Influenza A virus proteins PB1 and NS1 are subject to functionally important phosphorylation by protein kinase C

The virulence of influenza A viruses depends on the activity of the viral RNA polymerase complex and viral regulatory phosphoproteins. We identified that the protein kinase C (PKC) inhibitor Go6976 had a post-entry anti-influenza viral effect, by using a polymerase activity-based reporter assay. This inhibitory effect was observed for influenza virus-infected cells as well as for cells transiently transfected with constructs for the RNA polymerase complex. Importantly, the in vitro analysis of viral protein phosphorylation identified PKC alpha as a kinase phosphorylating PB1 and NS1, but not PB2, PA or NP. Go6976 was able to block PKC-specific phosphorylation in vitro. Thus, our data suggest that PKC contributes to the phosphorylation of influenza PB1 and NS1 proteins which appears to be functionally relevant for both viral RNA polymerase activity and efficient viral replication.