Purification and properties of ATPase from an alkalophilic Bacillus

Na+-stimulated ATPase of alkaliphilic halotolerant cyanobacterium Aphanothece halophytica translocates Na+ into proteoliposomes via Na+ uniport mechanism

Background

When cells are exposed to high salinity conditions, they develop a mechanism to extrude excess Na⁺ from cells to maintain the cytoplasmic Na⁺ concentration. Until now, the ATPase involved in Na⁺ transport in cyanobacteria has not been characterized. Here, the characterization of ATPase and its role in Na⁺ transport of alkaliphilic halotolerant Aphanothece halophytica were investigated to understand the survival mechanism of A. halophytica under high salinity conditions.

Results

The purified enzyme catalyzed the hydrolysis of ATP in the presence of Na⁺ but not K⁺, Li⁺ and Ca²⁺. The apparent K_m values for Na⁺ and ATP were 2.0 and 1.2 mM, respectively. The enzyme is likely the F₁F₀-ATPase based on the usual subunit pattern and the protection against N,N'-dicyclohexylcarbodiimide inhibition of ATPase activity by Na⁺ in a pH-dependent manner. Proteoliposomes reconstituted with the purified enzyme could take up Na⁺ upon the addition of ATP. The apparent K_m values for this uptake were 3.3 and 0.5 mM for Na⁺ and ATP, respectively. The mechanism of Na⁺ transport mediated by Na⁺-stimulated ATPase in A. halophytica was revealed. Using acridine orange as a probe, alkalization of the lumen of proteoliposomes reconstituted with Na⁺-stimulated ATPase was observed upon the addition of ATP with Na⁺ but not with K⁺, Li⁺ and Ca²⁺. The Na⁺- and ATP-dependent alkalization of the proteoliposome lumen was stimulated by carbonyl cyanide m - chlorophenylhydrazone (CCCP) but was inhibited by a permeant anion nitrate. The proteoliposomes showed both ATPase activity and ATP-dependent Na⁺ uptake activity. The uptake of Na⁺ was enhanced by CCCP and nitrate. On the other hand, both CCCP and nitrate were shown to dissipate the preformed electric potential generated by Na⁺-stimulated ATPase of the proteoliposomes.

Conclusion

The data demonstrate that Na⁺-stimulated ATPase from A. halophytica, a likely member of F-type ATPase, functions as an electrogenic Na⁺ pump which transports only Na⁺ upon hydrolysis of ATP. A secondary event, Na⁺- and ATP-dependent H⁺ efflux from proteoliposomes, is driven by the electric potential generated by Na⁺-stimulated ATPase.

Role of the charged amino acid residues in the cytoplasmic loop between putative transmembrane segments 6 and 7 of Na+-ATPase of an alkaliphilic bacterium, Exiguobacterium aurantiacum

ATPase activity of the membrane-bound Na(+)-ATPase of an alkaliphilic bacterium, Exiguobacterium aurantiacum, was measured in various concentrations of NaCl. Hill plot analysis showed a Hill number of 1.7 with 5.2 mM as the K(0.5) value for Na(+). When the site-directed mutagenesis of seven charged amino acid residues in the cytoplasmic loop (L6/7) between putative transmembrane segments 6 and 7 of the enzyme was conducted, all the mutated enzymes exhibited Hill numbers close to that of the wild-type enzyme (WT). When reconstituted with lecithin, all the mutants exhibited Na(+)-transport activity. While alanine substitution for several residues gave some significant effects on the enzyme function, the most remarkable effect was observed in the substitution for Glu-733. The K(0.5) value of E733A for Na(+) was 83.2 mM. The mutant exhibited only 8.5% of the ATPase activity and 54.0% of the energy-coupling efficiency for Na(+) transport as compared with those of WT, respectively. Drastic decreases of apparent affinity for Na(+) and energy efficiency of ion transport were also observed in E733K and E733T, respectively.

Presence of a Na+-stimulated P-type ATPase in the plasma membrane of the alkaliphilic halotolerant cyanobacterium Aphanothece halophytica

