Assembly of F0 sector of Escherichia coli H+ ATP synthase. Interdependence of subunit insertion into the membrane

Mutational analysis of a conserved positive charge in the c-ring of E. coli ATP synthase

F1Fo ATP synthase is a ubiquitous molecular motor that utilizes a rotary mechanism to synthesize adenosine triphosphate (ATP), the fundamental energy currency of life. The membrane-embedded Fo motor converts the electrochemical gradient of protons into rotation, which is then used to drive the conformational changes in the soluble F1 motor that catalyze ATP synthesis. In E. coli, the Fo motor is composed of a c10 ring (rotor) alongside subunit a (stator), which together provide two aqueous half channels that facilitate proton translocation. Previous work has suggested that Arg50 and Thr51 on the cytoplasmic side of each subunit c are involved in the proton translocation process, and positive charge is conserved in this region of subunit c. To further investigate the role of these residues and the chemical requirements for activity at these positions, we generated 13 substitution mutants and assayed their in vitro ATP synthesis, H+ pumping, and passive H+ permeability activities, as well as the ability of mutants to carry out oxidative phosphorylation in vivo. While polar and hydrophobic mutations were generally tolerated in either position, introduction of negative charge or removal of polarity caused a substantial defect. We discuss the possible effects of altered electrostatics on the interaction between the rotor and stator, water structure in the aqueous channel, and interaction of the rotor with cardiolipin.

Bacterial F-type ATP synthases follow a well-choreographed assembly pathway

F-type ATP synthases are multiprotein complexes composed of two separate coupled motors (F₁ and F_O) generating adenosine triphosphate (ATP) as the universal major energy source in a variety of relevant biological processes in mitochondria, bacteria and chloroplasts. While the structure of many ATPases is solved today, the precise assembly pathway of F₁F_O-ATP synthases is still largely unclear. Here, we probe the assembly of the F₁ complex from Acetobacterium woodii. Using laser induced liquid bead ion desorption (LILBID) mass spectrometry, we study the self-assembly of purified F₁ subunits in different environments under non-denaturing conditions. We report assembly requirements and identify important assembly intermediates in vitro and in cellula. Our data provide evidence that nucleotide binding is crucial for in vitro F₁ assembly, whereas ATP hydrolysis appears to be less critical. We correlate our results with activity measurements and propose a model for the assembly pathway of a functional F₁ complex.

ATPases are the macromolecular machines for cellular energy production. Here the authors investigate factors that govern the assembly of the F₁ complex from a bacterial F-type ATPase and relate differences in activity of complexes assembled in cells and in vitro to structural changes.

Analysis of an N-terminal deletion in subunit a of the Escherichia coli ATP synthase

Subunit a is a membrane-bound stator subunit of the ATP synthase and is essential for proton translocation. The N-terminus of subunit a in E. coli is localized to the periplasm, and contains a sequence motif that is conserved among some bacteria. Previous work has identified mutations in this region that impair enzyme activity. Here, an internal deletion was constructed in subunit a in which residues 6-20 were replaced by a single lysine residue, and this mutant was unable to grow on succinate minimal medium. Membrane vesicles prepared from this mutant lacked ATP synthesis and ATP-driven proton translocation, even though immunoblots showed a significant level of subunit a. Similar results were obtained after purification and reconstitution of the mutant ATP synthase into liposomes. The location of subunit a with respect to its neighboring subunits b and c was probed by introducing cysteine substitutions that were known to promote cross-linking: a_L207C + c_I55C, a_L121C + b_N4C, and a_T107C + b_V18C. The last pair was unable to form cross-links in the background of the deletion mutant. The results indicate that loss of the N-terminal region of subunit a does not generally disrupt its structure, but does alter interactions with subunit b.

Membrane Protein Production in E. coli for Applications in Drug Discovery

Producing high quality purified membrane proteins for structure-based drug design and biophysical assays compatible with typical timelines in drug discovery is a significant challenge. Escherichia coli has been an expression host of the utmost importance for soluble proteins and has applications for membrane proteins as well. However, membrane protein overexpression in E. coli may lead to toxicity and low yields of functional product. Here, we review the challenges encountered with heterologous overproduction of α-helical membrane proteins in E. coli and a range of strategies to overcome them. A detailed protocol is also provided for expression and screening of membrane proteins in E. coli using a His-specific fluorescent probe and fluorescent size-exclusion chromatography.

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.

Assembly of F1F0-ATP synthases

F1F0-ATP synthases are multimeric protein complexes and common prerequisites for their correct assembly are (i) provision of subunits in appropriate relative amounts, (ii) coordination of membrane insertion and (iii) avoidance of assembly intermediates that uncouple the proton gradient or wastefully hydrolyse ATP. Accessory factors facilitate these goals and assembly occurs in a modular fashion. Subcomplexes common to bacteria and mitochondria, but in part still elusive in chloroplasts, include a soluble F1 intermediate, a membrane-intrinsic, oligomeric c-ring, and a membrane-embedded subcomplex composed of stator subunits and subunit a. The final assembly step is thought to involve association of the preformed F1-c10-14 with the ab2 module (or the ab8-stator module in mitochondria)--mediated by binding of subunit δ in bacteria or OSCP in mitochondria, respectively. Despite the common evolutionary origin of F1F0-ATP synthases, the set of auxiliary factors required for their assembly in bacteria, mitochondria and chloroplasts shows clear signs of evolutionary divergence. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Assembly of the Escherichia coli FoF1 ATP synthase involves distinct subcomplex formation

The ATP synthase (FoF1) of Escherichia coli couples the translocation of protons across the cytoplasmic membrane by Fo to ATP synthesis or hydrolysis in F1. Whereas good knowledge of the nanostructure and the rotary mechanism of the ATP synthase is at hand, the assembly pathway of the 22 polypeptide chains present in a stoichiometry of ab2c10α3β3γδϵ has so far not received sufficient attention. In our studies, mutants that synthesize different sets of FoF1 subunits allowed the characterization of individually formed stable subcomplexes. Furthermore, the development of a time-delayed in vivo assembly system enabled the subsequent synthesis of particular missing subunits to allow the formation of functional ATP synthase complexes. These observations form the basis for a model that describes the assembly pathway of the E. coli ATP synthase from pre-formed subcomplexes, thereby avoiding membrane proton permeability by a concomitant assembly of the open H+-translocating unit within a coupled FoF1 complex.

