Importance of Aromatic Anchor Residue Identity and Location for the Tilt and Dynamics of Transmembrane Peptides

Abstract

Aromatic or polar amino acid residues often flank each side of a membrane-spanning alpha helix, whether in an integral membrane protein or a model peptide. The aromatic residues Trp, Tyr and to a lesser extent Phe tend to partition to the membrane-water interface and act as “anchors” to stabilize the transmembrane orientation. The synthetic model peptide, GWALP23 (acetyl-GGALW5(LA)6LW19LAGA-[ethanol]amide), has proven valuable for experimentation, with only one Trp “anchor” near each end of the transmembrane sequence. Indeed, with relatively minimal complications from the peptide dynamics, the average tilt of GWALP23 has been shown to vary systematically in lipid bilayer membranes of different thickness (see J. Biol. Chem. 285, 31723). We have employed 2H-alanines and solid-state NMR spectroscopy to investigate the consequences of moving or replacing W5 or W19 in GWALP23 with selected Tyr, Phe or Trp residues at the same or nearby locations. We find that GWALP23 peptides having Y5, F5 or W5 exhibit essentially the same average tilt in bilayer membranes of DOPC, DMPC or DLPC; with somewhat increased dynamics for the F5 peptide. When “double” anchors are present in Y4,5GWALP23 or F4,5GWALP23, the peptides appear less responsive to the bilayer thickness, as the dynamics become dramatically more extensive. Moving W19 to position 18, a 100° radial change, alters the direction of the helix tilt, as expected. We conclude that, in the absence of other functional groups, the aromatic residues determine the preferred orientations and dynamics of transmembrane peptides. Increased dynamics are observed when the ring hydrogen bonding is removed (Phe), or when two aromatic anchors are present on one side of the core transmembrane sequence.