The latest news from the sodium world

Metagenomics Revealed a New Genus 'Candidatus Thiocaldithrix dubininis' gen. nov., sp. nov. and a New Species 'Candidatus Thiothrix putei' sp. nov. in the Family Thiotrichaceae, Some Members of Which Have Traits of Both Na+- and H+-Motive Energetics

Two metagenome-assembled genomes (MAGs), GKL-01 and GKL-02, related to the family Thiotrichaceae have been assembled from the metagenome of bacterial mat obtained from a sulfide-rich thermal spring in the North Caucasus. Based on average amino acid identity (AAI) values and genome-based phylogeny, MAG GKL-01 represented a new genus within the Thiotrichaceae family. The GC content of the GKL-01 DNA (44%) differed significantly from that of other known members of the genus Thiothrix (50.1-55.6%). We proposed to assign GKL-01 to a new species and genus 'Candidatus Thiocaldithrix dubininis' gen. nov., sp. nov. GKL-01. The phylogenetic analysis and estimated distances between MAG GKL-02 and the genomes of the previously described species of the genus Thiothrix allowed assigning GKL-02 to a new species with the proposed name 'Candidatus Thiothrix putei' sp. nov. GKL-02 within the genus Thiothrix. Genome data first revealed the presence of both Na+-ATPases and H+-ATPases in several Thiothrix species. According to genomic analysis, bacteria GKL-01 and GKL-02 are metabolically versatile facultative aerobes capable of growing either chemolithoautotrophically or chemolithoheterotrophically in the presence of hydrogen sulfide and/or thiosulfate or chemoorganoheterotrophically.

Modulation of Potassium Channel Activity in the Balance of ROS and ATP Production by Durum Wheat Mitochondria-An Amazing Defense Tool Against Hyperosmotic Stress

In plants, the existence of a mitochondrial potassium channel was firstly demonstrated about 15 years ago in durum wheat as an ATP-dependent potassium channel (PmitoKATP). Since then, both properties of the original PmitoKATP and occurrence of different mitochondrial potassium channels in a number of plant species (monocotyledonous and dicotyledonous) and tissues/organs (etiolated and green) have been shown. Here, an overview of the current knowledge is reported; in particular, the issue of PmitoKATP physiological modulation is addressed. Similarities and differences with other potassium channels, as well as possible cross-regulation with other mitochondrial proteins (Plant Uncoupling Protein, Alternative Oxidase, Plant Inner Membrane Anion Channel) are also described. PmitoKATP is inhibited by ATP and activated by superoxide anion, as well as by free fatty acids (FFAs) and acyl-CoAs. Interestingly, channel activation increases electrophoretic potassium uptake across the inner membrane toward the matrix, so collapsing membrane potential (ΔΨ), the main component of the protonmotive force (Δp) in plant mitochondria; moreover, cooperation between PmitoKATP and the K(+)/H(+) antiporter allows a potassium cycle able to dissipate also ΔpH. Interestingly, ΔΨ collapse matches with an active control of mitochondrial reactive oxygen species (ROS) production. Fully open channel is able to lower superoxide anion up to 35-fold compared to a condition of ATP-inhibited channel. On the other hand, ΔΨ collapse by PmitoKATP was unexpectedly found to not affect ATP synthesis via oxidative phosphorylation. This may probably occur by means of a controlled collapse due to ATP inhibition of PmitoKATP; this brake to the channel activity may allow a loss of the bulk phase Δp, but may preserve a non-classically detectable localized driving force for ATP synthesis. This ability may become crucial under environmental/oxidative stress. In particular, under moderate hyperosmotic stress (mannitol or NaCl), PmitoKATP was found to be activated by ROS, so inhibiting further large-scale ROS production according to a feedback mechanism; moreover, a stress-activated phospholipase A2 may generate FFAs, further activating the channel. In conclusion, a main property of PmitoKATP is the ability to keep in balance the control of harmful ROS with the mitochondrial/cellular bioenergetics, thus preserving ATP for energetic needs of cell defense under stress.

