Anomalous subdiffusion in heterogeneous lipid Bilayers

Fluorescence photobleaching recovery (FPR) is commonly used to measure lipid and protein diffusion in cellular membranes. Typically, a model wherein diffusion is constant with time and the mean-squared displacement is directly proportional to time is used to analyze the results; however, in nonhomogeneous systems such as cellular membranes, anomalous subdiffusion may occur. In anomalous subdiffusion, the diffusion coefficient, D, decreases with time and thus the mean-squared displacement is proportional to some power of time less than 1. Although theory predicts that diffusion can be anomalous through protein interactions or obstruction, the complex composition of cellular membranes has made the actual origin and consequences of anomalous diffusion in phospholipid bilayers unclear. In this study, we use atomic force microscopy to detect and measure the amount of the solid phase in supported bilayers that contain coexisting fluid- and solid-phase lipids. Solid-phase domains in bilayers have been shown to act as obstacles to diffusion. We then use FPR to determine the diffusional. behavior of the obstructed bilayers. We find that at a solid-phase area fraction of similar to35% diffusion is anomalous at short times and becomes normal at longer times as predicted by theory and Monte Carlo simulations. Increasing the solid-phase area fraction increases the amount of time diffusion is anomalous before becoming normal. The results of this work imply that accurately measuring diffusion in cell membranes requires an understanding of the heterogeneity of the underlying membrane structure.