The plasma membrane ATPase of the thermoacidophilic archaebacterium Sulfolobus acidocaldarius. Purification and immunological relationships to F1-ATPases

Purification of a Crenarchaeal ATP Synthase in the Light of the Unique Bioenergetics of Ignicoccus Species

In this study, the ATP synthase of Ignicoccus hospitalis was purified, characterized, and structurally compared to the respective enzymes of the other Ignicoccus species, to shed light on energy conservation in this unique group of archaea. The crenarchaeal genus Ignicoccus comprises three described species, i.e., I. hospitalis and Ignicoccus islandicus from hot marine sediments near Iceland and Ignicoccus pacificus from a hydrothermal vent system in the Pacific Ocean. This genus is unique among all archaea due to the unusual cell envelope, consisting of two membranes that enclose a large intermembrane compartment (IMC). I. hospitalis is the best studied member of this genus, mainly because it is the only known host for the potentially parasitic archaeon Nanoarchaeum equitans I. hospitalis grows chemolithoautotrophically, and its sole energy-yielding reaction is the reduction of elemental sulfur with molecular hydrogen, forming large amounts of hydrogen sulfide. This reaction generates an electrochemical gradient, which is used by the ATP synthase, located in the outer cellular membrane, to generate ATP inside the IMC. The genome of I. hospitalis encodes nine subunits of an A-type ATP synthase, which we could identify in the purified complex. Although the maximal in vitro activity of the I. hospitalis enzyme was measured around pH 6, the optimal stability of the A₁A_O complex seemed to be at pH 9. Interestingly, the soluble A₁ subcomplexes of the different Ignicoccus species exhibited significant differences in their apparent molecular masses in native electrophoresis, although their behaviors in gel filtration and chromatography-mass spectrometry were very similar.IMPORTANCE The Crenarchaeota represent one of the major phyla within the Archaea domain. This study describes the successful purification of a crenarchaeal ATP synthase. To date, all information about A-type ATP synthases is from euryarchaeal enzymes. The fact that it has not been possible to purify this enzyme complex from a member of the Crenarchaeota until now points to significant differences in stability, possibly caused by structural alterations. Furthermore, the study subject I. hospitalis has a particular importance among crenarchaeotes, since it is the only known host of N. equitans The energy metabolism in this system is still poorly understood, and our results can help elucidate the unique relationship between these two microbes.

The a subunit of the A1AO ATP synthase of Methanosarcina mazei Gö1 contains two conserved arginine residues that are crucial for ATP synthesis

Like the evolutionary related F1FO ATP synthases and V1VO ATPases, the A1AO ATP synthases from archaea are multisubunit, membrane-bound transport machines that couple ion flow to the synthesis of ATP. Although the subunit composition is known for at least two species, nothing is known so far with respect to the function of individual subunits or amino acid residues. To pave the road for a functional analysis of A1AO ATP synthases, we have cloned the entire operon from Methanosarcina mazei into an expression vector and produced the enzyme in Escherichia coli. Inverted membrane vesicles of the recombinants catalyzed ATP synthesis driven by NADH oxidation as well as artificial driving forces. [Formula: see text] as well as ΔpH were used as driving forces which is consistent with the inhibition of NADH-driven ATP synthesis by protonophores. Exchange of the conserved glutamate in subunit c led to a complete loss of ATP synthesis, proving that this residue is essential for H+ translocation. Exchange of two conserved arginine residues in subunit a has different effects on ATP synthesis. The role of these residues in ion translocation is discussed.

