Introduction of a carboxyl group in the first transmembrane helix of Escherichia coli F1Fo ATPase subunit c and cytoplasmic pH regulation

Regulatory Mechanisms and Environmental Adaptation of the F-ATPase Family

The F-type ATPase family of enzymes, including ATP synthases, are found ubiquitously in biological membranes. ATP synthesis from ADP and inorganic phosphate is driven by an electrochemical H⁺ gradient or H⁺ motive force, in which intramolecular rotation of F-type ATPase is generated with H⁺ transport across the membranes. Because this rotation is essential for energy coupling between catalysis and H⁺-transport, regulation of the rotation is important to adapt to environmental changes and maintain ATP concentration. Recently, a series of cryo-electron microscopy images provided detailed insights into the structure of the H⁺ pathway and the multiple subunit arrangement. However, the regulatory mechanism of the rotation has not been clarified. This review describes the inhibition mechanism of ATP hydrolysis in bacterial enzymes. In addition, properties of the F-type ATPase of Streptococcus mutans, which acts as a H⁺-pump in an acidic environment, are described. These findings may help in the development of novel antimicrobial agents.

The carboxyl-terminal helical domain of the ATP synthase γ subunit is involved in ε subunit conformation and energy coupling

The γ subunit located at the center of ATP synthase (F_OF₁) plays critical roles in catalysis. Escherichia coli mutant with Pro substitution of the γ subunit residue γLeu218, which are located the rotor shaft near the c subunit ring, decreased NADH-driven ATP synthesis activity and ATP hydrolysis-dependent H⁺ transport of membranes to ~60% and ~40% of the wild type, respectively, without affecting F_OF₁ assembly. Consistently, the mutant was defective in growth by oxidative phosphorylation, indicating that energy coupling is impaired by the mutation. The ε subunit conformations in the γLeu218Pro mutant enzyme were investigated by cross-linking between cysteine residues introduced into both the ε subunit (εCys118 and εCys134, in the second helix and the hook segment, respectively) and the γ subunit (γCys99 and γCys260, located in the globular domain and the carboxyl-terminal helix, respectively). In the presence of ADP, the two γ260 and ε134 cysteine residues formed a disulfide bond in both the γLeu218Pro mutant and the wild type, indicating that the hook segment of ε subunit penetrates into the α₃β₃-ring along with the γ subunits in both enzymes. However, γ260/ε134 cross-linking in the γLeu218Pro mutant decreased significantly in the presence of ATP, whereas this effect was small in the wild type. These results suggested that the γ subunit carboxyl-terminal helix containing γLeu218 is involved in the conformation of the ε subunit hook region during ATP hydrolysis and, therefore, is required for energy coupling in F_OF₁.

Complementation of the Fo c subunit of Escherichia coli with that of Streptococcus mutans and properties of the hybrid FoF1 ATP synthase

The c subunit of Streptococcus mutans ATP synthase (FoF1) is functionally exchangeable with that of Escherichia coli, since E. coli with a hybrid FoF1 is able to grow on minimum succinate medium through oxidative phosphorylation. E. coli F1 bound to the hybrid Fo with the S. mutans c subunit showed N,N'-dicyclohexylcarbodiimide-sensitive ATPase activity similar to that of E. coli FoF1. Thus, the S. mutans c subunit assembled into a functional Fo together with the E. coli a and b subunits, forming a normal F1 binding site. Although the H(+) pathway should be functional, as was suggested by the growth on minimum succinate medium, ATP-driven H(+) transport could not be detected with inverted membrane vesicles in vitro. This observation is partly explained by the presence of an acidic residue (Glu-20) in the first transmembrane helix of the S. mutans c subunit, since the site-directed mutant carrying Gln-20 partly recovered the ATP-driven H(+) transport. Since S. mutans is recognized to be a primary etiological agent of human dental caries and is one cause of bacterial endocarditis, our system that expresses hybrid Fo with the S. mutans c subunit would be helpful to find antibiotics and chemicals specifically directed to S. mutans.

A new type of Na(+)-driven ATP synthase membrane rotor with a two-carboxylate ion-coupling motif

The anaerobic bacterium Fusobacterium nucleatum uses glutamate decarboxylation to generate a transmembrane gradient of Na⁺. Here, we demonstrate that this ion-motive force is directly coupled to ATP synthesis, via an F₁F₀-ATP synthase with a novel Na⁺ recognition motif, shared by other human pathogens. Molecular modeling and free-energy simulations of the rotary element of the enzyme, the c-ring, indicate Na⁺ specificity in physiological settings. Consistently, activity measurements showed Na⁺ stimulation of the enzyme, either membrane-embedded or isolated, and ATP synthesis was sensitive to the Na⁺ ionophore monensin. Furthermore, Na⁺ has a protective effect against inhibitors targeting the ion-binding sites, both in the complete ATP synthase and the isolated c-ring. Definitive evidence of Na⁺ coupling is provided by two identical crystal structures of the c₁₁ ring, solved by X-ray crystallography at 2.2 and 2.6 Å resolution, at pH 5.3 and 8.7, respectively. Na⁺ ions occupy all binding sites, each coordinated by four amino acids and a water molecule. Intriguingly, two carboxylates instead of one mediate ion binding. Simulations and experiments demonstrate that this motif implies that a proton is concurrently bound to all sites, although Na⁺ alone drives the rotary mechanism. The structure thus reveals a new mode of ion coupling in ATP synthases and provides a basis for drug-design efforts against this opportunistic pathogen.

