Mutations in the beta-subunit Thr(159) and Glu(184) of the Rhodospirillum rubrum F(0)F(1) ATP synthase reveal differences in ligands for the coupled Mg(2+)- and decoupled Ca(2+)-dependent F(0)F(1) activities

Cyclophilin D in Mitochondrial Dysfunction: A Key Player in Neurodegeneration?

Mitochondrial dysfunction plays a pivotal role in numerous complex diseases. Understanding the molecular mechanisms by which the "powerhouse of the cell" turns into the "factory of death" is an exciting yet challenging task that can unveil new therapeutic targets. The mitochondrial matrix protein CyPD is a peptidylprolyl cis-trans isomerase involved in the regulation of the permeability transition pore (mPTP). The mPTP is a multi-conductance channel in the inner mitochondrial membrane whose dysregulated opening can ultimately lead to cell death and whose involvement in pathology has been extensively documented over the past few decades. Moreover, several mPTP-independent CyPD interactions have been identified, indicating that CyPD could be involved in the fine regulation of several biochemical pathways. To further enrich the picture, CyPD undergoes several post-translational modifications that regulate both its activity and interaction with its clients. Here, we will dissect what is currently known about CyPD and critically review the most recent literature about its involvement in neurodegenerative disorders, focusing on Alzheimer's Disease and Parkinson's Disease, supporting the notion that CyPD could serve as a promising therapeutic target for the treatment of such conditions. Notably, significant efforts have been made to develop CyPD-specific inhibitors, which hold promise for the treatment of such complex disorders.

What happens when the mitochondrial H+-translocating F1FO-ATP(hydrol)ase becomes a molecular target of calcium? The pore opens

The F₁F_O-ATPase has Mg²⁺ cofactor as the natural divalent cation to support the bifunctional activity of ATP synthesis and hydrolysis. Different physio(patho)logical conditions permit the molecular interaction of Ca²⁺ with the enzyme and the modification of the biological role. Three distinct binding regions of Ca²⁺ have been localized on the enzyme complex: one in the F₁ catalytic sites and the other two sites in the membrane-embedded domain F_O. In all likelihood, Ca²⁺-activated enzyme most frequently works as an H⁺-translocating F₁F_O-ATP(hydrol)ase with a monofunctional activity that triggers the formation of mitochondrial permeability transition pore (mPTP) phenomenon. The protein(s) component of the mPTP is considered an arcane mystery. However, the F₁F_O-ATPase could reveal the molecular mechanism of pore opening when Ca²⁺ is bound to the enzyme. In this regard, the role of Ca²⁺-dependent function of the F₁F_O-ATPase in the formation of the mPTP is discussed.

The inhibition of gadolinium ion (Gd3+) on the mitochondrial F1FO-ATPase is linked to the modulation of the mitochondrial permeability transition pore

The mitochondrial permeability transition pore (PTP), which drives regulated cell death when Ca²⁺ concentration suddenly increases in mitochondria, was related to changes in the Ca²⁺-activated F₁F_O-ATPase. The effects of the gadolinium cation (Gd³⁺), widely used for diagnosis and therapy, and reported as PTP blocker, were evaluated on the F₁F_O-ATPase activated by Mg²⁺ or Ca²⁺ and on the PTP. Gd³⁺ more effectively inhibits the Ca²⁺-activated F₁F_O-ATPase than the Mg²⁺-activated F₁F_O-ATPase by a mixed-type inhibition on the former and by uncompetitive mechanism on the latter. Most likely Gd³⁺ binding to F₁, is favoured by Ca²⁺ insertion. The maximal inactivation rates (k_inact) of pseudo-first order inactivation are similar either when the F₁F_O-ATPase is activated by Ca²⁺ or by Mg²⁺. The half-maximal inactivator concentrations (K_I) are 2.35 ± 0.35 mM and 0.72 ± 0.11 mM, respectively. The potency of a mechanism-based inhibitor (k_inact/K_I) also highlights a higher inhibition efficiency of Gd³⁺ on the Ca²⁺-activated F₁F_O-ATPase (0.59 ± 0.09 mM^-1∙s^-1) than on the Mg²⁺-activated F₁F_O-ATPase (0.13 ± 0.02 mM^-1∙s^-1). Consistently, the PTP is desensitized in presence of Gd³⁺. The Gd³⁺ inhibition on both the mitochondrial Ca²⁺-activated F₁F_O-ATPase and the PTP strengthens the link between the PTP and the F₁F_O-ATPase when activated by Ca²⁺ and provides insights on the biological effects of Gd³⁺.

