The hydrophobic M1 peptide (H2N-Ile-Nle-Gln-Arg-Ile-Pro-Leu-Tyr-Phe-Val-Val-Asn-Val-Ile-Ile-Pro-Cys-Leu-Leu-Phe-Ser-Phe-Leu-Thr-Gly-Leu-Val-Phe-Tyr-Leu-Pro-Thr-Asp-Ser-Gly-COOH) is one of the transmembrane segments of the best characterized member of the ligand-gated ion channels, the NAChR (nicotinic acetylcholine receptor). The secondary structure of this peptide is a distorted α-helix. This distortion could be due by several Proline residues present in the sequence [1].
We have studied the surface properties and the protein-lipid interaction of the transmembrane M1 sequence of the nAChR by using the lipid monolayer technique. Previous works report that M1 peptide adopts α-helical structure in a membrane-like environment (Corbin and Méthod 1998). In absorption experiments, M1 shows a spontaneous adsorption into a lipid free interface reaching a maximal lateral surface pressure of about 27 mN/m and a surface potential of about 200 mV. Pure peptide monolayers show a molecular area of 210 Å2/molecule compatible with an α-helix structure at the interface at maximal packing. The high stability under lateral compression and the low surface potential could be induced by proline residues that distort the helix. Compression isotherms of mixed peptide-lipid monolayers show a dual behavior in the peptide-lipid interaction. The peptide was immiscible when lipids form a condensed interface (DPPC, DPPG) at low peptide proportion, but it was miscible at high peptide concentration. On the other hand, the peptide was miscible in all proportions if the lipid is in the liquid-expanded state (POPC, POPG). For immiscibility or miscibility behavior no significant lipid-peptide interactions were seen (ideal behavior). In peptide penetration experiments, M1 peptide interacts preferentially with POPC compared with the penetration of M1 into POPG monolayers. The results show that transmembrane M1 peptide behaves as an ?ideal partner? with zwitterionic PC in liquid state.
