Cytoplasmic pH and the regulation of the dictyostelium cell cycle

Cytoplasmic pH (pHl) was monitored during the cell cycle of synchronous populations of Dictyostelium discoideum by means of a pH “null point” method. There is a cycle of pHl that closely corresponds to the DNA replication cycle, with a minimum of pH 7.20 in interphase and a peak of pH 7.45 during S phase and mitosis. When DNA replication is blocked by hydroxyurea, pHl continues to oscillate with a similar period as the uninhibited division cycle. Even when protein synthesis is inhibited by cycloheximide the periodic pHl cycles persist. Artificially raising pHl in exponentially growing cells by 0.1 units causes a severalfold increase in the rates of protein and DNA synthesis. We conclude that an autonomous pHl oscillator exists in Dictyostelium cells, which operates independently of protein and DNA synthesis and which may have a major role in the timing and regulation of the cell cycle

Cytoplasmic pH and glycolysis in the Dictyostelium discoideum cell cycle

AbstractLactate production measurements during the cell cycle of synchronized populations of Dictyostelium discoideum cells reveal cyclic variations in glycolysis which correspond with pHi oscillations which were discovered by us previously [(1985) Cell, in press]. Aerobic lactate production varies about 6-fold during the cell cycle and the lactate maxima correlate with (~ 0.25 pH unit) cyclic increases in pH. However, artificially altering pHi using weak acids or bases does not influence the rate of lactate production in asynchronous cell populations. This result suggests that the cyclic variations in pHi and those in glycolytic rate are not causally related events

Pertussis toxin-sensitive activation of p21ras by G protein-coupled receptor agonists in fibroblasts

Ras-proteins are guanine nucleotide binding proteins, which, in the GTP bound state emit a strong mitogenic signal. In the GDP bound state, the protein appears inactive. We have found that stimulation by insulin of cells expressing elevated levels of insulin receptors results in a rapid conversion of Ras-GDP into Ras-GTP. This process is part of the signalling pathway leading to immediate-early gene expression and a mitogenic response. There seems to be no involvement of Ras-GTP formation in the process of insulin stimulated glucose transport. Though the precise mechanism by which Ras is converted to the GTP bound state remains to be established, a tight correlation exists between receptor autophosphorylation and Ras-GTP formation