Transmembrane topology of Escherichia coli H(+)-ATPase (ATP synthase) subunit a

Preparation of Uniformly Oriented Inverted Inner (Cytoplasmic) Membrane Vesicles from Gram-Negative Bacterial Cells

The complex double-membrane organization of the envelope in Gram-negative bacteria places unique biosynthetic and topological constraints that can affect translocation of lipids and proteins synthesized on cytoplasm facing leaflet of cytoplasmic (inner) membrane (IM), across IM and between IM and outer membrane (OM). Uniformly oriented inside-out (ISO) vesicles became functional requisite for many biochemical reconstitution functional assays, vectorial proteomics, and vectorial lipidomics. Due to these demands, it is necessary to develop simple and reliable approaches for preparation of uniformly oriented IM membrane vesicles and validation of their sidedness. The uniformly ISO oriented membrane vesicles which have the cytoplasmic face of the membrane on the outside and the periplasmic side facing the sealed lumen can be obtained following intact cell disruption by a single passage through a French pressure cell (French press) at desired total pressure. Although high-pressure lysis leads to the formation of mostly inverted membrane vesicles (designated and abbreviated usually as ISO vesicles, everted or inverted membrane vesicles (IMVs)), inconclusive results are quite common. This uncertainty is due mainly by applying a different pressures, using either intact cells or spheroplasts and presence or absence of sucrose during rupture procedure. Many E. coli envelope fractionation techniques result in heterogeneity among isolated IM membrane vesicles. In part, this is due to difficulties in simple validation of sidedness of oriented membrane preparations of unknown sidedness. The sidedness of various preparations of membrane vesicles can be inferred from the orientation of residing uniformly oriented transmembrane protein. We outline the method in which the orientation of membrane vesicles can be verified by mapping of uniform or mixed topologies of essential protein E. coli protein leader peptidase (LepB) by advanced SCAM™. Although the protocol discussed in this chapter has been developed using Escherichia coli and Yersinia pseudotuberculosis, it can be directly adapted to other Gram-negative bacteria including pathogens.

ATP Synthesis by Oxidative Phosphorylation

The F1F0-ATP synthase (EC 3.6.1.34) is a remarkable enzyme that functions as a rotary motor. It is found in the inner membranes of Escherichia coli and is responsible for the synthesis of ATP in response to an electrochemical proton gradient. Under some conditions, the enzyme functions reversibly and uses the energy of ATP hydrolysis to generate the gradient. The ATP synthase is composed of eight different polypeptide subunits in a stoichiometry of α3β3γδεab2c10. Traditionally they were divided into two physically separable units: an F1 that catalyzes ATP hydrolysis (α3β3γδε) and a membrane-bound F0 sector that transports protons (ab2c10). In terms of rotary function, the subunits can be divided into rotor subunits (γεc10) and stator subunits (α3β3δab2). The stator subunits include six nucleotide binding sites, three catalytic and three noncatalytic, formed primarily by the β and α subunits, respectively. The stator also includes a peripheral stalk composed of δ and b subunits, and part of the proton channel in subunit a. Among the rotor subunits, the c subunits form a ring in the membrane, and interact with subunit a to form the proton channel. Subunits γ and ε bind to the c-ring subunits, and also communicate with the catalytic sites through interactions with α and β subunits. The eight subunits are expressed from a single operon, and posttranscriptional processing and translational regulation ensure that the polypeptides are made at the proper stoichiometry. Recent studies, including those of other species, have elucidated many structural and rotary properties of this enzyme.

Osmomechanics of the Propionigenium modestum F(o) motor

In Propionigenium modestum, ATP is manufactured from ADP and phosphate by the enzyme ATP synthase using the free energy of an electrochemical gradient of Na+ ions. The P. modestum ATP synthase is a clear member of the family of F-type ATP synthases and the only major distinction is an extension of the coupling ion specificity to H+, Li+, or Na+, depending on the conditions. The use of Na+ as a coupling ion offers unique experimental options to decipher the ion-translocation mechanism and the osmotic and mechanical behavior of the enzyme. The single a subunit and the oligomer of c subunits are part of the stator and rotor, respectively, and operate together in the ion-translocation mechanism. During ATP synthesis, Na+ diffuses from the periplasm through the a subunit channel onto the Na+ binding site on a c subunit. From there it dissociates into the cytoplasm after the site has rotated out of the interface with subunit a. In the absence of a membrane potential, the rotor performs Brownian motions into either direction and Na+ ions are exchanged between the two compartments separated by the membrane. Upon applying voltage, however, the direction of Na+ flux and of rotation is biased by the potential. The motor generates torque to drive the rotation of the gamma subunit, thereby releasing tightly bound ATP from catalytic sites in F(1). Hence, the membrane potential plays a pivotal role in the torque-generating mechanism. This is corroborated by the fact that for ATP synthesis, at physiological rates, the membrane potential is indispensable. We propose a catalytic mechanism for torque generation by the F(o) motor that is in accord with all experimental data and is in quantitative agreement with the requirement for ATP synthesis.

