Sphingomyelin/Phosphatidylcholine/Cholesterol Phase Diagram: Boundaries and Composition of Lipid Rafts

Abstract

The ternary system palmitoylsphingomyelin (PSM)/palmitoyloleoylphosphatidylcholine (POPC)/cholesterol is used to model lipid rafts. The phase behavior of the three binary systems PSM/POPC, PSM/cholesterol, and POPC/cholesterol is first experimentally determined. Phase coexistence boundaries are then determined for ternary mixtures at room temperature (23 degrees C) and the ternary phase diagram at that temperature is obtained. From the diagram at 23 degrees C and the binary phase diagrams, a reasonable expectation is drawn for the ternary phase diagram at 37 degrees C. Several photophysical methodologies are employed that do not involve detergent extraction, in addition to literature data (e.g., differential scanning calorimetry) and thermodynamic rules. For the ternary phase diagrams, some tie-lines are calculated, including the one that contains the PSM/POPC/ cholesterol 1:1:1 mixture, which is often used in model raft studies. The diagrams here described are used to rationalize literature results, some of them apparently discrepant, and to discuss lipid rafts within the framework of liquid-ordered/liquid-disordered phase coexistence.

Rafts in oligodendrocytes: Evidence and structure–function relationship

“…found to be present in DRMs were associated with lipid rafts in the living cell as detergent extraction seems to dramatically alter the lipid composition of preexisting domains (de Almeida et al, 2003;Heerklotz, 2002;Heerklotz et al, 2003;Skwarek, 2004). However, others provide evidence that isolation of DRM from biological membranes by detergent-induced extraction is not an artefact (Staneva et al, 2005).…”

Membrane lipids and vesicular traffic

“…Unexpectedly, anterograde transport at the Golgi is exquisitely sensitive to changes in the cholesterol concentration [25,26], possibly because this leads to improper segregation of lipids and proteins [27] or to incorrect disposition of proteins on the cytosolic surface. In yeast, membrane flux through the exocytic pathway is regulated by cytosolic Sec14 [28], a family of cytosolic phosphatidylcholine/phosphatidylinositol transfer proteins that via their dual specificity may act as sensors of lipid composition and adapt lipid metabolism.…”

Visualizing Association of N-Ras in Lipid Microdomains: Influence of Domain Structure and Interfacial Adsorption

“…A phase diagram of the ternary system POPC/SM/Chol at room temperature is given in De Almeida et al (Figure 1A). 22 …”