Changes in the morphology of influenza particles induced at low pH.

At low pH influenza virus causes membrane fusion. This phenomenon is thought to reflect a part of the infection mechanism of the virus. To obtain more information on the effect of low pH on the virus, the change in morphology of influenza virus particles was studied by electron microscopy. Further, the degradation of haemagglutinin (HA) after trypsin digestion as a function of pH was studied by gel electrophoresis. The results showed that a threshold value existed below which both a change in morphology and an increase in trypsin sensitivity were observed. This threshold pH was found to be strain specific. A number of strains showed a heterogeneity in the particle population with respect to the threshold pH. The various subpopulations appeared to differ genetically. Virus particles with uncleaved precursor HA, HAo, were not effected by the low pH treatment.