Spin-label electron paramagnetic resonance and differential scanning calorimetry studies of the interaction between mitochondrial cytochrome c oxidase and adenosine triphosphate synthase complex

VJ21.089The subcompartmented oxphosomic model of the phosphorylating system organization in mitochondria

The oxidative phosphorylation (OXPHOS) system of mitochondria supports all the vitally important energy-consuming processes in eukaryotic cells, providing them with energy in the form of ATP. OXPHOS enzymes (complexes I-V) are located in the inner mitochondrial membrane, mainly in the cristae subcompartment. At present, there is a large body of data evidencing that the respiratory complexes I, III2 and IV under in vivo conditions can physically interact with each other in diverse stoichiometry, thereby forming supercomplexes. Despite active accumulation of knowledge about the structure of the main supercomplexes of the OXPHOS system, its physical and functional organization in vivo remains unclear. Contemporary models of the OXPHOS system's organization in the inner membrane of mitochondria are contradictory and presume the existence of either highly organized respiratory strings, or, by contrast, a set of randomly dispersed respiratory supercomplexes and complexes. Furthermore, it is assumed that ATP-synthase (complex V) does not form associations with respiratory enzymes and operates autonomously. Our latest data obtained on mitochondria of etiolated shoots of pea evidence the possibility of physical association between the respiratory supercomplexes and dimeric ATP-synthase. These data have allowed us to reconsider the contemporary concept of the phosphorylation system organization and propose a new subcompartmented oxphosomic model. According to this model, a substantial number of the OXPHOS complexes form oxphosomes, which in a def inite stoichiometry include complexes I-V and are located predominantly in the cristae subcompartment of mitochondria in the form of highly organized strings or patches. These suprastructures represent "mini-factories" for ATP production. It is assumed that such an organization (1) contributes to increasing the eff iciency of the OXPHOS system operation, (2) involves new levels of activity regulation, and (3) may determine the inner membrane morphology to some extent. The review discusses the proposed model in detail. For a better understanding of the matter, the history of development of concepts concerning the OXPHOS organization with the emphasis on recent contemporary models is brief ly considered. The principal experimental data accumulated over the past 40 years, which conf irm the validity of the oxphosomic hypothesis, are also provided.

New insights into the organisation of the oxidative phosphorylation system in the example of pea shoot mitochondria

The physical and functional organisation of the OXPHOS system in mitochondria in vivo remains elusive. At present, different models of OXPHOS arrangement, representing either highly ordered respiratory strings or, vice versa, a set of randomly dispersed supercomplexes and respiratory complexes, have been suggested. In the present study, we examined a supramolecular arrangement of the OXPHOS system in pea shoot mitochondria using digitonin solubilisation of its constituents, which were further analysed by classical BN-related techniques and a multidimensional gel electrophoresis system when required. As a result, in addition to supercomplexes I1III2, I1III2IVn and III2IV1-2, dimer V2, and individual complexes I-V previously detected in plant mitochondria, new OXPHOS structures were also revealed. Of them, (1) a megacomplex (IIxIIIyIVz)n including complex II, (2) respirasomes I2III4IVn with two copies of complex I and dimeric complex III2, (3) a minor new supercomplex IV1Va2 comigrating with I1III2, and (4) a second minor form of ATP synthase, Va, were found. The activity of singular complexes I, IV, and V was higher than the activity of the associated forms. The detection of new supercomplex IV1Va2, along with assemblies I1III2 and I1-2III2-4IVn, prompted us to suggest the occurrence of in vivo oxphosomes comprising complexes I, III2, IV, and V. The putative oxphosome's stoichiometry, historical background, assumed functional significance, and subcompartmental location are discussed herein.

