The structural effects of the fusion peptide of feline leukemia virus (FeLV) on lipid polymorphism were studied, using differential scanning calorimetry (DSC), 31 P nuclear magnetic resonance (NMR), and time-resolved X-ray diffraction. This peptide lowers the bilayer to inverted hexagonal phase transition temperature, T(H), of dipalmitoleoylphosphatidylethanolamine (DiPoPE) at peptide mole fractions of up to 1.5 x 10 -3 at pH 5.0 and at pH 7.4. The temperature at which isotropic 31 P NMR signals for monomethyldioleoylphosphatidylethanolamine (MeDOPE) first occurred is lowered by the FeLV peptide. The amount of isotropic signal seen at 40°C is directly correlated to the peptide:lipid molar ratio. In the peptide-containing samples, more lipid remains in the isotropic state over the whole recorded temperature range. Isotropic 31 P NMR signals were observed for DiPoPE in the presence of the FeLV peptide for the entire recorded temperature range of 35-50°C, while pure DiPoPE showed no significant amount of isotropic signal. X-ray studies of DiPoPE show the formation of a new lipid phase with peptide, which is not seen in the pure lipid samples. Disordering of the L(α) phase is evidenced by broadening of the diffraction peaks, and the hexagonal cell parameter is decreased with peptide present. Our results suggest that the FeLV peptide is increasing the negative curvature of the lipid system, which is thought to be crucial to the formation of highly bent, high-energy structural fusion intermediates, such as the 'stalk' model. Fusion activity for this putative fusogenic peptide was also demonstrated, using a resonance energy transfer (RET) lipid mixing assay. To our knowledge, this work provides the first published experimental evidence of both fusogenic activity and effects on lipid polymorphism for the FeLV fusion peptide.
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