A Synergistic Effect between Cholesterol and Tryptophan-Flanked Transmembrane Helices Modulates Membrane Curvature

Abstract

The aim of this study was to gain insight into the structural consequences of hydrophobic mismatch for membrane proteins in lipid bilayers that contain cholesterol. For this purpose, tryptophanflanked peptides, designed to mimic transmembrane segments of membrane proteins, were incorporated in model membranes of unsaturated phosphatidylcholine bilayers of varying thickness and containing varying amounts of cholesterol. Analysis of the lipid organization by 31 P NMR and cryo-TEM demonstrated the formation of an isotropic phase, most likely representing a cubic phase, which occurred exclusively in mixtures containing lipids with relatively long acyl chains. Formation of this phase was inhibited by incorporation of lysophosphatidylcholine. These results indicate that the isotropic phase is formed as a consequence of negative hydrophobic mismatch and that its formation is related to a negative membrane curvature. When either peptide or cholesterol was omitted from the mixture, isotropic-phase formation did not occur, not even when the concentrations of these compounds were significantly increased. This suggests that formation of the isotropic phase is the result of a synergistic effect between the peptides and cholesterol. Interestingly, isotropic-phase formation was not observed when the tryptophans in the peptide were replaced by either lysines or histidines. We propose a model for the mechanism of this synergistic effect, in which its dependence on the flanking residues is explained by preferential interactions between cholesterol and tryptophan residues.

Biogenesis of caveolae: a structural model for caveolin-induced domain formation

“…Substitution of tryptophan, rather than loss of cysteine, is important for this effect (Carozzi et al, 2002), which suggests that an additional tryptophan residue could influence association of cholesterol with caveolae. This is an interesting possibility given a recent in vitro study showing that tryptophan-cholesterol interactions can modulate bilayer curvature (van Duyl et al, 2005). Thus, the interaction of caveolin with cholesterol may be fundamental to the generation of caveolae.…”

“…Cholesterol clearly plays an important role in the formation of caveolae but its concentration in the cytoplasmic leaflet of the curved caveolar bulb is energetically less favourable owing to its negative spontaneous curvature (van Duyl et al, 2005). Differences in the lengths of the transmembrane segments of caveolin might therefore be functionally important.…”

“…Differences in the lengths of the transmembrane segments of caveolin might therefore be functionally important. For instance, a negative hydrophobic mismatch of helix ␣2b and/or the insertion of the scaffolding domain as an in-plane helix can generate local membrane deformations that have positive curvature, thus stabilising cholesterol in the vicinity (van Duyl et al, 2005). Alternatively, this could create a curved protein surface, as in the case of amphiphysin, in which membrane curvature is generated through the electrostatic interaction between phospholipids and the concave surface of the BAR (Bin/amphiphysin/Rvs) domain (Peter et al, 2004).…”

Effect of Lipid Composition on the Topography of Membrane-Associated Hydrophobic Helices: Stabilization of Transmembrane Topography by Anionic Lipids

“…These sequences had weakly hydrophobic cores composed of Ala with or without a few Leu (Table 1, sequences [10][11][12][13][14]. When these peptides were inserted into DOPS vesicles highly blue-shifted λmax and low Qratio values were generally observed (with the exception of KK pA 22 peptide which showed a somewhat more red shifted λmax and high Q-ratio), showing that a TM state predominated. In contrast, a non-TM state predominated in the presence of vesicles composed of DOPC (Table 2).…”

“…Synthetic hydrophobic peptides, frequently Lys-flanked polyLeu or polyLeu-Ala helices, have been successfully utilized as models for hydrophobic α-helical TM domains by our lab and many others [18][19][20][21][22][23][24] . In this report fluorescence spectroscopy was used to study the lipid compositiondependence of the TM stability of these and closely related sequences.…”