Characterization of the interface between gamma and epsilon subunits of Escherichia coli F1-ATPase

What is the role of epsilon in the Escherichia coli ATP synthase?

The ATP synthase from Escherichia coli is a prototype of the ATP synthases that are found in many bacteria, in the mitochondria of eukaryotes, and in the chloroplasts of plants. It contains eight different types of subunits that have traditionally been divided into F(1), a water-soluble catalytic sector, and F(o), a membrane-bound ion transporting sector. In the current rotary model for ATP synthesis, the subunits can be divided into rotor and stator subunits. Several lines of evidence indicate that epsilon is one of the three rotor subunits, which rotate through 360 degrees. The three-dimensional structure of epsilon is known and its interactions with other subunits have been explored by several approaches. In light of recent work by our group and that of others, the role of epsilon in the ATP synthase from E. coli is discussed.

Structures of the thermophilic F1-ATPase epsilon subunit suggesting ATP-regulated arm motion of its C-terminal domain in F1

The epsilon subunit of bacterial and chloroplast F(o)F(1)-ATP synthases modulates their ATP hydrolysis activity. Here, we report the crystal structure of the ATP-bound epsilon subunit from a thermophilic Bacillus PS3 at 1.9-A resolution. The C-terminal two alpha-helices were folded into a hairpin, sitting on the beta sandwich structure, as reported for Escherichia coli. A previously undescribed ATP binding motif, I(L)DXXRA, recognizes ATP together with three arginine and one glutamate residues. The E. coli epsilon subunit binds ATP in a similar manner, as judged on NMR. We also determined solution structures of the C-terminal domain of the PS3 epsilon subunit and relaxation parameters of the whole molecule by NMR. The two helices fold into a hairpin in the presence of ATP but extend in the absence of ATP. The latter structure has more helical regions and is much more flexible than the former. These results suggest that the epsilon C-terminal domain can undergo an arm-like motion in response to an ATP concentration change and thereby contribute to regulation of F(o)F(1)-ATP synthase.

Chemical modification of mono-cysteine mutants allows a more global look at conformations of the epsilon subunit of the ATP synthase from Escherichia coli

The epsilon subunit of the ATP synthase from E. coli undergoes conformational changes while rotating through 360 degrees during catalysis. The conformation of epsilon was probed in the membrane-bound ATP synthase by reaction of mono-cysteine mutants with 3-N-maleimidyl-propionyl biocytin (MPB) under resting conditions, during ATP hydrolysis, and after inhibition by ADP-AlF(3). The relative extents of labeling were quantified after electrophoresis and blotting of the partially purified epsilon subunit. Residues from the N-terminal beta-sandwich domain showed a position-specific pattern of labeling, consistent with prior structural studies. Some residues near the epsilon-gamma interface showed changes up to two-fold if labeling occurred during ATP hydrolysis or after inhibition by ADP-AlF(3). In contrast, residues found in the C-terminal alpha-helices were all labeled to a moderate or high level with a pattern that was consistent with a partially opened helical hairpin. The results indicate that the two C-terminal alpha-helices do not adopt a fixed conformation under resting conditions, but rather exhibit intrinsic flexibility.

Subunit movements in membrane-integrated EF0F1 during ATP synthesis detected by single-molecule spectroscopy

The H+ -ATPsynthase from E. coli was isolated and labelled at the gamma- or epsilon-subunit with tetramethylrhodamine, and at the b-subunits with bisCy5. The double labelled enzymes were incorporated into liposomes. They showed ATP hydrolysis activity, and, after energization of the membrane by DeltapH and Deltavarphi, also ATP synthesis activity was observed. Fluorescence resonance energy transfer (FRET) was used to investigate the movements of either the gamma-subunit or the epsilon-subunit relative to the b-subunits in single membrane-integrated enzymes. The results show that during catalysis, the gamma-epsilon complex rotates stepwise relative to the b-subunit. The direction of rotation during ATP synthesis is opposite to that during ATP hydrolysis. The stepwise motion is characterized by dwell times (docking time of the gamma-epsilon complex to one alphabeta pair) up to several hundred ms, followed by a rapid movement of the gamma- and epsilon-subunit to the next alphabeta pair within 0.2 ms. The same FRET levels (i.e., the same gamma-b and epsilon-b distances) are observed during proton transport-coupled ATP hydrolysis and ATP synthesis, indicating that the reaction proceeds via the same intermediates in both directions. Under non-catalytic conditions, i.e., in the absence of ATP or without energization also, three FRET levels are found, however, the distances differ from those under catalytic conditions. We conclude that this reflects a movement of the epsilon-subunit during active/inactive transition.

Substitutions of the conserved Gly47 affect the CF1 inhibitor and proton gate functions of the chloroplast ATP synthase epsilon subunit

The conserved residue Gly47 of the chloroplast ATP synthase beta subunit was substituted with Leu, Arg, Ala and Glu by site-directed mutagenesis. This process generated the mutants epsilon G47L, epsilon G47R, epsilon G47A and epsilon G47E, respectively. All the beta variants showed lower inhibitory effects on the soluble CF1(-epsilon) Ca2+-ATPase compared with wild-type epsilon. In reduced conditions, epsilon G47E and epsilon G47R had a lower inhibitory effect on the oxidized CF1(-epsilon) Ca2+-ATPase compared with wild-type epsilon. In contrast, epsilon G47L and epsilon G47A increased the Ca2+-ATPase activity of soluble oxidized CF1(-epsilon). The replacement of Gly47 significantly impaired the interaction between the subunit epsilon and gamma in an in vitro binding assay? Further study showed that all epsilon variants were more effective in blocking proton leakage from the thylakoid membranes. This enhanced ATP synthesis of the chloroplast and restored ATP synthesis activity of the reconstituted membranes to a level that was more efficient than that achieved by wild-type epsilon. These results indicate that the conserved Gly47 residue of the epsilon subunit is very important for maintaining the structure and function of the epsilon subunit and may affect the interaction between the epsilon subunit, beta subunit of CF1 and subunit III of CFo, thereby regulating the ATP hydrolysis and synthesis, as well as the proton translocation role of the subunit III of CFo.

Movements of the epsilon-subunit during catalysis and activation in single membrane-bound H(+)-ATP synthase

F0F1-ATP synthases catalyze proton transport-coupled ATP synthesis in bacteria, chloroplasts, and mitochondria. In these complexes, the epsilon-subunit is involved in the catalytic reaction and the activation of the enzyme. Fluorescence-labeled F0F1 from Escherichia coli was incorporated into liposomes. Single-molecule fluorescence resonance energy transfer (FRET) revealed that the epsilon-subunit rotates stepwise showing three distinct distances to the b-subunits in the peripheral stalk. Rotation occurred in opposite directions during ATP synthesis and hydrolysis. Analysis of the dwell times of each FRET state revealed different reactivities of the three catalytic sites that depended on the relative orientation of epsilon during rotation. Proton transport through the enzyme in the absence of nucleotides led to conformational changes of epsilon. When the enzyme was inactive (i.e. in the absence of substrates or without membrane energization), three distances were found again, which differed from those of the active enzyme. The three states of the inactive enzyme were unequally populated. We conclude that the active-inactive transition was associated with a conformational change of epsilon within the central stalk.

