Presence of a new conserved domain in CART1, a novel member of the tumor necrosis factor receptor-associated protein family, which is expressed in breast carcinoma

p53-inducible human homologue of Drosophila seven in absentia (Siah) inhibits cell growth: suppression by BAG-1

The Drosophila seven in absentia (sina) gene is required for R7 photoreceptor cell formation during Drosophila eye development, where it functions within the Ras/Raf pathway and targets other proteins for degradation via associations with a ubiquitin-conjugating enzyme. Recently, a mammalian sina homologue was reported to be a p53-inducible gene in a myeloid leukemia cell line. To explore the function of human SINA-homologous (Siah) proteins, expression plasmids encoding Siah-1A were transiently transfected into 293 epithelial cells and GM701 fibroblast cells, resulting in growth arrest without induction of apoptosis. We discovered that BAG-1, a ubiquitin-like Hsp70/Hsc70-regulating protein, is a negative regulator of Siah-1A. Siah-1A was identified as a BAG-1-binding protein via yeast two-hybrid methods. Specific interaction of BAG-1 with Siah-1A was also demonstrated by in vitro binding experiments using glutathione S-transferase fusion proteins and co-immunoprecipitation studies. Siah-1A-induced growth arrest in 293 and GM701 cells was abolished by co-transfection of wild-type BAG-1 with Siah-1A but not by a C-terminal deletion mutant of BAG-1 that fails to bind Siah-1A. Over-expression of BAG-1 significantly inhibited p53-induced growth arrest in 293 cells without preventing p53 transactivation of reporter gene plasmids. BAG-1 also prevented growth arrest following UV-irradiation-induced genotoxic injury without interfering with accumulation of p53 protein or p21(waf-1) expression. BAG-1 functions downstream of p53-induced gene expression to inhibit p53-mediated suppression of cell growth, presumably by suppressing the actions of Siah-1A. We suggest that Siah-1A may be an important mediator of p53-dependent cell-cycle arrest and demonstrate that Siah-1A is directly inhibited by BAG-1.

The mammalian numb phosphotyrosine-binding domain. Characterization of binding specificity and identification of a novel PDZ domain-containing numb binding protein, LNX

Numb is a phosphotyrosine-binding (PTB) domain-containing protein implicated in the control of cell fate decisions during development. A modified two-hybrid screen in yeast was used to identify Numb PTB domain-interacting proteins important for Numb function. Here we report the identification of a novel protein, LNX, which interacts specifically with the Numb PTB domain. Two differentially expressed LNX messages encode overlapping proteins with predicted molecular masses of 80 kDa (LNX) and 70 kDa (LNX-b). LNX and LNX-b contain unique amino-terminal sequences and share four PDZ domains. The unique amino-terminal region of LNX includes a RING finger domain. The Numb PTB domain binding region of LNX was mapped to the sequence motif LDNPAY, found in both protein isoforms. Mutational analysis of LNX and peptide competition experiments showed that phosphorylation of the tyrosine residue within this motif was not required for binding to the Numb PTB domain. Finally, we also provide evidence that tyrosine phosphorylation of the LDNPAY sequence motif in LNX could generate a binding site for the phosphorylation-dependent binding of other PTB domain-containing proteins such as SHC. We speculate that LNX may be important for clustering PTB-containing proteins with functionally related transmembrane proteins in specific membrane compartments.

Differential regulation of IkappaB kinase alpha and beta by two upstream kinases, NF-kappaB-inducing kinase and mitogen-activated protein kinase/ERK kinase kinase-1

NF-kappaB is activated by various stimuli including inflammatory cytokines and stresses. A key step in the activation of NF-kappaB is the phosphorylation of its inhibitors, IkappaBs, by an IkappaB kinase (IKK) complex. Recently, two closely related kinases, designated IKKalpha and IKKbeta, have been identified to be the components of the IKK complex that phosphorylate critical serine residues of IkappaBs for degradation. A previously identified NF-kappaB-inducing kinase (NIK), which mediates NF-kappaB activation by TNFalpha and IL-1, has been demonstrated to activate IKKalpha. Previous studies showed that mitogen-activated protein kinase/ERK kinase kinase-1 (MEKK1), which constitutes the c-Jun N-terminal kinase/stress-activated protein kinase pathway, also activates NF-kappaB by an undefined mechanism. Here, we show that overexpression of MEKK1 preferentially stimulates the kinase activity of IKKbeta, which resulted in phosphorylation of IkappaBs. Moreover, a catalytically inactive mutant of IKKbeta blocked the MEKK1-induced NF-kappaB activation. By contrast, overexpression of NIK stimulates kinase activities of both IKKalpha and IKKbeta comparably, suggesting a qualitative difference between NIK- and MEKK1-mediated NF-kappaB activation pathways. Collectively, these results indicate that NIK and MEKK1 independently activate the IKK complex and that the kinase activities of IKKalpha and IKKbeta are differentially regulated by two upstream kinases, NIK and MEKK1, which are responsive to distinct stimuli.

