This entry represents the Fas-associated death domain protein FADD. This is an apoptotic adaptor molecule that recruits caspase-8 or caspase-10 to the activated Fas (CD95) or TNFR-1 receptors. The resulting aggregate, called the death-inducing signalling complex (DISC), performs caspase-8 proteolytic activation. Active caspase-8 initiates the subsequent cascade of caspases mediating apoptosis. For further information see [, , , , ].
The death domain (DD) is a conserved region of about 80 residues found on death receptors, and which is required for death signalling, as well as a variety of non-apoptotic functions [, ]. Proteins containing this domain include the low affinity neurotrophin receptor p73, Fas, FADD (Fas-associated death domain protein), TNF-1 (tumour necrosis factor receptor-1), Pelle protein kinase, and the Tube adaptor protein [].The induction of apoptosis also relies on the presence of a second domain, called the death effector domain. The death effector domain (DED) occurs in proteins that regulate programmed cell death, including both pro- and anti-apoptotic proteins; many of these proteins are also involved in controlling cellular activation and proliferation pathways []. Proteins containing this domain include FADD (DED N-terminal, DD C-terminal), PEA-15 (phosphoproteins enriched in astrocytes 15kDa), caspases and FLIP.The induction of apoptosis results in the activation of caspases, a family of aspartyl-specific cysteine proteases that are the main executioners of apoptosis. For example, the DED of FADD recruits two DED-containing caspases, caspase-8 and caspase-10, to form the death-inducing signal complex, which initiates apoptosis. Proteins containing the caspase recruitment domain (CARD) are involved in the recruitment and activation of caspases during apoptosis []. Other CARD proteins participate in NF-kappaB signalling pathways associated with innate or adaptive immune responses. Proteins containing CARD include Raidd, APAF-1 (apoptotic protease activating factor 1), procaspase 9 and iceberg (inhibitor of interleukin-1-beta generation).The DD shows strong structural similarity to both DED and CARD. They all display a 6-helical closed bundle fold, with greek key topology and an internal psuedo two-fold symmetry. However, despite their overall similarity in topology, each domain forms specialised interactions, typically only with members of its own subfamily, for example DED with DED.This superfamily represents the death domain and other structurally similar domains, including DED, CARD and the DAPIN domain.
TRADD is a signalling adaptor protein involved in tumour necrosis factor-receptor I (TNFR1)-associated apoptosis and cell survival. The decision between apoptosis and cell survival involves the interplay between two sequential signalling complexes. The plasma membrane-bound complex I is comprised of TNFR1, TRADD, the kinase RIP1, and TRAF2, which together mediate the activation of NF-kappaB. Subsequently, complex II is formed in the cytoplasm, where TRADD and RIP1 associate with FADD and caspase-8. If NF-kappaB is activated by complex I, then complex II will associate with the caspase-8 inhibitor FLIP(L) and the cell survives, while the failure to activate NF-kappaB leads to apoptosis [].TRADD contains two functionally separate domains, which allow the protein to couple to two distinct signaling pathways. The TRADD C-terminal death domain is responsible for its association with TNFR1, and with the death-domain proteins FADD and RIP1, which promote apoptosis. The TRADD N-terminal domain binds TRAF2 and promotes TRAF2 recruitment to TNFR1, thereby mediating the activation of NK-kappaB and JNK/AP1, which promote cell survival []. The N-terminal TRADD domain is composed of an α/β sandwich, where the β-strands form an antiparallel β-sheet.