N-terminal deletion of the gamma subunit affects the stabilization and activity of chloroplast ATP synthase

Five truncation mutants of chloroplast ATP synthase gamma subunit from spinach (Spinacia oleracea) lacking 8, 12, 16, 20 or 60 N-terminal amino acids were generated by PCR by a mutagenesis method. The recombinant gamma genes were overexpressed in Escherichia coli and assembled with alphabeta subunits into a native complex. The wild-type (WT) alphabetagamma assembly i.e. alphabetagammaWT exhibited high (Mg2+)-dependent and (Ca2+)-dependent ATP hydrolytic activity. Deletions of eight residues of the gamma subunit N-terminus caused a decrease in rates of ATP hydrolysis to 30% of that of the alphabetaWT assembly. Furthermore, only approximately 6% of ATP hydrolytic activity was retained with the sequential deletions of gamma subunit up to 20 residues compared with the activity of the alphabetaWT assembly. The inhibitory effect of the epsilon subunit on ATP hydrolysis of these alphabetagamma assemblies varied to a large extent. These observations indicate that the N-terminus of the gamma subunit is very important, together with other regions of the gamma subunit, in stabilization of the enzyme complex or during cooperative catalysis. In addition, the in vitro binding assay showed that the gamma subunit N-terminus is not a crucial region in binding of the epsilon subunit.

Effects of site-directed mutation on the function of the chloroplast ATP synthase epsilon subunit

The previous work in our lab showed that the spinach chloroplast ATP synthase epsilon mutant with 3 amino acid residues deleted from the N-terminus had much lower ability to inhibit ATP hydrolysis and block proton leakage in comparison to a mutant with 1 or 2 residues deleted from the N-terminus. The present study aimed at determining whether there is special importance in the structure and function of the N-terminal third residue of the chloroplast epsilon subunit. The leucine residue at the N-terminal third site (Leu3) of the spinach chloroplast epsilon subunit was replaced with Ile, Phe, Thr, Arg, Glu or Pro by site-directed mutagenesis, forming mutants epsilonL3I, epsilonL3F, epsilonL3T, epsilonL3R, epsilonL3E and epsilonL3P, respectively. These epsilon variants all showed lower abilities to inhibit ATP hydrolysis and to block proton leakage, as compared to the wild type epsilon subunit (epsilonWT). The abilities of mutants epsilonL3I and epsilonL3F to restore the ATP synthesis activity of reconstituted membranes were higher than those of epsilonWT, but the abilities of the other epsilon variants were lower than that of epsilonWT. These results indicate that the hydrophobic and neutral characteristics of Leu3 of the chloroplast epsilon subunit are very important for its ability to inhibit ATP hydrolysis and block proton leakage, and for the ATP synthesis ability of ATP synthase.

Structural model of the transmembrane Fo rotary sector of H+-transporting ATP synthase derived by solution NMR and intersubunit cross-linking in situ

H(+)-transporting, F(1)F(o)-type ATP synthases utilize a transmembrane H(+) potential to drive ATP formation by a rotary catalytic mechanism. ATP is formed in alternating beta subunits of the extramembranous F(1) sector of the enzyme, synthesis being driven by rotation of the gamma subunit in the center of the F(1) molecule between the alternating catalytic sites. The H(+) electrochemical potential is thought to drive gamma subunit rotation by first coupling H(+) transport to rotation of an oligomeric rotor of c subunits within the transmembrane F(o) sector. The gamma subunit is forced to turn with the c-oligomeric rotor due to connections between subunit c and the gamma and epsilon subunits of F(1). In this essay we will review recent studies on the Escherichia coli F(o) sector. The monomeric structure of subunit c, determined by NMR, shows that subunit c folds in a helical hairpin with the proton carrying Asp(61) centered in the second transmembrane helix (TMH). A model for the structural organization of the c(10) oligomer in F(o) was deduced from extensive cross-linking studies and by molecular modeling. The model indicates that the H(+)-carrying carboxyl of subunit c is occluded between neighboring subunits of the c(10) oligomer and that two c subunits pack in a "front-to-back" manner to form the H(+) (cation) binding site. In order for protons to gain access to Asp(61) during the protonation/deprotonation cycle, we propose that the outer, Asp(61)-bearing TMH-2s of the c-ring and TMHs from subunits composing the inlet and outlet channels must turn relative to each other, and that the swiveling motion associated with Asp(61) protonation/deprotonation drives the rotation of the c-ring. The NMR structures of wild-type subunit c differs according to the protonation state of Asp(61). The idea that the conformational state of subunit c changes during the catalytic cycle is supported by the cross-linking evidence in situ, and two recent NMR structures of functional mutant proteins in which critical residues have been switched between TMH-1 and TMH-2. The structural information is considered in the context of the possible mechanism of rotary movement of the c(10) oligomer during coupled synthesis of ATP.

The role of the betaDELSEED motif of F1-ATPase: propagation of the inhibitory effect of the epsilon subunit

In F(1)-ATPase, a rotary motor enzyme, the region of the conserved DELSEED motif in the beta subunit moves and contacts the rotor gamma subunit when the nucleotide fills the catalytic site, and the acidic nature of the motif was previously assumed to play a critical role in rotation. Our previous work, however, disproved the assumption (Hara, K. Y., Noji, H., Bald, D., Yasuda, R., Kinosita, K., Jr., and Yoshida, M. (2000) J. Biol. Chem. 275, 14260-14263), and the role of this motif remained unknown. Here, we found that the epsilon subunit, an intrinsic inhibitor, was unable to inhibit the ATPase activity of a mutant thermophilic F(1)-ATPase in which all of the five acidic residues in the DELSEED motif were replaced with alanines, although the epsilon subunit in the mutant F(1)-ATPase assumed the inhibitory form. In addition, the replacement of basic residues in the C-terminal region of the epsilon subunit by alanines caused a decrease of the inhibitory effect. Partial replacement of the acidic residues in the DELSEED motif of the beta subunit or of the basic residues in the C-terminal alpha-helix of the epsilon subunit induced a partial effect. We here conclude that the epsilon subunit exerts its inhibitory effect through the electrostatic interaction with the DELSEED motif of the beta subunit.

Rotation of the c subunit oligomer in fully functional F1Fo ATP synthase

The F(1)F(o)-type ATP synthase is the smallest motor enzyme known. Previous studies had established that the central gamma and epsilon subunits of the F(1) part rotate relative to a stator of alpha(3)beta(3) and delta subunits during catalysis. We now show that the ring of c subunits in the F(o) part moves along with the gamma and epsilon subunits. This was demonstrated by linking the three rotor subunits with disulfide bridges between cysteine residues introduced genetically at the interfaces between the gamma, epsilon, and c subunits. Essentially complete cross-linking of the gamma, epsilon, and c subunits was achieved by using CuCl(2) to induce oxidation. This fixing of the three subunits together had no significant effect on ATP hydrolysis, proton translocation, or ATP synthesis, and each of these functions retained inhibitor sensitivity. These results unequivocally place the c subunit oligomer in the rotor part of this molecular machine.