Molecular cloning and characterization of a novel protein kinase C-interacting protein with structural motifs related to RBCC family proteins

A novel protein kinase C (PKC)-interacting protein was identified by the yeast two-hybrid screening using the regulatory domain of PKC beta I as a bait. The protein contained several structural motifs such as two putative coiled-coil regions, a RING-finger, a B-box, and a B-box-like motif in the order from NH2- to COOH-terminals. The molecular organization of the protein resembles the structure of the RBCC protein family proteins which usually have a RING-finger, a B-box, and a coiled-coil region. Therefore, the protein identified was designated as RBCK1 (RBCC protein interacting with PKC 1). Northern blot analysis showed that RBCK1 gene is expressed ubiquitously among rat tissues. RBCK1 protein associated with PKC beta I and PKC zeta when coexpressed in cultured mammalian cells. By the polymerase chain reaction-assisted DNA-binding site selection and the electrophoretic mobility shift assay, RBCK1 protein was shown to bind to several DNA fragments containing TGG-rich sequences. When the yeast GAL4 DNA-binding domain fused RBCK1 protein was expressed in COS-7 cells harboring the luciferase gene placed under a synthetic promoter containing GAL4-binding sites, the fusion protein showed enhanced transcriptional activity comparing with the GAL4 DNA-binding domain. These results suggest that RBCK1 protein might be a transcription factor that has a role in the signaling pathway through PKC.

Activation of OX40 signal transduction pathways leads to tumor necrosis factor receptor-associated factor (TRAF) 2- and TRAF5-mediated NF-kappaB activation

We investigated the intracellular signaling of OX40, a member of the tumor necrosis factor receptor family. Activation of NF-kappaB in OX40-transfected HSB-2 cells was detected by electrophoretic mobility shift assay within 30 min after the binding of the ligand gp34. In vitro binding experiments showed that tumor necrosis factor receptor-associated factor (TRAF) 1, TRAF2, TRAF3, and TRAF5 but not TRAF4 associated with glutathione S-transferase-OX40 fusion protein. The cotransfection experiments using human embryo kidney cell derived HEK 293T cells showed that TRAF2, TRAF3, and TRAF5 associated with OX40 in vivo. Studies with OX40 deletion mutants demonstrated that the cytoplasmic portion consisting of amino acid sequence 256-263 (GGSFRTPI) was required for the association with TRAFs and NF-kappaB activation. The introduction of the dominant negative mutants of TRAF2 and TRAF5 into HSB-2-OX40 cells suppressed NF-kappaB activation in a dose-dependent manner. In addition, the introduction of TRAF3 together with the dominant negative mutants of TRAF2 or TRAF5 further reduced NF-kappaB activation. These results indicate that the NF-kappaB activation resulting from OX40 stimulation is mediated by both TRAF2 and TRAF5, and is likely to be negatively modulated by TRAF3.

Peg3/Pw1 is an imprinted gene involved in the TNF-NFkappaB signal transduction pathway

Tumor necrosis factor (TNF) mediates a variety of biological activities including cell proliferation, differentiation and programmed cell death. The specific response to TNF depends upon cell type and reflects the presence of specific regulatory proteins that participate in the TNF response pathway. TNF signal transduction is mediated by TRAF2 which binds the TNF Receptor2 (TNFR2) and activates NFkappaB. We previously identified a gene Pw1, which encodes a large zinc-finger containing protein. We have determined that Pw1 is identical to Peg3, a paternally expressed gene of unknown function (and will therefore be referred to as Peg3 throughout this text). We report here that Peg3 associates specifically with TRAF2 but not with other TRAF family members. Peg3 expression activates NFkappaB via IkappaB-NFkappaB dissociation and acts synergistically with TRAF2. Transfection of a truncated Peg3 containing the TRAF2 interaction site, abolishes NFkappaB activation by TRAF2 and/or TNF. We conclude that Peg3 is a regulator of the TNF response. These data reveal the involvement of an imprinted gene in this pathway.

Tumor necrosis factor receptor (TNFR)-associated factor 2A (TRAF2A), a TRAF2 splice variant with an extended RING finger domain that inhibits TNFR2-mediated NF-kappaB activation

We describe here the identification and characterization of tumor necrosis factor receptor (TNFR)-associated factor 2A (TRAF2A), a splice variant of the TRAF2 molecule utilized for signal transduction by members of the TNFR family. TRAF2A and TRAF2 cDNAs are identical in sequence with the exception of an extra 21 base pairs of sequence encoding a 7-amino acid insert within the TRAF2A RING finger domain. TRAF2A mRNA expression is regulated in a tissue-specific manner, with relative TRAF2A mRNA levels being highest in spleen and lowest in brain. TRAF2A protein is capable of binding to the cytoplasmic domain of TNFR2 (p75) and is detectable in T-lymphoma cells stably transfected with the TRAF2A cDNA. Unlike TRAF2, TRAF2A has a short half-life ( approximately 100 min) in these cells and is expressed at only low levels in transiently transfected COS-7 cells. However, TRAF2A levels in transiently transfected COS-7 cells approach those of TRAF2 upon coexpression with TRAF1 and/or TRAF2, indicating that TRAF2A stability is regulated by the binding of other TRAF family proteins. Also in contrast to TRAF2, TRAF2A is unable to stimulate NF-kappaB activity when overexpressed in 293 cells and acts as a dominant inhibitor of TNFR2-dependent NF-kappaB activation. TRAF2A thus represents a novel signal transduction protein, the expression of which can act to inhibit TRAF2-dependent NF-kappaB activation.

Tumor necrosis factor receptor associated factor 4 (TRAF4) expression pattern during mouse development

This is the first in situ hybridization analysis of expression of a tumor necrosis factor (TNF) receptor associated factor (TRAF) during development. TRAF4 is observed throughout mouse embryogenesis, most notably during ontogenesis of the central (CNS) and peripheral (PNS) nervous system, and of nervous tissues of sensory organs. TRAF4 is preferentially expressed by post-mitotic undifferentiated neurons. Interestingly, TRAF4 remains expressed in the adult hippocampus and olfactory bulb, known to contain multipotential cells responsible for neoneurogenesis.