Aphanothece cells could take up Na(+) and this uptake was strongly inhibited by the protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Cells preloaded with Na(+) exhibited Na(+) extrusion ability upon energizing with glucose. Na(+) was also taken up by the plasma membranes supplied with ATP and the uptake was abolished by gramicidin D, monensin or Na(+)-ionophore. Orthovanadate and CCCP strongly inhibited Na(+) uptake, whereas N, N'-dicyclohexylcarbodiimide (DCCD) slightly inhibited the uptake. Plasma membranes could hydrolyse ATP in the presence of Na(+) but not with K(+), Ca(2+) and Li(+). The K(m) values for ATP and Na(+) were 1.66+/-0.12 and 25.0+/-1.8 mM, respectively, whereas the V(max) value was 0.66+/-0.05 mumol min(-1) mg(-1). Mg(2+) was required for ATPase activity whose optimal pH was 7.5. The ATPase was insensitive to N-ethylmaleimide, nitrate, thiocyanate, azide and ouabain, but was substantially inhibited by orthovanadate and DCCD. Amiloride, a Na(+)/H(+) antiporter inhibitor, and CCCP showed little or no effect. Gramicidin D and monensin stimulated ATPase activity. All these results suggest the existence of a P-type Na(+)-stimulated ATPase in Aphanothece halophytica. Plasma membranes from cells grown under salt stress condition showed higher ATPase activity than those from cells grown under nonstress condition.

Cloning, sequencing and functional expression in Escherichia coli of the gene for a P-type Na(+)-ATPase of a facultatively anaerobic alkaliphile, Exiguobacterium aurantiacum

Cloning and sequencing of the gene encoding a P-type Na(+)-ATPase of a facultatively anaerobic alkaliphile, Exiguobacterium aurantiacum, were conducted. The structural gene was composed of 2628 nucleotides. The deduced amino acid sequence (876 amino acid residues; Mr, 96,664) suggested that the enzyme possesses 10 membrane-spanning regions. When the amino acid sequences of the four putative membrane regions, M4, M5, M6 and M8, of BL77/1 ATPase were aligned with those of fungal Na(+)-ATPase, Na(+)/K(+)-ATPase, H(+)-ATPases and sarcoplasmic reticulum Ca(2+)-ATPase, it exhibited the highest homology with Ca(2+)-ATPase except M5 region. By the transformation of Escherichia coli with the expression vector (pQE30) containing the ATPase gene, the enzyme was functionally expressed in E. coli membranes.

Purification and characterization of a membrane-associated ATPase from Natronococcus occultus, a haloalkaliphilic archaeon

Isolated membranes of the extreme haloalkaliphilic archaeon Natronococcus occultus were able to hydrolyze ATP via an ATPase, which required the presence of Mg(2+), high concentrations of NaCl, and a pH value of 9. The native molecular mass of the purified ATPase was 130 kDa and was composed of 74- and 61-kDa subunits. Enzyme activity was specific for the hydrolysis of ATP with slight activity towards GTP, CTP, and ITP. The enzyme required NaCl for maximal activity but Na(2)SO(4) and (NH(4))(2)SO(4) could substitute. The enzyme showed no activity if Na(2)SO(3) or sodium citrate was substituted for NaCl. The ATPase from N. occultus was inhibited by NBD-Cl, NaN(3), and ouabain, and was sensitive to nitrate, vanadate, DCCD, and bafilomycin A(1). It was not inhibited by NEM in contrast to other previously characterized halophile ATPases. The ATPase had a K(M) of 0.5 mM and appeared to be non-competitively inhibited by NaN(3) with a K(I) of 3.1 mM.

Characterization of a P-type Na+-ATPase of a facultatively anaerobic alkaliphile, Exiguobacterium aurantiacum

A facultatively anaerobic alkaliphile, Exiguobacterium aurantiacum, possesses a P-type Na(+)-stimulated ATPase in the membrane (Koyama, N. (1999) Curr. Microbiol. 39, 27-30). In this study, we attempted to purify and characterize the enzyme. The ATPase appears to consist of a single polypeptide with an apparent molecular mass of 100 kDa. The enzyme exhibited an optimum pH for activity at around 9. The enzyme was strongly inhibited by vanadate (50% inhibition observed at 3 microm) and forms an acylphosphate intermediate, suggesting a P-type ATPase. The enzyme, when reconstituted into soybean phospholipid vesicles, exhibited ATP-dependent (22)Na(+) uptake, which was completely inhibited by gramicidin. The reconstituted vesicles exhibited a generation of membrane potential (positive, inside). The enzyme is likely to be involved in an electrogenic transport of Na(+).

Presence of Na(+)-stimulated V-type ATPase in the membrane of a facultatively anaerobic and halophilic alkaliphile

It was found that a facultatively anaerobic and halophilic alkaliphile, M-12 (Amphibacillus sp.), possesses a Na(+)-stimulated ATPase in the membrane. The ATPase activity was inhibited by NO3- and SCN- which are the inhibitors of V-type ATPase, but not by azide and vanadate, inhibitors of F-type ATPase and P-type ATPase, respectively. Upon the incubation of the membrane in buffer containing ATP and MgCl2, several polypeptides were released from the membrane. Among them, two major polypeptides with apparent molecular masses of 79 and 55 kDa crossreacted with an antiserum against the catalytic units (subunits A and B) of V-type ATPase from Enterococcus hirae. The N-terminal amino acid sequences of the 79 and 55 kDa polypeptides showed high similarity to those of subunits A and B of V-type ATPase from Enterococcus hirae, respectively. M-12 is likely to possess a V-type Na(+)-ATPase.

