title: Studies of an influenza A virus temperature-sensitive mutant identify a late role for NP in the formation of infectious virions.
abstract: The influenza A virus nucleoprotein (NP) is a single-stranded RNA-binding protein that encapsidates the virus genome and has essential functions in viral-RNA synthesis. Here, we report the characterization of a temperature-sensitive (ts) NP mutant (US3) originally generated in fowl plague virus (A/chicken/Rostock/34). Sequence analysis revealed a single mutation, M239L, in NP, consistent with earlier mapping studies assigning the ts lesion to segment 5. Introduction of this mutation into A/PR/8/34 virus by reverse genetics produced a ts phenotype, confirming the identity of the lesion. Despite an approximately 100-fold drop in the viral titer at the nonpermissive temperature, the mutant US3 polypeptide supported wild-type (WT) levels of genome transcription, replication, and protein synthesis, indicating a late-stage defect in function of the NP polypeptide. Nucleocytoplasmic trafficking of the US3 NP was also normal, and the virus actually assembled and released around sixfold more virus particles than the WT virus, with normal viral-RNA content. However, the particle/PFU ratio of these virions was 50-fold higher than that of WT virus, and many particles exhibited an abnormal morphology. Reverse-genetics studies in which A/PR/8/34 segment 7 was swapped with sequences from other strains of virus revealed a profound incompatibility between the M239L mutation and the A/Udorn/72 M1 gene, suggesting that the ts mutation affects M1-NP interactions. Thus, we have identified a late-acting defect in NP that, separate from its function in RNA synthesis, indicates a role for the polypeptide in virion assembly, most likely involving M1 as a partner.
authors.affiliation: Department of Pathology, Division of Virology, University of Cambridge, Cambridge, United Kingdom.
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Noton
authors.forename: Martha
authors.initials: M
authors.lastname: Simpson-Holley
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Medcalf
authors.forename: Helen M
authors.initials: HM
authors.lastname: Wise
authors.forename: Edward C
authors.initials: EC
authors.lastname: Hutchinson
authors.forename: John W
authors.initials: JW
authors.lastname: McCauley
authors.forename: Paul
authors.initials: P
authors.lastname: Digard
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D002460:Cell Line; D002642:Chick Embryo; D002645:Chickens; D004285:Dogs; D006358:Hot Temperature; D006801:Humans; D009980:Influenza A virus / Q000502:physiology*; D050505:Mutant Proteins / Q000502:physiology; D020125:Mutation, Missense; D019590:Nucleocapsid Proteins; D012367:RNA, Viral / Q000096:biosynthesis; D016601:RNA-Binding Proteins / Q000502:physiology*; D014758:Viral Core Proteins / Q000502:physiology*; D014763:Viral Matrix Proteins / Q000378:metabolism; D010948:Viral Plaque Assay; D014764:Viral Proteins / Q000096:biosynthesis; D014771:Virion / Q000648:ultrastructure; D019065:Virus Assembly; D014779:Virus Replication
chemical_list: C488936:M1 protein, Influenza A virus; D050505:Mutant Proteins; C048842:NP protein, Influenza A virus; D019590:Nucleocapsid Proteins; D012367:RNA, Viral; D016601:RNA-Binding Proteins; D014758:Viral Core Proteins; D014763:Viral Matrix Proteins; D014764:Viral Proteins