Role of the delta subunit in enhancing proton conduction through the F0 of the Escherichia coli F1F0 ATPase.

We studied the effect of the delta subunit of the Escherichia coli F1 ATPase on the proton permeability of the F0 proton channel synthesized and assembled in vivo. Membranes isolated from an unc deletion strain carrying a plasmid containing the genes for the F0 subunits and the delta subunit were significantly more permeable to protons than membranes isolated from the same strain carrying a plasmid containing the genes for the F0 subunits alone. This increased proton permeability could be blocked by treatment with either dicyclohexyl-carbodiimide or purified F1, both of which block proton conduction through the F0. After reconstitution with purified F1 in vitro, both membrane preparations could couple proton pumping to ATP hydrolysis. These results demonstrate that an interaction between the delta subunit and the F0 during synthesis and assembly produces a significant change in the proton permeability of the F0 proton channel.     

Proton leakiness caused by cloned genes for the F0 sector of the proton-translocating ATPase of Escherichia coli: requirement for F1 genes.

To study expression of uncG, the gene coding for the gamma subunit of the Escherichia coli proton-translocating ATPase, deletions were made in the intergenic region between uncA, the gene coding for the alpha subunit, and uncG. Two deletions which fused uncA and uncG coded for alpha-gamma fusion polypeptides which were synthesized well both in vitro and in vivo, demonstrating that uncG expression is normally controlled by nucleotides in the intergenic region. Multicopy plasmids carrying these fusion genes and the genes for the other subunits of the ATPase had a harmful effect on the growth of E. coli. The effect was overcome by N,N'-dicyclohexylcarbodiimide, indicating that the cells probably leaked protons. The deleterious effect was eliminated by making a nonpolar deletion in the upstream F0 gene uncB, or by cloning each of the uncA-uncG fusion genes onto a separate plasmid, removed from the F0 genes, thus demonstrating that the fusion genes were not primarily responsible for the proton permeability. A plasmid which carried F0 genes and the gene for the delta subunit caused deleterious proton leakiness in unc+ cells but not in cells from which the unc operon was deleted. The proton leakiness caused by these different plasmids was therefore due to the production of a leaky F0 proton channel and required the presence of F1 genes. The results support a model for ATPase assembly in which F1 genes or polypeptides are involved in the formation or opening of the F0 proton channel.     

Assembly of a functional F0 of the proton-translocating ATPase of Escherichia coli

We have investigated both structural and functional assembly of the F0 portion of the Escherichia coli proton-translocating ATPase in vivo. Fractionation of E. coli minicells containing plasmids which code for parts of the unc operon shows that each of the F0 peptides a, b, and c insert into the cytoplasmic membrane independent of each other and without the polypeptides which form the F1 portion of the complex alpha, beta, gamma, delta, and epsilon. Assays of membrane energization indicate that, while formation of a functional proton channel requires the presence of all three F0 polypeptides a, b and c, they are not sufficient. Synthesis of both the alpha and beta subunits of the F1 are required for formation of a functional proton channel.     

Mutants of Escherichia coli H+-ATPase defective in the delta subunit of F1 and the b subunit of F0

Complete nucleotide sequence of the genes for subunits of the H+ ATPase of E.coli has been determined and several hybrid plasmids carrying various portions of these genes have been constructed. Genetic complementation and recombination tests of about forty mutants of E.coli defective in the ATPase were performed using these plasmids for identifying the locations of the mutations. Two mutants defective in the delta subunit and a novel type of mutant defective in the b subunit of F0 were identified. The delta subunit mutants showed no proton conduction, suggesting that this subunit has an important role for the proton conduction. The ATPase of the b subunit mutant has a normal activity of proton channel portion, which phenotype is clearly different from that of mutants of the b subunit reported previously.     

Assembly of the F0 proton channel of the Escherichia coli F1F0 ATPase: low proton conductance of reconstituted Fo sectors synthesized and assembled in the absence of F1.

