Roles of peptide–peptide charge interaction and lipid phase separation in helix–helix association in lipid bilayer

Abstract

The roles of peptide–peptide charged interaction and lipid phase separation in helix–helix association in lipid bilayers were investigated using a model peptide, P 24, as a transmembrane α-helical peptide, and its four analogues. Fluorescence amino acids, tryptophan (P 24W) and pyrenylalanine (P 24Pya), were introduced into the sequence of P 24, respectively. Association of these peptides permits the resonance excitation energy transfer between tryptophan in P 24W and pyrenylalanine in P 24Pya or excimer formation between P 24Pya themselves. To evaluate the effect of charged interaction on the association between α-helical transmembrane segments in membrane proteins, charged amino acids, glutamic acid (P 24EW) and lysine (P 24KPya), were introduced into P 24W and P 24Pya, respectively. Energy transfer experiments indicated that the charged interaction between the positive charge of lysine residue in P 24KPya and the negative charge of glutamic acid residue in P 24EW did not affect the aggregation of transmembrane peptides in lipid membranes. As the content ratio of sphingomyelin (SM) and cholesterol (Ch) was increased in the egg phosphatidylcholine (PC), the stronger excimer fluorescence spectra of P 24Pya were observed, indicating that the co-existence of SM and Ch in PC liposomes, that is, the raft of SM and Ch, promotes the aggregation of the α-helical transmembrane peptides in lipid bilayers. Since the increase in the contents of SM and Ch leads to the decrease in the content of liquid crystalline-order phase, the moving area of transmembrane peptides might be limited in the liposomes, resulting in easy formation of the excimer in the presence of the lipid-raft.