Polymorphism and interactions of a viral fusion peptide in a compressed lipid monolayer

With a view toward possible new insights into viral fusion mechanisms, we have investigated the HIV-1 gp41 fusion peptide in a monomolecular film of the biomembrane lipid palmitoyloleoylphosphatidylcholine. Its surface activity at an air/water interface was measured under equilibrium conditions, using the conventional Langmuir trough technique. Through a novel thermodynamic analysis, the partial molecular area of the peptide in the lipid moiety could be determined as a function of the lateral pressure and the interfacial peptide/lipid ratio. This indicates an orientation of the peptide backbone parallel to the lipid hydrocarbon tails. The molecular area decreases significantly upon monolayer compression, suggesting a conformational transition from a somewhat compact configuration to a more extended, presumably beta-strand structure when a lipid packing density is approached that is generally believed to mimic the physical state of a biological membrane. Up to a lateral pressure of approximately 15 mN/m, practically all peptide inserts into the lipid monolayer. At higher compression a distinct partitioning into the aqueous subphase is observed. Under these conditions the data also reflect a strong aggregation of the lipid-associated peptide. Beyond a critical peptide/lipid ratio, the peptide's area requirement was found to become substantially enhanced, possibly because of the formation of water-filled pores.