Arrangement of the multicopy H+-translocating subunit c in the membrane sector of the Escherichia coli F1F0 ATP synthase

Molecular characterization of the yeast vacuolar H+-ATPase proton pore

The Saccharomyces cerevisiae vacuolar ATPase (V-ATPase) is composed of at least 13 polypeptides organized into two distinct domains, V(1) and V(0), that are structurally and mechanistically similar to the F(1)-F(0) domains of the F-type ATP synthases. The peripheral V(1) domain is responsible for ATP hydrolysis and is coupled to the mechanism of proton translocation. The integral V(0) domain is responsible for the translocation of protons across the membrane and is composed of five different polypeptides. Unlike the F(0) domain of the F-type ATP synthase, which contains 12 copies of a single 8-kDa proteolipid, the V-ATPase V(0) domain contains three proteolipid species, Vma3p, Vma11p, and Vma16p, with each proteolipid contributing to the mechanism of proton translocation (Hirata, R., Graham, L. A., Takatsuki, A., Stevens, T. H., and Anraku, Y. (1997) J. Biol. Chem. 272, 4795-4803). Experiments with hemagglutinin- and c-Myc epitope-tagged copies of the proteolipids revealed that each V(0) complex contains all three species of proteolipid with only one copy each of Vma11p and Vma16p but multiple copies of Vma3p. Since the proteolipids of the V(0) complex are predicted to possess four membrane-spanning alpha-helices, twice as many as a single F-ATPase proteolipid subunit, only six V-ATPase proteolipids would be required to form a hexameric ring-like structure similar to the F(0) domain. Therefore, each V(0) complex will likely be composed of four copies of the Vma3p proteolipid in addition to Vma11p and Vma16p. Structural differences within the membrane-spanning domains of both V(0) and F(0) may account for the unique properties of the ATP-hydrolyzing V-ATPase compared with the ATP-generating F-type ATP synthase.

Molecular contacts in the transmembrane c-subunit oligomer of F-ATPases identified by tryptophan substitution mutagenesis

When isolated in its monomeric form, subunit c of the proton transporting ATP synthase of Escherichia coli was shown to fold in a hairpin-like structure consisting of two hydrophobic membrane spanning helices and a short connecting hydrophilic loop. In the plasma membrane of Escherichia coli, however, about 9-12 c-subunit monomers form an oligomeric complex that functions in transmembrane proton conduction and in energy transduction to the catalytic F1 domain. The arrangement of the monomers and the molecular architecture of the complex were studied by tryptophan scanning mutagenesis and restrained MD simulations. Residues 12-24 of the N-terminal transmembrane segment of subunit c were individually substituted by the large and moderately hydrophobic tryptophan side chain. Effects on the activity of the mutant proteins were studied in selective growth experiments and various ATP synthase specific activity assays. The results identify potential intersubunit contacts and structurally non-distorted, accessible residues in the c-oligomer and add constraints to the arrangement of monomers in the oligomeric complex. Results from our mutagenesis experiments were interpreted in structural models of the c-oligomer that have been obtained by restrained MD simulations. Different stoichiometries and monomer orientations were applied in these calculations. A cylindrical complex consisting of 10 monomers that are arranged in two concentric rings with the N-terminal helices of the monomers located at the periphery shows the best match with the experimental data.

Kinetic model of ATP synthase: pH dependence of the rate of ATP synthesis

Recently, a novel molecular mechanism of torque generation in the F(0) portion of ATP synthase was proposed [Rohatgi, Saha and Nath (1998) Curr. Sci. 75, 716-718]. In this mechanism, rotation of the c-subunit was conceived to take place in 12 discrete steps of 30 degrees each due to the binding and unbinding of protons to/from the leading and trailing Asp-61 residues of the c-subunit, respectively. Based on this molecular mechanism, a kinetic scheme has been developed in this work. The scheme considers proton transport driven by a concentration gradient of protons across the proton half-channels, and the rotation of the c-subunit by changes in the electrical potential only. This kinetic scheme has been analyzed mathematically and an expression has been obtained to explain the pH dependence of the rate of ATP synthesis by ATP synthase under steady state operating conditions. For a single set of three enzymological kinetic parameters, this expression predicts the rates of ATP synthesis which agree well with the experimental data over a wide range of pH(in) and pH(out). A logical consequence of our analysis is that DeltapH and Deltapsi are kinetically inequivalent driving forces for ATP synthesis.

