Grouper TRAF5 exerts negative regulation on antiviral immune response against iridovirus

Tumor necrosis factor receptor-associated factor 5 (TRAF5) is an intracellular protein that binds to the cytoplasmic portion of tumor necrosis factor receptors and mediates the activation of downstream nuclear factor-kappa B (NF-κB), interferon regulatory factor 3, and mitogen activated protein kinase signaling pathways. Compared with other TRAF proteins, TRAF5 is largely unknown in teleosts. In the present study, a TRAF5 homologue (HgTRAF5) from the hybrid grouper (Epinephelus fuscoguttatus♂ × Epinephelus lanceolatus♀) was cloned and characterized. The open reading frame of HgTRAF5 consists of 1743 nucleotides encoding a 581 amino acid protein with a predicted molecular mass of 64.90 kDa. Similar to its mammalian counterpart, HgTRAF5 contains an N-terminal RING finger domain, a zinc finger domain, and a C-terminal TRAF domain, including a coiled-coil domain and a MATH domain. HgTRAF5 shares 99.83% identity with giant grouper (Epinephelus lanceolatus) TRAF5. Quantitative real-time PCR analysis indicated that HgTRAF5 mRNA was broadly expressed in all examined tissues. The expression of HgTRAF5 increased after Singapore grouper iridovirus (SGIV) infection in grouper spleen (GS) cells. Intracellular localization analysis demonstrated that the full-length HgTRAF5 protein mainly distributed in the cytoplasm. HgTRAF5 overexpression also promoted SGIV replication during viral infection in vitro. HgTRAF5 significantly promoted the activities of interferon-β, interferon-sensitive response element, and NF-κB. Taken together, these results are important for a better understanding of the function of TRAF5 in fish and reveal its involvement in the host response to immune challenge by SGIV.

Structural and biochemical characterization of TRAF5 from Notothenia coriiceps and its implications in fish immune cell signaling

Conserved immune cell signaling in fish was recently highlighted by the identification of various immune cell signaling molecules. Tumor necrosis factor (TNF) receptor-associated factor (TRAF) proteins are critical adaptor molecules in immune cell signaling and contain E3 ubiquitin ligase activity. Here, we report the first crystal structure of the TRAF5 TRAF domain from the black rockcod (Notothenia coriiceps; ncTRAF5). Our structure revealed both similarities and differences with mammalian TRAF5. Structural and biochemical analyses indicated that ncTRAF5 forms a functional trimer unit in solution, with a structural flexibility that might be critical for imparting resistance to cold temperature-induced stress. We also found conserved surface residues on ncTRAF5 that might be critical binding hot spots for interaction with various receptors.

Expression and functional analysis of tumor necrosis factor receptor (TNFR)-associated factor 5 from Nile tilapia, Oreochromis niloticus

Nile tilapia (Oreochromis niloticus) is a pivotal economic fish that has been plagued by Streptococcus infections. Tumor necrosis factor receptor-associated factor 5 (TRAF5) is a crucial adaptor molecule, which can trigger downstream signaling cascades involved in immune pathway. In this study, Nile tilapia TRAF5 coding sequence (named OnTRAF5) was obtained, which contained typical functional domains, such as RING, zinc finger, coiled-coil and MATH domain. Different from other TRAF molecules, OnTRAF5 had shown relatively low identify with its homolog, and it was clustered into other teleost TRAF5 proteins. qRT-PCR was used to analysis the expression level of OnTRAF5 in gill, skin, muscle, head kidney, heart, intestine, thymus, liver, spleen and brain, In healthy Nile tilapia, the expression level of OnTRAF5 in intestine, gill and spleen were significantly higher than other tissues. While under Streptococcus agalactiae infection, the expression level of OnTRAF5 was improved significantly in all detected organs. Additionally, over-expression WT OnTRAF5 activated NF-κB, deletion of RING or zinc finger caused the activity impaired. In conclusion, OnTRAF5 participate in anti-bacteria immune response and is crucial for the signaling transduction.

