Regulation of the mitochondrial ATP synthase/ATPase complex: cDNA cloning, sequence, overexpression, and secondary structural characterization of a functional protein inhibitor

The ATPase inhibitor protein of the rat liver mitochondrial ATP synthase/ATPase complex has been cloned from a rat liver cDNA library, and its nucleotide sequence determined. The sequence is highly homologous to both the bovine heart (approximately 70%) and the yeast inhibitor proteins (approximately 40%). The deduced protein sequence is 107 amino acids in length, and based on homology to the bovine heart protein, the first 25 N-terminal amino acids encode a putative mitochondrial targeting sequence. The "mature" protein (without the targeting sequence) fused to the maltose binding protein has been overexpressed in Escherichia coli. The maltose binding protein was used as a handle for the development of a rapid one-step purification of the fusion protein by affinity chromatography on an amylose resin. The purified fusion protein was cleaved with Factor Xa protease at the fusion junction, and the resulting ATPase inhibitor protein was purified to > 90% purity. The purified, overexpressed inhibitor protein displays normal inhibitor activity. The protein inhibits ATP hydrolysis catalyzed by the ATP synthase/ATPase complex in submitochondrial particles in a manner kinetically indistinguishable from the same protein purified from rat liver mitochondria, and exhibits a specific activity of approximately 10,000 units/mg. The secondary structure of the inhibitor protein was determined by circular dichroism spectropolarimetry. The experimentally determined structure shows a high content of alpha-helix and is in good agreement with sequence-based structural predictions. As the function of the inhibitor protein is known to exhibit a high dependence on pH, a study of the pH dependence of inhibitor secondary structure was performed. It is shown that as pH is lowered, conditions which activate inhibitory capacity, the protein loses significant alpha-helical structure. This is the first report of the overexpression in E. coli of a functional ATPase inhibitor protein. Secondary structural analysis of this protein indicates that conversion from its active to its inactive form involves a significant conformational change.

Effects of the delta F508 mutation on the structure, function, and folding of the first nucleotide-binding domain of CFTR

The fatal autosomal recessive disease cystic fibrosis (CF) is caused by mutations in the gene which encodes the cystic fibrosis transmembrane conductance regulator (CFTR). Many of these disease-causing mutations, including the deletion of F508 (delta F508) which accounts for approximately 70% of the disease alleles, occur in one of the two consensus nucleotide binding sequences. Peptide studies have directly demonstrated that the N-terminal nucleotide binding sequences bind adenine nucleotides. Structurally, circular dichroism spectropolarimetry indicates that this region of CFTR assumes a beta-stranded structure in solution. The delta F508 mutation causes a diminution in the amount of beta-stranded structure and a concomitant increase in the amount of random coil structure present, indicating that either the mutant peptide has a different native structure or that the conformational equilibrium is shifted toward a more disordered form. Furthermore, the mutant peptide is more sensitive to denaturation, indicating that delta F508 is a stability, or protein-folding mutant. Here we review these results and discuss their implications for interpreting the behavior of delta F508 in situ and for the rational design of new CF drugs.

Altered protein folding may be the molecular basis of most cases of cystic fibrosis

Experiments have demonstrated that the cystic fibrosis transmembrane conductance regulator protein (CFTR), containing the most common cystic fibrosis (CF)-causing mutation (delta F508), reaches the plasma membrane in reduced amounts. Studies of a peptide model of CFTR indicate that the delta F508 mutated region is more sensitive to denaturating conditions. This paper proposes that altered protein folding accounts for these findings, and, thus, most cases of CF. Significantly, the hypothesis makes specific predictions about the effect of stabilizing conditions on mutant CFTR, and, further, suggests a new class of pharmaceuticals that may prove effective in the treatment of this important genetic disease.

Mutational analysis of the consensus nucleotide binding sequences in the rat liver mitochondrial ATP synthase beta-subunit

The coupling step in the biosynthesis of ATP in biological systems is generally believed to involve an energy-requiring release of ATP bound to the beta-subunit of the ATP synthase complex. A molecular description of the ATP binding site on the beta-subunit is, therefore, critical to understanding the mechanism of coupling in the enzyme. Previously, we reported that a purified, bacterially expressed rat liver beta-subunit binds adenine nucleotides tightly and specifically (Garboczi, D. N., Hullihen, J. H., and Pedersen, P. L. (1988) J. Biol. Chem. 263, 15694-15698). In order to assess the contribution of various regions of the isolated beta-subunit to the ATP binding site we have systematically deleted four different regions: the N-terminal region, the Walker A consensus region, the Walker B consensus region (Walker, J. E., Saraste, M., Runswick, M. J., and Gay, N. (1982) EMBO J. 1, 945-951), and a "C" region, which, like the A and B regions, bears homology to adenylate kinase. Plasmids directing the expression of double deletions of A and B regions, and B and C regions were also constructed. In addition, 2 residues outside of these regions, His-177 and Tyr-345, which have been predicted to play a central role in nucleotide binding, were mutated. Rabbit antisera to synthetic peptides of the A and C regions verified the identity of the bacterially expressed mutant proteins. Seven of the eight mutant proteins overexpressed in Escherichia coli were resistant to E. coli proteases in the preparative stages, as predicted for compact folded proteins. Furthermore, circular dichroism spectropolarimetry revealed no profound structural alterations in the purified mutant proteins. Relative to the overexpressed full-length beta-subunit, the mutant lacking the A consensus region suffered a 30-fold loss of affinity for ATP and a loss of specificity for 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP) over 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-monophosphate. The mutant proteins lacking either the N-terminal region or the B region exhibited nucleotide binding properties similar to the full-length beta-subunit, whereas the mutant protein lacking the C region suffered an order of magnitude reduction in affinity for ATP. The affinity of the A and B region double deletion was indistinguishable from the A region deletion in regard to TNP-ATP binding, while the double deletion mutant lacking the B and C regions was not stably expressed in the E. coli SE6004.(ABSTRACT TRUNCATED AT 400 WORDS)

Quaternary structure of ATP synthases: symmetry and asymmetry in the F1 moiety

It has been proposed that during ATP synthesis/hydrolysis F1 ATPases experience a complex pattern of nucleotide binding and release during the catalytic cycle (binding change mechanism). This type of mechanism has implications that can be correlated with the structure of the enzyme. F1-ATPases (stoichiometry alpha 3 beta 3 gamma delta epsilon) are essentially a symmetrical trimer of pairs of the major subunits (alpha and beta); the minor subunits (gamma, delta and epsilon) are in single copies and interact with the trimer in an asymmetrical fashion. The asymmetry introduced by the minor subunits has important structural and functional consequences: (1) it introduces differences between the potentially equivalent binding and catalytic sites in the major subunits, (2) it restricts the ways in which a binding change mechanism can occur, and (3) it governs the way in which the F1 interacts with the (asymmetrical) F0 sector.

