Correlated Lateral Diffusion of Lipids

Abstract

Recent years have witnessed a proliferation of publications concerning the lateral diffusion of membrane lipids. The reported diffusion coefficient (D) values appear to depend on systems under study and on techniques of investigation. There is no clear consensus regarding their dynamics that would explain the different D values as well as the relation between mobility and the radius of the diffusing object.Here we focus on the lateral diffusion of lipids, which has been less scrutinized than that of membrane proteins. Using fluorescence recovery after photobleaching, we have obtained lateral diffusion coefficients of lipids inserted into bilayers of giant unilamellar vesicles. We demonstrate that lipids diffuse with various diffusion coefficients ranging from D = 3.7 ± 0.4 to 13.9 ± 0.6 μm2/s, and that these values depend on the type of lipid. Interestingly, a transmembrane peptide, having nearly the same radius as lipids and whose hydrophobic thickness matches that of the bilayer, exhibits a D value of 9.6 ± 0.4 μm2/s. Since the lipids and the transmembrane peptide possess a similar diameter and lipids do not span membranes, the peptide diffusion coefficient is expected to be smaller than that of lipids, which is obviously not the case. Our systematic study suggests that the slower diffusion of lipids (as compared to that of the transmembrane peptide) is caused by the formation of dynamic lipid nano-patches that diffuse like a single object with an increased radius. These nano-patches seem to form more spontaneously when lipids are saturated. Consequently, one should be cautious when comparing diffusion coefficient of transmembrane proteins and lipids. Transmembrane peptides should be used as a reference instead of lipids.