Acetate regulation of spore formation is under the control of the Ras/cyclic AMP/protein kinase A pathway and carbon dioxide in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the Ras/cyclic AMP (cAMP)/protein kinase A (PKA) pathway is a nutrient-sensitive signaling cascade that regulates vegetative growth, carbohydrate metabolism, and entry into meiosis. How this pathway controls later steps of meiotic development is largely unknown. Here, we have analyzed the role of the Ras/cAMP/PKA pathway in spore formation by the meiosis-specific manipulation of Ras and PKA or by the disturbance of cAMP production. We found that the regulation of spore formation by acetate takes place after commitment to meiosis and depends on PKA and appropriate A kinase activation by Ras/Cyr1 adenylyl cyclase but not by activation through the Gpa2/Gpr1 branch. We further discovered that spore formation is regulated by carbon dioxide/bicarbonate, and an analysis of mutants defective in acetate transport (ady2Δ) or carbonic anhydrase (nce103Δ) provided evidence that these metabolites are involved in connecting the nutritional state of the meiotic cell to spore number control. Finally, we observed that the potential PKA target Ady1 is required for the proper localization of the meiotic plaque proteins Mpc70 and Spo74 at spindle pole bodies and for the ability of these proteins to initiate spore formation. Overall, our investigation suggests that the Ras/cAMP/PKA pathway plays a crucial role in the regulation of spore formation by acetate and indicates that the control of meiotic development by this signaling cascade takes places at several steps and is more complex than previously anticipated.

Yeast alpha-mating factor receptor-linked G-protein signal transduction suppresses Ras-dependent activity

Homologues of mammalian Ras conserved in Saccharomyces cerevisiae mediate glucose-stimulated cyclic AMP formation and we used this response to test for regulation of yeast Ras activity by the alpha-mating factor signal transduction pathway. alpha-Mating factor suppresses glucose-stimulated cyclic AMP formation by up to 57 +/- 12.6% (n = 5) and similar inhibition was observed in four different yeast strains (MATa cells). Moreover, this response is potent (IC50 = 0.14 +/- 0.19 microM (n = 4)), rapid (maximal within 1-2 min), and displays an absolute requirement for both the alpha-mating factor receptor (STE2) and associated G-protein beta-subunit (STE4). Inhibition appears independent of both phosphodiesterase activation and alpha-mating factor-stimulated cytoplasmic alkalinization. Also, basal cyclic AMP levels are unaffected by pheromone. This is the first demonstration that a cell-surface receptor linked to a heterotrimeric G-protein can suppress Ras-dependent activity and could provide important insight into mechanisms controlling p21ras in man. Inhibition of Ras-dependent cyclic AMP formation could also be a key event facilitating responses characteristic of yeast mating.

Transmembrane signalling in Saccharomyces cerevisiae

Baker's yeast, a unicellular eukaryote, has been a model organism for biochemists, geneticists and most recently for molecular biologists. Pioneering biochemical studies were conducted on yeast, such as the study of glucose fermentation and amino acid metabolism. The powerful tools of yeast genetics have allowed a comprehensive study of important issues such as the cell cycle and meiosis. In recent years, it has been established that Saccharomyces cerevisiae, the most extensively characterized of the yeasts, shares key molecules and biochemical pathways with higher eukaryotes. For example, actin, tubulin, ubiquitin, calmodulin, GTP regulatory proteins, different protein kinases including protein tyrosine kinases, were all found to play central roles in yeast. Furthermore, structurally homologous proteins, as well as transcription regulating elements, of yeast and higher eukaryotes, including mammals, were shown to be structurally and functionally interchangeable. It has also been found that yeast can express human genes. Technically, yeasts are simple to handle, inexpensive to grow, complete a cell cycle within 90 min, and therefore can yield relatively quick results. These qualities are useful in biotechnological applications. Saccharomyces cerevisiae, can be genetically manipulated fairly easily, and has been tinkered with more than any other system. A cloned, in vitro mutated gene, can be transformed into wild type yeast and by homologous recombination, can replace the native gene and generate the desired mutant. Such manipulations, not possible yet in other eukaryotic cells, allow the precise definition of the role played by different genes and their domains. These unique features of Saccharomyces cerevisiae, together with rapidly evolving techniques of molecular biology, have made it a successful model organism for the study of numerous questions.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid intracellular alkalinization of Saccharomyces cerevisiae MATa cells in response to alpha-factor requires the CDC25 gene product

The alpha-factor mating pheromone induces a transient intracellular alkalinization of MATa cells within minutes after exposure to the pheromone, and is the earliest biochemical event that can be identified subsequent to the exposure. Dissipation of the pheromone induced pH gradient, using 2,4-dinitrophenol or sodium orthovanadate, does not inhibit the biological response of the yeast to the pheromone such as mating and 'schmoo' formation. These findings suggest that the pheromone mediated pH change per se is not a part of the transmembrane signalling but rather the consequence of a biochemical reaction triggered by the alpha-pheromone interaction with its receptor and may have a permissive effect on the pheromonal response. The cdc25ts mutation causes MATa cells to become nonresponsive to alpha-factor subsequent to a shift to the restrictive temperature, suggesting that the CDC25 gene product participates in the pheromone response pathway.

[Variations in cyclic AMP level and specific activities of adenylate cyclase and cyclic AMP phosphodiesterase during the cell cycle of an Actinomycete (author's transl)]

The variations in the concentrations of intra- and extracellular cyclic AMP and in he specific activities of adenylate cyclase (EC 4.6.1.1) and cyclic AMP phosphodiesterase (EC 3.1.4.17) have been monitored in synchronized cultures of Nocardia restricta, a prokaryote belonging to the group of Actinomycetes. At the beginning of the cell cycle, during a first period of RNA and protein synthesis, there is an increasing synthesis of adenylate cyclase which can be suppressed in the presence of chloramphenicol or rifampicin. Simultaneously, the specific activity of cyclic AMP phosphodiesterase decreases and the concentrations of intra- and extracellular cyclic AMP rise. After the end of DNA replication, during a second period of RNA and protein synthesis, the specific activity of cyclic AMP phosphodiesterase increases; during the same time, the specific activity of adenylate cyclase and the level of intracellular cyclic AMP drop. It appears that the overall metabolism of cyclic AMP is coordinated so that the cyclic AMP level will be high at the beginning of DNA replication and will fall thereafter. The results are discussed in comparison with known data about the variations of cyclic AMP during the cell cycle of mammalian cells in cultures.

Regulation of the biosynthesis of cytochrome P-450 in brewer's yeast. Role of cyclic AMP

The drug metabolising enzyme cytochrome P-450 has been studied in great detail in mammalian systems and its presence in microorganisms is also well established. However, neither its function nor its means of control in brewer's yeast, Saccharomyces cerevisiae, has been investigated. We demonstrate here using yeast protoplasts that it is the intracellular concentration of cyclic AMP which controls, by repression, the de novo synthesis of the enzyme, and also that cyclic AMP concentrations are in turn inversely related to the concentration of glucose in the yeast growth medium.