We have previously proposed that during assembly of the Escherichia coli F1F0 ATPase, the proton permeability of the Fo sector of the E. coli F1F0 ATPase is increased significantly by interactions with F1 subunits [Pati, S., & Brusilow, W.S.A. (1989) J. Biol. Chem 264, 2640-2644]. To test this model for Fo assembly, we purified F0 sectors synthesized in the presence and absence of F1 subunits and measured the abilities of these different preparations to bind purified F1 ATPase and to conduct protons when reconstituted into liposomes. The results of these studies demonstrated significant differences in proton-conducting abilities of the different Fo preparations. Fo sectors synthesized in the presence of F1 subunits were more permeable to protons than those synthesized in the absence of F1 subunits.     

Synthesis of a functional F0 sector of the Escherichia coli H+-ATPase does not require synthesis of the alpha or beta subunits of F1.

The uncB, E, F, and H genes of the Escherichia coli unc operon were cloned behind the lac promoter of plasmid pUC9, generating plasmid pBP101. These unc loci code, respectively, for the chi, omega, and psi subunits of the F0 sector and the delta subunit of the F1 sector of the H+-ATP synthase complex. Induction of expression of the four unc genes by the addition of isopropyl-beta-D-thiogalactoside resulted in inhibition of growth. During isopropyl-beta-D-thiogalactoside induction, the three subunits of F0 were integrated into the cytoplasmic membrane with a resultant increase in H+ permeability. A functional F0 was formed from plasmid pBP101 in a genetic background lacking all eight of the unc structural genes coding the F1F0 complex. In the unc deletion background, a reasonable correlation was observed between the amount of F0 incorporated into the membrane and the function measured, i.e., high-affinity binding of F1 and rate of F0-mediated H+ translocation. This correlation indicates that most or all of the F0 assembled in the membrane is active. Although the F0 assembled under these conditions binds F1, only partial restoration of NADH-dependent or ATP-dependent quenching of quinacrine fluorescence was observed with these membranes. Proteolysis of a fraction of the psi subunit may account for this partial deficiency. The experiments described demonstrate that a functional F0 can be assembled in vivo in E. coli strains lacking genes for the alpha, beta, gamma, and epsilon subunits of F1.     

Subunit delta of H(+)-ATPases: at the interface between proton flow and ATP synthesis

The ATP synthases in photophosphorylation and respiration are of the F-type with a membrane-bound proton channel, F0, and an extrinsic catalytic portion, F1. The properties of one particular subunit, delta (in chloroplasts and Escherichia coli) and OSCP (in mitochondria), are reviewed and the role of this subunit at the interface between F0 and F1 is discussed. Delta and OSCP from the three sources have in common the molecular mass (approximately 20 kDa), an elongated shape (axial ratio in solution about 3:1), one high-affinity binding site to F1 (Kd approximately 100 nM) plus probably one or two further low-affinity sites. When isolated delta is added to CF1-depleted thylakoid membranes, it can block proton flow through exposed CF0 channels, as do CF1 or CF1(-delta)+ delta. This identifies delta as part of the proton conductor or, alternatively, conformational energy transducer between F0 (proton flow) and F1 (ATP). Hybrid constructs as CF1(-delta)+ E. coli delta and EF1(-delta)+ chloroplast delta diminish proton flow through CF0.CF1(-delta) + E. coli delta does the same on EF0. Impairment of proton leaks either through CF0 or through EF0 causes "structural reconstitution' of ATP synthesis by remaining intact F0F1. Functional reconstitution (ATP synthesis by fully reconstructed F0F1), however, is absolutely dependent on the presence of subunit delta and is therefore observed only with CF1 or CF1(-delta) + chloroplast delta on CF0 and EF1 or EF1(-delta) + E. coli delta on EF0. The effect of hybrid constructs on F0 channels is surprising in view of the limited sequence homology between chloroplast and E. coli delta (36% conserved residues including conservative replacements). An analysis of the distribution of the conserved residues at present does not allow us to discriminate between the postulated conformational or proton-conductive roles of subunit delta.     

F0F1-ATPase from Vibrio alginolyticus. Subunit composition and proton pumping activity.

An F0F1-ATPase was isolated from the membranes of the marine bacterium Vibrio alginolyticus. Homology between the subunits of the F0-complexes from E. coli and V. alginolyticus was found using antibodies against subunits a, b and c of the E. coli F0F1-ATPase. The F0F1-complex from V. alginolyticus was reconstituted into proteoliposomes, which were competent in ATP-dependent proton uptake. This process was inhibited by triphenyltin, DCCD, and venturicidin. Na+ did not affect proton translocation.     

