Lipid interactions of the hemagglutinin HA2 NH2-terminal segment during influenza virus-induced membrane fusion

Fusion of influenza viruses with target membranes is induced by acid and involves complex changes in the viral fusion protein hemagglutinin (HA) and in the contact sites between viruses and target membranes (Stegmann, T., White, J. M., and Helenius, A. (1990) EMBO J. 9, 4231-4241). At 0 degrees C, in a first, kinetically distinct step, target membranes irreversibly adhere to the viruses. Fusion itself starts only after a lag-phase of several minutes (X-31 strain viruses) or after raising the temperature (PR8/34 strain viruses). We now provide evidence that the initial conformational change resulting in virus-target membrane adhesion is restricted to a (minor) subpopulation of the HA molecules. These molecules become susceptible to bromelain digestion, and they could be labeled with the photoactivatable reagent [3H]PTPC/11, a nonexchangeable lipid present in the target lipid bilayer (Harter, C., Bächi, T., Semenza, G., and Brunner, J. (1988) Biochemistry 27, 1856-1864). Only the HA2 subunit was labeled, and analyses of 2-nitro-5-thio-cyanobenzoic acid fragments derived thereof indicate that the HA2 NH2-terminal segment (fusion peptide) inserted into the target membrane bilayer. When the temperature was raised to trigger fusion of PR8/34 viruses, labeling of HA2 increased by a factor of 130. Most (74%) of that label was incorporated into the COOH-terminal membrane anchor region, but there was also a strong increase (about 30-fold) of NH2-terminal fusion peptide labeling. This suggests that fusion is preceded., or accompanied, by further changes in HA which lead to additional extensive lipid insertions of HA2 fusion peptides.