Efficient reconstitution of mitochondrial energy-transfer reactions from depleted membranes and F1-ATPase as a function of the amount of bound oligomycin sensitivity-conferring protein (OSCP)

Attenuated ADP-inhibition of FOF1 ATPase mitigates manifestations of mitochondrial dysfunction in yeast

Proton-translocating F_OF₁ ATP synthase (F-ATPase) couples ATP synthesis or hydrolysis to transmembrane proton transport in bacteria, chloroplasts, and mitochondria. The primary function of the mitochondrial F_OF₁ is ATP synthesis driven by protonmotive force (pmf) generated by the respiratory chain. However, when pmf is low or absent (e.g. during anoxia), F_OF₁ consumes ATP and functions as a proton-pumping ATPase. Several regulatory mechanisms suppress the ATPase activity of F_OF₁ at low pmf. In yeast mitochondria they include special inhibitory proteins Inh1p and Stf1p, and non-competitive inhibition of ATP hydrolysis by MgADP (ADP-inhibition). Presumably, these mechanisms help the cell to preserve the ATP pool upon membrane de-energization. However, no direct evidence was presented to support this hypothesis so far. Here we report that a point mutation Q263L in subunit beta of Saccharomyces cerevisiae ATP synthase significantly attenuated ADP-inhibition of the enzyme without major effect on the rate of ATP production by mitochondria. The mutation also decreased the sensitivity of the enzyme ATPase activity to azide. Similar effects of the corresponding mutations were observed in earlier studies in bacterial enzymes. This observation indicates that the molecular mechanism of ADP-inhibition is probably the same in mitochondrial and in bacterial F_OF₁. The mutant yeast strain had lower growth rate and had a longer lag period preceding exponential growth phase when starved cells were transferred to fresh growth medium. However, upon the loss of mitochondrial DNA (ρ⁰) the βQ263L mutation effect was reversed: the βQ263L ρ⁰ mutant grew faster than the wild-type ρ⁰ yeast. The results suggest that ADP-inhibition might play a role in prevention of wasteful ATP hydrolysis in the mitochondrial matrix.

ADP-Inhibition of H+-FOF1-ATP Synthase

H+-F_OF₁-ATP synthase (F-ATPase, F-type ATPase, F_OF₁ complex) catalyzes ATP synthesis from ADP and inorganic phosphate in eubacteria, mitochondria, chloroplasts, and some archaea. ATP synthesis is powered by the transmembrane proton transport driven by the proton motive force (PMF) generated by the respiratory or photosynthetic electron transport chains. When the PMF is decreased or absent, ATP synthase catalyzes the reverse reaction, working as an ATP-dependent proton pump. The ATPase activity of the enzyme is regulated by several mechanisms, of which the most conserved is the non-competitive inhibition by the MgADP complex (ADP-inhibition). When ADP binds to the catalytic site without phosphate, the enzyme may undergo conformational changes that lock bound ADP, resulting in enzyme inactivation. PMF can induce release of inhibitory ADP and reactivate ATP synthase; the threshold PMF value required for enzyme reactivation might exceed the PMF for ATP synthesis. Moreover, membrane energization increases the catalytic site affinity to phosphate, thereby reducing the probability of ADP binding without phosphate and preventing enzyme transition to the ADP-inhibited state. Besides phosphate, oxyanions (e.g., sulfite and bicarbonate), alcohols, lauryldimethylamine oxide, and a number of other detergents can weaken ADP-inhibition and increase ATPase activity of the enzyme. In this paper, we review the data on ADP-inhibition of ATP synthases from different organisms and discuss the in vivo role of this phenomenon and its relationship with other regulatory mechanisms, such as ATPase activity inhibition by subunit ε and nucleotide binding in the noncatalytic sites of the enzyme. It should be noted that in Escherichia coli enzyme, ADP-inhibition is relatively weak and rather enhanced than prevented by phosphate.

