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Charge Displacements during ATP-Hydrolysis and Synthesis of the Na⁺-Transporting F_oF₁-ATPase of Ilyobacter tartaricus

Christiane Burzik ^*, Georg Kaim , Peter Dimroth , Ernst Bamberg ^*  and Klaus Fendler ^*

^* Max-Planck-Institut für Biophysik, D-60596 Frankfurt/Main, Germany; Eidgenössische Technische Hochschule Zürich, CH-8092 Zürich, Switzerland; and Universität Frankfurt, D-60439 Frankfurt-Main, Germany

Correspondence: Address reprint requests to Klaus Fendler, Marie Curie Str. 15, Frankfurt/Main, Germany D-60439. Tel.: 49-69-6303-2035; Fax: 49-69-6303-2002; E-mail: klaus.fendler{at}mpibp-frankfurt.mpg.de.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Transient electrical currents generated by the Na⁺-transporting F_oF₁-ATPase of Ilyobacter tartaricus were observed in the hydrolytic and synthetic mode of the enzyme. Two techniques were applied: a photochemical ATP concentration jump on a planar lipid membrane and a rapid solution exchange on a solid supported membrane. We have identified an electrogenic reaction in the reaction cycle of the F_oF₁-ATPase that is related to the translocation of the cation through the membrane bound F_o subcomplex of the ATPase. In addition, we have determined rate constants for the process: For ATP hydrolysis this reaction has a rate constant of 15–30 s^-1 if H⁺ is transported and 30–60 s^-1 if Na⁺ is transported. For ATP synthesis the rate constant is 50–70 s^-1.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Driven by a transmembrane electrochemical proton gradient, F_oF₁-ATPases catalyze ATP synthesis in bacteria, mitochondria, or chloroplasts. The ATP synthases of a few anaerobic bacteria-like Propionigenium modestum (Laubinger and Dimroth, 1988) and Ilyobacter tartaricus (Neumann et al., 1998) have acquired the ability to use the energy stored in a Na⁺ electrochemical gradient for the synthesis of ATP. These Na⁺-F_oF₁-ATPases are close relatives of the H⁺ driven F_oF₁-ATPases in structure and properties. Like the latter they consist of a water soluble F₁ part (subunit stoichiometry ₃ß₃) that harbors the catalytic sites for ATP synthesis and hydrolysis and the membrane-embedded F_o component (subunit stoichiometry ab₂c₁₁), that is responsible for ion translocation (Stahlberg et al., 2001).

The high-resolution crystal structure from bovine mitochondrial F₁ (Abrahams et al., 1994) was in remarkable agreement with the binding change mechanism (Boyer, 1997) suggesting a rotary catalytic mechanism, which was proven experimentally (Noji et al., 1997). Subunits-, -, and the c₁₁ oligomer could be cross-linked without loss of function (Tsunoda et al., 2001) and were shown to represent the rotor of F_oF₁ complexes by direct visualization of rotation with an attached actin filament (Pänke et al., 2000; Sambongi et al., 1999). More recently, coupled ATP-driven rotation of the c₁₁ subassembly and rotation during ATP synthesis has been demonstrated for the first time with single F_oF₁ molecules (Börsch et al., 2002; Kaim et al., 2002). The a-subunit is connected laterally with the c₁₁ oligomer (Birkenhager et al., 1995; Singh et al., 1996; Takeyasu et al., 1996), where it is held in place by the two b-subunits that form a peripheral stalk connecting subunit-a with an -subunit of F₁ with the help of subunit- (Dunn and Chandler, 1998; Rodgers and Capaldi, 1998).

Since the pioneering work of Drachev et al. (1974), current measurements on planar lipid membranes (black lipid membrane (BLM)) have been used to investigate electrogenic membrane proteins. With an appropriate activation system they were applied to detect ion transport reactions or conformational transitions in the reaction cycle of ion translocating ATPases (for a review see Bamberg et al. (1993)). Because in F_oF₁-ATPase hydrolysis and synthesis of ATP are associated with ion translocation in the membrane-embedded F_o part, an electrical signal is expected after activation of the enzyme with ATP or ADP + P_i. This has been demonstrated previously using the F_oF₁-ATPase from Rhodospirillum rubrum (Christensen et al., 1988). Activation was accomplished with a photolytic ATP- or ADP-concentration jump using caged ATP and caged ADP, respectively. We have now applied the same technique to the Na⁺-F_oF₁-ATPase from I. tartaricus. This enzyme has the advantage of additional possibilities to modulate its function due to its Na⁺ sensitivity. In contrast to previous work we have used a short laser flash for the release of ATP from caged ATP, which allowed millisecond time resolution. In addition, we have complemented the measurements with a method for a substrate concentration jump using a solid supported membrane (SSM) (Pintschovius et al., 1999). Using both techniques we have detected Na⁺-dependent transmembrane charge displacements. We propose that the charge displacements are associated with the rotary movements of the ringlike assembled c₁₁ oligomer versus subunits ab₂ in the membrane-imbedded F_o part of the protein.

