Mitochondrial antiviral-signaling protein (MAVS) transmits signals from RIG-I-like receptors after RNA virus infections. However, the mechanism by which MAVS activates downstream components, such as TBK1 and IKKα/β, is unclear, although previous work suggests the involvement of NEMO or TBK1-binding proteins TANK, NAP1, and SINTBAD. Here, we report that MAVS-mediated innate immune activation is dependent on TRAFs, partially on NEMO, but not on TBK1-binding proteins. MAVS recruited TBK1/IKKε by TRAFs that were pre-associated with TBK1/IKKε via direct interaction between the coiled-coil domain of TRAFs and the SDD domain of TBK1/IKKε. TRAF2−/−3−/−5−/−6−/− cells completely lost RNA virus responses. TRAFs’ E3 ligase activity was required for NEMO activation by synthesizing ubiquitin chains that bound to NEMO for NF-κB and TBK1/IKKε activation. NEMO-activated IKKα/β were important for TBK1/IKKε activation through IKKα/β-mediated TBK1/IKKε phosphorylation. Moreover, individual TRAFs differently mediated TBK1/IKKε activation and thus fine-tuned antiviral immunity under physiological conditions.
Cytosolic dsDNA activates the cyclic GMP-AMP synthase (cGAS)-stimulator of IFN genes (STING) pathway to produce cytokines, including type I IFNs. The roles of many critical proteins, including NEMO, IKKβ, and TBK1, in this pathway are unclear because of the lack of an appropriate system to study. In this article, we report that lower FBS concentrations in culture medium conferred high sensitivities to dsDNA in otherwise unresponsive cells, whereas higher FBS levels abrogated this sensitivity. Based on this finding, we demonstrated genetically that NEMO was critically involved in the cGAS-STING pathway. Cytosolic DNA activated TRIM32 and TRIM56 to synthesize ubiquitin chains that bound NEMO and subsequently activated IKKβ. Activated IKKβ, but not IKKα, was required for TBK1 and NF-κB activation. In contrast, TBK1 was reciprocally required for NF-κB activation, probably by directly phosphorylating IKKβ. Thus, our findings identified a unique innate immune activation cascade in which TBK1-IKKβ formed a positive feedback loop to assure robust cytokine production during cGAS-STING activation.
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