MOLECULAR-BASIS FOR PROKARYOTIC SPECIFICITY OF MAGAININ-INDUCED LYSIS

Magainins and mastoparans are examples of peptide antibiotics and pept ide venoms, respectively. They have been grouped together as class L a mphipathic helixes [Segrest, J. P., et al. (1990) Proteins 8, 103-117] because of similarities in the distribution of Lys residues along the polar face of the helix. Class L venoms lyse both eukaryotic and prok aryotic cells whereas class L antibiotics specifically lyse bacteria. The structural basis for the specificity of class L antibiotics is not well understood. Sequence analysis showed that class L antibiotics ha ve a Glu residue on the nonpolar face of the amphipathic helix; this i s absent from class L venoms. We synthesized three model class L pepti des with or without Glu on the nonpolar face: 18L(MG) (LGSIWKFIKAFVGGI KKF), [E(14)]18L(MG) and [G(5),E(14)]18L(MG). Hemolysis, bacteriolysis , and bacteriostasis studies using these peptides showed that the spec ificity of lysis is due to both the presence of a Glu residue on the n onpolar face of the helix and the bulk of the nonpolar face. Studies u sing large unilamellar phospholipid vesicles showed that the inclusion of cholesterol greatly inhibited leakage by the two Glu-containing pe ptides. These results cannot be attributed to changes in the phase beh avior of the lipids caused by the inclusion of cholesterol or to diffe rences in the secondary structure of the peptides. These results sugge st that eukaryotic cells are resistant to lysis by magainins because o f peptide-cholesterol interactions in their membranes that inhibit the formation of peptide structures capable of lysis, perhaps by hydrogen bonding between Glu and cholesterol. Bacterial membranes, lacking cho lesterol, are susceptible to lysis by magainins.