TRIM41-Mediated Ubiquitination of Nucleoprotein Limits Influenza A Virus Infection.

Influenza A virus (IAV) is a highly transmissible respiratory pathogen and a major cause of morbidity and mortality around the world. Nucleoprotein (NP) is an abundant IAV protein essential for multiple steps of the viral life cycle. Our recent proteomic study of the IAV-host interaction network found that TRIM41 (tripartite motif-containing 41), a ubiquitin E3 ligase, interacted with NP. However, the role of TRIM41 in IAV infection is unknown. Here, we report that TRIM41 interacts with NP through its SPRY domain. Furthermore, TRIM41 is constitutively expressed in lung epithelial cells, and overexpression of TRIM41 inhibits IAV infection. Conversely, RNA interference (RNAi) and knockout of TRIM41 increase host susceptibility to IAV infection. As a ubiquitin E3 ligase, TRIM41 ubiquitinates NP in vitro and in cells. The TRIM41 mutant lacking E3 ligase activity fails to inhibit IAV infection, suggesting that the E3 ligase activity is indispensable for TRIM41 antiviral function. Mechanistic analysis further revealed that the polyubiquitination leads to NP protein degradation and viral inhibition. Taking these observations together, TRIM41 is a constitutively expressed intrinsic IAV restriction factor that targets NP for ubiquitination and protein degradation. IMPORTANCE Influenza control strategies rely on annual immunization and require frequent updates of the vaccine, which is not always a foolproof process. Furthermore, the current antivirals are also losing effectiveness as new viral strains are often refractory to conventional treatments. Thus, there is an urgent need to find new antiviral mechanisms and develop therapeutic drugs based on these mechanisms. Targeting the virus-host interface is an emerging new strategy because host factors controlling viral replication activity will be ideal candidates, and cellular proteins are less likely to mutate under drug-mediated selective pressure. Here, we show that the ubiquitin E3 ligase TRIM41 is an intrinsic host restriction factor to IAV. TRIM41 directly binds the viral nucleoprotein and targets it for ubiquitination and proteasomal degradation, thereby limiting viral infection. Exploitation of this natural defense pathway may open new avenues to develop antiviral drugs targeting the influenza virus.