Cells secrete prepackaged hormones and neurotransmitters by a process known as exocytosis. This process plays an important role in the function of most organ systems in the body. For example, the pancreas secretes hormones that regulate blood sugar. Secretion of neurotransmitters is the means by which the nervous system controls a whole host of function, including heart rate, blood pressure, breathing, and muscle movement. Despite its obvious importance, exocytosis is poorly understood. We know a great deal about the signaling systems which initiate the process, but very little about what happens at the biochemical level once secretion is triggered. In a resting secretory cell, hormones or neurotransmitters are stored in small, membrane- covered vesicles, waiting for an appropriate signal. During exocytosis, these vesicles make their way to the surface of the cell, where they bind and fuse with the cell membrane, releasing their contents outside the cell. Although it seems likely that one or more proteins participate in this process, it has proved difficult to demonstrate this experimentally. This proposal takes a new approach toward identifying such proteins. It takes advantage of the fact that certain bacterial toxins can partially or totally block one of the later stages of the secretory process. Although the exact mechanism by which the inhibition occurs is unknown, it almost certainly involves an interaction with the proteins which are of interest to us. I plan to use radiolabeled toxin interact. I will then purify these proteins and determine their subcellular localization and functions will markedly increase our understanding of, and thus our potential ability to control, the process of exocytosis. Eventually, these studies may lead to novel therapeutic approaches towards diseases such as hypertension which have a prominent secretory component.