Human 4-1BB (CD137) signals are mediated by TRAF2 and activate nuclear factor-kappa B

Human 4-1BB (CD137), a member of the tumor necrosis factor receptor (TNFR) superfamily, costimulates T cell activation. No apparent intrinsic kinase activity is seen with 4-1BB, which suggests that 4-1BB-associated molecules may be involved in 4-1BB-mediated signal transduction. We found that tumor necrosis factor receptor-associated factor (TRAF) 1, TRAF2, and TRAF3, all interacted with the cytoplasmic domain of 4-1BB. Mutation analysis showed that TRAF1, TRAF2, and TRAF3 were associated with one of two runs of acidic residues found in the cytoplasmic domain of 4-1BB. In addition, 4-1BB cross-linking with TCR signal in Jurkat cells and overexpression of 4-1BB in 293 cells were able to induce activation of the nuclear factor-kappa B (NF-kappa B). 4-1BB-mediated NF-kappa B activation was inhibited by a dominant negative-TRAF2 or -NF-kappa B-inducing kinase (NIK). These data suggest that 4-1BB functions may be mediated by NF-kappa B activation, which requires a TRAF2/NIK pathway.

Human IAP-like protein regulates programmed cell death downstream of Bcl-xL and cytochrome c

The gene encoding human IAP-like protein (hILP) is one of several mammalian genes with sequence homology to the baculovirus inhibitor-of-apoptosis protein (iap) genes. Here we show that hILP can block apoptosis induced by a variety of extracellular stimuli, including UV light, chemotoxic drugs, and activation of the tumor necrosis factor and Fas receptors. hILP also protected against cell death induced by members of the caspase family, cysteine proteases which are thought to be the principal effectors of apoptosis. hILP and Bcl-xL were compared for their ability to affect several steps in the apoptotic pathway. Redistribution of cytochrome c from mitochondria, an early event in apoptosis, was not blocked by overexpression of hILP but was inhibited by Bcl-xL. In contrast, hILP, but not Bcl-xL, inhibited apoptosis induced by microinjection of cytochrome c. These data suggest that while Bcl-xL may control mitochondrial integrity, hILP can function downstream of mitochondrial events to inhibit apoptosis.

4-1BB and Ox40 are members of a tumor necrosis factor (TNF)-nerve growth factor receptor subfamily that bind TNF receptor-associated factors and activate nuclear factor kappaB

Members of the tumor necrosis factor (TNF)-nerve growth factor (NGF) receptor family have been shown to be important costimulatory molecules for cellular activation. 4-1BB and Ox40 are two recently described members of this protein family which are expressed primarily on activated T cells. To gain insight into the signaling pathways employed by these factors, yeast two-hybrid library screens were performed with the cytoplasmic domains of 4-1BB and Ox40 as baits. TNF receptor-associated factor 2 (TRAF2) was identified as an interacting protein in both screens. The ability of both 4-1BB and Ox40 to interact with TRAF2 was confirmed in mammalian cells by coimmunoprecipitation studies. When the binding of the receptors to other TRAF proteins was investigated, 4-1BB and Ox40 displayed distinct binding patterns. While 4-1BB bound TRAF2 and TRAF1, Ox40 interacted with TRAF3 and TRAF2. Using deletion and alanine scanning analysis, we defined the elements in the cytoplasmic domains of both receptors that mediate these interactions. The 4-1BB receptor was found to have two independent stretches of acidic residues that can mediate association of the TRAF molecules. In contrast, a single TRAF binding domain was identified in the cytoplasmic tail of Ox40. The cytoplasmic domains of both receptors were shown to activate nuclear factor kappaB in a TRAF-dependent manner. Taken together, our results indicate that 4-1BB and Ox40 bind TRAF proteins to initiate a signaling cascade leading to activation of nuclear factor kappaB.

Dominant negative mutants of TRAF3 reveal an important role for the coiled coil domains in cell death signaling by the lymphotoxin-beta receptor

Ligation of the lymphotoxin-beta receptor (LTbetaR) recruits tumor necrosis factor receptor-associated factor-3 (TRAF3) and initiates cell death in HT29 adenocarcinoma cells. The minimal receptor binding domain (TRAF-C) defined by two hybrid analyses is not sufficient for direct recruitment to the ligated receptor. A series of TRAF3 deletion mutants reveal that a subregion of the coiled coil motif is required for efficient recruitment to the LTbetaR. Furthermore, the ability of TRAF3 to self-associate maps to an adjacent subregion. A TRAF3 deletion mutant that lacks the N-terminal zinc RING and zinc finger motifs, but retains the coiled coil and TRAF-C motifs, competitively displaces endogenous TRAF3 from the LTbetaR. A second TRAF3 mutant that lacks the receptor binding domain, yet contains the TRAF3 self-association domain, prevents TRAF3 homodimers from being recruited to the LTbetaR. Both of these mutants have a dominant negative effect on cell death and demonstrate that the recruitment of TRAF3 oligomers is necessary to initiate signal transduction that activates the cell death pathway.

