Single Tryptophan and Tyrosine Comparisons in the N-Terminal and C-Terminal Interface Regions of Transmembrane GWALP Peptides
2013
Gleason, Nicholas J. | Greathouse, Denise V. | Grant, Christopher V. | Opella, Stanley J. | Koeppe, Roger E.
Hydrophobic membrane-spanning helices often are flanked by interfacial aromatic or charged residues. In this paper, we compare the consequences of single Trp → Tyr substitutions at each interface for the properties of a defined transmembrane helix in the absence of charged residues. The choice of molecular framework is critical for these single-residue experiments because the presence of “too many” aromatic residues (more than one at either membrane-water interface) introduces excess dynamic averaging of solid state NMR observables. To this end, we compare the outcomes when changing W⁵ or W¹⁹, or both of them, to tyrosine in the well-characterized transmembrane peptide acetyl-GGALW⁵(LA)₆LW¹⁹LAGA-amide (“GWALP23”). By means of solid-state ²H and ¹⁵N NMR experiments, we find that Y¹⁹GW⁵ALP23 displays similar magnitudes of peptide helix tilt as Y⁵GW¹⁹ALP23 and responds similarly to changes in bilayer thickness, from DLPC to DMPC to DOPC. The presence of Y¹⁹ changes the azimuthal rotation angle ρ (about the helix axis) to a similar extent as Y⁵, but in the opposite direction. When tyrosines are substituted for both tryptophans to yield GY⁵,¹⁹ALP23, the helix tilt angle is again of comparable magnitude, and furthermore, the preferred azimuthal rotation angle ρ is relatively unchanged from that of GW⁵,¹⁹ALP23. The extent of dynamic averaging increases marginally when Tyr replaces Trp. Yet, importantly, all members of the peptide family having single Tyr or Trp residues near each interface exhibit only moderate and not highly extensive dynamic averaging. The results provide important benchmarks for evaluating conformational and dynamic control of membrane protein function.
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