Subunit δ is the key player for assembly of the H(+)-translocating unit of Escherichia coli F(O)F1 ATP synthase

The ATP synthase (F(O)F1) of Escherichia coli couples the translocation of protons across the cytoplasmic membrane to the synthesis or hydrolysis of ATP. This nanomotor is composed of the rotor c10γε and the stator ab2α3β3δ. To study the assembly of this multimeric enzyme complex consisting of membrane-integral as well as peripheral hydrophilic subunits, we combined nearest neighbor analyses by intermolecular disulfide bond formation or purification of partially assembled F(O)F1 complexes by affinity chromatography with the use of mutants synthesizing different sets of F(O)F1 subunits. Together with a time-delayed in vivo assembly system, the results demonstrate that F(O)F1 is assembled in a modular way via subcomplexes, thereby preventing the formation of a functional H(+)-translocating unit as intermediate product. Surprisingly, during the biogenesis of F(O)F1, F1 subunit δ is the key player in generating stable F(O). Subunit δ serves as clamp between ab2 and c10α3β3γε and guarantees that the open H(+) channel is concomitantly assembled within coupled F(O)F1 to maintain the low membrane proton permeability essential for viability, a general prerequisite for the assembly of multimeric H(+)-translocating enzymes.

Time-delayed in vivo assembly of subunit a into preformed Escherichia coli FoF1 ATP synthase

Escherichia coli F(O)F(1) ATP synthase, a rotary nanomachine, is composed of eight different subunits in a α3β3γδεab2c10 stoichiometry. Whereas F(O)F(1) has been studied in detail with regard to its structure and function, much less is known about how this multisubunit enzyme complex is assembled. Single-subunit atp deletion mutants are known to be arrested in assembly, thus leading to formation of partially assembled subcomplexes. To determine whether those subcomplexes are preserved in a stable standby mode, a time-delayed in vivo assembly system was developed. To establish this approach, we targeted the time-delayed assembly of membrane-integrated subunit a into preformed F(O)F(1) lacking subunit a (F(O)F(1)-a) which is known to form stable subcomplexes in vitro. Two expression systems (araBADp and T7p-laco) were adjusted to provide compatible, mutually independent, and sufficiently stringent induction and repression regimens. In detail, all structural atp genes except atpB (encoding subunit a) were expressed under the control of araBADp and induced by arabinose. Following synthesis of F(O)F(1)-a during growth, expression was repressed by glucose/d-fucose, and degradation of atp mRNA controlled by real-time reverse transcription-PCR. A time-delayed expression of atpB under T7p-laco control was subsequently induced in trans by addition of isopropyl-β-d-thiogalactopyranoside. Formation of fully assembled, and functional, F(O)F(1) complexes was verified. This demonstrates that all subunits of F(O)F(1)-a remain in a stable preformed state capable to integrate subunit a as the last subunit. The results reveal that the approach presented here can be applied as a general method to study the assembly of heteromultimeric protein complexes in vivo.

Roles of AtpI and two YidC-type proteins from alkaliphilic Bacillus pseudofirmus OF4 in ATP synthase assembly and nonfermentative growth

AtpI, a membrane protein encoded by many bacterial atp operons, is reported to be necessary for c-ring oligomer formation during assembly of some ATP synthase complexes. We investigated chaperone functions of AtpI and compared them to those of AtpZ, a protein encoded by a gene upstream of atpI that has a role in magnesium acquisition at near-neutral pH, and of SpoIIIJ and YqjG, two YidC/OxaI/Alb3 family proteins, in alkaliphilic Bacillus pseudofirmus OF4. A strain with a chromosomal deletion of atpI grew nonfermentatively, and its purified ATP synthase had a c-ring of normal size, indicating that AtpI is not absolutely required for ATP synthase function. However, deletion of atpI, but not atpZ, led to reduced stability of the ATP synthase rotor, reduced membrane association of the F(1) domain, reduced ATPase activity, and modestly reduced nonfermentative growth on malate at both pH 7.5 and 10.5. Both spoIIIJ and yqjG, but not atpI or atpZ, complemented a YidC-depleted Escherichia coli strain. Consistent with such overlapping functions, single deletions of spoIIIJ or yqjG in the alkaliphile did not affect membrane ATP synthase levels or activities, but functional specialization was indicated by YqjG and SpoIIIJ showing respectively greater roles in malate growth at pH 7.5 and 10.5. Expression of yqjG was elevated at pH 7.5 relative to that at pH 10.5 and in ΔspoIIIJ strains, but it was lower than constitutive spoIIIJ expression. Deletion of atpZ caused the largest increase among the mutants in magnesium concentrations needed for pH 7.5 growth. The basis for this phenotype is not yet resolved.

Interaction with monomeric subunit c drives insertion of ATP synthase subunit a into the membrane and primes a-c complex formation

Subunit a is the main part of the membrane stator of the ATP synthase molecular turbine. Subunit c is the building block of the membrane rotor. We have generated two molecular fusions of a and c subunits with different orientations of the helical hairpin of subunit c. The a/c fusion protein with correct orientation of transmembrane helices was inserted into the membrane, and co-incorporated into the F(0) complex of ATP synthase with wild type subunit c. The fused c subunit was incorporated into the c-ring tethering the ATP synthase rotor to the stator. The a/c fusion with incorrect orientation of the c-helices required wild type subunit c for insertion into the membrane. In this case, the fused c subunit remained on the periphery of the c-ring and did not interfere with rotor movement. Wild type subunit a inserted into the membrane equally well with wild type subunit c and c-ring assembly mutants that remained monomeric in the membrane. These results show that interaction with monomeric subunit c triggers insertion of subunit a into the membrane, and initiates formation of the a-c complex, the ion-translocating module of the ATP synthase. Correct assembly of the ATP synthase incorporating topologically correct fusion of subunits a and c validates using this model protein for high resolution structural studies of the ATP synthase proton channel.

