Phase Diagram of a 3-Component Lipid Mixture of PS/PE/CHOL to Model the Inner Leaflet of a Plasma Membrane

Abstract

The two leaflets of an animal cell plasma membrane are compositionally asymmetric: the outer leaflet has a relatively high concentration of phosphatidylcholine (PC) and sphingomyelin (SM), and the inner leaflet has most of this membrane's phosphatidylethanolamine (PE) and almost all of its phosphatidylserine (PS). The overall cholesterol mole fraction is high, with its distribution between the two leaflets uncertain. Whereas model membrane studies using lipids mimicking the outer leaflet composition have revealed complex mixing behavior including solid/liquid and liquid/liquid phase separations, the mixing behavior of the inner leaflet is still poorly understood. We have constructed a phase diagram for a model mixture of the inner leaflet using a high melting temperature PS, a low melting temperature PE, and cholesterol. Dipalmitoyl PS (DPPS) and palmitoyloleoyl PE (POPE) are in the solid (Lβ) and liquid disordered (Lα) phases, respectively, at the experimental temperature of 30°C. A combination of fluorescence microscopy and fluorescence resonance energy transfer (FRET) between fluorescent lipid probes was used to map all regions of the phase diagram. An Lβ/Lα coexistence region was observed up to at least 15% cholesterol.