STAT activation and differential complex formation dictate selectivity of interferon responses.

Interferons (IFNs) induce gene expression by phosphorylating latent transcription factors belonging to the signal transducer and activator of transcription (STAT) family, mediated by janus kinases (Jaks). STAT dimers directly activate genes containing the IFNgamma activation site (GAS) DNA element, with different STAT proteins displaying slightly different intrinsic DNA binding specificities. The combinatorial association of STATs with the additional DNA binding adaptor protein interferon regulatory factor (IRF)9 expands the range of enhancer elements that can be targeted by the JAK-STAT pathway to interferon-stimulated response element (ISRE) and IRF response element (IRE). Based on the amino-acid sequence similarity within the IRF family and functional overlap with the STAT family, in this paper we hypothesize that other IRF members could serve as adapter proteins for the STATs during IFN responses to redirect them to subsets of ISRE, GAS and/or IRE-containing IFN-stimulated genes (ISGs). In addition, the fact that STAT2 homodimers are not capable of binding consensus GAS sites leaves the possibility for a novel type of DNA-binding site bound by STAT2 homodimers and potentially other STAT complexes.