Isolation of a complete A1AO ATP synthase comprising nine subunits from the hyperthermophile Methanococcus jannaschii

Archaeal A(1)A(O) ATP synthase/ATPase operons are highly conserved among species and comprise at least nine genes encoding structural proteins. However, all A(1)A(O) ATPase preparations reported to date contained only three to six subunits and, therefore, the study of this unique class of secondary energy converters is still in its infancy. To improve the quality of A(1)A(O) ATPase preparations, we chose the hyperthermophilic, methanogenic archaeon Methanococcus jannaschii as a model organism. Individual subunits of the A(1)A(O) ATPase from M. jannaschii were produced in E. coli, purified, and antibodies were raised. The antibodies enabled the development of a protocol ensuring purification of the entire nine-subunit A(1)A(O) ATPase. The ATPase was solubilized from membranes of M. jannaschii by Triton X-100 and purified to apparent homogeneity by sucrose density gradient centrifugation, ion exchange chromatography, and gel filtration. Electron micrographs revealed the A(1) and A(O) domains and the central stalk, but also additional masses which could represent a second stalk. Inhibitor studies were used to demonstrate that the A(1) and A(O) domains are functionally coupled. This is the first description of an A(1)A(O) ATPase preparation in which the two domains (A(1) and A(O)) are fully conserved and functionally coupled.

Overproduction of a functional A1 ATPase from the archaeon Methanosarcina mazei Gö1 in Escherichia coli

Single subunits of the A1 ATPase from the archaeon Methanosarcina mazei Gö1 were produced in E. coli as MalE fusions and purified, and polyclonal antibodies were raised against the fusion proteins. A DNA fragment containing the genes ahaE, ahaC, ahaF, ahaA, ahaB, ahaD, and ahaG, encoding the hydrophilic A1 domain and part of the stalk of the A1AO ATPase of M. mazei Gö1, was constructed, cloned into an expression vector and transformed into different strains of Escherichia coli. In any case, a functional, ATP-hydrolysing A1 ATPase was produced. Western blots demonstrated the production of subunits A, B, C, and F in E. coli, and minicell analyses suggested that subunits D, E, and G were produced as well. This is the first demonstration of a heterologous production of a functional ATPase from an archaeon. The A1 ATPase was sensitive to freezing but lost only about 50% of its activity within 18 days on ice. Inhibitor studies revealed that the heterologously produced A1 ATPase is insensitive to azide, dicyclohexylcarbodiimide and bafilomycin A1, but sensitive to diethylstilbestrol and its analogues dienestrol and hexestrol. The expression system described here will open new avenues towards the functional and structural analyses of this unique class of enzymes.

Bioenergetics of the Archaea

In the late 1970s, on the basis of rRNA phylogeny, Archaea (archaebacteria) was identified as a distinct domain of life besides Bacteria (eubacteria) and Eucarya. Though forming a separate domain, Archaea display an enormous diversity of lifestyles and metabolic capabilities. Many archaeal species are adapted to extreme environments with respect to salinity, temperatures around the boiling point of water, and/or extremely alkaline or acidic pH. This has posed the challenge of studying the molecular and mechanistic bases on which these organisms can cope with such adverse conditions. This review considers our cumulative knowledge on archaeal mechanisms of primary energy conservation, in relationship to those of bacteria and eucarya. Although the universal principle of chemiosmotic energy conservation also holds for Archaea, distinct features have been discovered with respect to novel ion-transducing, membrane-residing protein complexes and the use of novel cofactors in bioenergetics of methanogenesis. From aerobically respiring Archaea, unusual electron-transporting supercomplexes could be isolated and functionally resolved, and a proposal on the organization of archaeal electron transport chains has been presented. The unique functions of archaeal rhodopsins as sensory systems and as proton or chloride pumps have been elucidated on the basis of recent structural information on the atomic scale. Whereas components of methanogenesis and of phototrophic energy transduction in halobacteria appear to be unique to Archaea, respiratory complexes and the ATP synthase exhibit some chimeric features with respect to their evolutionary origin. Nevertheless, archaeal ATP synthases are to be considered distinct members of this family of secondary energy transducers. A major challenge to future investigations is the development of archaeal genetic transformation systems, in order to gain access to the regulation of bioenergetic systems and to overproducers of archaeal membrane proteins as a prerequisite for their crystallization.

