The ATPase inhibitor protein in oxidative phosphorylation. The rate-limiting factor to phosphorylation in submitochondrial particles

Molecular mechanism on forcible ejection of ATPase inhibitory factor 1 from mitochondrial ATP synthase

IF₁ is a natural inhibitor protein for mitochondrial F_oF₁ ATP synthase that blocks catalysis and rotation of the F₁ by deeply inserting its N-terminal helices into F₁. A unique feature of IF₁ is condition-dependent inhibition; although IF₁ inhibits ATP hydrolysis by F₁, IF₁ inhibition is relieved under ATP synthesis conditions. To elucidate this condition-dependent inhibition mechanism, we have performed single-molecule manipulation experiments on IF₁-inhibited bovine mitochondrial F₁ (bMF₁). The results show that IF₁-inhibited F₁ is efficiently activated only when F₁ is rotated in the clockwise (ATP synthesis) direction, but not in the counterclockwise direction. The observed rotational-direction-dependent activation explains the condition-dependent mechanism of IF₁ inhibition. Investigation of mutant IF₁ with N-terminal truncations shows that the interaction with the γ subunit at the N-terminal regions is crucial for rotational-direction-dependent ejection, and the middle long helix is responsible for the inhibition of F₁.

Redesigned and reversed: architectural and functional oddities of the trypanosomal ATP synthase

Mitochondrial F-type adenosine triphosphate (ATP) synthases are commonly introduced as highly conserved membrane-embedded rotary machines generating the majority of cellular ATP. This simplified view neglects recently revealed striking compositional diversity of the enzyme and the fact that in specific life stages of some parasites, the physiological role of the enzyme is to maintain the mitochondrial membrane potential at the expense of ATP rather than to produce ATP. In addition, mitochondrial ATP synthases contribute indirectly to the organelle's other functions because they belong to major determinants of submitochondrial morphology. Here, we review current knowledge about the trypanosomal ATP synthase composition and architecture in the context of recent advances in the structural characterization of counterpart enzymes from several eukaryotic supergroups. We also discuss the physiological function of mitochondrial ATP synthases in three trypanosomatid parasites, Trypanosoma cruzi, Trypanosoma brucei and Leishmania, with a focus on their disease-causing life cycle stages. We highlight the reversed proton-pumping role of the ATP synthase in the T. brucei bloodstream form, the enzyme's potential link to the regulation of parasite's glycolysis and its role in generating mitochondrial membrane potential in the absence of mitochondrial DNA.

The F1Fo-ATPase inhibitor, IF1, is a critical regulator of energy metabolism in cancer cells

In the last two decades, IF1, the endogenous inhibitor of the mitochondrial F1Fo-ATPase (ATP synthase) has assumed greater and ever greater interest since it has been found to be overexpressed in many cancers. At present, several findings indicate that IF1 is capable of playing a central role in cancer cells by promoting metabolic reprogramming, proliferation and resistance to cell death. However, the mechanism(s) at the basis of this pro-oncogenic action of IF1 remains elusive. Here, we recall the main features of the mechanism of the action of IF1 when the ATP synthase works in reverse, and discuss the experimental evidence that support its relevance in cancer cells. In particular, a clear pro-oncogenic action of IF1 is to avoid wasting of ATP when cancer cells are exposed to anoxia or near anoxia conditions, therefore favoring cell survival and tumor growth. However, more recently, various papers have described IF1 as an inhibitor of the ATP synthase when it is working physiologically (i.e. synthethizing ATP), and therefore reprogramming cell metabolism to aerobic glycolysis. In contrast, other studies excluded IF1 as an inhibitor of ATP synthase under normoxia, providing the basis for a hot debate. This review focuses on the role of IF1 as a modulator of the ATP synthase in normoxic cancer cells with the awareness that the knowledge of the molecular action of IF1 on the ATP synthase is crucial in unravelling the molecular mechanism(s) responsible for the pro-oncogenic role of IF1 in cancer and in developing related anticancer strategies.

The ATPase Inhibitory Factor 1 (IF1): A master regulator of energy metabolism and of cell survival

In this contribution we summarize most of the findings reported for the molecular and cellular biology of the physiological inhibitor of the mitochondrial H(+)-ATP synthase, the engine of oxidative phosphorylation (OXPHOS) and gate of cell death. We first describe the structure and major mechanisms and molecules that regulate the activity of the ATP synthase placing the ATPase Inhibitory Factor 1 (IF1) as a major determinant in the regulation of the activity of the ATP synthase and hence of OXPHOS. Next, we summarize the post-transcriptional mechanisms that regulate the expression of IF1 and emphasize, in addition to the regulation afforded by the protonation state of histidine residues, that the activity of IF1 as an inhibitor of the ATP synthase is also regulated by phosphorylation of a serine residue. Phosphorylation of S39 in IF1 by the action of a mitochondrial cAMP-dependent protein kinase A hampers its interaction with the ATP synthase, i.e., only dephosphorylated IF1 interacts with the enzyme. Upon IF1 interaction with the ATP synthase both the synthetic and hydrolytic activities of the engine of OXPHOS are inhibited. These findings are further placed into the physiological context to stress the emerging roles played by IF1 in metabolic reprogramming in cancer, in hypoxia and in cellular differentiation. We review also the implication of IF1 in other cellular situations that involve the malfunctioning of mitochondria. Special emphasis is given to the role of IF1 as driver of the generation of a reactive oxygen species signal that, emanating from mitochondria, is able to reprogram the nucleus of the cell to confer by various signaling pathways a cell-death resistant phenotype against oxidative stress. Overall, our intention is to highlight the urgent need of further investigations in the molecular and cellular biology of IF1 and of its target, the ATP synthase, to unveil new therapeutic strategies in human pathology. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Molecular Regulation of the Mitochondrial F(1)F(o)-ATPsynthase: Physiological and Pathological Significance of the Inhibitory Factor 1 (IF(1))

In mammals, the mitochondrial F₁F_o-ATPsynthase sets out the energy homeostasis by producing the bulk of cellular ATP. As for every enzyme, the laws of thermodynamics command it; however, it is privileged to have a dedicated molecular regulator that controls its rotation. This is the so-called ATPase Inhibitory Factor 1 (IF₁) that blocks its reversal to avoid the consumption of cellular ATP when the enzyme acts as an ATP hydrolase. Recent evidence has also demonstrated that IF₁ may control the alignment of the enzyme along the mitochondrial inner membrane, thus increasing the interest for the molecule. We conceived this review to outline the fundamental knowledge of the F₁F_o-ATPsynthase and link it to the molecular mechanisms by which IF₁ regulates its way of function, with the ultimate goal to highlight this as an important and possibly unique means to control this indispensable enzyme in both physiological and pathological settings.

