title: At low pH, influenza virus matrix protein M1 undergoes a conformational change prior to dissociating from the membrane.
abstract: The M1 matrix protein of influenza A virus, which plays multiple roles in virion assembly and infection, underlies the viral envelope. However, previous studies have given differing accounts of the number of layers in the M1-envelope complex and their thicknesses and compositions. To resolve this issue, we performed cryo-electron microscopy and cryo-electron tomography on the self-same specimens. At neutral pH, there were two kinds of complexes, corresponding to a lipid bilayer with embedded glycoproteins, with and without a closely associated, 4-nm-thick sheet of M1 protein. The reported discrepancies arose from differences in imaging conditions, i.e., in defocus and in whole-particle projections versus thin tomographic slices. Exposure of virions to low pH (as in the endosome) promotes membrane fusion, and previous work has shown that the M2 ion channel causes the virion interior to acidify also. We found that after 5 min at pH 4.9, the proportion of virions lacking an M1 layer increased from 10% to 50%. In virions retaining an M1 layer under these conditions, the M1-envelope complex exhibited two states, viz, the original, neutral-pH state and one in which the M1 layer appeared thinner and/or closer to the membrane. These observations extend previous indications that acidic pH causes the M1 layer to dissociate, leaving the envelope more pliable and, consequently, fusion compatible, and they show that dissociation is preceded by a conformational change in M1.
authors.affiliation: Laboratory of Structural Biology, National Institute of Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland, USA.
authors.forename: Juan
authors.initials: J
authors.lastname: Fontana
authors.forename: Alasdair C
authors.initials: AC
authors.lastname: Steven
mesh_terms: D020285:Cryoelectron Microscopy; D055032:Electron Microscope Tomography; D006863:Hydrogen-Ion Concentration; D053122:Influenza A Virus, H3N2 Subtype / Q000187:drug effects / Q000648:ultrastructure*; D011487:Protein Conformation / Q000187:drug effects; D014763:Viral Matrix Proteins / Q000737:chemistry* / Q000648:ultrastructure*
chemical_list: C488936:M1 protein, Influenza A virus; D014763:Viral Matrix Proteins