Orientation and Motion of Tryptophan Interfacial Anchors in Membrane-Spanning Peptides

Abstract

The tryptophans of integral membrane proteins have been suggested to play specific roles as "interfacial anchors", based on their preference for a location near the lipid head groups. Still, the underlying mechanism behind this behavior remains unclear. NMR experiments can provide an important tool to study this interaction in an actual bilayer environment. Here solid-state deuterium nuclear magnetic resonance was used to study the tryptophans in membrane-spanning model peptides from the WALP family (acetyl-GWW(LA)nWWA-ethanolamide with n = 5 and 6.5) in samples of mechanically aligned dimyristoylphosphatidylcholine (DMPC) bilayers. The data indicate that the tryptophans near the C-terminal end of the peptide display a significantly different behavior from those near the N-terminus. This is reflected prominently in a large difference in the motion experienced by the indoles at either end of the peptide, highlighting the directionality of the helix. Nevertheless, our observations indicate high levels of motional freedom for all tryptophans in these membrane spanning domains that exceed the dynamics for the helix itself. These observations signify that steric and dynamic features of the polypeptide context modulate the tryptophan anchoring in the membrane interface. Measurements of WALP19 in the ether-linked DMPC analogue ditetradecylphosphatidylcholine (missing the lipid carbonyls) show very similar Trp dynamics and suggest similar orientations for some or all of the tryptophans. This suggests that the lipid acyl chain carbonyls play at most a minor role in the anchoring interaction between these Trp residues and the DMPC interfacial region.

Channel Replacement Therapy for Cystic Fibrosis

“…In addition to stabilizing the concentration of monomer in aqueous solutions, inclusion of any C-terminal aromatic residue provided a lipid/water interfacial anchor. As was seen with W, F residues are commonly present (and Y to a lesser extent) at the hydrophobic/hydrophilic interface for TM segments of membrane proteins (Braun and von Heijne, 1999;Mall et al, 2000;Demmers et al, 2001;de Planque et al, 2003;Granseth et al, 2005;van der Wel et al, 2007;Hong et al, 2007 Comparing all of the summarized data in Tables 3 and 4 and the profiles shown in Figs. 16 and 17, the sequences displaying the highest ion transport activity and the second and third most left-shifted k 1/2 values are both W containing peptide sequences, NK 4 -M2GlyR-p22 T19R, S22W and NK 4 -M2GlyR-p22 Q21W, S22R.…”

“…A propensity for tryptophans residing at the aqueous/lipid interface also had been observed and tested by others (Braun and von Heijne, 1999;Mall et al, 2000;Demmers et al, 2001;de Planque et al, 2003;Granseth et al, 2005;van der Wel et al, 2007;Hong et al, 2007). Placing a W at the Cterminus sets the registry of the TM segment that spans the bilayer.…”

The Preference of Tryptophan for Membrane Interfaces

“…More recently, it was shown that Trps near the C terminus of a membrane-spanning ␣-helix display a distinctly different behavior from those near the N terminus, reflected in large differences in the allowed side chain torsion angles and ring motions, highlighting the directionality of an ␣-helix relative to the bilayer/solution interface (67).…”

Amphipathic Solvation of Indole: Implications for the Role of Tryptophan in Membrane Proteins

“…13 Considering its predominantly hydrophobic structure, the Trp side chain is expected to have an inherent affinity for the lipid bilayer. 13 Although it has been long known that Trp locates preferentially at the membrane−water interface, the mechanism by which this binding occurs is less clear. Previous binding studies have shown that indole may be partly exposed to a polar environment in bilayers but have not specified its exact location.…”