Crystallographic analysis of CD40 recognition and signaling by human TRAF2

Tumor necrosis factor receptor superfamily members convey signals that promote diverse cellular responses. Receptor trimerization by extracellular ligands initiates signaling by recruiting members of the tumor necrosis factor receptor-associated factor (TRAF) family of adapter proteins to the receptor cytoplasmic domains. We report the 2.4-A crystal structure of a 22-kDa, receptor-binding fragment of TRAF2 complexed with a functionally defined peptide from the cytoplasmic domain of the CD40 receptor. TRAF2 forms a mushroom-shaped trimer consisting of a coiled coil and a unique beta-sandwich domain. Both domains mediate trimerization. The CD40 peptide binds in an extended conformation with every side chain in contact with a complementary groove on the rim of each TRAF monomer. The spacing between the CD40 binding sites on TRAF2 supports an elegant signaling mechanism in which trimeric, extracellular ligands preorganize the receptors to simultaneously recognize three sites on the TRAF trimer.