The role of unanchored polyubiquitin chains and the TRIM E3-ubiquitin ligase family of proteins in the innate immune response to influenza virus infection (INM3P.402)

Intracellular innate immune responses are essential to protect host cells against pathogens. Upon infection, pattern recognition receptors detect incoming microbes and trigger downstream signaling pathways leading to production of antiviral type-I interferons (IFNs).These signaling pathways are regulated by different posttranslational modifications including the ubiquitin (Ub) system. Proteins covalently attached to K48-linked polyUb are targeted for degradation by the proteasome, whereas protein modification with K63-linked polyUb may have non-proteolytic activating functions. Unanchored Ub chains that are not covalently attached to any protein are also important for kinase activation. Tripartite motif (TRIM) proteins, which function as E3-ubiquitin ligases, have been implicated in antiviral activity. We found that TRIM6 is important in the synthesis of unanchored K48-linked polyUb chains, which activate the IKKε kinase for STAT1 phosphorylation and induction of an antiviral response. These polyUb chains and IKKε interact in vivo in an IFN-I signaling dependent manner upon virus infection in mice. To better understand the physiological role of unanchored polyUb during virus infection, we developed a system to isolate ubiquitin-interacting proteins from the lungs of influenza-infected mice. Using a proteomics approach we identified new cellular factors that interact with unanchored polyUb chains in vivo and may be involved in the innate immune antiviral response.