Aqueous accessibility to the transmembrane regions of subunit c of the Escherichia coli F1F0 ATP synthase

Rotary catalysis in F(1)F(0) ATP synthase is powered by proton translocation through the membrane-embedded F(0) sector. Proton binding and release occur in the middle of the membrane at Asp-61 on transmembrane helix (TMH) 2 of subunit c. Previously the reactivity of Cys substituted into TMH2 revealed extensive aqueous access at the cytoplasmic side as probed with Ag(+) and other thiolate-directed reagents. The analysis of aqueous accessibility of membrane-embedded regions in subunit c was extended here to TMH1 and the periplasmic side of TMH2. The Ag(+) sensitivity of Cys substitutions was more limited on the periplasmic versus cytoplasmic side of TMH2. In TMH1, Ag(+) sensitivity was restricted to a pocket of four residues lying directly behind Asp-61. Aqueous accessibility was also probed using Cd(2+), a membrane-impermeant soft metal ion with properties similar to Ag(+). Cd(2+) inhibition was restricted to the I28C substitution in TMH1 and residues surrounding Asp-61 in TMH2. The overall pattern of inhibition, by all of the reagents tested, indicates highest accessibility on the cytoplasmic side of TMH2 and in a pocket of residues around Asp-61, including proximal residues in TMH1. Additionally subunit a was shown to mediate access to this region by the membrane-impermeant probe 2-(trimethylammonium)ethyl methanethiosulfonate. Based upon these results and other information, a pocket of aqueous accessible residues, bordered by the peripheral surface of TMH4 of subunit a, is proposed to extend from the cytoplasmic side of cTMH2 to Asp-61 in the center of the membrane.

Essential arginine in subunit a and aspartate in subunit c of FoF1 ATP synthase: effect of repositioning within helix 4 of subunit a and helix 2 of subunit c

FoF1 ATP synthase couples proton flow through the integral membrane portion Fo (ab2c10) to ATP-synthesis in the extrinsic F1-part ((alphabeta)3gammadeltaepsilon) (Escherichia coli nomenclature and stoichiometry). Coupling occurs by mechanical rotation of subunits c10gammaepsilon relative to (alphabeta)3deltaab2. Two residues were found to be essential for proton flow through ab2c10, namely Arg210 in subunit a (aR210) and Asp61 in subunits c (cD61). Their deletion abolishes proton flow, but "horizontal" repositioning, by anchoring them in adjacent transmembrane helices, restores function. Here, we investigated the effects of "vertical" repositioning aR210, cD61, or both by one helical turn towards the N- or C-termini of their original helices. Other than in the horizontal the vertical displacement changes the positions of the side chains within the depth of the membrane. Mutant aR210A/aN214R appeared to be short-circuited in that it supported proton conduction only through EF1-depleted EFo, but not in EFoEF1, nor ATP-driven proton pumping. Mutant cD61N/cM65D grew on succinate, retained the ability to synthesize ATP and supported passive proton conduction but apparently not ATP hydrolysis-driven proton pumping.

Random insertion of a TetR-inducing peptide tag into Escherichia coli proteins allows analysis of protein levels by induction of reporter gene expression

The insertion element InsTipalpha was constructed to generate protein expression data. It randomly fuses the TetR-inducing peptide Tip to the affected reading frame. Fusion protein expression is quantified by Tet-regulated reporter gene expression. The expression patterns of tagged Escherichia coli genes fully agree with published data from transcriptional fusions or microarrays, validating the Tip tag approach.

The F1Fo-ATP synthase of Mycobacterium smegmatis is essential for growth

The F(1)F(o)-ATP synthase plays an important role in a number of vital cellular processes in plants, animals, and microorganisms. In this study, we constructed a DeltaatpD mutant of Mycobacterium smegmatis and demonstrated that atpD encoding the beta subunit of the F(1)F(o)-ATP synthase is an essential gene in M. smegmatis during growth on nonfermentable and fermentable carbon sources.