Two-dimensional NMR, circular dichroism, and fluorescence studies of PP-50, a synthetic ATP-binding peptide from the beta-subunit of mitochondrial ATP synthase

PP-50, a peptide based on residues 141-190 of the beta-subunit of mitochondrial F1-ATPase, contains the GX4GKT consensus region for nucleoside triphosphate binding and has been shown to bind ATP [Garboczi, D.N., Shenbagamurthi, W.K., Hullihen, J., & Pedersen, P.L. (1988) J. Biol. Chem. 263, 812-816]. At pH 4.0, appropriate for NMR studies, PP-50 retains the ability to bind ATP tightly (KD = 17.5 microM) with a 1:1 stoichiometry as shown by titrations measuring the partial quenching of ATP fluorescence by PP-50. CD spectra of PP-50 at pH 4.0 and at low ionic strength show 5.8% helix, 30.2% beta-structure, and 64% coil. ATP binding increases the structure of PP-50, changing the CD to 7.5% helix, 44.5% beta-structure, and 48% coil. Increasing the ionic strength to 50 mM KCl also increases the structure, changing the CD to 7.4% helix, 64.4% beta-structure, and 28.2% coil. The 600-MHz proton NMR spectrum of PP-50, at pH 4.0 and low ionic strength, has been assigned by 2D methods (TOCSY, DQF-COSY, and NOESY with jump-return water suppression). Based on strong d alpha N NOEs, J alpha N values, and NH chemical shifts differing from random coil values, regions of extended structure are detected from residues 1-7 and 43-48. Based on dNN, dNN(i,i+2), and d alpha N(i,i+2) NOEs and 3J alpha N values, possible type I' and type I turns are found from residues 11-14 and 31-34, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

ATP synthase: structure-function relationships

Recent work has focused on obtaining a better understanding of the three-dimensional structural relationships between the alpha and beta subunits of the F1 moiety and the location of nucleotide binding domains within these subunits. Four types of approach are currently being pursued: X-ray crystallographic, chemical, molecular biological and biochemical. Here we briefly review some of the major conclusions of these studies, and point out some of the problems that must be resolved before an adequate model that relates structure to function in the ATP synthase molecule can be formulated.

The cystic fibrosis transmembrane conductance regulator. Effects of the most common cystic fibrosis-causing mutation on the secondary structure and stability of a synthetic peptide

Deletion of phenylalanine 508 (delta Phe-508) in the cystic fibrosis transmembrane conductance regulator (CFTR) protein causes approximately 70% of all cases of cystic fibrosis. This residue lies in a region of the protein that we have synthesized chemically and shown to bind adenine nucleotides (Thomas, P. J., Shenbagamurthi, P., Ysern, X., and Pedersen, P. L. (1991) Science 251, 555-557). A peptide lacking this critical residue, but otherwise corresponding to this crucial part of the protein, now also has been chemically synthesized and purified. This mutant peptide (P-66) exhibits a significant loss of beta-sheet structure as compared with the wild type peptide (P-67). Furthermore, urea denaturation of peptide structure reveals that P-66 is less stable than P-67. Although under non-denaturing conditions both peptides bind adenine nucleotides with high affinity, the loss of structural stability is reflected in the binding function of the peptides. Thus, P-67, in contrast to P-66, retains a significant capacity for nucleotide binding in 4 M urea. These results suggest a model for impaired delta Phe-508 CFTR function.

Overexpression of higher eukaryotic membrane proteins in bacteria. Novel insights obtained with the liver mitochondrial proton/phosphate symporter

In order to better understand why higher eukaryotic membrane proteins, in contrast to soluble proteins, are not readily expressed in Escherichia coli, the gene encoding the liver mitochondrial phosphate transporter (H+/Pi symporter) (Ferreira, G. C., Pratt, R. D., and Pedersen, P. L. (1989) J. Biol. Chem. 264, 15628-15633), was subcloned into a plasmid (pFOG402) containing the alkaline phosphatase promoter and leader sequence. Although this system is highly efficient in overexpressing soluble mitochondrial proteins in E. coli, e.g. alpha and beta subunits of the liver ATP synthase, it fails to express the H+/Pi transporter. Expression is not obtained by truncation of the transporter gene from either the 3' or 5' end, by fusing the mature transporter gene to genes encoding either the alpha or beta ATP synthase subunits, or by using different expression plasmids. Significantly, the H+/Pi transporter is overexpressed in E. coli provided its cDNA is first truncated at the 3' end (carboxyl-terminal end) and fused to a cDNA fragment derived from the ATP synthase alpha subunit gene. In fact, progressive deletions from the 3' end of the transporter cDNA produce a ladder of increasingly overexpressed fusion proteins which account from the largest to the smallest for approximately 2.5-14% of the total bacterial cell protein. The minimal truncation necessary from the 3' end is 192 base pairs corresponding to 64 COOH-terminal amino acids. This corresponds to 20% of the transporter and involves removal of one of the six predicted membrane-spanning segments. In a variety of additional experiments designed to define the molecular basis for E. coli's inability to express the complete liver H+/Pi transporter, problems related to cell toxicity and transcription were ruled out. However, in vitro transcription-translation assays revealed that the complete transporter is readily expressed when eukaryotic, but not prokaryotic, ribosomes are present. Significantly, the fused transporter gene (i.e. Pi transporter cDNA truncated at the 3' end + ATP synthase alpha subunit cDNA) is expressed when prokaryotic ribosomes are present. These results support the view that the difficulty in expressing higher eukaryotic membrane proteins in bacteria may be related in some cases to a problem at the level of translation.