Mitochondrial F1FO-ATPase and permeability transition pore response to sulfide in the midgut gland of Mytilus galloprovincialis

The molecular mechanisms which rule the formation and opening of the mitochondrial permeability transition pore (mPTP), the lethal mechanism which permeabilizes mitochondria to water and solutes and drives the cell to death, are still unclear and particularly little investigated in invertebrates. Since Ca²⁺ increase in mitochondria is accompanied by mPTP opening and the participation of the mitochondrial F₁F_O-ATPase in the mPTP is increasingly sustained, the substitution of the natural cofactor Mg²⁺ by Ca²⁺ in the F₁F_O-ATPase activation has been involved in the mPTP mechanism. In mussel midgut gland mitochondria the similar kinetic properties of the Mg²⁺- or Ca²⁺-dependent F₁F_O-ATPase activities, namely the same affinity for ATP and bi-site activation kinetics by the ATP substrate, in spite of the higher enzyme activity and coupling efficiency of the Mg²⁺-dependent F₁F_O-ATPase, suggest that both enzyme activities are involved in the bioenergetic machinery. Other than being a mitochondrial poison and environmental contaminant, sulfide at low concentrations acts as gaseous mediator and can induce post-translational modifications of proteins. The sulfide donor NaHS, at micromolar concentrations, does not alter the two F₁F_O-ATPase activities, but desensitizes the mPTP to Ca²⁺ input. Unexpectedly, NaHS, under the conditions tested, points out a chemical refractoriness of both F₁F_O-ATPase activities and a failed relationship between the Ca²⁺-dependent F₁F_O-ATPase and the mPTP in mussels. The findings suggest that mPTP role and regulation may be different in different taxa and that the F₁F_O-ATPase insensitivity to NaHS may allow mussels to cope with environmental sulfide.

Role of Mitochondrial Calcium and the Permeability Transition Pore in Regulating Cell Death

Adult cardiomyocytes are postmitotic cells that undergo very limited cell division. Thus, cardiomyocyte death as occurs during myocardial infarction has very detrimental consequences for the heart. Mitochondria have emerged as an important regulator of cardiovascular health and disease. Mitochondria are well established as bioenergetic hubs for generating ATP but have also been shown to regulate cell death pathways. Indeed many of the same signals used to regulate metabolism and ATP production, such as calcium and reactive oxygen species, are also key regulators of mitochondrial cell death pathways. It is widely hypothesized that an increase in calcium and reactive oxygen species activate a large conductance channel in the inner mitochondrial membrane known as the PTP (permeability transition pore) and that opening of this pore leads to necroptosis, a regulated form of necrotic cell death. Strategies to reduce PTP opening either by inhibition of PTP or inhibiting the rise in mitochondrial calcium or reactive oxygen species that activate PTP have been proposed. A major limitation of inhibiting the PTP is the lack of knowledge about the identity of the protein(s) that form the PTP and how they are activated by calcium and reactive oxygen species. This review will critically evaluate the candidates for the pore-forming unit of the PTP and discuss recent data suggesting that assumption that the PTP is formed by a single molecular identity may need to be reconsidered.

Mitochondrial Ca2+ -activated F1 FO -ATPase hydrolyzes ATP and promotes the permeability transition pore

The properties of the mitochondrial F₁ F_O -ATPase catalytic site, which can bind Mg²⁺ , Mn²⁺ , or Ca²⁺ and hydrolyze ATP, were explored by inhibition kinetic analyses to cast light on the Ca²⁺ -activated F₁ F_O -ATPase connection with the permeability transition pore (PTP) that initiates cascade events leading to cell death. While the natural cofactor Mg²⁺ activates the F₁ F_O -ATPase in competition with Mn²⁺ , Ca²⁺ is a noncompetitive inhibitor in the presence of Mg²⁺ . Selective F₁ inhibitors (Is-F₁ ), namely NBD-Cl, piceatannol, resveratrol, and quercetin, exerted different mechanisms (mixed and uncompetitive inhibition) on either Ca²⁺ - or Mg²⁺ -activated F₁ F_O -ATPase, consistent with the conclusion that the catalytic mechanism changes when Mg²⁺ is replaced by Ca²⁺ . In a partially purified F₁ domain preparation, Ca²⁺ -activated F₁ -ATPase maintained Is-F₁ sensitivity, and enzyme inhibition was accompanied by the maintenance of the mitochondrial calcium retention capacity and membrane potential. The data strengthen the structural relationship between Ca²⁺ -activated F₁ F_O -ATPase and the PTP, and, in turn, on consequences, such as physiopathological cellular changes.

