Lipid acyl chain protrusion induced by the influenza virus hemagglutinin fusion peptide detected by NMR paramagnetic relaxation enhancement

The glycoprotein spikes of membrane-enveloped viruses include a subunit that catalyzes fusion (joining) of the viral and target cell membranes. For influenza virus, this is subunit 2 of hemagglutinin which has a similar to 20-residue N-terminal fusion peptide (Fp) region that binds target membrane. An outstanding question is whether there are associated membrane changes important for fusion. Several computational studies have found increased "protrusion" of lipid acyl chains near Fp, i.e. one or more chain carbons are closer to the aqueous region than the headgroup phosphorus. Protrusion may accelerate initial joining of outer leaflets of the two membranes into a stalk intermediate. In this study, higher protrusion probability in membrane with vs. without Fp is convincingly detected by larger Mn2+-associated increases in chain C-13 NMR transverse relaxation rates (Gamma(2)'s). Data analysis provides a ratio Gamma(2,neighbor)/Gamma(2,distant) for lipids neighboring vs. more distant from the Fp. The calculated ratio depends on the number of Fp-neighboring lipids and the experimentally-derived range of 4 to 24 matches the range of increased protrusion probabilities from different simulations. For samples either with or without Fp, the Gamma(2) values are well-fitted by an exponential decay as the C-13 site moves closer to the chain terminus. The decays correlate with free-energy of protrusion proportional to the number of protruded -CH2 groups, with free energy per -CH2 of similar to 0.25 k(B)T. The NMR data support one major fusion role of the Fp to be much greater protrusion of lipid chains, with highest protrusion probability for chain regions closest to the headgroups.