TRADD is a signalling adaptor protein involved in tumour necrosis factor-receptor I (TNFR1)-associated apoptosis and cell survival. The decision between apoptosis and cell survival involves the interplay between two sequential signalling complexes. The plasma membrane-bound complex I is comprised of TNFR1, TRADD, the kinase RIP1, and TRAF2, which together mediate the activation of NF-kappaB. Subsequently, complex II is formed in the cytoplasm, where TRADD and RIP1 associate with FADD and caspase-8. If NF-kappaB is activated by complex I, then complex II will associate with the caspase-8 inhibitor FLIP(L) and the cell survives, while the failure to activate NF-kappaB leads to apoptosis [].TRADD contains two functionally separate domains, which allow the protein to couple to two distinct signaling pathways. The TRADD C-terminal death domain is responsible for its association with TNFR1, and with the death-domain proteins FADD and RIP1, which promote apoptosis. The TRADD N-terminal domain binds TRAF2 and promotes TRAF2 recruitment to TNFR1, thereby mediating the activation of NK-kappaB and JNK/AP1, which promote cell survival []. The N-terminal TRADD domain is composed of an α/β sandwich, where the β-strands form an antiparallel β-sheet.
This entry represents the death domain (DD) found in the FS7-associated cell surface antigen (FAS). FAS, also known as TNFRSF6 (TNF receptor superfamily member 6), APT1, CD95, FAS1, or APO-1, together with FADD (Fas-associating via Death Domain) and caspase 8, is an integral part of the death inducing signalling complex (DISC), which plays an important role in the induction of apoptosis and is activated by binding of the ligand FasL to FAS [, ]. FAS also plays a critical role in self-tolerance by eliminating cell types (autoreactive T and B cells) that contribute to autoimmunity [].DDs are protein-protein interaction domains found in a variety of domain architectures. Their common feature is that they form homodimers by self-association or heterodimers by associating with other members of the DD superfamily including CARD (Caspase activation and recruitment domain), DED (Death Effector Domain), and PYRIN. They serve as adaptors in signaling pathways and can recruit other proteins into signaling complexes [, ].
Caspase-8 (MEROPS identifier C14.009) is a cytoplasmic cysteine endopeptidase with a preference for aspartyl bonds, acing at neutral pH []. It is active as a homodimer or as a heterodimer in association with the long isoform of FLICE, and proteolytic processing of the caspase-8 precursor is required for stabilisation of the dimer [, , ]. It is one of the activator caspases. Caspase-8 has a strict requirement for Asp in P1, a preference for Glu in P3 and small residues in P1' []. The caspase-8 proenzyme has two N-terminal death effector domains which are removed upon activation along with a linker region between the large and small subunits of the C-terminal catalytic domain. The death inducing signalling complex, composed of a transmembrane death receptor and the adapter protein FADD assembles at the cell membrane following binding of a death ligand. Procaspase-8 is recruited to this complex, and becomes active by dimerisation. Active caspase-8 can then activate the executioner caspases -3 and -7 [].Caspase-8 cleaves RIPK1, which is crucial to inhibit RIPK1 kinase activity, limiting TNF-induced apoptosis, necroptosis and inflammatory response. In humans, non-cleavable RIPK1 leads to autoinflammatory disease characterized by hypersensitivity to apoptosis and necroptosis and increased inflammatory response [, ].
The tumour necrosis factor (TNF) receptor (TNFR) superfamily comprises more than 20 type-I transmembrane proteins. Family members are defined based on similarity in their extracellular domain - a region that contains many cysteine residues arranged in a specific repetitive pattern []. The cysteines allow formation of an extended rod-like structure, responsible for ligand binding []. Upon receptor activation, different intracellular signalling complexes are assembled for different members of the TNFR superfamily, depending on their intracellular domains and sequences []. Activation of TNFRs can therefore induce a range of disparate effects, including cell proliferation, differentiation, survival, or apoptotic cell death, depending upon the receptor involved [, ]. TNFRs are widely distributed and play important roles in many crucial biological processes, such as lymphoid and neuronal development, innate and adaptive immunity, and maintenance of cellular homeostasis []. Drugs that manipulate their signaling have potential roles in the prevention and treatment of many diseases, such as viral infections, coronary heart disease, transplant rejection, and immune disease []. TNF receptor 1A (also known as TNF-R1 and CD120a antigen) contains a death domain within its C-terminal region that mediates interactions with several proteins involved in signaling the downstream effects of TNF. Activation of the receptor may induce either cell survival or apoptosis, the latter proceeding via recruitment of the adaptor protein FADD and caspase-8 to the receptor complex []. Defects in TNF receptor 1A are the cause of familial hibernian fever (FHF) - an autosomal dominant disease characterised by recurrent fever, abdominal pain, localised tender skin lesions and myalgia.