Post-transcriptional regulation of the mitochondrial H(+)-ATP synthase: a key regulator of the metabolic phenotype in cancer

A distinctive metabolic trait of tumors is their enforced aerobic glycolysis. This phenotype was first reported by Otto Warburg, who suggested that the increased glucose consumption of cancer cells under aerobic conditions might result from an impaired bioenergetic activity of their mitochondria. A central player in defining the bioenergetic activity of the cell is the mitochondrial H(+)-ATP synthase. The expression of its catalytic subunit β-F1-ATPase is tightly regulated at post-transcriptional levels during mammalian development and in the cell cycle. Moreover, the down-regulation of β-F1-ATPase is a hallmark of most human carcinomas. In this review we summarize our present understanding of the molecular mechanisms that participate in promoting the "abnormal" aerobic glycolysis of prevalent human carcinomas. The role of the ATPase Inhibitor Factor 1 (IF1) and of Ras-GAP SH3 binding protein 1 (G3BP1), controlling the activity of the H(+)-ATP synthase and the translation of β-F1-ATPase mRNA respectively in cancer cells is emphasized. Furthermore, we underline the role of mitochondrial dysfunction as a pivotal player of tumorigenesis.

Requirement for a membrane potential for cellulose synthesis in intact cells of Acetobacter xylinum

The marked lability in cell-free preparations of the enzyme system involved in cellulose biosynthesis in most organisms studied led us to investigate factors responsible for loss of activity on cellular disruption. Previous studies have led to the suggestion that the existence of a transmembrane electrical potential (DeltaPsi) may be one factor responsible for maintaining an active system in intact cells. In this report, we show that dissipation of the DeltaPsi in metabolizing cells of Acetobacter xylinum results in severe inhibition of cellulose synthesis. The effect can be reversed by restoration of the DeltaPsi. Inhibition of cellulose biosynthesis by dissipation of the DeltaPsi can be observed under conditions in which no substantial impairment of energy metabolism occurs-i.e., under conditions in which a transmembrane pH gradient is of sufficient magnitude to maintain an adequate overall protonmotive force across the membrane. The inhibition of cellulose biosynthesis is specifically related to changes in the DeltaPsi, since the process can proceed normally in the absence of the pH gradient. These results support the suggestion that loss of the DeltaPsi on cellular disruption may be one of the factors responsible for the low capacity for cellulose synthesis in isolated membrane preparations and also raise the possibility that modulation of the DeltaPsi could be one means of regulating the rate of cellulose synthesis in vivo.

Effects of fluoxetine on the reproductive axis of female goldfish (Carassius auratus)

We investigated the effects of fluoxetine, a selective serotonin reuptake inhibitor, on neuroendocrine function and the reproductive axis in female goldfish. Fish were given intraperitoneal injections of fluoxetine twice a week for 14 days, resulting in five injections of 5 μg fluoxetine/g body wt. We measured the monoamine neurotransmitters serotonin, dopamine, and norepinephrine in addition to their metabolites with HPLC. Homovanillic acid, a metabolite in the dopaminergic pathway, increased significantly in the hypothalamus. Plasma estradiol levels were measured by radioimmunoassay and were significantly reduced approximately threefold after fluoxetine treatment. We found that fluoxetine also significantly reduced the expression of estrogen receptor (ER)β1 mRNA by 4-fold in both the hypothalamus and the telencephalon and ERα mRNA by 1.7-fold in the telencephalon. Fluoxetine had no effect on the expression of ERβ2 mRNA in the hypothalamus or telencephalon. Microarray analysis identified isotocin, a neuropeptide that stimulates reproductive behavior in fish, as a candidate gene affected by fluoxetine treatment. Real-time RT-PCR verified that isotocin mRNA was downregulated approximately sixfold in the hypothalamus and fivefold in the telencephalon. Intraperitoneal injection of isotocin (1 μg/g) increased plasma estradiol, providing a potential link between changes in isotocin gene expression and decreased circulating estrogen in fluoxetine-injected fish. Our results reveal targets of serotonergic modulation in the neuroendocrine brain and indicate that fluoxetine has the potential to affect sex hormones and modulate genes involved in reproductive function and behavior in the brain of female goldfish. We discuss these findings in the context of endocrine disruption because fluoxetine has been detected in the environment.

Regulation of mitochondrial structure and function by the F1Fo-ATPase inhibitor protein, IF1

When mitochondrial respiration is compromised, the F(1)F(o)-ATP synthase reverses and consumes ATP, serving to maintain the mitochondrial membrane potential (Delta psi(m)). This process is mitigated by IF(1). As little is known of the cell biology of IF(1), we have investigated the functional consequences of varying IF(1) expression. We report that, (1) during inhibition of respiration, IF(1) conserves ATP at the expense of Delta psi(m); (2) overexpression of IF(1) is protective against ischemic injury; (3) relative IF(1) expression level varies between tissues and cell types and dictates the response to inhibition of mitochondrial respiration; (4) the density of mitochondrial cristae is increased by IF(1) overexpression and decreased by IF(1) suppression; and (5) IF(1) overexpression increases the formation of dimeric ATP synthase complexes and increases F(1)F(o)-ATP synthase activity. Thus, IF(1) regulates mitochondrial function and structure under both physiological and pathological conditions.

A hinge of the endogeneous ATP synthase inhibitor protein: the link between inhibitory and anchoring domains

The ATP synthase of bovine heart mitochondria possesses a regulatory subunit called the endogenous inhibitory protein (IF(1)). This subunit regulates the catalytic activity of the F(1) sector in the mitochondrial inner membrane. When DeltamuH(+) falls, IF(1) binds to the enzyme and inhibits ATP hydrolysis. On the other hand, the establishment of a DeltamuH(+) induces the release of the inhibitory action of IF(1), allowing ATP synthesis to proceed. IF(1) is also involved in the dimerization of soluble F(1). Dynamic domain analysis and normal mode analysis of the reported crystallographic structure of IF(1) revealed that it has an effective hinge formed by residues 46-52. Molecular dynamics data of a 27 residue fragment confirmed the existence of the hinge. The hinge may act as a regulatory region that links the inhibitory and anchoring domains of IF(1). The residues assigned to the hinge are conserved between mammals, but not in other species, such as yeasts. Likewise, unlike the heart inhibitor, the yeast protein does not have the residues that allow it to form stable dimers through coiled-coil interactions. Collectively, the data suggest that the hinge and the dimerization domain of the inhibitor protein from bovine heart are related to its ability to form stable dimers and to interact with other subunits of the ATP synthase.

