Functional effects and cross-reactivity of antibody to purified subunit b (uncF protein) of Escherichia coli proton-ATPase

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.

ADP- and oligomycin-sensitive redox behavior of F0 b thiol in ATPsynthase depends on neighbored primary structure: investigations using 14-C-labeled alpha lipoic acid

Purified ATPsynthase of bovine heart mitochondria has been analyzed for its mobility and reactivity of oligomycin-sensitive sulfhydryl regions in presence of the substrate ADP and oligomycin. Labeling of thiol groups at the hydrophobic F_0 region of the ATPsynthase was increased in the enzyme initially treated with SDS, N-ethylmaleimide and dithiothreitol (modified enzyme). After dialysis or gel permeation the ATPsynthase was treated with [14C] alpha lipoic acid at a molar ratio of 35-85/1 (lipoic acid/ATPsynthase) corresponding to 4-8.6 nmol/mg protein. Under these conditions, ATPase activity of the native enzyme was significantly decreased. After preincubation with ADP, PAGE of the native, [14C] labeled enzyme revealed an increase of radioactivity at a region of 25 kDa deduced to Cys 197 of subunit b. In the modified enzyme the increase in radioactivity was found at 10 kDa. In this context, the sequence Lys-Cys-Ile around Cys 197 of subunit b suggests excessive reactivity of this thiol, as well as ready reversibility by -SH-S-S- interchange. Therefore, previously observed reaction by thiol reagents and antioxidants from outside the mitochondrion can be interpreted with Cys 197 of F0 b. It accounts for sulfhydryl unmasked by binding of ADP at F1.

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.

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.

Secondary structure composition of reconstituted subunit b of the Escherichia coli ATP synthase

Subunit b of the Escherichia coli ATP synthase was isolated by preparative gel electrophoresis, acetone precipitated and after ion-pair extraction redissolved in a buffer either containing n-dodecyl-beta-D-maltoside or sodium cholate. The secondary structure of isolated subunit b was shown to be the same as within the FO complex, but was strongly dependent on the detergent used for replacement of the phospholipid environment. This was shown by an identical tryptic digestion pattern, which was strongly influenced by the detergent used for solubilization. An influence of the detergent n-dodecyl-beta-D-maltoside on the secondary structure of the hydrophilic part of subunit b was also shown for the soluble part of the polypeptide comprising residues Val25 to Leu156 (bsol) using CD spectroscopy. In order to determine the secondary structure of subunit b in its native conformation, isolated subunit b was reconstituted into E. coli lipid vesicles and analyzed with CD spectroscopy. The resulting spectrum revealed a secondary structure composition of 80% alpha helix together with 14% beta turn conformation. These results suggest that subunit b is not a rigid rod-like alpha helix simply linking F1 to FO, but rather provides an inherent flexibility for the storage of elastic energy within the second stalk generated by rotational movements within the F1FO complex.

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

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

Angiostatin binds ATP synthase on the surface of human endothelial cells

Angiostatin, a proteolytic fragment of plasminogen, is a potent antagonist of angiogenesis and an inhibitor of endothelial cell migration and proliferation. To determine whether the mechanism by which angiostatin inhibits endothelial cell migration and/or proliferation involves binding to cell surface plasminogen receptors, we isolated the binding proteins for plasminogen and angiostatin from human umbilical vein endothelial cells. Binding studies demonstrated that plasminogen and angiostatin bound in a concentration-dependent, saturable manner. Plasminogen binding was unaffected by a 100-fold molar excess of angiostatin, indicating the presence of a distinct angiostatin binding site. This finding was confirmed by ligand blot analysis of isolated human umbilical vein endothelial cell plasma membrane fractions, which demonstrated that plasminogen bound to a 44-kDa protein, whereas angiostatin bound to a 55-kDa species. Amino-terminal sequencing coupled with peptide mass fingerprinting and immunologic analyses identified the plasminogen binding protein as annexin II and the angiostatin binding protein as the alpha/beta-subunits of ATP synthase. The presence of this protein on the cell surface was confirmed by flow cytometry and immunofluorescence analysis. Angiostatin also bound to the recombinant alpha-subunit of human ATP synthase, and this binding was not inhibited by a 2,500-fold molar excess of plasminogen. Angiostatin's antiproliferative effect on endothelial cells was inhibited by as much as 90% in the presence of anti-alpha-subunit ATP synthase antibody. Binding of angiostatin to the alpha/beta-subunits of ATP synthase on the cell surface may mediate its antiangiogenic effects and the down-regulation of endothelial cell proliferation and migration.