Functional characterization of an extremely thermophilic ATPase in membranes of the crenarchaeon Acidianus ambivalens

A plasma membrane-bound adenosine triphosphatase with specific activities up to 0.2 micromol min(-1) (mg protein)(-1) at 80 degrees C was detected in the thermoacidophilic crenarchaeon Acidianus ambivalens (DSM 3772). The enzymatic activity exhibited a broad pH-optimum in the neutral range with two suboptima at pH 5.5 and 7.0, respectively. Sulfite activation resulted in only one pH optimum at 6.25. In the presence of the divalent cations Mg2+ and Mn2+ the ATPase activity was maximal. Remarkably, the hydrolytic rates of GTP and ITP were substantially higher than for ATP. ADP and pyrophosphate were only hydrolyzed with small rates, whereas AMP was not hydrolyzed at all. Both activities could be weakly inhibited by the classical F-type ATPase inhibitor N,N'-dicyclohexylcarbodiimide, whereas azide had no influence at all. The classical inhibitor of V-type ATPases, nitrate, also exerted a small inhibitory effect. The strongly specific V-type ATPase inhibitor concanamycin A, however, showed no effect at all. The P-type ATPase inhibitor vanadate had no inhibitory effect on the ATPase activity at pH 7.0, whereas a remarkable inhibition at high concentrations could be observed for the activity at pH 5.5. Arrhenius plots for both membrane bound ATPase activities were linear up to 95 degrees C, reflecting the enormous thermostability of the enzyme.

Bioenergetics of the archaebacterium Sulfolobus

Archaea are forming one of the three kingdoms defining the universal phylogenetic tree of living organisms. Within itself this kingdom is heterogenous regarding the mechanisms for deriving energy from the environment for support of cellular functions. These comprise fermentative and chemolithotrophic pathways as well as light driven and respiratory energy conservation. Due to their extreme growth conditions access to the molecular machineries of energy transduction in archaea can be experimentally limited. Among the aerobic, extreme thermoacidophilic archaea, the genus Sulfolobus has been studied in greater detail than many others and provides a comprehensive picture of bioenergetics on the level of substrate metabolism, formation and utilization of high energy phosphate bonds, and primary energy conservation in respiratory electron transport. A number of novel metabolic reactions as well as unusual structures of respiratory enzyme complexes have been detected. Since their genomic organization and many other primary structures could be determined, these studies shed light on the evolution of various bioenergetic modules. It is the aim of this comprehensive review to bring the different aspects of Sulfolobus bioenergetics into focus as a representative example of, and point of comparison for closely related, aerobic archaea.

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.

Characterization of a membrane-associated ATPase from Methanococcus voltae, a methanogenic member of the Archaea

A membrane-associated ATPase with an M(r) of approximately 510,000 and containing subunits with M(r)s of 80,000 (alpha), 55,000 (beta), and 25,000 (gamma) was isolated from the methanogen Methanococcus voltae. Enzymatic activity was not affected by vanadate or azide, inhibitors of P- and F1-ATPase, respectively, but was inhibited by nitrate and bafilomycin A1, inhibitors of V1-type ATPases. Since dicyclohexylcarbodiimide inhibited the enzyme when it was present in membranes but not after the ATPase was solubilized, we suggest the presence of membrane-associated component analogous to the F0 and V0 components of both F-type and V-type ATPases. N-terminal amino acid sequence analysis of the alpha subunit showed a higher similarity to ATPases of the V-type family than to those of the F-type family.

Purification and enzymic characterization of the cytoplasmic pyrophosphatase from the thermoacidophilic archaebacterium Thermoplasma acidophilum

Cytoplasmic pyrophosphatase has been isolated from the thermoacidophilic archaebacterium Thermoplasma acidophilum. The enzyme was purified to electrophoretic homogeneity by combining ion-exchange and affinity-chromatographic separations. This soluble pyrophosphatase probably consists of six identical subunits, since SDS/PAGE gave an estimate of about 22 kDa for a single subunit and size-exclusion chromatography under non-denaturing conditions indicates a molecular mass of 110 +/- 5 kDa. The two most prominent catalytic features of this enzyme are the absolute requirement for divalent cations for catalytic action, Mg2+ conferring the highest activity, and the pronounced specificity for PPi. The catalytic behavior apparently follows simple Michaelis-Menten kinetics with a Km of about 7 microM for PPi and a specific activity of about 1200 U/mg at 56 degrees C. Surprisingly, maximum activity could be observed at 85 degrees C which is more than 20 degrees C above the temperature for optimal growth. Several cytoplasmic extracts of eubacteria and archaebacteria have been probed with a polyclonal antiserum raised against the purified archaebacterial protein. The only noticeable cross-reactivity could be detected with an extract from the methanogen Methanosarcina barkeri although this probably does not reflect the inferred phylogenetic relationship between methanogens and Thermoplasma acidophilum.

