Single amino acid insertions probe the alpha subunit of the Escherichia coli F1F0-ATP synthase

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.

Functional analysis of membranous Fo-a subunit of F1Fo-ATP synthase by in vitro protein synthesis

The a subunit of F(1)F(o) (F(1)F(o)-ATP synthase) is a highly hydrophobic protein with five putative transmembrane helices which plays a central role in H(+)-translocation coupled with ATP synthesis/hydrolysis. In the present paper, we show that the a subunit produced by the in vitro protease-free protein synthesis system (the PURE system) is integrated into a preformed F(o) a-less F(1)F(o) complex in Escherichia coli membrane vesicles and liposomes. The resulting F(1)F(o) has a H(+)-coupled ATP synthesis/hydrolysis activity that is approximately half that of the native F(1)F(o). By using this procedure, we analysed five mutations of F(1)F(o), where the conserved residues in the a subunit (Asn(90), Asp(112), Arg(169), Asn(173) and Gln(217)) were individually replaced with alanine. All of the mutant F(o) a subunits were successfully incorporated into F(1)F(o), showing the advantage over conventional expression in E. coli by which three (N90A, D112A, and Q217A) mutant a subunits were not found in F(1)F(o). The N173A mutant retained full activity and the mutants D112A and Q217A had weak, but detectable, activity. No activity was observed for the R169A and N90A mutants. Asn(90) is located in the middle of putative second transmembrane helix and likely to play an important role in H(+)-translocation. The present study exemplifies that the PURE system provides an alternative approach when in vivo expression of membranous components in protein complexes turns out to be difficult.

The role of transmembrane span 2 in the structure and function of subunit a of the ATP synthase from Escherichia coli

The importance of the second transmembrane span of subunit a of the ATP synthase from Escherichia coli has been established by two approaches. First, biochemical analysis of five cysteine-substitution mutants, four of which were previously constructed for labeling experiments, revealed that only D119C, found within the second transmembrane span, was deleterious to ATP synthase function. This mutant had a greatly reduced growth yield, indicating inefficient ATP synthesis, but it retained a significant level of ATP-driven proton translocation and sensitivity to N,N(')-dicyclohexyl-carbodiimide, indicating more robust function in the direction of ATP hydrolysis. Second, the entire second transmembrane span was probed by alanine-insertion mutagenesis at six different positions, from residues 98 to 122. Insertions at the central four positions from residues 107 to 117 resulted in the inability to grow on succinate minimal medium, although normal levels of membrane-bound ATPase activity and significant levels of subunit a were detected. Double mutants were constructed with a mutation that permits cross-linking to the b subunit. Cross-linked products in the mutant K74C/114iA were seen, indicating no major disruption of the a-b interface due to the insertion at 114. Analysis of the K74C/110iA double mutant indicated that K74C is a partial suppressor of 110iA. In summary, the results support a model in which the amino-terminal, cytoplasmic end of the second transmembrane span has close contact with subunit b, while the carboxy-terminal, periplasmic end is important for proton translocation.

Close proximity of a cytoplasmic loop of subunit a with c subunits of the ATP synthase from Escherichia coli

Interactions between subunit a and the c subunits of the Escherichia coli ATP synthase are thought to control proton translocation through the F(o) sector. In this study cysteine substitution mutagenesis was used to define the cytoplasmic ends of the first three transmembrane spans of subunit a, as judged by accessibility to 3-N-maleimidyl-propionyl biocytin. The cytoplasmic end of the fourth transmembrane span could not be defined in this way because of the limited extent of labeling of all residues between 186 and 206. In contrast, most of the preceding residues in that region, closer to transmembrane span 3, were labeled readily. The proximity of this region to other subunits in F(o) was tested by reacting mono-cysteine mutants with a photoactivated cross-linker. Residues 165, 169, 173, 174, 177, 178, and 182-184 could all be cross-linked to subunit c, but no sites were cross-linked to b subunits. Attempts using double mutants of subunit a to generate simultaneous cross-links to two different c subunits were unsuccessful. These results indicate that the cytoplasmic loop between transmembrane spans 3 and 4 of subunit a is in close proximity to at least one c subunit. It is likely that the more highly conserved, carboxyl-terminal region of this loop has limited surface accessibility due to protein-protein interactions. A model is presented for the interaction of subunit a with subunit c, and its implications for the mechanism of proton translocation are discussed.

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.

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.

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.

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.

Catalytic mechanism of F1-ATPase

The structure of the core catalytic unit of ATP synthase, alpha 3 beta 3 gamma, has been determined by X-ray crystallography, revealing a roughly symmetrical arrangement of alternating alpha and beta subunits around a central cavity in which helical portions of gamma are found. A low-resolution structural model of F0, based on electron spectroscopic imaging, locates subunit a and the two copies of subunit b outside of a subunit c oligomer. The structures of individual subunits epsilon and c (largely) have been solved by NMR spectroscopy, but the oligomeric structure of c is still unknown. The structures of subunits a and delta remain undefined, that of b has not yet been defined but biochemical evidence indicates a credible model. Subunits gamma, epsilon, b, and delta are at the interface between F1 and F0; gamma epsilon complex forms one element of the stalk, interacting with c at the base and alpha and beta at the top. The locations of b and delta are less clear. Elucidation of the structure F0, of the stalk, and of the entire F1F0 remains a challenging goal.

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.

Alanine-scanning mutagenesis of the epsilon subunit of the F1-F0 ATP synthase from Escherichia coli reveals two classes of mutants

Alanine-scanning mutagenesis was applied to the epsilon subunit of the F1-F0 ATP synthase from E. coli. Nineteen amino acid residues were changed to alanine, either singly or in pairs, between residues 10 and 93. All mutants, when expressed in the epsilon deletion strain XH1, were able to grow on succinate minimal medium. Membranes were prepared from all mutants and assayed for ATP-driven proton translocation, ATP hydrolysis +/- lauryldiethylamine oxide, and sensitivity of ATPase activity to N,N'-dicyclohexylcarbodiimide (DCCD). Most of the mutants fell into 2 distinct classes. The first group had inhibited ATPase activity, with near normal levels of membrane-bound F1, but decreased sensitivity to DCCD. The second group had stimulated ATPase activity, with a reduced level of membrane-bound F1, but normal sensitivity to DCCD. Membranes from all mutants were further characterized by immunoblotting using 2 monoclonal antibodies. A model for the secondary structure of epsilon and its role in the function of the ATP synthase has been developed. Some residues are important for the binding of epsilon to F1 and therefore for inhibition. Other residues, from Glu-59 through Glu-70, are important for the release of inhibition by epsilon that is part of the normal enzyme cycle.

Construction and plasmid-borne complementation of strains lacking the epsilon subunit of the Escherichia coli F1F0 ATP synthase

Two strains of Escherichia coli that lack the epsilon subunit of the F1F0 ATP synthase have been constructed. They are shown to be viable but with very low growth yields (28%). These strains can be complemented by plasmids carrying wild-type uncC, but not when epsilon is overproduced. These results indicate that epsilon is not essential for growth on minimal glucose medium and that the level of its expression affects the assembly of the ATP synthase.