Crystallographic analysis of CD40 recognition and signaling by human TRAF2

The signaling adaptors and pathways activated by TNF superfamily

Members of the TNF receptor superfamily play pivotal roles in numerous biological events in metazoan organisms. Ligand-mediated trimerization by corresponding homo- or heterotrimeric ligands, the TNF family ligands, causes recruitment of several intracellular adaptors, which activate multiple signal transduction pathways. While recruitment of death domain (DD) containing adaptors such as Fas associated death domain (FADD) and TNFR associated DD (TRADD) can lead to the activation of a signal transduction pathway that induces apoptosis, recruitment of TRAF family proteins can lead to the activation of transcription factors such as, NF-kappaB and JNK thereby promoting cell survival and differentiation as well as immune and inflammatory responses. Individual TNF receptors are expressed in different cell types and have a range of affinities for various intracellular adaptors, which provide tremendous signaling and biological specificities. In addition, numerous signaling modulators are involved in regulating activities of signal transduction pathways downstream of receptors in this superfamily. Most of the TNF receptor superfamily members as well as many of their signaling mediators, have been uncovered in the last two decades. However, much remains unknown about how individual signal transduction pathways are regulated upon activation by any particular TNF receptor, under physiological conditions.

Multiple structural states of S100A12: A key to its functional diversity

S100A12 is a member of the S100 family of EF-hand calcium-binding proteins. Together with two other calgranulins, S100A8 and S100A9, it is mostly expressed in human granulocytes, although there is increasing evidence of expression in keratinocytes and psoriatic lesions. It is involved in host-parasite response, and linked to corneal autoimmune diseases connected with filarial parasite infestation. Interaction of S100A12 with a multiligand receptor for advanced glycation end products (RAGE) mediates inflammation. Human recombinant S100A12 was found to induce neuritogenesis of cultured hippocampal cells, similar to two other S100 proteins, S100B and S100A4. X-ray structure of S100A12 has been solved in two crystal forms: R3 and P2(1). In the R3 crystal form S100A12 is a dimer, and in the P2(1) crystal form the dimers are arranged as a hexamer. The hexameric form suggests its role in receptor oligomerisation. S100A12 binds copper at the predicted zinc/copper binding site, which is located close to the surface of the protein. We propose copper-mediated generation of reactive oxygen species by S100A12 as its function in host-parasite response.

The Fn14 cytoplasmic tail binds tumour-necrosis-factor-receptor-associated factors 1, 2, 3 and 5 and mediates nuclear factor-kappaB activation

Fn14 is a growth-factor-inducible immediate-early-response gene encoding a 102-amino-acid type I transmembrane protein. The human Fn14 protein was recently identified as a cell-surface receptor for the tumour necrosis factor (TNF) superfamily member named TWEAK (TNF-like weak inducer of apoptosis). In the present paper, we report that the human TWEAK extracellular domain can also bind the murine Fn14 protein. Furthermore, site-specific mutagenesis and directed yeast two-hybrid interaction assays revealed that the TNFR-associated factor (TRAF) 1, 2, 3 and 5 adaptor molecules bind the murine Fn14 cytoplasmic tail at an overlapping, but non-identical, amino acid sequence motif. We also found that TWEAK treatment of quiescent NIH 3T3 cells stimulates inhibitory kappaBalpha phosphorylation and transcriptional activation of a nuclear factor-kappaB (NF-kappaB) enhancer/luciferase reporter construct. Fn14 overexpression in transiently transfected NIH 3T3 cells also promotes NF-kappaB activation, and this cellular response requires an intact TRAF binding site. These results indicate that Fn14 is a functional TWEAK receptor that can associate with four distinct TRAF family members and stimulate the NF-kappaB transcription factor signalling pathway.

Transmembrane domains 1 and 2 of the latent membrane protein 1 of Epstein-Barr virus contain a lipid raft targeting signal and play a critical role in cytostasis

The latent membrane protein 1 (LMP-1) oncoprotein of Epstein-Barr virus (EBV) is a constitutively active, CD40-like cell surface signaling protein essential for EBV-mediated human B-cell immortalization. Like ligand-activated CD40, LMP-1 activates NF-kappaB and Jun kinase signaling pathways via binding, as a constitutive oligomer, to tumor necrosis factor receptor-associated factors (TRAFs). LMP-1's lipid raft association and oligomerization have been linked to its activation of cell signaling pathways. Both oligomerization and lipid raft association require the function of LMP-1's polytopic multispanning transmembrane domain, a domain that is indispensable for LMP-1's growth-regulatory signaling activities. We have begun to address the sequence requirements of the polytopic hydrophobic transmembrane domain for LMP-1's signaling and biochemical activities. Here we report that transmembrane domains 1 and 2 are sufficient for LMP-1's lipid raft association and cytostatic activity. Transmembrane domains 1 and 2 support NF-kappaB activation, albeit less potently than does the entire polytopic transmembrane domain. Interestingly, LMP-1's first two transmembrane domains are not sufficient for oligomerization or TRAF binding. These results suggest that lipid raft association and oligomerization are mediated by distinct and separable activities of LMP-1's polytopic transmembrane domain. Additionally, lipid raft association, mediated by transmembrane domains 1 and 2, plays a significant role in LMP-1 activation, and LMP-1 can activate NF-kappaB via an oligomerization/TRAF binding-independent mechanism. To our knowledge, this is the first demonstration of an activity's being linked to individual membrane-spanning domains within LMP-1's polytopic transmembrane domain.

CD40 and its viral mimic, LMP1: similar means to different ends

CD40 is an important regulator of diverse aspects of the immune response including the T-cell-dependent humoral immune response, the development of antigen-presenting cells (APCs) and inflammation. Latent membrane protein 1 (LMP1), a protein encoded by Epstein-Barr Virus (EBV), appears to mimic CD40 in multiple ways. CD40 and LMP1 bind similar sets of cellular signalling proteins and activate overlapping signalling pathways. Despite many similarities shared between CD40 and LMP1, they also differ substantively. In this review, we will compare and contrast the signalling mediated by CD40 and LMP1.

The new MATH: homology suggests shared binding surfaces in meprin tetramers and TRAF trimers

Although apparently functionally unrelated, intracellular TRAFs and extracellular meprins share a region with conserved meprin and traf homology, MATH(1). Both TRAFs and meprins require subunit assembly for function. By structural analysis of the sequences, we provide an explanation of how meprins, which form tetramers, and TRAF molecules, which form trimers, can share homology. Our analysis suggests it is highly likely that the same oligomerization surface is used. The analysis has implications for the widely distributed group of proteins containing MATH domains.