	MATERIAL AND METHODS

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Protein purification and preparation of the proteoliposomes
The I. tartaricus cells (DSM 2382) were grown anaerobically at 30°C and collected as described (Neumann et al., 1998; Schink, 1984). Five grams of cells were broken by passing a suspension through a French pressure cell. The F_oF₁-ATPase was solubilized with Triton-X-100 (1%) and contaminating proteins were precipitated using polyethylene glycol PEG-6000. The protein activity was determined by a coupled spectrophotometric assay (Neumann et al., 1998). After further purification by gel chromatography an activity of 10–20 units mg^-1 protein was achieved. The protein concentration was determined by the bicinchoninic acid test.

Proteoliposomes were prepared by the freeze-thaw-sonication procedure as described by (Neumann et al., 1998) with a concentration of phosphatidylcholine (Sigma, St. Louis, MO; type IIS) of 30 mg·ml^-1 and a protein concentration of 1–2 mg·ml^-1.

BLM measurements
The preparation of bilayers, the electrical recording instrumentation, photolysis of caged ATP, and measurements of membrane conductivity were performed as described (Fendler et al., 1996; Fendler et al., 1985). Briefly, a planar lipid membrane (area 1 mm²) was formed between the two compartments of a cuvette (volume 1.5 ml each compartment) filled with electrolyte. A solution of 1,5% phosphatidylcholine and 0,025% octadecylamine in n-decane was used to form the membrane. Proteoliposomes were added to one side of the membrane and allowed to adsorb to the planar membrane by stirring the solution for 2 h. They are capacitively coupled to the measuring system via the planar membrane. The F_oF₁-ATPase in the proteoliposomes was activated by photolytic release of ATP or ADP in the millisecond range (1mM MgCl₂ and room temperature: 14 ms at pH 7.0 and 2.0 ms at pH 6.4) using a XeCl excimer laser at 308 nm. The irradiance at the membrane plane was 180–300 mJ/cm² leading to a fraction of released ATP in solution of 0.20–0.35. Transient pump currents could be observed that represent the electrical activity of the protein. The electrolyte contained buffer, salts, and various amounts of NPE-caged ATP (P³-1-(2-nitro)phenylethyladenosine-5'-triphosphate), Na⁺ salt, or caged ADP, K⁺ salt, purchased from Calbiochem. For experiments that required the absence of Na⁺ the (C₂H₅)₃NH⁺ salt of NPE-caged ATP was kindly supplied by E. Grell, MPI für Biophysik, Germany. In all ATP hydrolysis measurements the electroneutral H⁺/Na⁺ exchanger monensin (10 µM) was present to facilitate equilibration of Na⁺ between the cuvette and the internal volume of the liposomes (Fendler et al., 1996).

SSM measurements
The SSM consisted of an alkanethiol monolayer covalently bound to a gold surface via the sulfur atom, with a phospholipid monolayer on top of it. For the preparation we followed the procedure described previously (Pintschovius et al., 1999; Seifert et al., 1993). As in the case of the BLM proteoliposomes were allowed to adsorb to the SSM and transient currents were measured via capacitive coupling. For the activation of the F_oF₁-ATPase concentration jumps of ATP and ADP were generated via a rapid solution exchange at the surface of the SSM (Pintschovius et al., 1999). The cuvette was connected to the outlet of an electromagnetic valve, which allowed fast switching between activating and nonactivating solutions. The usual procedure for a concentration jump consisted of three steps: 1), washing the cuvette with the nonactivating solution (1 s); 2), switching to the activating solution (1 s); and 3), removing the activating substrate from the cuvette with the nonactivating solution (1 s).

