Evidence for a Partially Stalled γ Rotor in F1-ATPase from Hydrogen-Deuterium Exchange Experiments and Molecular Dynamics Simulations.

F1-ATPase uses ATP hydrolysis to drive rotation of the γ subunit. The γ C-terminal helix constitutes the rotor tip that is seated in an apical bearing formed by α3ß3. It remains uncertain to what extent the γ conformation during rotation differs from that seen in rigid crystal structures. Existing models assume that the entire γ subunit participates in every rotation. Here we interrogated E. coli F1-ATPase by hydrogen-deuterium exchange (HDX) mass spectrometry. Rotation of γ caused greatly enhanced deuteration in the γ C-terminal helix. The HDX kinetics implied that most F1 complexes operate with an intact rotor at any given time, but that the rotor tip is prone to occasional unfolding. A molecular dynamics (MD) strategy was developed to model the off-axis forces acting on γ. MD runs showed stalling of the rotor tip and unfolding of the γ C-terminal helix. MD-predicted H-bond opening events coincided with experimental HDX patterns. Our data suggest that in vitro operation of F1-ATPase is associated with significant rotational resistance in the apical bearing. These conditions cause the γ C-terminal helix to get "stuck" (and unfold) sporadically while the remainder of γ continues to rotate. This scenario contrasts the traditional "greasy bearing" model that envisions smooth rotation of the γ C-terminal helix. The fragility of the apical rotor tip in F1-ATPase is attributed to the absence of a c10 ring that stabilizes the rotation axis in intact FoF1. Overall, the MD/HDX strategy introduced here appears well suited for interrogating the inner workings of molecular motors.

Load-dependent destabilization of the γ-rotor shaft in FOF1 ATP synthase revealed by hydrogen/deuterium-exchange mass spectrometry.

FoF1 is a membrane-bound molecular motor that uses proton-motive force (PMF) to drive the synthesis of ATP from ADP and Pi. Reverse operation generates PMF via ATP hydrolysis. Catalysis in either direction involves rotation of the γε shaft that connects the α3ß3 head and the membrane-anchored cn ring. X-ray crystallography and other techniques have provided insights into the structure and function of FoF1 subcomplexes. However, interrogating the conformational dynamics of intact membrane-bound FoF1 during rotational catalysis has proven to be difficult. Here, we use hydrogen/deuterium exchange mass spectrometry to probe the inner workings of FoF1 in its natural membrane-bound state. A pronounced destabilization of the γ C-terminal helix during hydrolysis-driven rotation was observed. This behavior is attributed to torsional stress in γ, arising from γ⋅⋅⋅α3ß3 interactions that cause resistance during γ rotation within the apical bearing. Intriguingly, we find that destabilization of γ occurs only when FoF1 operates against a PMF-induced torque; the effect disappears when PMF is eliminated by an uncoupler. This behavior resembles the properties of automotive engines, where bearings inflict greater forces on the crankshaft when operated under load than during idling.

Stomatin-like protein 2 is required for in vivo mitochondrial respiratory chain supercomplex formation and optimal cell function.

Stomatin-like protein 2 (SLP-2) is a mainly mitochondrial protein that is widely expressed and is highly conserved across evolution. We have previously shown that SLP-2 binds the mitochondrial lipid cardiolipin and interacts with prohibitin-1 and -2 to form specialized membrane microdomains in the mitochondrial inner membrane, which are associated with optimal mitochondrial respiration. To determine how SLP-2 functions, we performed bioenergetic analysis of primary T cells from T cell-selective Slp-2 knockout mice under conditions that forced energy production to come almost exclusively from oxidative phosphorylation. These cells had a phenotype characterized by increased uncoupled mitochondrial respiration and decreased mitochondrial membrane potential. Since formation of mitochondrial respiratory chain supercomplexes (RCS) may correlate with more efficient electron transfer during oxidative phosphorylation, we hypothesized that the defect in mitochondrial respiration in SLP-2-deficient T cells was due to deficient RCS formation. We found that in the absence of SLP-2, T cells had decreased levels and activities of complex I-III2 and I-III2-IV(1-3) RCS but no defects in assembly of individual respiratory complexes. Impaired RCS formation in SLP-2-deficient T cells correlated with significantly delayed T cell proliferation in response to activation under conditions of limiting glycolysis. Altogether, our findings identify SLP-2 as a key regulator of the formation of RCS in vivo and show that these supercomplexes are required for optimal cell function.