TRAF2 is essential for JNK but not NF-kappaB activation and regulates lymphocyte proliferation and survival

TRAF2 is believed to mediate the activation of NF-kappaB and JNK induced by the tumor necrosis factor receptor (TNFR) superfamily, which elicits pleiotropic responses in lymphocytes. We have investigated the physiological roles of TRAF2 in these processes by expressing a lymphocyte-specific dominant negative form of TRAF2, thereby blocking this protein's effector function. We find that the TNFR superfamily signals require TRAF2 for activation of JNK but not NF-kappaB. In addition, we show that TRAF2 induces NF-kappaB-independent antiapoptotic pathways during TNF-induced apoptosis. Inhibition of TRAF2 leads to splenomegaly, lymphadenopathy, and an increased number of B cells. These findings indicate that TRAF2 is involved in the regulation of lymphocyte function and growth in vivo.

Early lethality, functional NF-kappaB activation, and increased sensitivity to TNF-induced cell death in TRAF2-deficient mice

TRAF2 is an intracellular signal-transducing protein recruited to the TNFR1 and TNFR2 receptors following TNF stimulation. To investigate the physiological role of TRAF2, we generated TRAF2-deficient mice. traf2-/- mice appeared normal at birth but became progressively runted and died prematurely. Atrophy of the thymus and spleen and depletion of B cell precursors also were observed. Thymocytes and other hematopoietic progenitors were highly sensitive to TNF-induced cell death and serum TNF levels were elevated in these TRAF2-deficient animals. Examination of traf2-/- cells revealed a severe reduction in TNF-mediated JNK/SAPK activation but a mild effect on NF-kappaB activation. These results suggest that TRAF2-independent pathways of NF-kappaB activation exist and that TRAF2 is required for an NF-kappaB-independent signal that protects against TNF-induced apoptosis.

Organization of the human tumour necrosis factor receptor-associated factor 1 (TRAF1) gene and mapping to chromosome 9q33-34

A new family of signal transducing proteins, associated with members of the tumour necrosis factor receptor (TNFR) superfamily, has recently been identified. The structural hallmark of these molecules is a novel C-terminal homology region of 230 bp designated as TRAF (TNF receptor-associated factor) domain, which is involved in a variety of specific protein-protein interactions. To elucidate the human TRAF1 gene structure for identification of potential regulatory elements, a set of genomic polymerase chain reaction (PCR) fragments was generated, which comprised the whole coding region of TRAF1. These fragments were cloned and partially sequenced to map splicing sites. The human TRAF1 gene was found to have a total length of approx. 12 kb. It is split into six exons, four of which encode for parts of the TRAF domain. Analysis of the genomic structure of the TRAF domains of human TRAF2 and 3 suggests that these domains are also encoded by several exons. The putative promotor region of the TRAFI gene was isolated by use of a PCR-based genomic walking approach. Fluorescence in situ hybridization was used to map this gene to chromosome 9q33-34.

Serine/threonine kinase activity associated with the cytoplasmic domain of the lymphotoxin-beta receptor in HepG2 cells

The lymphotoxin-beta receptor (LT-betaR) has been shown to be the receptor for the membrane-bound lymphotoxin heterotrimers LTalpha1/beta2 and LTalpha2/beta1. The extracellular domain of LT-betaR shows extensive similarity with members of the tumor necrosis factor receptor family, while its cytoplasmic domain is distinct and lacks any inherent enzymatic activity. This suggests that the interaction of LT-betaR with other molecules might be important for signal transduction. Here we demonstrate the association of a fusion protein, comprising glutathione S-transferase and the cytoplasmic domain of LT-betaR (GST-LT-betaR(CD)), with several proteins in the size range 29-80 kDa from HepG2 cell lysates. We present evidence that two of these proteins are serine/threonine kinases, which associate with amino acids 324-377 of the cytoplasmic domain of LT-betaR and phosphorylate this receptor. The characteristics of these novel kinases indicate that they are distinct from the previously described tumor necrosis factor receptor-associated kinases. This suggests the presence of novel signal transduction pathway(s) for LT-betaR.

Characterization of LMP-1's association with TRAF1, TRAF2, and TRAF3

The latent membrane protein 1 (LMP-1) of Epstein-Barr virus (EBV) contributes to the immortalizing activity of EBV in primary, human B lymphocytes. LMP-1 is targeted to the plasma membrane, where it influences signaling pathways of infected cells. LMP-1 has been found to associate with members of the tumor necrosis factor receptor-associated factor (TRAF) family of proteins. As with LMP-1, the TRAF molecules have been shown to participate in cell signaling pathways. We have characterized and mapped in detail a region of LMP-1 that associates with TRAF1, TRAF2, and TRAF3. TRAF3 alone associates with LMP-1 in a yeast two-hybrid assay, whereas all three TRAF molecules associate with LMP-1 under various conditions when they are assayed in extracts of human cells. TRAF1, TRAF2, and TRAF3 appear to associate independently with LMP-1 but bind an overlapping target site. TRAF3 associates with LMP-1 most avidly and can compete with TRAF1 and TRAF2 for binding to LMP-1. TRAF2 associates with truncated derivatives of the carboxy terminus of LMP-1 more efficiently than with the intact terminus, indicating that LMP-1's conformation may regulate its association with TRAF2. Finally, point mutations that decrease LMP-1's association with the three TRAF molecules to 3 to 20% of wild-type levels do not detectably affect otherwise intact LMP-1's induction of NF-kappaB activity. Therefore, these associations are not necessary for the majority of intact LMP-1's induction of this signaling pathway.

