Burrows FJ, Vaux DL, Choo KH. Survivin and the inner centromere protein INCENP show similar cell-cycle localization and gene knockout phenotype. Curr Biol 2000; 10:1319-28. [PMID: 11084331 DOI: 10.1016/s0960-9822(00)00769-7] [Citation(s) in RCA: 423] [Impact Index Per Article: 17.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
BACKGROUND Survivin is a mammalian protein that carries a motif typical of the inhibitor of apoptosis (IAP)proteins, first identified in baculoviruses. Although baculoviral IAP proteins regulate cell death, the yeast Survivin homolog Bir1 is involved in cell division. To determine the function of Survivin in mammals, we analyzed the pattern of localization of Survivin protein during the cell cycle, and deleted its gene by homologous recombination in mice. RESULTS In human cells, Survivin appeared first on centromeres bound to a novel para-polar axis during prophase/metaphase, relocated to the spindle midzone during anaphase/telophase, and disappeared at the end of telophase. In the mouse, Survivin was required for mitosis during development. Null embryos showed disrupted microtubule formation, became polyploid, and failed to survive beyond 4.5days post coitum. This phenotype, and the cell-cycle localization of Survivin, resembled closely those of INCENP. Because the yeast homolog of INCENP, Sli15, regulates the Aurora kinase homolog Ipl1p, and the yeast Survivin homolog Bir1 binds to Ndc10p, a substrate of Ipl1p, yeast Survivin, INCENP and Aurora homologs function in concert during cell division. CONCLUSIONS In vertebrates, Survivin and INCENP have related roles in mitosis, coordinating events such as microtubule organization, cleavage-furrow formation and cytokinesis. Like their yeast homologs Bir1 and Sli15, they may also act together with the Aurora kinase.
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Verhagen AM, Ekert PG, Pakusch M, Silke J, Connolly LM, Reid GE, Moritz RL, Simpson RJ, Vaux DL. Identification of DIABLO, a mammalian protein that promotes apoptosis by binding to and antagonizing IAP proteins. Cell 2000; 102:43-53. [PMID: 10929712 DOI: 10.1016/s0092-8674(00)00009-x] [Citation(s) in RCA: 1658] [Impact Index Per Article: 69.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
To identify proteins that bind mammalian IAP homolog A (MIHA, also known as XIAP), we used coimmuno-precipitation and 2D immobilized pH gradient/SDS PAGE, followed by electrospray ionization tandem mass spectrometry. DIABLO (direct IAP binding protein with low pI) is a novel protein that can bind MIHA and can also interact with MIHB and MIHC and the baculoviral IAP, OpIAP. The N-terminally processed, IAP-interacting form of DIABLO is concentrated in membrane fractions in healthy cells but released into the MIHA-containing cytosolic fractions upon UV irradiation. As transfection of cells with DIABLO was able to counter the protection afforded by MIHA against UV irradiation, DIABLO may promote apoptosis by binding to IAPs and preventing them from inhibiting caspases.
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Strasser A, Vaux DL. Death in the snow: report on Keystone Conference on 'Apoptosis and Programmed Cell Death' at Breckenridge, CO, April 6-11th 1999. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1470:R1-R11. [PMID: 10656989 DOI: 10.1016/s0304-419x(99)00029-3] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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Abstract
Caspases are the key effector molecules of the physiological death process known as apoptosis, although some are involved in activation of cytokines, rather than cell death. They exist in most of our cells as inactive precursors (zymogens) that kill the cell once activated. Caspases can be controlled in two ways. The processing and activation of a caspase can be regulated by molecules such as FADD, APAF-1, Bcl-2 family members, FLIP and IAPs. Active caspases can be controlled by a variety of inhibitors that directly interact with the protease. This review describes the later direct caspase inhibitors that have been identified, products of both viral and cellular genes, and artificial caspase inhibitors that have been developed both as research tools and as pharmaceutical agents to inhibit cell death in vivo.
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Day CL, Dupont C, Lackmann M, Vaux DL, Hinds MG. Solution structure and mutagenesis of the caspase recruitment domain (CARD) from Apaf-1. Cell Death Differ 1999; 6:1125-32. [PMID: 10578182 DOI: 10.1038/sj.cdd.4400584] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Activation of procaspase-9, a key component of the apoptosis mechanism, requires the interaction of its caspase recruitment domain (CARD) with the CARD in the adaptor protein Apaf-1. Using nuclear magnetic resonance spectroscopy and mutagenesis we have determined the structure of the CARD from Apaf-1 and the residues important for binding the CARD in procaspase-9. Apaf-1's CARD contains seven short alpha-helices with the core six helices arranged in an antiparallel manner. Residues in helix 2 have a central role in mediating interaction with procaspase-9 CARD. This interaction surface is distinct from that proposed based on the structure of the CARD from RAIDD, but is coincident with that of the structurally similar FADD death effector domain and the Apaf-1 CARD interface identified by crystallographic studies.
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Uren AG, Beilharz T, O'Connell MJ, Bugg SJ, van Driel R, Vaux DL, Lithgow T. Role for yeast inhibitor of apoptosis (IAP)-like proteins in cell division. Proc Natl Acad Sci U S A 1999; 96:10170-5. [PMID: 10468581 PMCID: PMC17861 DOI: 10.1073/pnas.96.18.10170] [Citation(s) in RCA: 162] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023] Open
Abstract
Inhibitors of apoptosis (IAPs) are a family of proteins that bear baculoviral IAP repeats (BIRs) and regulate apoptosis in vertebrates and Drosophila melanogaster. The yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe both encode a single IAP, designated BIR1 and bir1, respectively, each of which bears two BIRs. In rich medium, BIR1 mutant S. cerevisiae underwent normal vegetative growth and mitosis. Under starvation conditions, however, BIR1 mutant diploids formed spores inefficiently, instead undergoing pseudohyphal differentiation. Most spores that did form failed to survive beyond two divisions after germination. bir1 mutant S. pombe spores also died in the early divisions after spore germination and became blocked at the metaphase/anaphase transition because of an inability to elongate their mitotic spindle. Rather than inhibiting caspase-mediated cell death, yeast IAP proteins have roles in cell division and appear to act in a similar way to the IAPs from Caenorhabditis elegans and the mammalian IAP Survivin.
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Hinds MG, Norton RS, Vaux DL, Day CL. Solution structure of a baculoviral inhibitor of apoptosis (IAP) repeat. NATURE STRUCTURAL BIOLOGY 1999; 6:648-51. [PMID: 10404221 DOI: 10.1038/10701] [Citation(s) in RCA: 146] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Members of the inhibitor of apoptosis (IAP) family of proteins are able to inhibit cell death following viral infection, during development or in cell lines in vitro. All IAP proteins bear one or more baculoviral IAP repeats (BIRs). Here we describe the solution structure of the third BIR domain from the mammalian IAP homolog B (MIHB/c-IAP-1). The BIR domain has a novel fold that is stabilized by zinc tetrahedrally coordinated by one histidine and three cysteine residues. The structure consists of a series of short alpha-helices and turns with the zinc packed in an unusually hydrophobic environment created by residues that are highly conserved among all BIRs.
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