Molecular basis for unique specificity of human TRAF4 for platelets GPIbβ and GPVI

TRAF1 from a Structural Perspective

Tumor necrosis factor receptor-associated factor (TRAF) proteins play pivotal roles in a multitude of cellular signaling pathways, encompassing immune response, cell fate determination, development, and thrombosis. Their involvement in these processes hinges largely on their ability to interact directly with diverse receptors via the TRAF domain. Given the limited binding interface, understanding how specific TRAF domains engage with various receptors and how structurally similar binding interfaces of TRAF family members adapt their distinct binding partners has been the subject of extensive structural investigations over several decades. This review presents an in-depth exploration of the current insights into the structural and molecular diversity exhibited by the TRAF domain and TRAF-binding motifs across a range of receptors, with a specific focus on TRAF1.

Structure of fish TRAF4 and its implication in TRAF4-mediated immune cell and platelet signaling

Due to an increasing interest in immunity and signal transduction in teleost fish, important key signaling molecules associated with the immune response, including TRAF molecules, have been recently cloned and characterized. To better understand the role of TRAF4 in fish immune signaling and compare it with the human system, our study cloned the TRAF4 gene from the Antarctic yellowbelly rockcod Notothenia coriiceps (ncTRAF4) and purified the protein. Here, we report the first crystal structure of teleost fish TRAF4. Based on biochemical characterization, our findings elucidated the mechanisms through which signaling molecules gain cold adaptivity. Additionally, we identified a platelet receptor GPIbβ homolog in N. coriiceps (ncGPIbβ) and found that the "RRFERLFKEARRTS" region of this homolog directly binds to ncTRAF4, indicating that ncTRAF4 also recognizes the "RLXA" motif for receptor interactions and further TARF4-mediated cellular signaling. Collectively, our findings provide novel insights into the mechanisms of TRAF4-mediated immune cell and platelet signaling in fish and the structural flexibility-mediated cold adaptiveness of signaling molecules.

Signaling mechanisms of the platelet glycoprotein Ib-IX complex

The glycoprotein Ib-IX (GPIb-IX) complex mediates initial platelet adhesion to von Willebrand factor (VWF) immobilized on subendothelial matrix and endothelial surfaces, and transmits VWF binding-induced signals to stimulate platelet activation. GPIb-IX also functions as part of a mechanosensor to convert mechanical force received via VWF binding into intracellular signals, thereby greatly enhancing platelet activation. Thrombin binding to GPIb-IX initiates GPIb-IX signaling cooperatively with protease-activated receptors to synergistically stimulate the platelet response to low-dose thrombin. GPIb-IX signaling may also occur following the binding of other GPIb-IX ligands such as leukocyte integrin αMβ2 and red cell-derived semaphorin 7A, contributing to thrombo-inflammation. GPIb-IX signaling requires the interaction between the cytoplasmic domains of GPIb-IX and 14-3-3 protein and is mediated through Src family kinases, the Rho family of small GTPases, phosphoinositide 3-kinase-Akt-cGMP-mitogen-activated protein kinase, and LIM kinase 1 signaling pathways, leading to calcium mobilization, integrin activation, and granule secretion. This review summarizes the current understanding of GPIb-IX signaling.

TNF receptor agonists induce distinct receptor clusters to mediate differential agonistic activity

Monoclonal antibodies (mAb) and natural ligands targeting costimulatory tumor necrosis factor receptors (TNFR) exhibit a wide range of agonistic activities and antitumor responses. The mechanisms underlying these differential agonistic activities remain poorly understood. Here, we employ a panel of experimental and clinically-relevant molecules targeting human CD40, 4-1BB and OX40 to examine this issue. Confocal and STORM microscopy reveal that strongly agonistic reagents induce clusters characterized by small area and high receptor density. Using antibody pairs differing only in isotype we show that hIgG2 confers significantly more receptor clustering than hIgG1 across all three receptors, explaining its greater agonistic activity, with receptor clustering shielding the receptor-agonist complex from further molecular access. Nevertheless, discrete receptor clustering patterns are observed with different hIgG2 mAb, with a unique rod-shaped assembly observed with the most agonistic mAb. These findings dispel the notion that larger receptor clusters elicit greater agonism, and instead point to receptor density and subsequent super-structure as key determinants.