Biogenesis of bacterial inner-membrane proteins

All cells must traffic proteins into and across their membranes. In bacteria, several pathways have evolved to enable protein transfer across the inner membrane, the periplasm, and the outer membrane. The major route of protein translocation in and across the cytoplasmic membrane is the general secretion pathway (Sec-pathway). The biogenesis of membrane proteins not only requires protein translocation but also coordinated targeting to the membrane beforehand and folding and assembly into their protein complexes afterwards to function properly in the cell. All these processes are responsible for the biogenesis of membrane proteins that mediate essential functions of the cell such as selective transport, energy conversion, cell division, extracellular signal sensing, and motility. This review will highlight the most recent developments on the structure and function of bacterial membrane proteins, focusing on the journey that integral membrane proteins take to find their final destination in the inner membrane.

Assembly and oligomerization of human ATP synthase lacking mitochondrial subunits a and A6L

Here we study ATP synthase from human rho0 (rho zero) cells by clear native electrophoresis (CNE or CN-PAGE) and show that ATP synthase is almost fully assembled in spite of the absence of subunits a and A6L. This identifies subunits a and A6L as two of the last subunits to complete the ATP synthase assembly. Minor amounts of dimeric and even tetrameric forms of the large assembly intermediate were preserved under the conditions of CNE, suggesting that it associated further into higher order structures in the mitochondrial membrane. This result was reminiscent to the reduced amounts of dimeric and tetrameric ATP synthase from yeast null mutants of subunits e and g detected by CNE. The dimer/oligomer-stabilizing effects of subunits e/g and a/A6L seem additive in human and yeast cells. The mature IF1 inhibitor was specifically bound to the dimeric/oligomeric forms of ATP synthase and not to the monomer. Conversely, nonprocessed pre-IF1 still containing the mitochondrial targeting sequence was selectively bound to the monomeric assembly intermediate in rho0 cells and not to the dimeric form. This supports previous suggestions that IF1 plays an important role in the dimerization/oligomerization of mammalian ATP synthase and in the regulation of mitochondrial structure and function.

Biogenesis of membrane bound respiratory complexes in Escherichia coli

Escherichia coli is one of the preferred bacteria for studies on the energetics and regulation of respiration. Respiratory chains consist of primary dehydrogenases and terminal reductases or oxidases linked by quinones. In order to assemble this complex arrangement of protein complexes, synthesis of the subunits occurs in the cytoplasm followed by assembly in the cytoplasm and/or membrane, the incorporation of metal or organic cofactors and the anchoring of the complex to the membrane. In the case of exported metalloproteins, synthesis, assembly and incorporation of metal cofactors must be completed before translocation across the cytoplasmic membrane. Coordination data on these processes is, however, scarce. In this review, we discuss the various processes that respiratory proteins must undergo for correct assembly and functional coupling to the electron transport chain in E. coli. Targeting to and translocation across the membrane together with cofactor synthesis and insertion are discussed in a general manner followed by a review of the coordinated biogenesis of individual respiratory enzyme complexes. Lastly, we address the supramolecular organization of respiratory enzymes into supercomplexes and their localization to specialized domains in the membrane.

Subunit a of the F(1)F(0) ATP synthase requires YidC and SecYEG for membrane insertion

The insertion of inner membrane proteins in Escherichia coli occurs almost exclusively via the SecYEG pathway, while some membrane proteins require the membrane protein insertase YidC. In vitro analysis demonstrates that subunit a of the F(1)F(0) ATP synthase (F(0)a) is strictly dependent on Ffh, SecYEG and YidC for its membrane insertion but independent of the proton motive force. The insertion of the first transmembrane segment of F(0)a also depends on Ffh and SecYEG but not on YidC, whereas the insertion is strongly dependent on the proton motive force, unlike the full-length F(0)a protein. These data demonstrate an extensive role of YidC in the assembly of the F(0) sector of the F(1)F(0) ATP synthase.

Constant c10 ring stoichiometry in the Escherichia coli ATP synthase analyzed by cross-linking

The subunit c stoichiometry of Escherichia coli ATP synthase was studied by intermolecular cross-linking via oxidation of bi-cysteine-substituted subunit c (cA21C/cM65C). Independent of the carbon source used for growth and independent of the presence of other FoF1 subunits, an equal pattern of cross-link formation stopping at the formation of decamers was obtained.

Structural interactions between transmembrane helices 4 and 5 of subunit a and the subunit c ring of Escherichia coli ATP synthase

Subunit a plays a key role in promoting H+ transport and the coupled rotary motion of the subunit c ring in F1F0-ATP synthase. H+ binding and release occur at Asp-61 in the middle of the second transmembrane helix (TMH) of F0 subunit c. H+ are thought to reach Asp-61 via aqueous pathways mapping to the surfaces of TMHs 2-5 of subunit a. TMH4 of subunit a is thought to pack close to TMH2 of subunit c based upon disulfide cross-link formation between Cys substitutions in both TMHs. Here we substituted Cys into the fifth TMH of subunit a and the second TMH of subunit c and tested for cross-linking using bis-methanethiosulfonate (bis-MTS) reagents. A total of 62 Cys pairs were tested and 12 positive cross-links were identified with variable alkyl length linkers. Cross-linking was achieved near the middle of the bilayer for the Cys pairs a248C/c62C, a248C/ c63C, a248C/c65C, a251C/c57C, a251C/c59C, a251C/c62C, a252C/c62C, and a252C/c65C. Cross-linking was achieved near the cytoplasmic side of the bilayer for Cys pairs a262C/c53C, a262C/c54C, a262C/c55C, and a263C/c54C. We conclude that both aTMH4 and aTMH5 pack proximately to cTMH2 of the c-ring. In other experiments we demonstrate that aTMH4 and aTMH5 can be simultaneously cross-linked to different subunit c monomers in the c-ring. Five mutants showed pH-dependent cross-linking consistent with aTMH5 changing conformation at lower pH values to facilitate cross-linking. We suggest that the pH-dependent conformational change may be related to the proposed role of aTMH5 in gating H+ access from the periplasm to the cAsp-61 residue in cTMH2.

