Structure of Dimeric F1F0-ATP Synthase

The structure of the dimeric ATP synthase from yeast mitochondria was analyzed by transmission electron microscopy and single particle image analysis. In addition to the previously reported side views of the dimer, top view and intermediate projections served to resolve the arrangement of the rotary c₁₀ ring and the other stator subunits at the F₀-F₀ dimeric interface. A three-dimensional reconstruction of the complex was calculated from a data set of 9960 molecular images at a resolution of 27 Å. The structural model of the dimeric ATP synthase shows the two monomers arranged at an angle of ∼45°, consistent with our earlier analysis of the ATP synthase from bovine heart mitochondria (Minauro-Sanmiguel, F., Wilkens, S., and Garcia, J. J. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 12356–12358). In the ATP synthase dimer, the two peripheral stalks are located near the F₁-F₁ interface but are turned away from each other so that they are not in contact. Based on the three-dimensional reconstruction, a model of how dimeric ATP synthase assembles to form the higher order oligomeric structures that are required for mitochondrial cristae biogenesis is discussed.     

Properties of Bound Inorganic Phosphate on Bovine Mitochondrial F1F0-ATP Synthase

Beef-heart mitochondrial F₁F₀-ATP synthase contained six molecules of bound inorganic phosphate (P_i). This phosphate exchanged completely with exogenous ³²P_i when the enzyme was exposed to 30% (v/v) dimethyl sulfoxide (DMSO) and then returned to a DMSO-free buffer (Beharry and Bragg 2001). Only two molecules were replaced by ³²P_i when the enzyme was not pretreated with DMSO. These two molecules of ³²P_i were not displaced from the enzyme by the treatment with 1 mM ATP. Similarly, two molecules of bound ³²P_i remained on the DMSO-pretreated enzyme following addition of ATP, that is, four molecules of ³²P_i were displaced by ATP. The ATP-resistant ³²P_i was removed from the enzyme by pyrophosphate. It is proposed that these molecules of ³²P_i are bound at an unfilled adenine nucleotide-binding noncatalytic site on the enzyme. Brief exposure of the enzyme loaded with two molecules of ³²P_i to DMSO, followed by removal of the DMSO, resulted in the loss of the bound ³²P_i and in the formation of two molecules of bound ATP from exogenous ADP. A third catalytic site on the enzyme was occupied by ATP, which could undergo a P_i ATP exchange reaction with bound P_i The presence of two catalytic sites containing bound P_i is consistent with the X-ray crystallographic structure of F₁ (Bianchet, et al., 1998). Thus, five of the six molecules of bound P_i were accounted for. Three molecules of bound P_i were at catalytic sites and participated in ATP synthesis or P_i ATP exchange. Two other molecules of bound P_i were present at a noncatalytic adenine nucleotide-binding site. The location and role of the remaining molecule of bound P_i remains to be established. We were unable to demonstrate, using chemical modification of sulfhydryl groups by iodoacetic acid, any gross difference in the conformation of F₁F₀ in DMSO-containing compared with DMSO-free buffers.     

Single-molecule Analysis of F0F1-ATP Synthase Inhibited by N,N-Dicyclohexylcarbodiimide

N,N-Dicyclohexylcarbodiimide (DCCD) is a classical inhibitor of the F₀F₁-ATP synthase (F₀F₁), which covalently binds to the highly conserved carboxylic acid of the proteolipid subunit (c subunit) in F₀. Although it is well known that DCCD modification of the c subunit blocks proton translocation in F₀ and the coupled ATP hydrolysis activity of F₁, how DCCD inhibits the rotary dynamics of F₀F₁ remains elusive. Here, we carried out single-molecule rotation assays to characterize the DCCD inhibition of Escherichia coli F₀F₁. Upon the injection of DCCD, rotations irreversibly terminated with first order reaction kinetics, suggesting that the incorporation of a single DCCD moiety is sufficient to block the rotary catalysis of the F₀F₁. Individual molecules terminated at different angles relative to the three catalytic angles of F₁, suggesting that DCCD randomly reacts with one of the 10 c subunits. DCCD-inhibited F₀F₁ sometimes showed transient activation; molecules abruptly rotated and stopped after one revolution at the original termination angle, suggesting that hindrance by the DCCD moiety is released due to thermal fluctuation. To explore the mechanical activation of DCCD-inhibited molecules, we perturbed inhibited molecules using magnetic tweezers. The probability of transient activation increased upon a forward forcible rotation. Interestingly, during the termination F₀F₁, showed multiple positional shifts, which implies that F₁ stochastically changes the angular position of its rotor upon a catalytic reaction. This effect could be caused by balancing the angular positions of the F₁ and the F₀ rotors, which are connected via elastic elements.     

