Charged or aromatic anchor residue dependence of transmembrane peptide tilt

The membrane-spanning segments of integral membrane proteins often are flanked by aromatic or charged amino acid residues, which may "anchor" the transmembrane orientation. Single spanning transmembrane peptides such as those of the WALP family, acetyl-GWW(LA)(n)LWWA-amide, furthermore adopt a moderate average tilt within lipid bilayer membranes. To understand the anchor residue dependence of the tilt, we introduce Leu-Ala "spacers" between paired anchors and in some cases replace the outer tryptophans. The resulting peptides, acetyl-GX(2)ALW(LA)(6)LWLAX(22)A-amide, have Trp, Lys, Arg, or Gly in the two X positions. The apparent average orientations of the core helical sequences were determined in oriented phosphatidylcholine bilayer membranes of varying thickness using solid-state (2)H NMR spectroscopy. When X is Lys, Arg, or Gly, the direction of the tilt is essentially constant in different lipids and presumably is dictated by the tryptophans (Trp(5) and Trp(19)) that flank the inner helical core. The Leu-Ala spacers are no longer helical. The magnitude of the apparent helix tilt furthermore scales nicely with the bilayer thickness except when X is Trp. When X is Trp, the direction of tilt is less well defined in each phosphatidylcholine bilayer and varies up to 70° among 1,2-dioleoyl-sn-glycero-3-phosphocholine, 1,2-dimyristoyl-sn-glycero-3-phosphocholine, and 1,2-dilauroyl-sn-glycero-3-phosphocholine bilayer membranes. Indeed, the X = Trp case parallels earlier observations in which WALP family peptides having multiple Trp anchors show little dependence of the apparent tilt magnitude on bilayer thickness. The results shed new light on the interactions of arginine, lysine, tryptophan, and even glycine at lipid bilayer membrane interfaces.

Polar groups in membrane channels: consequences of replacing alanines with serines in membrane-spanning gramicidin channels

To explore the consequences of burying polar, hydrogen-bonding hydroxyl groups within the hydrocarbon core of lipid bilayer membranes, we examined the structural and functional effects of alanine-to-serine substitutions in bilayer-spanning gramicidin channels. A native Ala was replaced by Ser at position 3 or 5 in the gramicidin A (gA) sequence: formyl-VG(2)A(3)LA(5)VVVWLWLWLW-ethanolamide (d-residues underlined). In the head-to-head dimers that form the conducting, membrane-spanning gA channels, these sequence positions are located near the lipid bilayer center (and subunit interface). The sequence substitutions at positions 3 and 5 were tested within the context of having either Gly or d-Ala at position 2, because d-Ala(2) causes the channel lifetimes to increase 3-fold relative to Gly(2) [Mattice et al. (1995) Biochemistry 34, 6827]. Size-exclusion chromatograms and circular dichroism spectra show that the Ala --> Ser replacements are well tolerated and have little effect on channel structure. In planar bilayers, the Ser-substituted gramicidins form well-defined channels, with cation conductances that are approximately 60% of those of the reference channels. The Ser-substituted channels are structurally equivalent to native gramicidin channels, as demonstrated by the formation of heterodimeric channels between a Ser-containing subunit and a native gramicidin subunit. These hybrid channels exhibit rectification, attributable to asymmetric placement of the single Ser hydroxyl group with respect to the bilayer center. Compared to the corresponding Ala-containing reference channels, the polar Ser residues decrease the analogues' channel-forming potency by 3 orders of magnitude, indicating a substantial energetic penalty ( approximately 15 kJ/mol) for burying the polar Ser side chain in the bilayer hydrophobic core.

Comparison of "Polarization inversion with spin exchange at magic angle" and "geometric analysis of labeled alanines" methods for transmembrane helix alignment

Using the model alpha-helical peptide acetyl-GGALW5LALALALALALALW19LAGA-ethanolamide ("GWALP23"), we have compared the polarization inversion with spin exchange at magic angle method and geometric analysis of labeled alanines method for estimating the transmembrane helix orientation. For GWALP23 in bilayers of a short lipid, dilauroylphosphatidylcholine, we find general agreement between the two methods, with a static helix tilt of about 11degrees-13degrees with respect to the bilayer normal.