Rotary molecular motors

The F-, V-, and A-adenosine triphosphatases (ATPases) represent a family of evolutionarily related ion pumps found in every living cell. They either function to synthesize adenosine triphosphate (ATP) at the expense of an ion gradient or they act as primary ion pumps establishing transmembrane ion motive force at the expense of ATP hydrolysis. The A-, F-, and V-ATPases are rotary motor enzymes. Synthesis or hydrolysis of ATP taking place in the three catalytic sites of the membrane extrinsic domain is coupled to ion translocation across the single ion channel in the membrane-bound domain via rotation of a central part of the complex with respect to a static portion of the enzyme. This chapter reviews recent progress in the structure determination of several members of the family of F-, A-, and V-ATPases and our current understanding of the rotary mechanism of energy coupling.

The Na(+)-translocating F(1)F(0) ATP synthase of Propionigenium modestum: mechanochemical insights into the F(0) motor that drives ATP synthesis

The ATP synthase of Propionigenium modestum encloses a rotary motor involved in the production of ATP from ADP and inorganic phosphate utilizing the free energy of an electrochemical Na(+) ion gradient. This enzyme clearly belongs to the family of F(1)F(0) ATP synthases and uses exclusively Na(+) ions as the physiological coupling ion. The motor domain, F(0), comprises subunit a and the b subunit dimer which are part of the stator and the subunit c oligomer acting as part of the rotor. During ATP synthesis, Na(+) translocation through F(0) proceeds from the periplasm via the stator channel (subunit a) onto a Na(+) binding site of the rotor (subunit c). Upon rotation of the subunit c oligomer versus subunit a, the occupied rotor site leaves the interface with the stator and the Na(+) ion can freely dissociate into the cytoplasm. Recent experiments demonstrate that the membrane potential is crucial for ATP synthesis under physiological conditions. These findings support the view that voltage generates torque in F(0), which drives the rotation of the gamma subunit thus liberating tightly bound ATP from the catalytic sites in F(1). We suggest a mechanochemical model for the transduction of transmembrane Na(+)-motive force into rotary torque by the F(0) motor that can account quantitatively for the experimental data.

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.

Molecular models of the structural arrangement of subunits and the mechanism of proton translocation in the membrane domain of F(1)F(0) ATP synthase

Subunit c of the proton-transporting ATP synthase of Escherichia coli forms an oligomeric complex in the membrane domain that functions in transmembrane proton conduction. The arrangement of subunit c monomers in this oligomeric complex was studied by scanning mutagenesis. On the basis of these studies and structural information on subunit c, different molecular models for the potential arrangement of monomers in the c-oligomer are discussed. Intersubunit contacts in the F(0) domain that have been analysed in the past by chemical modification and mutagenesis studies are summarised. Transient contacts of the c-oligomer with subunit a might play a crucial role in the mechanism of proton translocation. Schematic models presented by several authors that interpret proton transport in the F(0) domain by a relative rotation of the c-subunit oligomer against subunit a are reviewed against the background of the molecular models of the oligomer.

Operation of the F(0) motor of the ATP synthase

ATP, the universal carrier of cell energy is manufactured from ADP and phosphate by the enzyme ATP synthase using the energy stored in a transmembrane ion gradient. The two components of the ion gradient (DeltapH or DeltapNa(+)) and the electrical potential difference Deltapsi are thermodynamically but not kinetically equivalent. In contrast to accepted wisdom, the electrical component is kinetically indispensable not only for bacterial ATP synthases but also for that from chloroplasts. Recent biochemical studies with the Na(+)-translocating ATP synthase of Propionigenium modestum have given a good idea of the ion translocation pathway in the F(0) motor. Taken together with biophysical data, the operating principles of the motor have been delineated.