F1F0-ATPase from Escherichia coli with mutant F0 subunits. Partial purification and immunoprecipitation of F1F0 complexes

Previously identified mutations in subunits a and b of the F0 sector of the F1F0-ATPase from Escherichia coli are further characterized by isolating detergent-solubilized, partially purified F1F0 complexes from cells bearing these mutations. The composition of the various F1F0 complexes was judged by quantitating the amount of each subunit present in the detergent-solubilized preparations. The composition of the F0 sectors containing altered polypeptides was determined by quantitating the F0 subunits that were immunoprecipitated by antibodies directed against the F1 portion. In this way, the relative amounts of F0 subunits (a, b, c) which survived the isolation procedure bound to F1 were determined for each mutation. This analysis indicates that both missense mutations in subunit a (aser206----leu and ahis245----tyr) resulted in the isolation of F1F0 complexes with normal subunit composition. The nonsense mutation in subunit a (atyr235----end) resulted in isolation of a complex containing the b and c subunits. The bgly131----asp mutation in the b subunit results in an F0 complex which does not assemble or survive the isolation. The isolated F1F0 complex containing the mutation bgly9----asp in the b subunit was defective in two regards: first, a reduction in F1 content relative to F0 and second, the absence of the a subunit. Immunoprecipitations of this preparation demonstrated that F1 interacts with both c and mutant b subunits. A strain carrying the mutation, bgly9----asp, and the compensating suppressor mutation apro240----leu (previously shown to be partially unc+) yielded an F1F0 ++ complex that remained partially defective in F1 binding to F0 but normal in the subunit composition of the F0 sector. The assembly, structure, and function of the F1F0-ATPase is discussed.     

Cross-reconstitution of the F0F1-ATP synthases of chloroplasts and Escherichia coli with special emphasis on subunit delta

F0F1-ATP synthases catalyse ATP formation from ADP and Pi by using the free energy supplied by the transmembrane electrochemical potential of the proton. The delta subunit of F1 plays an important role at the interface between the channel portion F0 and the catalytic portion F1. In chloroplasts it can plug the protonic conductance of CF0 and in Escherichia coli it is required for binding of EF1 to EF0. We wanted to know whether or not delta of one species was effective between F0 and F1 of the other species and vice versa. To this end the respective coupling membrane (thylakoids, everted vesicles from E. coli) was (partially) depleted of F1 and purified F1, F1(-delta), and delta were added in various combinations to the F1-depleted membranes. The efficiency or reconstitution was measured in thylakoids via the rate of phenazinemethosulfate-mediated cyclic photophosphorylation and in E. coli everted vesicles via the degree of 9-amino-6-chloro-2-methoxyacridine fluorescence quenching. Addition of CF1 to partially CF1-depleted thylakoid vesicles restored photophosphorylation to the highest extent. CF1(-delta)+chloroplast delta, EF1, EF1(-delta)+E. coli delta were also effective but to lesser extent. CF1(-delta)+E. coli delta and EF1(-delta)+chloroplast delta restored photophosphorylation to a small but still significant extent. With F1-depleted everted vesicles prepared by repeated EDTA treatment of E. coli membranes, addition of CF1, CF1 (-delta)+chloroplast delta and CF1(-delta)+E. coli delta gave approximately half the extent of 9-amino-6-chloro-2-methoxyacridine fluorescence quenching as compared to EF1 or EF1(-delta)+E. coli delta by energization of the vesicles with NADH, while Ef1(-delta)+chloroplast delta was ineffective. All 'mixed' combinations were probably reconstitutively active only by plugging the protonic leak through the exposed F0 (structural reconstitution) rather than by catalytic activity. Nevertheless, the cross-reconstitution is stunning in view of the weak sequence similarity between chloroplast delta and E. coli delta. It favors a role of delta as a conformational transducer rather than as a proton conductor between F0 and F1.     

Characterization of the H(+)-pumping F1F0 ATPase of Vibrio alginolyticus.

