Structural aspects of the interaction of bee venom peptide melittin with phospholipids

Structure, location, and lipid perturbations of melittin at the membrane interface

Melittin is arguably the most widely studied amphipathic, membrane-lytic alpha-helical peptide. Although several lines of evidence suggest an interfacial membrane location at low concentrations, melittin's exact position and depth of penetration into the hydrocarbon core are unknown. Furthermore, the structural basis for its lytic action remains largely a matter of conjecture. Using a novel x-ray absolute-scale refinement method, we have now determined the location, orientation, and likely conformation of monomeric melittin in oriented phosphocholine lipid multilayers. Its helical axis is aligned parallel to the bilayer plane at the depth of the glycerol groups, but its average conformation differs from the crystallographic structure. As observed earlier for another amphipathic alpha-helical peptide, the lipid perturbations induced by melittin are remarkably modest. Small bilayer perturbations thus appear to be a general feature of amphipathic helices at low concentrations. In contrast, a dimeric form of melittin causes larger structural perturbations under otherwise identical conditions. These results provide direct structural evidence that self-association of amphipathic helices may be the crucial initial step toward membrane lysis.

Measurement of the secondary structure of adsorbed protein by circular dichroism. 1. Measurements of the helix content of adsorbed melittin

A new circular dichroism (CD) technique is presented which quantifies, in situ, the changes in protein and peptide secondary structure upon adsorption at the quartz/liquid interface. Far-UV CD spectra of adsorbed proteins were recorded from several quartz interfaces contained in a specially constructed cell. Adsorbed, oriented alpha-helical spectra were recorded from hydrophilic and hydrophobic quartz using the bee venom peptide, melittin, which can be induced into an alpha-helical, tetrameric conformation in solution. The hydrophobic quartz provides a model system for oil-in-water emulsions and cell membranes. Surface concentrations were determined by radio-counting and were dependent on the nature of the surface. The characterization of these spectra has been partly achieved using far-UV CD spectra obtained from melittin adsorbed onto hydrophilic colloidal silica particles, where orientation effects are eliminated. Analysis of these spectra reveals considerable denaturation of the helical structures upon adsorption. Surface concentrations from the silica were determined from adsorption isotherms. The surface orientation of adsorbed melittin was dependent on the state of aggregation and hence degree of helicity of the molecule. These results support a model for the mode of action of melittin in lysing membranes.

Melittin induced voltage-dependent conductance in DOPC lipid bilayers

Melittin-induced conductance was measured on planar bilayers made from dioleoylphosphatidylcholine. Upon application of a fixed voltage, the current response was monophasic and remained so even after prolonged observation times. The conductance of melittin-doped bilayers increased exponentially with voltage. In addition, an ohmic contribution appeared after some current had passed. The voltage-dependent conductance increased e-fold every 22 mV and was proportional to the fourth power of the aqueous monomeric peptide concentration, for all salt concentrations investigated (0.4-1.8 M NaCl). Discrete conductance steps could be resolved at all these salt concentrations. The amplitudes of these steps were highly variable. In each experiment, conductance was initially only observed for potentials which were positive on the side of peptide addition. As more and more current passed across the bilayer, the current-voltage curves became symmetric. The system needed some time to reach stationary current-voltage characteristics: about 50 min at pH 7 but only about 15 min at pH 8, suggesting involvement of the N-terminus (pK around 7.5) of melittin in the slow formation of a 'prepore'.

Electrostatics of a peptide at a membrane/water interface. The pH dependence of melittin association with lipid vesicles

The association of the peptide melittin with small unilamellar DMPC vesicles was studied as a function of pH. The results are discussed quantitatively assuming a water-membrane partition equilibrium. Electrostatic surface charging is taken into account as more and more of the strongly basic peptide accumulates at the bilayer/water interface. The data could be well described in terms of a Gouy-Chapman approach involving an effective interfacial charge well below the actual physical charge carried by the individual peptide molecules. The partition coefficient turned out to be pH invariant, so that one can exclude deprotonation reactions upon insertion of the peptide into the bilayer. The effective interfacial charge per associated melittin molecule decreased over a broad range of pH (pH 7 to pH above 10). Contributions of the free amino terminus and of the arginine residues could be determined by comparing with results obtained using modified melittin (N-terminally formylated and fully acetylated). The data suggest approximately equal fractional contributions of the amino terminus and the three lysines to the effective interfacial charge. The two arginines contribute less. Thus, they may be located farther away from the interface or be closely associated with counter-ions. The analysis is extended to the effect of different ionic strengths.

Thermodynamic and kinetic studies on the association of melittin with a phospholipid bilayer

Association of the amphiphilic peptide melittin with unilamellar vesicles of dioleoylphosphatidylcholine has been experimentally investigated by means of circular dichroism, fluorescence energy transfer and stopped-flow experiments. Circular dichroism changes upon titration of the peptide with vesicles (at low salt concentration) were analyzed to yield thermodynamic association isotherms. These isotherms are quantitatively interpreted in terms of a monomer-monomer partitioning of melittin between the aqueous and bilayer media. The data can be very well fitted by theoretical curves based on a Gouy-Chapman surface potential. Energy transfer involving chemically modified tryptophan confirms a lack of aggregation of the associated peptide. According to the kinetic measurements the association proceeds in practice as a one-step process, which is rather fast but not fully diffusion-controlled. We propose a simple mechanism where the inherent conformational transition determines the overall rate.

The structure of melittin in membranes

The conformation of the polypeptide melittin in lipid membranes as determined by Raman spectroscopy is a bent alpha-helix formed by the mainly hydrophobic residues 1-21, and a nonhelical COOH-terminal segment of the hydrophilic residues 22-26. Fluorescence quenching experiments on residue Trp19 reveal that all COOH-termini are located on that side of a vesicular membrane to which melittin was added. By means of fluorescence energy transfer between unmodified and modified Trp19 residues, melittin is shown to aggregate in membranes predominantly in the form of tetramers. These and previous results on the location and orientation of melittin permit the development of a model for the structure of melittin tetramers in membranes. The hydrophilic sides of four bilayer-spanning helices face each other to form a hydrophilic pore through the membrane.