Stimulatory effect of NH4+ on the transport of leucine and glucose in an anaerobic alkaliphile

An anaerobic alkaliphile, EP01, specifically requires NH4+ for the acceleration of amino acid and glucose transport [Koyama, N. (1988) FEBS Lett. 253, 187-189]. In this paper, we attempted to clarify how NH4+ is involved in the transport system. The bacterium acidifies the cytoplasm, which was suggested to result in NH4+ accumulation when NH4Cl was added to the medium. Increase of the NH4Cl concentration administered to the medium caused the acceleration of leucine and glucose transport, which was accompanied by an increase in the internal pH and the absolute internal concentration of NH4+, whereas a decrease in the concentration ratio of internal NH4+/external NH4+ was observed. The addition of 3 mM NH4Cl, which resulted in significant stimulation of leucine and glucose transport, raised the internal NH4+ concentration by 42 mM, but the internal pH only by 0.1 units. It seems more likely that leucine and glucose transport are accelerated depending on the increase in the internal NH4+ concentration rather than the increase in the internal pH. By the imposition of an inwardly directed Na+ gradient, the K(+)-loaded membrane vesicles accumulated leucine and glucose, indicating that a sodium chemical potential is available for active transport. The membrane of the bacterium exhibited a Na(+)-stimulated ATPase activity which was remarkably enhanced by the addition of NH4Cl, depending on its concentration, and was inhibited by vanadate. Leucine and glucose transport were inhibited by vanadate. Based on these results, we propose a mechanism in which NH4+ contributes internally to leucine and glucose transport, depending on its concentration, by the activation of a Na(+)-translocating ATPase responsible for the generation of a sodium chemical potential.

The F1-type ATPase in anaerobic Lactobacillus casei

An ATPase from anaerobic Lactobacillus casei has been isolated and 100-times purified. The 400 kDa enzyme molecule was found to have a hexagonal structure 10 nm in diameter composed of at least six protein masses. SDS-electrophoresis reveals four or, under certain conditions, five types of subunit, of apparent molecular masses 57 (alpha), 55 (beta), 40 (gamma), 22 (delta) and 14 (epsilon) kDa with stoichiometry of 3 alpha, 3 beta, gamma, delta, epsilon. The following features resembling F1-ATPases from other sources were found to be inherent in the solubilized L. casei ATPase. (i) Detachment from the membrane desensitizes ATPase to low DCCD concentrations and sensitizes it to water-soluble carbodiimide. (ii) Soluble ATPase is inhibited by Nbf chloride and azide, is resistant to SH-modifiers and is activated by sulfite and octyl glucoside, the activating effect being much stronger than in the case of the membrane-bound ATPase. Substrate specificity of the enzyme is also similar to that of other factors F1. Divalent cations strongly activate the soluble enzyme when added at a concentration equal to that of ATP. An excess of Mn2+, Mg2+ or Co2+ inhibits ATPase activity of F1, whereas that of Ca2+ induces its further activation. No other F1-like ATPases are found in L. casei. It is concluded that this anaerobic bacterium possesses a typical F1-ATPase similar to those in mitochondria, chloroplasts, aerobic and photosynthetic eubacteria.

The Na+ cycle of extreme alkalophiles: a secondary Na+/H+ antiporter and Na+/solute symporters

Extremely alkalophilic bacteria that grow optimally at pH 10.5 and above are generally aerobic bacilli that grow at mesophilic temperatures and moderate salt levels. The adaptations to alkalophily in these organisms may be distinguished from responses to combined challenges of high pH together with other stresses such as salinity or anaerobiosis. These alkalophiles all possess a simple and physiologically crucial Na+ cycle that accomplishes the key task of pH homeostasis. An electrogenic, secondary Na+/H+ antiporter is energized by the electrochemical proton gradient formed by the proton-pumping respiratory chain. The antiporter facilitates maintenance of a pHin that is two or more pH units lower than pHout at optimal pH values for growth. It also largely converts the initial electrochemical proton gradient formed by respiration into an electrochemical sodium gradient that energizes motility as well as a plethora of Na+ solute symporters. These symporters catalyze solute accumulation and, importantly, reentry of Na+. The extreme nonmarine alkalophiles exhibit no primary sodium pumping dependent upon either respiration or ATP. ATP synthesis is not part of their Na+ cycle. Rather, the specific details of oxidative phosphorylation in these organisms are an interesting analogue of the same process in mitochondria, and may utilize some common features to optimize energy transduction.