The ATP synthase of Escherichia coli: structure and function of F(0) subunits

In this review we discuss recent work from our laboratory concerning the structure and/or function of the F(0) subunits of the proton-translocating ATP synthase of Escherichia coli. For the topology of subunit a a brief discussion gives (i) a detailed picture of the C-terminal two-thirds of the protein with four transmembrane helices and the C terminus exposed to the cytoplasm and (ii) an evaluation of the controversial results obtained for the localization of the N-terminal region of subunit a including its consequences on the number of transmembrane helices. The structure of membrane-bound subunit b has been determined by circular dichroism spectroscopy to be at least 75% alpha-helical. For this purpose a method was developed, which allows the determination of the structure composition of membrane proteins in proteoliposomes. Subunit b was purified to homogeneity by preparative SDS gel electrophoresis, precipitated with acetone, and redissolved in cholate-containing buffer, thereby retaining its native conformation as shown by functional coreconstitution with an ac subcomplex. Monoclonal antibodies, which have their epitopes located within the hydrophilic loop region of subunit c, and the F(1) part are bound simultaneously to the F(0) complex without an effect on the function of F(0), indicating that not all c subunits are involved in F(1) interaction. Consequences on the coupling mechanism between ATP synthesis/hydrolysis and proton translocation are discussed.

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.

Structural interpretations of F(0) rotary function in the Escherichia coli F(1)F(0) ATP synthase

F(1)F(0) ATP synthases are known to synthesize ATP by rotary catalysis in the F(1) sector of the enzyme. Proton translocation through the F(0) membrane sector is now proposed to drive rotation of an oligomer of c subunits, which in turn drives rotation of subunit gamma in F(1). The primary emphasis of this review will be on recent work from our laboratory on the structural organization of F(0), which proves to be consistent with the concept of a c(12) oligomeric rotor. From the NMR structure of subunit c and cross-linking studies, we can now suggest a detailed model for the organization of the c(12) oligomer in F(0) and some of the transmembrane interactions with subunits a and b. The structural model indicates that the H(+)-carrying carboxyl of subunit c is located between subunits of the c(12) oligomer and that two c subunits pack in a front-to-back manner to form the proton (cation) binding site. The proton carrying Asp61 side chain is occluded between subunits and access to it, for protonation and deprotonation via alternate entrance and exit half-channels, requires a swiveled opening of the packed c subunits and stepwise association with different transmembrane helices of subunit a. We suggest how some of the structural information can be incorporated into models of rotary movement of the c(12) oligomer during coupled synthesis of ATP in the F(1) portion of the molecule.

Reverse engineering a protein: the mechanochemistry of ATP synthase

ATP synthase comprises two rotary motors in one. The F(1) motor can generate a mechanical torque using the hydrolysis energy of ATP. The F(o) motor generates a rotary torque in the opposite direction, but it employs a transmembrane proton motive force. Each motor can be reversed: The F(o) motor can drive the F(1) motor in reverse to synthesize ATP, and the F(1) motor can drive the F(o) motor in reverse to pump protons. Thus ATP synthase exhibits two of the major energy transduction pathways employed by the cell to convert chemical energy into mechanical force. Here we show how a physical analysis of the F(1) and F(o) motors can provide a unified view of the mechanochemical principles underlying these energy transducers.

How Fo-ATPase generates rotary torque

The F-ATPases synthesize ATP using a transmembrane ionmotive force (IMF) established by the electron transport chain. This transduction involves first converting the IMF to a rotary torque in the transmembrane Fo portion. This torque is communicated from Fo to the F1 portion where the energy is used to release the newly synthesized ATP from the catalytic sites according to Boyer's binding change mechanism. Here we explain the principle by which an IMF generates this rotary torque in the Fo ion engine.

Mutations in single hairpin units of genetically fused subunit c provide support for a rotary catalytic mechanism in F(0)F(1) ATP synthase

Previously, we generated genetically fused dimers and trimers of subunit c of the Escherichia coli ATP synthase based upon the precedent of naturally occurring dimers in V-type H(+)-transporting ATPases. The c(2) and c(3) oligomers have proven useful in testing hypothesis regarding the mechanism of energy coupling. In the first part of this paper, the uncoupling Q42E substitution has been introduced into the second loop of the c(2) dimer or the third loop of the c(3) trimer. Both mutant proteins proved to be as functional as the wild type c(2) dimer or wild type c(3) trimer. The results argue against an obligatory movement of the epsilon subunit between loops of monomeric subunit c in the c(12) oligomer during rotary catalysis. Rather, the results support the hypothesis that the c-epsilon connection remains fixed as the c-oligomer rotates. In the second section of this paper, we report on the effect of substitution of the proton translocating Asp(61) in every second helical hairpin of the c(2) dimer, or in every third hairpin of the c(3) trimer. Based upon the precedent of V-type ATPases, where the c(2) dimer occurs naturally with a single proton translocating carboxyl in every second hairpin, these modified versions of the E. coli c(2) and c(3) fused proteins were predicted to have a functional H(+)-transporting ATPase activity, with a reduced H(+)/ATP stoichiometry, but to be inactive as ATP synthases. A variety of Asp(61)-substituted proteins proved to lack either activity indicating that the switch in function in V-type ATPases is a consequence of more than a single substitution.

