Indications for a K+/H+ cotransport system in plasma membranes from two acidophilic microorganisms

Escherichia coli proton-translocating F0F1-ATP synthase and its association with solute secondary transporters and/or enzymes of anaerobic oxidation–reduction under fermentation

The Escherichia coli proton-translocating F0F1-ATP synthase has a priority in H+ circulation through the membrane in maintaining proton-motive force in the context of ATP synthesis and hydrolysis. Recent advances in the study of this complex under fermentative growth have led to hypothesis that, in the absence of oxidative phosphorylation, F0F1 is implicated as an essential part of H+ movement and ATP hydrolysis, associated with solute secondary transporters and/or enzymes of anaerobic oxidation-reduction. These associations can result from a protein-protein interaction by dithiol-disulfide interchange. In such associations F0F1 has novel functions in bacterial cell physiology.

Intracellular pH homeostasis in the filamentous fungus Aspergillus niger

Intracellular pH homeostasis in the filamentous fungus Aspergillus niger was measured in real time by 31P NMR during perfusion in the NMR tube of fungal biomass immobilized in Ca2+-alginate beads. The fungus maintained constant cytoplasmic pH (pH(cyt)) and vacuolar pH (pH(vac)) values of 7.6 and 6.2, respectively, when the extracellular pH (pH(ex)) was varied between 1.5 and 7.0 in the presence of citrate. Intracellular metabolism did not collapse until a Delta pH over the cytoplasmic membrane of 6.6-6.7 was reached (pH(ex) 0.7-0.8). Maintenance of these large pH differences was possible without increased respiration compared to pH(ex) 5.8. Perfusion in the presence of various hexoses and pentoses (pH(ex) 5.8) revealed that the magnitude of Delta pH values over the cytoplasmic and vacuolar membrane could be linked to the carbon catabolite repressing properties of the carbon source. Also, larger Delta pH values coincided with a higher degree of respiration and increased accumulation of polyphosphate. Addition of protonophore (carbonyl cyanide m-chlorophenylhydrazone, CCCP) to the perfusion buffer led to decreased ATP levels, increased respiration and a partial (1 microm CCCP), transient (2 microm CCCP) or permanent (10 microm CCCP) collapse of the vacuolar membrane Delta pH. Nonlethal levels of the metabolic inhibitor azide (N3-, 0.1 mm) caused a transient decrease in pH(cyt) that was closely paralleled by a transient vacuolar acidification. Vacuolar H+ influx in response to cytoplasmic acidification, also observed during extreme medium acidification, indicates a role in pH homeostasis for this organelle. Finally, 31P NMR spectra of citric acid producing A. niger mycelium showed that despite a combination of low pH(ex) (1.8) and a high acid-secreting capacity, pH(cyt) and pH(vac) values were still well maintained (pH 7.5 and 6.4, respectively).

Characterization of a plasma membrane H(+)-ATPase from the extremely acidophilic alga Dunaliella acidophila

Dunaliella acidophila is an unicellular green alga which grows optimally at pH 0-1 while maintaining neutral internal pH. A plasma membrane preparation of this algae has been purified on sucrose density gradients. The preparation exhibits vanadate-sensitive ATPase activity of 2 mumol Pi/mg protein/min, an activity 15 to 30-fold higher than that in the related neutrophilic species D. salina. The following properties suggest that the ATPase is an electrogenic plasma membrane H+ pump. (i) ATP induces proton uptake and generates a positive-inside membrane potential as demonstrated with optical probes. (ii) ATP hydrolysis and proton uptake are inhibited by vanadate, diethylstilbestrol, dicyclohexylcarbodiimide and erythrosine but not by molybdate, azide or nitrate. (iii) ATP hydrolysis and proton uptake are stimulated by fussicoccin in a pH-dependent manner as found for plants plasma membrane H(+)-ATPase. Unusual properties of this enzyme are: (i) the Km for ATP is around 60 microM, considerably lower than in other plasma membrane H(+)-ATPases, and (ii) the ATPase activity and proton uptake are stimulated three to fourfold by K+ and to a smaller extent by other monovalent cations. These results suggest that D. acidophila possesses a vanadate-sensitive H(+)-ATPase with unusual features enabling it to maintain the large transmembrane pH gradient.