Contribution of proton-translocating proteins to the virulence of Salmonella enterica serovars Typhimurium, Gallinarum, and Dublin in chickens and mice

We investigated the attenuating effects of a range of respiratory chain mutations in three Salmonella serovars which might be used in the development of live vaccines. We tested mutations in nuoG, cydA, cyoA, atpB, and atpH in three serovars of Salmonella enterica: Typhimurium, Dublin, and Gallinarum. All three serovars were assessed for attenuation in their relevant virulence assays of typhoid-like infections. Serovar Typhimurium was assessed in 1-day-old chickens and the mouse. Serovar Gallinarum 9 was assessed in 3-week-old chickens, and serovar Dublin was assessed in 6-week-old mice. Our data show variation in attenuation for the nuoG, cydA, and cyoA mutations within the different serovar-host combinations. However, mutations in atpB and atpH were highly attenuating for all three serovars in the various virulence assays. Further investigation of the mutations in the atp operon showed that the bacteria were less invasive in vivo, showing reduced in vitro survival within phagocytic cells and reduced acid tolerance. We present data showing that this reduced acid tolerance is due to an inability to adapt to conditions rather than a general sensitivity to reduced pH. The data support the targeting of respiratory components for the production of live vaccines and suggest that mutations in the atp operon provide suitable candidates for broad-spectrum attenuation of a range of Salmonella serovars.

Introduction of a carboxyl group in the loop of the F0 c-subunit affects the H+/ATP coupling ratio of the ATP synthase from Synechocystis 6803

The proton translocation stoichiometry (H+/ATP ratio) was investigated in membrane vesicles from a Synechocystis 6803 mutant in which the serine at position 37 in the hydrophilic loop of the c-subunit from the wild type was replaced by a negatively charged glutamic acid residue (strain plc37). At this position the c-subunit of chloroplasts and the cyanobacterium Synechococcus 6716 already contains glutamic acid. H+/ATP ratios were determined with active ATP synthase in thermodynamic equilibrium between phosphate potential (deltaGp) and the proton gradient (deltamuH+) induced by acid-base transition. The mutant displayed a significantly higher H+/ATP ratio than the control strain (wild type with kanamycin resistance) at pH 8 (4.3 vs. 3.3); the higher ratio also being observed in chloroplasts and Synechococcus 6716. Furthermore, the pH dependence of the H+/ATP of strain plc37 resembles that of Synechococcus 6716. When the pH was increased from 7.6 to 8.4, the H+/ATP of the mutant increased from 4.2 to 4.6 whereas in the control strain the ratio decreased from 3.8 to 2.8. Differences in H+/ATP between the mutant and the control strain were confirmed by measuring the light-induced phosphorylation efficiency (P/2e), which changed as expected, i.e., the P/2e ratio in the mutant was significantly less than that in the wild type. The need for more H+ ions used per ATP in the mutant was also reflected by the significantly lower growth rate of the mutant strain. The results are discussed against the background of the present structural and functional models of proton translocation coupled to catalytic activity of the ATP synthase.

Mefloquine and new related compounds target the F(0) complex of the F(0)F(1) H(+)-ATPase of Streptococcus pneumoniae

The activities of mefloquine (MFL) and related compounds against previously characterized Streptococcus pneumoniae strains carrying defined amino acid substitutions in the c subunit of the F(0)F(1) H(+)-ATPase were studied. In addition, a series of MFL-resistant (Mfl(r)) strains were isolated and characterized. A good correlation was observed between inhibition of growth and inhibition of the membrane-associated F(0)F(1) H(+)-ATPase activity. MFL was about 10-fold more active than optochin and about 200-fold more active than quinine in inhibiting both the growth and the ATPase activities of laboratory pneumococcal strain R6. Mutant strains were inhibited by the different compounds to different degrees, depending on their specific mutations in the c subunit. The resistant strains studied had point mutations that changed amino acid residues in either the c subunit or the a subunit of the F(0) complex. Changes in the c subunit were located in one of the two transmembrane alpha helices: residues M13, G14, G20, M23, and N24 of helix 1 and residues M44, G47, V48, A49, and V57 of helix 2. Changes in the a subunit were also found in either of the transmembrane alpha helices, helix 5 or 6: residue L186 of helix 5 and residues W206, F209, and S214 of helix 6. These results suggest that the transmembrane helices of the c and a subunits interact and that the mutated residues are important for the structure of the F(0) complex and proton translocation.

Features of V-ATPases that distinguish them from F-ATPases

The general structure of F- and V-ATPases is quite similar and they may share a common mechanism of action that involves mechanochemical energy transduction. Both holoenzymes are composed of catalytic sectors, F1 and V1 respectively, and membrane sectors, F(o) and V(o) respectively. Although we assume that a similar mechanism underlies ATP-dependent proton pumping by F- and V-ATPases in eukaryotic cells, the latter cannot catalyze pmf-driven ATP synthesis. The loss of this ability is probably due to a proton slip that is a consequence of alterations in its membrane sector. The major events include gene duplication of the proteolipids and the presence of three distinct proteolipids in each complex.