ATP-induced dimerization of the F0F1 ε subunit from Bacillus PS3: a hydrogen exchange-mass spectrometry study.

F0F1 ATP synthase harnesses a transmembrane electrochemical gradient for the production of ATP. When operated in reverse, this multiprotein complex catalyzes ATP hydrolysis. In bacteria, the ε subunit is involved in regulating this ATPase activity. Also, ε is essential for coupling ATP hydrolysis (or synthesis) to proton translocation. The ε subunit consists of a ß sandwich and two C-terminal helices, α1 and α2. The protein can switch from a compact fold to an alternate conformation where α1 and α2 are separated, resulting in an extended structure. ε from the thermophile Bacillus PS3 (Tε) binds ATP with high affinity such that this protein may function as an intracellular ATP level sensor. ATP binding to isolated Tε triggers a major conformational transition. Earlier data were interpreted in terms of an ATP + Tεextended → ATP·Tεcompact transition that may mimic aspects of the regulatory switching within F0F1 (Yagi et al. (2007) Proc. Natl. Acad. Sci. U.S.A., 104, 11233­11238). In this work, we employ complementary biophysical techniques for examining the ATP-induced conformational switching of isolated Tε. CD spectroscopy confirmed the occurrence of a large-scale conformational transition upon ATP binding, consistent with the formation of stable helical structure. Hydrogen/deuterium exchange (HDX) mass spectrometry revealed that this transition is accompanied by a pronounced stabilization in the vicinity of the ATP-binding pocket. Surprisingly, dramatic stabilization is also seen in the ß8−ß9 region, which is remote from the site of ATP interaction. Analytical ultracentrifugation uncovered a previously unrecognized feature of Tε: a high propensity to undergo dimerization in the presence of ATP. Comparison with existing crystallography data strongly suggests that the unexpected ß8−ß9 HDX protection is due to newly formed protein­protein contacts. Hence, ATP binding to isolated Tε proceeds according to 2ATP + 2Tεextended → (ATP·Tεcompact)2. Implications of this dimerization propensity for the possible role of Tε as an antibiotic target are discussed.

Elastic rotation of Escherichia coli F(O)F(1) having ε subunit fused with cytochrome b(562) or flavodoxin reductase.

Intra-molecular rotation of FOF1 ATP synthase enables cooperative synthesis and hydrolysis of ATP. In this study, using a small gold bead probe, we observed fast rotation close to the real rate that would be exhibited without probes. Using this experimental system, we tested the rotation of FOF1 with the ε subunit connected to a globular protein [cytochrome b562 (ε-Cyt) or flavodoxin reductase (ε-FlavR)], which is apparently larger than the space between the central and the peripheral stalks. The enzymes containing ε-Cyt and ε-FlavR showed continual rotations with average rates of 185 and 148 rps, respectively, similar to the wild type (172 rps). However, the enzymes with ε-Cyt or ε-FlavR showed a reduced proton transport. These results indicate that the intra-molecular rotation is elastic but proton transport requires more strict subunit/subunit interaction.

Individual interactions of the b subunits within the stator of the Escherichia coli ATP synthase.

