Involvement of the ubiquitin-like domain of TBK1/IKK-i kinases in regulation of IFN-inducible genes

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Abstract
TANK‐binding kinase 1 (TBK1/NAK/T2K) and I‐κB Kinase (IKK‐ i /IKK‐ε) play important roles in the regulation of interferon (IFN)‐inducible genes during the immune response to bacterial and viral infections. Cell stimulation with ssRNA virus, dsDNA virus or gram‐negative bacteria leads to activation of TBK1 or IKK‐ i , which in turn phosphorylates the transcription factors, IFN‐regulatory factor (IRF) 3 and IRF7, promoting their translocation in the nucleus. To understand the molecular basis of activation of TBK1, we analyzed the sequence of TBK1 and IKK‐ i and identified a ubiquitin‐like domain (ULD) adjacent to their kinase domains. Deletion or mutations of the ULD in TBK1 or IKK‐ i impaired activation of respective kinases, failed to induce IRF3 phosphorylation and nuclear localization and to activate IFN‐β or RANTES promoters. The importance of the ULD of TBK1 in LPS‐ or poly(I:C)‐stimulated IFN‐β production was demonstrated by reconstitution experiments in TBK1‐IKK‐ i ‐deficient cells. We propose that the ULD is a regulatory component of the TBK1/IKK‐ i kinases involved in the control of the kinase activation, substrate presentation and downstream signaling pathways.