Regulation of the subcellular localization of tumor necrosis factor receptor-associated factor (TRAF)2 by TRAF1 reveals mechanisms of TRAF2 signaling

Tumor necrosis factor receptor-associated factor (TRAF)2 is a critical adaptor molecule for tumor necrosis factor (TNF) receptors in inflammatory and immune signaling. Upon receptor engagement, TRAF2 is recruited to CD40 and translocates to lipid rafts in a RING finger-dependent process, which enables the activation of downstream signaling cascades including c-Jun NH(2)-terminal kinase (JNK) and nuclear factor (NF)-kappaB. Although TRAF1 can displace TRAF2 and CD40 from raft fractions, it promotes the ability of TRAF2 activate signaling over a sustained period of time. Removal of the RING finger of TRAF2 prevents its translocation into detergent-insoluble complexes and renders it dominant negative for signaling. TRAF1(-/-) dendritic cells show attenuated responses to secondary stimulation by TRAF2-dependent factors and increased stimulus-dependent TRAF2 degradation. Replacement of the RING finger of TRAF2 with a raft-targeting signal restores JNK activation and association with the cyto-skeletal protein Filamin, but not NF-kappaB activation. These findings offer insights into the mechanism of TRAF2 signaling and identify a physiological role for TRAF1 as a regulator of the subcellular localization of TRAF2.

Intracellular localization and transcriptional regulation of tumor necrosis factor (TNF) receptor-associated factor 4 (TRAF4)

To gain insight in the subcellular localization of tumor necrosis factor receptor-associated factor (TRAF4) we analyzed GFP chimeras of full-length TRAF4 and various deletion mutants derived thereof. While TRAF4-GFP (T4-GFP) was clearly localized in the cytoplasm, the N-terminal deletion mutant, T4(259-470), comprising the TRAF domain of the molecule, and a C-terminal deletion mutant consisting mainly of the RING and zinc finger domains of TRAF4 were both localized predominantly to the nucleus. Passive nuclear localization of T4(259-470) can be ruled out as the TRAF domain of TRAF4 was sufficient to form high molecular weight complexes. T4(259-470) recruited full-length TRAF4 into the nucleus whereas TRAF4 was unable to change the nuclear localization of T4(259-470). Thus, it seems that individual T4(259-470) mutant molecules are sufficient to direct the respective TRAF4-T4(259-470) heteromeric complexes into the nucleus. In cells forming cell-cell contacts, TRAF4 was recruited to the sites of contact via its C-TRAF domain. The expression of some TRAF proteins is regulated by the NF-kappaB pathway. Thus, we investigated whether this pathway is also involved in the regulation of the TRAF4 gene. Indeed, in primary T-cells and Jurkat cells stimulated with the NF-kappaB inducers TNF or phorbol 12-myristate 13-acetate (PMA), TRAF4-mRNA was rapidly up-regulated. In Jurkat T-cells deficient for I-kappaB kinase gamma (IKKgamma, also known as NEMO), an essential component of the NF-kappaB-inducing-IKK complex, induction of TRAF4 was completely inhibited. In cells deficient for RIP (receptor interactive protein), an essential signaling intermediate of TNF-dependent NF-kappaB activation, TNF-, but not PMA-induced up-regulation of TRAF4 was blocked. These data suggest that activation of the NF-kappaB pathway is involved in up-regulation of TRAF4 in T-cells.

Structurally adaptive hot spots at a protein interaction interface on TRAF3

Tumor necrosis factor (TNF) signaling is controlled by receptors and intracellular signaling pathways that activate the NF-kappaB transcription factor. The resulting signals elicit immune responses and have important implications for disorders such as autoimmunity or allergic reactions. TNF-receptor-associated factors (TRAFs) bind to the cytoplasmic portion of TNFRs as well as downstream regulators and thus are co-inducers of the signal transduction. TRAF3 binds to diverse receptors and regulators by accomodating a conserved motif that is embedded in completely different structural frameworks. Thus, the protein-protein contact region on TRAF3 represents a binding interface that is structurally and functionally adaptive. In this report, three 'hot spots' at the TRAF3 protein-interaction interface are defined that provide the principal contact regions for different binding partners. The side-chains of residues at these 'hot spots' are flexible and undergo movements on binding the different partners. These side chain rearrangements provide a structural adaptability that promotes interaction with a variety of distinct proteins. It is proposed that similar adaptive 'hot spots' are also present on the binding surfaces of TRAF1, TRAF2 and TRAF5.

Adenovirus transduction to effect CD40 signalling improves the immune stimulatory activity of myeloma cells

Neoplastic plasma cells from patients with myeloma fail to stimulate an effective anti-myeloma immune response, which may be in part due to their deficient expression of immune accessory molecules. Attempting to alter this, we infected myeloma cell lines and patient-derived primary myeloma cells with an adenovirus encoding CD154 (Ad-CD154). Myeloma cells were made to express the CD154 transgene at multiplicity of infection (MOI) between 10 and 1000. Furthermore, infection of CD40(positive) myeloma cells with Ad-CD154, but not an adenovirus encoding an irrelevant transgene, beta-galactosidase (Ad-LacZ), induced enhanced expression of immune accessory molecules, such as CD54, HLA-DR and CD70. In addition, Ad-CD154-infected myeloma cells could activate bystander CD40(positive) antigen-presenting cells to express immune accessory molecules. Consequently, Ad-CD154 infected myeloma cells stimulated proliferation in allogeneic mixed lymphocyte reactions (MLR). Finally, co-infection of CD40(negative) myeloma cells with Ad-CD154 and an adenovirus encoding CD40 (Ad-CD40) induced expression of immune accessory molecules and enhanced the MLR stimulatory capacity of transduced myeloma cells. Collectively, these results indicate that infection of myeloma cells with Ad-CD154 or Ad-CD154/Ad-CD40 can induce changes in myeloma cells that enhance their ability to induce cellular immune activation. As such, this approach may have potential application for immune therapy of patients with this disease.

Distinct molecular mechanism for initiating TRAF6 signalling

Tumour-necrosis factor (TNF) receptor-associated factor 6 (TRAF6) is the only TRAF family member that participates in signal transduction of both the TNF receptor (TNFR) superfamily and the interleukin-1 receptor (IL-1R)/Toll-like receptor (TLR) superfamily; it is important for adaptive immunity, innate immunity and bone homeostasis. Here we report crystal structures of TRAF6, alone and in complex with TRAF6-binding peptides from CD40 and TRANCE-R (also known as RANK), members of the TNFR superfamily, to gain insight into the mechanism by which TRAF6 mediates several signalling cascades. A 40 degrees difference in the directions of the bound peptides in TRAF6 and TRAF2 shows that there are marked structural differences between receptor recognition by TRAF6 and other TRAFs. The structural determinant of the petide TRAF6 interaction reveals a Pro-X-Glu-X-X-(aromatic/acidic residue) TRAF6-binding motif, which is present not only in CD40 and TRANCE-R but also in the three IRAK adapter kinases for IL-1R/TLR signalling. Cell-permeable peptides with the TRAF6-binding motif inhibit TRAF6 signalling, which indicates their potential as therapeutic modulators. Our studies identify a universal mechanism by which TRAF6 regulates several signalling cascades in adaptive immunity, innate immunity and bone homeostasis.

