INFLUENZA HEMAGGLUTININ ASSUMES A TILTED CONFORMATION DURING MEMBRANE-FUSION AS DETERMINED BY ATTENUATED TOTAL-REFLECTION FTIR SPECTROSCOPY

Fusion of influenza virus with target membranes is mediated by an acid -induced conformational change of the viral fusion protein hemagglutin in (HA) involving an extensive reorganization of the alpha-helices, A 'spring-loaded' displacement over at least 100 Angstrom provides a mec hanism for the insertion of the fusion peptide into the target membran e, but does not explain how the two membranes are brought into fusion contact, Here we examine, by attenuated total reflection Fourier trans form infrared spectroscopy, the secondary structure and orientation of HA reconstituted in planar membranes, At neutral pH, the orientation of the HA trimers in planar membranes is approximately perpendicular t o the membrane, However, at the pH of fusion, the HA trimers are tilte d 55-70 degrees from the membrane normal in the presence or absence of bound target membranes, In the absence of target membranes, the overa ll secondary structure of HA at the fusion pH is similar to that at ne utral pH, but similar to 50-60 additional residues become alpha-helica l upon the conformational change in the presence of bound target membr anes, These results are discussed in terms of a structural model for t he fusion intermediate of influenza HA.