Effect of Variations in the Structure of a Polyleucine-Based α-Helical Transmembrane Peptide on Its Interaction with Phosphatidylethanolamine Bilayers

Abstract

High-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy were used to study the interaction of a cationic α-helical transmembrane peptide, acetyl-Lys 2-Leu 24-Lys 2-amide (L 24), and members of the homologous series of zwitterionic n-saturated diacyl phosphatidylethanolamines (PEs). Analogs of L 24, in which the lysine residues were replaced by 2,3-diaminopropionic acid (acetyl-DAP 2-Leu 24-DAP 2-amide (L 24DAP)) or in which a leucine residue at each end of the polyleucine sequence was replaced by a tryptophan (Ac-K 2-W-L 22-W-K 2-amide (WL 22W)), were also studied to investigate the roles of lysine side-chain snorkeling and aromatic side-chain interactions with the interfacial region of phospholipid bilayers. The gel/liquid-crystalline phase transition temperature of the PE bilayers is altered by these peptides in a hydrophobic mismatch-independent manner, in contrast to the hydrophobic mismatch-dependent manner observed previously with zwitterionic phosphatidylcholine (PC) and anionic phosphatidylglycerol (PG) bilayers. Moreover, all three peptides reduce the phase transition temperature to a greater extent in PE bilayers than in PC and PG bilayers, indicating a greater disruption of PE gel-phase bilayer organization. Moreover, the lysine-anchored L 24 reduces the phase transition temperature, enthalpy, and the cooperativity of PE bilayers to a much greater extent than DAP-anchored L 24DAP, whereas replacement of the terminal leucines by tryptophan residues (Ac-K 2-W-L 22-W-K 2-amide) only slightly attenuates the effects of this peptide on the chain-melting phase transition of the host PE bilayers. All three peptides form very stable α-helices in PE bilayers, but small conformational changes occur in response to mismatch between peptide hydrophobic length and gel-state lipid bilayer hydrophobic thickness. These results suggest that the lysine snorkeling plays a significant role in the peptide-PE interactions and that cation- π-interactions between lysine and tryptophan residues may modulate these interactions. Altogether, these results suggest that the lipid-peptide interactions are affected not only by the hydrophobic mismatch between these peptides and the host lipid bilayer but also by the electrostatic and hydrogen-bonding interactions between the positively charged lysine residues at the termini of these peptides and the polar headgroups of PE bilayers.