The low pH-induced fusion of influenza virus with intact erythrocyte plasma membranes is preceded by a delay time following pH reduction, that is itself pH- and temperature dependent. At 37°C/pH 4.8, lipid mixing between virus and target membranes begins < 2s after pH reduction, whereas at 4°C/pH 4.8, fusion does not commence until > 10 min after pH reduction. We have found that within this time period at 4°C, a population of virus acquires the capacity to subsequently undergo fusion with high efficiency at elevated temperatures and pH 7.4. Both the kinetics and the extent of this pH 7.4 fusion depend upon the time of pre-incubation at pH 4.8/4°C. Incubation at pH 7.4/4°C, following this pre-incubation does not result in fusion, but the capacity to fuse at pH 7.4/37°C is retained for a time period exceeding 1 h. The longevity of this fusion committed state makes it amenable to biochemical and immunological analysis. We have shown that it is insensitive to dithiothreitol, neuraminidase and trypsin, but is incapacitated by thermolysin or protease K. We conclude that only the HA2 sub-unit of influenza haemagglutinin is a necessary protein component of later stages of the fusion pathway.