Hypothyroidism Leads to a Decreased Expression of Mitochondrial F0F1-ATP Synthase in Rat Liver

In liver mitochondria isolated from hypothyroid rats, the rate of ATP synthesis is lower than in mitochondria from normal rats. Oligomycin-sensitive ATP hydrolase activity and passive proton permeability were significantly lower in submitochondrial particles from hypothyroid rats compared to those isolated from normal rats. In mitochondria from hypothyroid rats, the changes in catalytic activities of F₀F₁-ATP synthase are accompanied by a decrease in the amount of immunodetected -F₁, F₀1-PVP, and OSCP subunits of the complex. Northern blot hybridization shows a decrease in the relative cytosolic content of mRNA for -F₁ subunit in liver of hypothyroid rats. Administration of 3,5,3-triodo-L-thyronine to the hypothyroid rats tends to remedy the functional and structural defects of F₀F₁-ATP synthase observed in the hypothyroid rats. The results obtained indicate that hypothyroidism leads to a decreased expression of F₀F₁-ATP synthase complex in liver mitochondria and this contributes to the decrease of the efficiency of oxidative phosphorylation.     

Cyclophilin D Modulates Mitochondrial F0F1-ATP Synthase by Interacting with the Lateral Stalk of the Complex

Blue native gel electrophoresis purification and immunoprecipitation of F₀F₁-ATP synthase from bovine heart mitochondria revealed that cyclophilin (CyP) D associates to the complex. Treatment of intact mitochondria with the membrane-permeable bifunctional reagent dimethyl 3,3-dithiobis-propionimidate (DTBP) cross-linked CyPD with the lateral stalk of ATP synthase, whereas no interactions with F₁ sector subunits, the ATP synthase natural inhibitor protein IF1, and the ATP/ADP carrier were observed. The ATP synthase-CyPD interactions have functional consequences on enzyme catalysis and are modulated by phosphate (increased CyPD binding and decreased enzyme activity) and cyclosporin (Cs) A (decreased CyPD binding and increased enzyme activity). Treatment of MgATP submitochondrial particles or intact mitochondria with CsA displaced CyPD from membranes and activated both hydrolysis and synthesis of ATP sustained by the enzyme. No effect of CsA was detected in CyPD-null mitochondria, which displayed a higher specific activity of the ATP synthase than wild-type mitochondria. Modulation by CyPD binding appears to be independent of IF1, whose association to ATP synthase was not affected by CsA treatment. These findings demonstrate that CyPD association to the lateral stalk of ATP synthase modulates the activity of the complex.     

The Structural and Functional Connection between the Catalytic and Proton Translocating Sectors of the Mitochondrial F 1 F 0 -ATP Synthase

The structural and functional connection between the peripheral catalytic F₁ sector and theproton-translocating membrane sector F₀ of the mitochondrial ATP synthase is reviewed. Theobservations examined show that the N-terminus of subunit , the carboxy-terminal and centralregion of F₀I-PVP(b), OSCP, and part of subunit d constitute a continuous structure, the lateralstalk, which connects the peripheries of F₁ to F₀ and surrounds the central element of thestalk, constituted by subunits and . The ATPase inhibitor protein (IF₁) binds at one sideof the F₁F₀ connection. The carboxy-terminal segment of IF₁ apparently binds to OSCP. The42L-58K segment of IF₁, which is per se the most active domain of the protein, binds at thesurface of one of the three / pairs of F₁, thus preventing the cyclic interconversion of thecatalytic sites required for ATP hydrolysis.     

