N-terminal acetylation separately promotes nuclear localization and host shutoff activity of the influenza A virus ribonuclease PA-X

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 identified a requirement for N-terminal acetylation of PA-X for its host shutoff activity, but did not address how this modification promotes activity. Here, we report that PA-X N-terminal acetylation has two functions that can be separated based on the position of the acetylation, on the first amino acid, i.e. the initiator methionine, vs. the second amino acid following initiator methionine excision. Modification at either site is sufficient to ensure correct PA-X localization to the nucleus. However, modification at the second amino acid is not sufficient for host shutoff activity of ectopically expressed PA-X, which requires N-terminal acetylation of the initiator methionine specifically. Interestingly, during infection, N-terminal acetylation of PA-X at any position results in PA-X host shutoff activity, and this is, in part, due to a functional interaction with the influenza protein NS1. This reveals an unexpected role for other viral proteins in PA-X activity. Our studies highlight the multifaceted nature of PA-X N-terminal acetylation in its ability to regulate the host shutoff activity of PA-X through multiple avenues.

Influenza A Virus and Acetylation: The Picture Is Becoming Clearer

“…Finally, NS1, NS2 (NEP), PB1 [ 74 ], and PA-X [ 67 ] are acetylated at the N-terminus ( Table 3 ). The acetylation of IAV proteins promotes their stability [ 80 ] and function [ 67 , 76 , 77 , 78 , 79 , 81 ] and, consequently, IAV replication [ 67 , 77 , 78 ]. Specifically, lysine acetylation promotes the interferon-antagonizing function of NS1 [ 78 ] and stability [ 80 ], as well as the endonuclease activity [ 79 ] of PA.…”

“…Specifically, lysine acetylation promotes the interferon-antagonizing function of NS1 [ 78 ] and stability [ 80 ], as well as the endonuclease activity [ 79 ] of PA. Furthermore, N-terminal acetylation promotes the nuclear import as well as host shutoff activity of PA-X [ 67 , 81 ]. In addition, many host proteins, e.g., actin, eIF4GI, exportin, importin, mTOR (mammalian target of rapamycin), NHP2L1, nucleophosmin, SF3B2, and tubulin, are also hyper-acetylated at the lysine or serine residues or at the N-terminus in IAV-infected cells [ 50 , 55 , 74 ].…”