The uniqueness of the plant mitochondrial potassium channel

The ATP-inhibited Plant Mitochondrial K(+) Channel (PmitoKATP) was discovered about fifteen years ago in Durum Wheat Mitochondria (DWM). PmitoKATP catalyses the electrophoretic K(+) uniport through the inner mitochondrial membrane; moreover, the co-operation between PmitoKATP and K(+)/H(+) antiporter allows such a great operation of a K(+) cycle to collapse mitochondrial membrane potential (ΔΨ) and ΔpH, thus impairing protonmotive force (Δp). A possible physiological role of such ΔΨ control is the restriction of harmful reactive oxygen species (ROS) production under environmental/oxidative stress conditions. Interestingly, DWM lacking Δp were found to be nevertheless fully coupled and able to regularly accomplish ATP synthesis; this unexpected behaviour makes necessary to recast in some way the classical chemiosmotic model. In the whole, PmitoKATP may oppose to large scale ROS production by lowering ΔΨ under environmental/oxidative stress, but, when stress is moderate, this occurs without impairing ATP synthesis in a crucial moment for cell and mitochondrial bioenergetics.

Potassium channel-oxidative phosphorylation relationship in durum wheat mitochondria from control and hyperosmotic-stressed seedlings

Durum wheat mitochondria (DWM) possess an ATP-inhibited K(+) channel, the plant mitoK(ATP) (PmitoK(ATP) ), which is activated under environmental stress to control mitochondrial ROS production. To do this, PmitoK(ATP) collapses membrane potential (ΔΨ), thus suggesting mitochondrial uncoupling. We tested this point by studying oxidative phosphorylation (OXPHOS) in DWM purified from control seedlings and from seedlings subjected both to severe mannitol and NaCl stress. In severely-stressed DWM, the ATP synthesis via OXPHOS, continuously monitored by a spectrophotometric assay, was about 90% inhibited when the PmitoK(ATP) was activated by KCl. Contrarily, in control DWM, although PmitoK(ATP) collapsed ΔΨ, ATP synthesis, as well as coupling [respiratory control (RC) ratio and ratio between phosphorylated ADP and reduced oxygen (ADP/O)] checked by oxygen uptake experiments, were unaffected. We suggest that PmitoK(ATP) may play an important defensive role at the onset of the environmental/oxidative stress by preserving energy in a crucial moment for cell and mitochondrial bioenergetics. Consistently, under moderate mannitol stress, miming an early stress condition, the channel may efficiently control reactive oxygen species (ROS) generation (about 35-fold from fully open to closed state) without impairing ATP synthesis. Anyway, if the stress significantly proceeds, the PmitoK(ATP) becomes fully activated by decrease of ATP concentration (25-40%) and increase of activators [free fatty acids (FFAs) and superoxide anion], thus impairing ATP synthesis.

The energy spilling reactions of bacteria and other organisms

For many years it was assumed that living organisms always utilized ATP in a highly efficient manner, but simple growth studies with bacteria indicated that the efficiency of biomass production was often at least 3-fold lower than the amount that would be predicted from standard biosynthetic pathways. The utilization of energy for maintenance could only explain a small portion of this discrepancy particularly when the growth rate was high. These ideas and thermodynamic arguments indicated that cells might have another avenue of energy utilization. This phenomenon has also been called 'uncoupling', 'spillage' and 'overflow metabolism', but 'energy spilling' is probably the most descriptive term. It appears that many bacteria spill energy, and the few that do not can be killed (large and often rapid decrease in viability), if the growth medium is nitrogen-limited and the energy source is in 'excess'. The lactic acid bacterium, Streptococcus bovis, is an ideal bacterium for the study of energy spilling. Because it only uses substrate level phosphorylation to generate ATP, ATP generation can be calculated with a high degree of certainty. It does not store glucose as glycogen, and its cell membrane can be easily accessed. Comparative analysis of heat production, membrane voltage, ATP production and Ohm's law indicated that the energy spilling reaction of S. bovis is mediated by a futile cycle of protons through the cell membrane. Less is known about Escherichia coli, but in this bacterium energy spilling could be mediated by a futile cycle of potassium or ammonium ions. Energy spilling is not restricted to prokaryotes and appears to occur in yeasts and in higher organisms. In man, energy spilling may be related to cancer, ageing, ischemia and cardiac failure.