Hetero-oligomerization among the TIF family of RBCC/TRIM domain-containing nuclear cofactors: a potential mechanism for regulating the switch between coactivation and corepression

The RING-B box-coiled-coil (RBCC) motif (also re-named recently as the tripartite motif (TRIM)) is a widely distributed motif that is hypothesized to be a protein-protein interface. The RBCC/TRIM domain of the corepressor KAP-1 is both necessary and sufficient to interact directly with the transcription repressor KRAB domain. Each subdomain of the KAP-1-RBCC contributes directly to the oligomerization and/or ligand recognition. Little is known about the function or the natural binding ligands for the RBCC/TRIM domain of the other TIF family members. In order to investigate whether hetero-oligomerization might be a biological regulatory mechanism, we have evaluated the hetero-oligomerization potential of the TIF family members including KAP-1, TIF1alpha, TIF1gamma, and the RBCC/TRIM family members including PML1, and MID1. We have reconstituted and characterized the oligomerization for these proteins using baculovirus and mammalian expression systems by biochemical approaches. Our data indicate that the RBCC/TRIM domains of KAP-1, TIF1alpha and TIF1gamma exist in a homo-oligomeric state. However, there is little cross-talk between KAP-1 and other TIF family members, suggesting that a high degree of specificity for oligomerization interface and ligand recognition is intrinsically built into the RBCC/TRIM domain of KAP-1. Finally, we demonstrate that TIF1alpha interacts with TIF1gamma and the coiled-coil region of TIF1gamma is necessary for this interaction. The hetero-oligomerization between TIF1alpha and TIF1gamma implies a potential regulatory mechanism for transcriptional regulation.

A novel zinc finger protein that inhibits osteoclastogenesis and the function of tumor necrosis factor receptor-associated factor 6

A variety of surface receptors eliciting diverse cellular responses have been shown to recruit tumor necrosis factor receptor-associated factor (TRAF) adaptor molecules. However, a few TRAF-interacting intracellular proteins that serve as downstream targets or regulators of TRAF function have been identified. In search of new intracellular molecules that bind TRAF6, we carried out a yeast two-hybrid cDNA library screening with an N-terminal segment of TRAF6 as the bait. A novel human C(2)H(2)-type zinc finger family protein was identified, which when coexpressed with TRAF6 led to a suppression of TRAF6-induced activation of NF-kappa B and c-Jun N-terminal kinase. This novel protein was designated TIZ (for TRAF6-inhibitory zinc finger protein). TIZ expression also inhibited the signaling of RANK (receptor activator of NF-kappa B), which together with TRAF6 has been shown to be essential for osteoclastogenesis. Furthermore, the expression level of TIZ appeared to be regulated during the differentiation of human peripheral blood monocytes into osteoclasts. More significantly, transfection of TIZ into the monocyte/macrophage cell line Raw264.7 reduced the RANK ligand-induced osteoclastogenesis of this cell line. Our findings suggest that the novel zinc finger protein TIZ may play a role during osteoclast differentiation by modulating TRAF6 signaling activity.

Downstream regulator TANK binds to the CD40 recognition site on TRAF3

TRAFs (tumor necrosis factor receptor [TNFR]-associated factors) bind to the cytoplasmic portion of liganded TNFRs and stimulate activation of NF-kappaB or JNK pathways. A modulator of TRAF signaling, TANK, serves as either an enhancer or an inhibitor of TRAF-mediated signaling pathways. The crystal structure of a region of TANK bound to TRAF3 has been determined and compared to a similar CD40/TRAF3 complex. TANK and CD40 bind to the same crevice on TRAF3. The recognition motif PxQxT is presented in a boomerang-like structure in TANK that is markedly different from the hairpin loop that forms in CD40 upon binding to TRAF3. Critical TANK contact residues were confirmed by mutagenesis to be required for binding to TRAF3 or TRAF2. Binding affinity, measured by isothermal titration calorimetry and competition assays, demonstrated that TANK competes with CD40 for the TRAF binding site.

All TRAFs are not created equal: common and distinct molecular mechanisms of TRAF-mediated signal transduction

The tumor necrosis factor (TNF) receptor associated factors (TRAFs) have emerged as the major signal transducers for the TNF receptor superfamily and the interleukin-1 receptor/Toll-like receptor (IL-1R/TLR) superfamily. TRAFs collectively play important functions in both adaptive and innate immunity. Recent functional and structural studies have revealed the individuality of each of the mammalian TRAFs and advanced our understanding of the underlying molecular mechanisms. Here, we examine this functional divergence among TRAFs from a perspective of both upstream and downstream TRAF signal transduction pathways and of signaling-dependent regulation of TRAF trafficking. We raise additional questions and propose hypotheses regarding the molecular basis of TRAF signaling specificity.

Characterization of a CD40-dominant inhibitory receptor mutant

CD40 is an important mediator of immune and inflammatory responses. It is a costimulatory molecule for B cell proliferation and survival. Blockade of CD40 has been shown to induce tolerance and its role in other pathogenic conditions has led to the proposal that CD40 inhibition could be valuable therapeutically. As a first step to this end, we have characterized a CD40-dominant negative receptor. This inhibitory mutant lacks the identified CD40 signaling domains. It inhibits both cotransfected and endogenous CD40 activation of NF-kappaB. This mutant is specific, as it does not affect TNF or latent membrane protein 1 signaling. Its potential usefulness is illustrated by its ability to inhibit the CD40 ligand-stimulated increases of HLA and CD54 expression, molecules involved in Ag recognition and lymphocyte recruitment leading to organ rejection. The inhibitory mutant has no TNFR-associated factor 2-binding capabilities and inhibits the recruitment of TNFR-associated factor 2 to the CD40 signaling complex after stimulation. These studies show that the CD40 inhibitory receptor molecule is effective, specific, and useful both for research and potentially as a clinical tool. And furthermore, it is likely that similar dominant inhibitory receptors can be generated for all of the members of the TNFR superfamily.