FOF1 ATP synthases are rotary nanomotors that couple proton translocation across biological membranes to the synthesis/hydrolysis of ATP. During catalysis, the peripheral stalk, composed of two b subunits and subunit δ in Escherichia coli, counteracts the torque generated by the rotation of the central stalk. Here we characterize individual interactions of the b subunits within the stator by use of monoclonal antibodies and nearest neighbor analyses via intersubunit disulfide bond formation. Antibody binding studies revealed that the C-terminal region of one of the two b subunits is principally involved in the binding of subunit δ, whereas the other one is accessible to antibody binding without impact on the function of FOF1. Individually substituted cysteine pairs suitable for disulfide cross-linking between the b subunits and the other stator subunits (b-α, b-ß, b-δ, and b-a) were screened and combined with each other to discriminate between the two b subunits (i.e. bI and bII). The results show the b dimer to be located at a non-catalytic α/ß cleft, with bI close to subunit α, whereas bII is proximal to subunit ß. Furthermore, bI can be linked to subunit δ as well as to subunit a. Among the subcomplexes formed were a-bI-α, bII-ß, α-bI-bII-ß, and a-bI-δ. Taken together, the data obtained define the different positions of the two b subunits at a non-catalytic interface and imply that each b subunit has a different role in generating stability within the stator. We suggest that bI is functionally related to the single b subunit present in mitochondrial ATP synthase.

Stomatin-like protein 2 deficiency in T cells is associated with altered mitochondrial respiration and defective CD4+ T cell responses.

Stomatin-like protein 2 (SLP-2) is a mostly mitochondrial protein that regulates mitochondrial biogenesis and function and modulates T cell activation. To determine the mechanism of action of SLP-2, we generated T cell-specific SLP-2-deficient mice. These mice had normal numbers of thymocytes and T cells in the periphery. However, conventional SLP-2-deficient T cells had a posttranscriptional defect in IL-2 production in response to TCR ligation, and this translated into reduced CD4(+) T cell responses. SLP-2 deficiency was associated with impaired cardiolipin compartmentalization in mitochondrial membranes, decreased levels of the NADH dehydrogenase (ubiquinone) iron-sulfur protein 3, NADH dehydrogenase (ubiquinone) 1ß subcomplex subunit 8, and NADH dehydrogenase (ubiquinone) 1α subcomplex subunit 9 of respiratory complex I, and decreased activity of this complex as well as of complex II plus III of the respiratory chain. In addition, SLP-2-deficient T cells showed a significant increase in uncoupled mitochondrial respiration and a greater reliance on glycolysis. Based on these results, we propose that SLP-2 organizes the mitochondrial membrane compartmentalization of cardiolipin, which is required for optimal assembly and function of respiratory chain complexes. This function, in T cells, helps to ensure proper metabolic response during activation.

Elastic deformations of the rotary double motor of single F(o)F(1)-ATP synthases detected in real time by Förster resonance energy transfer.

Elastic conformational changes of the protein backbone are essential for catalytic activities of enzymes. To follow relative movements within the protein, Förster-type resonance energy transfer (FRET) between two specifically attached fluorophores can be applied. FRET provides a precise ruler between 3 and 8nm with subnanometer resolution. Corresponding submillisecond time resolution is sufficient to identify conformational changes in FRET time trajectories. Analyzing single enzymes circumvents the need for synchronization of various conformations. F(O)F(1)-ATP synthase is a rotary double motor which catalyzes the synthesis of adenosine triphosphate (ATP). A proton-driven 10-stepped rotary F(O) motor in the Escherichia coli enzyme is connected to a 3-stepped F(1) motor, where ATP is synthesized. To operate the double motor with a mismatch of step sizes smoothly, elastic deformations within the rotor parts have been proposed by W. Junge and coworkers. Here we extend a single-molecule FRET approach to observe both rotary motors simultaneously in individual F(O)F(1)-ATP synthases at work. We labeled this enzyme with two fluorophores specifically, that is, on the ε- and c-subunits of the two rotors. Alternating laser excitation was used to select the FRET-labeled enzymes. FRET changes indicated associated transient twisting within the rotors of single enzyme molecules during ATP hydrolysis and ATP synthesis. Supported by Monte Carlo simulations of the FRET experiments, these studies reveal that the rotor twisting is greater than 36° and is largely suppressed in the presence of the rotation inhibitor DCCD. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Stomatin-like protein 2 binds cardiolipin and regulates mitochondrial biogenesis and function.

