Phase studies of model biomembranes: Complex behavior of DSPC/DOPC/Cholesterol

Abstract

We have undertaken a series of experiments to examine the behavior of individual components of cell membranes. Here we report an initial stage of these experiments, in which the properties of a chemically simple lipid mixture are carefully mapped onto a phase diagram. Four different experimental methods were used to establish the phase behavior of the 3-component mixture DSPC/DOPC/chol: (1) confocal fluorescence microscopy observation of giant unilamellar vesicles, GUVs; (2) FRET from perylene to C20:0-DiI; (3) fluorescence of dilute dyes C18:2-DiO and C20:0-DiI; and (4) wide angle X-ray diffraction. This particular 3-component mixture was chosen, in part, for a high level of immiscibility of the components in order to facilitate solving the phase behavior at all compositions. At 23 degrees C, a large fraction of the possible compositions for this mixture give rise to a solid phase. A region of 3-phase coexistence of {Lalpha+Lbeta+Lo} was detected and defined based on a combination of fluorescence microscopy of GUVs, FRET, and dilute C20:0-DiI fluorescence. At very low cholesterol concentrations, the solid phase is the tilted-chain phase Lbeta'. Most of the phase boundaries have been determined to be within a few percent of the composition. Measurements of the perturbations of the boundaries of this accurate phase diagram could serve as a means to understand the behaviors of a range of added lipids and proteins.

Loss of hydroxyl groups from the ceramide moiety can modify the lateral diffusion of membrane proteins in S. cerevisiae

“…7 ; Tables 2, 3 ). Considering the phase diagrams of ternary mixture ( 39,41,(43)(44)(45), a suffi cient degree of ergosterol depletion would cause the transition from the L o phase to the L ␤ phase. The ergosterol content in the sur2 ⌬ csg1 ⌬ csh1 ⌬ mutant was decreased by ‫ف‬ 20-30% relative to WT cells.…”

“…This suggests that the extension of both the hydrophilic head group and the C4-OH are necessary for effective growth in limited nutrient conditions. Studies of model membranes show that liquid-disordered (L d ) and liquid-ordered (L o ) phases coexist in ternary mixtures of sterol (e.g., ergosterol), sphingolipids, and glycerolipids (e.g., dipalmitoylphosphatidylcholine) (37)(38)(39)(40)(41). While the L d phase is a fl uid state, the L o phase is an intermediate state between gel and liquid crystal phases, characterized by tight packing and rapid lateral diffusion .…”

Competition between line tension and curvature stabilizes modulated phase patterns on the surface of giant unilamellar vesicles: A simulation study

“…The disc-shaped region around the vertex with initial area A(v i ) is projected out a distance ∆R along the normals of the edges, as shown in figure 4. Let the two faces g (1) ij and g (2) ij be defined by the three points,…”

“…The three component lipid system, distearoyl-phosphatidylcholine (DSPC)/dioleoylphosphatidylcholine (DOPC)/cholesterol (CHOL) has a well characterized phase diagram, with a region of liquidliquid (L o + L d ) coexistence that is readily observable in giant unilamellar vesicle (GUV) studies as large round domains [1,2]. This mixture is a useful model for understanding the general nature of bilayers containing a high melting lipid (DSPC), a low melting lipid (DOPC), and cholesterol.…”

Ordered Membrane Domain-Forming Properties of the Lipids of Borrelia burgdorferi

“…To do this, we prepared model membrane vesicles composed of various combinations of B. burgdorferi lipids. Model membrane vesicles have proven to be very useful systems for defining the abilities of lipids to form ordered domains in membranes and identifying conditions in which such domains exist as nanoscale-size domains (38)(39)(40)(41)(42)(43)(44)(45). Using model membrane vesicles, we found that the major Borrelia lipids differ greatly in their ability to support ordered-domain/raft formation.…”