Canonical azimuthal rotations and flanking residues constrain the orientation of transmembrane helices

Comparisons of interfacial Phe, Tyr, and Trp residues as determinants of orientation and dynamics for GWALP transmembrane peptides

Aromatic amino acids often flank the transmembrane alpha helices of integral membrane proteins. By favoring locations within the membrane–water interface of the lipid bilayer, aromatic residues Trp, Tyr, and sometimes Phe may serve as anchors to help stabilize a transmembrane orientation. In this work, we compare the influence of interfacial Trp, Tyr, or Phe residues upon the properties of tilted helical transmembrane peptides. For such comparisons, it has been critical to start with no more than one interfacial aromatic residue near each end of a transmembrane helix, for example, that of GWALP23 (acetyl-GGALW⁵(LA)₆LW¹⁹LAGA-[ethanol]amide). To this end, we have employed ²H-labeled 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 proximate locations. We find that GWALP23 peptides having F5, Y5, or W5 exhibit essentially the same average tilt and similar dynamics in bilayer membranes of 1,2-dilauroylphosphatidylcholine (DLPC) or 1,2-dioleoylphosphatidylcholine (DOPC). When double Tyr anchors are present, in Y^4,5GWALP23 the NMR observables are markedly more subject to dynamic averaging and at the same time are less responsive to the bilayer thickness. Decreased dynamics are nevertheless observed when ring hydrogen bonding is removed, such that F^4,5GWALP23 exhibits a similar extent of low dynamic averaging as GWALP23 itself. When F5 is the sole aromatic group in the N-interfacial region, the dynamic averaging is (only) slightly more extensive than with W5, Y5, or Y4 alone or with F4,5, yet it is much less than that observed for Y^4,5GWALP23. Interestingly, moving Y5 to Y4 or W19 to W18, while retaining only one hydrogen-bond-capable aromatic ring at each interface, maintains the low level of dynamic averaging but alters the helix azimuthal rotation. The rotation change is about 40° for Y4 regardless of whether the host lipid bilayer is DLPC or DOPC. The rotational change (Δρ) is more dramatic and more complex when W19 is moved to W18, as Δρ is about +90° in DLPC but about −60° in DOPC. Possible reasons for this curious lipid-dependent helix rotation could include not only the separation distances between flanking aromatic or hydrophobic residues but also the absolute location of the W19 indole ring. For the more usual cases, when the helix azimuthal rotation shows little dependence on the host bilayer identity, excepting W¹⁸GWALP23, the transmembrane helices adapt to different lipids primarily by changing the magnitude of their tilt. We conclude that, in the absence of other functional groups, interfacial aromatic residues determine the preferred orientations and dynamics of membrane-spanning peptides. The results furthermore suggest possibilities for rotational and dynamic control of membrane protein function.

Single tryptophan and tyrosine comparisons in the N-terminal and C-terminal interface regions of transmembrane GWALP peptides

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 W(5) or W(19), or both of them, to tyrosine in the well-characterized transmembrane peptide acetyl-GGALW(5)(LA)6LW(19)LAGA-amide ("GWALP23"). By means of solid-state (2)H and (15)N NMR experiments, we find that Y(19)GW(5)ALP23 displays similar magnitudes of peptide helix tilt as Y(5)GW(19)ALP23 and responds similarly to changes in bilayer thickness, from DLPC to DMPC to DOPC. The presence of Y(19) changes the azimuthal rotation angle ρ (about the helix axis) to a similar extent as Y(5), but in the opposite direction. When tyrosines are substituted for both tryptophans to yield GY(5,19)ALP23, the helix tilt angle is again of comparable magnitude, and furthermore, the preferred azimuthal rotation angle ρ is relatively unchanged from that of GW(5,19)ALP23. 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.