Mechanical rotation of the c subunit oligomer in ATP synthase (F0F1): direct observation

F0F1, found in mitochondria or bacterial membranes, synthesizes adenosine 5'-triphosphate (ATP) coupling with an electrochemical proton gradient and also reversibly hydrolyzes ATP to form the gradient. An actin filament connected to a c subunit oligomer of F0 was able to rotate by using the energy of ATP hydrolysis. The rotary torque produced by the c subunit oligomer reached about 40 piconewton-nanometers, which is similar to that generated by the gamma subunit in the F1 motor. These results suggest that the gamma and c subunits rotate together during ATP hydrolysis and synthesis. Thus, coupled rotation may be essential for energy coupling between proton transport through F0 and ATP hydrolysis or synthesis in F1.

Molecular architecture of the rotary motor in ATP synthase

Adenosine triphosphate (ATP) synthase contains a rotary motor involved in biological energy conversion. Its membrane-embedded F0 sector has a rotation generator fueled by the proton-motive force, which provides the energy required for the synthesis of ATP by the F1 domain. An electron density map obtained from crystals of a subcomplex of yeast mitochondrial ATP synthase shows a ring of 10 c subunits. Each c subunit forms an alpha-helical hairpin. The interhelical loops of six to seven of the c subunits are in close contact with the gamma and delta subunits of the central stalk. The extensive contact between the c ring and the stalk suggests that they may rotate as an ensemble during catalysis.

Structural changes linked to proton translocation by subunit c of the ATP synthase

F1F0 ATP synthases use a transmembrane proton gradient to drive the synthesis of cellular ATP. The structure of the cytosolic F1 portion of the enzyme and the basic mechanism of ATP hydrolysis by F1 are now well established, but how proton translocation through the transmembrane F0 portion drives these catalytic changes is less clear. Here we describe the structural changes in the proton-translocating F0 subunit c that are induced by deprotonating the specific aspartic acid involved in proton transport. Conformational changes between the protonated and deprotonated forms of subunit c provide the structural basis for an explicit mechanism to explain coupling of proton translocation by F0 to the rotation of subunits within the core of F1. Rotation of these subunits within F1 causes the catalytic conformational changes in the active sites of F1 that result in ATP synthesis.

Helical interactions and membrane disposition of the 16-kDa proteolipid subunit of the vacuolar H(+)-ATPase analyzed by cysteine replacement mutagenesis

Theoretical mechanisms of proton translocation by the vacuolar H(+)-ATPase require that a transmembrane acidic residue of the multicopy 16-kDa proteolipid subunit be exposed at the exterior surface of the membrane sector of the enzyme, contacting the lipid phase. However, structural support for this theoretical mechanism is lacking. To address this, we have used cysteine mutagenesis to produce a molecular model of the 16-kDa proteolipid complex. Transmembrane helical contacts were determined using oxidative cysteine cross-linking, and accessibility of cysteines to the lipid phase was determined by their reactivity to the lipid-soluble probe N-(1-pyrenyl)maleimide. A single model for organization of the four helices of each monomeric proteolipid was the best fit to the experimental data, with helix 1 lining a central pore and helix 2 and helix 3 immediately external to it and forming the principal intermolecular contacts. Helix 4, containing the crucial acidic residue, is peripheral to the complex. The model is consistent not only with theoretical proton transport mechanisms, but has structural similarity to the dodecameric ring complex formed by the related 8-kDa proteolipid of the F(1)F(0)-ATPase. This suggests some commonality between the proton translocating mechanisms of the vacuolar and F(1)F(0)-ATPases.