Preparation of highly phosphorylating mitochondria from the yeast Schizosaccharomyces pombe

Schizosaccharomyces pombe yeast cells grown on either fermentable or respiratory media were efficiently converted to stable spheroplasts by the alpha-(1-->3)-glucanase Novozym 234 in the presence of 1.2 M sorbitol. Lysis of spheroplasts by gentle homogenization in dilute sorbitol resulted in the preparation of mitochondria with a structure similar to that observed within the starting yeast cells. The isolated mitochondria exhibited high oxidation rates with various respiratory substrates, NADH being the most efficient. The mitochondria appeared well coupled since the second State 4 rate observed after ADP consumption was identical to the initial one. The State 3 rate in the presence of ADP was completely inhibited by low oligomycin concentrations, similarly to the concomitant ATP synthesis of 900 nmol/min x mg protein. These NADH oxidation and dependent ATP-synthesis activities are much higher than those previously described for mitochondria isolated from Schizosaccharomyces pombe, and similar to the highest values reported for Saccharomyces cerevisiae.

Characterization of the oligomycin-sensitivity properties of the F1F0-ATPase in mitochondria from rats infected with the liver fluke Fasciola hepatica

The F1F0-ATPase activity of liver mitochondria isolated from rats infected with Fasciola hepatica at 3 and 4 weeks post-infection showed a marked loss of sensitivity to oligomycin and to N,N'-dicyclohexylcarbodiimide. A loss of sensitivity to diethylstilbestrol was also demonstrated at 4 weeks post-infection. Recovery was apparent in most cases by 6 weeks post-infection. No significant difference in latent ATPase activity was observed between mitochondria from control and infected livers at any stage of the infection. The mitochondria from infected livers were therefore considered to have a full complement of the F1 moiety of the F1F0-ATPase complex. Purification of the mitochondrial ATPase from 4-week infected livers resulted in a very low yield of an oligomycin-insensitive complex. This was due to a failure to enrich specific activity during purification. The evidence presented indicates that infection with Fasciola hepatica gives rise to alterations in the function of the host liver mitochondrial ATPase, namely loss of inhibitor sensitivity and apparent structural alterations of the ATPase complex.

Cross-reconstitution studies with polypeptides of Escherichia coli and bovine heart mitochondrial F0F1 ATP synthase

To characterize the role of supernumerary subunits of the mammalian F0F1 ATP synthase, cross-reconstitution of mitochondrial and bacterial F0F1 complexes has been carried out. Escherichia coli F1 (EcF1) can be reconstituted with F1-stripped everted membranes of E. coli (UPEc) and of bovine heart mitochondria (USMP). Bovine heart mitochondrial F1 (BHF1) can also be reconstituted with both membranes. Both EcF1 and BHF1, when reconstituted with UPEc, exhibited oligomycin-insensitive ATP-hydrolase activity. Subunits of the mammalian F0, in particular F0I-PVP protein, F6 and oligomycin-sensitivity-conferring protein (OSCP) conferred oligomycin sensitivity to the catalytic activity of EcF1 or BHF1 reconstituted with UPEc. Reaction of N,N'-dicyclohexylcarbodiimide and development of inhibition of passive H+ conduction was, in UPEc, considerably slower and exhibited a lower apparent affinity than in USMP. The ATP hydrolase activity of UPEc+EcF1 or UPEc+BHF1 was, also, less sensitive to inhibition by N,N'-dicyclohexylcarbodiimide than USMP+EcF1 or USMP+BHF1. Addition of mitochondrial F0I-PVP to UPEc enhanced the sensitivity of H+ conduction to oligomycin. F0I-PVP and OSCP added to UPEc, promoted inhibition by N,N'-dicyclohexylcarbodiimide of passive H+ conduction and increased its binding affinity to subunit c of E. coli F0. The presence of F0I-PVP and OSCP also promoted inhibition by N,N'-dicyclohexylcarbodiimide of the ATP-hydrolase activity of EcF1 or BHF1 reconstituted with UPEc.