A model for the structure of subunit a of the Escherichia coli ATP synthase and its role in proton translocation

Most of what is known about the structure and function of subunit a, of the ATP synthase, has come from the construction and isolation of mutations, and their analysis in the context of the ATP synthase complex. Three classes of mutants will be considered in this review. (1) Cys substitutions have been used for structural analysis of subunit a, and its interactions with subunit c. (2) Functional residues have been identified by extensive mutagenesis. These studies have included the identification of second-site suppressors within subunit a. (3) Disruptive mutations include deletions at both termini, internal deletions, and single amino acid insertions. The results of these studies, in conjunction with information about subunits b and c, can be incorporated into a model for the mechanism of proton translocation in the Escherichia coli ATP synthase.

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.

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.

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.

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.

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.

Topography of the yeast ATP synthase F0 sector

The interaction between the hydrophilic C-terminal part of subunit 4 (subunit b) and OSCP, which are two components of the connecting stalk of the yeast ATP synthase, was shown after reconstitution of the two over-expressed proteins and by the two-hybrid method. The organization of a part of the F0 sector was studied by the use of mutants containing cysteine residues in a loop connecting the two N-terminal postulated membrane-spanning segments. Labelling of the mutated subunits 4 by a maleimide fluorescent probe revealed that the sulfhydryl groups were modified upon incubation of intact mitochondria. In addition, non-permeant maleimide reagents labeled subunit 4D54C, thus showing a location of this residue in the intermembrane space. Cross-linking experiments revealed the proximity of subunits 4 and f. In addition, a disulfide bridge between subunit 4D54C and subunit 6 was evidenced, thus demonstrating near-neighbor relationships of the two subunits and a location of the N-terminal part of the mitochondrially-encoded subunit 6 in the intermembrane space.

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.

Transmembrane topography of subunit a in the Escherichia coli F1F0 ATP synthase

Subunit a is the least understood of the three subunits that compose the F0 sector in the Escherichia coli F0F1 ATP synthase. In this study, we have substituted Cys into predicted extramembranous loops of the protein and used chemical modification to obtain topographical information on the folding of subunit a. The extent of labeling of the substituted Cys residues by fluorescein-5'-maleimide was determined. The localization of reactive Cys residues was inferred from differences in the extent of labeling in inside out and right side out membrane vesicles. The NH2-terminal segment of subunit a was localized to the outside (periplasmic) surface and the COOH terminus to the cytoplasmic surface by these procedures. Loop residues in two periplasmic extramembranous loops and in two cytoplasmic extramembranous loops were also localized. The localization of two cytoplasmic Cys residues was confirmed by using 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid to block fluorescein-5'-maleimide labeling. From the localization of the Cys residues, a model for the topography is proposed that consists of five transmembrane segments with the NH2 terminus periplasmic and the COOH terminus cytoplasmic. The positions of second site suppressors, including several isolated here to the nonfunctional E219C and H245C substitutions, provide support for the topographical model proposed.

Identification of D-aspartate in rat pheochromocytoma PC12 cells

D-Aspartate is now known to be present in mammalian neuronal and endocrine cells in vivo, and may play some role(s) in neurocrine and endocrine functions. However, origin of D-aspartate is unknown. Here, we report that free D-aspartate (108 pmoles/3 x 10(7) cells) is present in the cultured PC12 cells, a rat pheochromocytoma cell line, as determined with immunohistochemical techniques as well as high performance liquid chromatography (HPLC) on a Pirkle-type chiral column. The amount of D-aspartate does not change with the passage. The culture medium does not contain D-aspartate. These results strongly suggest the presence of a de novo biosynthetic pathway for D-aspartate in the endocrine cells.

A triple mutation in the a subunit of the Escherichia coli/Propionigenium modestum F1Fo ATPase hybrid causes a switch from Na+ stimulation to Na+ inhibition

Previously we have shown that the Na+-translocating Escherichia coli (F1-delta)/Propionigenium modestum (Fo+delta) hybrid ATPase acquires a Na+-independent phenotype by the c subunit double mutation F84L, L87V that is reflected by Na+-independent growth of the mutant strain MPC8487 on succinate [Kaim, G., and Dimroth, P. (1995) J. Mol. Biol. 253, 726-738]. Here we describe a new class of mutants that were obtained by random mutagenesis and screening for Na+-independent growth on succinate. All six mutants of the new class contained four mutations in the a subunit (S89P, K220R, V264E, I278N). Results from site-specific mutagenesis revealed that the substitutions K220R, V264E, and I278N were sufficient to create the new phenotype. The resulting E. coli mutant strain MPA762 could only grow in the absence but not in the presence of Na+ ions on succinate minimal medium. This effect of Na+ ions on growth correlated with a Na+-specific inhibition of the mutant ATPase. The Ki for NaCl was 1. 5 mM at pH 6.5, similar to the Km for NaCl in activating the parent hybrid ATPase at this pH. On the other hand, activation by Li+ ions was retained in the new mutant ATPase. In the absence of Na+ or Li+, the mutant enzyme had the same pH optimum at pH 6.5 and twice the specific activity as the parent hybrid ATPase. In accordance with the kinetic data, the reconstituted mutant ATPase catalyzed H+ or Li+ transport but no Na+ transport. These results show for the first time that the coupling ion selectivity of F1Fo ATPases is determined by structural elements not only of the c subunit but also of the a subunit.