Determination of F_oF₁-ATPase activity in the presence and absence of caged ATP
The activity of the protein was determined by the coupled spectrophotometric assay (Neumann et al., 1998). The hydrolytic activity was measured in the presence of 1 mM MgCl₂, 2 mM NaCl and an ATP regenerating system (phosphoenolpyruvate, pyruvate kinase, lactate dehydrogenase) in addition to various amounts of ATP and NPE-caged ATP. For experimental conditions, see Neumann et al. (1998).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Transient currents of the Na⁺-F_oF₁-ATPase on the BLM
Proteoliposomes containing the F_oF₁-ATPase were allowed to adsorb to the planar bilayer as described in Material and Methods. Then caged ATP was added and the ion pump was activated via the photolytic release of ATP (see Fig. 1). Transient currents were observed in the absence and presence of 2 mM Na⁺. A typical signal obtained in the presence of Na⁺ is shown in Fig. 2 A. The polarity of the current corresponds to the transport of positive charge into the proteoliposomes consistent with the hydrolytic activity and ion transport of the F_oF₁-ATPase. Although the ATPase molecules are randomly oriented in the liposome membrane (orientation was tested according to Laubinger and Dimroth (1988)) only the inside-out pumps are activated by released ATP and contribute to the signal. The signal could be inhibited by 80% using 1 mM NaN₃ (F₁ inhibitor) in the presence of Na⁺ (Fig. 2, B and C). Note that traces B and C are from a different experiment than A, which explains the slightly different kinetics. Inhibition of the F_oF₁-ATPase by NaN₃ is also observed for the sodium translocating F_oF₁-ATPase from P. modestum (Laubinger and Dimroth, 1988). Similarly, DCCD (dicyclohexylcarbodiimide, an F_O-specific inhibitor) abolished the transient current in the absence of Na⁺. Na⁺ stimulation, specific inhibition, and the polarity of the transient currents demonstrate that these signals are indeed correlated with hydrolytic activity and ion transport of the Na⁺ dependent F_oF₁-ATPase of I. tartaricus.

View larger version (15K): [in this window] [in a new window]   FIGURE 1  BLM and adsorbed proteoliposomes. (A) ATPase in the hydrolytic mode. The ATPase is activated by caged ATP. The current I(t) is measured via the electrodes E. This circuit is omitted in B. (B) ATPase in the synthetic mode. "Background ATP" generates the driving force for ATP synthesis, which is activated by caged ADP.

View larger version (35K): [in this window] [in a new window]   FIGURE 2  (A) Transient current after activation with caged ATP at the BLM. Conditions: 50 mM HEPES, pH 7,0 (TRIS); 100 mM KCl; 2 mM NaCl; 1 mM MgCl2;1 mM DTT (dithiothreitole); 10 µM monensin; 360 µM caged ATP ( = 0,22). The solid line is a fit using a triexponential function. (B and C) Transient current in the absence (B) and presence (C) of 1 mM NaN3. Conditions as in A. Traces B and C are from a separate experiment.

Caged ATP binding to the Na⁺-F_oF₁-ATPase
We investigated the possibility that caged ATP binds to the Na⁺-F_oF₁-ATPase acting as a competitive inhibitor. This has been demonstrated previously for NaK-ATPase (Forbush, 1984), HK-ATPase (Stengelin et al., 1992), and Kdp-ATPase (Fendler et al., 1996). The interaction of caged ATP with the F_oF₁-ATPase can be studied using two different procedures: variation of the concentration of released ATP by addition of different amounts of caged ATP at constant light intensity or by variation of the light intensity at constant caged ATP concentration. The data are plotted according to the concentration of released ATP as shown in Fig. 3. Released ATP was calculated from the energy of the laser flash and the concentration of caged ATP as described previously (Friedrich et al., 1996). If only ATP binds to the enzyme the two traces should superimpose. This is not the case demonstrating that the pumping activity of the F_oF₁-ATP ATPase depends on both, the concentration of ATP and that of caged ATP, which implies that caged ATP binds to the Na⁺-F_oF₁-ATP ATPase from I. tartaricus.

View larger version (19K): [in this window] [in a new window]   FIGURE 3  Transient currents after activation with caged ATP at the BLM. (A) The peak currents are presented as a function of released ATP concentrations. (B) Relaxation rates of the transient currents as a function of released ATP concentrations. Conditions: 50 mM H3PO4/KOH, pH 7,0; 2 mM NaCl; 1 mM MgCl2; 1 mM DTT; 10 µM monensin. Filled circles: 0–2 mM caged ATP, = 0.23. Open circles: 2 mM caged ATP, = 0.23–0.