Sulfite inhibits the F1F0-ATP synthase and activates the F1F0-ATPase of Paracoccus denitrificans

The F₁F₀ complex of Paracoccus denitrificans (PdF₁F₀) is the fastest ATP synthase but the slowest ATPase. Sulfite exerts maximal activation of the PdF₁F₀-ATPase (Pacheco-Moisés, F., García, J. J., Rodríguez-Zavala, J. S., and Moreno-Sánchez, R. (2000). Eur. J. Biochem. 267, 993–1000) but its effect on the PdF₁F₀-ATP synthase activity remains unknown. Therefore, we studied the effect of sulfite on ATP synthesis and ³²Pi ATP exchange reactions of inside-out membrane vesicles of P. denitrificans. Sulfite inhibited both reactions under conditions of maximal pH and normal sensitivity to dicyclohexylcarbodiimide. Sulfite increased by 10- and 5-fold the K _0.5 for Mg²⁺-ADP and Pi during ATP synthesis, respectively, and by 4-fold the IC₅₀ of Mg²⁺-ADP for inhibition of the PdF₁F₀-ATPase activity. Thus, sulfite exerts opposite effects on the forward and reverse functioning of the PdF₁F₀ complex. These effects are not due to membrane or PdF₁F₀ uncoupling. Kinetic and structural modifications that could account for these results are discussed.     

Identification of F0 subunits in the rat liver mitochondrial F0F1-ATP synthase.

In order to identify the subunits constituting the rat liver F0F1-ATP synthase, the complex prepared by selective extraction from the mitochondrial membranes with a detergent followed by purification on a sucrose gradient has been compared to that obtained by immunoprecipitation with an anti-F1 serum. The subunits present in both preparations that are assumed to be authentic components of the complex have been identified. The results show that the total rat liver F0F1-ATP synthase contains at least 13 different proteins, seven of which can be attributed to F0. The following F0 subunits have been identified: the subunit b (migrating as a 24 kDa band in SDS-PAGE), the oligomycin-sensitivity-conferring protein (20 kDa), and F6 (9 kDa) that have N-terminal sequences homologous to the beef-heart ones; the mtDNA encoded subunits 6 (20 kDa) and 8 (less than 7 kDa) that can be synthesized in isolated mitochondria; an additional 20 kDa protein that could be equivalent to the beef heart subunit d.     

The F0F1-ATP Synthase Complex Contains Novel Subunits and Is Essential for Procyclic Trypanosoma brucei

The mitochondrial F₀F₁ ATP synthase is an essential multi-subunit protein complex in the vast majority of eukaryotes but little is known about its composition and role in Trypanosoma brucei, an early diverged eukaryotic pathogen. We purified the F₀F₁ ATP synthase by a combination of affinity purification, immunoprecipitation and blue-native gel electrophoresis and characterized its composition and function. We identified 22 proteins of which five are related to F₁ subunits, three to F₀ subunits, and 14 which have no obvious homology to proteins outside the kinetoplastids. RNAi silencing of expression of the F₁ α subunit or either of the two novel proteins showed that they are each essential for the viability of procyclic (insect stage) cells and are important for the structural integrity of the F₀F₁-ATP synthase complex. We also observed a dramatic decrease in ATP production by oxidative phosphorylation after silencing expression of each of these proteins while substrate phosphorylation was not severely affected. Our procyclic T. brucei cells were sensitive to the ATP synthase inhibitor oligomycin even in the presence of glucose contrary to earlier reports. Hence, the two novel proteins appear essential for the structural organization of the functional complex and regulation of mitochondrial energy generation in these organisms is more complicated than previously thought.     

Proton-powered subunit rotation in single membrane-bound F0F1-ATP synthase

Synthesis of ATP from ADP and phosphate, catalyzed by F(0)F(1)-ATP synthases, is the most abundant physiological reaction in almost any cell. F(0)F(1)-ATP synthases are membrane-bound enzymes that use the energy derived from an electrochemical proton gradient for ATP formation. We incorporated double-labeled F(0)F(1)-ATP synthases from Escherichia coli into liposomes and measured single-molecule fluorescence resonance energy transfer (FRET) during ATP synthesis and hydrolysis. The gamma subunit rotates stepwise during proton transport-powered ATP synthesis, showing three distinct distances to the b subunits in repeating sequences. The average durations of these steps correspond to catalytic turnover times upon ATP synthesis as well as ATP hydrolysis. The direction of rotation during ATP synthesis is opposite to that of ATP hydrolysis.     