Rotation of the c subunit oligomer in fully functional F1Fo ATP synthase

The F(1)F(o)-type ATP synthase is the smallest motor enzyme known. Previous studies had established that the central gamma and epsilon subunits of the F(1) part rotate relative to a stator of alpha(3)beta(3) and delta subunits during catalysis. We now show that the ring of c subunits in the F(o) part moves along with the gamma and epsilon subunits. This was demonstrated by linking the three rotor subunits with disulfide bridges between cysteine residues introduced genetically at the interfaces between the gamma, epsilon, and c subunits. Essentially complete cross-linking of the gamma, epsilon, and c subunits was achieved by using CuCl(2) to induce oxidation. This fixing of the three subunits together had no significant effect on ATP hydrolysis, proton translocation, or ATP synthesis, and each of these functions retained inhibitor sensitivity. These results unequivocally place the c subunit oligomer in the rotor part of this molecular machine.

The rotary mechanism of ATP synthase

Since the chemiosmotic theory was proposed by Peter Mitchell in the 1960s, a major objective has been to elucidate the mechanism of coupling of the transmembrane proton motive force, created by respiration or photosynthesis, to the synthesis of ATP from ADP and inorganic phosphate. Recently, significant progress has been made towards establishing the complete structure of ATP synthase and revealing its mechanism. The X-ray structure of the F(1) catalytic domain has been completed and an electron density map of the F(1)-c(10) subcomplex has provided a glimpse of the motor in the membrane domain. Direct microscopic observation of rotation has been extended to F(1)-ATPase and F(1)F(o)-ATPase complexes.

Movement of the helical domain of the epsilon subunit is required for the activation of thermophilic F1-ATPase

The inhibitory effect of epsilon subunit in F(1)-ATPase from thermophilic Bacillus PS3 was examined focusing on the structure-function relationship. For this purpose, we designed a mutant for epsilon subunit similar to the one constructed by Schulenberg and Capaldi (Schulenberg, B., and Capaldi, R. A. (1999) J. Biol. Chem. 274, 28351-28355). We introduced two cysteine residues at the interface of N-terminal beta-sandwich domain (S48C) and C-terminal alpha-helical domain (N125C) of epsilon subunit. The alpha(3)beta(3)gammaepsilon complex containing the reduced form of this mutant epsilon subunit showed suppressed ATPase activity and gradual activation during the measurement. This activation pattern was similar to the complex with the wild type epsilon subunit. The conformation of the mutant epsilon subunit must be fixed and similar to the reported three-dimensional structure of the isolated epsilon subunit, when the intramolecular disulfide bridge was formed on this subunit by oxidation. This oxidized mutant epsilon subunit could form the alpha(3)beta(3)gammaepsilon complex but did not show any inhibitory effect. The complex was converted to the activated state, and the cross-link in the mutant epsilon subunit in the complex was efficiently formed in the presence of ATP-Mg, whereas no cross-link was observed without ATP-Mg, suggesting the conformation of the oxidized mutant epsilon subunit must be similar to that in the activated state complex. A non-hydrolyzable analog of ATP, 5'-adenylyl-beta,gamma-imidodiphosphate, could stimulate the formation of the cross-link on the epsilon subunit. Furthermore, the cross-link formation was stimulated by nucleotides even when this mutant epsilon subunit was assembled with a mutant alpha(3)beta(3)gamma complex lacking non-catalytic sites. These results indicate that binding of ATP to the catalytic sites induces a conformational change in the epsilon subunit and triggers transition of the complex from the suppressed state to the activated state.

Mitochondrial F(0)F(1) ATP synthase. Subunit regions on the F1 motor shielded by F(0), Functional significance, and evidence for an involvement of the unique F(0) subunit F(6)

Studies reported here were undertaken to gain greater molecular insight into the complex structure of mitochondrial ATP synthase (F(0)F(1)) and its relationship to the enzyme's function and motor-related properties. Significantly, these studies, which employed N-terminal sequence, mass spectral, proteolytic, immunological, and functional analyses, led to the following novel findings. First, at the top of F(1) within F(0)F(1), all six N-terminal regions derived from alpha + beta subunits are shielded, indicating that one or more F(0) subunits forms a "cap." Second, at the bottom of F(1) within F(0)F(1), the N-terminal region of the single delta subunit and the C-terminal regions of all three alpha subunits are shielded also by F(0). Third, and in contrast, part of the gamma subunit located at the bottom of F(1) is already shielded in F(1), indicating that there is a preferential propensity for interaction with other F(1) subunits, most likely delta and epsilon. Fourth, and consistent with the first two conclusions above that specific regions at the top and bottom of F(1) are shielded by F(0), further proteolytic shaving of alpha and beta subunits at these locations eliminates the capacity of F(1) to couple a proton gradient to ATP synthesis. Finally, evidence was obtained that the F(0) subunit called "F(6)," unique to animal ATP synthases, is involved in shielding F(1). The significance of the studies reported here, in relation to current views about ATP synthase structure and function in animal mitochondria, is discussed.

Insights into the rotary catalytic mechanism of F0F1 ATP synthase from the cross-linking of subunits b and c in the Escherichia coli enzyme

The transmembrane sector of the F(0)F(1) rotary ATP synthase is proposed to organize with an oligomeric ring of c subunits, which function as a rotor, interacting with two b subunits at the periphery of the ring, the b subunits functioning as a stator. In this study, cysteines were introduced into the C-terminal region of subunit c and the N-terminal region of subunit b. Cys of N2C subunit b was cross-linked with Cys at positions 74, 75, and 78 of subunit c. In each case, a maximum of 50% of the b subunit could be cross-linked to subunit c, which suggests that either only one of the two b subunits lie adjacent to the c-ring or that both b subunits interact with a single subunit c. The results support a topological arrangement of these subunits, in which the respective N- and C-terminal ends of subunits b and c extend to the periplasmic surface of the membrane and cAsp-61 lies at the center of the membrane. The cross-linking of Cys between bN2C and cV78C was shown to inhibit ATP-driven proton pumping, as would be predicted from a rotary model for ATP synthase function, but unexpectedly, cross-linking did not lead to inhibition of ATPase activity. ATP hydrolysis and proton pumping are therefore uncoupled in the cross-linked enzyme. The c subunit lying adjacent to subunit b was shown to be mobile and to exchange with c subunits that initially occupied non-neighboring positions. The movement or exchange of subunits at the position adjacent to subunit b was blocked by dicyclohexylcarbodiimide. These experiments provide a biochemical verification that the oligomeric c-ring can move with respect to the b-stator and provide further support for a rotary catalytic mechanism in the ATP synthase.

