CUBIC PHASES IN PHOSPHATIDYLCHOLINE-CHOLESTEROL MIXTURES: CHOLESTEROL AS MEMBRANE "FUSOGEN"

¹ Northwestern University
² Givaudan, Inc.

^* To whom correspondence should be addressed. E-mail: b-tenchov{at}northwestern.edu.

Submitted on February 22, 2006
Revised on March 29, 2006
Accepted on 23 June 2006

	Abstract

X-ray diffraction reveals that mixtures of some unsaturated phosphatidylcholines (PCs) with cholesterol readily form inverted bicontinuous cubic phases that are stable under physiological conditions. This effect was studied in most detail for dioleoyl PC/Chol mixtures with molar ratios of 1:1 and 3:7. Facile formation of Im3m and Pn3m phases with lattice constants of 30-50 nm and 25-30 nm, respectively, took place in phosphate buffered saline, in sucrose solution and in water near the temperature of the L - H_II transition of the mixtures, as well as during cooling of the H_II phase. Once formed, the cubic phases displayed an ability to supercool and replace the initial L phase over broad range of physiological temperatures. Conversion into stable cubic phases was also observed for mixtures of cholesterol with dilinoleoyl PC, but not for mixtures with palmitoyl-linoleoyl PC or palmitoyl-oleoyl PC, for which only transient cubic traces were recorded at elevated temperatures. A saturated, branched-chain PC, diphytanoyl PC, also displayed a cubic phase in mixture with cholesterol. Unlike the PEs, the membrane PCs are intrinsically non-fusogenic lipids - in excess water they only form lamellar phases and not any of the inverted phases on their own. Thus, the finding that cholesterol induces cubic phases in mixtures with unsaturated PCs may have important implications for its role in fusion. In ternary mixtures, saturated PCs and sphingomyelin are known to separate into liquid-ordered domains along with cholesterol. Our results thus suggest that unsaturated PCs, which are excluded from these domains, could form fusogenic domains with cholesterol. Such a dual role of cholesterol may explain the seemingly paradoxical ability of cell membranes to simultaneously form rigid, low-curvature raft-like patches, while still being able to undergo facile membrane fusion.

Key Words: X-ray diffraction, cholesterol, inverted bicontinuous cubic phase, membrane curvature, membrane fusion, rafts

This article has been cited by other articles:

				D. P. Siegel The Gaussian Curvature Elastic Energy of Intermediates in Membrane Fusion Biophys. J., December 1, 2008; 95(11): 5200 - 5215. [Abstract] [Full Text] [PDF]

				P. Wessman, A. A. Stromstedt, M. Malmsten, and K. Edwards Melittin-Lipid Bilayer Interactions and the Role of Cholesterol Biophys. J., November 1, 2008; 95(9): 4324 - 4336. [Abstract] [Full Text] [PDF]

				C. Lynch III Meyer and Overton Revisited Anesth. Analg., September 1, 2008; 107(3): 864 - 867. [Full Text] [PDF]

				D. Armstrong and R. Zidovetzki Amplification of Diacylglycerol Activation of Protein Kinase C by Cholesterol Biophys. J., June 15, 2008; 94(12): 4700 - 4710. [Abstract] [Full Text] [PDF]

				M. A. Churchward, T. Rogasevskaia, D. M. Brandman, H. Khosravani, P. Nava, J. K. Atkinson, and J. R. Coorssen Specific Lipids Supply Critical Negative Spontaneous Curvature--An Essential Component of Native Ca2+-Triggered Membrane Fusion Biophys. J., May 15, 2008; 94(10): 3976 - 3986. [Abstract] [Full Text] [PDF]

				D. P. Siegel and B. G. Tenchov Influence of the Lamellar Phase Unbinding Energy on the Relative Stability of Lamellar and Inverted Cubic Phases Biophys. J., May 15, 2008; 94(10): 3987 - 3995. [Abstract] [Full Text] [PDF]

				L. Wessels, M. W. Elting, D. Scimeca, and K. Weninger Rapid Membrane Fusion of Individual Virus Particles with Supported Lipid Bilayers Biophys. J., July 15, 2007; 93(2): 526 - 538. [Abstract] [Full Text] [PDF]