Molecular characterization of CD40 signaling intermediates

Signal transduction through the CD40 receptor is initiated by binding of its trimeric ligand and propagated by interactions of tumor necrosis factor receptor-associated factor (TRAF) proteins with the multimerized CD40 cytoplasmic domain. Using defined multimeric constructs of the CD40 cytoplasmic domain expressed as either soluble or myristoylated proteins, we have addressed the extent of receptor multimerization needed to initiate signal transduction and identified components of CD40 signaling complexes. Signal transduction in human embryonic kidney 293 cells, measured by nuclear factor kappaB activation, was observed in cells expressing soluble trimeric CD40 cytoplasmic domain and to a lesser extent in cells expressing dimeric CD40 cytoplasmic domain. Nuclear factor kappaB activation was strongest in cells expressing myristoylated trimeric CD40 cytoplasmic domain. Signal transduction through trimeric CD40 cytoplasmic domains with various point mutations in the TRAF binding sites was similar to signal transduction through analogous full-length receptors. Transiently expressed soluble trimeric CD40 cytoplasmic domain was isolated as complexes that contained TRAF2, TRAF3, TRAF5, TRAF6, and the inhibitor of apoptosis protein (c-IAP1). Association of c-IAP1 with the CD40 cytoplasmic domain complex was indirect and dependent on the presence of an intact TRAF1/2/3 binding site. These results suggest that extracellular ligation of CD40 can be bypassed and that soluble trimerized CD40 complexes can be isolated and used to identify components that link CD40 with signaling pathways.

A diverse family of proteins containing tumor necrosis factor receptor-associated factor domains

We have identified three new tumor necrosis factor-receptor associated factor (TRAF) domain-containing proteins in humans using bioinformatics approaches, including: MUL, the product of the causative gene in Mulibrey Nanism syndrome; USP7 (HAUSP), an ubiquitin protease; and SPOP, a POZ domain-containing protein. Unlike classical TRAF family proteins involved in TNF family receptor (TNFR) signaling, the TRAF domains (TDs) of MUL, USP7, and SPOP are located near the NH(2) termini or central region of these proteins, rather than carboxyl end. MUL and USP7 are capable of binding in vitro via their TDs to all of the previously identified TRAF family proteins (TRAF1, TRAF2, TRAF3, TRAF4, TRAF5, and TRAF6), whereas the TD of SPOP interacts weakly with TRAF1 and TRAF6 only. The TD of MUL also interacted with itself, whereas the TDs of USP7 and SPOP did not self-associate. Analysis of various MUL and USP7 mutants by transient transfection assays indicated that the TDs of these proteins are necessary and sufficient for suppressing NF-kappaB induction by TRAF2 and TRAF6 as well as certain TRAF-binding TNF family receptors. In contrast, the TD of SPOP did not inhibit NF-kappaB induction. Immunofluorescence confocal microscopy indicated that MUL localizes to cytosolic bodies, with targeting to these structures mediated by a RBCC tripartite domain within the MUL protein. USP7 localized predominantly to the nucleus, in a TD-dependent manner. Data base searches revealed multiple proteins containing TDs homologous to those found in MUL, USP7, and SPOP throughout eukaryotes, including yeast, protists, plants, invertebrates, and mammals, suggesting that this branch of the TD family arose from an ancient gene. We propose the moniker TEFs (TD-encompassing factors) for this large family of proteins.

Surface map comparison: studying function diversity of homologous proteins

A simplified protein surface cartography approach has been developed to assist in the analysis of surface features in homologous families, and thus to predict conservation or divergence of protein functions and protein-protein interaction patterns. A spherical approximation of protein surface was used, with a focus on charged and hydrophobic residues. The resulting surface map allows for qualitative analysis and comparison of surfaces of proteins, but can also be used to define a simple numerical measure of map similarity between two or more proteins. The latter was shown to be useful for function based classifications within large protein families. Surface map analysis was tested on several test cases: haemoglobins, death domains and TRAF domains. It was shown that surface map comparison allows a better function prediction than general sequence analysis methods and can reproduce known examples of functional variation within a divergent group of proteins. In another example, we predict novel, unexpected sets of common functional properties for seemingly distant members of a large group of divergent proteins. The method was also shown to be robust enough to allow using protein models from comparative modelling instead of experimental structures.

VP4 differentially regulates TRAF2 signaling, disengaging JNK activation while directing NF-kappa B to effect rotavirus-specific cellular responses

Rotaviruses rapidly activate NF-kappaB and induce the secretion of selected chemokines after infection. The ability of rotavirus particles lacking genomic RNA to activate NF-kappaB suggested that rotavirus proteins direct cell signaling responses. We identified conserved TNFR-associated factor (TRAF) binding motifs within the rotavirus capsid protein VP4 and its N-terminal VP8* cleavage product. TRAFs (-1, -2, and -3) are bound by the rhesus rotavirus VP8* protein through three discrete TRAF binding domains. Expression of VP4 or VP8* from rhesus or human rotaviruses induced a 5-7-fold increase in NF-kappaB activity and synergistically enhanced TRAF2-mediated NF-kappaB activation. Mutagenesis of VP8* TRAF binding motifs abolished VP8* binding to TRAFs and the ability of the protein to activate NF-kappaB. Expression of pathway-specific dominant negative (DN) inhibitors DN-TRAF2 or DN-NF-kappaB-inducing kinase also abolished VP8*-, VP4-, or rotavirus-mediated NF-kappaB activation. These findings demonstrate that rotavirus primarily activates NF-kappaB through a TRAF2-NF-kappaB-inducing kinase signaling pathway and that VP4 and VP8* proteins direct pathway activation through interactions with cellular TRAFs. In contrast, transcriptional responses from AP-1 reporters were inhibited 5-fold by VP8* and were not activated by rotavirus infection, suggesting the differential regulation of TRAF2 signaling responses by VP8*. VP8* blocked JNK activation directed by TRAF2 or TRAF5 but had no effect on JNK activation directed by TRAF6 or MEKK1. This establishes that fully cytoplasmic rotaviruses selectively engage signaling pathways, which regulate cellular transcriptional responses. These findings also demonstrate that TRAF2 interactions can disengage JNK signaling from NF-kappaB activation and thereby provide a new means for TRAF2 interactions to determine pathway-specific responses.

The TNF-receptor-associated factor family: scaffold molecules for cytokine receptors, kinases and their regulators

The TNF-receptor-associated factor (TRAF) family is a phylogenetically conserved group of scaffold proteins that link receptors of the IL-1R/Toll and TNF receptor family to signalling cascades, leading to the activation of NF-kappaB and mitogen-activated protein kinases. Furthermore, TRAF proteins serve as a docking platform for a variety of regulators of these signalling pathways and are themselves often regulated at the transcriptional and posttranslational level. In this review, we address the structural and molecular basis of TRAF protein functions and highlight their role in cytokine signalling.

CD40 and LMP-1 both signal from lipid rafts but LMP-1 assembles a distinct, more efficient signaling complex

CD40, a member of the TNFR-1 receptor family, shares several features with LMP-1, an oncoprotein encoded by Epstein-Barr virus. CD40 and LMP-1 activate transcription by binding to TRAFs, JAK3 and/or TRADD. CD40's association with CD40L activates signaling. However, LMP-1 signals independently of a ligand but dependently on self-association. We demonstrate that activated CD40 and LMP-1 co-localize in lipid rafts and recruit TRAF3 there, findings consistent with signals of CD40 and LMP-1 being initiated from lipid rafts. To elucidate their signaling, we compared requirements for their aggregation and subcellular localization. Targeting CD40's monomeric C-terminal signaling domain to lipid rafts activates signaling, as does rendering it trimeric. Addition of both modifications supports signaling more efficiently. Parallel experiments with LMP-1 indicate that targeting the monomeric C-terminal signaling domain of LMP-1 to lipid rafts activates signaling, but trimerizing it does not. Fusing LMP-1's N-terminus and membrane-spanning domains to CD40's C-terminus supports signaling more efficiently than CD40 plus ligand or CD40's trimerized and/or localized derivatives. An activity of LMP-1's N-terminus and membrane-spanning domains other than trimerization must contribute to its efficient signaling.

