An additional acidic residue in the membrane portion of the b-subunit of the energy-transducing adenosine triphosphatase of Escherichia coli affects both assembly and function

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.

Deletions in the second stalk of F1F0-ATP synthase in Escherichia coli

In Escherichia coli F1F0-ATP synthase, the two b subunits form the second stalk spanning the distance between the membrane F0 sector and the bulk of F1. Current models predict that the stator should be relatively rigid and engaged in contact with F1 at fixed points. To test this hypothesis, we constructed a series of deletion mutations in the uncF(b) gene to remove segments from the middle of the second stalk of the subunit. Mutants with deletions of 7 amino acids were essentially normal, and those with deletions of up to 11 amino acids retained considerable activity. Membranes prepared from these strains had readily detectable levels of F1-ATPase activity and proton pumping activity. Removal of 12 or more amino acids resulted in loss of oxidative phosphorylation. Levels of membrane-associated F1-ATPase dropped precipitously for the longer deletions, and immunoblot analysis indicated that reductions in activity correlated with reduced levels of b subunit in the membranes. Assuming the likely alpha-helical conformation for this area of the b subunit, the 11-amino acid deletion would result in shortening the subunit by approximately 16 A. Since these deletions did not prevent the b subunit from participating in productive interactions with F1, we suggest that the b subunit is not a rigid rodlike structure, but has an inherent flexibility compatible with a dynamic role in coupling.

Insertion scanning mutagenesis of subunit a of the F1F0 ATP synthase near His245 and implications on gating of the proton channel

Subunit a of the E. coli F1F0 ATP synthase was probed by insertion scanning mutagenesis in a region between residues Glu219 and His245. A series of single amino acid insertions, of both alanine and aspartic acid, were constructed after the following residues: 225, 229, 233, 238, 243, and 245. The mutants were tested for growth yield, binding of F1 to membranes, dicyclohexylcarbodiimide sensitivity of ATPase activity, ATP-driven proton translocation, and passive proton permeability of membranes stripped of F1. Significant loss of function was seen only with insertions after positions 238 and 243. In contrast, both insertions after residue 225 and the alanine insertion after residue 245 were nearly identical in function to the wild type. The other insertions showed an intermediate loss of function. Missense mutations of His245 to serine and cysteine were nonfunctional, while the W241C mutant showed nearly normal ATPase function. Replacement of Leu162 by histidine failed to suppress the 245 mutants, but chemical rescue of H245S was partially successful using acetate. An interaction between Trp241 and His245 may be involved in gating a "half-channel" from the periplasmic surface of F0 to Asp61 of subunit a.

A mutation in which alanine 128 Is replaced by aspartic acid abolishes dimerization of the b-subunit of the F0F1-ATPase from Escherichia coli

Site-directed mutagenesis was used to investigate the roles of a short series of hydrophobic amino acids in the b-subunit of the Escherichia coli F0F1-ATPase. A mutation affecting one of these, G131D, had been previously characterized and was found to interrupt assembly of the F0F1-ATPase (Jans, D. A., Hatch, L., Fimmel, A. L., Gibson, D., and Cox, G. B. (1985) J. Bacteriol. 162, 420-426). To extend this work, aspartic acid was substituted for each one of the residues from positions 124 to 132. The properties of mutants in this series are consistent with the region from Val124 to Gly131 forming an alpha-helix. Two of the mutations, V124D and A128D, resulted in a similar phenotype to the G131D mutation. This suggested that Val124, Ala128, and Gly131 form a helical face which may have a role in inter- or intrasubunit interactions. This was tested by overexpressing and purifying the cytoplasmic domains of the wild type and A128D mutant b-subunits. Sedimentation equilibrium centrifugation indicated that the wild type domain formed a dimer whereas the mutant was present as a monomer.

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.

The chloroplast CF0I subunit can replace the b-subunit of the F0F1-ATPase in a mutant strain of Escherichia coli K12

The amino acid sequence of the CF0I subunit from the chloroplast F0F1-ATPase has only a low similarity to the amino acid sequence of the b-subunit of the E. coli F0F1-ATPase. However, secondary and tertiary structure predictions plus the distribution of hydrophobic and hydrophilic amino acids have indicated that these two subunits serve a similar function. This proposition was investigated directly. A cDNA clone for the chloroplast atpF gene, encoding the CF0I subunit, was altered by site-directed mutagensis such that the translation start site corresponded to the N-terminus of the mature protein. An E. coli mutant strain carrying a chain-terminating mutation in the uncF gene, encoding the b-subunit, was transformed with the plasmid carrying the altered atpF gene. The resultant transformant was able to grow on succinate and gave a growth yield similar to that of a wild-type control. Assays on membrane preparations from the transformant also clearly indicated that the mature CF0I subunit from spinach chloroplasts was able to replace the E. coli b-subunit in the E. coli F0F1-ATPase.