Manipulations in the peripheral stalk of the Saccharomyces cerevisiae F1F0-ATP synthase

The Saccharomyces cerevisiae F(1)F(0)-ATP synthase peripheral stalk is composed of the OSCP, h, d, and b subunits. The b subunit has two membrane-spanning domains and a large hydrophilic domain that extends along one side of the enzyme to the top of F(1). In contrast, the Escherichia coli peripheral stalk has two identical b subunits, and subunits with substantially altered lengths can be incorporated into a functional F(1)F(0)-ATP synthase. The differences in subunit structure between the eukaryotic and prokaryotic peripheral stalks raised a question about whether the two stalks have similar physical and functional properties. In the present work, the length of the S. cerevisiae b subunit has been manipulated to determine whether the F(1)F(0)-ATP synthase exhibited the same tolerances as in the bacterial enzyme. Plasmid shuffling was used for ectopic expression of altered b subunits in a strain carrying a chromosomal disruption of the ATP4 gene. Wild type growth phenotypes were observed for insertions of up to 11 and a deletion of four amino acids on a nonfermentable carbon source. In mitochondria-enriched fractions, abundant ATP hydrolysis activity was seen for the insertion mutants. ATPase activity was largely oligomycin-insensitive in these mitochondrial fractions. In addition, very poor complementation was seen in a mutant with an insertion of 14 amino acids. Lengthier deletions yielded a defective enzyme. The results suggest that although the eukaryotic peripheral stalk is near its minimum length, the b subunit can be extended a considerable distance.     

ATP synthesis driven by proton transport in F1F0-ATP synthase

Topical questions in ATP synthase research are: (1) how do protons cause subunit rotation and how does rotation generate ATP synthesis from ADP+Pi? (2) How does hydrolysis of ATP generate subunit rotation and how does rotation bring about uphill transport of protons? The finding that ATP synthase is not just an enzyme but rather a unique nanomotor is attracting a diverse group of researchers keen to find answers. Here we review the most recent work on rapidly developing areas within the field and present proposals for enzymatic and mechanoenzymatic mechanisms.     

Probing conformations of the beta subunit of F0F1-ATP synthase in catalysis

A subcomplex of F0F1-ATP synthase (F0F1), alpha3beta3gamma, was shown to undergo the conformation(s) during ATP hydrolysis in which two of the three beta subunits have the "Closed" conformation simultaneously (CC conformation) [S.P. Tsunoda, E. Muneyuki, T. Amano, M. Yoshida, H. Noji, Cross-linking of two beta subunits in the closed conformation in F1-ATPase, J. Biol. Chem. 274 (1999) 5701-5706]. This was examined by the inter-subunit disulfide cross-linking between two mutant beta(I386C)s that was formed readily only when the enzyme was in the CC conformation. Here, we adopted the same method for the holoenzyme F0F1 from Bacillus PS3 and found that the CC conformation was generated during ATP hydrolysis but barely during ATP synthesis. The experiments using F0F1 with the epsilon subunit lacking C-terminal helices further suggest that this difference is related to dynamic nature of the epsilon subunit and that ATP synthesis is accelerated when it takes the pathway involving the CC conformation.     

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.     

FRET reveals changes in the F1-stator stalk interaction during activity of F1F0-ATP synthase

A stator is proposed as necessary to prevent futile rotation of the F(1) catalytic sector of mitochondrial ATP synthase (mtATPase) during periods of ATP synthesis or ATP hydrolysis. Although the second stalk of mtATPase is generally believed to fulfil the role of a stator capable of withstanding the stress produced by rotation of the central rotor, there is little evidence to directly support this view. We show that interaction between two candidate proteins of the second stalk, OSCP and subunit b, fused at their C-termini to GFP variants and assembled into functional mtATPase can be monitored in mitochondria using fluorescence resonance energy transfer (FRET). Substitution of native OSCP with a variant containing a glycine 166 to asparagine (G166N) substitution yielded a metastable complex. In contrast to the enzyme containing native OSCP, FRET could be irreversibly lowered for the enzyme containing G166N at a rate that correlated closely with the rate of enzyme activity (ATP hydrolysis). The non-hydrolysable ATP analogue, AMP-PCP did not have this effect. We conclude that two candidate proteins of the stator stalk, OSCP and b, are subject to stresses during enzyme catalytic activity commensurate with their role as a part of a stator stalk.     