Amiloride resistance in the methanoarcheon Methanothermobacter thermoautotrophicus: characterization of membrane-associated proteins

An amiloride-resistant mutant with diminished Na+/H+ antiporter activity was isolated from Methanothermobacter thermoautotrophicus. To define the protein basis of amiloride resistance, the composition of membrane-associated proteins was partially characterized and compared with that of the wild type strain. An abundant 670-kDa membrane-associated protein that was present only in the mutant strain was analyzed by MALDI-TOF MS and identified as a coenzyme F420-reducing hydrogenase. The amiloride resistance was not accompanied by changes in protein size or changes in the level of subunits A or B of the A1A0-type ATP synthase; on the other hand, the SDS-PAGE patterns of the chloroform-methanol extract of membranes from both strains were different. Two bands with calculated molecular mass 16 and 11 kDa were identified as MtrD and AtpK, respectively. The observed over-expression of a 22.7-kDa protein in the mutant cells may represent the multimeric form of the MtrD subunit. These results show that the impairment of the Na+/H+ antiporter system in the amiloride-resistant mutant of Methanothermobacter thermoautotrophicus is accompanied by only small changes in a few membrane-associated proteins.

The high salt requirement of the moderate halophile Chromohalobacter salexigens DSM3043 can be met not only by NaCl but by other ions

The growth rate of Chromohalobacter salexigens DSM 3043 can be stimulated in media containing 0.3 M NaCl by a 0.7 M concentration of other salts of Na+, K+, Rb+, or NH4+, Cl-, Br-, NO3-, or SO4(2-) ions. To our knowledge, growth rate stimulation by a general high ion concentration has not been reported for any organism previously.

Evolution of the Na-P(i) cotransport systems

Membrane transport systems for P(i) transport are key elements in maintaining homeostasis of P(i) in organisms as diverse as bacteria and human. Two Na-P(i) cotransporter families with well-described functional properties in vertebrates, namely NaPi-II and NaPi-III, show conserved structural features with prokaryotic origin. A clear vertical relationship can be established among the mammalian protein family NaPi-III, a homologous system in C. elegans, the yeast system Pho89, and the bacterial P(i) transporter Pit. An alternative lineage connects the mammalian NaPi-II-related transporters with homologous proteins from Caenorhabditis elegans and Vibrio cholerae. The present review focuses on the molecular evolution of the NaPi-II protein family. Preliminary results indicate that the NaPi-II homologue cloned from V. cholerae is indeed a functional P(i) transporter when expressed in Xenopus oocytes. The closely related NaPi-II isoforms NaPi-IIa and NaPi-IIb are responsible for regulated epithelial Na-dependent P(i) transport in all vertebrates. Most species express two different NaPi-II proteins with the exception of the flounder and Xenopus laevis, which rely on only a single isoform. Using an RT-PCR-based approach with degenerate primers, we were able to identify NaPi-II-related mRNAs in a variety of vertebrates from different families. We hypothesize that the original NaPi-IIb-related gene was duplicated early in vertebrate development. The appearance of NaPi-IIa correlates with the development of the mammalian nephron.

Bacterial energetics at high pH: what happens to the H+ cycle when the extracellular H+ concentration decreases?

A decrease in the extracellular H+ concentration creates difficulties for membrane-linked energetics in bacteria employing H+ as the coupling ion. At high extracellular pH (pHo), H+ ions pumped from the cell by, say, the respiratory chain, are immediately neutralized by the alkaline extracellular medium. Under such conditions, the only driving force that might compel outer H+ ions to return to cytosol and perform their function is the electric potential difference across the cytoplasmic membrane (delta psi). However, when delta pH in the opposite direction is equal to, e.g., 2 pH units (intracellular pH = 7.5 at pHo = 9.5), delta psi would be so high that the risk of membrane electric breakdown would increase. This is why some bacteria deal with high pH by, for example, replacing H+ by Na+ as the coupling ion rather than by increasing delta psi. It has been shown in several species of bacteria that the alkalinization of the growth medium induces primary Na+ pumps (e.g. Na(+)-motive respiratory chain enzymes and Na+ ATPase). Electrogenic Na+ efflux via these pumps produces an electrochemical Na+ potential difference (delta mu Na+) composed of delta psi and delta pNa+. delta mu Na+ can be used to perform various types of membrane-linked work. The delta psi constituent of delta mu Na+ may maintain electrophoretic influx of H+ such that the alkalinization of cytoplasm is prevented. The latter function may be supported by a mechanism based on the uphill influx of Cl- instead of Na+. This seems to be the case for alkaliphilic and halophilic Natronobacter pharaonis. There is an indication that not only Na+ but also Ca2+ may substitute for H+ in Gleobacter violaceus growing at high pH.

