title: Stepwise priming by acidic pH and a high K+ concentration is required for efficient uncoating of influenza A virus cores after penetration.
abstract: Influenza A virus (IAV) uses the low pH in late endocytic vacuoles as a cue for penetration by membrane fusion. Here, we analyzed the prefusion reactions that prepare the core for uncoating after it has been delivered to the cytosol. We found that this priming process occurs in two steps that are mediated by the envelope-embedded M2 ion channel. The first weakens the interactions between the matrix protein, M1, and the viral ribonucleoprotein bundle. It involves a conformational change in a linker sequence and the C-terminal domain of M1 after exposure to a pH below 6.5. The second step is triggered by a pH of <6.0 and by the influx of K(+) ions. It causes additional changes in M1 as well as a loss of stability in the viral ribonucleoprotein bundle. Our results indicate that both the switch from Na(+) to K(+) in maturing endosomes and the decreasing pH are needed to prime IAV cores for efficient uncoating and infection of the host cell.
  
  
  OBJECTIVE
  The entry of IAV involves several steps, including endocytosis and fusion at late endosomes. Entry also includes disassembly of the viral core, which is composed of the viral ribonucleoproteins and the RNA genome. We have found that the uncoating process of IAV is initiated long before the core is delivered into the cytosol. M2, an ion channel in the viral membrane, is activated when the virus passes through early endosomes. Here, we show that protons entering the virus through M2 cause a conformational change in the matrix protein, M1. This weakens interactions between M1 and the viral ribonucleoproteins. A second change was found to occur when the virus enters late endosomes. The preacidified core is then exposed to a high concentration of K(+), which affects the interactions between the ribonucleoproteins. Thus, when cores are finally delivered to the cytosol, they are already partially destabilized and, therefore, uncoating competent and infectious.
authors.affiliation: Institute of Biochemistry, ETH Zurich, Zurich, Switzerland sarah.stauffer@bc.biol.ethz.ch ari.helenius@bc.biol.ethz.ch.
authors.forename: Sarah
authors.initials: S
authors.lastname: Stauffer
authors.affiliation: Institute of Biochemistry, ETH Zurich, Zurich, Switzerland.
authors.forename: Yuehan
authors.initials: Y
authors.lastname: Feng
authors.affiliation: Institute of Biochemistry, ETH Zurich, Zurich, Switzerland.
authors.forename: Firat
authors.initials: F
authors.lastname: Nebioglu
authors.affiliation: Institute of Biochemistry, ETH Zurich, Zurich, Switzerland.
authors.forename: Rosalie
authors.initials: R
authors.lastname: Heilig
authors.affiliation: Institute of Biochemistry, ETH Zurich, Zurich, Switzerland.
authors.forename: Paola
authors.initials: P
authors.lastname: Picotti
authors.affiliation: Institute of Biochemistry, ETH Zurich, Zurich, Switzerland sarah.stauffer@bc.biol.ethz.ch ari.helenius@bc.biol.ethz.ch.
authors.forename: Ari
authors.initials: A
authors.lastname: Helenius
mesh_terms: D000818:Animals; D011992:Endosomes / Q000378:metabolism* / Q000821:virology*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009980:Influenza A virus / Q000187:drug effects* / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D011485:Protein Binding / Q000187:drug effects; D011487:Protein Conformation / Q000187:drug effects; D014763:Viral Matrix Proteins / Q000378:metabolism*; D057646:Virus Uncoating / Q000187:drug effects*; D000086082:Viroporin Proteins
chemical_list: D011188:Potassium; D014763:Viral Matrix Proteins; C488937:M2 protein, Influenza A virus; D000086082:Viroporin Proteins