Abstract
Hydrophobic hydrocarbons are absorbed by cell membranes. The effects of hydrocarbons on biological membranes have been studied extensively, but less is known how these compounds affect lipid phase separation. Here, we show that pyrene and pyrene-like hydrocarbons can dissipate lipid domains in phase separating giant unilamellar vesicles at room temperature. In contrast, related aromatic compounds left the phase separation intact, even at high concentration. We hypothesize that this behavior is because pyrene and related compounds lack preference for either the liquid-ordered (Lo) or liquid-disordered (Ld) phase, while larger molecules prefer Lo, and smaller, less hydrophobic molecules prefer Ld. In addition, our data suggest that localization in the bilayer (depth) and the shape of the molecules might contribute to the effects of the aromatic compounds. Localization and shape of pyrene and related compounds are similar to cholesterol and therefore these molecules could behave as such.
Highlights
The plasma membrane is the main permeability barrier of the cell and consists of hundreds to thousands of different lipid species in addition to a wide range of proteins that allow the cell to sense the environment and transport specific molecules in and out of the cell
Triphenylene and benzo(e)pyrene prevented phase separation in giant unilamellar vesicles (GUVs), composed of DPPC, DOPC and cholesterol when added to the lipid mixture in a 1 to 1 molar ratio (Figure 3A)
In the GUVs analyzed, few vesicles displayed an intermediate appearance between phase separation and one phase, which is indicated by a pLo/Ld value between 0.2 and 0.8 (Figure 3B)
Summary
The plasma membrane is the main permeability barrier of the cell and consists of hundreds to thousands of different lipid species in addition to a wide range of proteins that allow the cell to sense the environment and transport specific molecules in and out of the cell. The lipids of the membrane are not randomly distributed but can form distinct domains, often referred to as lipid rafts, and associate with specific proteins [1,2,3]. Rafts are associated with specific membrane proteins, thereby affecting signaling and protein trafficking in the membrane as summarized by Levental and Veatch [4]. Hydrocarbons affect the membrane properties as they interfere with the interaction of proteins with their neighboring lipids. The hydrocarbons can bind to hydrophobic pockets or surfaces of proteins and thereby influence their activity. Local anesthetics exert their effects by e.g. decreasing the miscibility temperature of lipids as shown in giant plasma membrane vesicles [5], thereby increasing the membrane fluidity. Hydrocarbons alter membrane properties such as membrane thickness, head group hydration and fluidity, all of which can affect membrane proteins [7]
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