Role of Ions and Intracellular Proteins in Exocytosis

This chapter describes a new protein called synexin that induces Ca²⁺-dependent formation of pentalaminar complexes among secretory granule membranes. Synexin has proved distinct from other more conventional candidates for Ca²⁺-effect mediators, such as calmodulin, actomyosin, and tubulin. The chapter also describes the process of membrane breakage or fission that finally results in the release of secretory vesicle contents. Studies comparing the granule lysis reaction with predicted behavior of secreting chromaffin and other cells are briefly reviewed in the chapter. The biochemical data supporting the exocytosis hypothesis are that the storage organelles from different cells can be isolated and therefore shown to be discrete objects and when secretion occurs, the entire organelle contents ranging from small molecules to large proteins can be found in the extracellular medium. Calcium plays a critical role in regulating exocytosis in a number of systems.