The F1F0 ATPase of Vibrio alginolyticus was cloned from a chromosomal lambda library. The unc operon, which contains the structural genes for the ATPase, was sequenced and shown to have a gene organization of uncIBEFHAGDC. The sequence of each subunit was compared with those of other eubacterial ATPases. The V. alginolyticus unc genes exhibited greater similarity to the Escherichia coli unc genes than to any of the other bacterial unc genes for which the sequence is available. The ATPase was expressed in an E. coli unc deletion strain, and the ATP hydrolytic activity was characterized. It has a pH optimum of 7.6 and is stimulated by the addition of Triton X-100 or any of a variety of salts. The recombinant F1F0 was purified 30.4-fold and reconstituted into proteoliposomes. This enzyme catalyzed the pumping of protons coupled to ATP hydrolysis as measured in fluorescence quenching experiments but would not pump Na+ ions under similar conditions.     

All three subunits are required for the reconstitution of an active proton channel (F0) of Escherichia coli ATP synthase (F1F0).

The membrane-integrated, proton-translocating F0 portion of the ATP synthase (F1F0) from Escherichia coli is built up from three kinds of subunits a, b and c with the proposed stoichiometry of 1:2:10 +/- 1. We have dissociated the F0 complex by treatment with trichloroacetate (3 M) at pH 8.0, in the presence of deoxycholate (1%) and N-tetradecyl-N, N-dimethyl-3-ammonio-1-propanesulfonate (Zwittergent 3-14, 5%). The subunits were separated by gel filtration with trichloroacetate (1 M) included in the elution buffer. The homogeneity of the fractions was checked by rechromatography and SDS-gel electrophoresis. After integration into phospholipid vesicles each subunit alone as well as all possible combinations were tested for H+ translocating activity and binding of F1. A functional H+ channel could only be reconstituted by the combination a1b2c10 which corresponds to that of native F0.     

The Fo subunits of the Escherichia coli F1Fo-ATP synthase are sufficient to form a functional proton pore

The assembly of the Fo sector of the Escherichia coli ATP synthase has been studied using both structural and functional criteria for assembly. Cross-linking E. coli minicell membranes containing only the Fo subunits a, b, and c with dithiobis(succinimidyl propionate) (DSP) produces b2 and c2 dimers that are generated by cross-linking membranes containing the assembled holoenzyme. Five plasmids carrying the genes specifying the Fo polypeptides in a bacterial strain lacking all of the unc (ATP synthase) genes show a good correlation between Fo function and the amount of the membrane-bound Fo polypeptides. In this report we revise a conclusion reached previously (Klionsky, D.J., Brusilow, W.S.A., and Simoni, R.D. (1983) J. Biol. Chem. 258, 10136-10143) and present evidence that the Fo subunits alone are sufficient to assemble a functional proton pore.     

Complementation between uncF alleles affecting assembly of the F1F0-ATPase complex of Escherichia coli.

A mutant affected in the b subunit (coded by the uncF gene) of the F1F0-ATPase in Escherichia coli was isolated by a localized mutagenesis procedure in which a plasmid carrying the unc genes was mutagenized in vivo. The biochemical properties of cells carrying the uncF515 allele were examined in a strain carrying the allele on a multicopy plasmid and a mutator-induced polar unc mutation on the chromosome. The strain carrying the mutant unc allele was uncoupled with respect to oxidative phosphorylation. Membrane-bound ATPase activity was very low or absent, and membranes were somewhat proton permeable. It was concluded that the F0 sector was assembled. Determination of the DNA sequence of the uncF515 allele showed it differed from wild type in that a G----A substitution occurred at position 392, resulting in glycine being replaced by aspartate at position 131. Genetic complementation tests indicated that the uncF515 allele complemented the uncF476 allele (Gly 9----Asp). Two-dimensional gel electrophoresis of membrane preparations indicated that the uncF515 and uncF476 alleles interrupted assembly of the F1F0-ATPase at different stages.     

Defective gamma subunit of ATP synthase (F1F0) from Escherichia coli leads to resistance to aminoglycoside antibiotics.