Purified F-ATP synthase forms a Ca2+-dependent high-conductance channel matching the mitochondrial permeability transition pore

The molecular identity of the mitochondrial megachannel (MMC)/permeability transition pore (PTP), a key effector of cell death, remains controversial. By combining highly purified, fully active bovine F-ATP synthase with preformed liposomes we show that Ca²⁺ dissipates the H⁺ gradient generated by ATP hydrolysis. After incorporation of the same preparation into planar lipid bilayers Ca²⁺ elicits currents matching those of the MMC/PTP. Currents were fully reversible, were stabilized by benzodiazepine 423, a ligand of the OSCP subunit of F-ATP synthase that activates the MMC/PTP, and were inhibited by Mg²⁺ and adenine nucleotides, which also inhibit the PTP. Channel activity was insensitive to inhibitors of the adenine nucleotide translocase (ANT) and of the voltage-dependent anion channel (VDAC). Native gel-purified oligomers and dimers, but not monomers, gave rise to channel activity. These findings resolve the long-standing mystery of the MMC/PTP and demonstrate that Ca²⁺ can transform the energy-conserving F-ATP synthase into an energy-dissipating device.

The molecular identity of the mitochondrial megachannel (MMC)/permeability transition pore (PTP), a key effector of cell death, remains controversial. Here authors demonstrate that the membrane embedded bovine F-ATP synthase elicits Ca2 + -dependent currents matching those of the MMC/PTP.

Mitochondrial F-ATP Synthase and Its Transition into an Energy-Dissipating Molecular Machine

The mitochondrial F-ATP synthase is the principal energy-conserving nanomotor of cells that harnesses the proton motive force generated by the respiratory chain to make ATP from ADP and phosphate in a process known as oxidative phosphorylation. In the energy-converting membranes, F-ATP synthase is a multisubunit complex organized into a membrane-extrinsic F₁ sector and a membrane-intrinsic F_O domain, linked by central and peripheral stalks. Due to its essential role in the cellular metabolism, malfunction of F-ATP synthase has been associated with a variety of pathological conditions, and the enzyme is now considered as a promising drug target for multiple disease conditions and for the regulation of energy metabolism. We discuss structural and functional features of mitochondrial F-ATP synthase as well as several conditions that partially or fully inhibit the coupling between the F₁ catalytic activities and the F_O proton translocation, thus decreasing the cellular metabolic efficiency and transforming the enzyme into an energy-dissipating structure through molecular mechanisms that still remain to be defined.

From the Ca2+-activated F1FO-ATPase to the mitochondrial permeability transition pore: an overview

Based on recent advances on the Ca²⁺-activated F₁F_O-ATPase features, a novel multistep mechanism involving the mitochondrial F₁F_O complex in the formation and opening of the still enigmatic mitochondrial permeability transition pore (MPTP), is proposed. MPTP opening makes the inner mitochondrial membrane (IMM) permeable to ions and solutes and, through cascade events, addresses cell fate to death. Since MPTP forms when matrix Ca²⁺ concentration rises and ATP is hydrolyzed by the F₁F_O-ATPase, conformational changes, triggered by Ca²⁺ insertion in F₁, may be transmitted to F_O and locally modify the IMM curvature. These events would cause F₁F_O-ATPase dimer dissociation and MPTP opening.