Hexokinase receptors: preferential enzyme binding in normal cells to nonmitochondrial sites and in transformed cells to mitochondrial sites

Hexokinase plays an important role in normal glucose-utilizing tissues like brain and kidney, and an even more important role in highly malignant cancer cells where it is markedly overexpressed. In both cell types, normal and transformed, a significant portion of the total hexokinase activity is bound to particulate material that sediments upon differential centrifugation with the crude "mitochondrial" fraction. In the case of brain, particulate binding may constitute most of the total hexokinase activity of the cell, and in highly malignant tumor cells as much as 80 percent of the total. When a variety of techniques are rigorously applied to better define the particulate location of hexokinase within the crude "mitochondrial fraction," a striking difference is observed between the distribution of hexokinase in normal and transformed cells. Significantly, particulate hexokinase found in rat brain, kidney, or liver consistently distributes with nonmitochondrial membrane markers whereas the particulate hexokinase of highly glycolytic hepatoma cells distributes with outer mitochondrial membrane markers. These studies indicate that within normal tissues hexokinase binds preferentially to nonmitochondrial receptor sites but upon transformation of such cells to yield poorly differentiated, highly malignant tumors, the overexpressed enzyme binds preferentially to outer mitochondrial membrane receptors. These studies, taken together with the well-known observation that, once solubilized, the particulate hexokinase from a normal tissue can bind to isolated mitochondria, are consistent with the presence in normal tissues of at least two different types of particulate receptors for hexokinase with different subcellular locations. A model which explains this unique transformation-dependent shift in the intracellular location of hexokinase is proposed.

Mitochondrial ATP synthase. Quaternary structure of the F1 moiety at 3.6 A determined by x-ray diffraction analysis

The F1 moiety of the mitochondrial ATP synthase is composed of five different subunits with stoichiometry alpha 3 beta 3 gamma delta epsilon and exhibits the capacity to synthesize ATP from ADP and Pi. We have previously crystallized rat liver F1 and described its structure at 9-A resolution (Amzel, L. M., McKinney, M., Narayanan, P., and Pedersen, P. L. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 5852-5856). Here we present an x-ray map of this complex enzyme at 3.6 A, which provides a much more informative description of its quaternary structure. The overall dimensions of the F1 molecule are 120 A x 120 A x 74 A. The enzyme exhibits 3-fold symmetry relating the three copies of each of the two major subunits, alpha and beta. As the alpha subunits (but not the beta subunits) contain cysteine residues, it has been possible to identify the alpha subunits by heavy atom labeling with mersalyl and to relate their positions in the F1 molecule to the beta subunits. Significantly, the alpha and beta subunits each exist as trimeric arrays which are organized in two slightly offset, interdigitated layers along the 3-fold axis. In one trimeric layer the alpha subunits are located close to the axis with homologous subunits interacting with each other; in the other trimeric layer the beta subunits are far from the axis, and they interact only with alpha subunits and not with one another. At one end of the structure, part of the interface between each alpha and beta subunit encloses a space or "pocket" that is accessible to the solvent; at the other end the interfaces between the subunits are more open and exposed. The present work represents the highest resolution map reported to date for the F1 moiety of an ATP synthase complex.

Glucose phosphorylation. Site-directed mutations which impair the catalytic function of hexokinase

Recent studies from this and other laboratories have resulted in the cloning and sequencing of hexokinases from a variety of tissues including yeast, human kidney, rat brain, rat liver, and mouse hepatoma. Significantly, studies on the hepatoma enzyme conducted in this laboratory (Arora, K.K., Fanciulli, M., and Pedersen, P.L. (1990) J. Biol. Chem. 265, 6481-6488) resulted also in its overexpression in Escherichia coli in active form. We have now used site-directed mutagenesis for the first time in studies of hexokinase to evaluate the role of amino acid residues predicted to interact with either glucose or ATP. Four amino acid residues (Ser-603, Asp-657, Glu-708, and Glu-742) believed to interact with glucose were mutated to alanine or glycine, whereas a lysine residue (Lys-558) thought to be directly involved in binding ATP was mutated to either methionine or arginine. Of all the mutations in residues believed to interact with glucose, the Asp-657----Ala mutation is the most profound, reducing the hexokinase activity to a level less than 1% of the wild type. The relative Vmax values for Ser-603----Ala, Glu-708----Ala, and Glu-742----Ala enzymes are 6, 10, and 6.5%, respectively, of the wild-type enzyme. Glu-708 and Glu-742 mutations increase the apparent Km for glucose 50- and 14-fold, respectively, while the Ser-603----Ala mutation decreases the apparent Km for glucose 5-fold. At the putative ATP binding site, the relative Vmax for Lys-558----Arg and Lys-558----Met enzymes are 70 and 29%, respectively, of the wild-type enzyme with no changes in the apparent Km for glucose. No changes were observed in the apparent Km for ATP with any mutation. These results support the view that all 4 residues predicted to interact with glucose from earlier x-ray studies may play a role in binding and/or catalysis. The Asp-657 and Ser-603 residues may be involved in both, while Glu-708 and Glu-742 clearly contribute to binding but are not essential for catalysis. In contrast, Lys-558 appears to be essential neither for binding nor catalysis.

Cystic fibrosis transmembrane conductance regulator: nucleotide binding to a synthetic peptide

Multiple mutations in the gene responsible for cystic fibrosis are located within a region predicted to encode a nucleotide-binding fold in the amino terminal half of the cystic fibrosis transmembrane conductance regulator protein. A 67-amino acid peptide (P-67) that corresponds to the central region of this putative nucleotide binding site was chemically synthesized and purified. This peptide bound adenine nucleotides. The apparent dissociation constants (Kd's) for the trinitrophenyl (TNP) adenine nucleotides, TNP-adenosine triphosphate, TNP-adenosine diphosphate, and TNP-adenosine monophosphate, were 300 nanomolar, 200 nanomolar, and greater than 1 micromolar, respectively. The Kd for adenosine triphosphate was 300 micromolar. Circular dichroism spectroscopy was used to show that P-67 assumes a predominantly beta sheet structure in solution, a finding that is consistent with secondary structure predictions. On the basis of this information, the phenylalanine at position 508, which is deleted in approximately 70 percent of individuals with cystic fibrosis, was localized to a beta strand within the nucleotide binding peptide. Deletion of this residue is predicted to induce a significant structural change in the beta strand and altered nucleotide binding.

