Cellular antiviral responses against influenza A virus are countered at the posttranscriptional level by the viral NS1A protein via its binding to a cellular protein required for the 3' end processing of cellular pre-mRNAS.

The influenza A virus NS1 protein (NS1A protein) binds and inhibits the function of the 30-kDa subunit of CPSF, a cellular factor that is required for the 3'-end processing of cellular pre-mRNAs. Here we generate a recombinant influenza A/Udorn/72 virus that encodes an NS1A protein containing a mutated binding site for the 30-kDa subunit of CPSF. This mutant virus is substantially attenuated, indicating that this binding site in the NS1A protein is required for efficient virus replication. Using this mutant virus, we show that NS1A binding to CPSF mediates the viral posttranscriptional countermeasure against the initial cellular antiviral response--the interferon-alpha/beta (IFN-alpha/beta)-independent activation of the transcription of cellular antiviral genes, which requires the interferon regulatory factor-3 (IRF-3) transcription factor that is activated by virus infection. Whereas the posttranscriptional processing of these cellular antiviral pre-mRNAs is inhibited in cells infected by wild-type influenza A virus, functional antiviral mRNAs are produced in cells infected by the mutant virus. These results establish that the binding of 30-kDa CPSF to the NS1A protein is largely responsible for the posttranscriptional inhibition of the processing of these cellular antiviral pre-mRNAs. Mutation of this binding site in the NS1A protein also affects a second cellular antiviral response: in cells infected by the mutant virus, IFN-beta mRNA is produced earlier and in larger amounts.