Stoichiometry of the oligomycin-sensitivity-conferring protein (OSCP) in the mitochondrial F0F1-ATPase determined by an immunoelectrotransfer blot technique

The oligomycin axis of mitochondrial ATP synthase: OSCP and the proton channel

Oligomycin has long been known as an inhibitor of mitochondrial ATP synthase, putatively binding the F(o) subunits 9 and 6 that contribute to proton channel function of the complex. As its name implies, OSCP is the oligomycin sensitivity-conferring protein necessary for the intact enzyme complex to display sensitivity to oligomycin. Recent advances concerning the structure and mechanism of mitochondrial ATP synthase have led to OSCP now being considered a component of the peripheral stator stalk rather than a central stalk component. How OSCP confers oligomycin sensitivity on the enzyme is unknown, but probably reflects important protein-protein interactions made within the assembled complex and transmitted down the stator stalk, thereby influencing proton channel function. We review here our studies directed toward establishing the stoichiometry, assembly, and function of OSCP in the context of knowledge of the organization of the stator stalk and the proton channel.

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.

Single copies of subunits d, oligomycin-sensitivity conferring protein, and b are present in the Saccharomyces cerevisiae mitochondrial ATP synthase

In the mitochondrial ATP synthase (mtATPase) of the yeast Saccharomyces cerevisiae, the stoichiometry of subunits d, oligomycin-sensitivity conferring protein (OSCP), and b is poorly defined. We have investigated the stoichiometry of these subunits by the application of hexahistidine affinity purification technology. We have previously demonstrated that intact mtATPase complexes incorporating a Hex6-tagged subunit can be isolated via Ni2+-nitrilotriacetic acid affinity chromatography (Bateson, M., Devenish, R. J., Nagley, P., and Prescott, M. (1996) Anal. Biochem. 238, 14-18). Strains were constructed in which Hex6-tagged versions of subunits d, OSCP, and b were coexpressed with the corresponding wild-type subunit. This coexpression resulted in a mixed population of mtATPase complexes containing untagged wild-type and Hex6-tagged subunits. The stoichiometry of each subunit was then assessed by determining whether or not the untagged wild-type subunit could be recovered from Ni2+-nitrilotriacetic acid purifications as an integral component of those complexes absorbed by virtue of the Hex6-tagged subunit. As only the Hex6-tagged subunit was recovered from such purifications, we demonstrate that the stoichiometry of subunits d, OSCP, and b in yeast is 1 in each case.

The role of the stalk in the coupling mechanism of F1F0-ATPases

The extrinsic and intrinsic membrane sectors of F1F0-ATPases are linked by a slender stalk 40-50 A in length. The stalk transmits the energy produced by oxidative or photosynthetic phosphorylation from the intrinsic sector, F0, to the catalytic sites in the extrinsic F1 sector. How this is achieved is unknown, but long-range conformational changes linked to transmembrane proton transport may be involved. In bacterial and chloroplast F1F0-ATPases, the stalk is probably a composite of subunits delta and epsilon, part of the gamma-subunit, and the extrinsic membrane domains of 2 subunits (identical or non-identical according to the species) that are bound to the membrane by their N-terminal regions. The stalk in the bovine mitochondrial enzyme appears to be more complex, and the gamma, delta, epsilon, OSCP, F6, b and d subunits all contribute to it. A bovine stalk complex has been assembled in vitro from bacterially expressed OSCP, F6, b and d, both in the presence and in the absence of F1-ATPase. One molecule of each of these subunits is present in the assembled complex, as there is also in each native F1F0-ATPase assembly. Providing that suitable crystals can be obtained, the stalk complex and the F1.stalk complex may permit the high resolution structure of bovine F1-ATPase to be extended into the stalk domain.

Docking the mitochondrial inhibitor protein IF1 to a membrane receptor different from the F1-ATPase beta subunit

Monoclonal antibodies reacting with the inhibitor protein (IF1) of the mitochondrial ATPase/ATP synthase complex did not modify the IF1-induced inhibition of soluble F1 ATPase activity. On the contrary, they increased the ATPase activity of inverted electron-transport particles without inducing a significant release of IF1 from these particles. This suggested that IF1 could be linked to a membrane protein when it was not inhibiting the ATPase activity. IF1 antibodies have been used to show that IF1 can bind not only to the beta subunit of F1-ATPase [Klein, G., Satre, M., Dianoux, A. C. & Vignais, P. V. (1981) Biochemistry 20, 1339-1344] but also to a protein present in the inner-mitochondrial membrane. The cross-linking of IF1 to this membrane protein gave a product of M(r) 15000-16000 that migrated differently from IF1 and from the dimer of IF1 using SDS/PAGE. When the cross-linked product was obtained by using a cleavable cross-linking reagent, the complex between IF1 and the docking protein was partly dissociated and free IF1 was recovered. Considering the molecular mass of IF1, this docking protein for IF1 has apparent M(r) 5000-6000. The complex between IF1 and this receptor protein can be detected in low amounts by antibodies against IF1 in the absence of cross-linking reagent. Since this complex remained in the pellet after treatment of the membrane with Triton X-100, it should be a membrane protein. Therefore, IF1 can bind not only to its inhibitory-binding site at the beta subunit of F1, but also to a non inhibitory site which is a membrane protein of approximate M(r) 5000-6000.