Tumor necrosis factor receptor superfamily member 6 (TNFRSF6), also known as Fas cell surface death receptor (FasR) or Fas, APT1, CD95, FAS1, APO-1, FASTM, ALPS1A, contains a death domain and plays a central role in the physiological regulation of programmed cell death [, ]. It has been implicated in the pathogenesis of various malignancies and diseases of the immune system [, ]. The receptor interacts with the Fas ligand (FasL), allowing the formation of a death-inducing signaling complex that includes Fas-associated death domain protein (FADD), caspase 8, and caspase 10; autoproteolytic processing of the caspases in the complex triggers a downstream caspase cascade, leading to apoptosis. This receptor has also been shown to activate NF-kappaB, MAPK3/ERK1, and MAPK8/JNK, and is involved in transducing the proliferating signals in normal diploid fibroblast and T cells [, , ].In channel catfish and the Japanese rice fish medaka, homologues of Fas receptor (FasR), as well as FADD and caspase 8, have been identified and characterized, and likely constitute the teleost equivalent of the death-inducing signaling complex (DISC) [, ]. FasL/FasR are involved in the initiation of apoptosis and suggest that mechanisms of cell-mediated cytotoxicity in teleosts are similar to those used by mammals; presumably, the mechanism of apoptosis induction via death receptors was evolutionarily established during the appearance of vertebrates.This entry represents the N-terminal domain of TNFRSF6/Fas from teleosts. TNF-receptors are modular proteins. The N-terminal extracellular part contains a cysteine-rich region responsible for ligand-binding. This region is composed of small modules of about 40 residues containing 6 conserved cysteines; the number and type of modules can vary in different members of the family [, , ].
The tumour necrosis factor (TNF) receptor associated factors (TRAFs) are major signal transducers for the TNF receptor (TNFR) superfamily and the interleukin-1 receptor/Toll-like receptor superfamily in mammals []. TRAFs constitute a family of genetically conserved adapter proteins found in mammals (TRAF1-6) as well as in other multicellular organisms such as Drosophila [], Caenorhabditis elegans []. TRAF2 is the prototypical member of the family. Mammalian TRAF1 and TRAF2 were the first members initially identified by their association with TNFR2. The TRAF1/TRAF2 and TRAF3/TRAF5 gene pairs may have arisen from recent independent gene duplications and to share a common ancestral gene. TRAF4 and TRAF6 precursor genes may have arisen earlier during evolution, with the divergence of the TRAF6 precursor occurring earliest of all. Except TRAF1, this PIRSF has a general domain architecture containing one N-terminal RING finger, a variable number of middle region of TRAF-type zinc finger and C2H2 type of zinc finger, and one C-terminal MATH domain. TRAF1 is unique in the family in that it lacks the N-terminal RING and zinc-finger domains []. This has rendered TRAF1 unable to promote TNF receptor signalling and act as a "dominant negative"TRAF []. Also TRAF1 is a substrate for caspases activated by TNF family death receptors []. The larger C-terminal cleaved fragment can bind to and sequester TRAF2 from TNFR1 complex, therefore modulating TNF induced NFkB activation []. A wide range of biological functions, such as adaptive and innate immunity, embryonic development, stress response and bone metabolism, are mediated by TRAFs through the induction of cell survival, proliferation, differentiation and death. TRAFs are functionally divergent from a perspective of both upstream and downstream TRAF signal transduction pathways and of signalling-dependent regulation of TRAF trafficking. Each TRAF protein interacts with and mediates the signal transduction of multiple receptors, and in turn each receptor utilises multiple TRAFs for specific functions []. About 40 interaction partners of TRAF have been described thus far, including receptors, kinases, regulators and adaptor proteins.TRAF proteins can be recruited to and activated by ligand-engaged receptors in least three distinct ways []. 1) Members of the TNFR superfamily that do not contain intracellular death domains, such as TNFR2 and CD40, recruit TRAFs directly via short sequences in their intracellular tails []. 2) Those that contain an intracellular death domain, such as TNFR1, first recruit an adapter protein, TRADD, via a death-domain-death-domain interaction, which then serves as a central platform of the TNFR1 signalling complex, which assembles TRAF2 and RIP for survival signalling, and FADD and caspase-8 for the induction of apoptosis. 3) Members of the IL-1R/TLR superfamily contain a protein interaction module known as the TIR domain, which recruits, sequentially, MyD88, a TIR domain and death domain containing protein, and IRAKs, adapter Ser/Thr kinases with death domains. IRAKs in turn associate with TRAF6 to elicit signalling by IL-1 and pathogenic components such as LPS. A common mechanism for the membrane-proximal event in TRAF signalling has been revealed by the conserved trimeric association in the crystal structure of the TRAF domain of TRAF2 [].This entry represents the TNF receptor associated factors found in metazoa.
