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.

The purpose of this study was to investigate whether the spring-loaded conformational change in the influenza HA, the loop to helix transition of HA2 55-76 (Carr and Kim, 1993; Bullough et al., 1994) , is required for membrane fu-sion and, if so, to determine whether it is required for an early or a late stage of the fusion reaction (see Fig. 6 in Hernandez et al., 1996) . Our approach was to engineer site-specific mutations into the region of high coiled-coil propensity (Table I, Fig. 1 ) and to assess their effects on HA structure and membrane fusion activity. Our findings (summarized in Table III ) are discussed in terms of the role of the spring-loaded coiled-coil region for the structure and function of the influenza HA and other viral and cellular membrane fusion proteins. We interpret our results in support of the hypothesis that the spring-loaded conformational change is required for fusion, but we consider an alternate model.