Energy transduction in the methanogen Methanococcus voltae is based on a sodium current

We provide experimental support for the proposal that ATP production in Methanococcus voltae, a methanogenic member of the archaea, is based on an energetic system in which sodium ions, not protons, are the coupling ions. We show that when grown at a pH of 6.0, 7.1, or 8.2, M. voltae cells maintain a membrane potential of approximately -150 mV. The cells maintain a transmembrane pH gradient (pH(in) - pH(out)) of -0.1, -0.2, and -0.2, respectively, values not favorable to the inward movement of protons. The cells maintain a transmembrane sodium concentration gradient (sodium(out)/sodium(in)) of 1.2, 3.4, and 11.6, respectively. While the protonophore 3,3',4',5-tetrachlorosalicylanilide inhibits ATP formation in cells grown at pH 6.5, neither ATP formation nor growth is inhibited in cells grown in medium at pH 8.2. We show that when grown at pH 8.2, cells synthesize ATP in the absence of a favorably oriented proton motive force. Whether grown at pH 6.5 or pH 8.2, M. voltae extrudes Na+ via a primary pump whose activity does not depend on a proton motive force. The addition of protons to the cells leads to a harmaline-sensitive efflux of Na+ and vice versa, indicating the presence of Na+/H+ antiporter activity and, thus, a second mechanism for the translocation of Na+ across the cell membrane. M. voltae contains a membrane component that is immunologically related to the H(+)-translocating ATP synthase of the archaeabacterium Sulfolobus acidocaldarius. Since we demonstrated that ATP production can be driven by an artificially imposed membrane potential only in the presence of sodium ions, we propose that ATP production in M. voltae is mediated by an Na+-translocating ATP synthase whose function is coupled to a sodium motive force that is generated through a primary Na+ pump.

Structural conservation and functional diversity of V-ATPases

The vacuolar system of eukaryotic cells contains a large number of organelles that are primary energized by an H(+)-ATPase that was named V-ATPase. The structure and function of V-ATPases from various sources was extensively studied in the last few years. Several genes encoding subunits of the enzyme were cloned and sequenced. The sequence information revealed the relations between V-ATPases and F-ATPases that evolved from common ancestral genes. The two families of proton pumps share structural and functional similarity. They contain distinct peripheral catalytic sectors and hydrophobic membrane sectors. Genes encoding subunits of V-ATPase in yeast cells were interrupted to yield mutants that are devoid of the enzyme and are sensitive to pH and calcium concentrations in the medium. The mutants were used to study structure, function, molecular biology, and biogenesis of the V-ATPase. They also shed light on the functional assembly of the enzyme in the vacuolar system.

F-type or V-type? The chimeric nature of the archaebacterial ATP synthase

Archaebacterial plasma membranes contain an ATPase acting in vivo as a delta mu H(+)-driven ATP synthase. While functional features and their general structural design are resembling F-type ATPases, primary sequences of the two large polypeptides from the catalytic part are closely related to V-type ATPases from eucaryotic vacuolar membranes. The chimeric nature of archaebacterial ATPase from Sulfolobus was investigated in terms of nucleotide interactions and related to specific sequence parameters in a comparison to well known F- and V-type ATPases. The study disclosed a general difference of F- and V-type ATPases at one class of the nucleotide binding sites.