The tumour necrosis factor (TNF) receptor associated factors (TRAFs) are major signal transducers for the TNF receptor (TNFR) superfamily and the interleukin-1 receptor/Toll-like receptor superfamily in mammals []. TRAFs constitute a family of genetically conserved adapter proteinsfound in mammals (TRAF1-6) as well as in other multicellular organisms such as Drosophila [], Caenorhabditis elegans []. TRAF2 is the prototypical member of the family. Mammalian TRAF1 and TRAF2 were the first members initially identified by their association with TNFR2. The TRAF1/TRAF2 and TRAF3/TRAF5 gene pairs may have arisen from recent independent gene duplications and to share a common ancestral gene. TRAF4 and TRAF6 precursor genes may have arisen earlier during evolution, with the divergence of the TRAF6 precursor occurring earliest of all. Except TRAF1, this PIRSF has a general domain architecture containing one N-terminal RING finger, a variable number of middle region of TRAF-type zinc finger and C2H2 type of zinc finger, and one C-terminal MATH domain. TRAF1 is unique in the family in that it lacks the N-terminal RING and zinc-finger domains []. This has rendered TRAF1 unable to promote TNF receptor signalling and act as a "dominant negative"TRAF []. Also TRAF1 is a substrate for caspases activated by TNF family death receptors []. The larger C-terminal cleaved fragment can bind to and sequester TRAF2 from TNFR1 complex, therefore modulating TNF induced NFkB activation []. A wide range of biological functions, such as adaptive and innate immunity, embryonic development, stress response and bone metabolism, are mediated by TRAFs through the induction of cell survival, proliferation, differentiation and death. TRAFs are functionally divergent from a perspective of both upstream and downstream TRAF signal transduction pathways and of signalling-dependent regulation of TRAF trafficking. Each TRAF protein interacts with and mediates the signal transduction of multiple receptors, and in turn each receptor utilises multiple TRAFs for specific functions []. About 40 interaction partners of TRAF have been described thus far, including receptors, kinases, regulators and adaptor proteins.TRAF proteins can be recruited to and activated by ligand-engaged receptors in least three distinct ways []. 1) Members of the TNFR superfamily that do not contain intracellular death domains, such as TNFR2 and CD40, recruit TRAFs directly via short sequences in their intracellular tails []. 2) Those that contain an intracellular death domain, such as TNFR1, first recruit an adapter protein, TRADD, via a death-domain-death-domain interaction, which then serves as a central platform of the TNFR1 signalling complex, which assembles TRAF2 and RIP for survival signalling, and FADD and caspase-8 for the induction of apoptosis. 3) Members of the IL-1R/TLR superfamily contain a protein interaction module known as the TIR domain, which recruits, sequentially, MyD88, a TIR domain and death domain containing protein, and IRAKs, adapter Ser/Thr kinases with death domains. IRAKs in turn associate with TRAF6 to elicit signalling by IL-1 and pathogenic components such as LPS. A common mechanism for the membrane-proximal event in TRAF signalling has been revealed by the conserved trimeric association in the crystal structure of the TRAF domain of TRAF2 [].