Stomatin-like protein 2 (SLP-2) is a widely expressed mitochondrial inner membrane protein of unknown function. Here we show that human SLP-2 interacts with prohibitin-1 and -2 and binds to the mitochondrial membrane phospholipid cardiolipin. Upregulation of SLP-2 expression increases cardiolipin content and the formation of metabolically active mitochondrial membranes and induces mitochondrial biogenesis. In human T lymphocytes, these events correlate with increased complex I and II activities, increased intracellular ATP stores, and increased resistance to apoptosis through the intrinsic pathway, ultimately enhancing cellular responses. We propose that the function of SLP-2 is to recruit prohibitins to cardiolipin to form cardiolipin-enriched microdomains in which electron transport complexes are optimally assembled. Likely through the prohibitin functional interactome, SLP-2 then regulates mitochondrial biogenesis and function.

Two rotary motors in F-ATP synthase are elastically coupled by a flexible rotor and a stiff stator stalk.

ATP is synthesized by ATP synthase (F(O)F(1)-ATPase). Its rotary electromotor (F(O)) translocates protons (in some organisms sodium cations) and generates torque to drive the rotary chemical generator (F(1)). Elastic power transmission between F(O) and F(1) is essential for smoothing the cooperation of these stepping motors, thereby increasing their kinetic efficiency. A particularly compliant elastic domain is located on the central rotor (c(10-15)/ε/γ), right between the two sites of torque generation and consumption. The hinge on the active lever on subunit ß adds further compliance. It is under contention whether or not the peripheral stalk (and the "stator" as a whole) also serves as elastic buffer. In the enzyme from Escherichia coli, the most extended component of the stalk is the homodimer b(2), a right-handed α-helical coiled coil. By fluctuation analysis we determined the spring constant of the stator in response to twisting and bending, and compared wild-type with b-mutant enzymes. In both deformation modes, the stator was very stiff in the wild type. It was more compliant if b was elongated by 11 amino acid residues. Substitution of three consecutive residues in b by glycine, expected to destabilize its α-helical structure, further reduced the stiffness against bending deformation. In any case, the stator was at least 10-fold stiffer than the rotor, and the enzyme retained its proton-coupled activity.

Decreased stability and increased formation of soluble aggregates by immature superoxide dismutase do not account for disease severity in ALS.

Protein aggregation is a hallmark of many diseases, including amyotrophic lateral sclerosis (ALS), where aggregation of Cu/Zn superoxide dismutase (SOD1) is implicated in causing neurodegeneration. Recent studies have suggested that destabilization and aggregation of the most immature form of SOD1, the disulfide-reduced, unmetallated (apo) protein is particularly important in causing ALS. We report herein in depth analyses of the effects of chemically and structurally diverse ALS-associated mutations on the stability and aggregation of reduced apo SOD1. In contrast with previous studies, we find that various reduced apo SOD1 mutants undergo highly reversible thermal denaturation with little aggregation, enabling quantitative thermodynamic stability analyses. In the absence of ALS-associated mutations, reduced apo SOD1 is marginally stable but predominantly folded. Mutations generally result in slight decreases to substantial increases in the fraction of unfolded protein. Calorimetry, ultracentrifugation, and light scattering show that all mutations enhance aggregation propensity, with the effects varying widely, from subtle increases in most cases, to pronounced formation of 40-100 nm soluble aggregates by A4V, a mutation that is associated with particularly short disease duration. Interestingly, although there is a correlation between observed aggregation and stability, there is minimal to no correlation between observed aggregation, predicted aggregation propensity, and disease characteristics. These findings suggest that reduced apo SOD1 does not play a dominant role in modulating disease. Rather, additional and/or multiple forms of SOD1 and additional biophysical and biological factors are needed to account for the toxicity of mutant SOD1 in ALS.