Mitochondrial proton/phosphate transporter. An antibody directed against the COOH terminus and proteolytic cleavage experiments provides new insights about its membrane topology

Molecular cloning and sequencing of a full-length cDNA encoding the rat liver mitochondrial phosphate transporter (H+/Pi symporter) has revealed its primary structure (Ferreira, G. C. Pratt, R. D., and Pedersen, P. L. (1989) J. Biol. Chem. 264, 15628-15633). To date, no experimental data pertinent to the membrane topology of this transporter are available. For this reason, four different peptides which represent different regions of the H+/Pi symporter were synthesized and used to raise polyclonal antibodies. Each of the antipeptide antibodies exhibits immunoreactivity with its synthetic peptide antigen, but only antiserum against a COOH-terminal peptide reacts with the native transporter, suggesting that the other peptides are either conformally restricted or located in the interior of the protein. Competitive radioimmunoassays, using intact "mitoplasts" (outer membrane-free mitochondria) and inverted inner membrane vesicles, show that the COOH-terminal antibodies bind only to the cytoplasmic surface of the inner membrane, indicating that the COOH terminus of the protein is normally exposed to the mitochondrial intermembrane space. In support of this conclusion, tryptic digestion of mitoplasts but not of the inside-out vesicles, cleaves the antigenic site for the COOH-terminal antibodies. In other experiments, it was shown that N-ethylmalemide, a sulfhydryl alkylating agent known to inhibit the mitochondrial phosphate transporter, markedly reduces the accessibility of the COOH terminus to trypsin. These studies provide the first direct experimental data relevant to the membrane topology of the mitochondrial H+/Pi symporter. In addition, they support the view that alkylation of a reactive cysteine residue induces a significant conformational change in the transporter.

Rat liver mitochondrial ATP synthase. Effects of mutations in the glycine-rich region of a beta subunit peptide on its interaction with adenine nucleotides

The beta subunit of the rat liver mitochondrial ATP synthase contains a glycine-rich amino acid sequence implicated in binding nucleotides by its similarity to a sequence found in many other nucleotide-binding proteins. A C-terminal three-quarter-length rat liver beta subunit fragment (Glu122 through Ser479), containing this homology region, interacts with adenine nucleotides (Garboczi, D.N., Hullihen, J.H., and Pedersen, P.L. (1988) J. Biol. Chem. 263, 15694-15698). Here we directly test the involvement of the glycine-rich region in nucleotide binding by altering its amino acid sequence through mutation or deletion. Twenty-one mutations within the glycine-rich region of the beta subunit cDNA were randomly generated. Wild-type and mutant beta subunit proteins were purified from overexpressing Escherichia coli strains. The mutant proteins were screened for changes in their interaction with 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP), a fluorescent nucleotide analog. Only one mutant protein bearing two amino acid changes (Gly153----Val, Gly156----Arg) exhibited a fluorescence enhancement higher than that of the wild-type "control." Further analysis of this protein revealed a lower affinity for TNP-ATP (Kd = 10 microM) compared with wild type (Kd = 5 microM). In addition, a mutant from which amino acids Gly149-Lys214 had been deleted was prepared. This mutant protein, which lacks the entire glycine-rich region, also displayed a marked reduction in affinity for TNP-ATP (Kd greater than 60 microM). Prior addition of 0.5 mM ATP significantly reduced the binding of TNP-ATP to both the double and deletion mutants. The altered interaction of nucleotide with both glycine-rich region mutants points to the involvement of this region in the binding site. Further, this work shows that a beta subunit protein that lacks the glycine-rich homology region can still interact with nucleotide, indicating that one or more additional regions of this subunit contribute to the nucleotide binding site.

Glucose phosphorylation in tumor cells. Cloning, sequencing, and overexpression in active form of a full-length cDNA encoding a mitochondrial bindable form of hexokinase

In rapidly growing tumor cells exhibiting high glucose catabolic rates, the enzyme hexokinase is markedly elevated and bound in large amounts (50-80% of the total cell activity) to the outer mitochondrial membrane (Arora, K.K., and Pedersen, P.L. (1988) J. Biol. Chem. 263, 17422-17428; Parry, D.M., and Pedersen, P.L. (1983) J. Biol. Chem. 258, 10904-10912). In extending these studies, we have isolated a cDNA clone of hexokinase from a lambda gt11 library of the highly glycolytic, c37 mouse hepatoma cell line. This clone, comprising 4,198 base pairs, contains a single open reading frame of 2,754 nucleotides which encode a 918-amino acid hexokinase with a mass of 102,272 daltons. This enzyme exhibits, respectively, 68 and 32 amino acid differences, including several charge differences, from the recently sequenced human kidney and rat brain enzymes. The putative glucose and ATP binding domains present in the latter two enzymes and in rat liver glucokinase are conserved in the tumor enzyme. At its N-terminal region, tumor hexokinase has a 12-amino acid hydrophobic stretch which is present in the rat brain enzyme but absent in the rat liver glucokinase, a cytoplasmic enzyme. The mature tumor hexokinase protein has been overexpressed in active form in Escherichia coli and purified 9-fold. The overexpressed enzyme binds to rat liver mitochondria in the presence of MgCl2. This is the first report describing the cloning and sequencing of a tumor hexokinase, and the first report documenting the overexpression of any hexokinase type in E. coli. Questions pertinent to the enzyme's mechanism, regulation, binding to mitochondria, and its marked elevation in tumor cells can now be addressed.

Glucose phosphorylation. Interaction of a 50-amino acid peptide of yeast hexokinase with trinitrophenyl ATP

A 50-amino acid peptide predicted by chemical modification studies of yeast hexokinase to contain an ATP-binding site has been synthesized and purified. The peptide, which includes residues from glutamate 78 at the NH2-terminal end to leucine 127 at the COOH-terminal, resides within the smaller of the two lobes found in the three-dimensional structure of yeast hexokinase. It is this region which has been reported recently to exhibit significant sequence homology with hexokinase types I and IV of higher eukaryotic cells and sequence homology with the active site of protein kinases. Similar to native yeast hexokinase, the 50-amino acid peptide interacts strongly with the fluorescent analog TNP-ATP [2',(3')-O-(2,4,6-trinitrophenyl)-adenosine-5'-triphosphate]. A 5-fold enhancement is observed when 8 microM peptide interacts with 20 microM TNP-ATP. The stoichiometry of binding is very close to 1 mol of TNP-ATP/mol peptide. Also, similar to native yeast hexokinase, the fluorescent enhancement observed upon TNP-ATP binding to the synthetic peptide is greater than that observed upon TNP-ADP binding. Finally, TNP-AMP exhibits a much lower fluorescent enhancement in the presence of hexokinase or the synthetic peptide. The additional findings that ATP can readily prevent TNP-ATP binding and that TNP-ATP can substitute for ATP as a weak substrate for hexokinase in the phosphorylation of glucose indicate that the synthetic peptide described here comprises part of the catalytic site.

