IKKϵ negatively regulates RIG-I via direct phosphorylation

Inhibitor of nuclear factor kappa-B kinase Epsilon (IKKϵ) is an IKK-related kinase. Despite it was originally discovered as a kinase functionally related to TBK-1, studies entailing gene knockout mouse demonstrated that IKKϵ is dispensable for interferon induction by viral infection. In this study, we report that IKKϵ directly phosphorylates a key serine residue within the RNA-binding domain of RIG-I (retinoic acid-inducible gene 1) to inhibit RIG-I-mediate innate immune signaling. Using IKKϵ-deficient MEFs, we found that loss of IKKϵ resulted in increased cytokine production in response to the activation of cytosolic sensors. Biochemical analyses indicated that IKKϵ physically associated with and phosphorylated RIG-I. Mass spectrometry analysis identified that IKKϵ phosphorylated the serine 855 of the RNA-binding pocket of RIG-I carboxyl terminal domain, a residues known to impinge on RNA-binding via phosphorylation. Our findings collectively support the conclusion that IKKϵ modulates innate immune signaling cascades via phosphorylating the RIG-I cytosolic sensor, providing a feedback regulatory mechanism.