Regulation of PKR-dependent RNA translation inhibition by TRIM21 upon virus infection or other stress

The host always employs various ways to defend against viral infection and spread. However, viruses have evolved their own effective strategies, such as inhibition of RNA translation of the antiviral effectors, to destroy the host's defense barriers. Protein synthesis, commonly controlled by the alpha-subunit of eukaryotic translation initiation factor 2 (eIF2 alpha), is a basic cellular biological process among all species. In response to viral infection, in addition to inducing the transcription of antiviral cytokines by innate immunity, infected cells also inhibit the RNA translation of antiviral factors by activating the protein kinase R (PKR)-eIF2 alpha signaling pathway. Regulation of innate immunity has been well studied; however, regulation of the PKR-eIF2 alpha signaling pathway remains unclear. In this study, we found that the E3 ligase TRIM21 negatively regulates the PKR-eIF2 alpha signaling pathway. Mechanistically, TRIM21 interacts with the PKR phosphatase PP1 alpha and promotes K6-linked polyubiquitination of PP1 alpha. Ubiquitinated PP1 alpha augments its interaction with PKR, causing PKR dephosphorylation and subsequent translational inhibition release. Furthermore, TRIM21 can constitutively restrict viral infection by reversing PKR-dependent translational inhibition of various previously known and unknown antiviral factors. Our study highlights a previously undiscovered role of TRIM21 in regulating translation, which will provide new insights into the host antiviral response and novel targets for the treatment of translation-associated diseases in the clinic. Author summaryTranscriptional induction of antiviral cytokines, especially interferons (IFNs), is a hallmark of innate immunity upon viral infection. In addition, viral infection can inhibit RNA translation of the antiviral factors by activating the PKR-eIF2 alpha axis. Regulation of the innate immune signaling pathway has been well studied. However, regulation of the PKR signaling pathway remains unclear. Our previous study demonstrated that TRIM21, an E3 ligase, positively regulates the innate immune response by catalyzing K27-linked ubiquitination of MAVS. However, whether TRIM21 regulates the PKR signaling pathway is unknown.In this study, we found that TRIM21 can negatively regulate PKR activation by promoting K6-linked ubiquitination of the PKR phosphatase PP1 alpha, contributing to the enhancement of the PKR-PP1 alpha interaction, which leads to dephosphorylation of PKR. Moreover, our study identified various previously known and unknown antiviral factors regulated by the TRIM21-PKR axis. Our study highlights TRIM21-mediated PKR-dependent RNA translation in the antiviral response, which may provide a target for the treatment of translation-associated diseases in the clinic.