Towards the development of Bacillus subtilis as a cell factory for membrane proteins and protein complexes

Background

The Gram-positive bacterium Bacillus subtilis is an important producer of high quality industrial enzymes and a few eukaryotic proteins. Most of these proteins are secreted into the growth medium, but successful examples of cytoplasmic protein production are also known. Therefore, one may anticipate that the high protein production potential of B. subtilis can be exploited for protein complexes and membrane proteins to facilitate their functional and structural analysis. The high quality of proteins produced with B. subtilis results from the action of cellular quality control systems that efficiently remove misfolded or incompletely synthesized proteins. Paradoxically, cellular quality control systems also represent bottlenecks for the production of various heterologous proteins at significant concentrations.

Conclusion

While inactivation of quality control systems has the potential to improve protein production yields, this could be achieved at the expense of product quality. Mechanisms underlying degradation of secretory proteins are nowadays well understood and often controllable. It will therefore be a major challenge for future research to identify and modulate quality control systems of B. subtilis that limit the production of high quality protein complexes and membrane proteins, and to enhance those systems that facilitate assembly of these proteins.

The cytoplasmic loops of subunit a of Escherichia coli ATP synthase may participate in the proton translocating mechanism

Subunit a plays a key role in promoting H(+) transport and the coupled rotary motion of the subunit c ring in F(1)F(0)-ATP synthase. H(+) binding and release occur at Asp-61 in the middle of the second transmembrane helix (TMH) of F(0) subunit c. H(+) are thought to reach Asp-61 via aqueous pathways mapping to the surfaces of TMHs 2-5 of subunit a based upon the chemical reactivity of Cys substituted into these helices. Here we substituted Cys into loops connecting TMHs 1 and 2 (loop 1-2) and TMHs 3 and 4 (loop 3-4). A large segment of loop 3-4 extending from loop residue 192 loop to residue 203 in TMH4 at the lipid bilayer surface proved to be very sensitive to inhibition by Ag(+). Cys-161 and -165 at the other end of the loop bordering TMH3 were also sensitive to inhibition by Ag(+). Further Cys substitutions in residues 86 and 93 in the middle of the 1-2 loop proved to be Ag(+)-sensitive. We next asked whether the regions of Ag(+)-sensitive residues clustered together near the surface of the membrane by combining Cys substitutions from two domains and testing for cross-linking. Cys-161 and -165 in loop 3-4 were found to cross-link with Cys-202, -203, or -205, which extend into TMH4 from the cytoplasm. Further Cys at residues 86 and 93 in loop 1-2 were found to cross-link with Cys-195 in loop 3-4. We conclude that the Ag(+)-sensitive regions of loops 1-2 and 3-4 may pack in a single domain that packs at the ends of TMHs 3 and 4. We suggest that the Ag(+)-sensitive domain may be involved in gating H(+) release at the cytoplasmic side of the aqueous access channel extending through F(0).

Subunit a facilitates aqueous access to a membrane-embedded region of subunit c in Escherichia coli F1F0 ATP synthase

Rotary catalysis in F(1)F(0) ATP synthase is powered by proton translocation through the membrane-embedded F(0) sector. Proton binding and release occurs in the middle of the membrane at Asp-61 on transmembrane helix 2 of subunit c. Previously, the reactivity of cysteines substituted into F(0) subunit a revealed two regions of aqueous access, one extending from the periplasm to the middle of the membrane and a second extending from the middle of the membrane to the cytoplasm. To further characterize aqueous accessibility at the subunit a-c interface, we have substituted Cys for residues on the cytoplasmic side of transmembrane helix 2 of subunit c and probed the accessibility to these substituted positions using thiolate-reactive reagents. The Cys substitutions tested were uniformly inhibited by Ag(+) treatment, which suggested widespread aqueous access to this generally hydrophobic region. Sensitivity to N-ethylmaleimide (NEM) and methanethiosulfonate reagents was localized to a membrane-embedded pocket surrounding Asp-61. The cG58C substitution was profoundly inhibited by all the reagents tested, including membrane impermeant methanethiosulfonate reagents. Further studies of the highly reactive cG58C substitution revealed that NEM modification of a single c subunit in the oligomeric c-ring was sufficient to cause complete inhibition. In addition, NEM modification of subunit c was dependent upon the presence of subunit a. The results described here provide further evidence for an aqueous-accessible region at the interface of subunits a and c extending from the middle of the membrane to the cytoplasm.

ATP synthesis without R210 of subunit a in the Escherichia coli ATP synthase

Interactions between subunit a and oligomeric subunit c are essential for the coupling of proton translocation to rotary motion in the ATP synthase. A pair of previously described mutants, R210Q/Q252R and P204T/R210Q/Q252R [L.P. Hatch, G.B. Cox and S.M. Howitt, The essential arginine residue at position 210 in the a subunit of the Escherichia coli ATP synthase can be transferred to position 252 with partial retention of activity, J. Biol. Chem. 270 (1995) 29407-29412] has been constructed and further analyzed. These mutants, in which the essential arginine of subunit a, R210, was switched with a conserved glutamine residue, Q252, are shown here to be capable of both ATP synthesis by oxidative phosphorylation, and ATP-driven proton translocation. In addition, lysine can replace the arginine at position 252 with partial retention of both activities. The pH dependence of ATP-driven proton translocation was determined after purification of mutant enzymes, and reconstitution into liposomes. Proton translocation by the lysine mutant, and to a lesser extent the arginine mutant, dropped off sharply above pH 7.5, consistent with the requirement for a positive charge during function. Finally, the rates of ATP synthesis and of ATP-driven proton translocation were completely inhibited by treatment with DCCD (N,N'-dicyclohexylcarbodiimide), while rates of ATP hydrolysis by the mutants were not significantly affected, indicating that DCCD modification disrupts the F(1)-F(o) interface. The results suggest that minimal requirements for proton translocation by the ATP synthase include a positive charge in subunit a and a weak interface between subunit a and oligomeric subunit c.