Topography of the yeast ATP synthase F0 sector by using cysteine substitution mutants. Cross-linkings between subunits 4, 6, and f

The arrangement of the N-terminal part of subunit 4 (subunit b) has been studied by the use of mutants containing cysteine residues in a loop connecting the two N-terminal postulated membrane-spanning segments. Labelling of the mutated subunit 4 by the fluorescent probe N-(7-(dimethylamino)-4-methyl-3-coumarinyl)maleimide revealed that the sulfhydryl groups were modified upon incubation of intact mitochondria. In addition, the nonpermeant sulfhydryl reagent 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid prevented the 3-(N-maleimidylpropionyl)biocytin labeling of subunit 4D54C, thus showing a location of this residue in the intermembrane space. Cross-linking experiments revealed the proximity of subunits 4 and f. In addition a disulfide bridge between subunit 4D54C and subunit 6 was evidenced, thus demonstrating near-neighbor relationships of the two subunits and a location of the N-terminal part of the mitochondrially-encoded subunit 6 in the intermembrane space.

Interaction of the delta and b subunits contributes to F1 and F0 interaction in the Escherichia coli F1F0-ATPase

Interactions of the F1F0-ATPase subunits between the cytoplasmic domain of the b subunit (residues 26-156, bcyt) and other membrane peripheral subunits including alpha, beta, gamma, delta, epsilon, and putative cytoplasmic domains of the a subunit were analyzed with the yeast two-hybrid system and in vitro reconstitution of ATPase from the purified subunits as well. Only the combination of bcyt fused to the activation domain of the yeast GAL-4, and delta subunit fused to the DNA binding domain resulted in the strong expression of the beta-galactosidase reporter gene, suggesting a specific interaction of these subunits. Expression of bcyt fused to glutathione S-transferase (GST) together with the delta subunit in Escherichia coli resulted in the overproduction of these subunits in soluble form, whereas expression of the GST-bcyt fusion alone had no such effect, indicating that GST-bcyt was protected by the co-expressed delta subunit from proteolytic attack in the cell. These results indicated that the membrane peripheral domain of b subunit stably interacted with the delta subunit in the cell. The affinity purified GST-bcyt did not contain significant amounts of delta, suggesting that the interaction of these subunits was relatively weak. Binding of these subunits observed in a direct binding assay significantly supported the capability of binding of the subunits. The ATPase activity was reconstituted from the purified bcyt together with alpha, beta, gamma, delta, and epsilon, or with the same combination except epsilon. Specific elution of the ATPase activity from glutathione affinity column with the addition of glutathione after reconstitution demonstrated that the reconstituted ATPase formed a complex. The result indicated that interaction of b and delta was stabilized by F1 subunits other than epsilon and also suggested that b-delta interaction was important for F1-F0 interaction.

The ATP synthase--a splendid molecular machine

An X-ray structure of the F1 portion of the mitochondrial ATP synthase shows asymmetry and differences in nucleotide binding of the catalytic beta subunits that support the binding change mechanism with an internal rotation of the gamma subunit. Other structural and mutational probes of the F1 and F0 portions of the ATP synthase are reviewed, together with kinetic and other evaluations of catalytic site occupancy and behavior during hydrolysis or synthesis of ATP. Subunit function as related to proton translocation and rotational catalysis is considered. Physical demonstrations of the gamma subunit rotation have been achieved. The findings have implications for other enzymatic catalyses.

Subunit a of proton ATPase F0 sector is a substrate of the FtsH protease in Escherichia coli

Escherichia coli FtsH is a membrane-bound ATPase with a proteolytic activity against the SecY subunit of protein translocase. We now report that subunit a of the membrane-embedded Fo part of H+-ATPase is another substrate of FtsH. Pulse-chase experiments showed that subunit a is unstable when it alone (without Fo subunits b and c) was oversynthesized and that it is stabilized in the ftsH mutants. Selective and ATP-dependent degradation of subunit a by purified FtsH protein was demonstrated in vitro. These results suggest that FtsH serves as a quality-control mechanism to avoid potentially harmful accumulation of free subunit a in the membrane.