Ca2+ binding to F-ATP synthase β subunit triggers the mitochondrial permeability transition

F-ATP synthases convert the electrochemical energy of the H⁺ gradient into the chemical energy of ATP with remarkable efficiency. Mitochondrial F-ATP synthases can also undergo a Ca²⁺-dependent transformation to form channels with properties matching those of the permeability transition pore (PTP), a key player in cell death. The Ca²⁺ binding site and the mechanism(s) through which Ca²⁺ can transform the energy-conserving enzyme into a dissipative structure promoting cell death remain unknown. Through in vitro, in vivo and in silico studies we (i) pinpoint the "Ca²⁺-trigger site" of the PTP to the catalytic site of the F-ATP synthase β subunit and (ii) define a conformational change that propagates from the catalytic site through OSCP and the lateral stalk to the inner membrane. T163S mutants of the β subunit, which show a selective decrease in Ca²⁺-ATP hydrolysis, confer resistance to Ca²⁺-induced, PTP-dependent death in cells and developing zebrafish embryos. These findings are a major advance in the molecular definition of the transition of F-ATP synthase to a channel and of its role in cell death.

Calcium and regulation of the mitochondrial permeability transition

Recent years have seen renewed interest in the permeability transition pore, a high conductance channel responsible for permeabilization of the inner mitochondrial membrane, a process that leads to depolarization and Ca²⁺ release. Transient openings may be involved in physiological Ca²⁺ homeostasis while long-lasting openings may trigger and/or execute cell death. In this review we specifically focus (i) on the hypothesis that the PTP forms from the F-ATP synthase and (ii) on the mechanisms through which Ca²⁺ can reversibly switch this energy-conserving nanomachine into an energy-dissipating device.

Forty years later: Mitochondria as therapeutic targets in muscle diseases

The hypothesis that mitochondrial dysfunction can be a general mechanism for cell death in muscle diseases is 40 years old. The key elements of the proposed pathogenetic sequence (cytosolic Ca2+ overload followed by excess mitochondrial Ca2+ uptake, functional and then structural damage of mitochondria, energy shortage, worsened elevation of cytosolic Ca2+ levels, hypercontracture of muscle fibers, cell necrosis) have been confirmed in amazing detail by subsequent work in a variety of models. The explicit implication of the hypothesis was that it "may provide the basis for a more rational treatment for some conditions even before their primary causes are known" (Wrogemann and Pena, 1976, Lancet, 1, 672-674). This prediction is being fulfilled, and the potential of mitochondria as pharmacological targets in muscle diseases may soon become a reality, particularly through inhibition of the mitochondrial permeability transition pore and its regulator cyclophilin D.

On the structural possibility of pore-forming mitochondrial FoF1 ATP synthase

The mitochondrial permeability transition is an inner mitochondrial membrane event involving the opening of the permeability transition pore concomitant with a sudden efflux of matrix solutes and breakdown of membrane potential. The mitochondrial F(o)F(1) ATP synthase has been proposed as the molecular identity of the permeability transition pore. The likeliness of potential pore-forming sites in the mitochondrial F(o)F(1) ATP synthase is discussed and a new model, the death finger model, is described. In this model, movement of a p-side density that connects the lipid-plug of the c-ring with the distal membrane bending Fo domain allows reversible opening of the c-ring and structural cross-talk with OSCP and the catalytic (αβ)(3) hexamer. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

The c-Ring of the F1FO-ATP Synthase: Facts and Perspectives

The F1FO-ATP synthase is the only enzyme in nature endowed with bi-functional catalytic mechanism of synthesis and hydrolysis of ATP. The enzyme functions, not only confined to energy transduction, are tied to three intrinsic features of the annular arrangement of c subunits which constitutes the so-called c-ring, the core of the membrane-embedded FO domain: (i) the c-ring constitution is linked to the number of ions (H(+) or Na(+)) channeled across the membrane during the dissipation of the transmembrane electrochemical gradient, which in turn determines the species-specific bioenergetic cost of ATP, the "molecular currency unit" of energy transfer in all living beings; (ii) the c-ring is increasingly involved in the mitochondrial permeability transition, an event linked to cell death and to most mitochondrial dysfunctions; (iii) the c subunit species-specific amino acid sequence and susceptibility to post-translational modifications can address antibacterial drug design according to the model of enzyme inhibitors which target the c subunits. Therefore, the simple c-ring structure not only allows the F1FO-ATP synthase to perform the two opposite tasks of molecular machine of cell life and death, but it also amplifies the enzyme's potential role as a drug target.