The inorganic ion content of native aquatic bacteria

In this study we have quantified the ionic content and volume of native aquatic, and two cultured bacteria, by X-ray microanalysis (XRMA) in the transmission electron microscope (TEM). The cellular concentrations of magnesium (means of 630 and 710 mM) were more than an order of a magnitude higher than the outside concentrations. The internal concentrations of sodium were on average 50-180 mM, and the [K+]/[Na+] ratios were in the range of 0.1-0.5; lowest for apparently nonactive bacteria. Magnesium and chloride probably act as the major components of cell turgor, since no other inorganic ions were present in comparable amounts. Our carbon and nitrogen measurements indicated that organic solutes are not likely to be present at significant concentrations. The estimated charge of inorganic ions (Na, Mg, P, Cl, K, and Ca) gave a positive net internal charge for most cells. However, in cultures of Vibrio natriegens, the high internal chloride concentration made the net inorganic charge negative in these cells. Our results suggest that growing marine bacterioplankton have an internal environment in which magnesium is the dominating cation. These results suggest that actively growing marine bacteria are physiologically adapted to high internal concentrations of both magnesium and chloride.Key words: X-ray microanalysis, magnesium, osmolyte, marine bacteria.

Energetics of alkaliphilic Bacillus species: physiology and molecules

The challenge of maintaining a cytoplasmic pH that is much lower than the external pH is central to the adaptation of extremely alkaliphilic Bacillus species to growth at pH values above 10. The success with which this challenge is met may set the upper limit of pH for growth in these bacteria, all of which also exhibit a low content of basic amino acids in proteins or protein segments that are exposed to the outside bulk phase liquid. The requirement for an active Na(+)-dependent cycle and possible roles of acidic cell wall components in alkaliphile pH homeostasis are reviewed. The gene loci that encode Na+/H+ antiporters that function in the active cycle are described and compared with the less Na(+)-specific homologues thus far found in non-alkaliphilic Gram-positive prokaryotes. Alkaliphilic Bacillus species carry out oxidative phosphorylation using an exclusively H(+)-coupled ATPase (synthase). Nonetheless, ATP synthesis is more rapid and reaches a higher phosphorylation potential at highly alkaline pH than at near-neutral pH even though the bulk electrochemical proton gradient across the coupling membrane is lower at highly alkaline pH. It is possible that some of the protons extruded by the respiratory chain are conveyed to the ATP synthase without first equilibrating with the external bulk phase. Mechanisms that might apply to oxidative phosphorylation in this type of extensively studied alkaliphile are reviewed, and note is made of the possibility of different kinds of solutions to the problem that may be found in new alkaliphilic bacteria that are yet to be isolated or characterized.

Specific cation binding site in mammalian cytochrome oxidase

Calcium ion binds reversibly with cytochrome c oxidase from beef heart mitochondria (Kd approximately 2 microM) shifting alpha- and gamma-absorption bands of heme a to the red. Two sodium ions compete with one Ca2+ for the binding site with an average dissociation constant square root[K1(Na) x K2(Na)] approximately 3.6 mM. The Ca2+-induced spectral shift of heme a is specific for mammalian cytochrome c oxidase and is not observed in bacterial or yeast aa3 oxidases although the Ca2+-binding site has been revealed in the bacterial enzyme [Ostermeier, C., Harrenga, A., Ermler, U. and Michel, H. (1997) Proc. Natl. Acad. Sci. USA 94, 10547-10553]. As His-59 and Gln-63 involved in Ca2+ binding with Subunit I of P. denitrificans oxidase are not conserved in bovine oxidase, these residues have to be substituted by alternative ligands in mammalian enzyme, which is indeed the case as shown by refined structure of bovine heart cytochrome oxidase (S. Yoshikawa, personal communication). We propose that it is interaction of Ca2+ with the species-specific ligand(s) in bovine oxidase that accounts for perturbation of heme a. The Ca2+/Na2+-binding site may be functionally associated with the exit part of 'pore B' proton channel in subunit I of mammalian cytochrome c oxidase.

