Mechanism of endogenous regulation of the type I interferon response by suppressor of IκB kinase epsilon (SIKE), a novel substrate of TANK-binding kinase 1 (TBK1).

TANK-binding kinase 1 (TBK1) serves as a key convergence point in multiple innate immune signaling pathways. In response to receptor-mediated pathogen detection, TBK1 phosphorylation promotes production of pro-inflammatory cytokines and type I interferons. Increasingly, TBK1 dysregulation has been linked to autoimmune disorders and cancers, heightening the need to understand the regulatory controls of TBK1 activity. Here, we describe the mechanism by which suppressor of IKKε (SIKE) inhibits TBK1-mediated phosphorylation of interferon regulatory factor 3 (IRF3), which is essential to type I interferon production. Kinetic analyses showed that SIKE not only inhibits IRF3 phosphorylation but is also a high affinity TBK1 substrate. With respect to IRF3 phosphorylation, SIKE functioned as a mixed-type inhibitor (K(i, app) = 350 nM) rather than, given its status as a TBK1 substrate, as a competitive inhibitor. TBK1 phosphorylation of IRF3 and SIKE displayed negative cooperativity. Both substrates shared a similar Km value at low substrate concentrations (∼50 nM) but deviated >8-fold at higher substrate concentrations (IRF3 = 3.5 μM; SIKE = 0.4 μM). TBK1-SIKE interactions were modulated by SIKE phosphorylation, clustered in the C-terminal portion of SIKE (Ser-133, -185, -187, -188, -190, and -198). These sites exhibited striking homology to the phosphorylation motif of IRF3. Mutagenic probing revealed that phosphorylation of Ser-185 controlled TBK1-SIKE interactions. Taken together, our studies demonstrate for the first time that SIKE functions as a TBK1 substrate and inhibits TBK1-mediated IRF3 phosphorylation by forming a high affinity TBK1-SIKE complex. These findings provide key insights into the endogenous control of a critical catalytic hub that is achieved not by direct repression of activity but by redirection of catalysis through substrate affinity.