Effect of denaturants on multisite and unisite ATP hydrolysis by bovine heart submitochondrial particles with and without inhibitor protein

The effect of guanidinium hydrochloride (GdnHCl) on multisite and unisite ATPase activity by F0F1 of submitochondrial particles from bovine hearts was studied. In particles without control by the inhibitor protein, 50 mM GdnHCl inhibited multisite hydrolysis by about 85%; full inhibition required around 500 mM. In the range of 500-650 mM, GdnHCl enhanced the rate of unisite catalysis by promoting product release; it also increased the rate of hydrolysis of ATP bound to the catalytic site without GdnHCl. GdnHCl diminished the affinity of the enzyme for aurovertin. The effects of GdnHCl were irreversible. The results suggest that disruption of intersubunit contacts in F0F1 abolishes multisite hydrolysis and stimulates of unisite hydrolysis. Particles under control by the inhibitor protein were insensitive to concentrations of GdnHCl that induce the aforementioned alterations of F0F1 free of inhibitor protein, indicating that the protein stabilizes the global structure of particulate F1.

Overexpression of the inhibitor protein IF(1) in AS-30D hepatoma produces a higher association with mitochondrial F(1)F(0) ATP synthase compared to normal rat liver: functional and cross-linking studies

According to functional studies, the higher IF(1) content reported in mitochondria of cancerous cells is supposed to induce a higher association with the F(1)F(0) complex than in normal cells and therefore a better inhibition of its ATPase activity. The first structural evidence supporting this prediction is here presented. Densitometric analyses of Western blotting experiments indicated a 2-fold increase in IF(1) content of AS-30D submitochondrial particles compared to normal rat liver controls. The ratio of IF(1)/F(1) alpha subunit increased similarly as judged by Westernblot analyses. This IF(1) overexpression correlated with a slower rate of IF(1) release (F(1)F(0)-ATPase activation) from the F(1)F(0) complex in AS-30D than in normal rat liver submitochondrial particles. The IF(1)-IF(1), gamma-IF(1), and alpha-IF(1) cross-linkages previously formed with dithiobis(succinimidylpropionate) in bovine F(1)F(0)I and IF(1) complexes were reproduced in the F(1)F(0)I-ATP synthase of hepatoma AS-30D cells. However, a much lower yield of IF(1) cross-linkages was found in normal rat liver particles which made them almost undetectable in SMP as well as in the immunoprecipitated F(1)F(0)I complex. Modeling in vivo IF(1) overexpression of cancerous cells by in vitro reconstitution of excess recombinant IF(1) with rat liver submitochondrial particles devoid of IF(1) reproduced the same IF(1) cross-linkages observed in AS-30D particles.

An Inhibitor of the F1 subunit of ATP synthase (IF1) modulates the activity of angiostatin on the endothelial cell surface

Angiostatin binds to endothelial cell (EC) surface F(1)-F(0) ATP synthase, leading to inhibition of EC migration and proliferation during tumor angiogenesis. This has led to a search for angiostatin mimetics specific for this enzyme. A naturally occurring protein that binds to the F1 subunit of ATP synthase and blocks ATP hydrolysis in mitochondria is inhibitor of F1 (IF1). The present study explores the effect of IF1 on cell surface ATP synthase. IF1 protein bound to purified F(1) ATP synthase and inhibited F(1)-dependent ATP hydrolysis consistent with its reported activity in studies of mitochondria. Although exogenous IF1 did not inhibit ATP production on the surface of EC, it did conserve ATP on the cell surface, particularly at low extracellular pH. IF1 inhibited ATP hydrolysis but not ATP synthesis, in contrast to angiostatin, which inhibited both. In cell-based assays used to model angiogenesis in vitro, IF1 did not inhibit EC differentiation to form tubes and only slightly inhibited cell proliferation compared with angiostatin. From these data, we conclude that inhibition of ATP synthesis is necessary for an anti-angiogenic outcome in cell-based assays. We propose that IF1 is not an angiostatin mimetic, but it can serve a protective role for EC in the tumor microenvironment. This protection may be overridden in a concentration-dependent manner by angiostatin. In support of this hypothesis, we demonstrate that angiostatin blocks IF1 binding to ATP synthase and abolishes its ability to conserve ATP. These data suggest that there is a relationship between the binding sites of IF1 and angiostatin on ATP synthase and that IF1 could be employed to modulate angiogenesis.

Interconversion between dimers and monomers of endogenous mitochondrial F1-inhibitor protein complexes and the release of the inhibitor protein. Spectroscopic characteristics of the complexes

The F1-inhibitor protein complex (F1-IP) was purified from heart submitochondrial particles. Size exclusion chromatography of the endogenous complex showed that it contains dimers (D) and monomers (M) of F1-IP. Further chromatographic analysis showed that D and M interconvert. At high protein concentrations, the interconversion reaction is shifted toward the D species. The release of the inhibiting action of IP is faster at low than at high protein concentrations. During activation of F1, the M species accumulates through a process that is faster than the release of IP from F1. These findings indicate that the activation of F1-IP involves the transformation of D into M, which subsequently loses IP. The spectroscopic characteristics of D, M, and free F1 show that the binding of IP and dimerization modifies the fluorescence intensity of tyrosine residues and that of the single tryptophan of F1 which is far from the IP binding site.

Cross-linking of the endogenous inhibitor protein (IF1) with rotor (gamma, epsilon) and stator (alpha) subunits of the mitochondrial ATP synthase

The location of the endogenous inhibitor protein (IF1) in the rotor/stator architecture of the bovine mitochondrial ATP synthase was studied by reversible cross-linking with dithiobis(succinimidylpropionate) in soluble F1I and intact F1F0I complexes of submitochondrial particles. Reducing two-dimensional electrophoresis, Western blotting, and fluorescent cysteine labeling showed formation of alpha-IF1, IF1-IF1, gamma-IF1, and epsilon-IF1 cross-linkages in soluble F1I and in native F1F0I complexes. Cross-linking blocked the release of IF1 from its inhibitory site and therefore the activation of F1I and F1F0I complexes in a dithiothreitol-sensitive process. These results show that the endogenous IF1 is at a distance < or = 12 angstroms to gamma and epsilon subunits of the central rotor of the native mitochondrial ATP synthase. This finding strongly suggests that, without excluding the classical assumption that IF1 inhibits conformational changes of the catalytic beta subunits, the inhibitory mechanism of IF1 may involve the interference with rotation of the central stalk.

