In vitro activation of the Saccharomyces cerevisiae Ras/adenylate cyclase system by glucose and some of its analogues

Glucose activation of adenylate cyclase in Saccharomyces cerevisiae mutants lacking glucose-phosphorylating enzymes

The "in vitro" activation by glucose of the RAS-adenylate cyclase system in membranes from a strain of Saccharomyces cerevisiae lacking any functional glucose kinase activity presents similar features to those of the wild type. However, this triple mutant appears to be unable to produce the glucose-induced increase of cAMP levels "in vivo". The results obtained in vitro indicate that the signal transduction mechanism is active in the mutant cells and suggest that the absence of intracellular acidification in vivo might be responsible for the lack of response to glucose.

Interactions between glucose metabolism and oxidative phosphorylations on respiratory-competent Saccharomyces cerevisiae cells

The purpose of this work was to analyze the interactions between oxidative phosphorylations and glucose metabolism on yeast cells aerobically grown on lactate as carbon source and incubated in a resting cell medium. On such respiratory-competent yeast cells, four different metabolic steady states have particularly been studied: (a) glucose feeding under anaerobiosis, (b) ethanol supply under aerobiosis, (c) glucose supply under aerobiosis and (d) glucose plus ethanol under aerobiosis. For each condition, we measured: (a) the cellular ATP/ADP ratio and NADH content sustained under these conditions, (b) the glucose consumption rate (glucose conditions) and the respiratory rate (aerobic conditions). Under aerobic conditions, when ethanol is used as substrate, the ATP/ADP ratio and NADH level are very high as compared with glucose feeding. However, the rate of oxygen consumption is similar under both conditions. The main observation is a large increase in the respiratory rate when both glucose and ethanol are added. This increase corresponds to an ATP/ADP ratio and a NADH level lower than those observed with ethanol but higher than those with glucose. Therefore the response of the respiratory rate to the ATP/ADP ratio depends on the redox potential. We studied the way in which the ATP-consuming activity was increased under glucose+ethanol conditions. By NMR experiments, it appears that neither the futile cycle at the level of the phosphofructo-1-kinase/fructo-1,6-bisphosphatase couple nor the synthesis of carbohydrate stores could account for the increase in oxidative phosphorylation. However, it is shown that, in the presence of glucose+ethanol, ATP consumption is strongly stimulated. It is hypothesized that this consumption is essentially due to the combination of the well-known plasma membrane proton-ATPase activation by glucose and the high phosphate potential due to oxidative ethanol metabolism. While it is well documented that oxidative phosphorylations inhibit the glycolytic flux, i.e. the Pasteur effect, we clearly show in this work that the glycolytic pathway limits the ability of mitochondria to maintain a cellular phosphate potential.

Activation of adenylate cyclase in cdc25 mutants of Saccharomyces cerevisiae

The activation of adenylate cyclase by guanine nucleotides and 6-deoxyglucose was studied in membrane preparations from S. cerevisiae mutants lacking the CDC25 gene product. Adenylate cyclase from cdc25 ts membranes was activated by GTP and GppNHp in membranes from cells collected after glucose was exhausted from the medium. The activation was also observed in membranes from repressed cells at 2.5 mM Mg2+. It is also shown that 6-deoxyglucose can activate adenylate cyclase in the absence of CDC25 gene product. The relative amount of membrane-bound adenylate cyclase was drastically reduced in cdc25 ts membranes when subjected to the restrictive temperature, while no significant change was observed in the wild type. These data suggest that Cdc25 might not be required in certain conditions for the guanine nucleotide exchange reaction in Ras and that it might be implicated in anchoring the Ras/adenylate cyclase system to the plasma membrane.