F0 complex of the Escherichia coli ATP synthase. Not all monomers of the subunit c oligomer are involved in F1 interaction

The antigenic determinants of mAbs against subunit c of the Escherichia coli ATP synthase were mapped by ELISA using overlapping synthetic heptapeptides. All epitopes recognized are located in the hydrophilic loop region and are as follows: 31-LGGKFLE-37, 35-FLEGAAR-41, 36-LEGAAR-41 and 36-LEGAARQ-42. Binding studies with membrane vesicles of different orientation revealed that all mAbs bind to everted membrane vesicles independent of the presence or absence of the F1 part. Although the hydrophilic region of subunit c and particularly the highly conserved residues A40, R41, Q42 and P43 are known to interact with subunits gamma and epsilon of the F1 part, the mAb molecules have no effect on the function of F0. Furthermore, it could be demonstrated that the F1 part and the mAb molecule(s) are bound simultaneously to the F0 complex suggesting that not all c subunits are involved in F1 interaction. From the results obtained, it can be concluded that this interaction is fixed, which means that subunits gamma and epsilon do not switch between the c subunits during catalysis and furthermore, a complete rotation of the subunit c oligomer modified with mAb(s) along the stator of the F1F0 complex, proposed to be composed of at least subunits b and delta, seems to be unlikely.

Rotational coupling in the F0F1 ATP synthase

The F0F1 ATP synthase is a large multisubunit complex that couples translocation of protons down an electrochemical gradient to the synthesis of ATP. Recent advances in structural analyses have led to the demonstration that the enzyme utilizes a rotational catalytic mechanism. Kinetic and biochemical evidence is consistent with the expected equal participation of the three catalytic sites in the alpha 3 beta 3 hexamer, which operate in sequential, cooperative reaction pathways. The rotation of the core gamma subunit plays critical roles in establishing the conformation of the sites and the cooperative interactions. Mutational analyses have shown that the rotor subunits are responsible for coupling and in doing so transmit specific conformational information between transport and catalysis.

Structure of the subunit c oligomer in the F1Fo ATP synthase: model derived from solution structure of the monomer and cross-linking in the native enzyme

The structure of the subunit c oligomer of the H+-transporting ATP synthase of Escherichia coli has been modeled by molecular dynamics and energy minimization calculations from the solution structure of monomeric subunit c and 21 intersubunit distance constraints derived from cross-linking of subunits. Subunit c folds in a hairpin-like structure with two transmembrane helices. In the c12 oligomer model, the subunits pack to form a compact hollow cylinder with an outer diameter of 55-60 A and an inner space with a minimal diameter of 11-12 A. Phospholipids are presumed to pack in the inner space in the native membrane. The transmembrane helices pack in two concentric rings with helix 1 inside and helix 2 outside. The calculations strongly favor this structure versus a model with helix 2 inside and helix 1 outside. Asp-61, the H+-transporting residue, packs toward the center of the four transmembrane helices of two interacting subunits. From this position at the front face of one subunit, the Asp-61 carboxylate lies proximal to side chains of Ala-24, Ile-28, and Ala-62, projecting from the back face of a second subunit. These interactions were predicted from previous mutational analyses. The packing supports the suggestion that a c-c dimer is the functional unit. The positioning of the Asp-61 carboxyl in the center of the interacting transmembrane helices, rather than at the periphery of the cylinder, has important implications regarding possible mechanisms of H+-transport-driven rotation of the c oligomer during ATP synthesis.

Defining the domain of binding of F1 subunit epsilon with the polar loop of F0 subunit c in the Escherichia coli ATP synthase

We have previously shown that the E31C-substituted epsilon subunit of F1 can be cross-linked by disulfide bond formation to the Q42C-substituted c subunit of F0 in the Escherichia coli F1F0-ATP synthase complex (Zhang, Y., and Fillingame, R. H. (1995) J. Biol. Chem. 270, 24609-24614). The interactions of subunits epsilon and c are thought to be central to the coupling of H+ transport through F0 to ATP synthesis in F1. To further define the domains of interaction, we have introduced additional Cys into subunit epsilon and subunit c and tested for cross-link formation following sulfhydryl oxidation. The results show that Cys, in a continuous stretch of residues 26-33 in subunit epsilon, can be cross-linked to Cys at positions 40, 42, and 44 in the polar loop region of subunit c. The results are interpreted, and the subunit interaction is modeled using the NMR and x-ray diffraction structures of the monomeric subunits together with information on the packing arrangement of subunit c in a ring of 12 subunits. In the model, residues 26-33 form a turn of antiparallel beta-sheet which packs between the polar loop regions of adjacent subunit c at the cytoplasmic surface of the c12 oligomer.