Equilibrium between monomeric and dimeric mitochondrial F1-inhibitor protein complexes

Mg-ATP particles from bovine heart mitochondria have more than 95% of their F1 in complex with the inhibitor protein (IF1). The F1-IF1 complex was solubilized and purified. The question addressed was if this naturally occurring complex existed as monomers or dimers. Size exclusion chromatography and electron microscopy showed that most of the purified F1-IF1 complex was a dimer of two F1-IF1. As determined by the former method, the relative concentrations of dimeric and monomeric F1-IF1 depended on the concentration of protein that was applied to the column. Apparently, there is an equilibrium between the two forms of F1-IF1.

Large conformational changes of the epsilon subunit in the bacterial F1F0 ATP synthase provide a ratchet action to regulate this rotary motor enzyme

The F(1)F(0) ATP synthase is the smallest motor enzyme known. Previous studies had established that the central stalk, made of the gamma and epsilon subunits in the F(1) part and c subunit ring in the F(0) part, rotates relative to a stator composed of alpha(3)beta(3)deltaab(2) during ATP hydrolysis and synthesis. How this rotation is regulated has been less clear. Here, we show that the epsilon subunit plays a key role by acting as a switch of this motor. Two different arrangements of the epsilon subunit have been visualized recently. The first has been observed in beef heart mitochondrial F(1)-ATPase where the C-terminal portion is arranged as a two-alpha-helix hairpin structure that extends away from the alpha(3)beta(3) region, and toward the position of the c subunit ring in the intact F(1)F(0). The second arrangement was observed in a structure determination of a complex of the gamma and epsilon subunits of the Escherichia coli F(1)-ATPase. In this, the two C-terminal helices are apart and extend along the gamma to interact with the alpha and beta subunits in the intact complex. We have been able to trap these two arrangements by cross-linking after introducing appropriate Cys residues in E. coli F(1)F(0), confirming that both conformations of the epsilon subunit exist in the enzyme complex. With the C-terminal domain of epsilon toward the F(0), ATP hydrolysis is activated, but the enzyme is fully coupled in both ATP hydrolysis and synthesis. With the C-terminal domain toward the F(1) part, ATP hydrolysis is inhibited and yet the enzyme is fully functional in ATP synthesis; i.e., it works in one direction only. These results help explain the inhibitory action of the epsilon subunit in the F(1)F(0) complex and argue for a ratchet function of this subunit.

The activity of the ATP synthase from Escherichia coli is regulated by the transmembrane proton motive force

The ATP synthase from Escherichia coli was reconstituted into liposomes from phosphatidylcholine/phosphatidic acid. The proteoliposomes were energized by an acid-base transition and a K(+)/valinomycin diffusion potential, and one second after energization, the electrochemical proton gradient was dissipated by uncouplers, and the ATP hydrolysis measurement was started. In the presence of ADP and P(i), the initial rate of ATP hydrolysis was up to 9-fold higher with pre-energized proteoliposomes than with proteoliposomes that had not seen an electrochemical proton gradient. After dissipating the electrochemical proton gradient, the high rate of ATP hydrolysis decayed to the rate without pre-energization within about 15 s. During this decay the enzyme carried out approximately 100 turnovers. In the absence of ADP and P(i), the rate of ATP hydrolysis was already high and could not be significantly increased by pre-energization. It is concluded that ATP hydrolysis is inhibited when ADP and P(i) are bound to the enzyme and that a high Delta mu(H(+)) is required to release ADP and P(i) and to convert the enzyme into a high activity state. This high activity state is metastable and decays slowly when Delta mu(H(+)) is abolished. Thus, the proton motive force does not only supply energy for ATP synthesis but also regulates the fraction of active enzymes.

Anomalies of mitochondrial ATP synthase regulation in four different types of neuronal ceroid lipofuscinosis

Several neuronal ceroid lipofuscinoses (NCL) show storage of subunit c of mitochondrial ATP synthase. The neurodegenerative process, however, remains obscure. We previously reported a decreased basal ATP synthase activity in fibroblasts from late-infantile NCL (CLN2) and juvenile NCL (CLN3) patients. We have now extended the study of the ATP synthase system to an ovine NCL (a model for the late-infantile NCL variant, CLN6) and the infantile NCL (CLN1). In fibroblasts from healthy sheep, active regulation of ATP synthase in response to cellular energy demand was present similar to human cells: ATP synthase was down-regulated under conditions of anoxia or functional uncoupling and was up-regulated in response to calcium. In fibroblasts from NCL sheep, basal ATP synthase activity was slightly elevated and down-regulation in response to anoxia or uncoupling of mitochondria also occurred. Calcium produced an unexpected down-regulation to 55% of basal activity. Activities of respiratory chain enzymes did not differ between healthy and NCL sheep. In fibroblasts from CLN1 patients, basal ATP synthase activity was reduced and regulation of the enzyme was absent. Activities of respiratory chain complexes II and IV were reduced. The defect of ATP synthase regulation found in fibroblasts from NCL sheep and infantile NCL patients is different from the ATP synthase deficiencies demonstrated in late-infantile and juvenile NCL, but problems of mitochondrial energy production, if also expressed in brain, would be a common feature of several NCL forms. Deficient ATP supply could result in degeneration of neurons, especially in those with high energy requirements.

Pressure effects on the interaction between natural inhibitor protein and mitochondrial F1-ATPase

Pressure stability of the complex formed between F1-ATPase and the inhibitor protein (IP) was studied in the membrane-bound and soluble, purified forms of beef-heart mitochondrial enzymes. A latent preparation of submitochondrial particles (SMP-MgATP) initially exhibits low hydrolytic activity. Dissociation of IP increases the activity about 10-fold. This increase occurs in parallel with an increase in sensitivity to pressure inactivation. The membrane-bound, latent IP-F1-ATPase complex is activated 2.5-fold when incubated at a pressure of 1.7 kbar, suggesting dissociation of IP. A fully active preparation of submitochondrial particles depleted of IP (AS-particles) is highly pressure labile when compared with the latent form. In the absence of IP, soluble purified F1-ATPase is also inactivated by pressure. In contrast, the soluble IP-F1-ATPase complex is very resistant to pressure, as evidenced by enzymatic and fluorescence studies. Based on the pressure-titration experiments, binding of IP stabilizes the F1-ATPase complex by 1.54 kcal per mole of complex. The substrate MgATP confers additional protection on both preparations only in the presence of IP. Glycerol appears to prevent dissociation of IP and therefore protects SMP-MgATP from pressure inactivation. Our results demonstrate that in addition to its regulatory role in catalysis, IP stabilizes the structure of the F1-ATPase complex. The pressure-induced dissociation of IP from F1-ATPase and its prevention by glycerol suggest that nonpolar in addition to electrostatic interactions are important for the binding of IP to the regulatory site.