Characterization and purification of a membrane-bound archaebacterial pyrophosphatase from Sulfolobus acidocaldarius

Plasma membranes of the thermoacidophilic archaebacterium Sulfolobus acidocaldarius (DSM 639) display a pyrophosphate-hydrolyzing activity [M. Lübben & G. Schäfer (1987) Eur. J. Biochem. 164, 533-540]. In our present work, we solubilized and purified this pyrophosphatase to homogeneity. It consists of a single subunit with a molecular mass of 17-18 kDa, forming an oligomer of 70 kDa under native conditions. Edman degradation revealed 30 amino acids of the N-terminus. The enzyme cleaves phosphoric-acid-anhydride bonds independently of monovalent or divalent cations. Temperature and pH optima of 75 degrees C and 3.5-3.7, respectively, characterize it as an ectoenzyme. Membrane lipids of Sulfolobus stimulate the activity. The dolichol-pyrophosphate-complexing peptide-antibiotic bacitracin inhibited growth of Sulfolobus. A possible function of the acid pyrophosphatase is the hydrolysis of dolichol pyrophosphate in connection with glycosylation reactions of membrane proteins.

Evolution of organellar proton-ATPases

Proton ATPases function in biological energy conversion in every known living cell. Their ubiquity and antiquity make them a prime source for evolutionary studies. There are two related families of H(+)-ATPases; while the family of F-ATPases function in eubacteria chloroplasts and mitochondria, the family of V-ATPases are present in archaebacteria and the vacuolar system of eukaryotic cells. Sequence analysis of several subunits of V- and F-ATPases revealed several of the important steps in their evolution. Moreover, these studies shed light on the evolution of the various organelles of eukaryotes and suggested some events in the evolution of the three kingdoms of eubacteria, archaebacteria and eukaryotes.

Purification and properties of an ATPase from Sulfolobus solfataricus

A sulfite-activated ATPase isolated from Sulfolobus solfataricus had a relative molecular mass of 370,000. It was composed of three subunits whose relative molecular masses were 63,000, 48,000, and 24,000. The enzyme was inhibited by the vacuolar ATPase inhibitors nitrate and N-ethylmaleimide; 4-chloro-7-nitrobenzofurazan (NBD-Cl) was also inhibitory. N-Ethylmaleimide was predominately bound to the largest subunit while NBD-Cl was bound to both subunits. ATPase activity was inhibited by low concentrations of p-chloromercuriphenyl sulfonate and the inhibition was reversed by cysteine which suggested that thiol groups were essential for activity. While the ATPase from S. solfataricus shared several properties with the ATPase from S. acidocaldarius there were significant differences. The latter enzyme was activated by sulfate and chloride and was unaffected by N-ethylmaleimide, whereas the S. solfataricus ATPase was inhibited by these anions as well as N-ethylmaleimide. These differences as well as differences that occur in other vacuolar-like ATPases isolated from the methanogenic and the extremely halophilic bacteria suggest the existence of several types of archaeal ATPases, none of which have been demonstrated to synthesize ATP.

Purification and characterization of the F1 portion of the ATP synthase (F1Fo) of Streptomyces lividans

The F1 complex of the ATP synthase of Streptomyces lividans was isolated and purified. The procedure involved the solubilization of F1 from membranes with buffer of low ionic strength in the presence of EDTA, ion-exchange chromatography and gel filtration. The purified F1 complex from S. lividans (SLF1) consists of five subunits alpha, beta, gamma, delta and epsilon with molecular masses of 58,000, 50,000, 36,000, 28,000 and 13,000, respectively and exhibits immunological cross-reactivity with the F1 portion purified from Escherichia coli (ECF1). The enzymatic properties of SLF1 were determined by the use of microtiter-plate-based assay and compared with data obtained for ECF1. ATPase activity of SLF1 (specific activity: 20-30 U/mg) was only observed in the presence of high concentrations of Ca2+ (10mM). Stimulation of the ATPase activity by Mg2+ was not detectable; quite to the contrary, Mg2+ inhibited the Ca(2+)-stimulated activity of SLF1. SLF1 was re-bound to F1-stripped membranes of S. lividans, but not to F1-stripped membrane vesicles of E. coli. In contrast, ECF1 could be cross-reconstituted with F1-stripped membranes of S. lividans; however, a structural but not a functional reconstitution of the hybrid F1Fo complex was observed.

