1.C.18 The Melittin (Melittin) Family
Many organisms synthesize proteins (or peptides) which are degraded to relatively small hydrophobic or amphipathic, bioactive peptides. These peptides exhibit antibiotic, fungicidal, virucidal, hemolytic and/or tumoricidal activities by interacting with membranes and forming transmembrane channels that allow the free flow of electrolytes, metabolites and water across the phospholipid bilayers. Most of these peptides appear to function in biological warfare. There are many designations given to these bioactive peptides. They include the magainins, cecropins, melittins, defensins, bacteriocidins, etc. Certain common structural features observed between members of distinct families suggest that at least some of these families share a common ancestry. The process of pore formation for mellitin in lipid bilayers has been studied in some detail (Lee et al. 2013). Melittin (26 residues) is possibly the best studied of the insect peptide toxins. It is found in the venom of the European honey bee, Apis mellifera. Three-dimensional structures of melittin have been elucidated.
Time-dependent pore formation has been studied in individual giant unilamellar vesicles exposed to a melittin solution (Lee et al., 2008). An individual vescile first expanded its surface area at constant volume and then suddenly reversed expansion of its volume at constant area. The area expansion, the volume expansion, and the point of reversal all matched the results of equilibrium measurements performed on peptide-lipid mixtures. The mechanism includes negative feedback that makes peptide-induced pores stable with a well defined size. Melittin creates transient pores in lipid bilayers (Santo et al. 2013; Wiedman et al. 2013).
Sengupta et al. (2008) used molecular dynamics simulation to study the interaction of a specific class of melittin with a dipalmitoylphosphatidylcholine bilayer. Transmembrane pores spontaneously formed above a critical peptide to lipid ratio. The lipid molecules bent inwards to form a toroidally shaped pore but with only one or two peptides lining the pore, in contrast to the traditional models of toroidal pores in which the peptides are assumed to adopt a transmembrane orientation. Sengupta et al., 2008 reported that peptide aggregation, either prior to or after binding to the membrane surface, is a prerequisite for pore formation, but that the presence of a stable helical secondary structure of the peptide is not. Electrostatic interactions are important in the poration process; removing charges of the basic amino-acid residues of melittin prevents pore formation. In the absence of counter ions, pores not only form more rapidly, but lead to membrane rupture via a novel recursive poration pathway.
A 9-mus all-atom molecular-dynamics simulation starting from a closely packed transmembrane melittin tetramer in DMPC showed formation of a toroidal pore after 1 mus (Leveritt et al. 2015). The pore remains stable with a roughly constant radius for the rest of the simulation. One or two melittin monomers frequently transitioned between transmembrane and surface states. All four peptides were largely helical. A simulation in a DMPC/DMPG membrane did not lead to a stable pore, consistent with the experimentally observed lower activity of melittin in anionic membranes. Thus, a dynamic toroidal pore seems to account for the transport properties of melittin (Leveritt et al. 2015).
The generalized transport reaction catalyzed by channel-forming amphipathic peptides is:
small solutes, electrolytes and water (in) small solutes, electrolytes and water (out).