Differential display analysis of murine collagen-induced arthritis: cloning of the cDNA-encoding murine ATPase inhibitor

We used the differential display technique in order to detect a new gene involved in murine type II collagen-induced arthritis (CIA). In this study, we have identified a novel gene, IF1, whose expression level is increased during the natural course of CIA. Northern blot analyses suggest that IF1 is involved in the natural course of CIA but is not involved as a trigger of CIA. IF1 is considered to be the murine ATPase inhibitor gene for several reasons. First, IF1 shows an extremely high homology to the rat ATPase inhibitor; the highly conserved region between rat and bovine amino acid residues 22-45, which is the minimum sequence showing ATPase inhibitory activities, is also highly conserved in IF1. Second, IF1 possesses a histidine-rich region in the same area, which is thought to be important for regulation of mammalian inhibitors. Third, the tissue distribution of IF1 is very suggestive. The expression of IF1 was very strong in energetic organs such as the heart, brain and kidney, and the development of arthritis requires great amounts of ATP. As arthritis develops rapidly, the cellular ATP pool may be decreased. Before the ATP pool is exhausted, the ATPase inhibitor may serve as a brake for ATP hydrolysis. If the supply of free energy can be reduced, the inflammation of arthritis may in turn be restored. Our hypothesis is that the ATPase inhibitor is involved in regulating the inflammatory responses.

Functional regions of the H(+)-ATPase inhibitory protein from ox heart mitochondria

Derivatives of the inhibitor protein (IF1) of the mitochondrial H(+)-ATP synthase, bearing deletions at the N- or C-terminal ends, were tested for their abilities (a) to bind to the synthase, (b) to inhibit its ATPase activity and (c) to respond to energisation of the mitochondrial membrane. Deletion of nine residues from its N-terminus, or ten from its C-terminus had little effect on any of these three properties of IF1. Further deletions from the N-terminus (up to residue 17) led to an increase in binding affinity but a reduced ability to inhibit ATPase activity and to form a stable ATPase-IF1 complex. Removal of five more residues from the N-terminus (up to residue 22) reduced these abilities further, but also decreased binding affinity by an order of magnitude. It was concluded that residues 10-17 of IF1 interact with F1 in a way which modulates the stability and function of the interaction between F1 and IF1.

Histidine-49 is necessary for the pH-dependent transition between active and inactive states of the bovine F1-ATPase inhibitor protein

The role of the histidyl residue at position 49 (H49) of the bovine mitochondrial F1-ATPase inhibitor protein (F1I) was examined by site-directed mutagenesis. Six amino acids (Q, E, K, V, L, and I) were substituted for H49 and the activities of the resulting inhibitor proteins were characterized with respect to pH. Each of the six mutations abolished the pH sensitivity which is characteristic of wild-type F1I. At pH 8.0 each of the mutations caused an increase in apparent maximum inhibition and a decrease in apparent Ki relative to wild type. At pH 6.7 the hydrophilic substitutions had little effect on apparent Ki, while the hydrophobic substitutions caused increases of 3.5- to 8.5-fold relative to wild type. The ratios of apparent Ki at pH 8.0 to apparent Ki at pH 6.7 were in the range of 0.5 to 1.6 for the mutants, whereas the wild-type value is 15.0. The mutations appear to shift the equilibrium between active and inactive conformations of F1I toward the active state. We find that H49 is required by F1I for sensitivity to pH and that it may facilitate the transition between active and inactive states of F1I. A possible role for H49 in the stabilization of the inactive state through participation in a multivalent complex with Zn2+ is also discussed.

Inhibition by Ca2+ of the hydrolysis and the synthesis of ATP in Ehrlich ascites tumour mitochondria: relation to the Crabtree effect

Phosphorylation of ADP and hydrolysis of ATP by isolated mitochondria from Ehrlich ascites tumour cells is greatly reduced when the mitochondria have been preloaded with Ca2+ (50 nmol/mg protein or more). Translocation of ADP is diminished in Ca(2+)-loaded mitochondria. However, ATPase in toluene-permeabilized mitochondria and in inside-out submitochondrial particles is also strongly inhibited by micromolar concentrations of Ca2+, indicating that, independently of adenine nucleotide transport, F1Fo-ATPase is also affected. ATP hydrolysis by submitochondrial particles depleted of the inhibitory subunit of F1Fo-ATPase (the Pullman-Monroy protein inhibitor) is insensitive to Ca2+; however, this sensitivity is restored when the particles are supplemented with the inhibitory subunit isolated from beef heart mitochondria. In view of the previous observations that glucose elicits in Ehrlich ascites tumour cells an increase of cytoplasmic free Ca2+ (Teplova, V.V., Bogucka, K., Czyz, A., Evtodienko, Yu.V., Duszyński, J. and Wojtczak, L. (1993) Biochem. Biophys. Res. Commun. 196, 1148-1154) and that this calcium is then taken up by mitochondria, resulting in a strong inhibition of coupled respiration (Evtodienko, Yu.V., Teplova, V.V., Duszyński, J., Bogucka, K. and Wojtczak, L. (1994) Cell Calcium 15, 439-446), the present results are discussed in terms of the mechanism of the Crabtree effect in tumour cells.

