The Influenza A Virus Host Shutoff Factor PA-X Is Rapidly Turned Over in a Strain-Specific Manner

The influenza A endoribonuclease PA-X regulates virulence and trans-mission of the virus by reducing host gene expression and thus regulating immune responses to influenza A virus. Despite this key function in viral biology, the levels of the PA-X protein remain markedly low during infection. Previous results suggest that these low levels are not solely the result of regulation of the level of translation and RNA stability. How PA-X is regulated posttranslationally remains unknown. We now report that the PA-X protein is rapidly turned over. PA-Xs from multiple viral strains are short-lived, although the half-life of PA-X ranges from -30 minutes to -3.5 hours depending on the strain. Moreover, sequences in the variable PA-X C-terminal domain are primarily responsible for regulating PA-X half-life, although the N -termi-nal domain also accounts for some differences among strains. Interestingly, we find that the PA-X from the 2009 pandemic H1N1 strain has a longer half-life than the other variants we tested. This PA-X isoform has been reported to have a higher host shutoff activity, suggesting a role for protein turnover in regulating PA-X activity. Collectively, results of this study reveals a novel regulatory mechanism of PA-X protein levels that may impact host shutoff activity during influenza A virus infection. IMPORTANCE The PA-X protein from influenza A virus reduces host immune responses to infection through suppressing host gene expression, including genes encoding the antiviral response. Thus, it plays a central role in influenza A virus biol-ogy. Despite its key function, PA-X was only discovered in 2012 and much remains to be learned, including how PA-X activity is regulated to promote optimal levels of viral infection. In this study, we reveal that PA-X protein levels are very low likely because of rapid turnover. We show that instability is a conserved property among PA-X variants from different strains of influenza A virus but that the half-lives of PA -X variants differ. Moreover, the longer half-life of PA-X from the 2009 pandemic H1N1 strain correlates with its reported higher activity. Therefore, PA-X stability may be a way to regulate its activity and may contribute to the differential virulence of influenza A virus strains.

Automated summary

The PA-X protein from influenza A virus is rapidly turned over, with half-lives varying among different viral strains. The PA-X isoform from the 2009 pandemic H1N1 strain has a longer half-life and higher host shutoff activity. Sequences in the variable C-terminal domain primarily regulate PA-X half-life.