Molecular Associations and Membrane Domains

This chapter focuses on the models of membrane organization. The most widely cited model of the organization of cell membranes is termed “fluid mosaic.” It emphasizes the mobility and autonomy of membrane lipids and proteins and implies a role for molecular mobility in membrane function. Indeed, this aspect of the model has been the basis for proposals that reactions among membrane proteins may be coupled by a collision of diffusing species and that signaling from the cell surface may require clustering of mobile receptors by antigens or hormones. This chapter discusses results with a technique, fluorescence photobleaching and recovery (FPR) that was developed to quantitate D_lat in membranes and how this technique for measuring molecular mobility on a scale of micrometers per minute can be used to demonstrate molecular associations and molecular immobilization­­­­––that is, the way in which FPR can be used to investigate the spatial and temporal organization of cell surfaces. The FPR technique, which measures lateral mobility of membrane lipids and proteins, can be readily used to show that the cell surface is “grainy,” differentiated into regions of specialized composition and function. Examples show that FPR can be used as a qualitative tool for detecting molecular associations in surfaces. A minimum of two cross-linking antibodies are required, and ideally a third, irrelevant monoclonal antibody should be available to show that the effects of surface cross-linking are specific for the pair of molecules of interest.