The effect of lipid peroxidation on the activity of various membrane-bound ATPases in rat kidney

Peroxidation of membrane phospholipids is accompanied by alteration of the structural and functional characteristics of membranes. Lipid peroxidation changes the activities of various enzymes. The present study evaluates the effect of lipid peroxidation on the activity of various ATPases localized on kidney membranes. Our experiments were performed on crude preparation of rat kidney membranes which were exposed to Fe2+.ADP/NADPH-induced lipid peroxidation. The extent of peroxidation was estimated by measuring the thiobarbituric acid-reactive substances. Simultaneously the activities of different ATPases were determined and divided according to their ouabain sensitivity and Mg2+ dependency. We found that 10 min incubation of isolated rat kidney membranes at 37 degrees C with inductors of lipid peroxidation increased the production of thiobarbituric acid-reactive substances from 1.10 +/- 0.26 to 7.72 +/- 2.55 nmol malondialdehyde/mg prot. (+/- SD, n = 4). Under these conditions total ATPase activity was decreased from 681 +/- 77 to 507 +/- 82, ouabain-sensitive Mg(2+)-dependent ATPase (Na+,K(+)-ATPase) activity from 249 +/- 54 to 81 +/- 21 and ouabain-insensitive Mg(2+)-dependent activity from 287 +/- 48 to 173 +/- 58 whereas apparently Mg(2+)-independent ATPase activity was increased from 145 +/- 37 to 253 +/- 42 nmoles P/min/mg prot. (+/- SD, n = 4). The study indicates different mechanisms by which lipoperoxides affect the function of membrane-bound ATPases activities. It is concluded that the ATPases activities are changed during lipid peroxide formation.

Regulation of the mitochondrial ATP-synthase in human fibroblasts

The F1F0-ATPase activity of mitochondrial complex V can be rapidly measured in sonicated preparations of monolayer skin fibroblast cultures from children. We show that direct regulation at the level of ATP-synthase occurs in these cell preparations. ATP-synthase capacity is decreased in response to blocking of the respiratory chain by cyanide (mimicking anoxia) or uncoupling of mitochondria. ATP-synthase capacity falls to 60% and 35% of control, respectively. Up-regulation of ATP-synthase can be demonstrated in fibroblasts exposed to 4 mmol/l calcium (127% of control). Mitochondrial recovery was unchanged under the different incubation conditions as judged by the activity of mitochondrial marker enzymes. We conclude that direct regulation at the level of ATP-synthase occurs in vivo in human fibroblasts. The naturally occurring inhibitor protein IF1 and the calcium binding inhibitor protein CaBI may be involved in this regulation of ATP-synthase.

ATP synthesis and hydrolysis of the ATP-synthase from Micrococcus luteus regulated by an inhibitor subunit and membrane energization

After incubation for 70 min in Tris-HCl (pH 8.0), the rate of ATP hydrolysis of free and reconstituted ATP-synthase from Micrococcus luteus multiplied about three times. The apparent increase in activity is due to the reversible dissociation of the delta-subunit. Results of experiments on the temperature dependence of the ATP hydrolysis rate of substrate saturated ATP-synthase exhibited a discontinuity in the Arrhenius plot at 32 +/- 0.5 degrees C for the delta-subunit associated enzyme. Below 32 +/- 0.5 degrees C the activation energy, Ea, was 231.5 +/- 5 kJ mol-1, while above this temperature-level it decreased to 76.4 +/- 3 kJ mol-1. ATP synthesis and hydrolysis of the ATP-synthase, co-reconstituted with monomeric bacteriorhodopsin (Halobacterium halobium), showed a lag of 50 s upon the illumination with green light (505-575 nm). This retardation and the activity depended on the ATP-synthase concentration, being typical of the dissociation of an inhibitor protein. The N-terminal protein sequences of the delta- and epsilon-subunit of the ATP-synthase were identified by automated Edman degradation. Alignment of the amino acid sequence and secondary structure calculations for the delta-subunit did not reveal homology to other known ATP-synthase delta-subunits, but significant equivalence to the epsilon-subunit of E. coli. Sequence analysis of the epsilon-subunit from M. luteus showed homology to equivalent regions in delta-subunits and Oligomycin Sensitivity Conferring Protein (OSCP) of other organisms.

Deactivation of F0F1 ATPase in intact plant mitochondria. Effect of pH and inhibitors

By using a method especially adapted to intact (pea leaf) mitochondria, we studied the regulation of the F0F1 ATPase by the electrochemical proton gradient (delta mu H+) and by the matricial pH. The kinetics of decay of the ATP hydrolase activity was studied immediately after the collapse of the electrochemical proton gradient by an uncoupler. At pH 7.5, three inhibitors of the ATPase (venturicidin, tri-n-butyl tin and aurovertin), used at non-saturating concentrations, inhibited ATP hydrolysis to the same extent throughout the decay. This showed that the activity was totally controlled by the ATPase during all the decay and rules out any involvement of the phosphate or nucleotide carriers. This interpretation was confirmed by the fact that carboxyatractyloside, an inhibitor of the ATP/ADP antiporter, had a strong effect only on the initial rate of ATP hydrolysis, but not on the rate measured after some tens of seconds of decay. Oligomycin, at variance with the other ATPase inhibitors, interfered with the deactivation process, suggesting that its effect depends on the conformational state of the enzyme. Between pH 6.5 and 7.5, the hydrolase activity rose continuously and was still kinetically controlled by the ATPase. At higher pH value, the activity slightly decreased and appeared limited by at least one of the carriers. The activity of the ATPase itself, free of any transport process, seemed to increase monotonously with pH from 6.5 to 8. The electrochemical proton gradient is required to maintain the ATPase active, whereas no effect can be observed on transport processes. Matricial pH, while modulating the apparent catalytic turnover, has no marked effect on the rate of deactivation. These results, obtained with intact mitochondria, extend previous observations on the isolated enzyme and question the binding of IF1 as a rate-limiting step for ATPase deactivation.

Recombinant bovine heart mitochondrial F1-ATPase inhibitor protein: overproduction in Escherichia coli, purification, and structural studies

A synthetic gene coding for the inhibitor protein of bovine heart mitochondrial F1 adenosine triphosphatase was designed and cloned in Escherichia coli. Recombinant F1-ATPase inhibitor protein was overproduced in E. coli and secreted to the periplasmic space. Biologically active recombinant F1-ATPase inhibitor protein was recovered from the bacterial cells by osmotic shock and was purified to near homogeneity in a single cation-exchange chromatography step. The recombinant inhibitor protein was shown to inhibit bovine mitochondrial F1-ATPase in a pH-dependent manner, as well as Saccharomyces cerevisiae mitochondrial F1-ATPase. Thorough analysis of the amino acid sequence revealed a potential coiled-coil structure for the C-terminal portion of the protein. Experimental evidence obtained by circular dichroism analyses supports this prediction and suggests F1I to be a highly stable, mainly alpha-helical protein which displays C-terminal alpha-helical coiled-coil intermolecular interaction.