Mammalian mitogen-activated protein kinase signal transduction pathways activated by stress and inflammation

The molecular details of mammalian stress-activated signal transduction pathways have only begun to be dissected. This, despite the fact that the impact of these pathways on the pathology of chronic inflammation, heart disease, stroke, the debilitating effects of diabetes mellitus, and the side effects of cancer therapy, not to mention embryonic development, innate and acquired immunity, is profound. Cardiovascular disease and diabetes alone represent the most significant health care problems in the developed world. Thus it is not surprising that understanding these pathways has attracted wide interest, and in the past 10 years, dramatic progress has been made. Accordingly, it is now becoming possible to envisage the transition of these findings to the development of novel treatment strategies. This review focuses on the biochemical components and regulation of mammalian stress-regulated mitogen-activated protein kinase (MAPK) pathways. The nuclear factor-kappa B pathway, a second stress signaling paradigm, has been the subject of several excellent recent reviews (258, 260).

Kinetic analysis of adenovirus fiber binding to its receptor reveals an avidity mechanism for trimeric receptor-ligand interactions

Most adenoviruses bind to the N-terminal immunoglobulin domain D1 of the coxsackievirus and adenovirus receptor via the head part of their fiber proteins. Three receptor molecules can bind per fiber head. We expressed the D1 domain and the adenovirus type 2 fiber head in bacteria and studied binding interactions by surface plasmon resonance measurements. When receptor domains bind adenovirus fiber independently of each other, the dissociation constant is 20-25 nm. However, when adenovirus fiber binds to receptors immobilized on the sensor chip, a situation better mimicking adenovirus binding to receptors on the cell surface, the dissociation constant was around 1 nm. Kinetic analysis shows that this happens via an avidity mechanism; three identical interactions with high on and off rate constants lead to tight binding of one fiber head to three receptor molecules with a very low overall off rate. The avidity mechanism could be used by other viruses that have multimeric adhesion proteins to attach to target cells. It could also be more general to trimeric receptor-ligand interactions, including those involved in intracellular signaling.

Segregation of TRAF6-mediated signaling pathways clarifies its role in osteoclastogenesis

Signals emanating from the receptor for interleukin-1 (IL-1), lipopolysaccharide (LPS) or osteoclast differentiation factor/receptor activator of NF kappa B ligand (ODF/RANKL) stimulate transcription factors AP-1 through mitogen-activated protein kinase (MAPK) activation and NF kappa B through I kappa B kinase (IKK) activation. These kinases are thought to be activated by tumor necrosis factor receptor-associated factor 6 (TRAF6). However, molecular mechanisms by which TRAF6 activates various downstream kinases remain to be elucidated. We identified functional domains of TRAF6 under physiological conditions established by appropriate expression of TRAF6 mutants in TRAF6-deficient cells. In IL-1 and LPS signaling pathways, the RING finger and first zinc finger domains are not required for NF kappa B activation but are required for full activation of MAPK. However, IL-1 and LPS signals utilize distinct regions within the zinc finger domains of TRAF6 to activate NF kappa B. Furthermore, the RING finger domain is not required for differentiation of splenocytes to multinuclear osteoclasts, but is essential for osteoclast maturation. Thus, TRAF6 plays essential roles in both the differentiation and maturation of osteoclasts by activating various kinases via its multiple domains.

Interleukin 1 signal transduction--current concepts and relevance to periodontitis

This review examines a well-characterized factor, interleukin 1 (IL-1), that has recently received considerable attention. A level of understanding is emerging that goes beyond simple recognition that IL-1 plays a role in disease, and begins to explain the molecular mechanisms of function. This review summarizes some current information on the importance of IL-1 in periodontitis as well as the signal transduction of IL-1, from binding to its cell-surface receptors, to the activation of cytoplasmic mediators and transcription factors responsible for the induction of target genes. The effect of IL-1 signal transduction is ultimately the activation and repression of specific transcription factors that regulate genes responsible for cellular activities. As additional steps of signal transduction become better-characterized, these insights may facilitate the development of improved therapeutic approaches for controlling inflammation and connective tissue destruction in a variety of diseases.

Selenomethionine incorporation in Saccharomyces cerevisiae RNA polymerase II

A protocol for the incorporation of SeMet into yeast proteins is described. Incorporation at a level of about 50% suffices for the location of Se sites in an anomalous difference Fourier map of the 0.5 MDa yeast RNA polymerase II. This shows the utility of the approach as an aid in the model-building of large protein complexes.

The role of TNF-receptor family members and other TRAF-dependent receptors in bone resorption

The contribution of osteoclasts to the process of bone loss in inflammatory arthritis has recently been demonstrated. Studies in osteoclast biology have led to the identification of factors responsible for the differentiation and activation of osteoclasts, the most important of which is the receptor activator of NF-kappa B ligand/osteoclast differentiation factor (RANKL/ODF), a tumor necrosis factor (TNF)-like protein. The RANKL/ODF receptor, receptor activator of NF-kappa B (RANK), is a TNF-receptor family member present on both osteoclast precursors and mature osteoclasts. Like other TNF-family receptors and the IL-1 receptor, RANK mediates its signal transduction via TNF receptor-associated factor (TRAF) proteins, suggesting that the signaling pathways activated by RANK and other inflammatory cytokines involved in osteoclast differentiation and activation are interconnected.

The tumour necrosis factor receptor family: life or death choices

The past twelve months have seen a renewal of interest in the therapeutic potential of members of the tumour necrosis factor receptor family and their cytokine ligands. This biomedical interest has spawned a number of structural studies, which have significantly deepened our understanding of the molecular basis for the function of these cell-surface signalling systems. The fresh data have revealed unexpected mechanisms conferring ligand-receptor specificity and have highlighted the structural requirements for the initiation of intracellular signalling.

The multifaceted role of Fas signaling in immune cell homeostasis and autoimmunity

Originally identified as a cell surface receptor that triggered the death of lymphocytes and tumor cells, it is now recognized that Fas (also known as CD95 or Apo-I) has distinct functions in the life and death of different cell types in the immune system. Fas signaling may also be involved in T cell costimulation and proliferation. Although Fas deficiency in humans and mice predisposes them towards systemic autoimmunity, Fas-FasL interactions can also facilitate organ-specific immunopathology. Proximal signaling by Fas and related receptors depends on subunit preassembly, which accounts for the dominant-negative effect of pathogenic receptor mutants and natural splice variants.

