TRIM 25 nuclear translocation is hampered by localization to influenza A virus non-structural protein 1 cytosolic aggregates

Non-structural protein 1 (NS1) of influenza A virus (IAV) is involved in multiple blocking processes of cellular antiviral action. One of these includes the binding of cytosolic tripartite motif-containing protein 25 (TRIM25) to impede nuclear translocation and, thus, counteracting the block of viral RNA elongation taking place inside the nucleus. In addition, TRIM25 is part of the RIG-I signaling pathway and is an important component regarding the activation of interferon-induced genes and triggering cellular antiviral responses. In this study, we focus on the spatial localization of TRIM25 with respect to cytosolic NS1 aggregates found in IAV infected cells. Besides strain specific characteristics, we confirmed co-localization of TRIM25 to NS1 cytosolic aggregates at 16 hours post infection for all investigated IAV strains based on image analysis on single cell level. Immunoprecipitation further confirmed the interaction between NS1 and TRIM25 when comparing wt IAV and recombinant IAV devoid of NS1 expression. Of note, we combined fluorescence in situ hybridization targeting viral RNA with immunolabeling of TRIM25 suitable for nanoscopy and the diffraction limited detection of NS1 in one imaging approach. Our study reveals strain dependent differences in the block of IRF3 activation in respect to the accumulation of TRIM25 in cytosolic NS1 aggregates.