Electrostatic Forces Govern Assembly and Disintegration of the Influenza Virus Protein Scaffold to Provide Tension for Membrane Fusion

Influenza A virus is an enveloped negative strand RNA virus. Its outer envelope consists of the lipid membrane with incorporated glycoproteins and proton channel M2. The inner envelope of the virion is a membrane-associated scaffold of matrix protein M1, which contacts both the viral RNP and the lipid envelope. Formation and disintegration of the protein scaffold are essential processes for influenza replication and infection. Both involve interaction of M1 with the lipid membrane; both are controlled by pH. We investigate the physico-chemical mechanism of these processes using the combination of electrochemical and fluorescent measurements with AFM. In neutral media, the adsorption of M1 protein on the lipid bilayer was electrostatic in nature and reversible. Acidification drives conformational changes in M1 molecules and increase of their charge leading to partial desorption due to increased repulsion between M1 monomers still stuck to the membrane. This repulsive force could generate tension for membrane rupture, as it was demonstrated for lipid vesicles coated with M1. Thus, electrostatic forces could explain M1 protein scaffold disintegration at low pH and most likely stretch the lipid membrane, promoting fusion pore widening for RNP release.