title: Mutations within the conserved NS1 nuclear export signal lead to inhibition of influenza A virus replication.
abstract: BACKGROUND
  The influenza A virus NS1 protein is a virulence factor and an antagonist of host cell innate immune responses. During virus infection NS1 protein has several functions both in the nucleus and in the cytoplasm and its intracellular localization is regulated by one or two nuclear localization signals (NLS) and a nuclear export signal (NES).
  
  
  METHODS
  In order to investigate the role of NS1 NES in intracellular localization, virus life cycle and host interferon responses, we generated recombinant A/Udorn/72 viruses harboring point mutations in the NES sequence.
  
  
  RESULTS
  NS1 NES was found to be inactivated by several of the mutations resulting in nuclear retention of NS1 at late stages of infection confirming that this sequence is a bona fide functional NES. Some of the mutant viruses showed reduced growth properties in cell culture, inability to antagonize host cell interferon production and increased p-IRF3 levels, but no clear correlation between these phenotypes and NS1 localization could be made. Impaired activation of Akt phosphorylation by the replication-deficient viruses indicates possible disruption of NS1-p85β interaction by mutations in the NES region.
  
  
  CONCLUSIONS
  We conclude that mutations within the NS1 NES result in impairment of several NS1 functions which extends further from the NES site being only involved in regulating the nuclear-cytoplasmic trafficking of NS1.
authors.affiliation: Virology Unit, Department of Infectious Disease Surveillance and Control, National Institute for Health and Welfare (THL), Mannerheimintie 166, FIN-00300 Helsinki, Finland. janne.tynell@thl.fi.
authors.forename: Janne
authors.initials: J
authors.lastname: Tynell
authors.forename: Krister
authors.initials: K
authors.lastname: Melén
authors.forename: Ilkka
authors.initials: I
authors.lastname: Julkunen
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D039223:Cleavage And Polyadenylation Specificity Factor / Q000378:metabolism; D017124:Conserved Sequence; D003593:Cytoplasm / Q000378:metabolism; D006801:Humans; D009980:Influenza A virus / Q000235:genetics*; D007251:Influenza, Human / Q000821:virology; D007371:Interferon-gamma / Q000096:biosynthesis; D028884:Karyopherins / Q000378:metabolism; D008958:Models, Molecular; D009154:Mutation; D049790:Nuclear Export Signals / Q000235:genetics*; D011485:Protein Binding; D011487:Protein Conformation; D021381:Protein Transport; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D017361:Viral Nonstructural Proteins / Q000737:chemistry / Q000235:genetics*; D014779:Virus Replication / Q000235:genetics*; D000097606:Exportin 1 Protein
chemical_list: D039223:Cleavage And Polyadenylation Specificity Factor; D007371:Interferon-gamma; D028884:Karyopherins; D049790:Nuclear Export Signals; D018160:Receptors, Cytoplasmic and Nuclear; D017361:Viral Nonstructural Proteins; D000097606:Exportin 1 Protein; C085820:INS1 protein, influenza virus