Isolation and characterization of a neomycin-resistant mutant of Methanobacterium thermoautotrophicum with a lesion in Na+-translocating ATPase (synthase)

A mutant of Methanobacterium thermoautotrophicum with a lesion in membrane Na+-translocating ATPase (synthase) was isolated. The total ATPase activity in permeabilized cells of this mutant was elevated three-fold as compared with the wild-type strain. In contrast to wild-type cells, mutant ATPase was neither inhibited by DCCD nor stimulated by Na+ ions. The methane formation orate of the mutant cells at pH 7.5 under non-growing conditions was nearly twice that of the wild-type strain and was stimulated by sodium ions. On the other hand, the ATP synthesis driven by methanogenesis under the same conditions was lower that of the wild-type under the same conditions, and contrary to the wild-type was not stimulated by Na+ ions. ATP synthesis driven by a potassium diffusion potential in the presence of sodium ions was markedly diminished in the mutant cells. The membrane potential values of the wild-type and the mutant cells in the presence of 10 mM NaCl at pH 7.0 were comparable at energized conditions (-223 mV and -230 mV respectively). The Mg2+-dependent ATPase activity of the 10(5) x g supernatant of broken cells from the mutant cells was 30% higher than in the wild-type. On the other hand, two bands with Mg2+-dependent ATPase activity were identified by native PAGE in this fraction in both wild-type as well as in mutant. These data suggest that the binding of Na+-translocating ATPase (synthase) to the membrane spanning part is changed in the mutant strain.

The presence of H+ and Na+ -linked Ca2+ extruding systems in Methanobacterium thermoautotrophicum

The effects of monovalent cations (Na+, K+ and choline+) and the uncoupler 3,3',4',5-tetrachlorosalicylanilide (TCS) were tested on 45Ca2+ uptake by non-energized cells of Methanobacterium thermoautotrophicum. 45Ca2+ uptake was stimulated by the addition of K+ and (less) by choline+ while Na+ slowed down and even reversed it, thereby mimicking the energization of cells. The uncoupler agent, TCS, suppressed 45Ca2+ uptake in non-energized cells in the presence or absence of Na+ but in cells energized in an atmosphere of CO2+H2 it exerted a stimulating effect. Uncoupled 45Ca2+ efflux was measured in cells pre-loaded with 45Ca2+ by means of the divalent ionophore A23187 following its washing out by buffer containing serum albumin. The efflux was temperature-dependent and was stimulated by external 40Ca2+ and Na+. In the absence of Na+, the uncoupled efflux was completely inhibited by TCS, whereas in the presence of Na+, TCS was without any effect. The results are in agreement with the model in which the Ca2+ influx pathway is represented by a membrane potential-driven uniport whereas Ca2+ efflux is mediated by two transport systems - Na+/Ca2+ and H+/Ca2+ antiporters - whose participation in the total efflux is dependent on the energy of the corresponding gradients of driving ions.

The presence of H+ and Na(+)-translocating ATPases in Methanobacterium thermoautotrophicum and their possible function under alkaline conditions

Two ATPases with different apparent molecular masses of approx. 500 kDa and 400 kDa were identified in the EDTA extract of the cell membranes of Methanobacterium thermoautotrophicum. Western blotting with polyclonal antiserum reactive with beta-subunit of mitochondrial ATPase from rat liver and yeast was used for further analysis of these ATPases. A strong crossreactivity with a single protein band with an apparent molecular weight of about 53 kDa (similar to beta-subunit of F-type ATPase from other sources) was found in protein extracts of whole cells of Methanobacterium thermoautotrophicum strains delta H and Marburg, as well as of Methanospirillum hungatei. This indicates the presence of F-type ATPase in methanogens. ATP synthesis driven by membrane potential which was generated by artificially-imposed delta pH in the presence of protonophorous uncoupler and sodium ions was stimulated by bafilomycin A1, an inhibitor of V- and A-type ATPases, as well as by harmaline, an inhibitor of Na+/H+ antiporter. These results indicate that cells of Methanobacterium thermoautotrophicum strain delta H contain the F-type ATP synthase which is Na(+)-translocating in addition to V- or A-type ATP synthase which is H(+)-translocating.

Chemiosmotic concept of the membrane bioenergetics: what is already clear and what is still waiting for elucidation?

The present state of the chemiosmotic concept is reviewed. Special attention is paid to (i) further progress in studies on the Na(+)-coupled energetics and (ii) paradoxical bioenergetic effects when protonic or sodium potentials are utilized outside the coupling membrane (TonB-mediated uphill transports across the outer bacterial membrane). A hypothesis is put forward assuming that the same principle is employed in the bacterial flagellar motor.