The Mitochondrial Permeability Transition Pore: Channel Formation by F-ATP Synthase, Integration in Signal Transduction, and Role in Pathophysiology

The mitochondrial permeability transition (PT) is a permeability increase of the inner mitochondrial membrane mediated by a channel, the permeability transition pore (PTP). After a brief historical introduction, we cover the key regulatory features of the PTP and provide a critical assessment of putative protein components that have been tested by genetic analysis. The discovery that under conditions of oxidative stress the F-ATP synthases of mammals, yeast, and Drosophila can be turned into Ca(2+)-dependent channels, whose electrophysiological properties match those of the corresponding PTPs, opens new perspectives to the field. We discuss structural and functional features of F-ATP synthases that may provide clues to its transition from an energy-conserving into an energy-dissipating device as well as recent advances on signal transduction to the PTP and on its role in cellular pathophysiology.

From ATP to PTP and Back: A Dual Function for the Mitochondrial ATP Synthase

Mitochondria not only play a fundamental role in heart physiology but are also key effectors of dysfunction and death. This dual role assumes a new meaning after recent advances on the nature and regulation of the permeability transition pore, an inner membrane channel whose opening requires matrix Ca(2+) and is modulated by many effectors including reactive oxygen species, matrix cyclophilin D, Pi (inorganic phosphate), and matrix pH. The recent demonstration that the F-ATP synthase can reversibly undergo a Ca(2+)-dependent transition to form a channel that mediates the permeability transition opens new perspectives to the field. These findings demand a reassessment of the modifications of F-ATP synthase that take place in the heart under pathological conditions and of their potential role in determining the transition of F-ATP synthase from and energy-conserving into an energy-dissipating device.

The oligomycin-sensitivity conferring protein of mitochondrial ATP synthase: emerging new roles in mitochondrial pathophysiology

The oligomycin-sensitivity conferring protein (OSCP) of the mitochondrial F(O)F1 ATP synthase has long been recognized to be essential for the coupling of proton transport to ATP synthesis. Located on top of the catalytic F1 sector, it makes stable contacts with both F1 and the peripheral stalk, ensuring the structural and functional coupling between F(O) and F1, which is disrupted by the antibiotic, oligomycin. Recent data have established that OSCP is the binding target of cyclophilin (CyP) D, a well-characterized inducer of the mitochondrial permeability transition pore (PTP), whose opening can precipitate cell death. CyPD binding affects ATP synthase activity, and most importantly, it decreases the threshold matrix Ca²⁺ required for PTP opening, in striking analogy with benzodiazepine 423, an apoptosis-inducing agent that also binds OSCP. These findings are consistent with the demonstration that dimers of ATP synthase generate Ca²⁺-dependent currents with features indistinguishable from those of the PTP and suggest that ATP synthase is directly involved in PTP formation, although the underlying mechanism remains to be established. In this scenario, OSCP appears to play a fundamental role, sensing the signal(s) that switches the enzyme of life in a channel able to precipitate cell death.

Dimers of mitochondrial ATP synthase form the permeability transition pore

Here we define the molecular nature of the mitochondrial permeability transition pore (PTP), a key effector of cell death. The PTP is regulated by matrix cyclophilin D (CyPD), which also binds the lateral stalk of the FOF1 ATP synthase. We show that CyPD binds the oligomycin sensitivity-conferring protein subunit of the enzyme at the same site as the ATP synthase inhibitor benzodiazepine 423 (Bz-423), that Bz-423 sensitizes the PTP to Ca(2+) like CyPD itself, and that decreasing oligomycin sensitivity-conferring protein expression by RNAi increases the sensitivity of the PTP to Ca(2+). Purified dimers of the ATP synthase, which did not contain voltage-dependent anion channel or adenine nucleotide translocator, were reconstituted into lipid bilayers. In the presence of Ca(2+), addition of Bz-423 triggered opening of a channel with currents that were typical of the mitochondrial megachannel, which is the PTP electrophysiological equivalent. Channel openings were inhibited by the ATP synthase inhibitor AMP-PNP (γ-imino ATP, a nonhydrolyzable ATP analog) and Mg(2+)/ADP. These results indicate that the PTP forms from dimers of the ATP synthase.

