Trif-related adapter molecule is phosphorylated by PKCε during Toll-like receptor 4 signaling

PKCε has been shown to play a key role in the effect of the Gram-negative bacterial product LPS; however, the target for PKCε in LPS signaling is unknown. LPS signaling is mediated by Toll-like receptor 4, which uses four adapter proteins, MyD88, MyD88 adapter-like (Mal), Toll/IL-1R domain-containing adapter inducing IFN-β (Trif), and Trif-related adapter molecule (TRAM). Here we show that TRAM is transiently phosphorylated by PKCε on serine-16 in an LPS-dependent manner. Activation of IFN regulatory factor 3 and induction of the chemokine RANTES, which are both TRAM-dependent, were attenuated in PKCε-deficient cells. TRAMS16A is inactive when overexpressed and is attenuated in its ability to reconstitute signaling in TRAM-deficient cells. We have therefore uncovered a key process in Toll-like receptor 4 signaling, identifying TRAM as the target for PKCε.