The NMR structure of the TRADD death domain, a key protein in the TNF signaling pathway

Abstract

Tumor necrosis factor (TNF) is a member of a superfamily of cytokines that consists of more than 20 structurally related transmembrane and soluble proteins, which play a crucial role in various biological events by recruiting several intracellular adaptors, thus activating multiple signal transduction pathways (Wajant et al. 2003). TNF-a is a homotrimer produced by activated macrophages, and signals through two distinct cell surface receptor subgroups, TNFR1 and TNFR2. The cytoplasmic domain of these two receptors initiate intracellular events, with TNFR1 being the dominant receptor for TNF. The binding of TNF-a to TNFR1 triggers a series of intracellular events, involving many proteins which will recruit key enzymes to TNFR1 that are responsible for initiating signaling events, which leads to either apoptosis or activation of two transcription factors, NF-kB and AP-1. The cytoplasmic region of TNFR1 contains the death domain (DD) sequence, which forms a trimer upon formation of the TNF complex with TNFR1, recruiting other DD containing proteins such as TRADD (TNF receptor-associated DD), the adaptor protein RIP (receptor-interacting protein), and FADD (Fasassociated DD). These protein interactions occur mostly through their common domain, the DD, comprised of *90 amino acids, with low (*30%) sequence homology. The DD superfamily is comprised of several subfamilies: DD, caspase recruitment domain (CARD), death effector domain (DED) and pyrin domain (PYR). So far, about 86 proteins have been identified in humans (Park et al. 2007). The adaptor protein TRADD is a multifunctional 34 kDa protein that is recruited to TNFR1 upon ligand binding. It contains two separate domains, allowing it to bind to several protein partners and participate in different signaling pathways (Hsu et al. 1996). The C-terminal of TRADD contains a DD which is a central component in signaling for TNFR1 stimulation (Hsu et al. 1996). The DD of TRADD is recruited to TNFR1 through homotypic DD interactions, which then promotes subsequent binding to all other signal transducers. TRADD DD can associate to the death domains of TNFR1 and RIP (Hsu et al. 1996), while the N-terminal of TRADD interacts with TRAFs (TNF receptor associated factors), leading to NF-kB and MAP kinase activation. The signaling events generated from TNFR1 upon ligand binding ultimately result in the inflammatory and immunological responses. These responses contribute to both physiological homeostasis and can lead to inflammatory diseases when TNF-a is overproduced. While the role of TNF-a in inflammatory diseases such as rheumatoid arthritis is clinically validated by several anti-TNF therapies, the question of how it triggers these specific signaling responses from TNFR1 by the complex array of signaling proteins remains to be fully understood. Many investigations have focused on understanding the mechanism by which these protein–protein interactions occur, including structural studies of proteins in the TNFR1 pathway. The three dimensional structure of the FAS DD (Huang et al. 1996), FADD DD (Carrington et al. 2006), TNF DD (Sukits et al. D. H. H. Tsao (&) W.-T. Hum K. Malakian Structural Biology and Computational Chemistry, Chemical and Screening Sciences, Wyeth Research, 200 CambridgePark Drive, Cambridge, MA 02140, USA e-mail: dtsao@wyeth.com