Simulation of catalytic water activation in mitochondrial F1-ATPase using a hybrid quantum mechanics/molecular mechanics approach: an alternative role for β-Glu 188

The use of quantum mechanics/molecular mechanics simulations to study the free energy landscape of the water activation at the catalytic site of mitochondrial F(1)-ATPase affords us insight into the generation of the nucleophile OH(-) prior to ATP hydrolysis. As a result, the ATP molecule was found to be the final proton acceptor. In the simulated pathway, the transfer of a proton to the nucleotide was not direct but occurred via a second water molecule in a manner similar to the Grotthuss mechanism proposed for proton diffusion. Residue β-Glu 188, previously described as the putative catalytic base, was found to be involved in the stabilization of a transient hydronium ion during water activation. Simulations in the absence of the carboxylate moiety of β-Glu 188 support this role.

Solvent-Assisted Trypsin Digestion of Ricin for Forensic Identification by LC-ESI MS/MS

The castor bean plant (Ricinus communis) is used in large quantities for oil production and is also a common ornamental garden plant. However, the beans contain 1-3% of the highly toxic protein ricin, a type II ribosome-inactivating protein that is covered by the Chemical Weapons Convention, and there have been a number of reports concerning the use, or alleged use, of the toxin in terrorist and criminal activities. In the study reported here, we investigated the potential utility of organic solvent-assisted trypsin digestion of crude extracts containing the closely related toxins ricin or abrin to prepare samples for peptide analysis by liquid chromatography combined with electrospray ionization quadrupole time-of-flight tandem mass spectrometry. Diagnostic tryptic fragments of the toxins were detected and unambiguously identified by this procedure. The sample preparation protocol substantially reduces the sample preparation time, from overnight to an hour, and thus greatly reduces the total time required for analyses, to less than 2 h. Furthermore, the reported procedure leaves the disulfide bonds in the protein intact. This is highly relevant in the context of the Chemical Weapons Convention, since the disulfide bond connecting the two chains of ricin indicates the presence of an intact toxin and provides additional forensic evidence for the analytical results.

Observation of calcium-dependent unidirectional rotational motion in recombinant photosynthetic F1-ATPase molecules

ATP hydrolysis and synthesis by the F(0)F(1)-ATP synthase are coupled to proton translocation across the membrane in the presence of magnesium. Calcium is known, however, to disrupt this coupling in the photosynthetic enzyme in a unique way: it does not support ATP synthesis, and CaATP hydrolysis is decoupled from any proton translocation, but the membrane does not become leaky to protons. Understanding the molecular basis of these calcium-dependent effects can shed light on the as yet unclear mechanism of coupling between proton transport and rotational catalysis. We show here, using an actin filament gamma-rotation assay, that CaATP is capable of sustaining rotational motion in a highly active hybrid photosynthetic F(1)-ATPase consisting of alpha and beta subunits from Rhodospirillum rubrum and gamma subunit from spinach chloroplasts (alpha(R)(3)beta(R)(3)gamma(C)). The rotation was found to be similar to that induced by MgATP in Escherichia coli F(1)-ATPase molecules. Our results suggest a possible long range pathway that enables the bound CaATP to induce full rotational motion of gamma but might block transmission of this rotational motion into proton translocation by the F(0) part of the ATP synthase.

Assembled F1-(alpha beta ) and Hybrid F1-alpha 3beta 3gamma -ATPases from Rhodospirillum rubrum alpha, wild type or mutant beta, and chloroplast gamma subunits. Demonstration of Mg2+versus Ca2+-induced differences in catalytic site structure and function

Refolding together the expressed alpha and beta subunits of the Rhodospirillum rubrum F(1)(RF(1))-ATPase led to assembly of only alpha(1)beta(1) dimers, showing a stable low MgATPase activity. When incubated in the presence of AlCl(3), NaF and either MgAD(T)P or CaAD(T)P, all dimers associated into closed alpha(3)beta(3) hexamers, which also gained a low CaATPase activity. Both hexamer ATPase activities exhibited identical rates and properties to the open dimer MgATPase. These results indicate that: a) the hexamer, as the dimer, has no catalytic cooperativity; b) aluminium fluoride does not inhibit their MgATPase activity; and c) it does enable the assembly of RrF(1)-alpha(3)beta(3) hexamers by stabilizing their noncatalytic alpha/beta interfaces. Refolding of the RrF(1)-alpha and beta subunits together with the spinach chloroplast F(1) (CF(1))-gamma enabled a simple one-step assembly of two different hybrid RrF(1)-alpha(3)beta(3)/CF(1)gamma complexes, containing either wild type RrF(1)-beta or the catalytic site mutant RrF(1)beta-T159S. They exhibited over 100-fold higher CaATPase and MgATPase activities than the stabilized hexamers and showed very different catalytic properties. The hybrid wild type MgATPase activity was, as that of RrF(1) and CF(1) and unlike its higher CaATPase activity, regulated by excess free Mg(2+) ions, stimulated by sulfite, and inhibited by azide. The hybrid mutant had on the other hand a low CaATPase but an exceptionally high MgATPase activity, which was much less sensitive to the specific MgATPase effectors. All these very different ATPase activities were regulated by thiol modulation of the hybrid unique CF(1)-gamma disulfide bond. These hybrid complexes can provide information on the as yet unknown factors that couple ATP binding and hydrolysis to both thiol modulation and rotational motion of their CF(1)-gamma subunit.