TNFR-associated factor family protein expression in normal tissues and lymphoid malignancies

TNFR-associated factors (TRAFs) constitute a family of adapter proteins that associate with particular TNF family receptors. Humans and mice contain six TRAF genes, but little is known about their in vivo expression at the single cell level. The in vivo locations of TRAF1, TRAF2, TRAF5, and TRAF6 were determined in human and mouse tissues by immunohistochemistry. Striking diversity was observed in the patterns of immunostaining obtained for each TRAF family protein, suggesting their expression is independently regulated in a cell type-specific manner. Dynamic regulation of TRAFs was observed in cultured PBLs, where anti-CD3 Abs, mitogenic lectins, and ILs induced marked increases in the steady-state levels of TRAF1, TRAF2, TRAF5, and TRAF6. TRAF1 was also highly inducible by CD40 ligand in cultured germinal center B cells, whereas TRAF2, TRAF3, TRAF5, and TRAF6 were relatively unchanged. Analysis of 83 established human tumor cell lines by semiquantitative immunoblotting methods revealed tendencies of certain cancer types to express particular TRAFs. For example, expression of TRAF1 was highly restricted, with B cell lymphomas consistently expressing this TRAF family member. Consistent with results from tumor cell lines, immunohistochemical analysis of 232 non-Hodgkin lymphomas revealed TRAF1 overexpression in 112 (48%) cases. TRAF1 protein levels were also elevated in circulating B cell chronic lymphocytic leukemia specimens (n = 49) compared with normal peripheral blood B cells (p = 0.01), as determined by immunoblotting. These findings contribute to an improved understanding of the cell-specific roles of TRAFs in normal tissues and provide evidence of altered TRAF1 expression in lymphoid malignancies.

Molecular basis for CD40 signaling mediated by TRAF3

Tumor necrosis factor receptors (TNFR) are single transmembrane-spanning glycoproteins that bind cytokines and trigger multiple signal transduction pathways. Many of these TNFRs rely on interactions with TRAF proteins that bind to the intracellular domain of the receptors. CD40 is a member of the TNFR family that binds to several different TRAF proteins. We have determined the crystal structure of a 20-residue fragment from the cytoplasmic domain of CD40 in complex with the TRAF domain of TRAF3. The CD40 fragment binds as a hairpin loop across the surface of the TRAF domain. Residues shown by mutagenesis and deletion analysis to be critical for TRAF3 binding are involved either in direct contact with TRAF3 or in intramolecular interactions that stabilize the hairpin. Comparison of the interactions of CD40 with TRAF3 vs. TRAF2 suggests that CD40 may assume different conformations when bound to different TRAF family members. This molecular adaptation may influence binding affinity and specific cellular triggers.

CD40 induces apoptosis in carcinoma cells through activation of cytotoxic ligands of the tumor necrosis factor superfamily

CD40, a tumor necrosis factor (TNF) receptor (TNFR) family member, conveys signals regulating diverse cellular responses, ranging from proliferation and differentiation to growth suppression and cell death. The ability of CD40 to mediate apoptosis in carcinoma cells is intriguing given the fact that the CD40 cytoplasmic C terminus lacks a death domain homology with the cytotoxic members of the TNFR superfamily, such as Fas, TNFR1, and TNF-related apoptosis-inducing ligand (TRAIL) receptors. In this study, we have probed the mechanism by which CD40 transduces death signals. Using a trimeric recombinant soluble CD40 ligand to activate CD40, we have found that this phenomenon critically depends on the membrane proximal domain (amino acids 216 to 239) but not the TNFR-associated factor-interacting PXQXT motif in the CD40 cytoplasmic tail. CD40-mediated cytotoxicity is blocked by caspase inhibitors, such as zVAD-fmk and crmA, and involves activation of caspase 8 and caspase 3. Interestingly, CD40 ligation was found to induce functional Fas ligand, TRAIL (Apo-2L) and TNF in apoptosis-susceptible carcinoma cells and to up-regulate expression of Fas. These findings identify a novel proapoptotic mechanism which is induced by CD40 in carcinoma cells and depends on the endogenous production of cytotoxic cytokines and autocrine or paracrine induction of cell death.

TNF alpha and the TNF receptor superfamily: structure-function relationship(s)

Tumour Necrosis Factor alpha (TNF alpha), is an inflammatory cytokine produced by macrophages/monocytes during acute inflammation and is responsible for a diverse range of signalling events within cells, leading to necrosis or apoptosis. The protein is also important for resistance to infection and cancers. TNF alpha exerts many of its effects by binding, as a trimer, to either a 55 kDa cell membrane receptor termed TNFR-1 or a 75 kDa cell membrane receptor termed TNFR-2. Both these receptors belong to the so-called TNF receptor superfamily. The superfamily includes FAS, CD40, CD27, and RANK. The defining trait of these receptors is an extra cellular domain comprised of two to six repeats of cysteine rich motifs. Additionally, a number of structurally related "decoy receptors" exist that act to sequester TNF molecules, thereby rescuing cells from apoptosis. The crystal structures of TNF alpha, TNF beta, the extracellular domain of TNFR-1 (denoted sTNFR-1), and the TNF beta sTNFR-1 complex have been defined by crystallography. This article will review the structure/function relationships of the TNF alpha and the TNF receptor superfamily. It will also discuss insights as to how structural features play a role in the pleiotropic effects of TNF alpha.

Thermodynamic characterization of the interaction between TRAF2 and tumor necrosis factor receptor peptides by isothermal titration calorimetry

The tumor necrosis factor receptor (TNFR) superfamily can induce diverse biological effects, including cell survival, proliferation, differentiation, and apoptosis. The major signal transducers for TNFRs are the family of TNF receptor associated factors (TRAFs). The direct interaction between TRAFs and the intracellular tails of TNFRs is the first step of this signal relay process. Structural studies have revealed a trimeric nature of TRAF2 and a symmetrical mode of receptor binding, suggesting the involvement of trivalent TNFR2-receptor interaction in the signal transduction. In this study, using isothermal titration calorimetry (ITC), we report thermodynamic characterization of the interaction between TRAF2 and monomeric peptide sequences from TNFR members, including TNFR2, CD40, CD30, Ox40, and 4-1BB, and the Epstein-Barr virus (EBV)-transforming protein, latent infection membrane protein-1 (LMP1). The dissociation constants of the interaction were shown to range between 40 microM and 1.9 mM, which are substantially weaker than most protein-peptide interactions. The interaction is entirely driven by exothermic enthalpy, consistent with the abundance of polar contacts. The enthalpy of the interaction has a significant temperature dependence (DeltaCp = -245 cal/mol small middle dotK). The unfavorable entropy in the interaction and the comparison with structural energetics calculations suggest the involvement of conformational rearrangement in the interaction. The low affinity of TRAF2 to monomeric receptor peptides further supports the importance of avidity contribution in TRAF2 recruitment by these receptors upon ligand-induced trimerization or higher order oligomerization.

