Molecular basis for CD40 signaling mediated by TRAF3

Tumor necrosis factor receptors (TNFR) are single transmembrane-spanning gl ycoproteins that bind cytokines and trigger multiple signal transduction pa thways. Many of these TNFRs rely on interactions with TRAF proteins that bi nd to the intracellular domain of the receptors. CD40 is a member of the TN FR family that binds to several different TRAF proteins. We have determined the crystal structure of a 20-residue fragment from the cytoplasmic: domai n of CD40 in complex with the TRAF domain of TRAF3. The CD40 fragment binds as a hairpin loop across the surface of the TRAF domain. Residues shown by mutagenesis and deletion analysis to be critical for TRAF3 binding are inv olved either in direct contact with TRAF3 or in intramolecular interactions that stabilize the hairpin. Comparison of the interactions of CD40 with TR AF3 vs. TRAF2 suggests that CD40 may assume different conformations when bo und to different TRAF family members. This molecular adaptation may influen ce binding affinity and specific cellular triggers.