pH-dependent self-association of influenza hemagglutinin fusion peptides in lipid bilayers

We have recently designed a host-guest peptide system that allows us to qua ntitatively measure the energetics of interaction of viral fusion peptides with lipid bilayers. Here, we show that fusion peptides of influenza hemagg lutinin reversibly associate with one another at membrane surfaces above cr itical surface concentrations, which range from one to five peptides per 10 00 lipids in the systems that we investigated. It is further demonstrated b y using circular dichroism and Fourier transform infrared spectroscopy that monomeric peptides insert into the bilayers in a predominantly alpha -heli cal conformation, whereas self-associated fusion peptides adopt predominant ly antiparallel beta -sheet structures at the membrane surface. The two for ms are readily interconvertible and the equilibrium between them is determi ned by the pH and ionic strength of the surrounding solution. Lowering the pH favors the monomeric alpha -helical conformation, whereas increasing the ionic strength shifts the equilibrium towards the membrane-associated beta -aggregates. The binding data are interpreted in terms of a cooperative bi nding model that yields free energies of insertion and free energies of sel f-association for each of the peptides studied at pH 7.4 and pH 5. At pH 5 and 35 mM ionic strength, the insertion energy of the 20 residue influenza hemagglutinin fusion peptide is -7.2 kcal/mol and the self-association ener gy is -1.9 kcal/mol. We propose that self-association of fusion peptides co uld be a major driving force for recruiting a small number of hemagglutinin trimers into a fusion site. (C) 2000 Academic Press.