Characterization of the monomer-dimer equilibrium of recombinant histo-aspartic protease from Plasmodium falciparum.

Histo-aspartic protease (HAP) from Plasmodium falciparum is an intriguing aspartic protease due to its unique structure. Our previous study reported the first recombinant expression of soluble HAP, in its truncated form (lys77p-Leu328) (p denotes prosegment), as a thioredoxin (Trx) fusion protein Trx-tHAP. The present study found that the recombinant Trx-tHAP fusion protein aggregated during purification which could be prevented through the addition of 0.2% CHAPS. Trx-tHAP fusion protein was processed into a mature form of tHAP (mtHAP) by both autoactivation, and activation with either enterokinase or plasmepsin II. Using gel filtration chromatography as well as sedimentation velocity and equilibrium ultracentrifugation, it was shown that the recombinant mtHAP exists in a dynamic monomer-dimer equilibrium with an increasing dissociation constant in the presence of CHAPS. Enzymatic activity data indicated that HAP was most active as a monomer. The dominant monomeric form showed a K(m) of 2.0 microM and a turnover number, k(cat), of 0.036s(-1) using the internally quenched fluorescent synthetic peptide substrate EDANS-CO-CH(2)-CH(2)-CO-Ala-Leu-Glu-Arg-Met-Phe-Leu-Ser-Phe-Pro-Dap-(DABCYL)-OH (2837b) at pH 5.2.

Functionally compensating coevolving positions are neither homoplasic nor conserved in clades.

We demonstrated that a pair of positions in phosphoglycerate kinase that score highly by three nonparametric covariation measures are important for function even though the positions can be occupied by aliphatic, aromatic, or charged residues. Examination of these pairs suggested that the majority of the covariation scores could be explained by within-clade conservation. However, an analysis of diversity showed that the conservation within clades of covarying pairs was indistinguishable from pairs of positions that do not covary, thus ruling out both clade conservation and extensive homoplasy as means to identify covarying positions. Mutagenesis showed that the residues in the covarying pair were epistatic, with the type of epistasis being dependent on the initial pair. The results show that nonconserved covarying positions that affect protein function can be identified with high precision.

36 degrees step size of proton-driven c-ring rotation in FoF1-ATP synthase.

Synthesis of adenosine triphosphate ATP, the 'biological energy currency', is accomplished by F(o)F(1)-ATP synthase. In the plasma membrane of Escherichia coli, proton-driven rotation of a ring of 10 c subunits in the F(o) motor powers catalysis in the F(1) motor. Although F(1) uses 120 degrees stepping during ATP synthesis, models of F(o) predict either an incremental rotation of c subunits in 36 degrees steps or larger step sizes comprising several fast substeps. Using single-molecule fluorescence resonance energy transfer, we provide the first experimental determination of a 36 degrees sequential stepping mode of the c-ring during ATP synthesis.

The b subunits in the peripheral stalk of F1F0 ATP synthase preferentially adopt an offset relationship.

The peripheral stalk of F1F0 ATP synthase is essential for the binding of F1 to FO and for proper transfer of energy between the two sectors of the enzyme. The peripheral stalk of Escherichia coli is composed of a dimer of identical b subunits. In contrast, photosynthetic organisms express two b-like genes that form a heterodimeric peripheral stalk. Previously we generated chimeric peripheral stalks in which a portion of the tether and dimerization domains of the E. coli b subunits were replaced with homologous sequences from the b and b' subunits of Thermosynechococcus elongatus (Claggett, S. B., Grabar, T. B., Dunn, S. D., and Cain, B. D. (2007) J. Bacteriol. 189, 5463-5471). The spatial arrangement of the chimeric b and b' subunits, abbreviated Tb and Tb', has been investigated by Cu2+-mediated disulfide cross-link formation. Disulfide formation was studied both in soluble model polypeptides and between full-length subunits within intact functional F1F0 ATP synthase complexes. In both cases, disulfides were preferentially formed between TbA83C and Tb'A90C, indicating the existence of a staggered relationship between helices of the two chimeric subunits. Even under stringent conditions rapid formation of disulfides between these positions occurred. Importantly, formation of this cross-link had no detectable effect on ATP-driven proton pumping, indicating that the staggered conformation is compatible with normal enzymatic activity. Under less stringent reaction conditions, it was also possible to detect b subunits cross-linked through identical positions, suggesting that an in-register, nonstaggered parallel conformation may also exist.