The second stalk composed of the b- and delta-subunits connects F0 to F1 via an alpha-subunit in the Escherichia coli ATP synthase

The b- and delta-subunits of the Escherichia coli ATP synthase are critical for binding ECF1 to the F0 part, and appear to constitute the stator necessary for holding the alpha3beta3 hexamer as the c-epsilon-gamma domain rotates during catalysis. Previous studies have determined that the b-subunits are dimeric for a large part of their length, and interact with the F1 part through the delta-subunit (Rodgers, A. J. W., Wilkens, S., Aggeler, R., Morris, M. B., Howitt, S. M., and Capaldi, R. A. (1997) J. Biol. Chem. 272, 31058-31064). To further study b-subunit interactions, three mutants were constructed in which Ser-84, Ala-144, and Leu-156, respectively, were replaced by Cys. Treatment of purified ECF1F0 from all three mutants with CuCl2 induced disulfide formation resulting in b-subunit dimer cross-link products. In addition, the mutant bL156C formed a cross-link from a b-subunit to an alpha-subunit via alphaCys90. Neither b-b nor b-alpha cross-linking had significant effect on ATPase activities in any of the mutants. Proton pumping activities were measured in inner membranes from the three mutants. Dimerization of the b-subunit did not effect proton pumping in mutants bS84C or bA144C. In the mutant bL156C, CuCl2 treatment reduced proton pumping markedly, probably because of uncoupling caused by the b-alpha cross-link formation. The results show that the alpha-subunit forms part of the binding site on ECF1 for the b2delta domain and that the b-subunit extends all the way from the membrane to the top of the F1 structure. Some conformational flexibility in the connection between the second stalk and F1 appears to be required for coupled catalysis.

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.

The subunit delta-subunit b domain of the Escherichia coli F1F0 ATPase. The B subunits interact with F1 as a dimer and through the delta subunit

The delta and b subunits are both involved in binding the F1 to the F0 part in the Escherichia coli ATP synthase (ECF1F0). The interaction of the purified delta subunit and the isolated hydrophilic domain of the b subunit (bsol) has been studied here. Purified delta binds to bsol weakly in solution, as indicated by NMR studies and protease protection experiments. On F1, i.e. in the presence of ECF1-delta, delta, and bsol interact strongly, and a complex of ECF1.bsol can be isolated by native gel electrophoresis. Both delta subunit and bsol are protected from trypsin cleavage in this complex. In contrast, the delta subunit is rapidly degraded by the protease when bound to ECF1 when bsol is absent. The interaction of bsol with ECF1 involves the C-terminal domain of delta as delta(1-134) cannot replace intact delta in the binding experiments. As purified, bsol is a stable dimer with 80% alpha helix. A monomeric form of bsol can be obtained by introducing the mutation A128D (Howitt, S. M., Rodgers, A. J.,W., Jeffrey, P. D., and Cox, G. B. (1996) J. Biol. Chem. 271, 7038-7042). Monomeric bsol has less alpha helix, i.e. only 58%, is much more sensitive to trypsin cleavage than dimer, and unfolds at much lower temperatures than the dimer in circular dichroism melting studies, indicating a less stable structure. The bsol dimer, but not monomer, binds to delta in ECF1. To examine whether subunit b is a monomor or dimer in intact ECF1F0, CuCl2 was used to induce cross-link formation in the mutants bS60C, bQ104C, bA128C, bG131C, and bS146C. With the exception of bS60C, CuCl2 treatment resulted in formation of b subunit dimers in all mutants. Cross-linking yield was independent of nucleotide conditions and did not affect ATPase activity. These results show the b subunit to be dimeric for a large portion of the C terminus, with residues 124-131 likely forming a pair of parallel alpha helices.