A strain of Escherichia coli which was derived from a gentamicin-resistant clinical isolate was found to be cross-resistant to neomycin and streptomycin. The molecular nature of the genetic defect was found to be an insertion of two GC base pairs in the uncG gene of the mutant. The insertion led to the production of a truncated gamma subunit of 247 amino acids in length instead of the 286 amino acids that are present in the normal gamma subunit. A plasmid which carried the ATP synthase genes from the mutant produced resistance to aminoglycoside antibiotics when it was introduced into a strain with a chromosomal deletion of the ATP synthase genes. Removal of the genes coding for the beta and epsilon subunits abolished antibiotic resistance coded by the mutant plasmid. The relationship between antibiotic resistance and the gamma subunit was investigated by testing the antibiotic resistance of plasmids carrying various combinations of unc genes. The presence of genes for the F0 portion of the ATP synthase in the presence or absence of genes for the gamma subunit was not sufficient to cause antibiotic resistance. alpha, beta, and truncated gamma subunits were detected on washed membranes of the mutant by immunoblotting. The first 247 amino acid residues of the gamma subunit may be sufficient to allow its association with other F1 subunits in such a way that the proton gate of F0 is held open by the mutant F1.     

The roles of the alpha and gamma subunits in proton conduction through the Fo sector of the proton-translocating ATPase of Escherichia coli

Previous genetic and biochemical studies have shown that the Fo sector of the Escherichia coli H+-ATPase is synthesized and assembled in a nonleaky form from plasmid-borne genes. The proton channel then appears to be opened by an interaction of F1 subunits, especially the alpha subunit, with the nonleaky Fo (Brusilow, W. S. A. (1987) J. Bacteriol. 169, 4984-4990; Solomon, K. A., and Brusilow, W. S. A. (1988) J. Biol. Chem. 263, 5402-5407). To study the role of the alpha and gamma subunits in proton conduction, we constructed an inducible alpha plasmid. In an alpha-gamma- background, the induction of alpha synthesis caused lethal proton leakiness, as assayed by the loss of respiration-dependent acridine orange fluorescence quenching of E. coli membranes. The presence of a gamma subunit counteracted the lethal effects as if gamma were blocking the opened channel.     

Impaired proton conductivity resulting from mutations in the a subunit of F1F0 ATPase in Escherichia coli

Mutations in the uncB gene which encodes the a subunit of F1F0-ATPase in Escherichia coli were isolated and characterized. Eight mutations caused premature polypeptide chain termination. Two mutations were single amino acid substitutions resulting in the replacements of serine 206 with leucine (ser-206----leu) and histidine 245 with tyrosine (his-245----tyr). The ser-206----leu mutation does not alter F1 binding and allows ATP driven membrane energization at a low level. Stripping of F1 from membranes containing the ser-206----leu mutation does not render the membranes permeable to protons indicating impaired proton conductivity. The his-245----tyr mutation also blocks Fo-mediated proton conduction but has normal F1 binding properties. F1 bound to membranes with both ser-206----leu and his-245----tyr mutant a subunits is sensitive to dicyclohexylcarbodiimide. Apparently, both missense mutations impair proton conduction without altering assembly of the F1F0-ATPase complex. The direct involvement of the a subunit in proton translocation is discussed.     

Sec/SRP requirements and energetics of membrane insertion of subunits a, b, and c of the Escherichia coli F1F0 ATP synthase

Previously, the role of YidC in the membrane protein biogenesis of the F(0) sector of the Escherichia coli F(1)F(0) ATP synthase was investigated. Whereas subunits a and c of the F(1)F(0) ATP synthase were strictly dependent on YidC for membrane insertion, subunit b required YidC for efficient insertion (Yi, L., Jiang, F., Chen, M., Cain, B., Bolhuis, A., and Dalbey, R. E. (2003) Biochemistry 42, 10537-10544). In this paper, we investigated other protein components and energetics that are required in the membrane protein assembly of the F(0) sector subunits. We show here that the Sec translocase and the signal recognition particle (SRP) pathway are required for membrane insertion of subunits a and b. In contrast, subunit c required neither the Sec machinery nor the SRP pathway for insertion. While the proton motive force was not required for insertion of subunits b and c, it was required for translocation of the negatively charged periplasmic NH(2)-terminal tail of subunit a, whereas periplasmic loop 2 of subunit a could insert in a proton motive force-independent manner. Taken together, the in vivo data suggest that subunits a and b are inserted by the Sec/SRP pathway with the help of YidC, and subunit c is integrated into the membrane by the novel YidC pathway.