Role of the Glu residues of the influenza hemagglutinin fusion peptide in the pH dependence of fusion activity

To elucidate the role of the fusion peptide in Influenza hemagglutinin (HA) -mediated fusion, we compared pH-dependent conformational changes and fusio n mediated by wild-type and a mutant HA in which Glu residues at positions 11 and 15 of the fusion peptide are substituted for valine. The pH dependen ce of conformational changes and kinetics of fusion with erythrocytes was t he same for both forms of HA The time for commitment and the temperature de pendence of HA-mediated fusion were also the same. However, striking differ ences were observed between wild-type and mutant fusion peptides in their i nteractions with lipid membranes at neutral and acidic pH. Since eliminatio n of the negatively charged residues allows the exposed fusion peptide to p enetrate the bilayer at pH values closer to neutral, but does not affect co nformational changes and fusion activity in intact HA, we conclude that con formational changes are tightly coupled to fusion peptide insertion in the overall HA-mediated fusion cascade. (C) 2001 Academic Press.