Biological nano motor, ATP synthase F(o)F(1): from catalysis to gammaepsilonc(10-12) subunit assembly rotation

Proton translocating ATPase (ATP synthase), a chemiosmotic enzyme, synthesizes ATP from ADP and phosphate coupling with the electrochemical ion gradient across the membrane. This enzyme has been studied extensively by combined genetic, biochemical and biophysical approaches. Such studies revealed a unique mechanism which transforms an electrochemical ion gradient into chemical energy through the rotation of a subunit assembly. Thus, this enzyme can be defined as a nano motor capable of coupling a chemical reaction and ion translocation, or more simply, as a protein complex carrying out rotational catalysis. In this article, we briefly discuss our recent work, emphasizing the rotation of subunit assembly (gammaepsilonc(10-12)) which is formed from peripheral and intrinsic membrane subunits.

Partial assembly of the yeast mitochondrial ATP synthase

The mitochondrial ATP synthase is a molecular motor that drives the phosphorylation of ADP to ATP. The yeast mitochondrial ATP synthase is composed of at least 19 different peptides, which comprise the F1 catalytic domain, the F0 proton pore, and two stalks, one of which is thought to act as a stator to link and hold F1 to F0, and the other as a rotor. Genetic studies using yeast Saccharomyces cerevisiae have suggested the hypothesis that the yeast mitochondrial ATP synthase can be assembled in the absence of 1, and even 2, of the polypeptides that are thought to comprise the rotor. However, the enzyme complex assembled in the absence of the rotor is thought to be uncoupled, allowing protons to freely flow through F0 into the mitochondrial matrix. Left uncontrolled, this is a lethal process and the cell must eliminate this leak if it is to survive. In yeast, the cell is thought to lose or delete its mitochondrial DNA (the petite mutation) thereby eliminating the genes encoding essential components of F0. Recent biochemical studies in yeast, and prior studies in E. coli, have provided support for the assembly of a partial ATP synthase in which the ATP synthase is no longer coupled to proton translocation.

Subunit organization of the stator part of the F0 complex from Escherichia coli ATP synthase

Membrane-bound ATP synthases (F1F0) catalyze the synthesis of ATP via a rotary catalytic mechanism utilizing the energy of an electrochemical ion gradient. The transmembrane potential is supposed to propel rotation of a subunit c ring of F0 together with subunits gamma and epsilon of F1, thereby forming the rotor part of the enzyme, whereas the remainder of the F1F0 complex functions as a stator for compensation of the torque generated during rotation. This review focuses on our recent work on the stator part of the F0 complex, e.g., subunits a and b. Using epitope insertion and antibody binding, subunit a was shown to comprise six transmembrane helixes with both the N- and C-terminus oriented toward the cytoplasm. By use of circular dichroism (CD) spectroscopy, the secondary structure of subunit b incorporated into proteoliposomes was determined to be 80% alpha-helical together with 14% beta turn conformation, providing flexibility to the second stalk. Reconstituted subunit b together with isolated ac subcomplex was shown to be active in proton translocation and functional F1 binding revealing the native conformation of the polypeptide chain. Chemical crosslinking in everted membrane vesicles led to the formation of subunit b homodimers around residues bQ37 to bL65, whereas bA32C could be crosslinked to subunit a, indicating a close proximity of subunits a and b near the membrane. Further evidence for the proposed direct interaction between subunits a and b was obtained by purification of a stable ab2 subcomplex via affinity chromatography using His tags fused to subunit a or b. This ab2 subcomplex was shown to be active in proton translocation and F1 binding, when coreconstituted with subunit c. Consequences of crosslink formation and subunit interaction within the F1F0 complex are discussed.

Structural and functional features of the Escherichia coli F1-ATPase

The structural organization and overall dimensions of the Escherichia coli F1-ATPase in solution has been analyzed by synchroton X-ray scattering. Using an independent ab initio approach, the low-resolution shape of the hydrated enzyme was determined at 3.2 nm resolution. The shape permitted unequivocal identification of the volume occupied by the alpha3beta3gamma complex of the atomic model of the ECF1-ATPase. The position of the delta and epsilon subunits were found by interactive fitting of the solution scattering data and by cross-linking studies. Laser-induced covalent incorporation of 2-azido-ATP established a direct relationship between nucleotide binding affinity and the different interactions between the stalk subunits gamma and epsilon with the three catalytic subunits (beta) of the F1-ATPase. Mutants of the ECF1-ATPase with the introduction of Trp-for-Tyr replacement in the catalytic site of the complex made it possible to monitor the activated state for ATP synthesis (ATP conformation) in which the gamma and epsilon subunits are in close proximity to the alpha subunits and the ADP conformation, with the stalk subunits are linked to the beta subunit.

Molecular mechanisms of rotational catalysis in the F(0)F(1) ATP synthase

Rotation of the F(0)F(1) ATP synthase gamma subunit drives each of the three catalytic sites through their reaction pathways. The enzyme completes three cycles and synthesizes or hydrolyzes three ATP for each 360 degrees rotation of the gamma subunit. Mutagenesis studies have yielded considerable information on the roles of interactions between the rotor gamma subunit and the catalytic beta subunits. Amino acid substitutions, such as replacement of the conserved gammaMet-23 by Lys, cause altered interactions between gamma and beta subunits that have dramatic effects on the transition state of the steady state ATP synthesis and hydrolysis reactions. The mutations also perturb transmission of specific conformational information between subunits which is important for efficient conversion of energy between rotation and catalysis, and render the coupling between catalysis and transport inefficient. Amino acid replacements in the transport domain also affect the steady state catalytic transition state indicating that rotation is involved in coupling to transport.

The epsilon subunit of bacterial and chloroplast F(1)F(0) ATPases. Structure, arrangement, and role of the epsilon subunit in energy coupling within the complex

Recent studies show that the epsilon subunit of bacterial and chloroplast F(1)F(0) ATPases is a component of the central stalk that links the F(1) and F(0) parts. This subunit interacts with alpha, beta and gamma subunits of F(1) and the c subunit ring of F(0). Along with the gamma subunit, epsilon is a part of the rotor that couples events at the three catalytic sites sequentially with proton translocation through the F(0) part. Structural data on the epsilon subunit when separated from the complex and in situ are reviewed, and the functioning of this polypeptide in coupling within the ATP synthase is considered.

