Monitoring Ca2+ Signaling in Yeast

Calcium is one of the most important second messengers in all eukaryotic cells and, specifically, in budding yeast it plays essential roles in regulating many fundamental cellular processes, such as cell cycle, mating, glucose sensing, salt stress resistance, and cell survival. Like all eukaryotic cells, yeast cells actively maintain their cytosolic free Ca2+ concentration ([Ca2+]i) at extremely low levels, in the range of 50-200 nM, through a finely regulated homeostasis mechanism. Different stimuli can trigger an increase in [Ca2+]i, through calcium influx and/or calcium release from intracellular stores such as vacuoles or ER compartments. For example, glucose addition to nutrient-deprived cells induces a rapid and transient increase in the cytosolic Ca2+ level, largely dependent on an influx of calcium from the extracellular environment but also involving an IP3-dependent calcium release from intracellular stores. Conversely, hypotonic shock induces an increase in [Ca2+]i, mainly mediated by calcium release from intracellular stores, and partially by calcium influx through Cch1 channels on the plasma membrane. In the last few years, calcium signaling research in yeast has benefited from the development of in vivo bioluminescent assays based on the aequorin bioluminescent protein to monitor [Ca2+]i changes, allowing the investigation of known calcium channels and of still unidentified putative Ca2+ transporters.