Specific Single or Double Proline Substitutions in the “Spring-loaded” Coiled-Coil Region of the Influenza Hemagglutinin Impair or Abolish Membrane Fusion Activity

Qiao, Hui; Pelletier, Sandra L.; Hoffman, Lucas; Hacker, Jill; Armstrong, R. Todd; White, Judith M.

We interpret our results with V55P/S71P and V55P in support of the hypothesis that the spring-loaded conformational change, the loop to helix transition of HA2 55-76, is required for fusion. Additional electron microscopic, biophysical, structural, and mutational studies will be necessary to prove this hypothesis. Since we see defects with both V55P and V55P/S71P at the level of outer leaflet lipid mixing, we further interpret our data in support of the hypothesis that the spring-loaded conformational change is required for an early step in the fusion process. Experiments are planned to test whether it is required for HA to bind, hydrophobically, to target liposomes. Although our data support the hypothesis that the spring-loaded conformational change is necessary for fusion, it is likely that other parts of HA must move during fusion activation (White and Wilson, 1987; Steinhauer et al., 1996; Korte et al., 1997) . Moreover, in our recently proposed model for HA-mediated membrane fusion, we envision that the spring-loaded conformational change functions at an early step in fusion and that subsequent changes that would generate a TBHA2-like structure drive later stages of fusion (see Figs. 5 and 6 in Hernandez et al., 1996) .