title: Cytosolic 5'-triphosphate ended viral leader transcript of measles virus as activator of the RIG I-mediated interferon response.
abstract: BACKGROUND
  Double stranded RNA (dsRNA) is widely accepted as an RNA motif recognized as a danger signal by the cellular sentries. However, the biology of non-segmented negative strand RNA viruses, or Mononegavirales, is hardly compatible with the production of such dsRNA.
  
  
  RESULTS
  During measles virus infection, the IFN-beta gene transcription was found to be paralleled by the virus transcription, but not by the virus replication. Since the expression of every individual viral mRNA failed to activate the IFN-beta gene, we postulated the involvement of the leader RNA, which is a small not capped and not polyadenylated RNA firstly transcribed by Mononegavirales. The measles virus leader RNA, synthesized both in vitro and in vivo, was efficient in inducing the IFN-beta expression, provided that it was delivered into the cytosol as a 5'-trisphosphate ended RNA. The use of a human cell line expressing a debilitated RIG-I molecule, together with overexpression studies of wild type RIG-I, showed that the IFN-beta induction by virus infection or by leader RNA required RIG-I to be functional. RIG-I binds to leader RNA independently from being 5-trisphosphate ended; while a point mutant, Q299A, predicted to establish contacts with the RNA, fails to bind to leader RNA. Since the 5'-triphosphate is required for optimal RIG-I activation but not for leader RNA binding, our data support that RIG-I is activated upon recognition of the 5'-triphosphate RNA end.
  
  
  CONCLUSIONS
  RIG-I is proposed to recognize Mononegavirales transcription, which occurs in the cytosol, while scanning cytosolic RNAs, and to trigger an IFN response when encountering a free 5'-triphosphate RNA resulting from a mislocated transcription activity, which is therefore considered as the hallmark of a foreign invader.
authors.affiliation: VirPatH, Université Lyon 1, Centre National de la Recherche Scientifique (CNRS), Faculté de Médecine RTH Laennec, Lyon, France.
authors.forename: Sébastien
authors.initials: S
authors.lastname: Plumet
authors.forename: Florence
authors.initials: F
authors.lastname: Herschke
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Bourhis
authors.forename: Hélène
authors.initials: H
authors.lastname: Valentin
authors.forename: Sonia
authors.initials: S
authors.lastname: Longhi
authors.forename: Denis
authors.initials: D
authors.lastname: Gerlier
mesh_terms: D016679:Genome, Viral; D006801:Humans; D007113:Immunity, Innate; D016899:Interferon-beta / Q000235:genetics*; D008457:Measles / Q000235:genetics / Q000276:immunology; D008459:Measles virus / Q000235:genetics* / Q000276:immunology; D018096:Mononegavirales / Q000235:genetics; D012330:RNA, Double-Stranded / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics; D012367:RNA, Viral / Q000737:chemistry / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014158:Transcription, Genetic
chemical_list: D012330:RNA, Double-Stranded; D012333:RNA, Messenger; D012367:RNA, Viral; D016899:Interferon-beta