Mitochondrial ATP synthase. cDNA cloning, amino acid sequence, overexpression, and properties of the rat liver alpha subunit

The predicted amino acid sequence of the alpha subunit of the rat liver mitochondrial ATP synthase has been obtained by sequencing a cDNA for the alpha subunit. Analysis of the sequence shows that it contains the A and B consensus sequences found in many nucleotide-binding proteins. Twelve amino acids of the rat liver alpha subunit differ from the sequence of the bovine heart alpha subunit; four of these involve differences in charge. The rat liver alpha subunit, from arginine 15 to the C-terminal proline 510, has been overexpressed in Escherichia coli using the alkaline phosphatase promoter (phoA) and leader peptide to direct the export of the expressed protein to the bacterial periplasm. By treating the cells with lysozyme, osmotic shock, and alkaline pH washes, the alpha subunit can be extracted in high yield (greater than 25 mg/liter) and in a high state of purity. The expressed alpha subunit remains soluble at pH 9.5 or greater and precipitates when treated with Mg2+ ions at low millimolar concentration. The bacterially expressed alpha subunit interacts with 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP), resulting in a marked fluorescence enhancement upon binding. An enhancement of fluorescence is also observed upon the interaction of the alpha subunit with TNP-ADP. Preincubating the alpha subunit with 1.5 mM ATP significantly reduces the fluorescence enhancement seen with TNP-ATP. The alpha subunit binds TNP-ATP with an apparent Kd in the low micromolar range (1-5 microM) and binds TNP-ADP with an affinity at least 10-fold lower. This work shows that the rat liver alpha subunit can be overexpressed in E. coli to yield a large amount of functional protein. With the acquisition of the overexpressed alpha subunit, it is now possible to test the reconstitution of ATPase activity from a mixture of recombinant and rat liver-derived subunits and to test the formation of complexes by the overexpressed alpha and beta subunits of the rat liver F1-ATPase.

Glucose catabolism in brain. Intracellular localization of hexokinase

A major energy source in brain is glucose, which is committed to metabolism by hexokinase (Type I isozyme), an enzyme usually considered to be bound to the outer mitochondrial membrane. In this study, the subcellular location of hexokinase in brain has been rigorously investigated. Mitochondrial fractions containing hexokinase (greater than 500 milliunits/mg protein) were prepared by two different procedures, and then subjected to density gradient centrifugation before and after loading with barium phosphate, a technique designed to increase the density of the mitochondria. The gradient distribution patterns of both unloaded and loaded preparations show that brain hexokinase does not distribute exclusively with mitochondrial marker enzymes. This is particularly evident in the loaded preparations where there is a clear distinction between the peak activities of hexokinase and mitochondrial markers. The same observation was made when the mitochondrial fraction of either untreated or barium phosphate-loaded mitochondria was subjected to titration with digitonin. In fact, at concentrations of digitonin, which almost completely solubilize marker enzymes for both the inner and outer mitochondrial membranes, a significant fraction of the total hexokinase remains particulate bound. Electron microscopy confirmed that particulate material is still present under these conditions. Significantly, hexokinase is released from particulate material only at high concentrations of digitonin which solubilize the associated microsomal marker NADPH-cytochrome c reductase. Glucose 6-phosphate, which is known to release hexokinase from the brain "mitochondrial fraction" also releases hexokinase from this unidentified particulate component. These results on brain, a normal glucose utilizing tissue, differ from those obtained previously on highly glycolytic tumor cells where identical subfractionation procedures revealed a strictly outer mitochondrial membrane location for particulate hexokinase (Parry, D. M., and Pedersen, P. L. (1983) J. Biol. Chem. 258, 10904-10912). It is concluded that in brain, hexokinase has a greater propensity to localize at nonmitochondrial receptor sites than to those known to be associated with the outer mitochondrial membrane.

Energy-linked anion transport. Cloning, sequencing, and characterization of a full length cDNA encoding the rat liver mitochondrial proton/phosphate symporter

A full length cDNA clone encoding the precursor of the rat liver mitochondrial phosphate transporter (H+/Pi symporter) has been isolated from a cDNA library using a bovine heart partial length phosphate transporter clone as a hybridization probe. The entire clone is 1263 base pairs in length with 5'- and 3'-untranslated regions of 16 and 168 base pairs, respectively. The open reading frame encodes for the mature protein (312 amino acids) preceded by a presequence of 44 amino acids enriched in basic residues. The polypeptide sequence predicted from the DNA sequence was confirmed by analyzing the first 17 amino-terminal amino acids of the pure phosphate transporter protein. The rat liver phosphate transporter differs from the bovine heart transporter in 32 amino acids (i.e. approximately 10%). It contains a region from amino acid 139 to 159 which is 37% identical with the beta-subunit of the liver mitochondrial ATP synthase. Amino acid sequence comparisons of the Pi transporter with Pi binding proteins, other H+-linked symporters, and the human glucose transporter did not reveal significant sequence homology. Analysis of genomic DNA from both rat and S. cerevisiae by Southern blots using the rat liver mitochondrial Pi carrier cDNA as a probe revealed remarkably similar restriction patterns, a finding consistent with the presence in lower and higher eukaryotes of homologous Pi carrier proteins. This is the first report of the isolation, sequencing, and characterization of a full length cDNA coding for a protein involved in energy-coupled Pi transport.

F0 "proton channel" of rat liver mitochondria. Rapid purification of a functional complex and a study of its interaction with the unique probe diethylstilbestrol

The F0 portion of the rat liver mitochondrial ATP synthase (F0F1-ATPase) has been purified by a rapid, high yield procedure. F0 is selectively extracted from inner membrane vesicles with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) after prior treatment of the vesicles with guanidine HCl to remove F1. The resultant F0 is functional in proton translocation assays and separates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis into four major and three minor Coomassie-stainable bands, all with apparent molecular masses below 30 kDa. This CHAPS-purified F0 preparation was characterized in detail for its capacity to interact with the unique probe diethylstilbestrol (DES) which, depending on conditions, has been shown to interact with rat liver F0F1 to either inhibit or promote ATP hydrolysis (McEnery, M. W., and Pedersen, P.L. (1986) J. Biol. Chem. 261, 1745-1752). DES-inhibitory sensitivity could be conferred on F1-ATPase activity with the same concentration dependence on F0 as conferral of oligomycin sensitivity. DES was shown also to inhibit the magnitude of valinomycin induced proton influx, while initiating proton efflux in asolectin vesicles reconstituted with F0 and loaded with K+. The potency of DES in producing the latter effects was shown to be highly dependent on hydroxyl groups in "para" positions of the two benzene rings within the DES molecule. Finally, in the absence of F0, DES was shown to act as a catalyst of proton influx in K+-loaded asolectin vesicles upon addition of valinomycin. A model based on the structure of DES is presented to account for both the inhibitory and uncoupling properties of this compound.