CD154 (CD40 ligand)

CD40 ligand, a type II transmembrane protein recently renamed CD154, was originally considered restricted to activated T lymphocytes, functioning as a mediator of T cell-dependent B cell activation, proliferation, and differentiation. However, the spectrum of CD154 expression and function has broadened considerably during recent years, establishing new roles as a central mediator of immunity and inflammation for this member of the tumor necrosis factor (TNF) gene superfamily. The emerging picture indicates that ligation of the receptor CD40 via CD154, most potently in its trimeric form, functions in two ways. CD154 modulates physiologic processes, such as T cell-mediated effector functions and general immune responses required for appropriate host defense, but also triggers the expression of pro-inflammatory mediators, such as cytokines, adhesion molecules, and matrix degrading activities, all of which are associated with the pathogenesis of chronic inflammatory diseases, e.g., autoimmune disorders, arthritis, atherosclerosis, and cancer. Accordingly, CD40/CD154 interactions have advanced as a potential therapeutic target for these diseases, whereby two opposing strategies, interruption as well as enhancement of CD40 signaling, are explored for beneficial outcomes.

A novel mechanism of TRAF signaling revealed by structural and functional analyses of the TRADD-TRAF2 interaction

TRAF proteins are major mediators for the cell activation, cell survival, and antiapoptotic functions of the TNF receptor superfamily. They can be recruited to activated TNF receptors either by direct interactions with the receptors or indirectly via the adaptor protein TRADD. We now report the structure of the TRADD-TRAF2 complex, which is highly distinct from receptor-TRAF2 interactions. This interaction is significantly stronger and we show by an in vivo signaling assay that TRAF2 signaling is more readily initiated by TRADD than by direct receptor-TRAF2 interactions. TRADD is specific for TRAF1 and TRAF2, which ensures the recruitment of clAPs for the direct inhibition of caspase activation in the signaling complex. The stronger affinity and unique specificity of the TRADD-TRAF2 interaction are crucial for the suppression of apoptosis and provide a mechanistic basis for the perturbation of TRAF recruitment in sensitizing cell death induction.

TTRAP, a novel protein that associates with CD40, tumor necrosis factor (TNF) receptor-75 and TNF receptor-associated factors (TRAFs), and that inhibits nuclear factor-kappa B activation

CD40 belongs to the tumor necrosis factor (TNF) receptor family. CD40 signaling involves the recruitment of TNF receptor-associated factors (TRAFs) to its cytoplasmic domain. We have identified a novel intracellular CD40-binding protein termed TRAF and TNF receptor-associated protein (TTRAP) that also interacts with TNF-R75 and CD30. The region of the CD40 cytoplasmic domain that is required for TTRAP association overlaps with the TRAF6 recognition motif. Association of TTRAP with CD40 increases profoundly in response to treatment of cells with CD40L. Interestingly, TTRAP also associates with TRAFs, with the highest affinity for TRAF6. In transfected cells, TTRAP inhibits in a dose-dependent manner the transcriptional activation of a nuclear factor-kappaB (NF-kappaB)-dependent reporter mediated by CD40, TNF-R75 or Phorbol 12-myristate 13-acetate (PMA) and to a lesser extent by TRAF2, TRAF6, TNF-alpha, or interleukin-1beta (IL-1beta). TTRAP does not affect stimulation of NF-kappaB induced by overexpression of the NF-kappaB-inducing kinase (NIK), the IkappaB kinase alpha (IKKalpha), or the NF-kappaB subunit P65/RelA, suggesting it acts upstream of the latter proteins. Our results indicate that we have isolated a novel regulatory factor that is involved in signal transduction by distinct members of the TNF receptor family.

Solution structure of N-TRADD and characterization of the interaction of N-TRADD and C-TRAF2, a key step in the TNFR1 signaling pathway

TRADD is a multifunctional signaling adaptor protein that is recruited to TNFR1 upon ligand binding. The C-terminal of TRADD comprises the "death domain" that is responsible for association of TNFR1 and other death domain-containing proteins such as FADD and RIP. The N-terminal domain (N-TRADD) promotes the recruitment of TRAF2 to TNFR1 by binding to the C-terminal of TRAF2, leading to the activation of JNK/AP1 and NF-kappa B. The solution structure of N-TRADD was determined, revealing a novel protein fold. A combination of NMR, BIAcore, and mutagenesis experiments was used to help identify the site of interaction of N-TRADD with C-TRAF2, providing a framework for future attempts to selectively inhibit the TNF signaling pathways.

The Drosophila tumor necrosis factor receptor-associated factor-1 (DTRAF1) interacts with Pelle and regulates NFkappaB activity

A member of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) family was identified in Drosophila. DTRAF1 contains 7 zinc finger domains followed by a TRAF domain, similar to mammalian TRAFs and other members of the family identified in data bases from Caenorhabditis elegans, Arabidopsis, and Dictyostelium. Analysis of DTRAF1 binding to different members of the human TNF receptor family showed that this protein can interact through its TRAF domain with the p75 neurotrophin receptor and weakly with the lymphotoxin-beta receptor. DTRAF1 can also self-associate and binds to human TRAF1, TRAF2, and TRAF4. Interestingly, DTRAF1 interacts with human cIAP-1 and cIAP-2 but not with Drosophila DIAP-1 and -2. By itself, DTRAF1 did not induce significant NFkappaB activation when overexpressed in mammalian cells, although it specifically increased NFkappaB induction by TRAF6. In contrast, TRAF2-mediated NFkappaB induction was partially inhibited by DTRAF1. Mutants of DTRAF1 lacking the N-terminal region inhibited NFkappaB induction by either TRAF2 or TRAF6. DTRAF1 specifically associated with the regulatory N-terminal domain of Pelle, a Drosophila homolog of the human kinase interleukin-1 receptor-associated kinase (IRAK). Interestingly, though Pelle and DTRAF1 individually were unable to induce NFkappaB in a human cell line, co-expression of Pelle and DTRAF1 resulted in significant NFkappaB activity. Interactions of DTRAF1 with human TRAF-, TNF receptor-, and IAP-family proteins imply strong evolutionary conservation of TRAF protein structure and function throughout Metazoan evolution.