ATAR, a novel tumor necrosis factor receptor family member, signals through TRAF2 and TRAF5

Members of tumor necrosis factor receptor (TNFR) family signal largely through interactions with death domain proteins and TRAF proteins. Here we report the identification of a novel TNFR family member ATAR. Human and mouse ATAR contain 283 and 276 amino acids, respectively, making them the shortest known members of the TNFR superfamily. The receptor is expressed mainly in spleen, thymus, bone marrow, lung, and small intestine. The intracellular domains of human and mouse ATAR share only 25% identity, yet both interact with TRAF5 and TRAF2. This TRAF interaction domain resides at the C-terminal 20 amino acids. Like most other TRAF-interacting receptors, overexpression of ATAR activates the transcription factor NF-kappaB. Co-expression of ATAR with TRAF5, but not TRAF2, results in synergistic activation of NF-kappaB, suggesting potentially different roles for TRAF2 and TRAF5 in post-receptor signaling.

A regulatory role for TRAF1 in antigen-induced apoptosis of T cells

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) and TRAF1 were found as components of the TNFR2 signaling complex, which exerts multiple biological effects on cells such as cell proliferation, cytokine production, and cell death. In the TNFR2-mediated signaling pathways, TRAF2 works as a mediator for activation signals such as NF-kappaB, but the role of TRAF1 has not been previously determined. Here we show in transgenic mice that TRAF1 overexpression inhibits antigen-induced apoptosis of CD8(+) T lymphocytes. Our results demonstrate a biological role for TRAF1 as a regulator of apoptotic signals and also support the hypothesis that the combination of TRAF proteins in a given cell type determines distinct biological effects triggered by members of the TNF receptor superfamily.

Human TNF receptor-associated factor 5 (TRAF5): cDNA cloning, expression and assignment of the TRAF5 gene to chromosome 1q32

Tumor necrosis factor (TNF) receptor-associated factors (TRAFs) are signal transducers for members of the TNF receptor superfamily. We previously identified murine TRAF5 (mTRAF5) and showed that it specifically interacts with the lymphotoxin-beta receptor (LT-beta R) and activates the transcription factor NF-kappa B. Here we have cloned the human TRAF5 homologue (hTRAF5) by cross hybridization with mTRAF5 cDNA. hTRAF5 cDNA is composed of 2894 nucleotides with a 557-amino-acid open reading frame that exhibits 77.5 and 80% identity to mTRAF5 at the nucleotide and amino acid levels, respectively. Northern blot analysis revealed that hTRAF5 mRNA is expressed in all visceral organs. Western blotting revealed that hTRAF5 protein was abundantly expressed in the human follicular dentritic cell line, FDC-1, and to a much lesser degree in several tumor cell lines. Interspecific backcross mapping revealed that Traf5 is located in the distal region of mouse chromosome 1, which shares a region of homology with human chromosome 1q. Fluorescence in situ hybridization confirmed regional localization to human chromosome 1q32.

Binding sites of cytoplasmic effectors TRAF1, 2, and 3 on CD30 and other members of the TNF receptor superfamily

CD30 is present on the surfaces of malignant cells from patients with Hodgkin's lymphoma, anaplastic large cell lymphoma, and other lymphomas. The yeast two hybrid genetic screen method was used to identify molecular effectors which mediate CD30 signalling events. Clones corresponding to genes coding for TRAF1, TRAF2, and TRAF3 molecules, postulated to be involved in signalling via the TNF and CD40 receptors, were isolated. In this report, we show that the CD30 intracellular tail contains two motifs that bind TRAFs. The more amino terminal motif, 558PHYPEQET565, binds TRAF2 and 3, while the more carboxyl terminal motif, 576MLSVEEEG583, binds TRAF1 and 2. We show that these amino acid motifs are conserved in TNFRp75 and CD40 and that sequences in these receptors homologous to TRAF-binding sequences found in CD30 can selectively bind the TRAFs in a predictable manner.

MLN64 exhibits homology with the steroidogenic acute regulatory protein (STAR) and is over-expressed in human breast carcinomas

The MLN64 gene, which is localized in q12-q21 of the human chromosome 17, encodes a novel protein containing 2 distinct domains. At the N-terminal, MLN64 exhibits a potential trans-membrane region, while at the C-terminal, it shares homology with the F26F4.4 protein of Coenorhabditis elegans and the steroidogenic acute regulatory (StAR) protein, a mitochondrial protein which is involved in steroid-hormone synthesis. By comparing the C-terminal part of these proteins, we defined a novel protein domain, which we termed SHD for "StAR Homology Domain". Of the 93 primary invasive breast carcinomas that were examined, 14 were found to over-express MLN64. These 14 tumors also expressed high c-erbB-2 transcript levels, which were not detected in the MLN64-negative tumors. MLN64 mRNA and protein were specifically detected in malignant cells of breast carcinomas. MLN64 protein was localized within bundle-like structures distributed throughout the cell cytoplasm and condensed in a perinuclear patch, suggesting an association with a specific cell compartment. When the N-terminal part of MLN64 was deleted, MLN64 was uniformly distributed in the cell cytoplasm, indicating that N-terminal part is involved in the specific cytoplasmic localization of MLN64. The homology between the C-terminal part of MLN64 and the functional StAR domain (SHD) suggests that MLN64 and StAR, although distributed in different cellular compartments, may both play a role in steroidogenesis. In this case, the high levels of MLN64 observed in some breast carcinomas could contribute to the progression of these tumors through increased intratumoral steroidogenesis.