Interestingly, the shape of the signal is independent from the concentration of released ATP whereas its magnitude changes. This becomes apparent in Fig. 3 B, where the relaxation rates k₁, k₂, and k₃ are shown. They are virtually independent from the concentration of released ATP as well as the ratio of their amplitudes A₂/A₁ and A₃/A₁ (not shown). Such a behavior rules out a second order process (binding of ATP released in solution). In contrast it suggests that the ATP dependence of the signal reflects the different number of ATPase molecules activated.

Na⁺-, temperature-, and pH-dependence of the transient currents measured on the BLM
Transient currents were measured in the presence of Na⁺ and in its absence (<20 µM Na⁺) where protons are translocated. The magnitude of the peak current increases by a factor of two upon addition of Na⁺ (Fig. 4 A). Also the relaxation rate of the decaying phase (k₂) increases by a factor of two (see Fig. 4 B). Consequently, the charge transported is the same in the absence and presence of Na⁺, but the charge translocation is faster in its presence. This observation is in line with a single turnover activity of the F_oF₁-ATPase. The Na⁺ sensitivity of k₂ suggests that this relaxation rate is related to the transport of ions or a conformational change depending on the translocated ion with a rate constant of 30–60 s^-1 in the presence of Na⁺ and 15–30 s^-1 in its absence (H⁺ translocation).

View larger version (20K): [in this window] [in a new window]   FIGURE 4  Transient currents after activation with caged ATP at the BLM at different Na+ concentrations. (A) Peak currents. (B) Relaxation rates. The curved solid lines in A and B represent fits using hyperbolic functions with an offset. Conditions: 50 mM HEPES, pH 7,0 (TRIS); 100 mM KCl; various concentrations of NaCl, 1 mM MgCl2; 1 mM DTT; 10 µM monensin; 500 µM caged ATP ( = 0,35).

For a further identification of different processes represented by the different time constants of the electrical signal, a temperature dependence of the signal was analyzed. The measurement was performed at pH 7.0 and 6.4, that is under conditions where the release of ATP in solution was expected to be rate limiting (release of ATP 70 s^-1 at pH 7.0) and not rate limiting (release of ATP 500 s^-1 at pH 6.4) for the kinetics of the ATPase. Linear Arrhenius plots were obtained and the activation energies determined at pH 6.4 and 7.0 (Table 1). In particular, k₃ was independent from the temperature (zero activation energy within the limits of experimental error), which agrees with its assignment to the time constant of charging of the membrane capacitances. The rising phase (k₁) has the largest temperature dependence (110 kJ/mol at pH 6.4, 75 kJ/mol at pH 7.0) but also k₂ shows a temperature dependence that is in the range of values expected for an enzymatic reaction (42 kJ/mol at pH 6.4 and at pH 7.0)

View larger version (10K): [in this window] [in a new window]   FIGURE 5  Relaxation rates of the transient currents after activation with caged ATP at the BLM at different pH. The dashed line is the rate of ATP released from caged ATP in solution calculated according to Walker et al. (1988). Conditions: 50 mM HEPES, pH 7,0 (TRIS); 100 mM KCl; 1 mM MgCl2; 2 mM NaCl; 1 mM DTT; 10 µM monensin; 400 µM caged ATP ( = 0,26).

View larger version (20K): [in this window] [in a new window]   FIGURE 6  Transient currents after activation with caged ADP at the BLM. Conditions: 50 mM HEPES, pH 7,0 (TRIS); 100 mM NaCl; 1 mM MgCl2; 1 mM DTT. After adsorption of the proteoliposomes the cuvette was perfused with 2 ml protein free solution. The transient currents were consecutively recorded after addition of (a) 500 µM caged ADP, (b) 50 µM "background" ATP, and (c) 0.5 mM H3PO4(KOH).

ATP concentration jumps on the SSM
Proteoliposomes containing the F_oF₁-ATPase were adsorbed to the SSM and the ion pumps were then activated using a rapid solution exchange from the nonactivating solution (no ATP) to the activating solution (containing ATP) and back. Transient currents were observed in the absence and presence of Na⁺ upon addition (on-signal) and withdrawal of ATP (off-signal) as shown in Fig. 7, A and B. Note that the traces recorded in the presence and absence of Na⁺ (Fig. 7, A and B) are from different experiments. The amplitudes can, therefore, not be directly compared. Furthermore, these signals could be inhibited partially by NaN₃ and (in the absence of Na⁺) completely by DCCD (Fig. 7 C). Partial inhibition by NaN₃ was also observed on BLM and by other authors (Laubinger and Dimroth, 1988). Note that SSM measurements do not require caged ATP. These experiments, therefore, rule out that the electrical signals are due to an electrogenic process associated with the photolytic release of ATP in the binding site.

