Diffusion of transferrin receptor clusters.

Two dimensional motion of membrane receptors provides a mechanism for interaction among receptors in the plane of the membrane. In some cases the lateral diffusion leads to formation of clusters which may also be mobile. We have used image cross-correlation (ICCS) spectroscopy technique to measure the translational motion of transferrin receptors in the membrane of 3T3 fibroblasts and HEp2 carcinoma cells. The technique is based on the measurement and analysis of fluctuations in the intensity observed in fluorescence confocal microscope images measured as a function of time. The fluorescence fluctuations arise from stochastic concentration fluctuations about the equilibrium concentration caused by movement of receptors. The amplitude of the fluctuations depend on the number of fluorescent molecules in the observation volume and the dynamics provide the rate of movement. The diffusion observed by this analysis is orders of magnitude slower than that measured by conventional photobleaching techniques. The slower motion corresponds to the diffusion of receptor clusters which provide the more dominant fluctuations.