Membrane Interaction of Influenza Virus M1 Protein

The M1 protein of influenza virus is thought to make contact with the cytoplasmic tails of the glycoprotein spikes, lipid molecules in the viral membrane, and the internal ribonucleoprotein particles. Here we show electron micrographs of negatively stained virus particles in which M1 is visualized as a 60-A-long rod that touches the membrane but apparently is not membrane inserted. Photolabeling with a membrane restricted reagent resulted in labeling of the transmembrane region of haemagglutinin but not of M1, also suggesting that most of M1 is not embedded into the hydrophobic core of the viral membrane. Finally, in vitro reconstitution experiments using soluble M1 protein and synthetic liposomes or Madin-Darby canine kidney cell membranes suggest that M1 can bind to negatively charged liposomes and to the cellular membranes and that this binding can be prevented under high-salt conditions. Although none of these experiments prove that there does not exist a minor fraction of M1 that is membrane inserted, it appears that most of M1 in the virus is membrane associated through electrostatic interactions.