Important subunit interactions in the chloroplast ATP synthase

General structural features of the chloroplast ATP synthase are summarized highlighting differences between the chloroplast enzyme and other ATP synthases. Much of the review is focused on the important interactions between the epsilon and gamma subunits of the chloroplast coupling factor 1 (CF(1)) which are involved in regulating the ATP hydrolytic activity of the enzyme and also in transferring energy from the membrane segment, chloroplast coupling factor 0 (CF(0)), to the catalytic sites on CF(1). A simple model is presented which summarizes properties of three known states of activation of the membrane-bound form of CF(1). The three states can be explained in terms of three different bound conformational states of the epsilon subunit. One of the three states, the fully active state, is only found in the membrane-bound form of CF(1). The lack of this state in the isolated form of CF(1), together with the confirmed presence of permanent asymmetry among the alpha, beta and gamma subunits of isolated CF(1), indicate that ATP hydrolysis by isolated CF(1) may involve only two of the three potential catalytic sites on the enzyme. Thus isolated CF(1) may be different from other F(1) enzymes in that it only operates on 'two cylinders' whereby the gamma subunit does not rotate through a full 360 degrees during the catalytic cycle. On the membrane in the presence of a light-induced proton gradient the enzyme assumes a conformation which may involve all three catalytic sites and a full 360 degrees rotation of gamma during catalysis.

The role of the DELSEED motif of the beta subunit in rotation of F1-ATPase

F(1)-ATPase is a rotary motor protein, and ATP hydrolysis generates torque at the interface between the gamma subunit, a rotor shaft, and the alpha(3)beta(3) substructure, a stator ring. The region of conserved acidic "DELSEED" motif of the beta subunit has a contact with gamma subunit and has been assumed to be involved in torque generation. Using the thermophilic alpha(3)beta(3)gamma complex in which the corresponding sequence is DELSDED, we replaced each residue and all five acidic residues in this sequence with alanine. In addition, each of two conserved residues at the counterpart contact position of gamma subunit was also replaced. Surprisingly, all of these mutants rotated with as much torque as the wild-type. We conclude that side chains of the DELSEED motif of the beta subunit do not have a direct role in torque generation.

ATPase activity of a highly stable alpha(3)beta(3)gamma subcomplex of thermophilic F(1) can be regulated by the introduced regulatory region of gamma subunit of chloroplast F(1)

A mutant F(1)-ATPase alpha(3)beta(3)gamma subcomplex from the thermophilic Bacillus PS3 was constructed, in which 111 amino acid residues (Val(92) to Phe(202)) from the central region of the gamma subunit were replaced by the 148 amino acid residues of the homologous region from spinach chloroplast F(1)-ATPase gamma subunit, including the regulatory stretch, and were designated as alpha(3)beta(3)gamma((TCT)) (Thermophilic-Chloroplast-Thermophilic). By the insertion of this regulatory region into the gamma subunit of thermophilic F(1), we could confer the thiol modulation property to the thermophilic alpha(3)beta(3)gamma subcomplex. The overexpressed alpha(3)beta(3)gamma((TCT)) was easily purified in large scale, and the ATP hydrolyzing activity of the obtained complex was shown to increase up to 3-fold upon treatment with chloroplast thioredoxin-f and dithiothreitol. No loss of thermostability compared with the wild type subcomplex was found, and activation by dithiothreitol was functional at temperatures up to 80 degrees C. alpha(3)beta(3)gamma((TCT)) was inhibited by the epsilon subunit from chloroplast F(1)-ATPase but not by the one from the thermophilic F(1)-ATPase, indicating that the introduced amino acid residues from chloroplast F(1)-gamma subunit are important for functional interaction with the epsilon subunit.

F-ATPase: specific observation of the rotating c subunit oligomer of EF(o)EF(1)

The rotary motion in response to ATP hydrolysis of the ring of c subunits of the membrane portion, F(o), of ATP synthase, F(o)F(1), is still under contention. It was studied with EF(o)EF(1) (Escherichia coli) using microvideography with a fluorescent actin filament. To overcome the limited specificity of actin attachment through a Cys-maleimide couple which might have hampered the interpretation of previous work, we engineered a 'strep-tag' sequence into the C-terminal end of subunit c. It served (a) to purify the holoenzyme and (b) to monospecifically attach a fluorescent actin filament to subunit c. EF(o)EF(1) was immobilized on a Ni-NTA-coated glass slide by the engineered His-tag at the N-terminus of subunit beta. In the presence of MgATP we observed up to five counterclockwise rotating actin filaments per picture frame of 2000 microm(2) size, in some cases yielding a proportion of 5% rotating over total filaments. The rotation was unequivocally attributable to the ring of subunit c. The new, doubly engineered construct serves as a firmer basis for ongoing studies on torque and angular elastic distortions between F(1) and F(o).

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.

The gammaepsilon-c subunit interface in the ATP synthase of Escherichia coli. cross-linking of the epsilon subunit to the c subunit ring does not impair enzyme function, that of gamma to c subunits leads to uncoupling

Mutants with a cysteine residue in the gamma subunit at position 207 and the epsilon subunit at position 31 were expressed in combination with a c-dimer construct, which contains a single cysteine at position 42 of the second c subunit. These mutants are called gammaY207C/cc'Q42C and epsilonE31C/cc'Q42C, respectively. Cross-linking of epsilon to the c subunit ring was obtained almost to completion without significant effect on any enzyme function, i.e. ATP hydrolysis, ATP synthesis, and ATP hydrolysis-driven proton translocation were all close to that of wild type. The gamma subunit could also be linked to the c subunit ring in more than 90% yield, but this affected coupling. Thus, ATP hydrolysis was increased 2. 5-fold, ATP synthesis was dramatically decreased, and ATP hydrolysis-driven proton translocation was abolished, as measured by the 9-amino-6-chloro-2-methoxyacridinequenching method. These results for epsilonE31C/cc'Q42C indicate that the c subunit ring rotates with the central stalk element. That the gamma-epsilon cross-linked enzyme retains ATPase activity also argues for a gammaepsilon-c subunit rotor. However, the uncoupling induced by cross-linking of gamma to the c subunit ring points to important conformational changes taking place in the gammaepsilon-c subunit interface during this. Blocking these structural changes by cross-linking leads to a proton leak within the F(0).

Epsilon subunit, an endogenous inhibitor of bacterial F(1)-ATPase, also inhibits F(0)F(1)-ATPase

Since the report by Sternweis and Smith (Sternweis, P. C., and Smith, J. B. (1980) Biochemistry 19, 526-531), the epsilon subunit, an endogenous inhibitor of bacterial F(1)-ATPase, has long been thought not to inhibit activity of the holo-enzyme, F(0)F(1)-ATPase. However, we report here that the epsilon subunit is exerting inhibition in F(0)F(1)-ATPase. We prepared a C-terminal half-truncated epsilon subunit (epsilon(DeltaC)) of the thermophilic Bacillus PS3 F(0)F(1)-ATPase and reconstituted F(1)- and F(0)F(1)-ATPase containing epsilon(DeltaC). Compared with F(1)- and F(0)F(1)-ATPase containing intact epsilon, those containing epsilon(DeltaC) showed uninhibited activity; severalfold higher rate of ATP hydrolysis at low ATP concentration and the start of ATP hydrolysis without an initial lag at high ATP concentration. The F(0)F(1)-ATPase containing epsilon(DeltaC) was capable of ATP-driven H(+) pumping. The time-course of pumping at low ATP concentration was faster than that by the F(0)F(1)-ATPase containing intact epsilon. Thus, the comparison with noninhibitory epsilon(DeltaC) mutant shed light on the inhibitory role of the intact epsilon subunit in F(0)F(1)-ATPase.

