Fusion Peptides Promote Formation of Bilayer Cubic Phases in Lipid Dispersions. An X-Ray Diffraction Study

Small angle x-ray diffraction revealed a strong influence of the N-terminal influenza hemagglutinin fusion peptide on the formation of nonlamellar lipid phases. Comparative measurements were made on a series of three peptides, a 20-residue wild-type X-31 influenza virus fusion peptide, GLFGAIAGFIENGWEGMIDG, and its two point-mutant, fusion-incompetent peptides G1E and G13L, in mixtures with hydrated phospholipids, either dipalmitoleoylphosphatidylethanolamine (DPoPE), or monomethylated dioleoyl phosphatidylethanolamine (DOPE-Me), at lipid/peptide molar ratios of 200:1 and 50:1. All three peptides suppressed the H-II phase and shifted the L-alpha-H-II transition to higher temperatures, simultaneously promoting formation of inverted bicontinuous cubic phases, Q(II), which becomes inserted between the L-alpha and H-II phases on the temperature scale. Peptide-induced Q(II) had strongly reduced lattice constants in comparison to the Q(II) phases that form in pure lipids. Q(II) formation was favored at the expense of both L-alpha and H-II phases. The wild-type fusion peptide, WT-20, was distinguished from G1E and G13L by the markedly greater magnitude of its effect. WT-20 disordered the L-alpha phase and completely abolished the H-II phase in DOPE-Me/WT-20 50:1 dispersions, converted the Q(II) phase type from Im3m to Pn3m and reduced the unit cell size from similar to 38 nm for the Im3m phase of DOPE-Me dispersions to similar to 15 nm for the Pn3m phase in DOPE-Me/WT-20 peptide mixtures. The strong reduction of the cubic phase lattice parameter suggests that the fusion-promoting WT-20 peptide may function by favoring bilayer states of more negative Gaussian curvature and promoting fusion along pathways involving Pn3m phase-like fusion pore intermediates rather than pathways involving H-II phase-like intermediates.