TRAF-interacting protein (TRIP): a novel component of the tumor necrosis factor receptor (TNFR)- and CD30-TRAF signaling complexes that inhibits TRAF2-mediated NF-kappaB activation

Through their interaction with the TNF receptor-associated factor (TRAF) family, members of the tumor necrosis factor receptor (TNFR) superfamily elicit a wide range of biological effects including differentiation, proliferation, activation, or cell death. We have identified and characterized a novel component of the receptor-TRAF signaling complex, designated TRIP (TRAF-interacting protein), which contains a RING finger motif and an extended coiled-coil domain. TRIP associates with the TNFR2 or CD30 signaling complex through its interaction with TRAF proteins. When associated, TRIP inhibits the TRAF2-mediated NF-kappaB activation that is required for cell activation and also for protection against apoptosis. Thus, TRIP acts as a receptor-proximal regulator that may influence signals responsible for cell activation/proliferation and cell death induced by members of the TNFR superfamily.

Transducing signals of life and death

The molecules that form signaling complexes with the cytoplasmic domains of tumor necrosis factor (TNF) receptors (TNF-Rs) and CD95 have been identified recently. The death-signaling pathways induced by TNF-R1 and CD95 involve a group of death domain containing proteins, including caspase-8, a member of the interleukin-1beta-converting enzyme family. TNF-R1 and TNF-R2 also interact with the members of both the TNF-R associated factor family and the inhibitor of apoptosis protein family; these interactions lead to cell survival.

CD30 induction of human immunodeficiency virus gene transcription is mediated by TRAF2

CD30 is a member of the tumor necrosis factor receptor (TNFR) superfamily expressed on activated T and B lymphocytes and natural killer cells. Ligation of CD30 was previously shown to induce NF-kappaB activation and HIV expression in chronically infected T lymphocytes. In this study, we report that two members of the TNFR-associated factor (TRAF) family of proteins, TRAF1 and TRAF2, independently bind to the intracellular domain of CD30 (CD30IC). Transient overexpression of TRAF2, but not TRAF1, induced NF-kappaB activation and HIV-1-long terminal repeat-driven transcription in the T cell line, KT3. Moreover, dominant negative mutants consisting of the TRAF domain of TRAF1 and TRAF2 inhibited CD30 induction of NF-kappaB activation and HIV-1 transcription. These results suggest that CD30 ligation may enhance the expression of HIV via TRAF-2-mediated activation of NF-kappaB.

Activation of SAPK/JNK by TNF receptor 1 through a noncytotoxic TRAF2-dependent pathway

Interaction of the p55 tumor necrosis factor receptor 1 (TNF-R1)-associated signal transducer TRADD with FADD signals apoptosis, whereas the TNF receptor-associated factor 2 protein (TRAF2) is required for activation of the nuclear transcription factor nuclear factor kappa B. TNF-induced activation of the stress-activated protein kinase (SAPK) was shown to occur through a noncytotoxic TRAF2-dependent pathway. TRAF2 was both sufficient and necessary for activation of SAPK by TNF-R1; conversely, expression of a dominant-negative FADD mutant, which blocks apoptosis, did not interfere with SAPK activation. Therefore, SAPK activation occurs through a pathway that is not required for TNF-R1-induced apoptosis.

Apoptosis and disease: regulation and clinical relevance of programmed cell death

Regulation of the homeostatic balance between cell proliferation and cell death is essential for development and maintenance of multicellular organisms. Physiologic, or programmed, cell death is dependent on a genetically encoded and evolutionarily conserved pathway that induces a form of cellular suicide known as apoptosis. In the past decade, it has become clear that the regulatory mechanisms controlling programmed cell death are as fundamental, and as complex, as those regulating cell proliferation. Perturbation of the signaling cascades regulating apoptosis, whether by extracellular triggers, acquired or germline genetic mutations, or viral mimicry of signaling molecules, can result in a wide variety of human diseases. Analysis of these regulatory pathways has led to a better understanding of the etiology and pathogenesis of many human diseases, notably cancers, infectious diseases including AIDS, autoimmune diseases, and neurodegenerative/neurodevelopmental diseases. Our understanding of the regulation of programmed cell death in health and disease is far from complete, and the challenge of converting that understanding into new therapeutic modalities has only begun to be approached.

Targeted disruption of TRAF3 leads to postnatal lethality and defective T-dependent immune responses

TRAF3 was found as a protein that binds to the cytoplasmic tail of CD40 but is part of a family of proteins with common structure and activity. To clarify the physiological roles of TRAF3, we introduced a TRAF3 null mutation in mice through homologous recombination. TRAF3-deficient mice appear normal at birth but become progressively runted, correlating with progressive hypoglycemia and depletion of peripheral white cells. The mutant mice die by 10 days of age. Fetal liver cells from TRAF3-deficient embryos can reconstitute all hematopoietic lineages in lethally irradiated mice. However, these reconstituted mice are impaired in their immune responses to T-dependent antigen, and their T cells are functionally defective. These findings indicate that TRAF3 is required for postnatal development and for a competent immune system.

TRAF6 is a signal transducer for interleukin-1

Many cytokines signal through different cell-surface receptors to activate the transcription factor NF-kappaB. Members of the TRAF protein family have been implicated in the activation of NF-kappaB by the tumour-necrosis factor (TNF)-receptor superfamily. Here we report the identification of a new TRAF family member, designated TRAF6. When overexpressed in human 293 cells, TRAF6 activates NF-kappaB. A dominant-negative mutant of TRAF6 inhibits NF-kappaB activation signalled by interleukin-1 (IL-1) but not by TNF. IL-1 treatment of 293 cells induces the association of TRAF6 with IRAK, a serine/threonine kinase that is rapidly recruited to the IL-1 receptor after IL-1 induction. These findings indicate that TRAF proteins may function as signal transducers for distinct receptor families and that TRAF6 participates in IL-1 signalling.