Fluidity of structure and swiveling of helices in the subunit c ring of Escherichia coli ATP synthase as revealed by cysteine-cysteine cross-linking

Subunit c in the membrane-traversing F(0) sector of Escherichia coli ATP synthase is known to fold with two transmembrane helices and form an oligomeric ring of 10 or more subunits in the membrane. Models for the E. coli ring structure have been proposed based upon NMR solution structures and intersubunit cross-linking of Cys residues in the membrane. The E. coli models differ from the recent x-ray diffraction structure of the isolated Ilyobacter tartaricus c-ring. Furthermore, key cross-linking results supporting the E. coli model prove to be incompatible with the I. tartaricus structure. To test the applicability of the I. tartaricus model to the E. coli c-ring, we compared the cross-linking of a pair of doubly Cys substituted c-subunits, each of which was compatible with one model but not the other. The key finding of this study is that both A21C/M65C and A21C/I66C doubly substituted c-subunits form high yield oligomeric structures, c(2), c(3)... c(10), via intersubunit disulfide bond formation. The results indicate that helical swiveling, with resultant interconversion of the two conformers predicted by the E. coli and I. tartaricus models, must be occurring over the time course of the cross-linking experiment. In the additional experiments reported here, we tried to ascertain the preferred conformation in the membrane to help define the most likely structural model. We conclude that both structures must be able to form in the membrane, but that the helical swiveling that promotes their interconversion may not be necessary during rotary function.

Cross-linking between helices within subunit a of Escherichia coli ATP synthase defines the transmembrane packing of a four-helix bundle

Subunit a of F(1)F(0) ATP synthase is required in the H(+) transport driven rotation of the c-ring of F(0), the rotation of which is coupled to ATP synthesis in F(1). The three-dimensional structure of subunit a is unknown. In this study, Cys substitutions were introduced into two different transmembrane helices (TMHs) of subunit a, and the proximity of the thiol side chains was tested via attempted oxidative cross-linking to form the disulfide bond. Pairs of Cys substitutions were made in TMHs 2/3, 2/4, 2/5, 3/4, 3/5, and 4/5. Cu(+2)-catalyzed oxidation led to cross-link formation between Cys pairs L120C(TMH2) and S144C(TMH3), L120C(TMH2) and G218C(TMH4), L120C(TMH2) and H245C(TMH5), L120C(TMH2) and I246C(TMH5), N148C(TMH3) and E219C(TMH4), N148C(TMH3) and H245C(TMH5), and G218C(TMH4) and I248C(TMH5). Iodine, but not Cu(+2), was found to catalyze cross-link formation between D119C(TMH2) and G218C(TMH4). The results suggest that TMHs 2, 3, 4, and 5 form a four-helix bundle with one set of key functional residues in TMH4 (Ser-206, Arg-210, and Asn-214) located at the periphery facing subunit c. Other key residues in TMHs 2, 4, and 5, which were concluded previously to compose a possible aqueous access pathway from the periplasm, were found to locate to the inside of the four-helix bundle.

Supercomplexes and subcomplexes of mitochondrial oxidative phosphorylation

Dimerization or oligomerization of ATP synthase has been proposed to play an important role for mitochondrial cristae formation and to be involved in regulating ATP synthase activity. We found comparable oligomycin-sensitive ATPase activity for monomeric and oligomeric ATP synthase suggesting that oligomerization/monomerization dynamics are not directly involved in regulating ATP synthase activity. Binding of the natural IF1 inhibitor protein has been shown to induce dimerization of F1-subcomplexes. This suggested that binding of IF1 might also dimerize holo ATP synthase, and possibly link dimerization and inhibition. Analyzing mitochondria of human rho zero cells that contain mitochondria but lack mitochondrial DNA, we identified three subcomplexes of ATP synthase: (i) F1 catalytic domain, (ii) F1-domain with bound IF1, and (iii) F1-c subcomplex with bound IF1 and a ring of subunits c. Since both IF1 containing subcomplexes were present in monomeric state and exhibited considerably reduced ATPase activity as compared to the third subcomplex lacking IF1, we postulate that inhibition and induction of dimerization of F1-subcomplexes by IF1 are independent events. F1-subcomplexes were also found in mitochondria of patients with specific mitochondrial disorders, and turned out to be useful for the clinical differentiation between various types of mitochondrial biosynthesis disorders. Supramolecular associations of respiratory complexes, the "respirasomes", seem not to be the largest assemblies in the structural organization of the respiratory chain, as suggested by differential solubilization of mitochondria and electron microscopic analyses of whole mitochondria. We present a model for a higher supramolecular association of respirasomes into a "respiratory string".

YidC-mediated membrane insertion of assembly mutants of subunit c of the F1F0 ATPase

YidC is a member of the OxaI family of membrane proteins that has been implicated in the membrane insertion of inner membrane proteins in Escherichia coli. We have recently demonstrated that proteoliposomes containing only YidC support both the stable membrane insertion and the oligomerization of the c subunit of the F(1)F(0) ATP synthase (F(0)c). Here we have shown that two mutants of F(0)c unable to form a functional F(1)F(0) ATPase interact with YidC, require YidC for membrane insertion, but fail to oligomerize. These data show that oligomerization is not essential for the stable YidC-dependent membrane insertion of F(0)c consistent with a function of YidC as a membrane protein insertase.

Subcomplexes of human ATP synthase mark mitochondrial biosynthesis disorders

OBJECTIVE

METHODS

We describe biochemically and clinically relevant aspects of mitochondrial ATP synthase, the enzyme that supplies most ATP for the cells energy demand.

