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103:245-259. ISG54, was induced; this difference was observed at both protein and mRNA levels. These studies have revealed unexpected complexity in IFN-stimulated gene induction in vivo. For the first time we showed that the two closely related genes are expressed in a tissue-specific and inducer-specific manner. Furthermore, our findings provide the first evidence of a differential pattern of expression of ISG54 and ISG56 genes by IFN- and IFN-. The interferon (IFN) system is the first line of defense against virus infection in mammalian cells (15, 44). The antiviral effects of interferons are mediated by proteins encoded by IFN-stimulated genes Tranylcypromine hydrochloride (ISGs), whose transcription is induced by the Jak-STAT pathway (9, 10, 36, 49). The binding of type I IFNs, IFN- and IFN-, to their cell surface receptor (IFNAR) leads to Jak1- and Tyk2-mediated tyrosine phosphorylation of STAT1 and STAT2, which heterodimerize, bind to IFN regulatory factor 9 (IRF-9, or p48) to form the IFN-stimulated gene factor 3 (ISGF3) and translocate to the nucleus (8, 30). Once in the nucleus, ISGF3 binds to the interferon-stimulated response element (ISRE) present in the promoter regions of all ISGs Tranylcypromine hydrochloride and activates their transcription (7, 29, 37). IRF-9 is the component of ISGF3 which recognizes ISREs, and these elements can be recognized by other members of the IRF family, most notably IRF-3 and IRF-7 (16, 42, 45). As such, signaling pathways Tranylcypromine hydrochloride which lead to the activation of other IRFs can induce transcription of ISRE-containing genes without the involvement of IFNs. These viral stress-inducible genes (VSIGs) are induced by many viruses and other infectious agents, even in the absence of functional Jak-STAT signaling (46). Among the most highly induced VSIGs IL25 antibody are the members of the ISG56 gene family. Four members of the family have been identified in humans, (ISG56/IFIT-1, ISG54/IFIT-2, ISG58/IFIT-5, and ISG60/IFIT-4), whereas in the mouse there are three members (ISG56/IFIT-1, ISG54/IFIT-2, and ISG49/IFIT-3) (5, 11, 29, 41, 57). These genes are phylogenetically related, clustered on the same chromosomes, and often coordinately induced in response to IFNs, dsRNA, or viral infection (12, 18, 27, 38, 48, 52, 56). Several partially overlapping signaling pathways involved in antiviral defense can activate IRF-3 or IRF-7, resulting in the induction of VSIG transcription. Toll-like receptor 3 (TLR3) is a receptor for dsRNA located on endosome membranes (2). Downstream of TLR3 the adaptor protein TRIF recruits the protein kinase TBK-1, which phosphorylates IRF-3, causing its dimerization and nuclear translocation (31, 33). Complete activation of IRF-3 requires its additional phosphorylation by a phosphatidylinositol 3-kinase-mediated pathway (40). Viral single-stranded RNA can bind to TLR7 or TLR8 to activate similar pathways to TLR3 (19, 53). Viral CpG DNA or glycoproteins can trigger TLR9 or TLR4, respectively, to activate similar signaling pathways (20, 22). Signaling by all of these receptors converges on TBK-1 and IRF-3/IRF-7. Several cytoplasmic dsRNA-binding proteins, such as PKR, RIG-I, and Mda-5, have also been implicated in dsRNA-mediated and antiviral signaling (43, 58, 59). Among these alternative pathways, the ones initiated by the cytoplasmic RNA helicases RIG-I and Mda-5 appear most important for induction of IRFs (3, 14, 59). They use the adaptor protein IPS-1 to recruit TBK-1 and activate IRF-3 (25). The most highly homologous proteins encoded by the ISG56 family members show only 70% sequence identity. However, they all contain multiple tetratricopeptide repeat motifs, which are degenerate protein interaction modules facilitating specific interactions with other cellular proteins (47). Human and mouse p56 and p54 inhibit initiation of translation by binding to various subunits of the translation initiation factor, eIF3, a large multisubunit protein complex with multiple functions in translation initiation (21, 34). Binding of these proteins to different subunits of eIF3 has diverse functional consequences. Human p56 and p54, both of which bind to the e subunit, block eIF3-mediated stabilization of the eIF2GTPMet-tRNA ternary complex (17, 23, 51). In contrast, mouse p56 and p54 and human p54, all of which bind to the c subunit of eIF3, block the ability of eIF3 Tranylcypromine hydrochloride to promote the formation of the 48S preinitiation complex containing the 40S ribosomal subunit, the ternary complex, eIF4F, and mRNA (24, 51, 52). The translation-inhibitory effect of human p56 has been suggested to be one of the major antiviral mechanisms used by IFNs to block the replication of.