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 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.

Altered properties of mitochondrial ATP-synthase in patients with a T-->G mutation in the ATPase 6 (subunit a) gene at position 8993 of mtDNA

A family is described with a T-->G mutation at position 8993 of mtDNA. This mutation is located in the ATPase 6 gene of mtDNA which encodes subunit a of the ATP-synthase complex (FlFo-ATPase). Clinically, the patients showed severe infantile lactate acidosis and encephalomyopathy in a form that was different from the classical Leigh syndrome. In 3 affected boys, ranging in age from 3 months to 8 years, the mutation was found in 95-99% of the mtDNA population. The clinical symptoms correlated with the mtDNA heteroplasmy and in the healthy mother 50% of the mtDNA was mutated. The rate of mitochondrial ATP production by cultured skin fibroblasts containing 99% of mutated mtDNA was about 2-fold lower than that in normal fibroblasts. Native electrophoresis of the mitochondrial enzyme complexes revealed instability of the FlFo-ATPase in all the tissues of the patient that were investigated (heart, muscle, kidney, liver). Only a small portion of the ATP-synthase complex was present in the complete, intact form (620 kDa). Incomplete forms of the enzyme were present as subcomplexes with approx. molecular weights of 460, 390 and 150 kDa, respectively, which differed in the content of F1 and Fo subunits. Immunochemical analysis of the subunits of the FlFo-ATPase further revealed a markedly decreased content of the Fo subunit b in mitochondria from muscle and heart, and an increased content of the Fo subunit c in muscle mitochondria, respectively. These results indicate that in this family the T-->G point mutation at position 8993 in the mitochondrial ATPase 6 gene is accompanied by structural instability and altered assembly of the enzyme complex, that are both most likely due to changes in the properties of subunit a of the membrane sector part of the ATP-synthase.

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

The F0 sector of the Escherichia coli H+ transporting ATP synthase is composed of a complex of three subunits, each of which traverses the inner membrane. We have studied the interdependence of subunit insertion into the membrane in a series of chromosomal mutants in which the primary mutation prevented insertion of one of the F0 subunits. Subunit insertion was assessed using Western blots of mutant membrane preparations. Subunit b and subunit c were found to insert into the membrane independently of the other two F0 subunits. On the other hand, subunit a was not inserted into membranes that lacked either subunit b or subunit c. The conclusion that subunit a insertion is dependent upon the co-insertion of subunits b and c differs from the conclusion of several studies, where subunits were expressed from multicopy plasmids.

Plant mitochondrial F0F1 ATP synthase. Identification of the individual subunits and properties of the purified spinach leaf mitochondrial ATP synthase