The ATP synthase of Halobacterium salinarium (halobium) is an archaebacterial type as revealed from the amino acid sequences of its two major subunits

The head piece of the A-type ATP synthase in an extremely halophilic archaebacterium, namely Halobacterium salinarium (halobium), is composed of two kinds of subunit, alpha and beta, and is associated with ATP-hydrolyzing activity. The genes encoding these subunits with hydrolytic activity have been cloned and sequenced. The putative amino acid sequences of the alpha and beta subunits deduced from the nucleotide sequences of the genomic DNA consist of 585 and 471 residues, respectively. The amino acid sequence of the alpha subunit of the halobacterial ATPase is 63 and 49% identical to the alpha subunits of ATPases from two other archaebacteria, Methanosarcina barkeri and Sulfolobus acidocaldarius, respectively. The sequence of the beta subunit is 66 and 55% identical to the beta subunits from these respective organisms. The homology between the alpha and beta subunits is around 30%. In contrast, the sequences of the halobacterial ATPase is less than 30% identical to F1 ATPase when any combination of subunits is considered. However, they are greater than 50% identical to a eukaryotic vacuolar ATPase when alpha and a, beta and b combinations are considered. These data fully confirm the first demonstration of this kind of relationship which was achieved by immunoblotting with an antibody raised against the halobacterial ATPase. We concluded that the archaebacterial ATP synthase is an A-type and not an F-type ATPase. This classification is also demonstrated by a "rooted" phylogenetic tree where halobacteria locate close to other archaebacteria and eukaryotes and distant from eubacteria.

Relationship of the membrane ATPase from Halobacterium saccharovorum to vacuolar ATPases

Polyclonal antiserum against subunit A (67 kDa) of the vacuolar ATPase from Neurospora crassa reacted with subunit I (87 kDa) from a membrane ATPase of the extremely halophilic archaebacterium Halobacterium saccharovorum. The halobacterial ATPase was inhibited by nitrate and N-ethylmaleimide; the extent of the latter inhibition was diminished in the presence of adenosine di- or triphosphates. 4-Chloro-7-nitrobenzofurazan inhibited the halobacterial ATPase also in a nucleotide-protectable manner; the bulk of inhibitor was associated with subunit II (60 kDa). The data suggested that this halobacterial ATPase may have conserved structural features from both the vacuolar and the F-type ATPases.

Chemiosmotic energy conversion and the membrane ATPase of Methanolobus tindarius

Electron transport phosphorylation has been demonstrated to drive ATP synthesis for the methanogenic archaebacterium Methanolobus tindarius: Protonophores evoked uncoupler effects and lowered the membrane potential delta psi. Under the influence of N,N'-dicyclohexylcarbodiimide [(cHxN)2C] the membrane potential increased while methanol turnover was inhibited. 2-Bromoethanesulfonate, an inhibitor of methanogenesis, had no effect on the membrane potential but, like (cHxN)2C and protonophores, decreased the intracellular ATP concentration. Labeling experiments with (cHxN)2(14)C showed membranes to contain a proteolipid, with a molecular mass of 5.5 kDa, that resembles known (cHxN)2C-binding proteins of F0-F1 ATPases. The (cHxN)2-sensitive membrane ATPase hydrolysed Mg.ATP at a pH optimum of 5.0 with a Km (ATP) of 2.5 mM (V = 77 mU/mg). It was inhibited competitively by ADP; Ki (ADP) = 0.65 mM. Azide or vanadate caused no significant loss in ATPase activity, but millimolar concentrations of nitrate showed an inhibitory effect, suggesting a relationship to ATPases from vacuolar membranes. In contrast, no inhibition occurred in the presence of bafilomycin A1. The ATPase was extractable with EDTA at low salt concentrations. The purified enzyme consists of four different subunits, alpha (67 kDa), beta (52 kDa), gamma (20 kDa) and beta (less than 10 kDa), as determined from SDS gel electrophoresis.