Random mutagenesis of the gene for the beta-subunit of F1-ATPase from Escherichia coli

ATP synthesis by oxidative phosphorylation in Escherichia coli occurs in catalytic sites on the beta-subunits of F1-ATPase. Random mutagenesis of the beta-subunit combined with phenotypic screening is potentially important for studies of the catalytic mechanism. However, when applied to haploid strains, this approach is hampered by a preponderance of mutants in which assembly of F1-ATPase in vivo is defective, precluding enzyme purification. Here we mutagenized plasmids carrying the uncD (beta-subunit) gene with hydroxylamine or N-methyl-N'-nitro-N-nitrosoguanidine and isolated, by phenotypic screening and complementation tests, six plasmids carrying mutant uncD alleles. When the mutant plasmids were used to transform a suitable uncD- strain, assembly of F1-ATPase in vivo occurred in each case. Moreover, in one case (beta Gly-223----Asp) F1-ATPase assembly proceeded although it had previously been reported that this mutation, when present on the chromosome of a haploid strain, prevented assembly of the enzyme in vivo. Therefore, this work demonstrates an improved approach for random mutagenesis of the F1-beta-subunit. Six new mutant uncD alleles were identified: beta Cys-137----Tyr; beta Gly-142----Asp; beta Gly-146----Ser; beta Gly-207----Asp; beta-Gly-223----Asp; and a double mutant beta Pro-403----Ser,Gly-415----Asp which we could not separate. The first five of these lie within or very close to the predicted catalytic nucleotide-binding domain of the beta-subunit. The double mutant lies outside this domain; we speculate that the region around residues beta 403-415 is part of an alpha-beta intersubunit contact surface. Membrane ATPase and ATP-driven proton pumping activities were impaired by all six mutations. Purified F1-ATPase was obtained from each mutant and shown to have impaired specific ATPase activity.

The proton pore of the F0F1-ATPase of Escherichia coli: Ser-206 is not required for proton translocation

A series of experiments was carried out to investigate the role of some polar amino acids in the a-subunit of the ATP synthase of Escherichia coli. Site-directed mutagenesis resulted in the amino acid substitutions Ser-199----Ala, Ser-202----Ala, Ser-206----Ala, Arg-61----Gln or Asp-44----Asn. None of these amino acid substitutions affected the ability of the cells to carry out oxidative phosphorylation. It was concluded therefore that the effect of the substitution of leucine for Ser-206 reported previously (Cain, B.D. and Simoni, R.D. (1986) J. Biol. Chem. 261, 10043-10050) was due to the presence of the leucine rather than the absence of serine. Even though cells carrying the Asp-44----Asn substitution were able to carry out oxidative phosphorylation, membranes from such cells remained proton-impermeable after removal of the F1-ATPase. It appears likely that the proton pore of the F0 of the ATP synthase of E. coli consists of four amino acids, namely Arg-219, Glu-210 and His-245 of the a-subunit and Asp-61 of the c-subunit.

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

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The mechanism of ATP synthase: a reassessment of the functions of the b and a subunits

A model for the mechanism of ATP synthase was proposed previously (Cox, G.B., Jans, D.A., Fimmel, A.L., Gibson, F. and Hatch, L. (1984) Biochim. Biophys. Acta 768, 201-208) in which the b subunit of the Fo of Escherichia coli rotated. The driving force was proposed to be an interaction between two charged residues in the membrane, namely, Lys-23 of the b subunit and Asp-61 of the c subunit. To test this proposal the Lys-23 of the b subunit was replaced by threonine using site-directed mutagenesis. The resulting mutant, although it had an impairment in the assembly of the F1F0-ATPase, was normal with respect to oxidative phosphorylation. The role of the a subunit, which had been previously proposed to be a structural one, was reassessed by examination of the possible secondary and tertiary structure of the analogous proteins from several sources. Not only did these subunits appear to have very similar structures, but in each there was a highly conserved helical arm on one of the transmembrane helices which could form a proton channel if it interacted with the Asp-61 of the c subunit. A revised model is therefore presented in which five transmembrane helices from the a subunit and two from the b subunit are surrounded by a ring of c subunits. The highly conserved nature of the structures of the a, b and c subunits from various organisms suggests that the model may have relevance for ATP synthases from bacterial plasma membranes, mitochondria and chloroplasts.

Modification of subunit b of the F0 complex from Escherichia coli ATP synthase by a hydrophobic maleimide and its effects on F0 functions

Purified F0 from Escherichia coli ATP synthase was labelled with N-(7-dimethylamino-4-methyl-coumarinyl)-maleimide (DACM), a hydrophobic reagent which forms a stable, strongly fluorescent adduct with SH groups. Sodium dodecyl sulfate gel electrophoresis clearly demonstrated that subunit b was exclusively labelled, most likely at Cys-21, the only cysteine residue in E. coli F0. The amount of two molecules of DACM bound per F0, which was calculated from the absorption spectrum at 380 nm, is in full agreement with the postulated stoichiometry of two copies of subunit b/F0 complex. Thus the label provides a useful tool for simply detecting subunit b in protein chemical studies. DACM-labelled F0 was incorporated into liposomes and assayed for H+ translocating activity and its capacity to bind purified F1. Whereas the initial rate of H+ uptake was inhibited about 40% the reconstitution of a dicyclohexylcarbodiimide-sensitive F1F0 ATPase activity was completely unaffected. In a second set of experiments we reconstituted an F0 complex from DACM-labelled purified subunit b and an ac complex. In contrast to the results obtained with intact, DACM-labelled F0, both H+ translocating activity and F1 binding capacity were greatly reduced. Our data indicate that cysteine-21, probably together with other amino acids, is involved in maintaining a proper interaction of the hydrophobic N-terminal region of subunit b with the ac complex. This interplay seems to be a prerequisite for at least the in vitro assembly of a functional F0 complex.

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.