Spinach leaf mitochondrial F0F1 ATPase has been purified and is shown to consist of twelve polypeptides. Five of the polypeptides constitute the F1 part of the enzyme. The remaining polypeptides, with molecular masses of 28 kDa, 23 kDa, 18.5 kDa, 15 kDa, 10.5 kDa, 9.5 kDa and 8.5 kDa, belong to the F0 part of the enzyme. This is the first report concerning identification of the subunits of the plant mitochondrial F0. The identification of the components is achieved on the basis of the N-terminal amino acid sequence analysis and Western blot technique using monospecific antibodies against proteins characterized in other sources. The 28-kDa protein crossreacts with antibodies against the subunit of bovine heart ATPase with N-terminal Pro-Val-Pro- which corresponds to subunit F0b of Escherichia coli F0F1. Sequence analysis of the N-terminal 32 amino acids of the 23-kDa protein reveals that this protein is similar to mammalian oligomycin-sensitivity-conferring protein and corresponds to the F1 delta subunit of the chloroplast and E. coli ATPases. The 18.5-kDa protein crossreacts with antibodies against subunit 6 of the beef heart F0 and its N-terminal sequence of 14 amino acids shows a high degree of sequence similarity to the conserved regions at N-terminus of the ATPase subunits 6 from different sources. ATPase subunit 6 corresponds to subunit F0a of the E. coli enzyme. The 15-kDa protein and the 10.5-kDa protein crossreact with antibodies against F6 and the endogenous ATPase inhibitor protein of beef heart F0F1-ATPase, respectively. The 9.5-kDa protein is an N,N'-dicyclohexylcarbodiimide-binding protein corresponding to subunit F0c of the E. coli enzyme. The 8.5-kDa protein is of unknown identity. The isolated spinach mitochondrial F0F1 ATPase catalyzes oligomycin-sensitive ATPase activity of 3.5 mumol.mg-1.min-1. The enzyme catalyzes also hydrolysis of GTP (7.5 mumol.mg-1.min-1) and ITP (4.4 mumol.mg-1.min-1). Hydrolysis of ATP was stimulated fivefold in the presence of amphiphilic detergents, however the hydrolysis of other nucleotides could not be stimulated by these agents. These results show that the plant mitochondrial F0F1 ATPase complex differs in composition from the other mitochondrial, chloroplast and bacterial ATPases. The enzyme is, however, more closely related to the yeast mitochondrial ATPase and to the animal mitochondrial ATPase than to the chloroplast enzyme. The plant mitochondrial enzyme, however, exhibits catalytic properties which are characteristic for the chloroplast enzyme.

Substitution of the cysteinyl residue (Cys21) of subunit b of the ATP synthase from Escherichia coli

The Fo complex of the ATP synthase (F1Fo) of Escherichia coli contains only two cysteinyl residues, Cys21, of the two copies of subunit b. Modification of Cys21 with the hydrophobic maleimide N-(7-dimethylamino-4-methyl-coumarinyl)maleimide resulted in impairment of Fo functions [Schneider, E. & Altendorf, K. (1985) Eur. J. Biochim. 153, 105-109]. We replaced this residue (via cassette mutagenesis) by Ser, Gly, Ala, Thr, Asp and Pro. None of the replacements resulted in detectable alterations of the function of the ATP synthase, making a functional role for these sulfhydryl residues unlikely. Due to its high tolerance towards amino acid substitutions, the region around Cys21 seems not to be a protein-protein contact area.

Targeted mutagenesis of the b subunit of F1F0 ATP synthase in Escherichia coli: Glu-77 through Gln-85

Subunit b of Escherichia coli F1F0 ATP synthase contains a large hydrophilic region thought to be involved in the interaction between F1 and F0. Oligonucleotide-directed mutagenesis was used to evaluate the functional importance of a segment of this region from Glu-77 through Gln-85. The mutagenesis procedure employed a phagemid DNA template and a doped oligonucleotide primer designed to generate a predetermined collection of missense mutations in the target segment. Sixty-one mutant phagemids were identified and shown to contain nucleotide substitutions encoding 37 novel missense mutations. Mutations were isolated singly or in combinations of up to four mutations per recombinant phagemid. F1F0 ATP synthase function was studied by mutant phagemid complementation of a novel E. coli strain in which the uncF (b) gene was deleted. Complementation was assessed by observing growth on solid succinate minimal medium. Many phagemid-encoded uncF (b) gene mutations in the targeted segment resulted in growth phenotypes indistinguishable from those of strains expressing the native b subunit, suggesting abundant F1F0 ATP synthase activity. In contrast, several specific mutations were associated with a loss of enzyme function. Phagemids specifying the Ala-79----Pro, Arg-82----Pro, Arg-83----Pro, or Gln-85----Pro mutation failed to complement uncF (b) gene-deficient E. coli. F1F0 ATP synthase displayed the greatest sensitivity to mutations altering a single site in the target segment, Ala-79. The evidence suggests that Ala-79 occupies a restricted position in the enzyme complex.