View larger version (24K): [in this window] [in a new window]   FIGURE 7  Transient currents after activation with a rapid concentration jump at the SSM. (A) SSM signal after an ATP jump (1 mM ATP) in the absence of NaCl. (B) SSM signal after an ATP jump (0,5 mM ATP) in the presence of 2 mM NaCl. (C) Inhibition of A by 60 µM DCCD. Activating solution for A–C: 50 mM HEPES, pH 7,0 (TRIS); 300 mM KCl; 0 or 2 mM NaCl; 3 mM MgCl2; 0.5 or 1 mM ATP. Nonactivating solution: as activating solution but no ATP. (D) SSM signal after an ADP jump (0.3 mM ADP) in the presence of NaCl, Pi, and "background ATP." (E) Without "background ATP". Activating solution for D and E: 50 mM HEPES, pH 7,0 (TRIS); 300 mM KCl; 2 mM NaCl; 3 mM MgCl2; 1 mM H3PO4(KOH); 0/30 µM external ATP; 0,3 mM ADP. Nonactivating solution: as activating solution but no ADP.

In the liposomes, the F_oF₁-ATPase is inside out as well as rightside out oriented. However, only the inside-out population is activated by ATP and contributes to the pump current. The positive transient current measured after addition of ATP corresponds to the transport of positive charges into the liposome in agreement with the transport of Na⁺ out of the bacteria in the hydrolytic mode of the F_oF₁-ATPase.

An ATP dependence of the amplitude of the peak currents in the presence of Na⁺ (data not shown) yielded a hyperbolic relationship characterized by a half-saturation concentration K_0,5^ATP of 103 ± 15 µM in good agreement with published dissociation constants of F_oF₁-ATPases for ATP (see e.g., 140 µM, (Muneyuki et al., 1989) and 38 µM, (Weber et al., 1993) for bacterial F_oF₁-ATPases).

Na⁺ concentration jumps on the SSM
In the presence of ATP the F_oF₁-ATPase can be activated on the SSM by addition of Na⁺ (data not shown). Under these conditions, the enzyme is already hydrolyzing ATP at a slow rate when addition of Na⁺ speeds up this reaction. Because the F_oF₁-ATPase is already translocating protons before switching to the activating (in this case the Na⁺-containing) solution a potential (positive inside the liposomes) is already present when Na⁺ is added. This experiment, therefore, represents a Na⁺ activation of the F_oF₁-ATPase in the presence of a retarding potential. The peak currents at different Na⁺ concentrations saturate with a hyperbolic concentration dependence characterized by a half-saturation concentration of K_0,5^Na 350 ± 50 µM. This is close to the values found in our BLM study (Fig. 4; peak current: K_0,5^Na = 220 ± 74 µM, k₂: K_0,5^Na = 160 ± 33 µM) and that reported previously (270 µM at pH 8.0, (Neumann et al., 1998)). As a control, Na⁺ concentration jumps were performed in the absence of ATP. No electrical signals were observed under these conditions.

ADP concentration jumps on the SSM
As in the case of the BLM measurements, ADP concentration jumps in the presence of "background ATP" and P_i lead to electrical signals with negative polarity (Fig. 7 D) corresponding to the transport of positive charge out of the liposomes. These signals most probably represent charge translocation of the F_oF₁-ATPase in the synthetic mode. They show the same characteristics as observed on the BLM, namely they are not observed in the absence of "background ATP" (Fig. 7 E). These measurements corroborate the results obtained on the BLM. As stated for the ATP concentration jumps also, these measurements exclude that the transient current measured on the BLM is due to an electrogenic process associated with the photolytic release of the nucleotide in the binding site.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Charge movements during the enzymatic activity of ion translocating membrane proteins indicate the translocation of an ion in the protein or an electrogenic conformational change in the reaction cycle. Both processes are interesting in terms of the mechanism of ion translocation. With BLM and SSM techniques we have tools at hand to measure direction and speed of charge translocation during the transport cycle of the protein that can be interpreted on the basis of existing models for the transport process. We have measured electrical signals during the enzymatic activity of the F_oF₁-ATPase. It will be shown below that the characteristics of these signals suggest that they have to be assigned to a partial reaction of the Na⁺/H⁺ translocation process. We give a rate constant for this partial reaction and make a tentative assignment to a microscopic process during the rotary ion transport mechanism of the F_oF₁-ATPase.

