Exocytosis and Endocytosis: Membrane Fusion Events Captured in Rapidly Frozen Cells

This chapter focuses on the structural analysis of the events in endocytosis and exocytosis. Exocytosis and endocytosis present some mechanistic problems that are solved in a similar manner. Both utilize membrane fusion events that occur in highly localized environments. During exocytosis, membrane fusion creates an exceedingly small pore that leads to the formation of a narrow, tubular neck. During endocytosis, the formation of a narrow tubular neck is required before fusion will take place. Yet, they are fundamentally different. Exocytosis involves the release of potential energy that is stored in the highly organized structure of the granule core. The contents of nearly all secretory granules are packaged so as to limit hydration and to maximize interaction between constituents. In some cases, the granule membrane is designed to maintain ion gradients between the interior of the granule and the cytoplasm. In addition, the formation of a narrow neck during endocytosis or enlargement of the pore during exocytosis requires rapid movements of phospholipids into or out of the region. In this region, the bilayer must be highly mobile, a characteristic that may be of importance in the fusion event itself.