Levene Rachel Emily, Shrestha Shailab D, Gaglia Marta Maria
Graduate Program in Molecular Microbiology, Tufts University Graduate School of Biomedical Sciences, Boston, MA, USA.
Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, MA, USA.
J Virol. 2021 Mar 25;95(8). doi: 10.1128/JVI.02312-20. Epub 2021 Jan 27.
The influenza A endoribonuclease PA-X regulates virulence and transmission 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 PA-X protein remain markedly low during infection, and 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 post-translationally remains unknown. We now report that the PA-X protein is rapidly turned over. PA-X 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-terminal 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 compared to 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, this study reveals a novel regulatory mechanism of PA-X protein levels that may impact host shutoff activity during influenza A virus infection. 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 biology. 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.
甲型流感病毒内切核糖核酸酶PA-X通过降低宿主基因表达从而调节对甲型流感病毒的免疫反应，进而调控病毒的毒力和传播。尽管PA-X在病毒生物学中具有这一关键功能，但在感染过程中PA-X蛋白水平仍显著较低，并且先前的研究结果表明，这些低水平并非仅仅是翻译水平和RNA稳定性调控的结果。PA-X在翻译后是如何被调控的仍不清楚。我们现在报告称，PA-X蛋白周转迅速。来自多个病毒株的PA-X寿命较短，尽管根据病毒株的不同，PA-X的半衰期在约30分钟至约3.5小时之间。此外，可变的PA-X C末端结构域中的序列主要负责调节PA-X的半衰期，尽管N末端结构域也导致了不同病毒株之间的一些差异。有趣的是，我们发现与我们测试的其他变体相比，2009年大流行H1N1病毒株的PA-X半衰期更长。据报道，这种PA-X异构体具有更高的宿主关闭活性，这表明蛋白质周转在调节PA-X活性中发挥作用。总的来说，这项研究揭示了一种PA-X蛋白水平的新型调控机制，这可能会影响甲型流感病毒感染期间的宿主关闭活性。甲型流感病毒的PA-X蛋白通过抑制宿主基因表达，包括编码抗病毒反应的基因，来降低宿主对感染的免疫反应。因此，它在甲型流感病毒生物学中起着核心作用。尽管PA-X具有关键功能，但它直到2012年才被发现，仍有许多有待了解的地方，包括PA-X活性是如何被调控以促进病毒感染达到最佳水平的。在这项研究中，我们揭示了PA-X蛋白水平非常低可能是由于周转迅速。我们表明，不稳定性是甲型流感病毒不同毒株的PA-X变体之间的一个保守特性，但PA-X变体的半衰期不同。此外，2009年大流行H1N1病毒株的PA-X半衰期较长与其报道的较高活性相关。因此，PA-X的稳定性可能是调节其活性的一种方式，并且可能导致甲型流感病毒株毒力的差异。
