文摘
Recently, several indications have been found that suggest a preferential interaction betweencholesterol and tryptophan residues located near the membrane-water interface. The aim of this studywas to investigate by direct methods how tryptophan and cholesterol interact with each other and whatthe possible consequences are for membrane organization. For this purpose, we used cholesterol-containingmodel membranes of dimyristoylphosphatidylcholine (DMPC) in which a transmembrane model peptidewith flanking tryptophans [acetyl-GWW(LA)8LWWA-amide], called WALP23, was incorporated to mimicinterfacial tryptophans of membrane proteins. These model systems were studied with two complementarymethods. (1) Steady-state and time-resolved Förster resonance energy transfer (FRET) experimentsemploying the fluorescent cholesterol analogue dehydroergosterol (DHE) in combination with a competitionexperiment with cholesterol were used to obtain information about the distribution of cholesterol in thebilayer in the presence of WALP23. The results were consistent with a random distribution of cholesterolwhich indicates that cholesterol and interfacial tryptophans are not preferentially located next to eachother in these bilayer systems. (2) Solid-state 2H NMR experiments employing either deuterated cholesterolor indole ring-deuterated WALP23 peptides were performed to study the orientation and dynamics ofboth molecules. The results showed that the quadrupolar splittings of labeled cholesterol were not affectedby an interaction with tryptophan-flanked peptides and, vice versa, that the quadrupolar splittings of labeledindole 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 systemsstudied here, there is no evidence for a preferential interaction between cholesterol and tryptophans locatedat the bilayer interface.
