ABSTRACT
The discovery of the two major contractile proteins, actin and myosin, in such secretory cells as platelets, chromaffin cells, leukocytes, and the islets of Langerhans suggests that contractile proteins have a central role in energy-dependent exocytosis from these cells. In order to discuss the role of the contractile system of proteins in secretion, it is necessary to describe the biochemistry of the proteins of the contractile system. Intact cell studies of contractile function in leukocytes have primarily relied on the introduction of cytochalasins into the cell. Phalloidin is a bicyclic peptide from toadstools that binds to and stabilizes actin filaments. The structure of nonmuscle myosin has been extensively studied in only a few cells where its structure is similar to that of muscle myosin. Tropomyosin enhances the Adenosine triphosphase of platelet actomyosin possibly by stiffening the actin filament and facilitating interaction with myosin filaments.
