SARS-CoV-2 Nucleocapsid Protein Antagonizes GADD34-Mediated Innate Immune Pathway through Atypical Foci.

The integrated stress response, especially stress granules (SGs), contributes to host immunity. Typical G3BP1 ⁺ stress granules (tSGs) are usually formed after virus infection to restrain viral replication and stimulate innate immunity. Recently, several SG-like foci or atypical SGs (aSGs) with proviral function have been found during viral infection. We have shown that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) protein induces atypical N ⁺ /G3BP1 ⁺ foci (N ⁺ foci), leading to the inhibition of host immunity and facilitation of viral infection. However, the precise mechanism has not been well clarified yet. In this study, we showed that the SARS-CoV-2 N (SARS2-N) protein inhibits dsRNA-induced growth arrest and DNA damage-inducible 34 (GADD34) expression. Mechanistically, the SARS2-N protein promotes the interaction between GADD34 mRNA and G3BP1, sequestering GADD34 mRNA into the N ⁺ foci. Importantly, we found that GADD34 participates in IRF3 nuclear translocation through its KVRF motif and promotes the transcription of downstream interferon genes. The suppression of GADD34 expression by the SARS2-N protein impairs the nuclear localization of IRF3 and compromises the host's innate immune response, which facilitates viral replication. Taking these findings together, our study revealed a novel mechanism by which the SARS2-N protein antagonized the GADD34-mediated innate immune pathway via induction of N ⁺ foci. We think this is a critical strategy for viral pathogenesis and has potential therapeutic implications.