Is there a preferential interaction between cholesterol and tryptophan residues in membrane proteins?

Recently, several indications have been found that suggest a preferential interaction between cholesterol and tryptophan residues located near the membrane-water interface. The aim of this study was to investigate by direct methods how tryptophan and cholesterol interact with each other and what the possible consequences are for membrane organization. For this purpose, we used cholesterol-containing model membranes of dimyristoylphosphatidylcholine (DMPC) in which a transmembrane model peptide with flanking tryptophans [acetyl-GWW(LA)8LWWA-amide], called WALP23, was incorporated to mimic interfacial tryptophans of membrane proteins. These model systems were studied with two complementary methods. (1) Steady-state and time-resolved Förster resonance energy transfer (FRET) experiments employing the fluorescent cholesterol analogue dehydroergosterol (DHE) in combination with a competition experiment with cholesterol were used to obtain information about the distribution of cholesterol in the bilayer in the presence of WALP23. The results were consistent with a random distribution of cholesterol which indicates that cholesterol and interfacial tryptophans are not preferentially located next to each other in these bilayer systems. (2) Solid-state 2H NMR experiments employing either deuterated cholesterol or indole ring-deuterated WALP23 peptides were performed to study the orientation and dynamics of both molecules. The results showed that the quadrupolar splittings of labeled cholesterol were not affected by an interaction with tryptophan-flanked peptides and, vice versa, that the quadrupolar splittings of labeled indole rings in WALP23 are not significantly influenced by addition of cholesterol to the bilayer. Therefore, both NMR and fluorescence spectroscopy results independently show that, at least in the model systems studied here, there is no evidence for a preferential interaction between cholesterol and tryptophans located at the bilayer interface.

Helical distortion in tryptophan- and lysine-anchored membrane-spanning alpha-helices as a function of hydrophobic mismatch: a solid-state deuterium NMR investigation using the geometric analysis of labeled alanines method

We used solid-state deuterium NMR spectroscopy and geometric analysis of labeled alanines to investigate the structure and orientation of a designed synthetic hydrophobic, membrane-spanning alpha-helical peptide that is anchored within phosphatidylcholine (PC) bilayers using both Trp and Lys side chains near the membrane/water interface. The 23-amino-acid peptide consists of an alternating Leu/Ala core sequence that is expected to be alpha-helical, flanked by aromatic and then cationic anchors at both ends of the peptide: acetyl-GKALW(LA)(6)LWLAKA-amide (KWALP23). The geometric analysis of labeled alanines method was elaborated to permit the incorporation and assignment of multiple alanine labels within a single synthetic peptide. Peptides were incorporated into oriented bilayers of dilauroyl- (di-C12:0-), dimyristoyl- (di-C14:0-), or dioleoyl- (di-C18:1c-) PC. In the C12:0 and C14:0 lipids, the (2)H-NMR quadrupolar splittings for the set of six core alanines could not be fit to a canonical undistorted alpha-helix. Rather, we found that a model containing a helical distortion, such as a localized discontinuity or "kink" near the peptide and bilayer center, could fit the data for KWALP23 in these shorter lipids. The suggestion of helix distortion was confirmed by (2)H-NMR spectra for KWALP23 in which Leu(8) was changed to deuterated Ala(8). Further analysis involving reexamination of earlier data led to a similar conclusion that acetyl-GWW(LA)(8)LWWA-amide (WALP23) is distorted in dilauroyl-PC, allowing significant improvement in the fitting of the (2)H-NMR results. In contrast, WALP23 and KWALP23 are well represented as undistorted alpha-helices in dioleoyl-PC, suggesting that the distortion could be a response to hydrophobic mismatch between peptide and lipids.