Structural and functional characterization of subunits of the F0 sector of the mitochondrial F0F1-ATP synthase

Proteolytic digestion of F1-depleted submitochondrial particles (USMP), reconstitution with isolated subunits and titration with inhibitors show that the nuclear-encoded PVP protein, previously identified as an intrinsic component of bovine heart F0 (F01) (Zanotti, F. et al. (1988) FEBS Lett. 237, 9-14), is critically involved in maintaining the proper H+ translocating configuration of this sector and its correct binding to the F1 catalytic moiety. Trypsin digestion of USMP, under conditions leading to cleavage of the carboxyl region of the PVP protein and partial inhibition of transmembrane H+ translocation, results in general loss of sensitivity of this process to F0 inhibitors. This is restored by addition of the isolated PVP protein. Trypsin digestion of USMP causes also loss of oligomycin sensitivity of the catalytic activity of membrane reconstituted soluble F1, which can be restored by the combined addition of PVP and OSCP, or PVP and F6. Amino acid sequence analysis shows that, in USMP, modification by [14C] N,N'-dicyclohexylcarbodiimide of subunit c of F0 induces the formation of a dimer of this protein, which retains the 14C-labelled group. Chemical modification of cysteine-64 of subunit c results in inhibition of H+ conduction by F0. The results indicate that proton conduction in mitochondrial F0 depends on interaction of subunit c with the PVP protein.

Interaction between delta and epsilon subunits of F1-ATPase from pig heart mitochondria. Circular dichroism and intrinsic fluorescence of purified and reconstituted delta epsilon complex

A delta epsilon complex has been purified as a molecular entity from pig heart mitochondrial F1-ATPase. This delta epsilon complex has also been reconstituted from purified delta and epsilon subunits. Both isolated and reconstituted delta epsilon complexes have delta 1 epsilon 1 stoichiometry and are indistinguishable by their chromatographic behavior, their circular dichroism spectra (CD spectra), and their intrinsic fluorescence features. The content of secondary structures deduced from CD spectra of the delta epsilon complex appears to be the sum of the respective contributions of purified delta and epsilon subunits. All intrinsic fluorescence studies carried out on isolated epsilon subunit and delta epsilon complex show that the single tryptophan residue located on epsilon is involved in the interaction between delta and epsilon subunits. Results obtained with F1-ATPase are in favor of the same delta epsilon interaction in the entire enzyme.

Subunit delta of H(+)-ATPases: at the interface between proton flow and ATP synthesis

The ATP synthases in photophosphorylation and respiration are of the F-type with a membrane-bound proton channel, F0, and an extrinsic catalytic portion, F1. The properties of one particular subunit, delta (in chloroplasts and Escherichia coli) and OSCP (in mitochondria), are reviewed and the role of this subunit at the interface between F0 and F1 is discussed. Delta and OSCP from the three sources have in common the molecular mass (approximately 20 kDa), an elongated shape (axial ratio in solution about 3:1), one high-affinity binding site to F1 (Kd approximately 100 nM) plus probably one or two further low-affinity sites. When isolated delta is added to CF1-depleted thylakoid membranes, it can block proton flow through exposed CF0 channels, as do CF1 or CF1(-delta)+ delta. This identifies delta as part of the proton conductor or, alternatively, conformational energy transducer between F0 (proton flow) and F1 (ATP). Hybrid constructs as CF1(-delta)+ E. coli delta and EF1(-delta)+ chloroplast delta diminish proton flow through CF0.CF1(-delta) + E. coli delta does the same on EF0. Impairment of proton leaks either through CF0 or through EF0 causes "structural reconstitution' of ATP synthesis by remaining intact F0F1. Functional reconstitution (ATP synthesis by fully reconstructed F0F1), however, is absolutely dependent on the presence of subunit delta and is therefore observed only with CF1 or CF1(-delta) + chloroplast delta on CF0 and EF1 or EF1(-delta) + E. coli delta on EF0. The effect of hybrid constructs on F0 channels is surprising in view of the limited sequence homology between chloroplast and E. coli delta (36% conserved residues including conservative replacements). An analysis of the distribution of the conserved residues at present does not allow us to discriminate between the postulated conformational or proton-conductive roles of subunit delta.

