Selective Exocytosis and Spatiotemporal Regulation of Protein Recycling at the Plasma Membrane

Abstract

The concentration of membrane proteins on the cell surface and time they spend in the plasma membrane is defined by dynamic equilibrium between exocytotic delivery and endocytic recycling processes. However, the mechanisms by which cell is able to differentially regulate recycling kinetics of its plasma membrane components is not well understood.In the current study we present new insights how selective exocytosis and spatial redistribution of proteins in the plasma membrane may regulate their recycling kinetics independent of amount of protein in the plasma membrane. We applied fluorescence photo-activation localization microscopy (FPALM) together with multi-color total-internal reflection fluorescence microscopy to track individual proteins and detect their quantum delivery and internalization via exo- and endocytosis. We showed that upon fusion of exocytic vesicle membrane cargo can be either released into the plasma membrane or retained at the site of fusion. We further provide evidence that protein-specific retention serves as a fusion-associated mechanism for formation of dynamic clusters of proteins in the plasma membrane. These clusters of proteins were found to mediate a dynamic exchange of monomers (protein molecules) with the rest of the plasma membrane and also showed transient association with clathrin. Clathrin assembly at the sites of clusters led to formation of endocytic vesicles that mediated quantum internalization of the clustered protein.Taken together, these data provide novel mechanism by which spatiotemporal retention and release of membrane proteins during selective exocytosis regulates protein-specific recycling at the plasma membrane. Different models of protein-specific retention/release will be compared and discussed.