Depth-Dependent Membrane Ordering by Hemagglutinin Fusion Peptide Promotes Fusion

Membrane fusion, one of the most fundamental processes in life, occurs when two separate lipid membranes merge into a single continuous bilayer. Membrane fusion is essential for the entry of lipid-sheathed viruses such as influenza and HIV. Influenza virus is internalized via receptor-mediated endocytosis and then fuses with the endosomal membrane at low pH. Hemagglutinin, a glycoprotein found on the surface of influenza virus, is responsible for the fusion of the viral sheath with the endosomal membrane. The ∼20 amino acid long N-terminus of hemagglutinin, known as the fusion peptide, plays a crucial role in the viral fusion process. Although there exists vast literature on the importance and role of the fusion peptide in promoting membrane fusion, there is no consensus on the mechanism by which it promotes fusion. A recent report suggested that the fusion peptide occupies and orders space in the outer leaflets of contacting bilayers so as to promote acyl chain protrusion into interbilayer space and promote fusion "stalk" formation. We report here the effect of the wild type, G1S, G1V, and W14A mutants of hemagglutinin fusion peptide on depth-dependent ordering of model membranes along the bilayer normal. We utilized fluorescence anisotropy, lifetime measurements, and lifetime distribution analyses of different anthroyloxy stearic acid probes (n-AS) in order to examine the effect of fusion peptides at various depths along the bilayer normal. Wild type peptide uniquely ordered a region ∼12 Å from the bilayer midpoint, W14A and G1S mutants mainly ordered the bilayer interface, while G1V had little ordering influence. On the basis of recent analysis of the effects of these peptides on fusion, ordering of the mid-upper region of the bilayer appears to promote fusion pore formation, while ordering of the bilayer interface inhibits it.