Fluorescence correlation spectroscopy with single-molecule sensitivity on cell and model membranes

We report on the successful application of fluorescence correlation spectro scopy (FCS) to the analysis of single fluorescently labeled lipid analogue molecules diffusing laterally in lipid bilayers, as exemplified by time tra ces of fluorescence bursts of individual molecules entering and leaving the excitation area. FCS measurements performed on lipid probes in rat basophi lic leukemia cell membranes showed deviations from two-dimensional Brownian motion with a single uniform diffusion constant. Giant unilamellar vesicle s were employed as model systems to characterize diffusion of fluorescent l ipid analogues in both homogeneous and mixed lipid phases with diffusion he terogeneity. Comparing the results of cell membrane diffusion with the find ings on the model systems suggests possible explanations for the observatio ns. (a) anomalous subdiffusion in which evanescent attractive interactions with disparate mobile molecules modifies the diffusion statistics; (b) alte rnatively, probe molecules are localized in microdomains of submicroscopic size, possibly in heterogeneous membrane phases. Cytometry 36:176-182, 1999 . (C) 1999 Wiley-Liss, Inc.