The electrochemical-proton-gradient-activated states of F0F1 ATPase in plant mitochondria as revealed by detergents

ATP hydrolysis, triggered by the addition of polyoxyethylene-9-lauryl ether (Lubrol) or lauryldimethylamine oxide (LDAO) to energized plant mitochondria was studied in some details. The membrane disruption was quasi-instantaneous (2-3 s) with both detergents, as shown by the decrease of turbidity and the stopping of respiration. In pea leaf mitochondria, Lubrol triggered ATP hydrolysis in almost the same way as valinomycin plus nigericin, except that the activity was slightly stimulated and became insensitive to carboxyatractyloside. This allowed investigations of ATP hydrolysis without any interference of the ATP/ADP antiporter or the phosphate carrier. Lubrol did not prevent the ATPase from deactivating in pea leaf mitochondria, and did not trigger any ATP hydrolysis in potato tuber mitochondria. At variance with Lubrol, LDAO changed the properties of the F0F1 ATPase. It made the enzyme oligomycin insensitive and froze it in an activated state. The activity was also 5-8-times stimulated in pea leaf mitochondria. Moreover, LDAO revealed an important ATP hydrolase activity when added to energized potato tuber mitochondria. Despite the specific effect of LDAO, the activity triggered by this detergent strongly depended on the energized state of the organelles before detergent addition. From this study, it is concluded that the electrochemical proton gradient is completely necessary to activate the F0F1-ATPase in intact plant mitochondria, as known in chloroplasts and suggested by some reports in animal mitochondria. Moreover, it is suggested that the main difference between the enzymes of pea leaf and potato tuber mitochondria is their rate of deactivation after the collapse of the transmembrane electrochemical potential difference. Finally, when properly used, detergents appear to be a powerful tool to probe the state of the ATPase in intact mitochondria, and maybe in more integrated systems.

Binding of adenine nucleotides to the F1-inhibitor protein complex of bovine heart submitochondrial particles

The binding of ATP radiolabeled in the adenine ring or in the gamma- or alpha-phosphate to F1-ATPase in complex with the endogenous inhibitor protein was measured in bovine heart submitochondrial particles by filtration in Sephadex centrifuge columns or by Millipore filtration techniques. These particles had 0.44 +/- 0.05 nmol of F1 mg-1 as determined by the method of Ferguson et al. [(1976) Biochem. J. 153, 347]. By incubation of the particles with 50 microM ATP, and low magnesium concentrations (less than 0.1 microM MgATP), it was possible to observe that 3.5 mol of [gamma-32P]ATP was tightly bound per mole of F1 before the completion of one catalytic cycle. With [gamma-32P]ITP, only one tight binding site was detected. Half-maximal binding of adenine nucleotides took place with about 10 microM. All the bound radioactive nucleotides were released from the enzyme after a chase with cold ATP or ADP; 1.5 sites exchanged with a rate constant of 2.8 s-1 and 2 with a rate constant of 0.45 s-1. Only one of the tightly bound adenine nucleotides was released by 1 mM ITP; the rate constant was 3.2 s-1. It was also observed that two of the bound [gamma-32P]ATP were slowly hydrolyzed after removal of medium ATP; when the same experiment was repeated with [alpha-32P]ATP, all the label remained bound to F1, suggesting that ADP remained bound after completion of ATP hydrolysis. Particles in which the natural ATPase inhibitor protein had been released bound tightly only one adenine nucleotide per enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

An investigation of the relationships between rate and driving force in simple uncatalysed and enzyme-catalysed reactions with applications of the findings to chemiosmotic reactions

Both the rate and the driving force of a reaction can be expressed in terms of the concentrations of the reactants and products. Consequently, rate and driving force can be expressed as a function of each other. This has been done for a single-reactant, single-product, uncatalysed reaction and its enzyme-catalysed equivalent using the van't Hoff reaction isotherm and Haldane's generalized Michaelis-Menten rate equation, the primary objective being explanation of the exponential and sigmoidal relationships between reaction rate and delta mu H+ commonly observed in studies on chemiosmotic reactions. Acquisition of a purely thermodynamic rate vs. driving-force relationship requires recognition of the intensive and extensive variables and maintenance of the extensive variables constant. This relationship is identical for the two reactions and is hyperbolic or sigmoidal, depending on whether the equilibrium constant is smaller or larger than unity. In the case of the catalysed reaction, acquisition of the purely thermodynamic relationship requires the assumption that the enzyme be equally effective in catalysing the forward and backward reactions. If this condition is not met, the relationship is modified by the enzyme in a manner which can be determined from the ratio of the Michaelis constants of the reactant and product. Under conditions of enzyme saturation in respect to reactant+product, the rate vs. driving-force relationship is determined exclusively by the thermodynamics of the reaction and a single kinetic parameter, the magnitude of which is determined by the relative effectiveness of the enzyme in catalysing the forward and backward reactions. In view of this finding, it is pointed out that, since the catalytic components of chemiosmotic reactions appear to be saturated with respect to the reactant-product pair that is varied in experimental rate vs. delta mu H+ determinations, and that, since many complex enzymic reactions conform to the simple Michaelis-Menten equation with respect to a single reactant-product pair when the concentrations of all other reactants and products are maintained constant, one might expect to be capable of simulating the experimental relationships simply from knowledge of the thermodynamics of the reaction and the relative effectiveness of the catalytic component in catalysing the forward and backward reactions using the simple Michaelis-Menten equation. That this expectation appears to be largely correct is demonstrated with model reactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of mitochondrial ATP synthesis in the heart

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Evidence for an endogenous ATPase inhibitor protein in plant mitochondria. Purification and characterization

An endogenous ATPase inhibitor protein has been identified and isolated for the first time from plant mitochondria. The inhibitor protein was isolated from potato (Solanum tuberosum) tuber mitochondria and purified to homogeneity. The isolated inhibitor is a heat-stable, trypsin-sensitive, basic protein, with a molecular mass approximately 8.3 kDa. Amino acid analysis reveals a high content of glutamic acid, lysine and arginine and the absence of proline; threonine and leucine. The interaction of the inhibitor with F1-ATPase requires the presence of Mg2(+)-ATP in the incubation medium. The ATPase activity of isolated F1 is inhibited to 50% in the presence of 14 micrograms inhibitor/mg F1. A stoichiometry of 1.3 mol inhibitor/mol F1 for complete inhibition can be calculated from this value. The potato ATPase inhibitor is also a potent inhibitor of the ATPase activity of the isolated yeast F1. The inhibitor resembles the ATPase inhibitors of yeast and mammalian mitochondria, and does not seem to be related to the inhibitory peptide, epsilon subunit, of chloroplast ATPase.