A new model for mitochondrial membrane potential production and storage

Mitochondrial membrane potential (MMP) is the most reliable indicator of mitochondrial function. The MMP value range of -136 to -140mV has been considered optimal for maximum ATP production for all living organisms. Even small changes from the above range result in a large fall in ATP production and a large increase in ROS production. The resulting bioenergetic deregulation is considered as the causative agent for numerous major human diseases. Normalization of MMP value improves mitochondrial function and gives excellent therapeutic results. In order for a systematic effective treatment of these diseases to be developed, a detailed knowledge of the mechanism of MMP production is absolutely necessary. However, despite the long-standing research efforts, a concrete mechanism for MMP production has not been found yet. The present paper proposes a novel mechanism of MMP production based on new considerations underlying the function of the two basic players of MMP production, the electron transport chain (ETC) and the F1F0 ATP synthase. Under normal conditions, MMP is almost exclusively produced by the electron flow through ETC complexes I-IV, creating a direct electric current that stops in subunit II of complex IV and gradually charges MMP. However, upon ETC dysfunction F1F0 ATP synthase reverses its action and starts to hydrolyze ATP. ATP hydrolysis further produces electric energy which is transferred, in the form of a direct electric current, from F1 to F0 where is used to charge MMP. This new model is expected to redirect current experimental research on mitochondrial bioenergetics and indicate new therapeutic schemes for mitochondrial disorders.

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.

F1F0-ATP synthases of alkaliphilic bacteria: lessons from their adaptations

This review focuses on the ATP synthases of alkaliphilic bacteria and, in particular, those that successfully overcome the bioenergetic challenges of achieving robust H+-coupled ATP synthesis at external pH values>10. At such pH values the protonmotive force, which is posited to provide the energetic driving force for ATP synthesis, is too low to account for the ATP synthesis observed. The protonmotive force is lowered at a very high pH by the need to maintain a cytoplasmic pH well below the pH outside, which results in an energetically adverse pH gradient. Several anticipated solutions to this bioenergetic conundrum have been ruled out. Although the transmembrane sodium motive force is high under alkaline conditions, respiratory alkaliphilic bacteria do not use Na+- instead of H+-coupled ATP synthases. Nor do they offset the adverse pH gradient with a compensatory increase in the transmembrane electrical potential component of the protonmotive force. Moreover, studies of ATP synthase rotors indicate that alkaliphiles cannot fully resolve the energetic problem by using an ATP synthase with a large number of c-subunits in the synthase rotor ring. Increased attention now focuses on delocalized gradients near the membrane surface and H+ transfers to ATP synthases via membrane-associated microcircuits between the H+ pumping complexes and synthases. Microcircuits likely depend upon proximity of pumps and synthases, specific membrane properties and specific adaptations of the participating enzyme complexes. ATP synthesis in alkaliphiles depends upon alkaliphile-specific adaptations of the ATP synthase and there is also evidence for alkaliphile-specific adaptations of respiratory chain components.

Structural and functional organization of the mitochondrial respiratory chain: a dynamic super-assembly

The structural organization of the mitochondrial oxidative phosphorylation (OXPHOS) system has received large attention in the past and most investigations led to the conclusion that the respiratory enzymatic complexes are randomly dispersed in the lipid bilayer of the inner membrane and functionally connected by fast diffusion of smaller redox components, Coenzyme Q and cytochrome c. More recent investigations by native gel electrophoresis, however, have shown the existence of supramolecular associations of the respiratory complexes, confirmed by electron microscopy analysis and single particle image processing. Flux control analysis has demonstrated that Complexes I and III in mammalian mitochondria and Complexes I, III, and IV in plant mitochondria kinetically behave as single units with control coefficients approaching unity for each single component, suggesting the existence of substrate channelling within the supercomplexes. The reasons why the presence of substrate channelling for Coenzyme Q and cytochrome c was overlooked in the past are analytically discussed. The review also discusses the forces and the conditions responsible for the formation of the supramolecular units. The function of the supercomplexes appears not to be restricted to kinetic advantages in electron transfer: we discuss evidence on their role in the stability and assembly of the individual complexes and in preventing excess oxygen radical formation. Finally, there is increasing evidence that disruption of the supercomplex organization leads to functional derangements responsible for pathological changes.

Introduction of the chloroplast redox regulatory region in the yeast ATP synthase impairs cytochrome c oxidase

The ATP synthase is under a number of mechanisms of regulation. The chloroplast ATPase has a unique mode of regulation in which activity is controlled by the redox state in the organelle. This mode of regulation is determined by a small unique region within the gamma-subunit and this region contains two cysteine residues. Introduction of this region within the yeast gamma-subunit causes a defect in oxidative phosphorylation. Oxidative phosphorylation is restored if the cysteine residues are replaced with serine. Biochemical analysis of the chimeric mitochondrial ATPase indicates that the ATP synthase is not largely altered with the cysteine residues in either the oxidized or reduced states. However, the level and activity of cytochrome c oxidase are decreased by about 90%, whereas that of NADH dehydrogenase and cytochrome c reductase are unchanged as compared with the wild-type enzymes. The level and activity of cytochrome c oxidase are restored with replacement of the cysteine residues with serine in the regulatory region. These results indicate that the chimeric ATP synthase containing cysteine, but not serine, decreases the expression or assembly of cytochrome c oxidase with little effect on the activity of the ATP synthase.

