Our present description of the plasma membrane includes heterogeneities or mechanisms that may hinder the diffusion of membrane particles. Indeed, the diffusion of transmembrane proteins may be impeded by the actin-based membrane skeleton, whereas lipid microdomains are thought to transiently sequester molecules which are involved in signaling pathways.Fluorescence correlation spectroscopy (FCS) is a mature and powerful technique for measuring diffusion coefficients. However, the study becomes uneasy when the diffusion is not free as it is the case for the cell membrane components. We have shown that doing FCS measurements at different sizes of observation volumes gives access to the diffusion laws of molecules in live cell membranes. Our method enables to distinguish between different mechanisms responsible for the confinement of particles in the plasma membrane and to measure different characteristics of these confining structures, such as their size or the average confinement time. It has also been possible to study the reorganization of the cell membrane during a signaling event.
