Pirh2 restricts influenza A virus replication by modulating short-chain ubiquitination of its nucleoprotein.

Influenza A viruses (IAVs) rely on viral ribonucleoprotein (vRNP) complexes to control transcription and replication. Each vRNP consists of one viral genomic RNA segment associated with multiple nucleoproteins (NP) and a trimeric IAV RNA polymerase complex. Previous studies showed that post-translational modifications of vRNP components, such as NP, by host factors would in turn affect the IAV life cycle or modulate host anti-viral response. In this study, we found host E3 ubiquitin ligase Pirh2 interacted with NP and mediated short-chain ubiquitination of NP at lysine 351, which suppressed NP-PB2 interaction and vRNP formation. In addition, we showed that knockdown of Pirh2 promoted IAV replication, whereas overexpression of Pirh2 inhibited IAV replication. However, Pirh2-ΔRING lacking E3 ligase activity failed to inhibit IAV infection. Moreover, we showed that Pirh2 had no effect on the replication of a rescued virus, WSN-K351R, carrying lysine-to-arginine substitution at residue 351. Interestingly, by analyzing human and avian IAVs from 2011 to 2020 in influenza research databases, we found that 99.18% of 26 977 human IAVs encode lysine, but 95.3% of 9956 avian IAVs encode arginine at residue 351 of NP protein. Consistently, knockdown of Pirh2 failed to promote propagation of two avian-like influenza viruses, H9N2-W1 and H9N2-C1, which naturally encode arginine at residue 351 of NP. Taken together, we demonstrated that Pirh2 is a host factor restricting IAV infection by modulating short-chain ubiquitination of NP. Meanwhile, it is noteworthy that residue 351 of NP targeted by Pirh2 may associate with the evasion of human anti-viral response against avian-like influenza viruses.