A novel labeling approach supports the five-transmembrane model of subunit a of the Escherichia coli ATP synthase

Cysteine mutagenesis and surface labeling has been used to define more precisely the transmembrane spans of subunit a of the Escherichia coli ATP synthase. Regions of subunit a that are exposed to the periplasmic space have been identified by a new procedure, in which cells are incubated with polymyxin B nonapeptide (PMBN), an antibiotic derivative that partially permeabilizes the outer membrane of E. coli, along with a sulfhydryl reagent, 3-(N-maleimidylpropionyl) biocytin (MPB). This procedure permits reaction of sulfhydryl groups in the periplasmic space with MPB, but residues in the cytoplasm are not labeled. Using this procedure, residues 8, 27, 37, 127, 131, 230, 231, and 232 were labeled and so are thought to be exposed in the periplasm. Using inside-out membrane vesicles, residues near the end of transmembrane spans 1, 64, 67, 68, 69, and 70 and residues near the end of transmembrane spans 5, 260, 263, and 265 were labeled. Residues 62 and 257 were not labeled. None of these residues were labeled in PMBN-permeabilized cells. These results provide a more detailed view of the transmembrane spans of subunit a and also provide a simple and reliable technique for detection of periplasmic regions of inner membrane proteins in E. coli.

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

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

Structure of the membrane domain of subunit b of the Escherichia coli F0F1 ATP synthase

The structure of the N-terminal transmembrane domain (residues 1-34) of subunit b of the Escherichia coli F0F1-ATP synthase has been solved by two-dimensional 1H NMR in a membrane mimetic solvent mixture of chloroform/methanol/H2O (4:4:1). Residues 4-22 form an alpha-helix, which is likely to span the hydrophobic domain of the lipid bilayer to anchor the largely hydrophilic subunit b in the membrane. The helical structure is interrupted by a rigid bend in the region of residues 23-26 with alpha-helical structure resuming at Pro-27 at an angle offset by 20 degrees from the transmembrane helix. In native subunit b, the hinge region and C-terminal alpha-helical segment would connect the transmembrane helix to the cytoplasmic domain. The transmembrane domains of the two subunit b in F0 were shown to be close to each other by cross-linking experiments in which single Cys were substituted for residues 2-21 of the native subunit and b-b dimer formation tested after oxidation with Cu(II)(phenanthroline)2. Cys residues that formed disulfide cross-links were found with a periodicity indicative of one face of an alpha-helix, over the span of residues 2-18, where Cys at positions 2, 6, and 10 formed dimers in highest yield. A model for the dimer is presented based upon the NMR structure and distance constraints from the cross-linking data. The transmembrane alpha-helices are positioned at a 23 degrees angle to each other with the side chains of Thr-6, Gln-10, Phe-14, and Phe-17 at the interface between subunits. The change in direction of helical packing at the hinge region may be important in the functional interaction of the cytoplasmic domains.

Energy transduction in the sodium F-ATPase of Propionigenium modestum

The F-ATPase of the bacterium Propionigenium modestum is driven by an electrochemical sodium gradient between the cell interior and its environment. Here we present a mechanochemical model for the transduction of transmembrane sodium-motive force into rotary torque. The same mechanism is likely to operate in other F-ATPases, including the proton-driven F-ATPases of Escherichia coli.

Proton ATPases in bacteria: comparison to Escherichia coli F1F0 as the prototype

The F1F0 ATP synthase complex of Escherichia coli functions reversibly in coupling proton translocation to ATP synthesis or hydrolysis. The structural organization and subunit composition corresponds to that seen in many other bacteria, i.e. a membrane extrinsic F1 sector with five subunits in an alpha 3 beta 3 gamma delta epsilon stoichiometry, and a membrane-traversing F0 sector with three subunits in an a1b2c12 stoichiometry. The structure of much of the F1 sector is known from a X-ray diffraction model. During function, The gamma subunit is known to rotate within a hexameric ring of alternating alpha and beta subunits to promote sequential substrate binding and product release from catalytic sites on the three beta subunits. Proton transport through F0 must be coupled to this rotation. Subunit c folds in the membrane as a hairpin to two alpha helices to generate the proton-binding site in F0. Its structure was determined by NMR, and the structure of the c oligomer was deduced by cross-linking experiments and molecular mechanics calculations. The implications of the oligomeric structure of subunit c will be considered and related to the H+/ATP pumping ratio, P/O ratios and the cation-binding site in other types of F0. The possible limits of the structure in changing the ion-binding specificity, stoichiometry and routes of proton entrance/exit to the binding site will be considered.