RESULTS

Analyzing human Rho zero cells we could identify three subcomplexes of ATP synthase: F1 catalytic domain, F1 domain with bound natural IF1 inhibitor protein, and F1-c subcomplex, an assembly of F1 domain and a ring of F(O)-subunits c. Large amounts of F1 subcomplexes accumulated also in mitochondria of patients with specific mitochondrial disorders. By quantifying the F1 subcomplexes and other oxidative phosphorylation complexes in parallel, we were able to discriminate three classes of defects in mitochondrial biosynthesis, namely, mitochondrial DNA depletion, mitochondrial transfer RNA (tRNA) mutations, and mutations in the mitochondrial ATP6 gene.

INTERPRETATION

The relatively simple electrophoretic assay used here is a straightforward approach to differentiate between various types of genetic alterations affecting the biosynthesis of oxidative phosphorylation complexes and will be useful to guide molecular genetic diagnostics in the field of mitochondrial neuromuscular disorders.

YidC--an evolutionary conserved device for the assembly of energy-transducing membrane protein complexes

Members of the YidC/Oxa1/Alb3 membrane protein family are multifunctional mediators of membrane protein integration, folding and assembly into large complexes. Their evolutionary conserved and physiologically important role appears to relate to the assembly of major energy-transducing membrane protein complexes.

HMW1 is required for stability and localization of HMW2 to the attachment organelle of Mycoplasma pneumoniae

The cytoskeletal proteins HMW1 and HMW2 are components of the terminal organelle of the cell wall-less bacterium Mycoplasma pneumoniae. HMW1 is required for a tapered, filamentous morphology but exhibits accelerated turnover in the absence of HMW2. Here, we report that a reciprocal dependency exists between HMW1 and HMW2, with HMW2 subject to accelerated turnover with the loss of HMW1. Furthermore, the instability of HMW2 correlated with its failure to localize to the attachment organelle. The C-terminal domain of HMW1 is essential for both function and its accelerated turnover in the absence of HMW2. We constructed HMW1 deletion derivatives lacking portions of this domain and examined each for stability and function. The C-terminal 41 residues were particularly important for proper localization and function in cell morphology and P1 localization, but the entire C-terminal domain was required to stabilize HMW2. The significance of these findings in the context of attachment organelle assembly is considered.

YidC family members are involved in the membrane insertion, lateral integration, folding, and assembly of membrane proteins

Members of the YidC family exist in all three domains of life, where they control the assembly of a large variety of membrane protein complexes that function as transporters, energy devices, or sensor proteins. Recent studies in bacteria have shown that YidC functions on its own as a membrane protein insertase independent of the Sec protein–conducting channel. YidC can also assist in the lateral integration and folding of membrane proteins that insert into the membrane via the Sec pathway.

F1F0 ATP synthase subunit c is a substrate of the novel YidC pathway for membrane protein biogenesis

The Escherichia coli YidC protein belongs to the Oxa1 family of membrane proteins that have been suggested to facilitate the insertion and assembly of membrane proteins either in cooperation with the Sec translocase or as a separate entity. Recently, we have shown that depletion of YidC causes a specific defect in the functional assembly of F₁F₀ ATP synthase and cytochrome o oxidase. We now demonstrate that the insertion of in vitro–synthesized F₁F₀ ATP synthase subunit c (F₀c) into inner membrane vesicles requires YidC. Insertion is independent of the proton motive force, and proteoliposomes containing only YidC catalyze the membrane insertion of F₀c in its native transmembrane topology whereupon it assembles into large oligomers. Co-reconstituted SecYEG has no significant effect on the insertion efficiency. Remarkably, signal recognition particle and its membrane-bound receptor FtsY are not required for the membrane insertion of F₀c. In conclusion, a novel membrane protein insertion pathway in E. coli is described in which YidC plays an exclusive role.

Atp10p assists assembly of Atp6p into the F0 unit of the yeast mitochondrial ATPase

The F(0)F(1)-ATPase complex of yeast mitochondria contains three mitochondrial and at least 17 nuclear gene products. The coordinate assembly of mitochondrial and cytosolic translation products relies on chaperones and specific factors that stabilize the pools of some unassembled subunits. Atp10p was identified as a mitochondrial inner membrane component necessary for the biogenesis of the hydrophobic F(0) sector of the ATPase. Here we show that, following its synthesis on mitochondrial ribosomes, subunit 6 of the ATPase (Atp6p) can be cross-linked to Atp10p. This interaction is required for the integration of Atp6p into a partially assembled subcomplex of the ATPase. Pulse labeling and chase of mitochondrial translation products in vivo indicate that Atp6p is less stable and more rapidly degraded in an atp10 null mutant than in wild type. Based on these observations, we propose Atp10p to be an Atp6p-specific chaperone that facilitates the incorporation of Atp6p into an intermediate subcomplex of ATPase subunits.

YidC is strictly required for membrane insertion of subunits a and c of the F(1)F(0)ATP synthase and SecE of the SecYEG translocase

YidC was previously discovered to play a critical role for the insertion of the Sec-independent M13 procoat and Pf3 coat phage proteins into the Escherichia coli inner membrane. To determine whether there is an absolute requirement of YidC for membrane protein insertion of any endogenous E. coli proteins, we investigated a few representative membrane proteins. We found that membrane subunits of the F(0) sector of the F(1)F(0)ATP synthase and the SecE protein of the SecYEG translocase are highly dependent on YidC for membrane insertion, based on protease mapping and immunoblot analysis. We found that the SecE dependency on YidC for membrane insertion does not contradict the observation that depletion of YidC does not block SecYEG-dependent protein export at 37 degrees C. YidC depletion does not decrease the SecE level low enough to block export at 37 degrees C. In contrast, we found that protein export of OmpA is severely blocked at 25 degrees C when YidC is depleted, which may be due to the decreased SecE level, as a 50% decrease in the SecE levels drastically affects protein export at the cold temperature [Schatz, P. J., Bieker, K. L., Ottemann, K. M., Silhavy, T. J., and Beckwith, J. (1991) EMBO J. 10, 1749-57]. These studies reported here establish that physiological substrates of YidC include subunits of the ATP synthase and the SecYEG translocase, demonstrating that YidC plays a vital role for insertion of endogenous membrane proteins in bacteria.

