TNF receptor-associated factor 2 (TRAF2) regulates activation of NF-kappa-B and JNK and plays a central role in the regulation of cell survival and apoptosis [, ].TRAF2 is heavily regulated by ubiquitin signals. It has E3 ubiquitin-protein ligase activity and promotes K63-linked ubiquitination of target proteins. Alternatively, TRAF2 may be required for recruiting other E3 ligases, such as A20, to help processing and turnover. Furthermore, TRAF2 is regulated by both K63 and K48 type ubiquitin chains. K63 linkage may be mediated by TRAF2 auto-ubiquitination, and is required for TRAF2 activation. c-IAPl and Siah2 are known E3 ligases that can K48-ubiquitinate TRAF2 to target it for proteasome-dependent degradation [, , ]. TRAF2 contains a RING finger domain, five zinc finger domains, and a TRAF domain.
TNF receptor-associated factor 2 (TRAF2) regulates activation of NF-kappa-B and JNK and plays a central role in the regulation of cell survival and apoptosis [, ].TRAF2 is heavily regulated by ubiquitin signals. It has E3 ubiquitin-protein ligase activity and promotes K63-linked ubiquitination of target proteins. Alternatively, TRAF2 may be required for recruiting other E3 ligases, such as A20, to help processing and turnover. Furthermore, TRAF2 is regulated by both K63 and K48 type ubiquitin chains. K63 linkage may be mediated by TRAF2 auto-ubiquitination, and is required for TRAF2 activation. c-IAPl and Siah2 are known E3 ligases that can K48-ubiquitinate TRAF2 to target it for proteasome-dependent degradation [, , ]. TRAF2 contains a RING finger domain, five zinc finger domains, and a TRAF domain. The TRAF domain can be divided into a more divergent N-terminal alpha helical region (TRAF-N), and a highly conserved C-terminal MATH subdomain (TRAF-C) with an eight-stranded β-sandwich structure. TRAF-N mediates trimerization while TRAF-C interacts with receptors [, ].
TAB2 (TGF-beta-activated kinase 1 and MAP3K7-binding protein 2) is an adaptor protein that regulates activation of TAK1, a MAP kinase kinase kinase (MAPKKK), through linking TAK1 to TRAF6 in the Interleukin-1 (IL-1) induced NF-kappaB activation pathway []. TAB3 is a TAB2-like TAK1-binding protein that activates NF-kappaB similar to TAB2 []. It activates TAK1 and regulates its association with TRAF2 and TRAF6. Moreover, TAB3 interacts with TRAF6 and TRAF2 in an IL-1- and a TNF-dependent manner, respectively. TAB2 and TAB3 function redundantly as mediators of TAK1 activation in IL-1 and TNF signal transduction []. Both of them contain an N-terminal CUE domain, a coiled-coil (CC) region, a TAK1-binding domain and a C-terminal Npl4 zinc finger (NZF) ubiquitin-binding domain (UBD) [].This entry represents the CUE domain found in TAB2 and TAB3.
PP4R1 is a regulatory subunit of serine/threonine-protein phosphatase 4 (PP4). The catalytic subunit of PP4 (PP4C) belongs to the PP2A family of serine/threonine protein phosphatases []. Although 65% identical to the catalytic subunit of PP2AC, PP4C does not associate with the regulatory subunit of PP2A. Instead, PP4C associates with two other regulatory subunits, PP4R1 []and PP4R2 [].PP4R1 targets TRAF2 and TRAF6 to mediate inhibition of NF-kB activation [].
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.
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 [].
TNF receptor-associated factor 1 (TRAF1) plays a role in the regulation of cell survival and apoptosis []. TRAF1 is unique among TRAF proteins in that it lacks a RING domain found in the N-terminal regions of other TRAFs []. The heterotrimer formed by TRAF1 and TRAF2 is part of a E3 ubiquitin-protein ligase complex that promotes ubiquitination of target proteins, such as MAP3K14 [, ].TRAF1 is unique among the TRAFs in that it lacks a RING domain, which is critical for the activation of nuclear factor-kappaB and Jun NH2-terminal kinase. Studies on TRAF1-deficient mice suggest that TRAF1 has a negative regulatory role in TNFR-mediated signaling events []. TRAF1 contains one zinc finger and one TRAF domain.The TRAF domain can be divided into a more divergent N-terminal alpha helical region (TRAF-N), and a highly conserved C-terminal MATH subdomain (TRAF-C) with an eight-stranded β-sandwich structure. TRAF-N mediates trimerization while TRAF-C interacts with receptors [, ].