Structural features of the gamma subunit of the Escherichia coli F(1) ATPase revealed by a 4.4-A resolution map obtained by x-ray crystallography

The F(1) part of the F(1)F(O) ATP synthase from Escherichia coli has been crystallized and its structure determined to 4.4-A resolution by using molecular replacement based on the structure of the beef-heart mitochondrial enzyme. The bacterial F(1) consists of five subunits with stoichiometry alpha(3), beta(3), gamma, delta, and epsilon. delta was removed before crystallization. In agreement with the structure of the beef-heart mitochondrial enzyme, although not that from rat liver, the present study suggests that the alpha and beta subunits are arranged in a hexagonal barrel but depart from exact 3-fold symmetry. In the structures of both beef heart and rat-liver mitochondrial F(1), less than half of the structure of the gamma subunit was seen because of presumed disorder in the crystals. The present electron-density map includes a number of rod-shaped features which appear to correspond to additional alpha-helical regions within the gamma subunit. These suggest that the gamma subunit traverses the full length of the stalk that links the F(1) and F(O) parts and makes significant contacts with the c subunit ring of F(O).

The epsilon subunit of the F(1)F(0) complex of Escherichia coli. cross-linking studies show the same structure in situ as when isolated

Four double mutants in the epsilon subunit were generated, each containing two cysteines, which, based on the NMR structure of this subunit, should form internal disulfide bonds. Two of these were designed to generate interdomain cross-links that lock the C-terminal alpha-helical domain against the beta-sandwich (epsilonM49C/A126C and epsilonF61C/V130C). The second set should give cross-linking between the two C-terminal alpha-helices (epsilonA94C/L128C and epsilonA101C/L121C). All four mutants cross-linked with 90-100% efficiency upon CuCl(2) treatment in isolated Escherichia coli ATP synthase. This shows that the structure obtained for isolated epsilon is essentially the same as in the assembled complex. Functional studies revealed increased ATP hydrolysis after cross-linking between the two domains of the subunit but not after cross-linking between the C-terminal alpha-helices. None of the cross-links had any effect on proton pumping-coupled ATP hydrolysis, on DCCD sensitivity of this activity, or on ATP synthesis rates. Therefore, big conformational changes within epsilon can be ruled out as a part of the enzyme function. Protease digestion studies, however, showed that subtle changes do occur, since the epsilon subunit could be locked in an ADP or 5'-adenylyl-beta,gamma-imidodiphosphate conformation by the cross-linking with resulting differences in cleavage rates.

The dimerization domain of the b subunit of the Escherichia coli F(1)F(0)-ATPase

In this study a series of N- and/or C-terminal truncations of the cytoplasmic domain of the b subunit of the Escherichia coli F(1)F(0) ATP synthase were tested for their ability to form dimers using sedimentation equilibrium ultracentrifugation. The deletion of residues between positions 53 and 122 resulted in a strongly decreased tendency to form dimers, whereas all the polypeptides that included that sequence exhibited high levels of dimer formation. b dimers existed in a reversible monomer-dimer equilibrium and when mixed with other b truncations formed heterodimers efficiently, provided both constructs included the 53-122 sequence. Sedimentation velocity and (15)N NMR relaxation measurements indicated that the dimerization region is highly extended in solution, consistent with an elongated second stalk structure. A cysteine introduced at position 105 was found to readily form intersubunit disulfides, whereas other single cysteines at positions 103-110 failed to form disulfides either with the identical mutant or when mixed with the other 103-110 cysteine mutants. These studies establish that the b subunit dimer depends on interactions that occur between residues in the 53-122 sequence and that the two subunits are oriented in a highly specific manner at the dimer interface.

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.

Mutation of the mitochrondrially encoded ATPase 6 gene modeled in the ATP synthase of Escherichia coli

Defects of respiratory chain protein complexes and the ATP synthase are becoming increasingly implicated in human disease. Recently, mutations in the ATPase 6 gene have been shown to cause several different neurological disorders. The product of this gene is homologous to the a subunit of the ATP synthase of Escherichia coli. Here, mutations equivalent to those described in humans have been introduced into the a subunit of E. coli by site-directed mutagenesis, and the effects of these mutations on the ATPase activity, ATP synthesis and ability of the enzyme to pump protons studied in detail. The effects of the mutations varied considerably. The mutation L262P (9185 T-C equivalent) caused a 70% loss of ATP synthesis activity, reduced DCCD sensitivity, and lowered proton pumping activity. The L207P (8993 T-C equivalent) reduced ATP synthesis by 50%, affected DCCD sensitivity, while proton pumping was only marginally affected when measured by the standard AMCA quenching assay. The other mutations studied affected the functioning of the ATP synthase much less. The results confirm that modeling of these point mutations in the E. coli enzyme is a useful approach to determining how alterations in the ATPase 6 gene affect enzyme function and, therefore, how a pathogenic effect can be exerted.

Relevance of divalent cations to ATP-driven proton pumping in beef heart mitochondrial F0F1-ATPase

The ATP hydrolysis rate and the ATP hydrolysis-linked proton translocation by the F0F1-ATPase of beef heart submitochondrial particles were examined in the presence of several divalent metal cations. All Me-ATP complexes tested sustained ATP hydrolysis, although to a different extent. However, only Mg- and Mn-ATP-dependent hydrolysis could sustain a high level of proton pumping activity, as determined by acridine fluorescence quenching. Moreover, the Km of the Me-ATP hydrolysis-induced proton pumping activity was very similar to the Km value of Me-ATP hydrolysis. Both oligomycin and DCCD caused the full recovery of the fluorescence, providing clear evidence for the association of Mg-ATP hydrolysis with proton translocation through the F0F1-ATPase complex. In contrast, with other Me-ATP complexes, including Ca-ATP as substrate, the proton pumping activity was undetectable, implicating an uncoupling nature for these substrates. Attempts to demonstrate the involvement of the epsilon subunit of the enzyme in the coupling mechanism failed, suggesting that the participation of at least the N-terminal segment of the subunit in the coupling mechanism of the mitochondrial enzyme is unlikely.

