IFIT3 controls IFIT1 accumulation and specificity preventing self mRNA targeting during the innate immune response.

Abstract

IFIT1 is among the highest expressed proteins following virus detection and interferon (IFN) induction and binds the 5-end of non-self viral cap0-mRNAs lacking 2-O-methylation, blocking their translation. IFIT1 hetero-complexes with IFIT3, which enhances IFIT1 cap0-binding. Here, we demonstrate that IFIT3 is a master regulator of IFIT1, dictating its activity and stability. When expressed at high levels in the absence of IFIT3, IFIT1 can inhibit Semliki Forest virus replication, but can also inhibit translation of certain other self IFN-stimulated genes (ISGs), including the important innate immune proteins ISG15 and IFITM1 as exemplars. However, IFIT1:IFIT3 complexing rescues ISG15 and IFITM1 from IFIT1 translation inhibition. We demonstrate that IFIT1 is degraded by the proteasome in the absence of IFIT3, but that direct binding to IFIT3 protects IFIT1 from degradation, ensuring IFIT1 accumulation only occurs along with IFIT3, demonstrating a mechanism to control the fine balance between antiviral activity and self-targeting in the face of unrelenting viral evolution.