The Different Substrate Specificities Of Human Influenza Virus PA And PA‐X Endonucleases Support Distinct Roles In The Viral Life Cycle

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The PA subunit of the RNA dependent RNA polymerase of influenza virus is an endonuclease required for the cap‐snatching mechanism leading to the synthesis of viral mRNA. Recently, the PA‐X protein has been discovered in human influenza, which is translated from an alternative open reading frame within the PA gene coding sequence, resulting from a ribosomal frameshifting. The PA‐X retains the N‐terminal endonucleolytic domain of PA, but has a different and shorter C‐terminal part. It has been proposed that PA‐X degrades cellular mRNA, contributing to the attenuation of viral pathogenesis in the host. In human isolates from the 2009 pandemic H1N1 virus, a naturally truncated form of PA‐X is present. Here we express the recombinant PA and PA‐X proteins, the latter both in its full length and deleted form, from human seasonal influenza virus and show that their endonucleolytic activities have distinct substrate specificities, which depend on the structure of the RNA to be cleaved. Collectively, our results support distinct roles of PA and PA‐X in the viral life cycle.

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