A number of different influenza C virus strains were tested for their fusion properties using a resonance energy assay which allows direct monitoring of fusion between virus membranes and artificial lipid vesicles. The fusion pH of various strains was found to range between 5.6 and 6.1. Haemolytic activity of the different strains with chicken erythrocytes was observed at slightly lower pH values and varied between 5.1 and 5.7. Studies of the kinetics of influenza C virus fusion showed distinct characteristics in fusion activity. A lag before onset of fusion was found with influenza C virus which was not observed for influenza A or B viruses. In addition, studies on the rate of conformational change of the influenza C virus glycoprotein, as determined by morphological changes and endogenous tryptophan fluorescence, suggest that the conformational change is rate-limiting in the fusion process, whereas for influenza A viruses the glycoprotein conformational change is fast and a later step in the fusion process is rate-limiting. Monitoring the conformational change of influenza C virus glycoprotein by the onset of trypsin susceptibility showed, however, that membrane fusion occurred in some cases without onset of trypsin susceptibility, indicating that the trypsin-susceptible conformation is a post-fusogenic conformation.


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