Are Leucine and Isoleucine Equivalent in Binding of Amphipathic Peptides to Membranes

Abstract

Lysette is a 22 amino acid peptide derived from staphylococcal δ-lysin that forms an amphipathic α-helix when bound at membrane-water interfaces. We previously found that the experimentally determined ΔG° of binding for lysette is more favorable than that predicted by the Wimley-White interfacial hydrophobicity scale by about 4 kcal/mol. Closer investigation of the amino acid composition of other peptides that are well described by the Wimley-White interfacial scale, such as melittin, led us to hypothesize that a preponderance of isoleucine (Ile) over leucine residues (Leu), as found in lysette, may be responsible for the deviation from the Wimley-White prediction. To test our hypothesis, we designed lysette variants lysette-I and lysette-L. In lysette-I, all Leu residues were replaced by Ile, and in lysette-L, all Ile were replaced by Leu. We also designed a melittin variant, iso-melittin, in which all Leu residues were replace by Ile. Peptide-lipid equilibrium dissociation constants and helicities of peptides bound to zwitterionic phosphatidylcholine (POPC) vesicles were determined by stopped-flow fluorescence and circular dichroism. If the hypothesis were correct, Lysette-I and iso-melittin should bind significantly better to zwitterionic bilayers than their Leu-rich counterparts, if the helicities of the lipid-bound peptides were not influenced by the Leu-Ile substitution. We found that the lysette-I bound significantly better to POPC vesicles than the lysette-L variant, as predicted. In the case of melittin, however, iso-melittin bound with roughly the same affinity to POPC bilayers as the original Leu-rich melittin. The results are discussed in terms of peptide structures in solution and when bound at the water-lipid interface, and the location of the lipid-bound states in the membrane.This work was supported in part by NIH grants AI088567 and GM072507.