Solution structure of the epsilon subunit of the F1-ATPase from Escherichia coli and interactions of this subunit with beta subunits in the complex

The solution structure of the epsilon subunit of the Escherichia coli F1-ATPase has been determined by NMR spectroscopy. This subunit has a two-domain structure with an N-terminal 10-stranded beta sandwich and a C-terminal antiparallel two alpha-helix hairpin, as described previously (Wilkens, S., Dahlquist, F. W., McIntosh, L. P., Donaldson, L. W., and Capaldi, R. A. (1995) Nat. Struct. Biol. 2, 961-967). New data show that the two domains interact in solution in an interface formed by beta strand 7 and the very C-terminal alpha-helix. This interface involves only hydrophobic interactions. The dynamics of the epsilon subunit in solution were examined. The two domains are relatively tightly associated with little or no flexibility relative to one another. The epsilon subunit can exist in two states in the ECF1F0 complex depending on whether ATP or ADP occupies catalytic sites. Proteolysis of the epsilon subunit in solution and when bound to the core F1 complex indicates that the conformation of the polypeptide in solution closely resembles the conformation of epsilon when bound to the F1 in the ADP state. Chemical and photo-cross-linking show that the epsilon subunit spans and interacts with two beta subunits in the ADP state. These interactions are disrupted on binding of ATP + Mg2+, as is the interaction between the N- and C-terminal domains of the epsilon subunit.

Direct observation of the rotation of epsilon subunit in F1-ATPase

Rotation of the epsilon subunit in F1-ATPase from thermophilic Bacillus strain PS3 (TF1) was observed under a fluorescence microscope by the method used for observation of the gamma subunit rotation (Noji, H., Yasuda, R., Yoshida, M., and Kinosita, K., Jr. (1997) Nature 386, 299-302). The alpha3 beta3 gamma epsilon complex of TF1 was fixed to a solid surface, and fluorescently labeled actin filament was attached to the epsilon subunit through biotin-streptavidin. In the presence of ATP, the filament attached to epsilon subunit rotated in a unidirection. The direction of the rotation was the same as that observed for the gamma subunit. The rotational velocity was slightly slower than the filament attached to the gamma subunit, probably due to the experimental setup used. Thus, as suggested from biochemical studies (Aggeler, R., Ogilvie, I. , and Capaldi, R. A. (1997) J. Biol. Chem. 272, 19621-19624), the epsilon subunit rotates with the gamma subunit in F1-ATPase during catalysis.

Membrane topology of subunit a of the F1F0 ATP synthase as determined by labeling of unique cysteine residues

The membrane topology of the a subunit of the F1F0 ATP synthase from Escherichia coli has been probed by surface labeling using 3-(N-maleimidylpropionyl) biocytin. Subunit a has no naturally occurring cysteine residues, allowing unique cysteines to be introduced at the following positions: 8, 24, 27, 69, 89, 128, 131, 172, 176, 196, 238, 241, and 277 (following the COOH-terminal 271 and a hexahistidine tag). None of the single mutations affected the function of the enzyme, as judged by growth on succinate minimal medium. Membrane vesicles with an exposed cytoplasmic surface were prepared using a French pressure cell. Before labeling, the membranes were incubated with or without a highly charged sulfhydryl reagent, 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid. After labeling with the less polar biotin maleimide, the samples were solubilized with octyl glucoside/cholate and the subunit a was purified via the oligohistidine at its COOH terminus using immobilized nickel chromatography. The purified samples were electrophoresed and transferred to nitrocellulose for detection by avidin conjugated to alkaline phosphatase. Results indicated cytoplasmic accessibility for residues 69, 172, 176, and 277 and periplasmic accessibility for residues 8, 24, 27, and 131. On the basis of these and earlier results, a transmembrane topology for the subunit a is proposed.

The formation or the reduction of a disulfide bridge on the gamma subunit of chloroplast ATP synthase affects the inhibitory effect of the epsilon subunit

We have studied the change of the catalytic activity of chimeric complexes that were formed by chloroplast coupling factor 1 (CF1) -gamma, alpha and beta subunits of thermophilic bacterial F1 after formation or reduction of the disulfide bridge of different gamma subunits modified by oligonucleotide-directed mutagenesis techniques. For this purpose, three mutant gamma subunits were produced: gamma Delta194-230, here 37 amino acids from Pro-194 to Ile-230 are deleted, gammaC199A, Cys-199 is changed to Ala, and gamma Delta200-204, amino acids from Asp-200 to Lys-204 are deleted. All of the chimeric subunit complexes produced from each of these mutant CF1-gamma subunits and alpha and beta subunits from thermophilic bacterial F1 lost the sensitivity against thiol reagents when compared with the complex containing wild-type CF1-gamma. The pH optimum (pH 8.5-9.0) and the concentration of methanol to stimulate ATPase activities were not affected by these mutations. These indicate that the introduction of the mutations did not change the main features of ATPase activity of the chimeric complex. However, the interaction between gamma subunit and epsilon subunit was strongly influenced by the type of gamma subunit itself. Although the ATPase activity of the chimeric complex that contained gamma Delta200-204 or gammaC199A was inhibited by the addition of recombinant epsilon subunit from CF1 similarly to complexes containing the reduced wild-type gamma subunit, the recombinant epsilon subunit did not inhibit the ATPase of the complex, which contained the oxidized form of gamma subunit. Therefore the affinity of the epsilon subunit to the gamma subunit may be dependent on the state of the gamma subunit or the epsilon subunit may bind to the oxidized form of gamma subunit in a mode that does not inhibit the activity. The ATPase activity of the complex that contains gamma Delta194-230 was not efficiently inhibited by epsilon subunit. These results show that the formation or reduction of the disulfide bond on the gamma subunit may induce a conformational change in the region that directly affects the interaction of this subunit with the adjacent epsilon subunit.

ATP synthase--past and future

This paper gives an overview of a lecture scheduled for the opening of the 10th European Bioenergetics Congress. In this lecture I plan to first reflect on the accomplishments of some of the individuals who were involved in research on the ATP synthase during the past 50 years. Then I will give a brief view of the present information about rotational catalysis by the ATP synthase. This will be followed by a discussion of some results from my laboratory that call for additional experimentation. Finally I will direct attention to other questions about the ATP synthase that should be addressed in future studies.

Subunit interactions of Escherichia coli F1-ATPase: mutants of the gamma subunits defective in interaction with the epsilon subunit isolated by the yeast two-hybrid system

Previously, we established a method to detect subunit interactions of F1-ATPase by the yeast two-hybrid system (Moritani, C., et al. Biochim. Biophys. Acta 1274, 67-72, 1996). Here, we isolated mutants of the gamma subunits defective in interaction with the epsilon subunit by this new procedure to study the molecular basis of coupling mechanisms of the F1F0-ATPase. Based on the intensities of the reporter gene expression in this system, five mutants of the gamma subunit with different levels of gamma-epsilon interactions were isolated and their single base substitutions were determined. Mutants with a substitution of Pro-55 for Leu, Thr-102 for Met, Val-141 for Asp, or Gln-235 for Leu exhibited decreased reporter gene expression, suggesting decreased levels of interaction, while Asp-85 for Gly mutation caused a higher level of expression, suggesting increased interaction. Among these point mutations, G85D, M102T, or D141V mutations were introduced into the gamma subunit gene in the plasmid carrying whole unc operon. Transformants carrying a deletion mutant of the whole unc operon with these expression plasmids were analyzed. Mutations M102T and D141V with decreased gamma-epsilon interaction caused increases of membrane-bound F1-ATPase activity and proton pumping activity, while G85D with increased gamma-epsilon interaction exhibited lower levels of F1-ATPase activity in the membranes. Molecular assembly of the F1 subunits on the mutant membranes detected by Western blotting exhibited no defect for all three mutants. These results suggested that the correlation between the ATPase activity and gamma-epsilon interaction is reciprocal and this interaction may regulate the ATPase activity. The topological and functional importance of Gly-85, Met-102, and Asp-141 together with Leu-55 and Leu-235 in gamma-epsilon interaction is discussed.

