As a second messenger, cGAMP binds to the adaptor protein STING (stimulator of IFN genes) located on the endoplasmic reticulum (ER) membrane and directs the activation of transcription factor IRF-3 (IFN regulatory factor 3) through the protein kinase TBK1 (TANK-binding kinase 1) ( 7, 11, 14 – 16). For example, microbial dsDNA in the cytosol binds to and activates the enzyme cGAS (cGMP–AMP synthase), which catalyzes the synthesis of a cyclic dinucleotide, cGAMP (cyclic ) ( 9 – 13). A number of innate sensing pathways stimulate the induction of type I IFNs ( 1, 3, 7, 8). Type I IFNs, such as IFN-α and -β, are a major family of cytokines mediating antiviral immunity ( 5, 6). The innate immune system of metazoans detects molecules from viral or bacterial pathogens using pattern recognition receptors to initiate antimicrobial responses ( 1 – 4). These results suggest a concerted mechanism for the recruitment and activation of IRF-3 that can be subverted by viral proteins to evade innate immune responses. Moreover, rotavirus NSP1 (nonstructural protein 1) employs a pL xIS motif to target IRF-3 for degradation, but phosphorylation of NSP1 is not required for its activity. The structure of the IRF-3 phosphomimetic mutant S386/396E bound to the cAMP response element binding protein (CREB)-binding protein reveals that the pL xIS motif also mediates IRF-3 dimerization and activation. Here we show that the pL xIS motif of phosphorylated STING, MAVS, and TRIF binds to IRF-3 in a similar manner, whereas residues upstream of the motif confer specificity. The adaptor proteins STING (stimulator of IFN genes), MAVS (mitochondrial antiviral signaling), and TRIF (TIR domain-containing adaptor inducing IFN-β) mediate the recruitment of IRF-3 through a conserved pL xIS motif. These signaling pathways converge at the recruitment and activation of the transcription factor IRF-3 (IFN regulatory factor 3).
cGMP-AMP synthase, ritinoic acid-inducible protein 1 (RIG-I)–like receptors, and Toll-like receptors recognize microbial double-stranded (ds)DNA, dsRNA, and LPS to induce the expression of type I IFNs. Type I IFNs are key cytokines mediating innate antiviral immunity.