The electrical signals on the BLM and SSM are generated by the hydrolytic or synthetic activity of the F_oF₁-ATPase
After photolytic generation of substrates at the BLM or after a rapid exchange to a substrate containing solution at the SSM, a transient current is measured in the absence and presence of Na⁺. We have characterized this signal using different caged ATP, caged ADP, and Na⁺ concentrations. The affinities of the F_oF₁-ATPase for ATP and Na⁺ determined on the BLM and SSM agree well with the values found in literature. Furthermore, the specific inhibitors of the F_oF₁-ATPase NaN₃ and DCCD inhibited the electrical signals. Taken together, these results show that the electrical signals observed on the BLM and on the SSM reflect the electrical activity of the Na⁺-F_oF₁-ATPase from I. tartaricus.

Electrical signals could be measured under conditions of ATP hydrolysis and ATP synthesis. As expected, these signals are of opposite polarity. The polarity of the signals is consistent with cation transport to the periplasmic side of the protein under hydrolysis conditions and with cation transport to the cytoplasmic side of the protein under synthesis conditions. In addition, synthesis signals required an additional electrochemical driving force.

From their ATP, pH, and temperature dependence the relaxation rates k₁ and k₃ of the electrical signal were assigned to the photolytic release of ATP in the binding site and to the charging of the proteoliposomes due to the pumping activity of the enzyme, respectively. Only the relaxation rate k₂ is dependent on the nature of the transported cation and is assigned to the electrical activity of the F_oF₁-ATPase. Also its activation energy of 45 kJ/mol corroborates this assignment. It is suggested that k₂ represents the rate constant of an electrogenic partial reaction of the ATPase reaction cycle. The fact that the rate constant depends on the transported cation demonstrates that this reaction is limited by the electrogenic process itself rather than by the nucleotide related processes in F₁. For ATP hydrolysis this reaction has a rate constant of 15–30 s^-1 if H⁺ is transported and 30–60 s^-1 if Na⁺ is transported. For ATP synthesis the rate constant is 50–70 s^-1 (only determined in the presence of Na⁺).

The electrical signal measured on the BLM is dominated by a single turnover of the F_oF₁-ATPase activated by caged ATP photolyzed in the binding site
Caged ATP binds to various ATPases and acts as a competitive inhibitor (Fendler et al., 1996; Forbush, 1984; Nagel et al., 1987). Because at our conditions only 30% of the caged ATP is transformed to ATP by the ultraviolet light flash, the remaining caged ATP competes for the ATP binding sites on F₁. Caged ATP already bound at the binding sites may be photolytically released directly in the binding sites, or the unphotolyzed fraction of caged ATP may prevent access to the binding site for ATP released in solution. All these processes have to be taken into account for the interpretation of the time dependence of the electrical currents generated by photolytical release of ATP. In the following, we propose that the F_oF₁-ATPase is activated by caged ATP photolyzed in the binding site and that the electrical signal is dominated by a single turnover of the enzyme.

The linear decrease of the signal upon reduction of the light intensity (see Fig. 3 A, open circles) shows that the electrical signal scales with the number of photolyzed caged ATP molecules. This suggests that the fraction of photolyzed caged ATP in the binding sites determines the signal ("release in-site model"). If binding of released ATP was involved, a hyperbolic dependence would have been expected instead. Consequently, the increase of the signal with rising caged ATP concentration (see Fig. 3 A, filled circles) reflects the increasing number of binding sites occupied with caged ATP. We can, therefore, determine from this dependency the caged ATP affinity of the F_oF₁-ATPase. Under the conditions of the experiment 23% of caged ATP in solution are transformed to ATP. A half-saturation concentration of 81 µM determined from Fig. 3 A yields a dissociation constant for caged ATP of 350 µM (pH 7,0). In activity measurements using the proteoliposomes at different ATP and caged ATP concentrations competitive inhibition by caged ATP with a comparable inhibition constant (350 ± 50 µM) was found.

Further support of the "release in-site model" comes from the analysis of the shape of the current traces. Only the magnitude of the curves changes at different caged ATP concentration or different light intensity. The time constants and relative amplitudes remain the same (Fig. 3). At the same time, the transported charge changes with ATP concentration. If the ATP released in solution was available for the enzyme, an ATP dependent time constant would be expected. This is not observed. We therefore conclude that on the timescale of the current measurements (1 s) mainly ATP released from caged ATP bound in the binding site contributes to the transient current. This implies that a single turnover dominates the signal after activation of the F_oF₁-ATPase.

