CNOT4-Mediated Ubiquitination of Influenza A Virus Nucleoprotein Promotes Viral RNA Replication.

Influenza A virus (IAV) RNA segments are individually packaged with viral nucleoprotein (NP) and RNA polymerases to form a viral ribonucleoprotein (vRNP) complex. We previously reported that NP is a monoubiquitinated protein which can be deubiquitinated by a cellular ubiquitin protease, USP11. In this study, we identified an E3 ubiquitin ligase, CNOT4 (Ccr4-Not transcription complex subunit 4), which can ubiquitinate NP. We found that the levels of viral RNA, protein, viral particles, and RNA polymerase activity in CNOT4 knockdown cells were lower than those in the control cells upon IAV infection. Conversely, overexpression of CNOT4 rescued viral RNP activity. In addition, CNOT4 interacted with the NP in the cell. An in vitro ubiquitination assay also showed that NP could be ubiquitinated by in vitro -translated CNOT4, but ubiquitination did not affect the protein stability of NP. Significantly, CNOT4 increased NP ubiquitination, whereas USP11 decreased it. Mass spectrometry analysis of ubiquitinated NP revealed multiple ubiquitination sites on the various lysine residues of NP. Three of these, K184, K227, and K273, are located on the RNA-binding groove of NP. Mutations of these sites to arginine reduced viral RNA replication. These results indicate that CNOT4 is a ubiquitin ligase of NP, and ubiquitination of NP plays a positive role in viral RNA replication. IMPORTANCE Influenza virus, particularly influenza A virus, causes severe and frequent outbreaks among human and avian species. Finding potential target sites for antiviral agents is of utmost importance from the public health point of view. We previously found that viral nucleoprotein (NP) is ubiquitinated, and ubiquitination enhances viral RNA replication. In this study, we found a cellular ubiquitin ligase, CNOT4, capable of ubiquitinating NP. The ubiquitination sites are scattered on the surface of the NP molecule, which is critical for RNA replication. CNOT4 and a ubiquitin protease, USP11, together regulate the extent of NP ubiquitination and thereby the efficiency of RNA replication. This study thus identifies a potential antiviral target site and reveals a novel posttranslational mechanism for regulating viral replication. This represents a novel finding in the literature of influenza virus research.