Chemiosmotic energy conversion of the archaebacterial thermoacidophile Sulfolobus acidocaldarius: oxidative phosphorylation and the presence of an F0-related N,N'-dicyclohexylcarbodiimide-binding proteolipid

The energy-transducing mechanism of the thermoacidophilic archaebacterium Sulfolobus acidocaldarius DSM 639 has been studied, addressing the question whether chemiosmotic proton gradients serve as an intermediate energy store driving an F0F1-analogous ATP synthase. At pH 3.5, respiring S. acidocaldarius cells developed an electrochemical potential of H+ ions, consisting mainly of a proton gradient and a small inside-negative membrane potential. The steady-state proton motive force of 140 to 160 mV was collapsed by protonophores, while N,N'-dicyclohexylcarbodiimide (DCCD) caused a hyperpolarization of the membrane, as expected for a reagent commonly used to inhibit the flux through proton channels of F0F1-type ATP synthases. Cellular ATP content was strongly related to the proton motive force generated by respiration and declined rapidly, either by uncoupling or by action of DCCD, which in turn induced a marked respiratory control effect. This observation strongly supports the operation of chemiosmotic ATP synthesis with H+ as the coupling ion. The inhibition of ATP synthesis by [14C]DCCD was correlated with covalent reactions with membrane proteins. The extraction of labeled membranes with organic solvents specifically yielded a readily aggregating proteolipid of 6 to 7 kilodaltons apparent molecular mass. Its amino acid composition revealed significant similarity to the proteolipid found in eubacteria, such as Escherichia coli, as an extremely hydrophobic constituent of the F0 proton channel. Moreover, the N-terminal amino acid sequence of the Sulfolobus proteolipid displays a high degree of homology to eubacterial sequences, as well as to one derived from nucleic acid sequencing of another Sulfolobus strain (K. Denda, J. Konishi, T. Oshima, T. Date, and M. Yoshida, J. Biol. Chem. 264:7119-7121, 1989). Despite certain structural similarities between eucaryotic vacuolar ATPases and the F1-analogous ATPase from Sulfolobus sp. described earlier, the results reported here promote the view that the archaebacterial ATP-synthesizing complex functionally belongs to the F0F1 class of ATPases. These may be considered as phylogenetically conserved catalysts of energy transduction present in all kingdoms of organisms.

Structure, molecular genetics, and evolution of vacuolar H+-ATPases

Proton-ATPases can be divided into three classes denoted as P-, F-, and V-ATPases. The P-ATPases are evolutionarily distinct from the F- and V-type ATPases which have been shown to be related, probably evolved from a common ancestral enzyme. Like F-ATPases, V-ATPases are composed of two distinct structures: a catalytic sector that is hydrophilic in nature and a hydrophobic membrane sector which functions in proton conduction. Recent studies on the molecular biology of vacuolar H+-ATPases revealed surprising findings about the evolution of pronon pumps as well as important clues for the evolution of eukaryotic cells.

The role of sodium ions in methanogenesis. Formaldehyde oxidation to CO2 and 2H2 in methanogenic bacteria is coupled with primary electrogenic Na+ translocation at a stoichiometry of 2-3 Na+/CO2

Cell suspensions of Methanosarcina barkeri were found to oxidize formaldehyde to CO2 and 2H2 (delta G0' = -27 kJ/mol CO2), when methanogenesis was inhibited by 2-bromoethanesulfonate. We report here that this reaction is coupled with (a) primary electrogenic Na+ translocation at a stoichiometry of 2-3 Na+/CO2, (b) with secondary H+ translocation via a Na+/H+ antiporter and (c) with ATP synthesis driven by an electrochemical proton potential. This is concluded from the following findings. Formaldehyde oxidation to CO2 and 2H2 was dependent on Na+ ions, 2-3 mol Na+/mol formaldehyde oxidized were extruded. Na+ translocation was inhibited by Na+ ionophores, but not affected by protonophores of Na+/H+ antiport inhibitors. Formaldehyde oxidation was associated with the build up of a membrane potential in the order of 100 mV (inside negative), which could be dissipated by sodium ionophores rather than by protonophores. Formaldehyde oxidation was coupled with ATP synthesis, which could be inhibited by Na+ ionophores, Na+/H+ antiport inhibitors, by protonophores and by the H+-translocating-ATP-synthase inhibitor, dicyclohexylcarbodiimide. With cell suspensions of Methanobacterium thermoautotrophicum similar results were obtained.

