Influence of Lipid/Peptide Hydrophobic Mismatch on the Thickness of Diacylphosphatidylcholine Bilayers. A ²H NMR and ESR Study Using Designed Transmembrane α-Helical Peptides and Gramicidin A

Abstract

We have investigated the effect of a series of hydrophobic polypeptides (WALP peptides) on the mean hydrophobic thickness of (chain-perdeuterated) phosphatidylcholines (PCs) with different acyl chain length, using 2H NMR and ESR techniques. The WALP peptides are uncharged and consist of a sequence with variable length of alternating leucine and alanine, flanked on both sides by two tryptophans, and with the N- and C-termini blocked, e.g., FmAW2(LA)nW2AEtn. 2H NMR measurements showed that the shortest peptide with a total length of 16 amino acids (WALP16) causes an increase of 0.6 A in bilayer thickness in di-C12-PC, a smaller increase in di-C14-PC, no effect in di-C16-PC, and a decrease of 0.4 A in di-C18-PC, which was the largest decrease observed in any of the peptide/lipid systems. The longest peptide, WALP19, in di-C12-PC caused the largest increase in thickness of the series (+1.4 A), which decreased again for longer lipids toward di-C18-PC, in which no effect was noticed. WALP17 displayed an influence intermediate between that of WALP16 and WALP19. Altogether, incorporation of the WALP peptides was found to result in small but very systematic changes in bilayer thickness and area per lipid molecule, depending on the difference in hydrophobic length between the peptide and the lipid bilayer in the liquid-crystalline phase. ESR measurements with spin-labeled lipid probes confirmed this result. Because thickness is expected to be influenced most at the lipids directly adjacent to the peptides, also the maximal adaptation of these first-shell lipids was estimated. The calculation was based on the assumption that there is little or no aggregation of the WALP peptides, as was supported by ESR, and that lipid exchange is rapid on the 2H NMR time scale. It was found that even the maximal possible changes in first-shell lipid length were relatively small and represented only a partial response to mismatch. The synthetic WALP peptides are structurally related to the gramicidin channel, which was therefore used for comparison. In most lipid systems, gramicidin proved to be a stronger perturber of bilayer thickness than WALP19, although its length should approximate that of the shorter WALP16. The effects of gramicidin and WALP peptides on bilayer thickness were evaluated with respect to previous 31P NMR studies on the effects of these peptides on macroscopic lipid phase behavior. Both approaches indicate that, in addition to the effective hydrophobic length, also the physical nature of the peptide surface is a modulator of lipid order.

Self-association of Transmembrane α-Helices in Model Membranes

“…Dimer formation, rather than trimer or oligomer formation, has also been proposed for Trp-or dibromotyrosine-containing Lys-flanked poly-Leu peptide analogues in liquid crystalline bilayers (12). This is also in agreement with previous ESR measurements, which indicated that WALP and L 24 (Ac-K 2 L 24 K 2 -amide) peptides are present as monomers or dimers, even at high peptide/ lipid ratios (20,42,43).…”

Photo-Crosslinking Analysis of Preferential Interactions between a Transmembrane Peptide and Matching Lipids

“…Also previous ESR studies on boundary lipids can be taken as support of the hypothesis that larger membrane proteins allow a more stable interaction with surrounding lipids. In these studies boundary lipids are generally readily identified in large multispanning proteins, but they are often not observed for single-span peptides, such as the WALP peptides (12). Together, these results imply that within biological membranes the size and rigidity of the transmembrane domains of membrane proteins are major determinants for the interaction with the membrane lipids.…”

Modulation of the bilayer thickness of exocytic pathway membranes by membrane proteins rather than cholesterol

“…Supporting this possibility, Weiss et al (61) have recently shown, using proteosomes consisting of pure phospholipid and the membrane protein gramicidin, that phospholipids can clearly modulate their bilayer thickness to match the hydrophobic thickness of gramicidin channels. Likely, transmembrane domains modulate bilayer thickness by modulating lipid acyl chain conformation and packing by means of the hydrophobic effect (19,62). The negligible contribution of transmembrane domains to bilayer thickness in ER͞apical plasma membranes is probably due to the large percentage of dolichol͞sphingomyelin in these membranes, both of which may diminish the malleability of the bulk lipid to conform to the hydrophobic length of membrane protein components.…”