Interaction between cytochrome caa3 and F1F0-ATP synthase of alkaliphilic Bacillus pseudofirmus OF4 is demonstrated by saturation transfer electron paramagnetic resonance and differential scanning calorimetry assays

Interaction between the cytochrome caa3 respiratory chain complex and F1F0-ATP synthase from extremely alkaliphilic Bacillus pseudofirmus OF4 has been hypothesized to be required for robust ATP synthesis by this alkaliphile under conditions of very low protonmotive force. Here, such an interaction was probed by differential scanning calorimetry (DSC) and by saturation transfer electron paramagnetic resonance (STEPR). When the two purified complexes were embedded in phospholipid vesicles individually [(caa3)PL, (F1F0)PL)] or in combination [(caa3 + F1F0)PL] and subjected to DSC analysis, they underwent exothermic thermodenaturation with transition temperatures at 69, 57, and 46/75 degrees C, respectively. The enthalpy change, deltaH (-8.8 kcal/mmol), of protein-phospholipid vesicles containing both cytochrome caa3 and F1F0 was smaller than that (-12.4 kcal/mmol) of a mixture of protein-phospholipid vesicles formed from each individual electron transfer complex [(caa3)PL + (F1F0)PL]. The rotational correlation time of spin-labeled caa3 (65 micros) in STEPR studies increased significantly when the complex was mixed with F1F0 prior to being embedded in phospholipid vesicles (270 micros). When the complexes were reconstituted separately and then mixed together, or either mitochondrial cytochrome bc1 or F1F0 was substituted for the alkaliphile F1F0, the correlation time was unchanged (65-70 micros). Varying the ratio of the two alkaliphile complexes in both the DSC and STEPR experiments indicated that the optimal stoichiometry is 1:1. These results demonstrate a physical interaction between the cytochrome caa3 and F1F0-ATP synthase from B. pseudofirmus OF4 in a reconstituted system. They support the suggestion that such an interaction between these complexes may contribute to sequestered proton transfers during alkaliphile oxidative phosphorylation at high pH.

Kinetics of integrated electron transfer in the mitochondrial respiratory chain: random collisions vs. solid state electron channeling

Recent evidence, mainly based on native electrophoresis, has suggested that the mitochondrial respiratory chain is organized in the form of supercomplexes, due to the aggregation of the main respiratory chain enzymatic complexes. This evidence strongly contrasts the previously accepted model, the Random Diffusion Model, largely based on kinetic studies, stating that the complexes are randomly distributed in the lipid bilayer of the inner membrane and functionally connected by lateral diffusion of small redox molecules, i.e., coenzyme Q and cytochrome c. This review critically examines the experimental evidence, both structural and functional, pertaining to the two models and attempts to provide an updated view of the organization of the respiratory chain and of its kinetic consequences. The conclusion that structural respiratory assemblies exist is overwhelming, whereas the expected functional consequence of substrate channeling between the assembled enzymes is controversial. Examination of the available evidence suggests that, although the supercomplexes are structurally stable, their kinetic competence in substrate channeling is more labile and may depend on the system under investigation and the assay conditions.