Functional molecular masses of vacuolar membrane H+-ATPase from Saccharomyces cerevisiae as studied by radiation inactivation analysis

The functional molecular masses of the vacuolar membrane H+-ATPase in Saccharomyces cerevisiae under two kinetic conditions for ATP hydrolysis were measured by radiation inactivation. When vacuolar membrane vesicles were exposed to gamma-rays from 60Co, the activities catalyzing a single-cycle and multi-cycles of ATP hydrolysis both decreased as single-exponential functions of the radiation dosage. By applying the target theory, the functional molecular masses for single- and multi-cycle hydrolyses of ATP were determined to be approx. 0.9-1.1 X 10(5) and 4.1-5.3 X 10(5) Da, respectively. N,N'-Dicyclohexylcarbodiimide (DCCD) did not inhibit the former reaction but strongly inhibited the latter. It is suggested that the ATPase with a minimal composite of subunits a and b, in which subunit c is not necessarily involved operationally, can catalyze single-cycle hydrolysis of ATP, whereas for multi-cycle hydrolysis of ATP, the ATPase requires a properly organized oligomeric structure with subunits a-c, which may direct a positive cooperative mechanism of ATP hydrolysis and coupled H+ translocation in a DCCD-sensitive manner.

A comparison of an ATPase from the archaebacterium Halobacterium saccharovorum with the F1 moiety from the Escherichia coli ATP synthase

A purified ATPase associated with membranes from Halobacterium saccharovorum was compared with the F1 moiety from the Escherichia coli ATP synthase. The halobacterial enzyme was composed of two major (I and II) and two minor subunits (III and IV), whose molecular masses were 87 kDa, 60 kDa, 29 kDa and 20 kDa, respectively. The isoelectric points of these subunits ranged from 4.1 to 4.8, which in the case of the subunits I and II was consistent with the presence of an excess of acidic amino acids (20-22 mol/100 mol). Peptide mapping of subunits I and II denatured with sodium dodecyl sulfate showed no relationship between the primary structures of the individual halobacterial subunits or similarities to the subunits of the F1 ATPase from E. coli. Trypsin inactivation of the halobacterial ATPase was accompanied by the partial degradation of the major subunits. This observation, taken in conjunction with molecular masses of the subunits and the native enzyme, was consistent with the previously proposed stoichiometry of 2:2:1:1. These results suggest that H. saccharovorum, and possibly, halobacteria in general, possess an ATPase which is unlike the ubiquitous F0F1 ATP synthase.

Evidence from immunological studies of structure-mechanism relationship of F1 and F1F0

Monoclonal and polyclonal antibodies directed against peptides of F1-ATPase of F1F0-ATPase synthase provide new and efficient tools to study structure-function relationships and mechanisms of such complex membrane enzymes. This review summarizes the main results obtained using this approach. Antibodies have permitted the determination of the nature of subunits involved in the complex, their stoichiometry, their organization, neighboring interactions, and vectorial distribution within or on either face of the membrane. Moreover, in a few cases, amino acid sequences exposed on a face of the membrane or buried inside the complex have been identified. Antibodies are very useful for detecting the role of each subunit, especially for those subunits which appear to have no direct involvement in the catalytic mechanism. Concerning the mechanisms, the availability of monoclonal antibodies which inhibit (or activate) ATP hydrolysis or ATP synthesis, which modify nucleotide binding or regulation of activities, which detect specific conformations, etc. brings many new ways of understanding the precise functions. The specific recognition by monoclonal antibodies on the beta subunit of epitopes in the proximity of, or in the catalytic site, gives information on this site. The use of anti-alpha monoclonal antibodies has shown asymmetry of alpha in the complex as already shown for beta. In addition, the involvement of alpha with respect to nucleotide site cooperativity has been detected. Finally, the formation of F1F0-antibody complexes of various masses, seems to exclude the functional rotation of F1 around F0 during catalysis.