Response of GWALP transmembrane peptides to changes in the tryptophan anchor positions

While the interfacial partitioning of charged or aromatic anchor residues may determine the preferred orientations of transmembrane peptide helices, the dependence of helix orientation on anchor residue position is not well understood. When anchor residue locations are changed systematically, some adaptations of the peptide-lipid interactions may be required to compensate for the altered interfacial interactions. Recently, we have developed a novel transmembrane peptide, termed GW ^5,19ALP23 (acetyl-GGALW ⁵LALALALALALALW ¹⁹LAGA-ethanolamide), which proves to be a well-behaved sequence for an orderly investigation of protein-lipid interactions. Its roughly symmetric nature allows for shifting the anchoring Trp residues by one Leu-Ala pair inward (GW ^7,17ALP23) or outward (GW ^3,21ALP23), thus providing fine adjustments of the formal distance between the tryptophan residues. With no other obvious anchoring features present, we postulate that the inter-Trp distance may be crucial for aspects of the peptide-lipid interaction. Importantly, the amino acid composition is identical for each of the resulting related GWALP23 sequences, and the radial separation between the pairs of Trp residues on each side of the transmembrane α-helix remains similar. Here we address the adaptation of the aforementioned peptides to the varying Trp locations by means of solid-state ²H nuclear magnetic resonance experiments in varying lipid bilayer membrane environments. All of the GW ^x,yALP23 sequence isomers adopt transmembrane orientations in DOPC, DMPC, and DLPC environments, even when the Trp residues are quite closely spaced, in GW ^7,17ALP23. Furthermore, the dynamics for each peptide isomer are less extensive than for peptides possessing additional interfacial Trp residues. The helical secondary structure is maintained more strongly within the Trp-flanked core region than outside of the Trp boundaries. Deuterium-labeled tryptophan indole rings in the GW ^x,yALP23 peptides provide additional insights into the behavior of the Trp side chains. A Trp side chain near the C-terminus adopts a different orientation and undergoes somewhat faster dynamics than a corresponding Trp side chain located an equivalent distance from the N-terminus. In contrast, as the inter-Trp distance changes, the variations among the average orientations of the Trp indole rings at either terminus are systematic yet fairly small. We conclude that subtle adjustments to the peptide tilt, and to the N- and C-terminal Trp side chain torsion angles, permit the GW ^x,yALP23 peptides to maintain preferred transmembrane orientations while adapting to lipid bilayers with differing hydrophobic thicknesses.