Characterization of the membrane-bound ATPase from a facultatively anaerobic alkalophile

We have studied the properties of membrane-bound ATPase of a facultatively anaerobic alkalophile. The enzyme could not be solubilized without detergent, suggesting an integral membrane protein. The activity was accelerated by NH4+ and acetate anion, and inhibited by NH3-. The enzyme required Mg2+ or Mn2+ as a divalent cation for the maximal activity. In addition to ATP, the enzyme utilized other triphosphates of nucleosides as a substrate, but not di- nor monophosphates. The enzyme was suggested to crossreact with an antibody against the alpha-subunit of Na+/K+-ATPase from dog kidney.

The bioenergetics of alkalophilic bacilli

A summary, cum speculation, of the major bioenergetic characteristics of alkalophilic bacilli is presented in Figure 5. Further progress will depend heavily on the purification and characterization of the relevant proteins that catalyze the ion fluxes and on the development of much more potent genetic approaches to the outstanding issues of this interesting group of bacteria.

Bioenergetics of alkalophilic bacteria

The central problem for organisms which grow optimally, and in some cases obligately, at pH values of 10 to 11, is the maintenance of a relatively acidified cytoplasm. A key component of the pH homeostatic mechanism is an electrogenic Na+/H+ antiporter which--by virtue of kinetic properties and/or its concentration in the membrane--catalyzes net proton uptake while the organisms extrude protons during respiration. The antiporter is also capable of maintaining a constant pHin during profound elevations in pHout as long as Na+ entry is facilitated by the presence of solutes which are taken up with Na+. Secondary to the problem of acidifying the interior is the adverse effect of the large pH gradient, acid in, on the total pmf of alkalophile cells. For the purposes of solute uptake and motility, the organisms appear to largely bypass the problem of a low pmf by utilizing a sodium motive force for energization. However, ATP synthesis appears not to resolve the energetics problem by using Na+ or by incorporating the proton-translocating ATPase into intracellular organelles. The current data suggest that effective proton pumping carried out by the alkalophile respiratory chain at high pH may deliver at least some portion of the protons to the proton-utilizing catalysts, i.e., the F1F0-ATPase and the Na+/H+ antiporter, by some localized pathway.

Biocompatibility screening of poly (vinyl chloride) implants

It was proposed to test the biocompatibility of a polymeric implant by assaying enzyme activity in the tissue homogenate. Medical grade poly(vinyl chloride) was implanted in rat gluteal muscle and muscle specimens taken from the implanted and non-implanted sites after 7, 14 and 21 days. In the muscle homogenate taken in suitable solvents, the activities of alkaline phosphatase, acid phosphatase, Ca2+ ATPase, leucine aminopeptidase (LAP) and succinic dehydrogenase (SDH) were tested and compared at implanted and non-implanted sites. Increased activity of acid phosphatase, alkaline phosphatase, LAP and Ca2+ ATPase were seen at the implanted site but the SDH activity was reduced. The enzyme activity at both sites was maximum around 14 days and the highest increase in activity at the implant site compared to the control site was observed for alkaline phosphatase and acid phosphatase. After 21 days, the activity decreased gradually. In case of SDH, the inhibition was more after 14 days and gradually decreased after 21 days.

Oxidative phosphorylation by membrane vesicles from Bacillus alcalophilus

ADP and Pi-loaded membrane vesicles from t-malate-grown Bacillus alcalophilus synthesized ATP upon energization with ascorbate/N,N,N',N'-tetra-methyl-P-phenylenediamine. ATP synthesis occurred over a range of external pH from 6.0 to 11.0, under conditions in which the total protonmotive force delta-mu-H+ was as low as -30 mV. The phosphate potentials (delta Gp) were calculated to be 11 and 12 kcal/mol at pH 10.5 and 9.0, respectively, whereas the delta-mu-H+ values in vesicles at these two pH values were quite different (-40 +/- 20 mV at pH 10.5 and -125 +/- 20 mV at pH 9.0). ATP synthesis was inhibited by KCN, gramicidin, and by N,N1-dicyclohexylcarbodiimide. Inward translocation of protons, concomitant with ATP synthesis, was demonstrated using direct pH monitoring and fluorescence methods. No dependence upon the presence of Na+ or K+ was found. Thus, ATP synthesis in B. alcalophilus appears to involve a proton-translocating ATPase which functions at low delta-mu-H+.