Structural studies on membrane‐embedded influenza hemagglutinin and its fragments

The mechanism of influenza virus hemagglutinin (HA)‐mediated membrane fusion has been inferred in part from studies examining pH‐induced structural changes in soluble HA derivatives lacking the viral membrane anchor and, sometimes, the fusion peptide (the C‐ and N‐terminal residues of the HA 2 chain, respectively). To reconcile structure‐based mechanisms of HA‐mediated membrane fusion with structural implications of functional studies performed on membrane‐embedded HA, we have undertaken attenuated total reflection Fourier transform infrared (ATR‐FTIR) spectroscopic analyses of membrane‐embedded HA (strain X:31) and its fragments reconstituted into supported lipid bilayers. The fragments correspond to proteolytic products with the majority of the HA 1 chain and, in some cases, the fusion peptide removed (THA 2 and THA F− 2 , respectively). In combination with R18 fluorescence dequenching to monitor the functional implications of HA, subunit removal, we have assessed the influence of pH and target membrane presentation on the secondary structures, orientations relative to the membrane, and dynamics of these molecules. We find that X:31 HA is more tilted towards the plane of the membrane under fusion than under resting conditions, that the tilting of HA depends on the presence of the HA 1 chain, that the residues connecting the membrane‐inserted fusion peptide with the crystallographically determined coiled coil probably adopt an a‐helical conformation, and that several changes in the secondary structure and the amide H/D exchange kinetics occur as a result of acidification and target membrane presentation, which can be interpreted as small changes and a release of strain in the static and dynamic structure of membrane‐bound HA. THA 2 mediates fusion, but less efficiently and with less pH‐selectivity than HA.