TRAF5, a novel tumor necrosis factor receptor-associated factor family protein, mediates CD40 signaling

Signals emanating from CD40 play crucial roles in B-cell function. To identify molecules that transduce CD40 signalings, we have used the yeast two-hybrid system to done cDNAs encoding proteins that bind the cytoplasmic tail of CD40. A cDNA encoding a putative signal transducer protein, designated TRAF5, has been molecularly cloned. TRAF5 has a tumor necrosis factor receptor-associated factor (TRAF) domain in its carboxyl terminus and is most homologous to TRAF3, also known as CRAF1, CD40bp, or LAP-1, a previously identified CD40-associated factor. The amino terminus has a RING finger domain, a cluster of zinc fingers and a coiled-coil domain, which are also present in other members of the TRAF family protein except for TRAF1. In vitro binding assays revealed that TRAF5 associates with the cytoplasmic tail of CD40, but not with the cytoplasmic tail of tumor receptor factor receptor type 2, which associates with TRAF2. Based on analysis of the association between TRAF5 and various CD40 mutants, residues 230-269 of CD40 are required for the association with TRAF5. In contrast to TRAF3, overexpression of TRAF5 activates transcription factor nuclear factor kappa B. Furthermore, amino-terminally truncated forms of TRAF5 suppress the CD40-mediated induction of CD23 expression, as is the case with TRAF3. These results suggest that TRAF5 and TRAF3 could be involved in both common and distinct signaling pathways emanating from CD40.

Molecular, structural, and biological characteristics of the tumor necrosis factor ligand superfamily

The tumor necrosis factor receptor superfamily at present consists of ten different transmembrane (type I) glycoproteins with characteristic limited sequence homology for the cysteine-rich repeats in the extracellular domain. In parallel the tumor necrosis factor ligand super-family has been recognized by discovery of ligands for all members of the receptor superfamily. These molecules are also transmembrane (type II) glycoproteins, with the exception of lymphotoxin-alpha which is the only entirely secreted protein of the tumor necrosis factor-like proteins. Several members of the ligand superfamily, including tumor necrosis factor and CD95L also exist in a biologically active soluble form. The tumor necrosis factor ligand superfamily contains at present ten different proteins. In addition, NGFR p75 binds to a second family of proteins (neurotrophins). These nerve growth factor-like dimeric soluble molecules are basic neurotrophic factors and the five members (NGF, BDNF, NT-3, NT-4, NT-5) are not related to the tumor necrosis factor superfamily ligands. The members of the tumor necrosis factor ligand superfamily (TNF, LT-alpha, LT-beta, CD27L, CD30L, CD40L, CD95L, 4-IBB, OX40L, TRAIL) share common biological activities, but some properties are shared by only some ligands, while others are unique. The diverse biological activities triggered through tumor necrosis factor receptors have been linked to the regulation of cellular activation, including immune responses and inflammatory reactions, but also with the pathology of a series of human diseases.

The tumor necrosis factor-inducible zinc finger protein A20 interacts with TRAF1/TRAF2 and inhibits NF-kappaB activation

TRAF1 and TRAF2 form an oligomeric complex that associates with the cytoplasmic domains of various members of the tumor necrosis factor (TNF) receptor superfamily. TRAF2 action is required for activation of the transcription factor NF-kappaB triggered by TNF and the CD40 ligand. Here we show that TRAF1 and TRAF2 interact with A20, a zinc finger protein, whose expression is induced by agents that activate NF-kappaB. Mutational analysis revealed that the N-terminal half of A20 interacts with the conserved C-terminal TRAF domain of TRAF1 and TRAF2. In cotransfection experiments, A20 blocked TRAF2-mediated NF-kappaB activation. A20 also inhibited TNF and IL-1-induced NF-kappaB activation, suggesting that it may inhibit NF-kappaB activation signaled by diverse stimuli. The ability of A20 to block NF-kappaB activation was mapped to its C-terminal zinc finger domain. Thus, A20 is composed of two functionally distinct domains, an N-terminal TRAF binding domain that recruits A20 to the TRAF2-TRAF1 complex and a C-terminal domain that mediates inhibition of NF-kappaB activation. Our findings suggest a possible molecular mechanism that could explain A20's ability to negatively regulate its own TNF-inducible expression.

TRAF5, an activator of NF-kappaB and putative signal transducer for the lymphotoxin-beta receptor

Tumor necrosis factor (TNF) receptor-associated factors (TRAFs) are signal transducers for several members of the TNF receptor superfamily. We have identified a novel member of the TRAF family by degenerate oligonucleotide polymerase chain reaction amplification that contains a zinc RING finger and zinc finger motifs, a coiled-coil region, and a C-terminal "TRAF" homology domain. In vitro translated TRAF5 binds to the cytoplasmic region of the lymphotoxin-beta receptor (LT-betaR) but not to several other related receptors including CD40, both TNF receptors, Fas, and nerve growth factor receptor. TRAF5 and LT-betaR coimmunoprecipitate when overexpressed in COS7 cells. TRAF5 mRNA expression is found in all visceral organs and overlaps with LT-betaR. These features distinguish TRAF5 from the other members of the TRAF family. The transcription factor NF-kappaB is activated in HEK293 cells by overexpression of full-length TRAF5 but not a truncated form lacking the zinc binding region. Furthermore, overexpression of LT-betaR in HEK293 cells also results in activation of NF-kappaB, which is partially inhibited by the truncated TRAF5 mutant. These results show TRAF5 is functionally similar to TRAF2 in that both mediate activation NF-kappaB and implicate TRAF5 as a signal transducer for LT-betaR.

