Influenza virus-model membrane interaction. A morphological approach using modern cryotechniques

The membrane fusion activity of influenza virus was characterized morphologically using a model system composed of a highly purified influenza B virus suspension and ganglioside-containing zwitterionic liposomes. Electron microscopical analysis was performed after a combination of fast-freezing with either freeze-fracture or freeze-substitution-thin sectioning, ensuring maximal time resolution and avoiding preparation artifacts. In a parallel fluorescence 'lipid mixing' fusion assay, influenza virus-membrane fusion was characterized biochemically. Biochemical and morphological data are in full agreement, indicating negligible membrane fusion activity at neutral pH and high fusion activity at low pH. The freeze-fracture morphology strongly suggests a local point contact between viral and liposomal membrane at neutral pH, and a local point fusion mechanism for influenza virus-membrane fusion upon lowering of the pH. Fusion is followed by lipid mixing, lateral diffusion of viral spike proteins and exposure of viral contents at the inner liposomal surface.