N-terminal acetylation of the influenza ribonuclease PA-X promotes nuclear localization and host shutoff activity in a multifaceted manner

Abstract

To counteract host antiviral responses, influenza A virus triggers a global reduction of cellular gene expression, a process termed "host shutoff." A key effector of influenza A virus host shutoff is the viral endoribonuclease PA-X, which degrades host mRNAs. While many of the molecular determinants of PA-X activity remain unknown, a previous study found that N-terminal acetylation of PA-X is required for its host shutoff activity. However, it remains unclear how this co-translational modification promotes PA-X activity. Here, we report that PA-X N-terminal acetylation has two functions that can be separated based on whether acetylation occurs on the first amino acid, the initiator methionine, or the second amino acid following initiator methionine excision. Modification at either site is sufficient to ensure PA-X localization to the nucleus. However, modification of the second amino acid is not sufficient for host shutoff activity of ectopically expressed PA-X, which specifically requires N-terminal acetylation of the initiator methionine. Interestingly, during infection N-terminal acetylation of PA-X at any position results in host shutoff activity, suggesting that additional factors during infection can augment the host shutoff activity of PA-X. Our studies thus uncover a multifaceted role for PA-X N-terminal acetylation in regulation of this important immunomodulatory factor. ImportanceInfluenza A viruses pose a significant threat to human health in the form of seasonal epidemics and recurrent pandemics, leading to large burdens on the healthcare system. During infection, host immune and inflammatory responses have a key role in disease outcome. They are needed to clear the virus, but when excessive they can cause lung damage. Influenza A viruses encode several factors that modulate the host immune response, influencing viral replication and pathogenesis. In this study, we focused on one of these influenza factors, PA-X, which destroys cellular mRNAs and reduces cellular gene expression (a phenomenon called "host shutoff") to control immune responses. This protein is modified with an acetylation at its N-terminus, and this modification is needed for its activity, but it has remained unclear why. Our results demonstrate that N-terminal acetylation of PA-X is needed for multiple aspects of its function. It ensures that PA-X goes to the nucleus, where it accesses its RNA targets, but it also separately contributes to host shutoff activity. However, for host shutoff activity, the specific location of the modification matters, whereas for entry in the nucleus it does not. For full activity, the modification must be placed on the very first amino acid of the protein, and this amino acid must be methionine. These findings uncover how influenza A viruses exploit a widespread protein modification to support the host shutoff activity of one of its important immunomodulatory proteins.