Sterol Structure Determines Miscibility vs. Melting Transitions in Lipid Vesicles

¹ University of Washington

^* To whom correspondence should be addressed. E-mail: slkeller{at}chem.washington.edu.

Submitted on July 13, 2004
Revised on August 23, 2004
Accepted on 11 May 2005

	Abstract

Lipid bilayer membranes composed of DOPC, DPPC, and certain sterols demix into coexisting liquid phases below a miscibility transition temperature. We use fluorescence microscopy to directly observe phase transitions in vesicles of 1:1:1 DOPC:DPPC:sterol within giant unilamellar vesicles (GUVs). We show that vesicles containing the "promoter" sterols cholesterol, ergosterol, 25-hydroxycholesterol, epicholesterol, or dihydrocholesterol demix into coexisting liquid phases as temperature is lowered through the miscibility transition. In contrast, vesicles containing the "inhibitor" sterols androstenol, coprostanol, cholestenone, or cholestane form coexisting gel (solid) and liquid phases. Vesicles containing lanosterol, a sterol found in the cholesterol and ergosterol synthesis pathways, do not exhibit coexisting phases over a wide range of temperatures and compositions. Although more detailed phase diagrams and precise distinctions between gel and liquid phases are required to fully define the phase behavior of these sterols in vesicles, we find that our classifications of promoter and inhibitor sterols are consistent with previous designations based on fluorescence quenching and detergent resistance (X. Xu and London, E. (2000) Biochemistry 39, 843-849). We find no trend in the liquid-liquid or gel-liquid transition temperatures of membranes with promoter or inhibitor sterols, and measure the surface fraction of coexisting phases. We find that the vesicle phase behavior is related to the structure of the sterols. Promoter sterols have flat, fused rings, a hydroxyl headgroup, an acyl tail, and a small molecular area, which are all attributes of "membrane active" sterols (Y. Barenholz (2002) Prog. Lipid Res. 41, 1-5).

Key Words: Cholesterol, Ergosterol, Lanosterol, Liquid, Phase Behavior, Solid

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