Difffusion Dynamics of AChR Receptors on Live Muscle Cell Membrane

Abstract

The dynamic structure of a cell membrane allows it to become an effective platform for various biological functions, such as signal transduction, molecule transportation and endocytosis. We report here a single-molecular tracking experiment on a quantum-dot-labeled transmembrane protein, acetylcholine receptor (AChR), in cultured Xenopus muscle cells. We carried out a complete statistical analysis on a large set of AChR trajectories with more than 500 cells examined. Various drug treatments were used to perturb F-actin and scaffold proteins and examine their roles in regulating the motion of the AChRs. The diffusion dynamics of AChRs was characterized by three quantities: the mean-square displacement 〈Δr2(τ)〉, the probability density function P(Dx) of instantaneous displacement Dx(τ) and the probability distribution f(δ) of instantaneous diffusion coefficient δ. After a careful analysis, we conclude that (1) AChRs show a hindered motion by the surrounding membrane molecules at short time and become diffusive at long time. (2) The mobile AChRs have a broad distribution in diffusion coefficient δ with a long exponential tail, which is universal and independent of different sample conditions. (3) The exponential distribution f(δ) leads to an exponential distribution P(Dx). Our measurements of membrane diffusion based on a large number of single molecular trajectories provide a complete statistical description of dynamic heterogeneity on live cell membrane. By combining all the experimental results available, we propose a dynamic picket-fence model of membrane organization involving slow active remodeling of the underlying cortical actin network to explain the observed non-Gaussian statistics and dynamic heterogeneity. (Work was supported by the Research Grants Council of Hong Kong SAR.)