Ca 2+ is known to have several important intracellular regulatory roles in all eucaryotes. Some of the cellular processes and functions controlled by Ca 2+ include cellular differentiation, cell movement, endocytosis, morphogenesis, mitosis, secretion, cell division, mating, cell and nuclear fusion. However, the precise function of Ca 2+ in most of these processes has not been established, and the situation is further complicated because all these Ca 2+ mediated processes occur in conjunction with one another. The model eucaryote, S. cerevisiae is probably the best experimental system in which to dissect these elusive but ubiquitous cellular responses to Ca 2+. However, to understand completely the role of Ca 2+ as a secondary messenger we must first elucidate the mechanisms responsible for regulating cytosolic Ca 2+ levels. This will require an understanding of the mechanism, and regulation of Ca 2+ movement across the plasma membrane and Ca 2+ uptake and release by the various yeast organelles. The specific goal of the research project is to isolate and characterize mutants defective in regulation of cytosolic Ca 2+ levels, and to use these mutants to identify genes (and their encoded proteins) which are responsible for maintaining normal cytosolic Ca 2+ levels.