Influence of α-helical Transmembrane Peptides on the Affinity of Sterols for Phospholipid Bilayers

Abstract

It is well known that lipids can segregate laterally into nanoscopic domains or different phases. Yet very little is known about proteins can influence the lateral organization in cellular membranes. As most biomembranes contain relatively high concentrations of transmembrane proteins it is important to learn more about how lipid-protein interplay affects the lateral organization in membranes. Cholesterol is thought to have an important role in lateral organization of eukaryotic cell membranes. As cholesterol also has been implicated to take part in the sorting of cellular transmembrane proteins it is a good starting point to determine how transmembrane proteins influence the lateral sorting of cholesterol in phospholipid bilayers. Insight into this can be obtained by studying how cholesterol interacts with bilayer membranes of different composition in the presence of different transmembrane peptides, mimicking the transmembrane helices of proteins. For this purpose an assay, in which the partitioning of the fluorescent cholesterol analogue cholestatrienol (CTL) between large unilamellar vesicles (LUVs) and methyl-β-cyclodextrin (CD) can be measured, has been developed. The partition assay showed that CTL partition preferentially into fluid phospholipid bilayers with a more ordered acyl chain region, as has been observed previously with cholesterol. It is known that proteins can decrease or increase the order in lipid bilayers and that the nature of this effect is dependent on both the structure of the protein and the composition of the bilayer. In order to assess how such protein induced order changes in the lipid bilayer affects cholesterol partitioning we have measured CTL's affinity for bilayers with varying lipid composition and containing various transmembrane peptides.