Respiratory Na+ pump and Na+-dependent energetics in Vibrio alginolyticus

Physiological roles of three Na+/H+ antiporters in the halophilic bacterium Vibrio parahaemolyticus

Vibrio parahaemolyticus mutants lacking three Na+/H+ antiporters (NhaA, NhaB, NhaD) were constructed. The DeltanhaA strains showed significantly higher sensitivity to LiCl regarding their growth compared to the parental strain. The DeltanhaA and DeltanhaB strains exhibited higher sensitivities to LiCl. The mutant XACabd lacking all of the three antiporters could not grow in the presence of 500 mM LiCl at pH 7.0, or 50 mM at pH 8.5. The XACabd mutant was also sensitive to 1.0 M NaCl at pH 8.5. These results suggest that Na+/H+ antiporters, especially NhaA, are responsible for resistance to LiCl and to high concentrations of NaCl. Reduced Na+/H+ and Li+/H+ antiport activities were observed with everted membrane vesicles of DeltanhaB strains. However, Li+/H+ antiport activities of DeltanhaB strains were two times higher than those of DeltanhaA strains when cells were cultured at pH 8.5. It seems that expression of nhaA and nhaB is dependent on medium pH to some extent. In addition, HQNO (2-heptyl-4-hydroxyquinoline N-oxide), which is a potent inhibitor of the respiratory Na+ pump, inhibited growth of XACabd, but not of the wild type strain. Moreover, survival rate of XACabd under hypoosmotic stress was lower than that of wild type strain. It is likely that the Na+/H+ antiporters are involved in osmoregulation under hypoosmotic stress. Based on these findings, we propose that the Na+/H+ antiporters cooperate with the respiratory Na+ pump in ionic homeostasis in V. parahaemolyticus.

Na(+)-driven flagellar motor of Vibrio

Bacterial flagellar motors are molecular machines powered by the electrochemical potential gradient of specific ions across the membrane. Bacteria move using rotating helical flagellar filaments. The flagellar motor is located at the base of the filament and is buried in the cytoplasmic membrane. Flagellar motors are classified into two types according to the coupling ion: namely the H(+)-driven motor and the Na(+)-driven motor. Analysis of the flagellar motor at the molecular level is far more advanced in the H(+)-driven motor than in the Na(+)-driven motor. Recently, the genes of the Na(+)-driven motor have been cloned from a marine bacterium of Vibrio sp. and some of the motor proteins have been purified and characterized. In this review, we summarize recent studies of the Na(+)-driven flagellar motor.

Cloning and characterization of motY, a gene coding for a component of the sodium-driven flagellar motor in Vibrio alginolyticus

The bacterial flagellar motor is a molecular machine that couples proton or sodium influx to force generation for driving rotation of the helical flagellar filament. In this study, we cloned a gene (motY) encoding a component of the sodium-driven polar flagellar motor in Vibrio alginolyticus. Nucleotide sequence analysis revealed that the gene encodes a 293-amino-acid polypeptide with a single putative transmembrane segment that is very similar (94.5% identity) to the recently described MotY of V. parahaemolyticus. Their C-terminal domains were similar to the C-terminal domains of many peptidoglycan-interacting proteins, e.g., Escherichia coli MotB and OmpA, suggesting that MotY may interact with peptidoglycan for anchoring the motor. By using the lac promoter-repressor system, motY expression was controlled in V. alginolyticus cells. Swimming ability increased with increasing concentrations of the inducer isopropyl-beta-D-thiogalactopyranoside, and the swimming fraction increased after induction. These results are consistent with the notion that MotY is a component of the force-generating unit. V. alginolyticus motY complemented the motY mutation of V. parahaemolyticus. However, motY appeared to lack a region corresponding to the proposed motY promoter of V. parahaemolyticus. Instead, sequences similar to the sigma54 consensus were found in the upstream regions of both species. We propose that they are transcribed from the sigma54 -specific promoters.

Induction of the Escherichia coli cytochrome d by low delta mu H+ and by sodium ions

Regulation of synthesis of cytochrome d in Escherichia coli has been studied using mutants with cytochrome-d--beta-galactosidase gene fusions. It was shown that various protonophorous uncouplers, when added to the growth medium, cause induction of the cytochrome d synthesis. The cytochrome-d-inducing activity of uncouplers correlates with their ability to inhibit such a delta mu (H+)-driven function as motility of the E. coli cells. An increase in the Na+ concentration in the growth medium from 1.5 mM to 25 mM results in induction of the cytochrome d synthesis. The cytochrome-d-inducing effect of uncouplers is much more pronounced when the Na+ concentration is high than when it is low. These data are in agreement with the assumption that cytochrome d is involved in the Na+ energetics substituting for the H+ energetics when the latter appears to be inefficient. Mutations in arcA or arcB genes (but not in fnr gene) completely prevent the increase in the cytochrome d level induced by uncouplers but are without effect on that induced by Na+. It is assumed that in the control of the cytochrome d synthesis, the Arc system is involved in the delta mu H+ sensing whereas sensing of delta mu Na+ (or of the Na+ concentration) is mediated by some other receptor system.

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.

Na+/H+ antiporters, molecular devices that couple the Na+ and H+ circulation in cells

Na+/H+ antiporters are universal devices involved in the Na+ and H+ circulation of both eukaryotes and prokaryotes, thus playing an essential role in the pH and Na+ homeostasis of cells. This review focuses on the major impact of the application of molecular biology tools in the study of the antiporters. These tools permit the verification of the role of the antiporters and provide insights into their unique biology. A novel signal transduction to Na+ involving nhaR, a positive regulator, controls the expression of nhaA in E. coli. A "pH sensor" regulates the activity of Na+/H+ antiporters, both in eukaryotes and prokaryotes. A most intricate signal transduction to pH involving phosphorylation steps controls the activity of nhel in higher mammals. The identification of Histidine 226 in the "pH sensor" of NhaA is a step forward towards the understanding of the pH regulation of these proteins.

