Ca²⁺ Signal is Generated Only Once in the Mating Pheromone Response Pathway in Saccharomyces cerevisiae

抄録

The mating pheromone, a-factor, of the yeast Saccharomyces cerevisiae binds to the heterotrimeric G protein-coupled cell surface receptor of MATa cells and induces cellular responses necessary for mating. In higher eukaryotic cells, many hormones and growth factors rapidly mobilize a second messenger, Ca²⁺, by means of receptor-G protein signaling. Although striking similarities between the mechanisms of the receptor-G protein signaling in yeast and higher eukaryotes have long been known, it is still uncertain whether the pheromone rapidly mobilizes Ca²⁺ necessary for early events of the pheromone response. Here we reexamine this problem using sensitive methods for detecting Ca²⁺ fluxes and mobilization, and find no evidence that there is rapid Ca²⁺ influx leading to a rapid increase in the cytosolic free Ca²⁺ concentration. In addition, the yeast PLC1 deletion mutant lacking phosphoinositide-specific phospholipase C, a key enzyme for generating Ca²⁺ signals in higher eukaryotic cells, responds normally to the pheromone. These findings suggest that the receptor-G protein signaling does not utilize Ca²⁺ as a second messenger in the early stage of the pheromone response pathway. Since the receptor-G protein signaling does stimulate Ca²⁺ influx after early events have finished and this stimulation is essential for late events in the pheromone response pathway {Iida et al., (1990) J. Biol. Chem., 265: 13391-13399} Ca²⁺ may be used only once in the signal transduction pathway in unicellular eukaryotes such as yeast.