| Calcium ions are ubiquitously used by eukaryotic cells as a way to signal information about their environment and physiological state. Typically, calcium is kept very low in the interior of cells at rest, allowing very small increases to be perceived and subsequently deciphered. These intracellular calcium signals can be more specifically defined as transient increases in cytosolic free calcium concentrations that are translated into cellular responses. Calcium signaling is employed to regulate a wide variety of functions including, but not limited to, gene expression, exocytosis, cytoskeletal rearrangement, cell physiology, and fertilization. Calcium signals also occur during the mating process of the yeast Saccharomyces cerevisiae. Mating in yeast is initiated by the binding of secreted peptide pheromones (a -factor or α-factor) to haploid cells and subsequent activation of the pheromone signaling cascade which leads to gene transcription, cell cycle arrest, and polarized growth toward the mating partner.; After initiation of pheromone signaling, the rate of Ca2+ influx increases, leading to activation of calcium signaling pathways that are necessary for the survival of haploid cells that fail to find a mating partner. A high affinity calcium influx system composed of at least two yeast proteins, Cch1p and Mid1p, was shown to contribute to the production of these calcium signals. Cch1p/Mid1p channel activity was much greater after inactivation or inhibition of the calcium regulated phosphatase calcineurin, suggesting that this channel is subject to direct or indirect regulation by calcineurin. Additionally, a novel low-affinity Ca2+-influx system, termed LACS, was stimulated by pheromone signaling. LACS activity was insensitive to calcineurin activity, independent of Cch1p and Mid1p, and sufficient to elevate cytosolic Ca2+ concentrations. To identify components and regulators of LACS, a collection of yeast mutants was screened for defects in Ca2+-influx after treatment with α-factor. Numerous factors required for polarized morphogenesis were found to be necessary for LACS activity, but not for Cch1p/Mid1p activity. One such factor, Fig1p, was identified as being genetically most closely linked to calcium influx through LACS. (Abstract shortened by UMI.)... |