Domain compliance and elastic power transmission in rotary F(O)F(1)-ATPase.

The 2 nanomotors of rotary ATP synthase, ionmotive F(O) and chemically active F(1), are mechanically coupled by a central rotor and an eccentric bearing. Both motors rotate, with 3 steps in F(1) and 10-15 in F(O). Simulation by statistical mechanics has revealed that an elastic power transmission is required for a high rate of coupled turnover. Here, we investigate the distribution in the F(O)F(1) structure of compliant and stiff domains. The compliance of certain domains was restricted by engineered disulfide bridges between rotor and stator, and the torsional stiffness (kappa) of unrestricted domains was determined by analyzing their thermal rotary fluctuations. A fluorescent magnetic bead was attached to single molecules of F(1) and a fluorescent actin filament to F(O)F(1), respectively. They served to probe first the functional rotation and, after formation of the given disulfide bridge, the stochastic rotational motion. Most parts of the enzyme, in particular the central shaft in F(1), and the long eccentric bearing were rather stiff (torsional stiffness kappa > 750 pNnm). One domain of the rotor, namely where the globular portions of subunits gamma and epsilon of F(1) contact the c-ring of F(O), was more compliant (kappa congruent with 68 pNnm). This elastic buffer smoothes the cooperation of the 2 stepping motors. It is located were needed, between the 2 sites where the power strokes in F(O) and F(1) are generated and consumed.

The proton-translocating a subunit of F0F1-ATP synthase is allocated asymmetrically to the peripheral stalk.

The position of the a subunit of the membrane-integral F0 sector of Escherichia coli ATP synthase was investigated by single molecule fluorescence resonance energy transfer studies utilizing a fusion of enhanced green fluorescent protein to the C terminus of the a subunit and fluorescent labels attached to specific positions of the epsilon or gamma subunits. Three fluorescence resonance energy transfer levels were observed during rotation driven by ATP hydrolysis corresponding to the three resting positions of the rotor subunits, gamma or epsilon, relative to the a subunit of the stator. Comparison of these positions of the rotor sites with those previously determined relative to the b subunit dimer indicates the position of a as adjacent to the b dimer on its counterclockwise side when the enzyme is viewed from the cytoplasm. This relationship provides stability to the membrane interface between a and b2, allowing it to withstand the torque imparted by the rotor during ATP synthesis as well as ATP hydrolysis.

Tethering polypeptides through bifunctional PEG cross-linking agents to probe protein function: application to ATP synthase.

Chemical crosslinking mediated by short bifunctional reagents has been widely used for determining physical relationships among polypeptides in multisubunit proteins, but less often for functional studies. Here we introduce the approach of tethering polypeptides by using bifunctional reagents containing a lengthy, flexible PEG linker as a form of crosslinking especially suited to functional analyses. The rotary molecular motor ATP synthase was used as a model subject. Single cysteine residues were introduced into selected positions of ATP synthase epsilon subunit, a component of the rotor subcomplex of the enzyme, and the unrelated maltose binding protein (MBP), then the two purified recombinant proteins were crosslinked by means of a dimaleimido-PEG cross-linking agent. Following purification, the epsilon-PEG-MBP was incorporated into membrane-bound ATP synthase by reconstitution with epsilon-depleted F(1)-ATPase and membrane vesicles that had been stripped of endogenous F(1). ATP synthase reconstituted using epsilon-PEG-MBP had reduced ATP hydrolytic activity that was uncoupled from the pumping of H(+), indicating the physical blockage of rotation of the gammaepsilonc(10) rotor by the conjugated MBP, whereas enzyme reconstituted with epsilon-PEG was normal. These results directly demonstrate the feasibility of studying mechanistic features of molecular motors through PEG-based conjugation of unrelated proteins. Since tethering polypeptides provides a means of maintaining proximity without directly specifying or modifying interactions, application of the general method to other types of protein functional studies is envisioned.

