Direct Visualization of Mixing of Cholesterol and Phospholipids

Cell membranes are composed of various kinds of lipids and cholesterols. Mixing and demixing of such different lipids (e.g. lipid rafts and/or micron-sized domains) in cell memebranes have been known to be crucial for many cellular processes, but dynamics of such processes is not well understood. We have recentely developed a new technique, called FRAM (Fluorescence Recovery After Merging a phospholipid covered droplet), to measure diffusion of lipid monolayers and to visualize different lipid monolayers being mixed. Using this technique, we studied the mixing of phospholipids anc cholesterols. Direct visualization revealed that various phospholipids could mix/demix with cholesterols depending on their saturation of alkyl chains. We also show that various physical parameters (e.g. line tension, partition coefficient, diffusion coefficient, etc) could be computed from the visualization of mixing.