Chloroplast ATP synthase contains one single copy of subunit delta that is indispensable for photophosphorylation

F0F1 ATP synthases synthesize ATP in their F1 portion at the expense of free energy supplied by proton flow which enters the enzyme through their channel portion F0. The smaller subunits of F1, especially subunit delta, may act as energy transducers between these rather distant functional units. We have previously shown that chloroplast delta, when added to thylakoids partially depleted of the coupling factor CF1, can reconstitute photophosphorylation by inhibiting proton leakage through exposed coupling factor CF0. In view of controversies in the literature, we reinvestigated two further aspects related to subunit delta, namely (a) its stoichiometry in CF0CF1 and (b) whether or not delta is required for photophosphorylation. By rocket immunoelectrophoresis of thylakoid membranes and calibration against purified delta, we confirmed a stoichiometry of one delta per CF0CF1. In CF1-depleted thylakoids photophosphorylation could be reconstituted not only by adding CF1 and subunit delta but, surprisingly, also by CF1 (-delta). We found that the latter was attributable to a contamination of CF1 (-delta) preparations with integral CF1. To lesser extent CF1 (-delta) acted by complementary rebinding to CF0 channels that were closed because they contained delta [CF0(+delta)]. This added catalytic capacity to proton-tight thylakoid vesicles. The ability of subunit delta to control proton flow through CF0 and the absolute requirement for delta in restoration of photophosphorylation suggest an essential role of this small subunit at the interface between the large portions of ATP synthase: delta may be part of the coupling site between electrochemical, conformational and chemical events in this enzyme.

Spectral properties of fluorescent derivatives of the oligomycin sensitivity conferring protein and analysis of their interaction with the F1 and F0 sectors of the mitochondrial ATPase complex

In order to study the kinetics and the nature of the interactions between the oligomycin sensitivity conferring protein (OSCP) and the F0 and F1 sectors of the mitochondrial ATPase complex, fluorescent derivatives of OSCP, which are fully biologically active, have been prepared by reaction of OSCP with the following fluorescent thiol reagents: 6-acryloyl-2-(dimethylamino)naphthalene (acrylodan), 2-(4-maleimidylanilino)naphthalene-6-sulfonic acid (Mal-ANS), N-(1-pyrenyl)maleimide (Mal-pyrene), 7-(diethylamino)-3-(4-maleimidylphenyl)-4-methylcoumarin (Mal-coumarin), and fluorescein 5-maleimide (Mal-fluorescein). The preparation of these derivatives was based on the previous finding that the single cysteinyl residue of OSCP, Cys 118, can be covalently modified by alkylating reagents without loss of biological activity [Dupuis, A., Issartel, J. P., Lunardi, J., Satre, M., & Vignais, P. V. (1985) Biochemistry 24, 728-733]. For all fluorescent probes used, except Mal-pyrene and Mal-fluorescein, the emission spectra of conjugated OSCP were blue-shifted relative to those of the corresponding mercaptoethanol adducts, indicating that the fluorophores attached to Cys 118 were located in a hydrophobic pocket. These results were consistent with the high quantum yields and the increased fluorescence lifetimes of conjugated OSCP compared to mercaptoethanol adducts in aqueous buffer. They also fit with quenching data obtained with potassium iodide which showed that the fluorophore is shielded from the aqueous medium when it is attached to Cys 118 of OSCP. Especially noticeable was the wide half-width of the OSCP-acrylodan emission peak compared to that of mercaptoethanol-acrylodan.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

IF1 inhibition of mitochondrial F1-ATPase is correlated to entrapment of four adenine- or guanine-nucleotides including at least one triphosphate