The molecular mechanism of ATP synthesis by F1F0-ATP synthase

ATP synthesis by oxidative phosphorylation and photophosphorylation, catalyzed by F1F0-ATP synthase, is the fundamental means of cell energy production. Earlier mutagenesis studies had gone some way to describing the mechanism. More recently, several X-ray structures at atomic resolution have pictured the catalytic sites, and real-time video recordings of subunit rotation have left no doubt of the nature of energy coupling between the transmembrane proton gradient and the catalytic sites in this extraordinary molecular motor. Nonetheless, the molecular events that are required to accomplish the chemical synthesis of ATP remain undefined. In this review we summarize current state of knowledge and present a hypothesis for the molecular mechanism of ATP synthesis.     

Atrazine binds to F1F0-ATP synthase and inhibits mitochondrial function in sperm

Atrazine is a widely used triazine herbicide. Although controversy still exists, a number of recent studies have described its adverse effects on various animals including humans. Of particular interest is its effects on reproductive capacity. In this study, we investigated the mechanisms underlying the adverse effects of atrazine, with a focus on its effects on sperm. Here we show evidence that mitochondrial F₁F₀-ATP synthase is a molecular target of atrazine. A series of experiments with sperm and isolated mitochondria suggest that atrazine inhibits mitochondrial function through F₁F₀-ATP synthase. Moreover, affinity purification using atrazine as a ligand demonstrates that F₁F₀-ATP synthase is a major atrazine-binding protein in cells. The inhibitory activity against mitochondria and F₁F₀-ATP synthase is not limited to atrazine but is likely to be applicable to other triazine-based compounds. Thus, our findings may have wide relevance to pharmacology and toxicology.     

Subunit rotation in F0F1-ATP synthases as a means of coupling proton transport through F0 to the binding changes in F1

The rotation of an asymmetric core of subunits in F₀F₁-ATP synthases has been proposed as a means of coupling the exergonic transport of protons through F₀ to the endergonic conformational changes in F₁ required for substrate binding and product release. Here we review earlier evidence both for and against subunit rotation and then discuss our most recent studies using reversible intersubunit disulfide cross-links to test for rotation. We conclude that the subunit of F₁ rotates relative to the surrounding catalytic subunits during catalytic turnover by both soluble F₁ and membrane-bound F₀F₁. Furthermore, the inhibition of this rotation by the modification of F₀ with DCCD suggests that rotation in F₁ is obligatorily coupled to rotation in F₀ as an integral part of the coupling mechanism.     

Energy-dependent transformation of the catalytic activities of the mitochondrial F0 x F1-ATP synthase

The ADP(Mg2+)-deactivated, azide-trapped F0 x F1-ATPase of coupled submitochondrial particles is capable of ATP synthesis being incapable of ATP hydrolysis and ATP-dependent delta muH+ generation [FEBS Lett. (1995) 366, 29-32]. This puzzling phenomenon was studied further. No ATPase activity of the submitochondrial particles catalyzing succinate-supported oxidative phosphorylation in the presence of azide was observed when ATP was added to the assay mixture after an uncoupler. Rapid ATP hydrolysis was detected in the same system when ATP followed by an uncoupler was added. Less than 5% of the original ATPase activity was seen when the reaction (assayed with ATP-regenerating system) was initiated by the addition of ATP to the azide-trapped coupled particles oxidizing succinate either in the presence or in the absence of the uncoupler. High ATP hydrolytic activity was revealed when the reaction was started by the simultaneous addition of the ATP plus uncoupler to the particles generating delta muH+. The energy-dependent conversion of the enzyme into latent uncoupler-activated ATPase was prevented by free ADP (Ki approximately 20 microM) and was greatly enhanced after multiple turnovers in oxidative phosphorylation. The results suggest that the catalytic properties of F0 x F1 are delta muH+-dependent which is in accord with our hypothesis on different conformational states of the enzyme participating in ATP synthesis or hydrolysis.