Energy-driven subunit rotation at the interface between subunit a and the c oligomer in the F(O) sector of Escherichia coli ATP synthase

Subunit rotation within the F(1) catalytic sector of the ATP synthase has been well documented, identifying the synthase as the smallest known rotary motor. In the membrane-embedded F(O) sector, it is thought that proton transport occurs at a rotor/stator interface between the oligomeric ring of c subunits (rotor) and the single-copy a subunit (stator). Here we report evidence for an energy-dependent rotation at this interface. F(O)F(1) was expressed with a pair of substituted cysteines positioned to allow an intersubunit disulfide crosslink between subunit a and a c subunit [aN214C/cM65C; Jiang, W. & Fillingame, R. H. (1998) Proc. Natl. Acad. Sci. USA 95, 6607--6612]. Membranes were treated with N,N'-dicyclohexyl-[(14)C]carbodiimide to radiolabel the D61 residue on less than 20% of the c subunits. After oxidation to form an a--c crosslink, the c subunit properly aligned to crosslink to subunit a was found to contain very little (14)C label relative to other members of the c ring. However, exposure to MgATP before oxidation significantly increased the radiolabel in the a-c crosslink, indicating that a different c subunit was now aligned with subunit a. This increase was not induced by exposure to MgADP/P(i). Furthermore, preincubation with MgADP and azide to inhibit F(1) or with high concentrations of N,N'-dicyclohexylcarbodiimide to label most c subunits prevented the ATP effect. These results provide evidence for an energy-dependent rotation of the c ring relative to subunit a.

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.

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.

Structure, functioning, and assembly of the ATP synthase in cells from patients with the T8993G mitochondrial DNA mutation. Comparison with the enzyme in Rho(0) cells completely lacking mtdna

The structure and functioning of the ATP synthase of human fibroblast cell lines with 91 and 100%, respectively, of the T8993G mutation have been studied, with MRC5 human fibroblasts and Rho(0) cells derived from this cell line as controls. ATP hydrolysis was normal but ATP synthesis was reduced by 60% in the 100% mutants. Both activities were highly oligomycin-sensitive. The levels of F(1)F(0) were close to normal, and the enzyme was stable. It is concluded that the loss of ATP synthesis is because of disruption of the proton translocation step within the F(0) part. This is supported by membrane potential measurements using the dye JC-1. Cells with a 91% mutation load grew well and showed only a 25% loss in ATP synthesis. This much reduced effect for only a 9% difference in mutation load mirrors the reduced pathogenicity in patients. F(1)F(0) has been purified for the first time from human cell lines. A partial complex was obtained from Rho(0) cells containing the F(1) subunits associated with several stalk, as well as F(0) subunits, including oligomycin sensitivity conferring protein, b, and c subunits. This partial complex no longer binds inhibitor protein.

His(15) of subunit a of the Escherichia coli ATP synthase is important for the structure or assembly of the membrane sector F(o)

Approximately 37 amino acids at the amino-terminus of subunit a of the Escherichia coli ATP synthase are found localized to the periplasm. Results indicate that a single amino acid substitution, H15D, disrupts assembly of subunit a and causes a loss of ATP synthase function. In this study, a conserved region of nine amino acids, 11-19, was initially mutagenized randomly, generating no mutants that could grow on succinate-minimal medium. Subsequent mutagenesis, confined to residues His(14), His(15), and Asn(17), indicated that constructs containing H15D were the most deleterious. Four single mutants were constructed and analyzed: H15A, H14D, H15A, and H15D. Only H15D was significantly impaired, with respect to ATP-driven proton translocation, passive proton permeability through F(o), and sensitivity of membrane-bound ATPase to DCCD. Immunoblot analysis indicated very low levels of subunit a from H15D. Cysteine mutations were constructed at positions 14, 15, 17, and 18. Residues 14, 15, and 17 were shown to be accessible in the periplasmic space, while residue 18 was not, indicating that this region was stably folded. While both His(14) and His(15) are conserved among a group of bacteria, results presented here indicate that they are not equivalent, and that a specific role for His(15) in the assembly or structure of the ATP synthase is supported.

Modeling the Leigh syndrome nt8993 T-->C mutation in Escherichia coli F1F0 ATP synthase

The mutations in human mitochondrial DNA at nt8993 are associated with a range of neuromuscular disorders. One mutation encodes a proline in place of a leucine conserved in all animal mitochondrial ATPase-6 subunits and bacterial a subunits of F1F0 ATP synthases. This conserved site is leu-156 and leu-207 in humans and Escherichia coli, respectively. An aleu-207-->pro substitution mutation has been constructed in the E. coli F1F0 ATP synthase in order to model the biochemical basis of the human disease mutation. The phenotype of the aleu-207-->pro substitution has been compared to that of the previously studied aleu-207-->arg substitution (Hartzog and Cain, 1993, Journal of Biological Chemistry 268, 12250-12252). The leu-207-->pro mutation resulted in approximately a 35% decrease in the number of intact enzyme complexes as determined by N, N'-dicyclohexylcarbodiimide-sensitive membrane associated ATP hydrolysis activity and western analysis using an anti-a subunit antibody. A 75% reduction in the efficiency of proton translocation through F1F0 ATP synthase was observed in ATP-driven proton pumping assays. Interestingly, the loss in F1F0 ATP synthase activity resulting from the leu-207-->pro substitution was markedly less dramatic than had been observed for the leu-207-->arg mutation studied earlier. By analogy, the human enzyme may also be affected by the leu-156-->pro substitution to a lesser extent than the leu-156-->arg substitution, and this would account for the milder clinical manifestations of the human leu-156-->pro disease mutations.