Structural feature of TRAFs, their related human diseases and therapeutic intervention

Several studies have been conducted over the years to unravel the structural information on the receptors that bind to tumor necrosis factor receptor-associated factor (TRAF) and the driving forces for the TRAF/receptor complex. In addition, studies have also been performed to highlight the influence of TRAF malfunctioning and mutations on the development of human disease. However, a holistic study that systematically summarizes the available information and the existing clinical trends towards development of the TRAF-targeting drugs has not been conducted to date. Herein, I reviewed existing research that focused on the structural information of various receptors recognized by the different members of the TRAF family. I also reviewed studies on the different human diseases that occur due to TRAF malfunctioning or mutations as well as the clinical trials undertaken to treat TRAF-associated diseases.

Comparison of Target Recognition by TRAF1 and TRAF2

Although TRAF1 and TRAF2 share common receptors and have extremely conserved amino acid residues, recent studies have shown that key differences in receptor binding preferences with different affinities exist, which might be important for their different functions in TRAF-mediated signal transduction. To better understand TRAF1 and TRAF2 signaling, we analyzed and compared their receptor binding-affinities. Our study revealed that TRADD, TANK, and caspase-2 bind to both TRAF1 and TRAF2 with different affinities in vitro. Sequence and structural analyses revealed that S454 on TRAF2 (corresponding to A369 of TRAF1) is critical for the binding of TRADD, and F347 on TRAF1 (corresponding to L432 of TRAF2) is a critical determinant for high affinity binding of TANK and caspase-2.

TRAF4 binds to the juxtamembrane region of EGFR directly and promotes kinase activation

The activation of the epidermal growth factor receptor (EGFR) is crucial for triggering diverse cellular functions, including cell proliferation, migration, and differentiation, and up-regulation of EGFR expression or activity is a key factor in triggering the development of cancer. Here we show that overexpression of a scaffold protein, tumor necrosis factor receptor (TNF-R)-associated factor 4 (TRAF4), promotes EGF-induced autophosphorylation of EGFR (activation) and downstream signaling, whereas TRAF4 deficiency attenuates EGFR activation and EGF-driven cell proliferation. Using structure-based sequence alignment and NMR spectroscopy, we identified a TRAF4 binding site in the C-terminal half of the juxtamembrane (JM) segment of EGFR, a region known to promote asymmetric dimerization and subsequent activation. Deletion of the TRAF4 binding site led to dramatic defects in EGFR activation and EGF-driven cell proliferation. Specific point mutations in the TRAF4 binding site also resulted in significant attenuation of EGFR activation. Detailed structural examination of the inactive versus active forms of EGFR suggests that TRAF4 binding probably induces a conformational rearrangement of the JM region to promote EGFR dimerization. These results identify a novel mechanism of TRAF4-mediated EGFR activation and signaling.

Structure of TRAF Family: Current Understanding of Receptor Recognition

Tumor necrosis factor receptor–associated factor (TRAF) proteins are key signaling molecules that function in various cellular signaling events including immune response, cell death and survival, development, and thrombosis. Their roles in cellular signaling are mediated mostly by direct interactions with various receptors via the TRAF domain. To determine how specific TRAF domains can interact with various receptors with a limited binding interface and how similar binding interfaces of TRAF family members can recognize their specific binding partners, extensive structural studies on TRAF family proteins have been conducted for several decades. In this review, we discuss the current understanding of the structural and molecular diversity of the TRAF domain and TRAF-binding motifs in many receptors according to available structural information.

In Vitro Inhibitory Mechanism Effect of TRAIP on the Function of TRAF2 Revealed by Characterization of Interaction Domains

TRAF-interacting protein (TRAIP), a negative regulator of TNF-induced-nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation, inhibits adaptor protein TRAF2 by direct interaction and is critical in apoptosis, cell proliferation, antiviral response, and embryonic development. Although the critical function of TRAIP in NF-κB signaling is well-known, the molecular inhibitory mechanism of TRAIP remains unclear. We found that the TRAIP coiled-coil domain altered its stoichiometry between dimer and trimer in a concentration-dependent manner. Additionally, the TRAIP RING domain induced even higher-ordered assembly, which was necessary for interacting with the TRAF-N domain of TRAF2 but not TRAF1. Characterization of the TRAF-N domains of TRAF1 and TRAF2, the tentative TRAIP-binding region of TRAFs, suggested the molecular basis of the inhibitory effect of TRAIP on TRAF2 in NF-κB signaling.