Investigation of the substrate structure and metal cofactor requirements of the rat liver mitochondrial ATP synthase/ATPase complex

The F1 moiety of the rat liver mitochondrial ATP synthase/ATPase complex contains as isolated 2 mol Mg2+/mol F1, 1 mol of which is nonexchangeable and the other which is exchangeable (N. Williams, J. Hullihen, and P.L. Pedersen, (1987) Biochemistry 26, 162-169). In addition, the enzyme binds 1 mol ADP/mol F1 and 3 mol AMP.PNP, the latter of which can bind in complex formation with divalent cation and displace the Mg2+ at the exchangeable site. Thus, in terms of ligand binding sites the fully loaded rat liver F1 complex contains 3 mol MgAMP.PNP, 1 mol ADP, and 1 mol Mg2+. In this study we have used several metal ATP complexes or analogs thereof to gain further insight into the ligand binding domains of rat liver F1 and the mechanism by which it catalyzes ATP hydrolysis in soluble and membrane bound form. Studies with LaATP confirmed that MgATP is the most likely substrate for rat liver F1, and provided evidence that the enzyme may contain additional Mg2+ binding sites, undetected in previous studies of F1-ATPases, that are required for catalytic activity. Thus, F1 containing the thermodynamically stable LaATP complex in place of MgATP requires added Mg2+ to induce ATP hydrolysis. As Mg2+ cannot readily displace La2+ under these conditions there appears to be a catalytically important class of Mg2+ binding sites on rat liver F1, distinct from the nonexchangeable Mg2+ site and the sites involved in binding MgATP. Additional studies carried out with exchange inert metal-nucleotide complexes involving rhodium and the Mg2+ and Cd2+ complexes of ATP beta S and ATP alpha S imply that the rate-limiting step in the ATPase reaction pathway occurs subsequent to the P gamma-O-P beta bond cleavage steps, perhaps at the level of Mg(ADP)(Pi) hydrolysis or MgADP release. Evidence is presented that Mg2+ remains coordinated to the leaving group of the reaction, i.e., the beta phosphoryl group. Finally, in contrast to soluble F1, F1 bound to F0 in the inner mitochondrial membrane failed to discriminate between the Mg2+ complexes of the ATP beta S isomers. This indicates that a fundamental difference may exist between the catalytic or kinetic mechanism of F1 and the more physiologically intact F0F1 complex.

Inhibition of Na+-dependent phosphate transport by group-specific covalent reagents in rat kidney brush border membrane vesicles. Evidence for the involvement of tyrosine and sulfhydryl groups on the interior of the membrane

The effects of tyrosine- and sulfhydryl-specific reagents on the Na+-dependent transport of phosphate in brush border membrane vesicles prepared from rat renal cortex were investigated. This study is the first to show that the tyrosine-specific reagents 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole and tetranitromethane inactivate the transporter in a concentration- and time-dependent fashion while the membrane impermeant tyrosine reagent, N-acetylimidazole, has no effect on phosphate uptake. The membrane permeant sulfhydryl reagent N-ethylmaleimide also caused a time- and concentration-dependent inactivation of this transport process but the membrane impermeant reagents 7-chloro-4-sulfobenzo-2-oxa-1,3-diazole and eosin-5-maleimide had little effect on phosphate uptake. The inhibitory effects of both tyrosine- and sulfhydryl-specific reagents were additive, but no protection from inactivation by tyrosine-specific reagents could be achieved by preincubation of the vesicles with the substrates of the transporter or with competitive inhibitors of the transport process. These results suggest that the amino acids modified by these agents are located either within the membrane or on the cytosolic surface of the transporter. These residues may not participate in substrate binding, but may be important for the conformational change of the transporter necessary for the translocation of phosphate across these membranes. This study also shows that Na+-dependent phosphate transport can be inactivated by other reagents which covalently modify histidine, carboxyl, and amino groups on proteins.

Functional significance of mitochondrial bound hexokinase in tumor cell metabolism. Evidence for preferential phosphorylation of glucose by intramitochondrially generated ATP

Previous studies from this laboratory have shown that mitochondrial bound hexokinase is markedly elevated in highly glycolytic hepatoma cells (Parry, D. M., and Pedersen, P.L. (1983) J. Biol. Chem. 258, 10904-10912). A pore-forming protein, porin, within the outer membrane appears to comprise at least part of the receptor site (Nakashima, R.A., Mangan, P.S., Colombini, M., and Pedersen, P.L. (1986). Biochemistry 25, 1015-1021). In studies reported here experiments were carried out to assess the functional significance of mitochondrial bound tumor hexokinase. Two approaches were used to determine whether the bound enzyme has preferred access to mitochondrially generated ATP relative to cytosolic ATP. The first approach compared the time course of glucose 6-phosphate formation by AS-30D hepatoma mitochondria under conditions where ATP was regenerated endogenously via oxidative phosphorylation or exogenously by added pyruvate kinase and phosphoenolpyruvate. The second approach involved the measurement of the specific radioactivity of glucose 6-phosphate formed following the addition of [gamma-32P]ATP to either phosphorylating or nonphosphorylating AS-30D mitochondria. Both approaches provided results which show that the source of ATP for bound hexokinase is derived preferentially from the ATP synthase residing within the inner mitochondrial membrane compartment rather than from the medium (i.e. from the cytosolic compartment). These results provide the first direct demonstration that the exceptionally high level of hexokinase bound to mitochondria of highly glycolytic tumor cells has preferred access to mitochondrially generated ATP, a finding that may have rather profound metabolic significance for such tumors.

Purification in a single step and kinetic characterization of the pyruvate kinase of Trypanosoma brucei

The pyruvate kinase of Trypanosoma brucei can be purified to homogeneity in one step by affinity elution from a phosphocellulose column with the substrate phosphoenolpyruvate (PEP) and the allosteric activator fructose-2,6-diphosphate (FDP). The purified enzyme has a specific activity of 175 mumol min-1 (mg protein)-1 and a subunit molecular mass of 59 kDa as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Kinetic studies of the pure enzyme show that an increase in the PEP concentration decreases the apparent Km for adenosine diphosphate (ADP) and that an increase in the ADP concentration decreases the half saturation point (S0.5) for PEP. Likewise, the allosteric activator FDP decreases both the apparent Km for ADP and the S0.5 for PEP. ADP concentrations above 0.2 mM inhibit trypanosomal pyruvate kinase.