TNF receptor-associated factor 1 (TRAF1) plays a role in the regulation of cell survival and apoptosis []. TRAF1 is unique among TRAF proteins in that it lacks a RING domain found in the N-terminal regions of other TRAFs []. The heterotrimer formed by TRAF1 and TRAF2 is part of a E3 ubiquitin-protein ligase complex that promotes ubiquitination of target proteins, such as MAP3K14 [, ].TRAF1 is unique among the TRAFs in that it lacks a RING domain, which is critical for the activation of nuclear factor-kappaB and Jun NH2-terminal kinase. Studies on TRAF1-deficient mice suggest that TRAF1 has a negative regulatory role in TNFR-mediated signaling events []. TRAF1 contains one zinc finger and one TRAF domain.
This subfamily of tumor necrosis factor receptor 1A (TNFRSF1, also known as type I TNFR, TNFR1, DR1, TNFRSF1A, CD120a, p55) is found in teleosts. It binds TNF-alpha, through the death domain (DD), and activates NF-kappaB, mediates apoptosis and activates signaling pathways controlling inflammatory, immune, and stress responses. It mediates signal transduction by interacting with antiapoptotic protein BCL2-associated athanogene 4 (BAG4/SODD) and adaptor proteins TRAF2 and TRADD that play regulatory roles [, ].Knockout studies in zebrafish embryos have shown that a signaling balance between TNFRSF1A and TNFRSF1B is required for endothelial cell integrity. TNFRSF1A signals apoptosis through caspase-8, whereas TNFRSF1B signals survival via NF-kappaB in endothelial cells. Thus, this apoptotic pathway seems to be evolutionarily conserved, as TNFalpha promotes apoptosis of human endothelial cells and triggers caspase-2 and P53 activation in these cells via TNFRSF1A [].This entry represents the N-terminal domain of TNFR1A 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 [, , ].
Tumor necrosis factor receptor superfamily member 1A (TNFRSF1A, also known as type I TNFR, TNFR1, DR1, CD120a, p55) binds TNF-alpha, through the death domain (DD), and activates NF-kappaB, mediates apoptosis and activates signaling pathways controlling inflammatory, immune, and stress responses. It mediates signal transduction by interacting with antiapoptotic protein BCL2-associated athanogene 4 (BAG4/SODD) and adaptor proteins TRAF2 and TRADD that play regulatory roles [, ].The human genetic disorder called tumor necrosis factor associated periodic syndrome (TRAPS), or periodic fever syndrome, is associated with germline mutations of the extracellular domains of this receptor, possibly due to impaired receptor clearance []. TNFRSF1A polymorphisms rs1800693 and rs4149584 are associated with elevated risk of multiple sclerosis []. Serum levels of TNFRSF1A are elevated in schizophrenia and bipolar disorder, and high levels are also associated with cognitive impairment and dementia [, ]. Patients with idiopathic recurrent acute pericarditis (IRAP), presumed to be an autoimmune process, have also been shown to carry rare mutations (R104Q and D12E) in the TNFRSF1A gene [].This entry represents the N-terminal domain of TNFR1A. 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 [, , ].
Tumor necrosis factor receptor superfamily member 7 (TNFRSF7), also known as CD27, T14, S152, Tp55, S152 or LPFS2, has a key role in the generation of immunological memory via effects on T-cell expansion and survival, and B cell development [, ]. It binds to ligand CD70, and plays a key role in regulating B-cell activation and immunoglobulin synthesis. CD27 transduces signals that lead to the activation of NF-kappaB and MAPK8/JNK, and mediates the signaling process through adaptor proteins TRAF2 and TRAF5. CD27-binding protein (SIVA), a pro-apoptotic protein, can bind to CD27 and may play an important role in the apoptosis induced by this receptor []. The potential role of the CD27/CD70 pathway in the course of inflammatory diseases, such as arthritis, and inflammatory bowel disease, suggests that CD70 may be a target for immune intervention. The expression of CD27 and CD44 molecules correlates with the differentiation stage of B cell precursors and has been shown to have a biological significance in acute lymphoblastic leukemia [].This entry represents the N-terminal domain of TNFRSF7. 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 [, , ].