Finally, the same amount of charge is transported whether Na⁺ is present or not. This can be deduced from the fact that peak current and relaxation rate k₂ yield a Na⁺ dependence characterized by a nearly identical K_0.5^Na (Fig. 4). Under these conditions the product amplitude x relaxation time, which is proportional to the translocated charge, is constant at all Na⁺ concentrations. This finding additionally supports the notion that the electrical signal represents a single turnover of the enzyme.

The electrical signal is generated in F_o
At the ionic strength of our experiment (100 mM) the Debye length is 1 nm. The F₁ complex of the ATPase, however, protrudes into the solution more than 45 nm (Weber and Senior, 1997). It is, therefore, unlikely that movement of a charged portion of F₁ brings about the electrical signal because this part of the protein is effectively screened by the surrounding solution. The charge displacement has to take place in the membrane bound F_o complex, potentially in the "rotor" consisting of 11 c-subunits (Stahlberg et al., 2001) or in the membrane inserted part of the "stator," i.e., the a- or the b-subunits.

At concentrations <100 µM DCCD inhibits transport in the F_o portion of the ATPase by binding to the ion binding sites in the c-subunits. (Weber et al., 1993). It binds to protonated carboxylates and consequently Na⁺ and Li⁺ protect against DCCD inhibition in P. modestum and I. tartaricus (Kluge and Dimroth, 1993; Neumann et al., 1998). This was also observed in the present study. Inhibition of the electrical signals by DCCD therefore demonstrates that the ion binding sites are directly involved. A charge displacement in subunits-a, -b, and -c due to a displacement of charged residues by the torsional forces exerted by F₁ can be excluded.

A kinetic model for the activation of the F_oF₁-ATPase using caged ATP
The kinetic analysis will be based on transient currents observed on the BLM because of the superior time resolution of this technique. At time zero the current is zero, rises to its peak, and decays. This requires a kinetic model where an electroneutral process precedes the electrogenic reaction. Based on its dependence on Na⁺, pH, and temperature, we have assigned the rising phase (k₁) to the release of ATP from caged ATP. The fast decay (k₂) was attributed to an intrinsic reaction of the F_oF₁-ATPase. Taking into account that the release of ATP has to precede the intrinsic reaction of the protein the following kinetic model is obtained: electroneutral release of ATP from caged ATP with a rate constant k₁ = 250–600 s^-1 is followed by a charge translocation with rate constant k₂ = 15–60 s^-1.

Various models have been proposed for the hydrolytic reaction mechanism of the F_oF₁-Atpase (for a review see Bianchet et al. (2000)). All have in common that ATP is hydrolyzed in the tight binding site when the open site is occupied by ATP. In the subsequent step the tight site is transformed into a loose one concomitant with energy transduction. We have taken this core mechanism and have tried to adapt it to the situation of the BLM experiment (see Fig. 8 A). Initially only caged ATP is present at high concentrations (0.4–2 mM), which is assumed to be sufficient to bind to all sites including the open ATP binding site. When ATP is released from caged ATP in the tight site by ultraviolet irradiation it can be hydrolyzed providing energy for ion translocation. The role of ATP in the open binding site is here played by caged ATP that acts as a nonhydrolyzable ATP analog. Note that because of the requirement of an occupied open binding site our assumption of caged ATP binding to all three sites makes sense. In a subsequent reaction ADP and Pi are released and replaced by caged ATP thereby regenerating the initial configuration. In the model of Boyer and Walker (Abrahams et al., 1994; Boyer and Kohlbrenner, 1981) ADP and Pi are released from the loose site whereas later models propose release out of the open site (Duncan et al., 1995), which is thermodynamically more favorable. It has to be noted, however, that in our BLM experiments, ADP and Pi are virtually absent in the solution allowing dissociation of the substrates from the loose site.

View larger version (27K): [in this window] [in a new window]   FIGURE 8  (A) Kinetic model describing the activation of the FoF1-ATPase using a photolytic concentration jump with caged ATP. (B) Structure of FoF1-ATPase and rotary mechanism of Na+ transport during the process characterized by the rate constant k2. For clarity the -, ß-, -, and -subunits of F1 have been omitted. L, T, and O refer to the loose, tight, and open states, respectively, of the ATP binding site.

