The role of acidic residues in the "fusion segment" of influenza A virus hemagglutinin in low-pH-dependent membrane fusion.

To clarify the role of acidic amino acid residues in the "fusion segment" of hemagglutinin (HA) of influenza A virus (H1N1) in pH-dependent membrane fusion, we have constructed and expressed five mutant HA cDNAs in CV-1 cells by SV40-HA virus vectors (SVHA). Fusion activities of the five mutant HAs were examined by lipid mixing and polykaryon formation assays. In spite of the substitution of Gly and Lys for the acidic residues, all the mutants were found to retain their low-pH-dependent fusion activity by lipid mixing assay. Although SVHA-G19(HA(2)19D-->G), -K11 (HA(2)11E-->K) and -K19(HA(2)19D-->K) induced polykaryon formation at low pH as wild type HA did, SVHA-G11(HA(2)11E-->G) induced limited polykaryon formation and SVHA-G11,19 (HA(2)11E-->G, 19D-->G) did not. The substitution of Gly for Glu at position 11 inhibited widening of the initial fusion pore. However, Lys mutants induced the formation of an initial fusion pore and widened it at low pH where Lys residues might have positive charges. These results suggest that the neutralization of the charges on acidic residues in the "fusion segment" at low pH is not important for interaction of the "fusion segment" with the target lipid bilayer or for triggering the membrane fusion.