We are interested in understanding how secretory vesicle fusion is activated at the plasma membrane and prevented everywhere else in the cell. To address this question, we use a combination of genetics, cell biology and biochemistry to identify and characterize the interactions between the molecules required for secretion in the model system, Saccharomyces cerevisiae. Currently, our work focuses on the function of the Sec1 family of proteins, which are essential for membrane fusion, in vivo. Sec1 proteins interact genetically and physically with the core of the membrane fusion machinery (the SNARE complex), making them excellent candidates for membrane fusion regulators. We have found that yeast Sec1p binds to plasma-membrane SNARE complexes at sites of secretion, suggesting that Sec1p is required to activate SNARE-induced fusion between secretory vesicles and the plasma membrane. In order to determine how fusion is regulated, we are developing a cell-free assay that reconstitutes Sec1p-dependent membrane fusion, in vitro. Because many of the molecules required for intracellular membrane fusion, such as Sec1p, are conserved, we expect that our studies with yeast will reveal general principles behind the regulation of vesicle fusion in cells as disparate as yeast and neurons.