Cytochrome c and bioenergetic hypothetical model for alkaliphilic Bacillus spp

Although a bioenergetic parameter is unfavorable for production of ATP (DeltapH<0), the growth rate and yield of alkaliphilic Bacillus strains are higher than those of neutralophilic Bacillus subtilis. This finding suggests that alkaliphiles possess a unique energy-producing machinery taking advantage of the alkaline environment. Expected bioenergetic parameters for the production of ATP (DeltapH and DeltaPsi) do not reflect the actual parameters for energy production. Certain strains of alkaliphilic Bacillus spp. possess large amounts of cytochrome c when grown at a high pH. The growth rate and yield are higher at pH 10 than at pH 7 in facultative alkaliphiles. These findings suggest that a large amount of cytochrome c at high pHs (e.g., pH 10) may be advantageous for sustaining growth. To date, isolated cytochromes c of alkaliphiles have a very low midpoint redox potential (less than +100 mV) compared with those of neutralophiles (approximately +220 mV). On the other hand, the redox potential of the electron acceptor from cytochrome c, that is, cytochrome c oxidase, seems to be normal (redox potential of cytochrome a=+250 mV). This large difference in midpoint redox potential between cytochrome c and cytochrome a concomitant with the configuration (e.g., a larger negative ion capacity at the inner surface membrane than at the outer surface for the attraction of H+ to the intracellular membrane and a large amount of cyrochrome c) supporting H+-coupled electron transfer of cytochrome c may have an important meaning in the adaptation of alkaliphiles at high pHs. This respiratory system includes a more rapid and efficient H+ and e- flow across the membrane in alkaliphiles than in neutralophiles.

Supercomplex organization of the mitochondrial respiratory chain and the role of the Coenzyme Q pool: pathophysiological implications

In this review we examine early and recent evidence for an aggregated organization of the mitochondrial respiratory chain. Blue Native Electrophoresis suggests that in several types of mitochondria Complexes I, III and IV are aggregated as fixed supramolecular units having stoichiometric proportions of each individual complex. Kinetic evidence by flux control analysis agrees with this view, however the presence of Complex IV in bovine mitochondria cannot be demonstrated, presumably due to high levels of free Complex. Since most Coenzyme Q appears to be largely free in the lipid bilayer of the inner membrane, binding of Coenzyme Q molecules to the Complex I-III aggregate is forced by its dissociation equilibrium; furthermore free Coenzyme Q is required for succinate-supported respiration and reverse electron transfer. The advantage of the supercomplex organization is in a more efficient electron transfer by channelling of the redox intermediates and in the requirement of a supramolecular structure for the correct assembly of the individual complexes. Preliminary evidence suggests that dilution of the membrane proteins with extra phospholipids and lipid peroxidation may disrupt the supercomplex organization. This finding has pathophysiological implications, in view of the role of oxidative stress in the pathogenesis of many diseases.

Replacement of amino acid sequence features of a- and c-subunits of ATP synthases of Alkaliphilic Bacillus with the Bacillus consensus sequence results in defective oxidative phosphorylation and non-fermentative growth at pH 10.5

Mitchell's (Mitchell, P. (1961) Nature 191, 144-148) chemiosmotic model of energy coupling posits a bulk electrochemical proton gradient (Deltap) as the sole driving force for proton-coupled ATP synthesis via oxidative phosphorylation (OXPHOS) and for other bioenergetic work. Two properties of proton-coupled OXPHOS by alkaliphilic Bacillus species pose a challenge to this tenet: robust ATP synthesis at pH 10.5 that does not correlate with the magnitude of the Deltap and the failure of artificially imposed potentials to substitute for respiration-generated potentials in energizing ATP synthesis at high pH (Krulwich, T. (1995) Mol. Microbiol. 15, 403-410). Here we show that these properties, in alkaliphilic Bacillus pseudofirmus OF4, depend upon alkaliphile-specific features in the proton pathway through the a- and c-subunits of ATP synthase. Site-directed changes were made in six such features to the corresponding sequence in Bacillus megaterium, which reflects the consensus sequence for non-alkaliphilic Bacillus. Five of the six single mutants assembled an active ATPase/ATP synthase, and four of these mutants exhibited a specific defect in non-fermentative growth at high pH. Most of these mutants lost the ability to generate the high phosphorylation potentials at low bulk Deltap that are characteristic of alkaliphiles. The aLys(180) and aGly(212) residues that are predicted to be in the proton uptake pathway of the a-subunit were specifically implicated in pH-dependent restriction of proton flux through the ATP synthase to and from the bulk phase. The evidence included greatly enhanced ATP synthesis in response to an artificially imposed potential at high pH. The findings demonstrate that the ATP synthase of extreme alkaliphiles has special features that are required for non-fermentative growth and OXPHOS at high pH.