Mitochondrial ATP synthase. Overexpression in Escherichia coli of a rat liver beta subunit peptide and its interaction with adenine nucleotides

The C-terminal two-thirds of the rat liver ATP synthase beta subunit has been overexpressed and exported to the Escherichia coli periplasm under the direction of the alkaline phosphatase (phoA) promoter and leader peptide. The processed soluble protein contains the 358 amino acids from glutamate 122 to the rat liver beta C-terminal serine 479, including all the regions that have been predicted by chemical and genetic modification studies to be involved in nucleotide, Pi, and Mg2+ binding. Through a simple sequence of Tris/EDTA/lysozyme treatment, osmotic lysis, and alkaline pH washes, the processed beta subunit fragment can be prepared in greater than 95% purity and at a yield of greater than 20 mg/liter of culture. It interacts with 2'(3')-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) which exhibits a strong enhancement of fluorescence upon binding. A similar enhancement is observed upon interaction with TNP-ADP. Enhancement observed with both TNP-nucleotides is markedly reduced by prior addition of either ATP or ADP and almost completely prevented by the ATP synthase inhibitor 7-chloro-4-nitrobenz-2-oxa-1,3-diazole. Both TNP-ATP and TNP-ADP bind at a stoichiometry of approximately 1 mol of nucleotide/mol of beta subunit fragment. Under the same conditions, TNP-AMP does not exhibit a fluorescence enhancement. This work demonstrates that, in the absence of interaction with other ATP synthase subunits, the rat liver beta subunit sequence from glutamate 122 to the C terminus exhibits no more than one readily detectable nucleotide binding domain. This success in producing a "functional" beta subunit fragment has significance for the pursuit of genetic and physical studies focused on the structure and function of the rat liver ATP synthase beta subunit.

ATP synthases--structure of the F1-moiety and its relationship to function and mechanism

A great deal of progress has been made in understanding both the structure and the mechanism of F1-ATPase. The primary structure is now fully known for at least five species. Sequence comparison between chloroplast, photobacteria, aerobic bacteria, and mitochondrial representatives allow us to infer more general functional relationships and evolutionary trends. Although the F1 moiety is the most studied segment of the H+-ATPase complex, there is not a full understanding of the mechanism and regulation of its hydrolytic activity. The beta subunit is now known to contain one and probably two nucleotide binding domains, one of which is believed to be a catalytic site. Recently, two similar models have been proposed to attempt to describe the "active" part of the beta subunits. These models are mainly an attempt to use the structure of adenylate kinase to represent a more general working model for nucleotide binding phosphotransferases. Labelling experiments seem to indicate that several critical residues outside the region described by the "adenylate kinase" part of this model are also actively involved in the ATPase activity. New models will have to be introduced to include these regions. Finally, it seems that a consensus has been reached with regard to a broad acceptance of the asymmetric structure of the F1-moiety. In addition, recent experimental evidence points toward the presence of nonequivalent subunits to describe the functional activity of the F1-ATPase. A summary diagram of the conformational and binding states of the enzyme including the nonequivalent beta subunit is presented. Additional research is essential to establish the role of the minor subunits--and of the asymmetry they introduce in F1--on the physiological function of the enzyme.

Purification and characterization of a bindable form of mitochondrial bound hexokinase from the highly glycolytic AS-30D rat hepatoma cell line

Recent studies from this laboratory have demonstrated that a form of hexokinase characteristic of rapidly growing, highly glycolytic tumor cells is bound to an outer mitochondrial membrane receptor complex containing a Mr 35,000 pore protein (D. M. Parry and P. L. Pedersen, J. Biol. Chem., 258: 10904-10912, 1983; R. A. Nakashima, et al., Biochemistry, 25: 1015-1021, 1986). In new studies reported here the specificity of this receptor complex for binding hexokinase is defined, and a purification scheme is described which leads to a homogeneous and bindable form of the tumor hexokinase. In the AS-30D hepatoma, hexokinase activity is elevated more than 100-fold relative to liver tissue. The relative increase in hexokinase activity is 8 times greater than that of any other glycolytic enzyme. Hexokinase is the only glycolytic enzyme of AS-30D cells to exhibit a mitochondrial/cytoplasmic specific activity ratio greater than 1, showing a 3.5-fold elevation in the mitochondrial fraction. Purification of hexokinase is accomplished by preferential solubilization of the mitochondrial bound enzyme with glucose-6-phosphate, followed by high-performance liquid chromatography on gel permeation and anion exchange columns. The final fraction has a specific activity of 144 units per mg of protein, with a Km for glucose of 0.13 mM and for ATP of 1.4 mM. The purified tumor enzyme migrates as a single species upon sodium dodecyl sulfate: polyacrylamide gel electrophoresis with an apparent molecular weight of 98,000. Significantly, the purified tumor enzyme retains its activity for mitochondrial binding. Additional results derived from chromatographic, polyclonal antibody, and amino acid analysis studies indicate that the predominant rat hepatoma hexokinase species is related most closely to isozymic form(s) of the enzyme commonly referred to as type II, and least related to the liver type IV isozyme (glucokinase).

Beta subunit of rat liver mitochondrial ATP synthase: cDNA cloning, amino acid sequence, expression in Escherichia coli, and structural relationship to adenylate kinase

The amino acid sequence of all but a few N-terminal residues of the beta subunit of rat liver ATP synthase has been determined from cDNA clones. Rat liver F1-beta is shown to contain 17 amino acid differences from that reported for F1-beta of bovine heart, 2 differences of which involve differences in charge. This may account in part for the observation that bovine heart F1 binds nucleotides with much greater affinity than the rat liver enzyme. Rat liver F1-beta also contains homologous regions with another nucleotide binding protein, adenylate kinase, for which high-resolution structural studies are available. Adjacent to one of these homologous regions is an eight amino acid stretch which bears striking homology to the phosphorylation region of the (Na+,K+)-ATPase. The combination of these two homology regions may constitute at least part of a nucleotide binding domain in F1-beta. Significantly, both rat liver and bovine heart beta contain these regions of homology, whereas the 17 amino acid differences between the two enzymes lie outside this region. The possibility of a second nucleotide binding domain which differs between the two enzymes is discussed. A cDNA clone containing all the regions of homology as well as 11 of the 17 amino acid differences between the bovine heart and rat liver beta subunits has been ligated into the bacterial expression vector pKK223-3. After transformation of a protease-deficient strain of Escherichia coli, this cDNA clone is expressed as a 36-kilodalton protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial ATP synthase. Interaction of a synthetic 50-amino acid, beta-subunit peptide with ATP

