Identification of the minimal phosphoacceptor site required for in vivo activation of interferon regulatory factor 3 in response to virus and double-stranded RNA

The ubiquitously expressed latent interferon regulatory factor (IRF) 3 transcription factor is activated in response to virus infection by phosphorylation events that target a cluster of Ser/Thr residues, ³⁸²GGASSLEN-TVDLHISNSHPLSLTSDQY⁴⁰⁸ at the C-terminal end of the protein. To delineate the minimal phosphoacceptor sites required for IRF-3 activation, several point mutations were generated and tested for transactivation potential and cAMP-response element-binding protein-binding protein/p300 coactivator association. Expression of the IRF-3 S396D mutant alone was sufficient to induce type I IFN β, IFNα1, RANTES, and the interferon-stimulated gene 561 promoters. Using SDS-PAGE and immunoblotting with a novel phosphospecific antibody, we show for the first time that, in vivo, IRF-3 is phosphorylated on Ser³⁹⁶ following Sendai virus infection, expression of viral nucleocapsid, and double-stranded RNA treatment. These results demonstrate that Ser³⁹⁶ within the C-terminal Ser/Thr cluster is targeted in vivo for phosphorylation following virus infection and plays an essential role in IRF-3 activation. The inability of the phosphospecific antibody to detect Ser³⁹⁶ phosphorylation in lipopolysaccharide-treated cells suggests that other major pathways may be involved in IRF-3 activation following Toll-like receptor 4 stimulation.