Insights into ATP synthase assembly and function through the molecular genetic manipulation of subunits of the yeast mitochondrial enzyme complex

Development of an increasingly detailed understanding of the eucaryotic mitochondrial ATP synthase requires a detailed knowledge of the stoichiometry, structure and function of F(0) sector subunits in the contexts of the proton channel and the stator stalk. Still to be resolved are the precise locations and roles of other supernumerary subunits present in mitochondrial ATP synthase complexes, but not found in the bacterial or chloroplast enzymes. The highly developed system of molecular genetic manipulation available in the yeast Saccharomyces cerevisiae, a unicellular eucaryote, permits testing for gene function based on the effects of gene disruption or deletion. In addition, the genes encoding ATP synthase subunits can be manipulated to introduce specific amino acids at desired positions within a subunit, or to add epitope or affinity tags at the C-terminus, enabling questions of stoichiometry, structure and function to be addressed. Newly emerging technologies, such as fusions of subunits with GFP are being applied to probe the dynamic interactions within mitochondrial ATP synthase, between ATP synthase complexes, and between ATP synthase and other mitochondrial enzyme complexes.

Identification of subunit g of yeast mitochondrial F1F0-ATP synthase, a protein required for maximal activity of cytochrome c oxidase

By means of a yeast genome database search, we have identified an open reading frame located on chromosome XVI of Saccharomyces cerevisiae that encodes a protein with 53% amino acid similarity to the 11.3-kDa subunit g of bovine mitochondrial F1F0-ATP synthase. We have designated this ORF ATP20, and its product subunit g. A null mutant strain, constructed by insertion of the HIS3 gene into the coding region of ATP20, retained oxidative phosphorylation function. Assembly of F1F0-ATP synthase in the atp20-null strain was not affected in the absence of subunit g and levels of oligomycin-sensitive ATP hydrolase activity in mitochondria were normal. Immunoprecipitation of F1F0-ATP synthase from mitochondrial lysates prepared from atp20-null cells expressing a variant of subunit g with a hexahistidine motif indicated that this polypeptide was associated with other well-characterized subunits of the yeast complex. Whilst mitochondria isolated from the atp20-null strain had the same oxidative phosphorylation efficiency (ATP : O) as that of the control strain, the atp20-null strain displayed approximately a 30% reduction in both respiratory capacity and ATP synthetic rate. The absence of subunit g also reduced the activity of cytochrome c oxidase, and altered the kinetic control of this complex as demonstrated by experiments titrating ATP synthetic activity with cyanide. These results indicate that subunit g is associated with F1F0-ATP synthase and is required for maximal levels of respiration, ATP synthesis and cytochrome c oxidase activity in yeast.

Mutations in the membrane anchor of yeast cytochrome c1 compensate for the absence of Oxa1p and generate carbonate-extractable forms of cytochrome c1

Oxa1p is a mitochondrial inner membrane protein that is mainly required for the insertion/assembly of complex IV and ATP synthase and is functionally conserved in yeasts, humans, and plants. We have isolated several independent suppressors that compensate for the absence of Oxa1p. Molecular cloning and sequencing reveal that the suppressor mutations (CYT1-1 to -6) correspond to amino acid substitutions that are all located in the membrane anchor of cytochrome c1 and decrease the hydrophobicity of this anchor. Cytochrome c1 is a catalytic subunit of complex III, but the CYT1-1 mutation does not seem to affect the electron transfer activity. The double-mutant cyt1-1,164, which has a drastically reduced electron transfer activity, still retains the suppressor activity. Altogether, these results suggest that the suppressor function of cytochrome c1 is independent of its electron transfer activity. In addition to the membrane-bound cytochrome c1, carbonate-extractable forms accumulate in all the suppressor strains. We propose that these carbonate-extractable forms of cytochrome c1 are responsible for the suppressor function by preventing the degradation of the respiratory complex subunits that occur in the absence of Oxa1p.

Potassium collapses the deltaP in yeast mitochondria while the rate of ATP synthesis is inhibited only partially: modulation by phosphate