Protein sequence motifs

Protein sequence motifs are signatures of protein families and can often be used as tools for the prediction of protein function. The generalization and modification of already known motifs are becoming major trends in the literature, even though new motifs are still being discovered at an approximately linear rate. The emphasis of motif analysis appears to be shifting from metabolic enzymes, in which motifs are associated with catalytic functions and thus often readily recognizable, to structural and regulatory proteins, which contain more divergent motifs. The consideration of structural information increasingly contributes to the identification of motifs and their sensitivity. Genome sequencing provides the basis for a systematic analysis of all motifs that are present in a particular organism. A systematically derived motif database is therefore feasible, allowing the classification of the majority of the newly appearing protein sequences into known families.

The RING finger domain: a recent example of a sequence-structure family

In the past 18 months, two RING finger structures have been solved. They represent the first reported structures for this novel zinc-binding sequence motif. Both structures are significantly different from other zinc-binding domains, in terms of both their zinc-ligation scheme and their three-dimensional structures. The RING finger domain appears to be a convenient scaffold which can be altered to provide functional specificity in those proteins that contain the motif.

CD30 contains two binding sites with different specificities for members of the tumor necrosis factor receptor-associated factor family of signal transducing proteins

CD30 is a member of the tumor necrosis factor (TNF) receptor family of proteins. CD30 can regulate proliferation of lymphocytes and may also play an important role in human immunodeficiency virus replication. However, little is known about CD30 signal transduction. We performed a yeast two-hybrid library screen with the cytoplasmic domain of CD30 and isolated multiple independent cDNAs encoding human tumor necrosis factor receptor-associated factor (TRAF) 1, TRAF2, and CRAF1 (TRAF3). The ability of TRAF1, TRAF2, and CRAF1 to associate with CD30 was confirmed using an in vitro coprecipitation assay, further demonstrating that the interaction was specific and direct. The TRAF-binding domain of CD30 was mapped to the COOH-terminal 36 amino acid residues, which contained two independent binding sites. CRAF1 bound only a single site, which contained the sequence PEQET, whereas TRAF1 and TRAF2 were capable of binding to either the PEQET site or an additional downstream domain. These data indicate that the TRAF protein binding pattern of CD30 differs from other TNF receptor family members and suggest that signaling specificity through TNF receptor family proteins may be achieved through differences in their abilities to bind TRAF proteins.

Structural and biological features of the TNF receptor and TNF ligand superfamilies: interactive signals in the pathobiology of Hodgkin's disease

Members of the TNF receptor superfamily are type I membrane glycoproteins with limited homology (overall homologies: 25%-30%) in the extracellular domain containing variable numbers of cysteine-rich repeats. In contrast, the TNF ligand superfamily members (with the exception of LT-alpha) are type II membrane glycoproteins with limited homology to TNF (overall homologies: 20%) in the extracellular region. TNF and LT-alpha are trimeric proteins and are composed of beta-strands forming a beta-jellyroll, the homology of the beta-strand regions for the TNF ligand superfamily members suggests a similar trimeric or multimeric complex formation for the other members. A genetic linkage, as evidence for evolutionary relatedness, is also found by chromosomal cluster for CD30, CD120b, 4-1BB and OX40 to 1p36; CD27, CD120a and TNFR-RP to 12p13; TNF, LT-alpha and LT-beta to 6p21; CD27L and 4-1BBL to 19p13; CD95L and OX40L to 1q25. TNF, LT-alpha and LT-beta and their receptors (CD120a, CD120b, TNFR-RP) interact in a complex fashion. Other family members, however, show a one ligand/one receptor binding principle. Signals can also be transduced through at least some of the ligands. TNF superfamily ligands are involved in induction of cytokine secretion, upregulation of adhesion molecules, activation antigens and costimulatory proteins, all known to amplify stimulatory and regulatory signals that occur during immune responses. On the other hand, differences in the distribution, kinetics of induction and requirements for induction support the view of a defined role for each of the ligands for T-cell-mediated immune activities. The shedding of members of the TNF receptor superfamily could limit the signals mediated by the corresponding ligands, as a functional regulatory mechanism. Induction of cytotoxic cell death is another common functional feature of this cytokine family (TNF, LT-alpha, CD30L, CD95L and 4-1BBL). Further studies have to identify unique versus redundant biological and physiological functions for each of the TNF superfamily ligands. In addition to other cytokines primary H-RS cell frequently express at least TNF, LT-alpha, CD27L and CD30L, but not CD40L. Furthermore, H-RS cells express several TNF receptors, such as CD30, CD40, CD95, CD120a, CD120b and 4-1BB. The TNF-like ligands might support growth and activation of HD-associated tumor cells and/or interact with surrounding reactive bystander cells, particularly T-cells. The different interactions between H-RS cells and surrounding reactive bystander cells are part of the pathobiology of HD. Detailed functional analysis have to confirm the predicted biological activities of TNF, LT-alpha, CD27L, CD30L, CD40L, CD95L, 4-1BBL and gp34/OX40L for the H-RS cell/T-cell interactions with impact on tumor growth and pathogenesis of HD. TNF and LT-alpha/CD120a and CD120b, CD30/CD30L, and CD40/CD40L are clearly critical elements in the deregulated network of interactive signals between H-RS cells and surrounding bystander cells with membrane-associated and cytokine-mediated events. Several TNFR superfamily members are also candidates for novel treatment protocols, including CD30 and CD40.