Discrete signaling regions in the lymphotoxin-beta receptor for tumor necrosis factor receptor-associated factor binding, subcellular localization, and activation of cell death and NF-kappaB pathways

Lymphotoxin-beta receptor (LTbetaR), a member of the tumor necrosis factor receptor superfamily, is essential for the development and organization of secondary lymphoid tissue. Wild type and mutant LTbetaR containing successive truncations of the cytoplasmic domain were investigated by retrovirus-mediated gene transfer into HT29.14s and in 293T cells by transfection. Wild type receptors accumulated in perinuclear compartments and enhanced responsiveness to ligand-induced cell death and ligand-independent activation of NFkappaB p50 dimers. Coimmunoprecipitation and confocal microscopy mapped the TRAF3 binding site to amino acids PEEGDPG at position 389. However, LTbetaR truncated at position Pro(379) acted as a dominant positive mutant that down-modulated surface expression and recruited TRAF3 to endogenous LTbetaR. This mutant exhibited ligand-independent cell death and activated NF-kappaB p50 dimers. By contrast, truncation at Gly(359) created a dominant-negative mutant that inhibited ligand-induced cell death and activation of NF-kappaB p50/p65 heterodimers. This mutant also blocked accumulation of wild type receptor into perinuclear compartments, suggesting subcellular localization may be crucial for signal transduction. A cryptic TRAF-independent NF-kappaB activating region was identified. These mutants define discrete subregions of a novel proline-rich domain that is required for subcellular localization and signal transduction by the LTbetaR.

Requirement of tumor necrosis factor receptor-associated factor (TRAF)6 in interleukin 17 signal transduction

Signaling through its widely distributed cell surface receptor, interleukin (IL)-17 enhances the transcription of genes encoding proinflammatory molecules. Although it has been well documented that IL-17 activates the transcription factor nuclear factor (NF)-kappaB and c-Jun NH(2)-terminal kinase (JNK), the upstream signaling events are largely unknown. Here we report the requirement of tumor necrosis factor receptor-associated factor (TRAF)6 in IL-17-induced NF-kappaB and JNK activation. In embryonic fibroblasts (EFs) derived from TRAF6 knockout mice, IL-17 failed to activate the IkappaB kinases (IKKs) and JNK. Consequently, IL-17-induced IL-6 and intercellular adhesion molecule 1 expression in the TRAF6-deficient cells was abolished. Lack of TRAF6 appeared to be the sole defect responsible for the observed failure to respond to IL-17, because transient transfection of TRAF6 expression plasmid into the TRAF6-deficient cells restored IL-17-induced NF-kappaB activation in a luciferase reporter assay. Furthermore, the levels of IL-17 receptor (IL-17R) on the TRAF6-deficient EFs were comparable to those on the wild-type control cells. Defect in IL-17 response was not observed in TRAF2-deficient EFs. Moreover, when TRAF6 and IL-17R were coexpressed in 293 cells, TRAF6 coimmunoprecipitated with IL-17R. Together, these results indicate that TRAF6, but not TRAF2, is a crucial component in the IL-17 signaling pathway leading to proinflammatory responses.

Activation of apoptosis signal-regulating kinase 1 (ASK1) by tumor necrosis factor receptor-associated factor 2 requires prior dissociation of the ASK1 inhibitor thioredoxin

The stress-activated protein kinases (SAPKs, also called c-Jun NH(2)-terminal kinases) and the p38s, two mitogen-activated protein kinase (MAPK) subgroups activated by cytokines of the tumor necrosis factor (TNF) family, are pivotal to the de novo gene expression elicited as part of the inflammatory response. Apoptosis signal-regulating kinase 1 (ASK1) is a MAPK kinase kinase (MAP3K) that activates both the SAPKs and p38s in vivo. Here we show that TNF receptor (TNFR) associated factor 2 (TRAF2), an adapter protein that couples TNFRs to the SAPKs and p38s, can activate ASK1 in vivo and can interact in vivo with the amino- and carboxyl-terminal noncatalytic domains of the ASK1 polypeptide. Expression of the amino-terminal noncatalytic domain of ASK1 can inhibit TNF and TRAF2 activation of SAPK. TNF can stimulate the production of reactive oxygen species (ROS), and the redox-sensing enzyme thioredoxin (Trx) is an endogenous inhibitor of ASK1. We also show that expression of TRAF2 fosters the production of ROS in transfected cells. We demonstrate that Trx significantly inhibits TRAF2 activation of SAPK and blocks the ASK1-TRAF2 interaction in a reaction reversed by oxidants. Finally, the mechanism of ASK1 activation involves, in part, homo-oligomerization. We show that expression of ASK1 with TRAF2 enhances in vivo ASK1 homo-oligomerization in a manner dependent, in part, upon the TRAF2 RING effector domain and the generation of ROS. Thus, activation of ASK1 by TNF requires the ROS-mediated dissociation of Trx possibly followed by the binding of TRAF2 and consequent ASK1 homo-oligomerization.

Reconstitution of the KRAB-KAP-1 repressor complex: a model system for defining the molecular anatomy of RING-B box-coiled-coil domain-mediated protein-protein interactions

The KRAB domain is a 75 amino acid residue transcriptional repression module commonly found in eukaryotic zinc-finger proteins. KRAB-mediated gene silencing requires binding to the corepressor KAP-1. The KRAB:KAP-1 interaction requires the RING-B box-coiled coil (RBCC) domain of KAP-1, which is a widely distributed motif, hypothesized to be a protein-protein interface. Little is known about RBCC-mediated ligand binding and the role of the individual sub-domains in recognition and specificity. We have addressed these issues by reconstituting and characterizing the KRAB:KAP-1-RBCC interaction using purified components. Our results show that KRAB binding to KAP-1 is direct and specific, as the related RBCC domains from TIF1alpha and MID1 do not bind the KRAB domain. A combination of gel filtration, analytical ultracentrifugation, chemical cross-linking, non-denaturing gel electrophoresis, and site-directed mutagenesis techniques has revealed that the KAP-1-RBCC must oligomerize likely as a homo-trimer in order to bind the KRAB domain. The RING finger, B2 box, and coiled-coil region are required for oligomerization of KAP-1-RBCC and KRAB binding, as mutations in these domains concomitantly abolished these functions. KRAB domain binding stabilized the homo-oligomeric state of the KAP-1-RBCC as detected by chemical cross-linking and velocity sedimentation studies. Mutant KAP-1-RBCC molecules hetero-oligomerize with the wild-type KAP-1, but these complexes were inactive for KRAB binding, suggesting a potential dominant negative activity. Substitution of the coiled-coil region with heterologous dimerization, trimerization, or tetramerization domains failed to recapitulate KRAB domain binding. Chimeric KAP-1-RBCC proteins containing either the RING, RING-B box, or coiled-coil regions from MID1 also failed to bind the KRAB domain. The KAP-1-RBCC mediates a highly specific, direct interaction with the KRAB domain, and it appears to function as an integrated, possibly cooperative structural unit wherein each sub-domain contributes to oligomerization and/or ligand recognition. These observations provide the first principles for RBCC domain-mediated protein-protein interaction and have implications for identifying new ligands for RBCC domain proteins.