Addition of increasing concentrations of K+ to yeast mitochondria in the presence of 0 to 400 microM phosphate and 200 microM Mg2+ led to uncoupled respiration and decreased protonmotive force (deltaP):at 0 K+ deltaP = 213 mV, negative inside, where deltapsi = 180 mV and deltapH = 33 mV, while at 20 mM K+ deltaP = 28 mV, where deltapsi = 16 mV and deltapH = 12 mV. In contrast, the synthesis of ATP resulted in smaller values for the Km and the Vmax in 400 microM Pi and increasing ADP: in 0 K+, Km = 18.6 microM and Vmax = 75.4 nmol (min x mg protein)-1, while in 20 mM K+, Km = 5.2 microM and Vmax = 46.0 nmol (min x mg protein)-1, i.e., when K+ depleted most of the deltaP, and at ADP concentrations below the Km, the rate of ATP synthesis was essentially the same as in the absence of K+. At saturating ADP, the rate of ATP synthesis in the presence of K+ was about 60% of the rate observed without K+. The synthesis of ATP by yeast mitochondria was inhibited by oligomycin or uncouplers. K+ had no effects on rat liver mitochondria. Adenylate kinase activity was much smaller in yeast mitochondria than in rat liver mitochondria and thus did not account for the synthesis of ATP observed in the presence of K+. The effects of K+ on the deltaP of yeast mitochondria were prevented by increasing concentrations of phosphate (1 to 4 mM). At 4 mM phosphate, the deltaP was always above 200 mV and the kinetics of ATP synthesis were as follows: 0 K+ Km = 10.0 microM and Vmax = 88.3 nmol (min x mg protein)-1. At 20 mM K+, Km = 7.4 microM and Vmax = 133 nmol (min x mg protein)-1.

The nuclear ABC1 gene is essential for the correct conformation and functioning of the cytochrome bc1 complex and the neighbouring complexes II and IV in the mitochondrial respiratory chain

The nuclear ABC1 gene was isolated as a multicopy suppressor of a cytochrome b mRNA translation defect. Its inactivation leads to a respiratory deficiency suggesting a block in the bc1 segment of the respiratory chain [Bousquet, I., Dujardin, G. & Slonimski, P. P. (1991) EMBO J. 10, 2023-2031]. In the present study, we established that deleting the ABC1 chromosomal gene from Saccharomyces cerevisiae does not prevent the assembly of the bc1 complex (complex III) but markedly impairs the kinetics of its high-potential electron transfer pathway occurring on the positive, outer, side of the membrane, which results in reduced activity of the bc1 complex. In addition, the activity of complex II and its cytochrome b560 decrease drastically and complex IV activity is halved. It is also observed that the binding of the quinol to the bc1 complex ubiquinol oxidation site is affected and that adding exogenous quinones partially compensates for the respiratory deficiency in vitro, although the quinone content of mutant and wild-type mitochondria are similar. Lastly, complexes II, III and IV are found to be thermosensitive and the bc1 complex exhibits greater sensitivity than the wild-type strain to center N and P inhibitors, suggesting that the three multisubunit complexes have undergone structural modifications. The data suggest that the ABC1 gene product acts as a chaperone-like protein essential for the proper conformation and efficient functioning of the bc1 complex and the effects of the Abc1 protein on the complexes II and IV might result from interactions with the modified bc1 complex.

Alkaliphiles: 'basic' molecular problems of pH tolerance and bioenergetics

Alkaliphilic Bacillus species provide experimental opportunities for examination of physiological processes under conditions in which the stress of the extreme environment brings issues of general biological importance into special focus. The alkaliphile, like many other cells, uses Na+/H+ antiporters in pH regulation, but its array of these porters, and other ion-flux pathways that energize and support their activity, result in an extraordinary capacity for pH homeostasis; this process nonetheless becomes the factor that limits growth at the upper edge of the pH range. Above pH 9.5, aerobic alkaliphiles maintain a cytoplasmic pH that is two or more units below the external pH. This chemiosmotically adverse delta pH is bypassed by use of an electrochemical gradient of Na+ rather than of protons to energize solute uptake and motility. By contrast, ATP synthesis occurs via completely proton-coupled oxidative phosphorylation that proceeds just as well, or better, at pH 10 and above as it does in the same bacteria growing at lower pH, without the adverse pH gradient. Various mechanisms that might explain this conundrum are described, and the current state of the evidence supporting them is summarized.

