The interferon-induced exonuclease, ISG20, exerts antiviral activity through upregulation of type I interferon response proteins

Type I interferon-stimulated genes (ISGs) have critical roles in inhibiting virus replication and dissemination. Despite advances in understanding the molecular basis of ISG restriction, the antiviral mechanisms of many remain unclear. The 20 kDa ISG, ISG20, is a nuclear 3''-5''exonuclease with preference for single stranded RNA (ssRNA) and has been implicated in the IFN-mediated restriction of several RNA viruses. Although the exonuclease activity of ISG20 has been shown to degrade viral RNA in vitro, evidence has yet to be presented that virus inhibition in cells requires this activity. Here, we utilized a combination of an inducible, ectopic expression system and newly generated Isg20-/- mice to investigate mechanisms and consequences of ISG20-mediated restriction. Ectopically expressed ISG20 localized primarily to Cajal bodies in the nucleus and restricted replication of chikungunya and Venezuelan equine encephalitis viruses. Although restriction by ISG20 was associated with inhibition of translation of infecting genomic RNA, degradation of viral RNAs was not observed. Instead, translation inhibition of viral RNA was associated with ISG20-induced upregulation of over 100 other genes, many of which encode known antiviral effectors. ISG20 modulated the production of IFIT1, an ISG that suppresses translation of alphavirus RNAs. Consistent with this observation, the pathogenicity of IFIT1-sensitive alphaviruses was increased in Isg20-/- mice compared to wild-type viruses, but not in ISG20 ectopic-expressing cells. Our findings establish an indirect role for ISG20 in the early restriction of RNA virus replication by regulating expressionof other ISGs that inhibit translation and possibly other activities in the replication cycle. Overall design: Two clones each of tet-inducible MEFs overexpressing eGFP (control), Isg20, and Isg20(D94G) were induced by tetracycline removal for 72 hours. rRNA was depleted with RiboMinus Eukaryote kit (Life Technologies) and prepared for Illumina directional 100bp paired-end HiSeq2000 reads.

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Weiss CM, Trobaugh DW, Sun C, Lucas TM, Diamond MS, Ryman KD, Klimstra WB