Correlating Sterol Structure with Membrane Solubility Limits and Phase Behavior in Ternary Model Membranes

Abstract

Evolution selected for an array of structurally similar sterols in cell membranes. While animal cells rely on cholesterol, yeast utilize ergosterol, and plants incorporate a variety of phytosterols including stigmasterol and s-sitosterol. These sterols differ by one or two degrees of unsaturation and/or by an added methyl or ethyl group on their tail; these subtle changes have substantial effects on their membrane solubility. A previous study found the solubility limits of cholesterol, ergosterol, stigmasterol, and s-sitosterol in 1:1 DOPC/DPPC bilayers to be 65-70 mol %, 30-35 mol %, 20-25 mol %, and ∼40 mol % [Stevens, M. M.; Honerkamp-Smith, A. R.; Keller, S. L. Soft Matter, 2010, 6, 5882-5890]. Here we use fluorescence microscopy to correlate the structural features of ergosterol, stigmasterol, s-sitosterol with the miscibility temperature of ternary membranes containing DOPC, DPPC, and one of the aforementioned sterols. We map the full ternary phase diagram of each system and compare it with that of the well-characterized DOPC/DPPC/cholesterol system. By dark-field microscopy we determine upper bounds of the solubility limits of each sterol in DOPC/DPPC membranes, and we also provide qualitative tie lines for each system. Our data reveal that slight changes in sterol structure have strong effects on phase behavior and membrane solubility. Our results provide insight into intermolecular interactions between sterols and PC-lipids. This research was supported by NSF MCB-1402059.