Molecular characteristics and function study of TNF receptor-associated factor 5 from grouper (Epinephelus coioides)

Tumor necrosis factor receptor-associated factor 5 (TRAF5) is a key adapter molecule that participates in numerous signaling pathways. The function of TRAF5 in fish is largely unknown. In the present study, a TRAF5 cDNA sequence (EcTRAF5) was identified in grouper (Epinephelus coioides). Similar to its mammalian counterpart, EcTRAF5 contained an N-terminal RING finger domain, a zinc finger domain, a C-terminal TRAF domain, including a coiled-coil domain and a MATH domain. The EcTRAF5 protein shared relatively low sequence identity with that of other species, but clustered with TRAF5 sequences from other fish. Real-time PCR analysis revealed that EcTRAF5 mRNA was broadly expressed in numerous tissues, with relatively high expression in skin, hindgut, and head kidney. Additionally, the expression of EcTRAF5 was up-regulated in gills and head kidney after infection with Cryptocaryon irritans. Intracellular localization analysis demonstrated that the full-length EcTRAF5 protein was uniformly distributed in the cytoplasm; while a deletion mutant of the coiled-coil domain of EcTRAF5 was observed uniformly distributed in the cytoplasm and the nucleus. After exogenous expression in HEK293T cells, TRAF5 significantly activated NF-κB. The deletion of the EcTRAF5 RING domain or of the zinc finger domain dramatically impaired its ability to activate NF-κB, implying that the RING domain and the zinc finger domain are required for EcTRAF5 signaling.

Comparison of the peptide binding preferences of three closely related TRAF paralogs: TRAF2, TRAF3, and TRAF5

Tumor necrosis factor receptor-associated factors (TRAFs) constitute a family of adapter proteins that act in numerous signaling pathways important in human biology and disease. The MATH domain of TRAF proteins binds peptides found in the cytoplasmic domains of signaling receptors, thereby connecting extracellular signals to downstream effectors. Beyond several very general motifs, the peptide binding preferences of TRAFs have not been extensively characterized, and differences between the binding preferences of TRAF paralogs are poorly understood. Here we report a screening system that we established to explore TRAF peptide-binding specificity using deep mutational scanning of TRAF-peptide ligands. We displayed single- and double-mutant peptide libraries based on the TRAF-binding sites of CD40 or TANK on the surface of Escherichia coli and screened them for binding to TRAF2, TRAF3, and TRAF5. Enrichment analysis of the library sequencing results showed differences in the permitted substitution patterns in the TANK versus CD40 backgrounds. The three TRAF proteins also demonstrated different preferences for binding to members of the CD40 library, and three peptides from that library that were analyzed individually showed striking differences in affinity for the three TRAFs. These results illustrate a previously unappreciated level of binding specificity between these close paralogs and demonstrate that established motifs are overly simplistic. The results from this work begin to outline differences between TRAF family members, and the experimental approach established herein will enable future efforts to investigate and redesign TRAF peptide-binding specificity.

Physiological roles and mechanisms of signaling by TRAF2 and TRAF5

RAF2 and TRAF5 are closely related members of the TRAF family of proteins. They are important signal transducers for a wide range of TNF receptor superfamily members, including TNFR1, TNFR2, CD40 and other lymphocyte costimulatory receptors, RANK/TRANCE-R, EDAR, LTbetaR, LMP-1 and IRE1. TRAF2 andTRAF5 therefore regulate diverse physiological roles, ranging from T and B cell signaling and inflammatory responses to organogenesis and cell survival. The major pathways mediated by TRAF2 and TRAF5 are the classical and alternative pathways of NF-kappaB activation, and MAPK and JNK activation. TRAF2 is heavily regulated by ubiquitin signals, and many of the signaling functions of TRAF2 are mediated through its RING domain and likely its own role as an E3 ubiquitin ligase.

Inhibition of RANKL-mediated osteoclast differentiation by selective TRAF6 decoy peptides

RANK and RANKL are essential mediators of osteoclastogenesis. RANK interacts with members of the tumor necrosis factor receptor-associated factor (TRAF) family, of which TRAF6 is the critical signaling molecule. We identified a unique TRAF6-binding motif in RANK, which was subsequently co-crystallized with TRAF6 revealing distinct molecular interactions. A cell-permeable TRAF6 decoy peptide (T6DP) was shown to specifically target TRAF6 and inhibit RANKL-mediated signaling. In this study, we identified a core motif for binding to TRAF6 by generating a series of deletion mutants linked via palmitate as a means to internalize the peptide, thus making a smaller scaffold for intracellular delivery. The core motif of RKIPTEDEY inhibited RANKL-mediated osteoclastogenesis and bone resorption. In contrast, TRAF2/5 decoy peptides appeared to have no affect. Thus, disruption of the RANK-TRAF6 interaction may prove useful as a novel target for the development of a small molecule therapeutic agent for the treatment of bone-related diseases.

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.