ATP-synthase of Rhodobacter capsulatus: coupling of proton flow through F0 to reactions in F1 under the ATP synthesis and slip conditions

A stepwise increasing membrane potential was generated in chromatophores of the phototrophic bacterium Rhodobacter capsulatus by illumination with short flashes of light. Proton transfer through ATP-synthase (measured by electrochromic carotenoid bandshift and by pH-indicators) and ATP release (measured by luminescence of luciferin-luciferase) were monitored. The ratio between the amount of protons translocated by F0F1 and the ATP yield decreased with the flash number from an apparent value of 13 after the first flash to about 5 when averaged over three flashes. In the absence of ADP, protons slipped through F0F1. The proton transfer through F0F1 after the first flash contained two kinetic components, of about 6 ms and 20 ms both under the ATP synthesis conditions and under slip. The slower component of proton transfer was substantially suppressed in the absence of ADP. We attribute our observations to the mechanism of energy storage in the ATP-synthase needed to couple the transfer of four protons with the synthesis of one molecule of ATP. Most probably, the transfer of initial protons of each tetrad creates a strain in the enzyme that slows the translocation of the following protons.

Genetic fusions of subunit c in the F0 sector of H+-transporting ATP synthase. Functional dimers and trimers and determination of stoichiometry by cross-linking analysis

The multicopy c subunit of the H+-transporting ATP synthase of Escherichia coli folds through the transmembrane F0 sector as a hairpin of two hydrophobic alpha-helices with the proton-translocating aspartyl-61 side chain centered in the second transmembrane helix. The number of subunits c in the F0 complex, which is thought to determine the H+-pumping/ATP stoichiometry, was previously not determined with exactness but thought to range from 9-12. The studies described here indicate that the exact number is 12. Based upon the precedent of the subunit c in vacuolar-type ATPases, which are composed of four transmembrane helices and seem to have evolved by gene duplication of an F0-type progenitor gene, we constructed genetically fused dimers and trimers of E. coli subunit c. Both the dimeric and trimeric forms proved to be functional. These results indicate that the total number of subunit c in F0 should be a multiple of 2 and 3. Based upon a previous study in which the oligomeric organization of c subunits in F0 was determined by cross-linking of Cys-substituted subunits (Jones, P. C. , Jiang, W., and Fillingame, R. H. (1998) J. Biol. Chem. 273, 17178-17185), we introduced Cys into the first and last transmembrane helices of subunit c monomers, dimers, and trimers and attempted to generate cross-linked products by oxidation with Cu(II)-(1,10-phenanthroline)2. Double Cys substitutions at two sets of positions gave rise to extensive cross-linked multimers. Multimers of the monomer that extended up to the position of c12 were correlated and calibrated with distinct cross-linked species of the appropriate doubly Cys-substituted dimers (i.e. c2, c4, . c12) and doubly Cys-substituted trimers (i.e. c3, c6, c9, c12). The results show that there are 12 copies of subunit c per F0 in E. coli, the exact number having both mechanistic and structural significance.

Voltage-generated torque drives the motor of the ATP synthase

The mechanism by which ion-flux through the membrane-bound motor module (F0) induces rotational torque, driving the rotation of the gamma subunit, was probed with a Na+-translocating hybrid ATP synthase. The ATP-dependent occlusion of 1 (22)Na+ per ATP synthase persisted after modification of the c subunit ring with dicyclohexylcarbodiimide (DCCD), when 22Na+ was added first and ATP second, but not if the order of addition was reversed. These results support the model of ATP-driven rotation of the c subunit oligomer (rotor) versus subunit a (stator) that stops when either a 22Na+-loaded or a DCCD-modified rotor subunit reaches the Na+-impermeable stator. The ATP synthase with a Na+-permeable stator catalyzed 22Na+out/Na+in-exchange after reconstitution into proteoliposomes, which was not significantly affected by DCCD modification of the c subunit oligomer, but was abolished by the additional presence of ATP or by a membrane potential (DeltaPsi) of 90 mV. We propose that in the idling mode of the motor, Na+ ions are shuttled across the membrane by limited back and forth movements of the rotor against the stator. This motional flexibility is arrested if either ATP or DeltaPsi induces the switch from idling into a directed rotation. The Propionigenium modestum ATP synthase catalyzed ATP formation with DeltaPsi of 60-125 mV but not with DeltapNa+ of 195 mV. These results demonstrate that electric forces are essential for ATP synthesis and lead to a new concept of rotary-torque generation in the ATP synthase motor.