Low pH deforms the influenza virus envelope.

Influenza virus membrane fusion is induced by low pH, which triggers an irreversible conformational change in the viral haemagglutinin (HA). The result of this change is the extrusion of the HA fusion peptide, after which it may act in the fusion of virus and endosomal membranes. Here we describe electron microscopic observations on low pH-treated virus after negative staining or cryo-electron microscopy of virus in the frozen hydrated state. The results indicate a destabilization of the virus membrane at low pH that can be reversed by returning the pH to neutral.