Nuclear functions of the influenza A and B viruses NS1 proteins: Do they play a role in viral mRNA export?

Although it is known for decades that influenza viruses replicate and transcribe their genome in the nucleus of the host cell, there is little knowledge about the cellular and viral factors mediating the nuclear transport of viral mRNA transcripts to the cytoplasm. Efficient export of mature cellular mRNA is coupled to their synthesis by the RNA polymerase II and subsequent processing events such as splicing. This linkage necessitated influenza viruses to evolve a strategy to integrate their unspliced mRNAs generated by the viral polymerase into a cellular mRNA export pathway. Recent findings suggest that the major cellular mRNA export receptor Tap/NXF1 promotes the influenza virus mRNA export. Here, we review functions of the NS1 proteins of influenza A and B viruses and discuss the emerging evidence supporting a role of these viral factors in the export of viral mRNAs.