Probing the functional tolerance of the b subunit of Escherichia coli ATP synthase for sequence manipulation through a chimera approach.

A dimer of 156-residue b subunits forms the peripheral stator stalk of eubacterial ATP synthase. Dimerization is mediated by a sequence with an unusual 11-residue (hendecad) repeat pattern, implying a right-handed coiled coil structure. We investigated the potential for producing functional chimeras in the b subunit of Escherichia coli ATP synthase by replacing parts of its sequence with corresponding regions of the b subunits from other eubacteria, sequences from other polypeptides having similar hendecad patterns, and sequences forming left-handed coiled coils. Replacement of positions 55-110 with corresponding sequences from Bacillus subtilis and Thermotoga maritima b subunits resulted in fully functional chimeras, judged by support of growth on nonfermentable carbon sources. Extension of the T. maritima sequence N-terminally to position 37 or C-terminally to position 124 resulted in slower but significant growth, indicating retention of some capacity for oxidative phosphorylation. Portions of the dimerization domain between 55 and 95 could be functionally replaced by segments from two other proteins having a hendecad pattern, the distantly related E subunit of the Chlamydia pneumoniae V-type ATPase and the unrelated Ag84 protein of Mycobacterium tuberculosis. Extension of such sequences to position 110 resulted in loss of function. None of the chimeras that incorporated the leucine zipper of yeast GCN4, or other left-handed coiled coils, supported oxidative phosphorylation, but substantial ATP-dependent proton pumping was observed in membrane vesicles prepared from cells expressing such chimeras. Characterization of chimeric soluble b polypeptides in vitro showed their retention of a predominantly helical structure. The T. maritima b subunit chimera melted cooperatively with a midpoint more than 20 degrees C higher than the normal E. coli sequence. The GCN4 construct melted at a similarly high temperature, but with much reduced cooperativity, suggesting a degree of structural disruption. These studies provide insight into the structural and sequential requirements for stator stalk function.

The stator complex of the A1A0-ATP synthase--structural characterization of the E and H subunits.

Archaeal ATP synthase (A-ATPase) is the functional homolog to the ATP synthase found in bacteria, mitochondria and chloroplasts, but the enzyme is structurally more related to the proton-pumping vacuolar ATPase found in the endomembrane system of eukaryotes. We have cloned, overexpressed and characterized the stator-forming subunits E and H of the A-ATPase from the thermoacidophilic Archaeon, Thermoplasma acidophilum. Size exclusion chromatography, CD, matrix-assisted laser desorption ionization time-of-flight mass spectrometry and NMR spectroscopic experiments indicate that both polypeptides have a tendency to form dimers and higher oligomers in solution. However, when expressed together or reconstituted, the two individual polypeptides interact with high affinity to form a stable heterodimer. Analyses by gel filtration chromatography and analytical ultracentrifugation show the heterodimer to have an elongated shape, and the preparation to be monodisperse. Thermal denaturation analyses by CD and differential scanning calorimetry revealed the more cooperative unfolding transitions of the heterodimer in comparison to those of the individual polypeptides. The data are consistent with the EH heterodimer forming the peripheral stalk(s) in the A-ATPase in a fashion analogous to that of the related vacuolar ATPase.