IκB Kinase α Phosphorylation of TRAF4 Downregulates Innate Immune Signaling

Despite their homology, I kappa B kinase alpha (IKK alpha) and IKK beta have divergent roles in NF-kappa B signaling. IKK beta strongly activates NF-kappa B while IKK alpha can downregulate NF-kappa B under certain circumstances. Given this, identifying independent substrates for these kinases could help delineate their divergent roles. Peptide substrate array technology followed by bioinformatic screening identified TRAF4 as a substrate for IKK alpha. Like IKK alpha, TRAF4 is atypical within its family because it is the only TRAF family member to negatively regulate innate immune signaling. IKK alpha's phosphorylation of serine-426 on TRAF4 was required for this negative regulation. Binding to the Crohn's disease susceptibility protein, NOD2, is required for TRAF4 phosphorylation and subsequent inhibition of NOD2 signaling. Structurally, serine-426 resides within an exaggerated beta-bulge in TRAF4 that is not present in the other TRAF proteins, and phosphorylation of this site provides a structural basis for the atypical function of TRAF4 and its atypical role in NOD2 signaling.