Tumor necrosis factor receptor superfamily member 8 (TNFRSF8), also known as CD30, Ki-1 or D1S166E, is expressed by activated T and B cells. It transduces signals that lead to the activation of NF-kappaB, mediated by the adaptor proteins TRAF2 and TRAF5 []. This receptor has been shown to limit the proliferative potential of auto-reactive CD8 effector T cells and protect the body against autoimmunity []. Two alternatively spliced transcript variants of this gene encoding distinct isoforms have been reported. CD30 is expressed in malignant Hodgkin and Reed-Sternberg cells on the surface of extracellular vesicles, facilitating CD30-CD30L interaction between cell types []. This receptor is also associated with anaplastic large cell lymphoma. It is expressed in embryonal carcinoma, but not in seminoma, making it a useful marker in distinguishing between these germ cell tumors [, ]. Since CD30 has restricted expression in normal tissues, it is an optimal target for selectively eliminating CD30-expressing neoplastic cells by specific toxin-conjugated monoclonal antibodies (mAbs) [, ].This entry represents the N-terminal domain of TNFRSF8. 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 [, , ].
Tumor necrosis factor receptor superfamily member 4 (TNFRSF4), also known as OX40, ACT35, CD134, IMD16 or TXGP1L, activates NF-kappaB through its interaction with adaptor proteins TRAF2 and TRAF5 []. It also promotes the expression of apoptosis inhibitors BCL2 and BCL2lL1/BCL2-XL, and thus suppresses apoptosis []. It is primarily expressed on activated CD4+ and CD8+ T cells, where it is transiently expressed and upregulated on the most recently antigen-activated T cells within inflammatory lesions. This makes it an attractive target to modulate immune responses, i.e. TNFRSF4 (OX40) blocking agents to inhibit adverse inflammation or agonists to enhance immune responses [, ]. An artificially created biologic fusion protein, OX40-immunoglobulin (OX40-Ig), prevents OX40 from reaching the T-cell receptors, thus reducing the T-cell response. Some single nucleotide polymorphisms (SNPs) of its natural ligand OX40 ligand (OX40L, CD252), which is also found on activated T cells, have been associated with systemic lupus erythematosus [].This entry represents the N-terminal domain of TNFRSF4. 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 of modules can vary in number and type in different members of the family [, , ].
Although apparently functionally unrelated, intracellular TRAFs and extracellular meprins share a conserved region of about 180 residues, the meprin and TRAF homology (MATH) domain []. Meprins are mammalian tissue-specific metalloendopeptidases of the astacin family implicated in developmental, normal and pathological processes by hydrolysing a variety of proteins. Various growth factors, cytokines, and extracellular matrix proteins are substrates for meprins. They are composed of five structural domains: an N-terminal endopeptidase domain, a MAM domain (see ), a MATH domain, an EGF-like domain (see ) and a C-terminal transmembrane region. Meprin A and B form membrane bound homotetramer whereas homooligomers of meprin A are secreted. A proteolitic site adjacent to the MATH domain, only present in meprin A, allows the release of the protein from the membrane [].TRAF proteins were first isolated by their ability to interact with TNF receptors []. They promote cell survival by the activation of downstream protein kinases and, finally, transcription factors of the NF-kB and AP-1 family. The TRAF proteins are composed of 3 structural domains: a RING finger (see ) in the N-terminal part of the protein, one to seven TRAF zinc fingers (see ) in the middle and the MATH domain in the C-terminal part []. The MATH domain is necessary and sufficient for self-association and receptor interaction. From the structural analysis two consensus sequence recognised by the TRAF domain have been defined: a major one, [PSAT]x[QE]E and a minor one, PxQxxD [].The structure of the TRAF2 protein reveals a trimeric self-association of the MATH domain []. The domain forms a new, light-stranded antiparallel β-sandwich structure. A coiled-coil region adjacent to the MATH domain is also important for the trimerisation. The oligomerisation is essential for establishing appropriate connections to form signalling complexes with TNF receptor-1. The ligand binding surface of TRAF proteins is located in β-strands 6 and 7 [].