A 50-amino acid peptide predicted by chemical modification studies of F1 and by comparison with adenylate kinase to comprise part of an ATP-binding domain within the beta-subunit of mitochondrial ATP synthase has been synthesized and purified. In the numbering system used for bovine heart beta, the peptide consists of amino acid residues from aspartate 141 at the N-terminal end to threonine 190 at the carboxyl end. In Tris-Cl buffer, pH 7.4, the peptide undergoes a dramatic reaction with ATP resulting in precipitate formation. Analysis of the precipitate shows it to contain both peptide and ATP. Similar to the ATPase activity of F1 and the binding of nucleotide to the enzyme, the capacity of ATP to induce precipitation of the peptide is decreased markedly by lowering pH. Interaction of the peptide with the fluorescent ATP analog, TNP-ATP (2'(3')-O-(2,4-6-trinitrophenyl)-adenosine 5'-triphosphate), can be demonstrated in solution at low concentrations. A 7-fold enhancement in fluorescence is observed when 2.5 microM TNP-ATP interacts with 2.5 microM peptide. Divalent cation is neither required for ATP-induced precipitation of the peptide nor for demonstrating interaction between TNP-ATP and peptide, just as Mg2+ is not required for nucleotide binding to F1. These results indicate that the beta-subunit peptide studied here comprises at least part of a nucleotide-binding domain within the mitochondrial ATP synthase complex.

Mitochondrial ATP synthase: dramatic Mg2+-induced alterations in the structure and function of the F1-ATPase moiety

The ATPase activity of the F1 moiety of rat liver ATP synthase is inactivated when incubated prior to assay at 25 degrees C in the presence of MgCl2. The concentration of MgCl2 (130 microM) required to induce half-maximal inactivation is over 30 times higher than the apparent Km (MgCl2) during catalysis. Moreover, the relative efficacy of divalent cations in inducing inactivation during prior incubation follows an order significantly different from that promoting catalysis. Inactivation of F1-ATPase activity by Mg2+ is accompanied by the dramatic dissociation from the F1 complex of alpha subunits and part of the gamma-subunit population. The latter form a precipitate while the beta, delta, and epsilon subunits, and the remaining part of the gamma-subunit population, remain soluble. Dissociation is not a sudden "all or none" event but parallels loss of ATPase activity until alpha subunits have almost completely dissociated together with about 50% of the gamma-subunit population. Mg2+-induced loss of F1-ATPase activity cannot be prevented by including either the hydrolytic substrates ATP, GTP, or ITP in the incubation medium or the product ADP. Ethylenediaminetetraacetic acid, mercaptoethanol, and dithiothreitol are also ineffective in preventing loss of ATPase activity. Significantly, KPi at high concentration (greater than or equal to 200 mM) is effective in partially protecting F1 against inactivation. However, the most effective means of preventing Mg2+-induced inactivation of F1-ATPase activity is to rebind F1 to its F0 moiety in F1-depleted particles. When bound to F0, F1 is protected completely against divalent cation induced inactivation.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial ATP synthase complex: interaction of its F1 adenosinetriphosphatase moiety with the heavy atom iodine

Studies were carried out to determine whether a simple electron-dense "heavy atom" like iodine could be introduced selectively into one or more of the subunits of the mitochondrial ATP synthase complex of rat liver. Surprisingly, very low amounts of iodine are incorporated into the isolated F1 moiety of this complex under conditions which result in a marked loss of catalytic activity. ATPase activity is inactivated in a concentration-dependent manner at pH 7.5 with half-maximal inactivation occurring at about 40 microM iodine. A maximum of only 10 atoms of iodine are incorporated per F1 molecule under conditions where inhibition of ATPase activity is linearly related to iodine incorporation. The molecular size of F1 after iodination is unchanged, indicating that inactivation is due to modification of essential amino acid residues rather than subunit dissociation. Treatment of F1, with 20-50 microM [125I]iodine followed sequentially by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography showed that the beta subunit is preferentially labeled. Significantly, about two atoms of iodine per beta subunit are incorporated. Some iodine amounting to less than 23% of the total radioactivity placed on the gels is recovered in the alpha and gamma subunits whereas no radioactivity is detected in the delta and epsilon subunits. Iodination of F1 appears to modify essential residues other than those involved in substrate or product binding per se. Thus, nucleotide binding to F1 is unaltered by iodine, and neither phosphate, MgADP, nor MgATP protects F1 against inhibition by this agent.(ABSTRACT TRUNCATED AT 250 WORDS)

Ligand binding studies of the F1 moiety of rat liver ATP synthase: implications about the enzyme's structure and mechanism

F1-ATPase of rat liver was examined for its capacity to interact with both metal ions and nucleotides and for the effect of covalent ATPase inhibitors on these interactions. As isolated, rat liver F1 contains about 2 mol of Mg2+/mol of F1, 1 mol of which can be removed or exchanged. The remaining mole of Mg2+ per mole of F1 remains very tightly associated with F1 and is recovered in the alpha gamma fraction after cold denaturation. Rat liver F1 also contains as isolated a nearly equivalent amount of nucleotide (approximately 1.7 mol/mol of F1) which is readily removed by incubation at room temperature followed by column centrifugation. The "2 Mg2+ enzyme" binds almost 3 mol of 5'-adenylyl imidodiphosphate (AMP-PNP)/mol of F1 in the presence or absence of added divalent cation. When divalent cation is present as Co2+, an equivalent activator to Mg2+ in the ATPase reaction, 1 mol of F1 binds 3 mol of both AMP-PNP and Co2+. under these conditions, the very tight Mg2+ site remains loaded, the exchangeable Mg2+ site is replaced with AMP-PNPCo, and two additional AMP-PNPCo sites are filled. At this point, ADP can be loaded onto the enzyme as a fourth nucleotide at a site separate and distinct from the AMP-PNP sites. Significantly, rat liver F1 contains only a single readily detectable ADP binding site in the presence or absence of divalent cation.(ABSTRACT TRUNCATED AT 250 WORDS)