The interaction with phospholipids of bee venom melittin: a structural study of the Peptide and lipid components.

Upon examination by circular dichroism, photon correlation spectroscopy and nuclear magnetic resonance spectroscopy, melittin appeared to exist, in pure water at neutral pH, as a flexible random-coil monomer. In dilute NaCl it was still monomeric and essentially random-coil, but presented a pronounced rigidity of structure, and could be approximated to a prolate ellipsoid. At high ionic strength or in the presence of divalent anions, melittin molecules associated into compact disk-like tetramers, where each protomer consisted essentially of two right-handed helical stretches, connected presumably by a 135° bend at the level of Pro14. Correlations could be established between the binding of phosphate ions, evaluated by 31P-NMR, and the structural variations undergone by the peptide. A somewhat different helical tetramer was also obtained at alkaline pH. Upon binding to phospholipids, the conformation of each melittin protomer was similar to that occurring in aqueous phosphate solutions, with a grouping of polar residues along one face of the molecule. The Gln and Lys residues were however more strongly immobilized, and there was no NMR evidence for any self-aggregation of the peptide. Although melittin exerted a dramatic effect on the permeability of phospholipid vesicles to water and to water solutes, the bilayer structure of the vesicles was preserved, as demonstrated by low-angle x-ray and neutron diffraction analyses. Melittin could be shown to be present both at the center of the bilayers and in the aqueous region separating them. The increased permeability to water corresponded, when examined by low-angle neutron diffraction in H2O/D2O exchange experiments, to a deeper penetration of water in the lipid region of the bilayers, up to 0.4 nm from the center (at 95% relative humidity). The presence of water appeared in all cases essential for the incorporation of melittin in the bilayers.