Inhibition of the synthesis of a cytochrome-c-oxidase subunit isoform by antisense RNA

To investigate the role of subunit VIIe, an oxygen-regulated subunit isoform of Dictyostelium discoideum cytochrome-c oxidase, the full-length cDNA was inserted into an expression vector under the control of an actin promoter in the sense and antisense orientation. The DNA constructs were used for stable transformation of the slime mold amoebae. In most of the 28 antisense clones tested, the concentration of cytochrome-c oxidase was lowered compared to the wild type, while no significant changes were found in the sense mutants. Antisense RNA was abundantly expressed, leading to a drastic reduction of the steady-state level of the endogenous subunit VIIe mRNA, which was decreased up to 20-30% the level observed in parent cells. In these transformants, the amount of the target polypeptide and cytochrome c oxidase was 40-50% and 60-70% of control, respectively. A similar decrease was found in the level of the remaining nuclear and mitochondrial subunits. Unexpectedly, these changes affected neither basal nor uncoupled cell respiration suggesting an increase of the enzyme specific activity. Hypoxia completely relieved the cytochrome-c-oxidase deficit. These results indicate that subunit VII is needed for an efficient assembly of the protein complex and provide evidence for its involvement in the modulation of the enzyme activity.

Mitochondrial ubiquinol-cytochrome c reductase complex: crystallization and protein: ubiquinone interaction

The ubiquinol-cytochrome c reductase complex was crystallized in a thin plate form, which diffracts X-rays to 7 A resolution in the presence of mother liquor. This crystalline complex contains ten protein subunits and 140 nmol phospholipid per milligram protein. Over 90% of the phospholipid and ubiquinone in the reductase can be removed by repeated ammonium sulfate precipitation in the presence of 0.5% sodium cholate. The delipidated complex has no enzymatic activity and shows significant changes in the circular dichroism spectrum in the near UV region and in the EPR characteristics of both cytochromes b. Enzyme activity and spectral characteristics can be restored by replenishing the phospholipid and ubiquinone. The structural requirements of ubiquinone for electron transport were studied by measuring the ability of a variety of synthetic ubiquinone derivatives to restore the enzymatic activity and native spectroscopic signatures to the delipidated complex. Q-binding proteins and binding domains were identified using photoaffinity labeled Q-derivatives and HPLC separation of photolabeled peptides. Interaction between ubiquinol-cytochrome c reductase and succinate-Q reductase was established by differential scanning calorimetry and saturation transfer EPR using spin-labeled ubiquinol-cytochrome c reductase. Involvement of iron-sulfur protein in proton translocation by ubiquinol-cytochrome c reductase was investigated by hematorporphyrin-promoted photoinactivation of the complex. The cDNAs encoding the Rieske iron-sulfur protein and a small molecular mass Q-binding protein (QPc-9.5 kDa) were isolated and their nucleotide sequences determined. These will be useful in future structural and mechanistic studies of ubiquinol-cytochrome c reductase via in vitro reconstitution between an over-expressed, mutated subunit and a specific subunit-depleted reductase.

The effect of positive and negative pH-gradients on the stability of small unilamellar vesicles of negatively charged phospholipids

The stability of small unilamellar vesicles (SUV) made from negatively-charged phosphatidate by ultrasonication or pH-jump has been investigated. As criteria for the vesicle stability are used: (I) the bilayer integrity as judged from the permeability of the fluorescent probe carboxyfluorescein (CF) and (II) the susceptibility of the phospholipid vesicles to fusion as judged by gel filtration and freeze-fracture electron microscopy. Egg phosphatidate SUV (PA-SUV) whose internal cavity is in equilibrium with the dispersion medium are strictly speaking thermodynamically unstable by these criteria. They may, however, be regarded as stable from a practical point of view. CF-release is observed with a half-time of 14 days and also some vesicle fusion, particularly at low temperature (4 degrees C). The small effects observed, e.g., the small tendency of the vesicles to undergo fusion is probably due to the high surface charge density of PA bilayers. A main finding of this work is that the same positive pH-gradient which is used in the pH-jump method to drive the formation of SUV from large phosphatidic acid bilayer sheets has a stabilizing effect on the resulting PA-SUV. Stabilization is achieved by positive pH-gradients of about two pH-units or more with the pH of the external medium exceeding the pH of the vesicle cavity. Under these conditions, up to about 8 weeks no significant loss of entrapped CF and no fusion of SUV was observed both at 4 degrees C and room temperature. In contrast, a reverse or negative pH-gradient of several pH units applied to PA-SUV (with the external pH being lower than that of the vesicle cavity) destabilizes PA-SUV. Such a gradient can be shown to lead to a dramatic perturbation of the lipid bilayer packing as evident from a significant increase in CF permeability. The local perturbation of the phospholipid bilayer is accompanied by massive vesicle fusion which is prominent at low temperature (4 degrees C).