Viral targeting of DEAD box protein 3 reveals its role in TBK1/IKKepsilon-mediated IRF activation

Viruses are detected by different classes of pattern recog-nition receptors (PRRs), such as Toll-like receptors and RIG-like helicases. Engagement of PRRs leads to activation of interferon (IFN)-regulatory factor 3 (IRF3) and IRF7 through IKKe and TBK1 and consequently IFN-b induction. Vaccinia virus (VACV) encodes proteins that manipulate host signalling, sometimes by targeting uncharacterised proteins. Here, we describe a novel VACV protein, K7, which can inhibit PRR-induced IFN-b induction by pre-venting TBK1/IKKe-mediated IRF activation. We identified DEAD box protein 3 (DDX3) as a host target of K7. Expression of DDX3 enhanced Ifnb promoter induction by TBK1/IKKe, whereas knockdown of DDX3 inhibited this, and virus- or dsRNA-induced IRF3 activation. Further, dominant-negative DDX3 inhibited virus-, dsRNA- and cytosolic DNA-stimulated Ccl5 promoter induction, which is also TBK1/IKKe dependent. Both K7 binding and enhancement of Ifnb induction mapped to the N-terminus of DDX3. Furthermore, virus infection induced an associa-tion between DDX3 and IKKe. Therefore, this study shows for the first time the involvement of a DEAD box helicase in TBK1/IKKe-mediated IRF activation and Ifnb promoter induction