ATP synthase: an electrochemical transducer with rotatory mechanics

ATP synthase (F0F1-ATPase) uses proton- or sodium-motive force to produce ATP form ADP and P(i). Three lines of experiment have recently demonstrated large-scale intersubunit rotation during ATP hydrolysis by F1. We discuss how ion flow through the membrane-intrinsic portion, F0, may generate torque and how this might be transmitted between stator and rotor to finally expel spontaneously formed ATP from F1 into water.

Thermophilic F1-ATPase is activated without dissociation of an endogenous inhibitor, epsilon subunit

Subunit complexes (alpha3beta3gamma, alpha3beta3gammadelta, alpha3beta3gammaepsilon, and alpha3beta3gammadeltaepsilon) of thermophilic F1-ATPase were prepared, and their catalytic properties were compared to know the role of delta and epsilon subunits in catalysis. The presence of delta subunit in the complexes had slight inhibitory effect on the ATPase activity. The effect of epsilon subunit was more profound. The (-epsilon) complexes, alpha3beta3gamma and alpha3beta3gammadelta, initiated ATP hydrolysis without a lag. In contrast, the (+epsilon) complexes, alpha3beta3gammaepsilon and alpha3beta3gammadeltaepsilon, started hydrolysis of ATP (<700 microM) with a lag phase that was gradually activated during catalytic turnover. As ATP concentration increased, the lag phase of the (+epsilon) complexes became shorter, and it was not observed above 1 mM ATP. Analysis of binding and hydrolysis of the ATP analog, 2',3'-O-(2,4,6-trinitrophenyl)-ATP, suggested that the (+epsilon) complexes bound substrate only slowly. Differing from Escherichia coli F1-ATPase, the activation of the (+epsilon) complexes from the lag phase was not due to dissociation of epsilon subunit since the re-isolated activated complex retained epsilon subunit. This indicates that there are two alternative forms of the (+epsilon) complex, inhibited form and activated form, and the inhibited one is converted to the activated one during catalytic turnover.

Structure and arrangement of the delta subunit in the E. coli ATP synthase (ECF1F0)

F1F0 type ATPases are made up of two parts, an F1, which contains three catalytic sites on beta subunits, and an F0 which contains the proton channel. These two domains have been visualized in electron microscopy as linked by a narrow stalk of around 45 A in length. Biochemical studies have provided clear evidence that the gamma and epsilon subunits are components of this stalk. There is an emerging consensus that the gamma and epsilon subunits rotate relative to the alpha 3 beta 3 domain as part of the cooperativity and energy coupling within the complex. Two other subunits are required to link the F1 to F0 in the E. coli enzyme, and these are the delta and b subunits. The structure of a major part of the delta subunit (residues 1-134) has now been obtained by NMR spectroscopy. The main feature is a six alpha-helix bundle, which provides the N-terminal domain of the delta subunit. This domain interacts with the F1 core via the N-terminal part of the alpha subunit. The C-terminal domain of delta is less well defined. This part is required for binding to the F0 part by direct interaction with the b subunits. It is argued that delta and the two copies of the b subunit are components of a second stalk linking the F1 and F0 parts, which acts as a stator to allow the energy-linked rotational movements of delta and epsilon subunits.

Cross-linking of chloroplast F0F1-ATPase subunit epsilon to gamma without effect on activity. Epsilon and gamma are parts of the rotor

Cys residues were directed into positions 17, 28, 41 and 85 of a Cys6-->Ser mutant of subunit epsilon of spinach chloroplast F0F1 ATP synthase. Wild-type and engineered epsilon were expressed in Escherichia coli, purified in the presence of urea, refolded and reassembled with spinach chloroplast F1 lacking the epsilon subunit [F1(-epsilon)]. Cys-containing epsilon variants were modified with a sulfhydryl-reactive photolabile cross-linker. Photocross-linking of epsilon to F1(-epsilon) yielded the same SDS gel pattern of cross-link products independent of the presence or absence of Mg2+ x ADP, phosphate and Mg2+ x ATP. Epsilon (wild type) [Ser6,Cys28]epsilon and [Ser6,Cys41]epsilon were cross-linked with subunit gamma. With chloroplast F0F1 the same cross-link pattern was obtained, except for one extra cross-link, probably between [Ser6,Cys28]epsilon and F0 subunit III. [Ser6,Cys17]epsilon and [Ser6,Cys85]epsilon did not produce cross-links. Cross-linking of epsilon, [Ser6,Cys28]epsilon, [Ser6,Cys41]epsilon to gamma in soluble chloroplast F1 impaired the ability of epsilon to inhibit Ca2+-ATPase activity. The Mg2+-ATPase activity of soluble F1 (measured in the presence of 30% MeOH) was not affected by cross-linking epsilon with gamma. Functional reconstitution of photophosphorylation in F1-depleted thylakoids was observed with F1 in which gamma was cross-linked to [Ser6,Cys28]epsilon or [Ser6,Cys41]epsilon but not with wild-type epsilon. In view of the intersubunit rotation of gamma relative to (alphabeta)3, which is driven by ATP hydrolysis, gamma and epsilon would seem to act concertedly as parts of the 'rotor' relative to the 'stator' (alphabeta)3.

ATP synthase: a tentative structural model

Adenosine triphosphate (ATP) synthase produces ATP from ADP and inorganic phosphate at the expense of proton- or sodium-motive force across the respective coupling membrane in Archaea, Bacteria and Eucarya. Cation flow through the intrinsic membrane portion of this enzyme (Fo, subunits ab2c9-12) and substrate turnover in the headpiece (F1, subunits alpha3beta3 gammadeltaepsilon) are mechanically coupled by the rotation of subunit gamma in the center of the catalytic hexagon of subunits (alphabeta)3 in F1. ATP synthase is the smallest rotatory engine in nature. With respect to the headpiece alone, it probably operates with three steps. Partial structures of six out of its at least eight different subunits have been published and a 3-dimensional structure is available for the assembly (alphabeta)3gamma. In this article, we review the available structural data and build a tentative topological model of the holoenzyme. The rotor portion is proposed to consist of a wheel of at least nine copies of subunits c, epsilon and a portion of gamma as a spoke, and another portion of gamma as a crankshaft. The stator is made up from a, the transmembrane portion of b2, delta and the catalytic hexagon of (alphabeta)3. As an educated guess, the model may be of heuristic value for ongoing studies on this fascinating electrochemical-to-mechanical-to-chemical transducer.