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.

Unlike HA, the fusion proteins of retro-, paramyxo-, and coronaviruses induce fusion at neutral pH, and their fusion reactions are or appear to be triggered by interactions with virus receptors instead of low pH (Hernandez et al., 1996 (Hernandez et al., , 1997 . Whereas the conformational change in X:31 HA is irreversible (Stegmann et al., 1987; White and Wilson, 1987) and appears to be under kinetic control (Baker and Agard, 1994; Chen et al., 1995) , the conforma-tional changes of "neutral-pH" fusion proteins could be thermodynamically controlled and/or reversible. The cores of the Env glycoproteins of three retroviruses form trimeric coiled coils Fass and Kim, 1995; Lu et al., 1995; Fass et al., 1996; Weissenhorn et al., 1996) . Although these coiled-coils appear to have melting temperatures similar to that of the HA coiled-coil, they differ in length, pitch, helix packing, and other specific features (Bullough et al., 1994; Kim et al., 1996) . Any of these differences might contribute to the apparently increased resilience of the HA coiled-coil to mutation.