Ubiquitination Upregulates Influenza Virus Polymerase Function

The influenza A virus polymerase plays an essential role in the virus life cycle, directing synthesis of viral mRNAs and genomes. It is a trimeric complex composed of subunits PA, PB1, and PB2 and associates with viral RNAs and nucleoprotein (NP) to form higher-order ribonucleoprotein (RNP) complexes. The polymerase is regulated temporally over the course of infection to ensure coordinated expression of viral genes as well as replication of the viral genome. Various host factors and processes have been implicated in regulation of the IAV polymerase function, including posttranslational modifications; however, the mechanisms are not fully understood. Here we demonstrate that ubiquitination plays an important role in stimulating polymerase activity. We show that all protein subunits in the RNP are ubiquitinated, but ubiquitination does not significantly alter protein levels. Instead, ubiquitination and an active proteasome enhance polymerase activity. Expression of ubiquitin upregulates polymerase function in a dose-dependent fashion, causing increased accumulation of viral RNA (vRNA), cRNA, and mRNA and enhanced viral gene expression during infection. Ubiquitin expression directly affects polymerase activity independent of nucleoprotein (NP) or ribonucleoprotein (RNP) assembly. Ubiquitination and the ubiquitin-proteasome pathway play key roles during multiple stages of influenza virus infection, and data presented here now demonstrate that these processes modulate viral polymerase activity independent of protein degradation. IMPORTANCE The cellular ubiquitin-proteasome pathway impacts steps during the entire influenza virus life cycle. Ubiquitination suppresses replication by targeting viral proteins for degradation and stimulating innate antiviral signaling pathways. Ubiquitination also enhances replication by facilitating viral entry and virion disassembly. We identify here an addition proviral role of the ubiquitin-proteasome system, showing that all of the proteins in the viral replication machinery are subject to ubiquitination and this is crucial for optimal viral polymerase activity. Manipulation of the ubiquitin machinery for therapeutic benefit is therefore likely to disrupt the function of multiple viral proteins at stages throughout the course of infection.