Formation and properties of hybrid photosynthetic F1-ATPases. Demonstration of different structural requirements for stimulation and inhibition by tentoxin

A hybrid ATPase composed of cloned chloroplast ATP synthase beta and gamma subunits (betaC and gammaC) and the cloned alpha subunit from the Rhodospirillum rubrum ATP synthase (alphaR) was assembled using solubilized inclusion bodies and a simple single-step folding procedure. The catalytic properties of the assembled alpha3Rbeta3CgammaC were compared to those of the core alpha3Cbeta3CgammaC complex of the native chloroplast coupling factor 1 (CF1) and to another recently described hybrid enzyme containing R. rubrum alpha and beta subunits and the CF1 gamma subunit (alpha3Rbeta3RgammaC). All three enzymes were similarly stimulated by dithiothreitol and inhibited by copper chloride in response to reduction and oxidation, respectively, of the disulfide bond in the chloroplast gamma subunit. In addition, all three enzymes exhibited the same concentration dependence for inhibition by the CF1 epsilon subunit. Thus the CF1 gamma subunit conferred full redox regulation and normal epsilon binding to the two hybrid enzymes. Only the native CF1 alpha3Cbeta3CgammaC complex was inhibited by tentoxin, confirming the requirement for both CF1 alpha and beta subunits for tentoxin inhibition. However, the alpha3Rbeta3CgammaC complex, like the alpha3Cbeta3CgammaC complex, was stimulated by tentoxin at concentrations in excess of 10 microm. In addition, replacement of the aspartate at position 83 in betaC with leucine resulted in the loss of stimulation in the alpha3Rbeta3CgammaC hybrid. The results indicate that both inhibition and stimulation by tentoxin require a similar structural contribution from the beta subunit, but differ in their requirements for alpha subunit structure.

Hybrid Rhodospirillum rubrum F(0)F(1) ATP synthases containing spinach chloroplast F(1) beta or alpha and beta subunits reveal the essential role of the alpha subunit in ATP synthesis and tentoxin sensitivity

Trace amounts ( approximately 5%) of the chloroplast alpha subunit were found to be absolutely required for effective restoration of catalytic function to LiCl-treated chromatophores of Rhodospirillum rubrum with the chloroplast beta subunit (Avital, S., and Gromet-Elhanan, Z. (1991) J. Biol. Chem. 266, 7067-7072). To clarify the role of the alpha subunit in the rebinding of beta, restoration of catalytic function, and conferral of sensitivity to the chloroplast-specific inhibitor tentoxin, LiCl-treated chromatophores were analyzed by immunoblotting before and after reconstitution with mixtures of R. rubrum and chloroplast alpha and beta subunits. The treated chromatophores were found to have lost, in addition to most of their beta subunits, approximately a third of the alpha subunits, and restoration of catalytic activity required rebinding of both subunits. The hybrid reconstituted with the R. rubrum alpha and chloroplast beta subunits was active in ATP synthesis as well as hydrolysis, and both activities were completely resistant to tentoxin. In contrast, a hybrid reconstituted with both chloroplast alpha and beta subunits restored only a MgATPase activity, which was fully inhibited by tentoxin. These results indicate that all three copies of the R. rubrum alpha subunit are required for proton-coupled ATP synthesis, whereas for conferral of tentoxin sensitivity at least one copy of the chloroplast alpha subunit is required together with the chloroplast beta subunit. The hybrid system was further used to examine the effects of amino acid substitution at position 83 of the beta subunit on sensitivity to tentoxin.