This paper demonstrates that the inhibition of F1 ATPase activity by the natural inhibitor protein IF1 is correlated to triphosphate nucleotide entrapment in F1. The complete balance of nucleotides bound after preincubation with Mg-[alpha-32P]GTP or Mg-[alpha-32P]ATP, used to promote IF1 inhibition, has been established on purified F1 containing 0.7 mol of non-exchangeable endogenous nucleotides. As many as 4 mol of labelled guanine- or adenine- nucleotides are trapped in F1; at least one of these nucleotides is a triphosphate. On the contrary, in the absence of IF1, no triphosphate nucleotide is significantly retained and the diphosphate nucleotides bound are mainly exchangeable.

Purified subunit delta of chloroplast coupling factor CF1 reconstitutes photophosphorylation in partially CF1-depleted membranes

The ATP synthase of chloroplasts consists of the proton channel, CF0, and the catalytic part, CF1, which carries nucleotide-binding sites on subunits alpha and beta. The still poorly understood interaction between CF0 and the catalytic sites on CF1 is mediated by the smaller subunits gamma, delta and epsilon of CF1. We investigated the ability of purified delta to block proton leakage through CF0 channels after their exposure by removal of the CF1 counterpart. Thylakoids were partially depleted of CF1 by EDTA treatment. This increased their proton permeability and thereby reduced the rate of photophosphorylation. Subunit delta was isolated and purified by FPLC [Engelbrecht, S. and Junge, W. (1987) FEBS Lett. 219, 321-325]. Addition of delta to EDTA-treated thylakoids reconstituted high rates of phenazine-methosulfate-mediated photophosphorylation. Since delta does not interact with nucleotides by itself, the reconstitution was due to a reduction of the proton leakage through open CF0 channels. The molar ratio of purified delta over exposed CF0, which started to elicit this effect, was 3:1. However, if delta was added together with purified CF1 lacking delta, in a 1:1 molar ratio, the relative amount over exposed CF0 was as low as 0.06. This corroborated our previous conclusion [Lill, H., Engelbrecht, S., Schönknecht, G. and Junge, W. (1986) Eur. J. Biochem. 160, 627-634] that only a very small fraction of exposed CF0 was actually proton-conducting but with a very high unit conductance. CF1 including delta was apparently rebound preferentially to open CF0 channels. Although the ability of delta to control proton conduction through CF0 was evident, it remains to be established whether delta acts as a gated proton valve or as a conformational transducer in the integral CF0CF1 ATPase.

Availability to monoclonal antibodies of antigenic sites of the alpha and beta subunits in active, denatured or membrane-bound mitochondrial F1-ATPase

The binding of five monoclonal antibodies to mitochondrial F1-ATPase has been studied. Competition experiments between monoclonal antibodies demonstrate that these antibodies recognize four different antigenic sites and provide information on the proximity of these sites. The accessibility of the epitopes has been compared for F1 integrated in the mitochondrial membrane, for purified beta-subunit and for purified F1 maintained in its active form by the presence of nucleotides or inactivated either by dilution in the absence of ATP or by urea treatment. The three anti-beta monoclonal antibodies bound more easily to the beta-subunit than to active F1, and recognized equally active F1 and F1 integrated in the membrane, indicating that their antigenic sites are partly buried similarly in purified or membrane-bound F1 and better exposed in the isolated beta-subunit. In addition, unfolding F1 by urea strongly increased the binding of one anti-beta monoclonal antibody (14 D5) indicating that this domain is at least partly shielded inside the beta-subunit. One anti-alpha monoclonal antibody (20 D6) bound poorly to F1 integrated in the membrane, while the other (7 B3) had a higher affinity for F1 integrated in the membrane than for soluble F1. Therefore, 20 D6 recognizes an epitope of the alpha-subunit buried inside F1 integrated in the membrane, while 7 B3 binds to a domain of the alpha-subunit well exposed at the surface of the inner face of the mitochondrial membrane.