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.

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.

Electron microscopy and image analysis of the complexes I and V of the mitochondrial respiratory chain

The results of Section IV can be summarized in a simple ATP synthase model. This model implies that either the alpha or the beta subunits must be closer to the membrane. The work of Gao and Bauerlein (1987) indicates that the alpha subunits are closer to the membrane. Although the overall structure is more or less clear, important questions need to be clarified. First, the number and the arrangement of the subunits in the F0 part must be known. Second, the exact shape of F1, and particularly the shape of the large subunits needs to be elucidated. On the basis of fluorescence resonance energy transfer measurements by McCarty and Hammes (1987), a model was presented showing large oblong subunits. Such 'banana-shaped' subunits, which are also presented in the many phantasy models (e.g. Walker et al., 1982), are very unlikely in view of the electron microscopical results, although the large subunits do not need to be exactly spherical. The third and most interesting central question is on the changes in the structure that take place during the different steps in the synthesis of ATP. It can now be taken as proven that the energy transmitted to the ATP synthase is used to induce a conformational change in the latter enzyme, in such a way as to bring about the energy-requiring dissociation of already synthesized ATP (Penefsky, 1985 and reviewed in Slater, 1987). But the way in which the three parts of the ATP synthase are involved is completely unknown. It is rather puzzling that such a long distance exists between the catalytic sites, which are on the interface of the alpha and beta subunits and the F0 part where the proton movements occur, which, according to Mitchell's theory (1961), is the driving force for the synthesis of ATP. Perhaps alternative mechanisms such as the collision hypothesis formulated by Herweijer et al. (1985) are more realistic in describing the mechanism of ATP synthesis. It would bring the complexes I and V close together, not only in the artificial way treated in this paper, but in a useful way for energy conversion.

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.

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)

Pig heart mitochondrial membranes depleted of F1 and OSCP by various treatments were analyzed for their content in alpha and beta subunits of F1 and in OSCP using monoclonal antibodies. Membrane treatments and conditions of rebinding of F1 and OSCP were optimized to reconstitute efficient NADH- and ATP-dependent proton fluxes, ATP synthesis and oligomycin-sensitive ATPase activity. F1 and OSCP can be rebound independently to depleted membranes but to avoid unspecific binding of F1 to depleted membranes (ASUA) which is not efficient for ATP synthesis, F1 must be rebound before the addition of OSCP. The rebinding of OSCP to depleted membranes reconstituted with F1 inhibits the ATPase activity of rebound F1, while it restores the ATP-driven proton flux measured by the quenching of ACMA fluorescence. The rebinding of OSCP also renders the ATPase activity of bound F1 sensitive to uncouplers. The rebinding of OSCP alone or F1 alone, does not modify the NADH-dependent proton flux, while the rebinding of both F1 and OSCP controls this flux, inducing an inhibition of the rate of NADH oxidation. Similarly, oligomycin, which seals the F0 channel even in the absence of F1 and OSCP, inhibits the rate of NADH oxidation. OSCP is required to adjust the fitting of F1 to F0 for a correct channelling of protons efficient for ATP synthesis. All reconstituted energy-transfer reactions reach their optimal value for the same amount of OSCP. This amount is consistent with a stoichiometry of two OSCP per F1 in the F0-F1 complex.

Reconstitution of CF1-depleted thylakoid membranes with complete and fragmented chloroplast ATPase. The role of the delta subunit for proton conduction through CF0

Chloroplast ATPase (CF1) was isolated from spinach, pea and maize thylakoids by EDTA extraction followed by anion-exchange chromatography. CF1 was purified and resolved by HPLC into integral CF1, and CF1 lacking the delta & epsilon subunits: CF1(-delta) and CF1(-epsilon). Washing Mono-Q-bound CF1 with alcohol-containing buffers followed by elution without alcohol produced the beta subunit and in separate peaks CF1(-delta) and CF1(-epsilon). Elution from Mono Q in the presence of tenside yielded a beta delta fragment, CF1(-delta) and CF1(-delta epsilon). Chloroplasts were CF1-depleted by EDTA extraction. Reconstitution of photophosphorylation in these 'EDTA vesicles' was obtained by addition of CF1 and its fragments. CF1, CF1(-delta) and CF1(-delta epsilon) were active with cross-reactivity between spinach, pea and maize. delta-containing CF1 always reconstituted higher activities than delta-deficient CF1. The beta delta fragment and dicyclohexylcarbodiimide (DCCD)-inhibited CF1 also were reconstitutively active while beta and DCCD-inhibited CF1(-delta) were not. These results support the notion that subunit delta can function as a stopcock to the CF0 proton channel as proposed by Junge, W., Hong, Y. Q., Qian, L. P. and Viale, A. [(1984) Proc. Natl Acad. Sci. USA 81, 3078-3082].

Structure and function of the ATPase-ATP synthase complex of mitochondria as compared to chloroplasts and bacteria

An overview of the structure and function of the mitochondrial ATPase-ATP synthase complex is presented. Attempts are made to identify the analogies and differences between mitochondrial, chloroplastic and bacterial complexes. The relatively more precise information available on the structure of the E. coli enzyme is used to try and understand the apparently more complex structure of the mitochondrial enzyme. Recent ideas on the mechanism of ATP hydrolysis and ATP synthesis will be summarized.