Characterization of Escherichia coli ATP synthase beta-subunit mutations using a chromosomal deletion strain

(1) We constructed Escherichia coli strain JP17 with a deletion in the ATP synthase beta-subunit gene. JP17 is completely deficient in ATP synthase activity and expresses no beta-subunit. Expression of normal beta-subunit from a plasmid restores haploid levels of ATP synthase in membranes. JP17 was shown to be efficacious for studies of beta-subunit mutations. Site-directed mutants were studied directly in JP17. Randomly generated chromosomal mutants were identified by PCR and DNA sequencing, cloned, and expressed in JP17. (2) Eight novel mutations occurring within the putative catalytic nucleotide-binding domain were characterized with respect to their effects on catalysis and structure. The mutations beta C137S, beta G152D, beta G152R, beta E161Q, beta E161R, and beta G251D each impaired catalysis without affecting enzyme assembly or oligomeric structure and are of interest for future studies of catalytic mechanism. The mutations beta D301V and beta D302V, involving strongly conserved carboxyl residues, caused oligomeric instability of F1. However, growth characteristics of these mutants suggested that neither carboxyl side chain is critical for catalysis. (3) The mutations beta R398C and beta R398W rendered ATP synthase resistant to aurovertin, giving strong support to the view that beta R398 is a key residue in the aurovertin-binding site. Neither beta R398C or beta R398W impaired catalysis significantly.

Mutations in alpha-subunit of Escherichia coli F1-ATPase obtained by hydroxylamine-mutagenesis of plasmids carrying the uncA gene

In order to generate mutants randomly in the Escherichia coli uncA gene (encoding the alpha-subunit of F1-ATPase), plasmids carrying uncA were treated in vitro with hydroxylamine. Restriction fragments of the mutated uncA gene were then reconstructed into plasmid pDP34, which expresses all of the F1F0 structural genes, and the reconstructed mutant plasmids were expressed in a strain carrying a deletion of chromosomal uncA. Each of the mutations was characterized by DNA sequencing, growth assays, and biochemical assays of membrane preparations. Three nonsense and one frameshift mutation were identified and their properties were studied briefly. Eight new missense mutations were identified and characterization of their properties is described. These eight mutations were R139H, A177V, R210C, R303C, A306V, T343I, G351S, and P370L.

Mutations in three of the putative transmembrane helices of subunit a of the Escherichia coli F1F0-ATPase disrupt ATP-driven proton translocation

Three missense mutants in subunit a of the Escherichia coli F1F0-ATPase were isolated and characterized after hydroxylamine mutagenesis of a plasmid carrying the uncB (subunit a) gene. The mutations resulted in Asp119----His, Ser152----Phe, or Gly197----Arg substitutions in subunit a. Function was not completely abolished by any of the mutations. The F0 membrane sector was assembled in all three cases as judged by restoration of dicyclohexylcarbodiimide sensitivity to the F1F0-ATPase. The H+ translocation capacity of F0 was reduced in all three mutants. ATP-driven H+-translocation was also reduced, with the response in the Gly197----Arg mutant being almost nil and that in the Asp119----His and Ser152----Phe mutants less severely affected. The substituted residues are predicted to lie in the second, third, and fourth transmembrane helices suggested in most models for subunit a. The Gly197----Arg mutation lies in a very conserved region of the protein and the substitution may disrupt a structure that is critical to function. The Asp119----His and Ser152----Phe mutations also lie in areas with sequence conservation. A further analysis of randomly generated mutants may provide more information on regions of the protein that are crucial to function. Heterodiploid transformants, carrying plasmids with either the wild-type uncB gene or mutant uncB genes in an uncB (Trp231----stop) background, were characterized biochemically. The truncated subunit a was not detected in membranes of the background strain by Western blotting, and the uncB+ plasmid complemented strain showed normal biochemistry. The uncB mutant genes were shown to cause equivalent defects in either the heterodiploid background configuration, or after incorporation into an otherwise wild-type unc operon. The subunit a (Trp231----stop) background strain was shown to bind F1-ATPase nearly normally despite lacking subunit a in its membrane.