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

The multicopy subunit c of the H+-transporting F1F0 ATP synthase of Escherichia coli is thought to fold across the membrane as a hairpin of two hydrophobic alpha-helices. The conserved Asp61, centered in the second transmembrane helix, is essential for H+ transport. In this study, we have made sequential Cys substitutions across both transmembrane helices and used disulfide cross-link formation to determine the oligomeric arrangement of the c subunits. Cross-link formation between single Cys substitutions in helix 1 provided initial limitations on how the subunits could be arranged. Double Cys substitutions at positions 14/16, 16/18, and 21/23 in helix 1 and 70/72 in helix 2 led to the formation of cross-linked multimers upon oxidation. Double Cys substitutions in helix 1 and helix 2, at residues 14/72, 21/65, and 20/66, respectively, also formed cross-linked multimers. These results indicate that at least 10 and probably 12 subunits c interact in a front-to-back fashion to form a ring-like arrangement in F0. Helix 1 packs at the interior and helix 2 at the periphery of the ring. The model indicates that the Asp61 carboxylate is centered between the helical faces of adjacent subunit c at the center of a four-helix bundle.

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.

Interacting helical faces of subunits a and c in the F1Fo ATP synthase of Escherichia coli defined by disulfide cross-linking

Subunits a and c of Fo are thought to cooperatively catalyze proton translocation during ATP synthesis by the Escherichia coli F1Fo ATP synthase. Optimizing mutations in subunit a at residues A217, I221, and L224 improves the partial function of the cA24D/cD61G double mutant and, on this basis, these three residues were proposed to lie on one face of a transmembrane helix of subunit a, which then interacted with the transmembrane helix of subunit c anchoring the essential aspartyl group. To test this model, in the present work Cys residues were introduced into the second transmembrane helix of subunit c and the predicted fourth transmembrane helix of subunit a. After treating the membrane vesicles of these mutants with Cu(1, 10-phenanthroline)2SO4 at 0 degrees, 10 degrees, or 20 degreesC, strong a-c dimer formation was observed at all three temperatures in membranes of 7 of the 65 double mutants constructed, i.e., in the aS207C/cI55C, aN214C/cA62C, aN214C/cM65C, aI221C/cG69C, aI223C/cL72C, aL224C/cY73C, and aI225C/cY73C double mutant proteins. The pattern of cross-linking aligns the helices in a parallel fashion over a span of 19 residues with the aN214C residue lying close to the cA62C and cM65C residues in the middle of the membrane. Lesser a-c dimer formation was observed in nine other double mutants after treatment at 20 degreesC in a pattern generally supporting that indicated by the seven landmark residues cited above. Cross-link formation was not observed between helix-1 of subunit c and helix-4 of subunit a in 19 additional combinations of doubly Cys-substituted proteins. These results provide direct chemical evidence that helix-2 of subunit c and helix-4 of subunit a pack close enough to each other in the membrane to interact during function. The proximity of helices supports the possibility of an interaction between Arg210 in helix-4 of subunit a and Asp61 in helix-2 of subunit c during proton translocation, as has been suggested previously.

On the role of Arg-210 and Glu-219 of subunit a in proton translocation by the Escherichia coli F0F1-ATP synthase

A strain of Escherichia coli was constructed which had a complete deletion of the chromosomal uncB gene encoding subunit a of the F0F1-ATP synthase. Gene replacement was facilitated by a selection protocol that utilized the sacB gene of Bacillus subtilis cloned in a kanamycin resistance cartridge (Ried, J. L., and Collmer, A. (1987) Gene (Amst.) 57, 239-246). F0 subunits b and c inserted normally into the membrane in the DeltauncB strain. This observation confirms a previous report (Hermolin, J., and Fillingame, R. H. (1995) J. Biol. Chem. 270, 2815-2817) that subunit a is not required for the insertion of subunits b and c. The DeltauncB strain has been used to characterize mutations in Arg-210 and Glu-219 of subunit a, residues previously postulated to be essential in proton translocation. The aE219G and aE219K mutants grew on a succinate carbon source via oxidative phosphorylation and membranes from these mutants exhibited ATPase-coupled proton translocation (i.e. ATP driven 9-amino-6-chloromethoxyacridine quenching responses that were 60-80% of wild type membranes). We conclude that the aGlu-219 residue cannot play a critical role in proton translocation. The aR210A mutant did not grow on succinate and membranes exhibited no ATPase-coupled proton translocation. However, on removal of F1 from membrane, the aR210A mutant F0 was active in passive proton translocation, i.e. in dissipating the DeltapH normally established by NADH oxidation with these membrane vesicles. aR210A membranes with F1 bound were also proton permeable. Arg-210 of subunit a may play a critical role in active H+ transport that is coupled to ATP synthesis or hydrolysis, but is not essential for the translocation of protons across the membranes.

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.

The F0F1-type ATP synthases of bacteria: structure and function of the F0 complex

Membrane-bound ATP synthases (F0F1-ATPases) of bacteria serve two important physiological functions. The enzyme catalyzes the synthesis of ATP from ADP and inorganic phosphate utilizing the energy of an electrochemical ion gradient. On the other hand, under conditions of low driving force, ATP synthases function as ATPases, thereby generating a transmembrane ion gradient at the expense of ATP hydrolysis. The enzyme complex consists of two structurally and functionally distinct parts: the membrane-integrated ion-translocating F0 complex and the peripheral F1 complex, which carries the catalytic sites for ATP synthesis and hydrolysis. The ATP synthase of Escherichia coli, which has been the most intensively studied one, is composed of eight different subunits, five of which belong to F1, subunits alpha, beta, gamma, delta, and epsilon (3:3:1:1:1), and three to F0, subunits a, b, and c (1:2:10 +/- 1). The similar overall structure and the high amino acid sequence homology indicate that the mechanism of ion translocation and catalysis and their mode of coupling is the same in all organisms.