IKKE negatively regulates RIG-I via direct phosphorylation

Inhibitor of nuclear factor kappa-B kinase Epsilon (IKKE) 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 IKKE is dispensable for interferon induction by viral infection. In this study, we report that IKKE 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 IKKE-deficient MEFs, we found that loss of IKKE resulted in increased cytokine production in response to the activation of cytosolic sensors. Biochemical analyses indicated that IKKE physically associated with and phosphorylated RIG-I. Mass spectrometry analysis identified that IKKE 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 IKKE modulates innate immune signaling cascades via phosphorylating the RIG-I cytosolic sensor, providing a feedback regulatory mechanism. J. Med. Virol. 88:712-718, 2016. (c) 2015 Wiley Periodicals, Inc.