Evidence from immunological studies of structure-mechanism relationship of F1 and F1F0

Monoclonal and polyclonal antibodies directed against peptides of F1-ATPase of F1F0-ATPase synthase provide new and efficient tools to study structure-function relationships and mechanisms of such complex membrane enzymes. This review summarizes the main results obtained using this approach. Antibodies have permitted the determination of the nature of subunits involved in the complex, their stoichiometry, their organization, neighboring interactions, and vectorial distribution within or on either face of the membrane. Moreover, in a few cases, amino acid sequences exposed on a face of the membrane or buried inside the complex have been identified. Antibodies are very useful for detecting the role of each subunit, especially for those subunits which appear to have no direct involvement in the catalytic mechanism. Concerning the mechanisms, the availability of monoclonal antibodies which inhibit (or activate) ATP hydrolysis or ATP synthesis, which modify nucleotide binding or regulation of activities, which detect specific conformations, etc. brings many new ways of understanding the precise functions. The specific recognition by monoclonal antibodies on the beta subunit of epitopes in the proximity of, or in the catalytic site, gives information on this site. The use of anti-alpha monoclonal antibodies has shown asymmetry of alpha in the complex as already shown for beta. In addition, the involvement of alpha with respect to nucleotide site cooperativity has been detected. Finally, the formation of F1F0-antibody complexes of various masses, seems to exclude the functional rotation of F1 around F0 during catalysis.

Topological and functional characterization of the F0I subunit of the membrane moiety of the mitochondrial H+-ATP synthase

Using isolated polypeptides of the F0 sector of bovine heart mitochondrial H+-ATPase, antisera were developed detecting specifically two components of F0. These two components were identified as F0I and oligomycin-sensitivity-conferring protein (OSCP) respectively. Both F0I and OSCP were digested by mild trypsin treatment of submitochondrial particles depleted of the catalytic part of H+-ATPase (USMP). Proteolysis was largely prevented by binding of F1 to F0. Proteolysis of F0I resulted in the formation of three immunoreactive, membrane-bound fragments of apparently 26 kDa, 25.5 kDa and 18 kDa, respectively, indicating that F0I contains trypsin-accessible Arg or Lys residues located close to the end and the middle part of the protein, respectively, which are in intimate contact with F1. Digestion of USMP with trypsin resulted in depression of passive H+ conduction through F0 which could be ascribed to proteolysis of F0I.

F0 part of the ATP synthase from Escherichia coli. Influence of subunits a, and b, on the structure of subunit c

Four different sets of proteoliposomes were prepared from F0, subunit c, a complex of subunits a and c (ac complex) and an ac complex supplemented with subunit b. Only liposomes containing intact F0 or all subunits of F0 were active in proton translocation and F1 binding [Schneider, E. and Altendorf, K. (1985) EMBO J. 4, 515-518]. The conformation of subunit c in the different preparations was analyzed by labelling the proteoliposomes with the hydrophobic photoactivatable reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID). Subsequent isolation and Edman degradation of this polypeptide revealed distinct radioactive labelling patterns over the entire amino acid sequence. In the F0 complex and in the ac complex subunit c retains a labelling pattern which is related to that found in TID-labelled membrane vesicles of Escherichia coli [Hoppe et al. (1984) Biochemistry 23, 5610-5616]. In the absence of subunit a, considerably more and different amino acid residues of subunit c are modified. The labelling data are discussed in relation to structural aspects of F0 and functional properties of proteoliposomes reconstituted with F0 or individual subunits.

