1. Department of Biochemistry, Weill Medical College of Cornell University, New York, New York 10021, USA
2. Tri-Institutional MD-PhD Program, The Rockefeller University, New York, New York 10021, USA
3. Laboratory of Immunology, The Rockefeller University, New York, New York 10021, USA
4. Abramson Family Cancer Research Institute, Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA
5. Department of Bioimmunotherapy, The University of Texas MD Anderson Cancer Center, Box 143, 1515 Holcombe Boulevard, Houston, Texas 77030, USA
6. Istituto di Strutturistica Chimica 'Giordano Giacomello', CNR, CP 10, Monterotondo Stazione, Italy
7. Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, USA
8. Imgenex Corporation, 11185 Flintkote Ave, Suite E, San Diego, California 92121, USA
9. These authors contributed equally to this work
10. Present address: Department of Pharmacology, Yale University, New Haven, Connecticut 06520, USA.

Correspondence to: Hao Wu¹ Correspondence and requests for materials should be addressed to H.W. (e-mail: Email: haowu@med.cornell.edu). The atomic coordinates have been deposited in the Protein Data Bank under accession numbers 1LB4 (native TRAF6), 1LB5 (TRAF6–TRANCE-R complex) and 1LB6 (TRAF6–CD40 complex).

Abstract

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^{1, 2, 3, 4, 5}; 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° 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.