The translocation process is complex. It comprises release of ions to the periplasm, rotation of c₁₁, and finally uptake of ions from the cytoplasm. Which of these is associated with a charge translocation? Periplasmic release through a narrow channel in subunit-a as well as cytoplasmic uptake through a channel formed by tightly packed c-subunits are plausible candidates for an electrogenic reaction. In this respect, comparison with NaK-ATPase is of interest, where electrogenic Na⁺ uptake and release has previously been described (Gadsby et al., 1993; Pintschovius et al., 1999). Rotation of c₁₁, on the other hand, could also involve an electrogenic step, namely the transition of the bound cation across the "hydrophobic strip" as proposed by Dimroth et al. (1999). Whether all three processes contribute to the generation of the observed current or one of them dominates the charge translocation cannot yet be decided at present.

The kinetic model shown in Fig. 8 only accounts for a single turnover of the ATPase after release of ATP from caged ATP in the binding site. This process dominates the observed transient current. However, at a longer timescale, ATP released in solution may exchange for caged ATP in the binding sites and allow for a stationary activity as suggested by the slow component k₃ in the electrical signal.

The potential can act as the only driving force of the ATP synthesis activity of the F_oF₁-ATPase
It has been shown recently that the kinetically relevant driving force for ATP synthesis of the F_oF₁-ATPase is the electrical potential (Kaim and Dimroth, 1998; Kaim and Dimroth, 1999). We have, therefore, performed experiments at the BLM and SSM to test this hypothesis. Generation of an ion gradient or of a potential across the proteoliposomal membrane is possible through the F_oF₁-ATPase itself. For that purpose, a few F_oF₁-ATPase molecules are put into the hydrolytic mode by adding a small concentration of "background ATP." These generate a membrane potential and a Na⁺ ion gradient (Fig. 1). After the flash and release of ADP (for the BLM experiments) or after switching to an ADP containing solution (for the SSM experiments) the activated F_oF₁-ATPases are now in the synthetic mode driven by the membrane potential and the ion gradient.

Which one of the two driving forces is dominant? By a simple calculation it can be shown that the main driving force generated by means of the "background ATP" is the membrane potential. Transport of 1 mM Na⁺ into the liposomes (diameter 100–300 nm) generates a potential of 0,4–1,2 V (Fendler et al., 1996). This is certainly sufficient to drive ATP synthesis (under the condition of the SSM experiment shown in Fig. 7, ATP synthesis requires a free energy equivalent to 120 mV). At the same time this potential is high enough to prevent further ion transport driven by "background ATP." Hence, a considerable potential but only a negligible Na⁺ gradient of 81 mM inside versus 80 mM outside the liposomes is created. The measurements presented in Fig. 6, therefore, support the notion that a potential is sufficient for the synthetic activity of the Na⁺-F_oF₁-ATPase of I. tartaricus. It does, however, not rule out the possibility that a sufficiently high Na⁺ gradient in the absence of a potential could drive ATP synthesis.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
We have identified an electrogenic reaction in the reaction cycle of the Na⁺-transporting F_oF₁-ATPase of I. tartaricus. This process is related to the translocation of the cation through the membrane bound subcomplex of the F_oF₁-ATPase. The reaction could possibly be the transport of the Na⁺ or H⁺ ions through the putative access channel in the "stator," the a-subunit, the binding or release of the cation through a narrow channel formed by the c-subunits, or the transition of the bound cations across the "hydrophobic strip" (Dimroth et al., 1999). In addition, we have been able to determine rate constants for the process: for ATP hydrolysis this reaction has a rate constant of 15–30 s^-1 if H⁺ is transported and 30–60 s^-1 if Na⁺ is transported. For ATP synthesis the rate constant is 50–70 s^-1.

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
We thank Y. Appoldt for help with the preparation of the protein, Jennifer Mc Manus for excellent technical assistance, and E. Grell for providing the Na⁺-free NPE-caged ATP.

This work has been financially supported by the Deutsche Forschungsgemeinschaft (SFB 47Z).

	FOOTNOTES

 
Abbreviations used: BLM, black lipid membrane; SSM, solid supported membrane; NPE-caged ATP, P³-1-(2-nitro)phenylethyladenosine-5'-triphosphate); background ATP, ATP present in small amounts before and after activation of the ATPase; , rate constant of release of ATP from caged ATP; , fraction of ATP released from caged ATP; DCCD, dicyclohexylcarbodiimide; DTT, dithiothreitole.

Submitted on August 16, 2002; accepted for publication April 3, 2003.

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