Bacterial adenosine 5'-triphosphate synthase (F1F0): purification and reconstitution of F0 complexes and biochemical and functional characterization of their subunits

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F0 portion of Escherichia coli ATP synthase: orientation of subunit c in the membrane

Incubation of right-side-out oriented membrane vesicles of Escherichia coli with tetranitromethane resulted in the nitration of tyrosine residues (Tyr-10 and Tyr-73) of subunit c from the ATP synthase. Cleavage of the protein with cyanogen bromide and separation of the resulting fragments, especially of the tyrosine-containing peptides, clearly demonstrated that the distribution of the nitro groups is similar at any time and at any pH value chosen for the analysis. Furthermore, the percentage of 3-nitrotyrosine present in the two peptide fragments was in good agreement with that obtained for the intact polypeptide chain. While the modification of the tyrosine residues in subunit c with the lipophilic tetranitromethane is independent of the orientation of the membrane vesicles, the subsequent partial conversion of the 3-nitrotyrosine to the amino form only occurred when membrane vesicles with right-side-out orientation were treated with the ionic, water-soluble sodium dithionite, which at certain concentrations cannot penetrate biological membranes. Cleavage of subunit c isolated from nitrated and subsequently reduced membrane vesicles and separation of the resulting fragments by high-pressure liquid chromatography showed that the 3-nitrotyrosine in the Tyr-73-containing peptides has been completely reduced, while the nitro group in peptides containing Tyr-10 remained nearly unaffected.

The defective proton-ATPase of uncA mutants of Escherichia coli: ATP-binding and ATP-induced conformational change in mutant alpha-subunits

Mutations in the uncA gene of Escherichia coli cause loss of both oxidative phosphorylation and ATP-driven generation of the transmembrane proton gradient. The uncA gene encodes the alpha-subunit of the F1-sector of the E. coli membrane proton-ATPase. F1-alpha-subunit from normal (unc+) E. coli binds ATP tightly (KD = 0.1 microM) and undergoes a large ATP-induced conformational change, but the functional role of the ATP-binding site is currently unknown. There is disagreement in the literature as to whether the ATP-binding site is present or lacking in F1-alpha-subunit from uncA mutant strains. One obstacle in studying this question is the difficulty of purifying mutant alpha-subunits in native form. In order to circumvent this difficulty we have studied ATP binding and ATP-induced conformational changes in mixtures of F1 subunits obtained by dissociating uncA mutant F1. Anti-alpha antibody was used in conjunction with immunoblotting to identify the alpha-subunits in the mixtures. Retention of native conformation by the alpha-subunits was demonstrated by the fact that the dissociated alpha-subunits were fully competent to repolymerize with other F1 subunits to yield intact F1 aggregate. The results show that, contrary to previous reports, alpha-subunits from three catalytically defective uncA mutants do indeed bind ATP and do undergo an ATP-induced conformational change. The binding affinity of alpha-subunit for ATP was lower than normal in each of the three mutants, but this is not likely to be a significant factor under physiological conditions.

Accessibility of F0 subunits from Escherichia coli ATP synthase. A study with subunit specific antisera

Antisera have been raised against denatured and non-denatured subunits a, b and c of the F0 complex of the ATP synthase from Escherichia coli. The subunit specificity of the antibodies has been established with immunoblot analysis or enzyme-linked immunosorbent assay (ELISA). In inside-out oriented membrane vesicles the binding avidities of both sets of antisera, against denatured and non-denatured subunits of F0, were similar in the presence as well as in the absence of the F1 part. F1-depleted everted membrane vesicles always produced an efficient binding of the different antisera. In the presence of F1 no antibody recognition could be observed with the anti-a antisera, while anti-b and anti-c antisera showed strong binding. However, a higher membrane protein concentration was necessary for the same antibody binding as in F1-stripped vesicles. In membrane vesicles with right-side-out orientation the recognition of the three F0 subunits was dependent on the antisera set used. Antisera raised against denatured subunits showed no binding to the membrane vesicles, only in case of anti-(dodecylsulfate-denatured b) antiserum could a slight affinity be detected. An antigen-antibody recognition with all three F0 subunits occurred when the antisera against non-denatured subunits were incubated with membrane vesicles of right-side-out orientation. The membrane protein concentration which was necessary to produce a significant binding was 10-100-fold higher compared to that of F1-depleted everted membrane vesicles.