The laws of cell energetics

Recent progress in membrane bioenergetics studies has resulted in the important discovery that Na+ can effectively substitute for H+ as the energy coupling ion. This means that living cells can possess three convertible energy currencies, i.e. ATP, protonic and sodium potentials. Analysis of interrelations of these components in various types of living cells allows bioenergetic laws of universal applicability to be inferred.

Chemiosmotic systems in bioenergetics: H(+)-cycles and Na(+)-cycles

The development of membrane bioenergetic studies during the last 25 years has clearly demonstrated the validity of the Mitchellian chemiosmotic H+ cycle concept. The circulation of H+ ions was shown to couple respiration-dependent or light-dependent energy-releasing reactions to ATP formation and performance of other types of membrane-linked work in mitochondria, chloroplasts, some bacteria, tonoplasts, secretory granules and plant and fungal outer cell membranes. A concrete version of the direct chemiosmotic mechanism, in which H+ potential formation is a simple consequence of the chemistry of the energy-releasing reaction, is already proved for the photosynthetic reaction centre complexes. Recent progress in the studies on chemiosmotic systems has made it possible to extend the coupling-ion principle to an ion other than H+. It was found that, in certain bacteria, as well as in the outer membrane of the animal cell, Na+ effectively substitutes for H+ as the coupling ion (the chemiosmotic Na+ cycle). A precedent is set when the Na+ cycle appears to be the only mechanism of energy production in the bacterial cell. In the more typical case, however, the H+ and Na+ cycles coexist in one and the same membrane (bacteria) or in two different membranes of one and the same cell (animals). The sets of delta mu H+ and delta mu Na+ generators as well as delta mu H+ and delta mu Na+ consumers found in different types of biomembranes, are listed and discussed.

Light-dependent delta mu Na-generation and utilization in the marine cyanobacterium Oscillatoria brevis

Light-dependent Na+ and H+ transports, membrane potential (delta psi) and motility have been studied in the cells of the marine cyanobacterium Oscillatoria brevis. In the presence of a protonophorous uncoupler, carbonyl cyanide-m-chlorophenylhydrazone, the intracellular Na+ level is shown to increase in the dark and decrease in the light. The Na+/H+ antiporter, monensin, stimulates the dark CCCP-dependent [Na+]in increase and abolishes the light-dependent [Na+]in decrease. Na+ ions are necessary for the fast light-induced delta psi generation and H+ uptake by the cells. This uptake is inhibited by monensin being resistant to CCCP. Monensin sensitizes the delta psi level and the motility rate to low CCCP concentrations. The obtained data are consistent with the assumption that O. brevis possesses a primary Na+ pump which utilizes (directly or indirectly) the light energy.

The sodium cycle: a novel type of bacterial energetics

The progress of bioenergetic studies on the role of Na+ in bacteria is reviewed. Experiments performed over the past decade on several bacterial species of quite different taxonomic positions show that Na+ can, under certain conditions, substitute for H+ as the coupling ion. Various primary Na+ pumps (delta mu Na+ generators) are described, i.e., Na+ -motive decarboxylases, NADH-quinone reductase, terminal oxidase, and ATPase. The delta mu Na+ formed is shown to be consumed by Na+ driven ATP-synthase, Na+ flagellar motor, numerous Na+, solute symporters, and the methanogenesis-linked reverse electron transfer system. In Vibrio alginolyticus, it was found that delta mu Na+, generated by NADH-quinone reductase, can be utilized to support all three types of membrane-linked work, i.e., chemical (ATP synthesis), osmotic (Na+, solute symports), and mechanical (rotation of the flagellum). In Propionigenum modestum, circulation of Na+ proved to be the only mechanism of energy coupling. In other species studied, the Na+ cycle seems to coexist with the H+ cycle. For instance, in V. alginolyticus the initial and terminal steps of the respiratory chain are Na+ - and H+ -motive, respectively, whereas ATP hydrolysis is competent in the uphill transfer of Na+ as well as of H+. In the alkalo- and halotolerant Bacillus FTU, there are H+ - and Na+ -motive terminal oxidases. Sometimes, the Na+ -translocating enzyme strongly differs from its H+ -translocating homolog. So, the Na+ -motive and H+ -motive NADH-quinone reductases are composed of different subunits and prosthetic groups. The H+ -motive and Na+ -motive terminal oxidases differ in that the former is of aa3-type and sensitive to micromolar cyanide whereas the latter is of another type and sensitive to millimolar cyanide. At the same time, both Na+ and H+ can be translocated by one and the same P. modestum ATPase which is of the F0F1-type and sensitive to DCCD. The sodium cycle, i.e., a system composed of primary delta mu Na+ generator(s) and delta mu Na+ consumer(s), is already described in many species of marine aerobic and anaerobic eubacteria and archaebacteria belonging to the following genera: Vibrio, Bacillus, Alcaligenes, Alteromonas, Salmonella, Klebsiella, Propionigenum, Clostridium, Veilonella, Acidaminococcus, Streptococcus, Peptococcus, Exiguobacterium, Fusobacterium, Methanobacterium, Methanococcus, Methanosarcina, etc. Thus, the "sodium world" seems to occupy a rather extensive area in the biosphere.