Regulation of the mitochondrial ATP synthase in intact rat cardiomyocytes

The ATP synthase capacity of rat heart myocytes can be measured in sonicated cell suspensions and in sonicated preparations of cultured cardiomyocytes. This procedure allows the rapid measurement of mitochondrial function in response to changes in the metabolic status of the cell. In cultured myocytes, transitions in ATP synthase capacity (with no detectable change in cellular ATP concentration) accompany a change to anoxia or electrically stimulated contraction (rise of 70%). These changes are reversed on returning to the original conditions. Exposure of myocytes to low pH has little effect on basal ATP synthase capacity (down to values less than pH 6), but markedly affects cellular ATP levels and the response of the cells to anoxia and reoxygenation, possibly mimicking changes seen in ischaemic heart. Similar effects are seen in suspensions of freshly prepared myocytes, but these preparations are less stable and more pH-sensitive than are cells in culture. It is proposed that mitochondria in vivo are directly regulated at the level of the ATP synthase, and that a regulator protein, the naturally occurring inhibitor protein from mitochondria, may be responsible for this regulation.

Reversible modulation of the mitochondrial ATP synthase with energy demand in cultured rat cardiomyocytes

The ATP synthase capacity of rat heart myocytes can be measured in sonicates of cultured cardiomyocytes. In these cells, transitions in ATP synthase capacity occur on changing to the anoxic or uncoupled state (drop in ATP synthase capacity of over 40%) or on electrically stimulating the cells to contract (rise of 70%). These changes occur rapidly (half time less than 1 min) and are completely reversed on returning to the original conditions. It is proposed that mitochondria in vivo are directly regulated at the level of the ATP synthase. The naturally occurring inhibitor protein from mitochondria may be responsible for this regulation.

Effect of lithium and of other drugs used in the treatment of manic illness on the cation-transporting properties of Na+,K+-ATPase in mouse brain synaptosomes

We have developed and used a novel technique to investigate the effects of lithium and other psychotropic drugs on the cation-transporting properties of the sodium- and potassium-activated ATPase enzyme (Na+,K+-ATPase) in intact synaptosomes. Rubidium-86 uptake into intact synaptosomes is an active process and is inhibited by approximately 75% in the presence of the Na+,K+-ATPase inhibitor acetylstrophanthidin. In vitro addition of lithium to synaptosomes prepared from untreated mice causes a progressive inhibition of acetylstrophanthidin-sensitive 86Rb uptake, but only at concentrations higher than the clinical therapeutic range. However, pretreatment of mice for 14 days in vivo with lithium, carbamazepine, and haloperidol, but not phenytoin, causes a significant stimulation of 86Rb uptake into synaptosomes via Na+,K+-ATPase.

Inhibition of H+-transporting ATPase by formation of a tight nucleoside diphosphate-fluoroaluminate complex at the catalytic site

Inhibition of the mitochondrial and bacterial F1-type ATPases [of ATP phosphohydrolase (H+-transporting), EC 3.6.1.34] by fluoride was found to depend on the presence of aluminum and ADP at the catalytic site(s) of F1-type ATPase. AIF-4 was demonstrated to be the active fluoroaluminate species. The identical pattern of inhibition of F1-type ATPase activity obtained in the presence of ADP and NaF with beryllium, a metal that forms fluoride complexes strictly tetracoordinated, suggests that aluminum acts through a tetrahedral complex. Inhibition of isolated F1-type ATPase by AIF-4 in the presence of ADP cannot be reversed by ADP, ATP, or chelators of aluminum. However, the inhibition of the ATPase activity of the F1 sector in submitochondrial particles caused by AIF-4 and ADP was reversed upon addition of an oxidizable substrate. Uncouplers prevented the reversal of inhibition, suggesting that the protonmotive force generated by respiration was responsible for the relief of inhibition. Because of structural similarities between AIF4- and , AIF4- is postulated to mimic the phosphate group of ATP and form an abortive complex with ADP at the active site(s) of F1-type ATPase.

Pre-steady-state studies of the adenosine triphosphatase activity of coupled submitochondrial particles. Regulation by ADP

ATPase activities were measured in 10 mM MgCl2, 5 mM ATP, 1 mM ADP, and 1 microM FCCP with submitochondrial particles from bovine heart that had been stimulated by delta mu H+-forming substrates and with particles whose natural inhibitor protein was partially removed by heating. The activities were not linear with time. With both particles, the rate of ATP hydrolysis in the 7-fold greater than that in the steady state. Pre-steady-state and steady-state kinetic studies showed that the decrease of ATPase activity was due to the binding of ADP in a high-affinity site of the enzyme (K0.5 of 10 microM). Inhibition of ATP hydrolysis was accompanied by the binding of approximately 1 mol of ADP/mol of particulate F1; 10 microM ADP gave half-maximal binding. ADP could be replaced by IDP, but with an affinity 50-fold lower (K0.5 of 0.5 mM). Maximal inhibition by ADP and IDP was achieved in less than 5 s. Inhibition was enhanced by uncouplers. Even in the presence of pyruvate kinase and phosphoenolpyruvate, the rates of hydrolysis were about 2.5-fold higher in the first seconds of reaction than in the steady state. This decrease of ATPase activity also correlated with the binding of nearly 1 mol of ADP/mol of F1. This inhibitory ADP remained bound to the enzyme after several thousand turnovers. Apparently, it is possible to observe maximal rates of hydrolysis only in the first few catalytic cycles of the enzyme.

Kinetics of the release of the mitochondrial inhibitor protein. Correlation with synthesis and hydrolysis of ATP

(1) The kinetics of the release of the mitochondrial inhibitor protein (IF1) is studied in bovine heart submitochondrial vesicles supplemented with 125I-labelled IF1, using a method for rapidly 'freezing' the state of F1-IF1 interaction. It is shown that generation of a protonmotive force leads to release of IF1 from F1 into solution, following an exponential process. (2) In one set of experiments the rate of IF1 release, in IF1 supplemented vesicles generating a protonmotive force, is correlated with the induction of ATP hydrolytic capacity. It is found that, even under different metabolic states (phosphorylating and non-phosphorylating conditions), both processes follow the same time-course (half-time of around 40 s) and that there is a direct correlation between induced ATPase capacity and IF1 released. This finding rules out the possibility of a non-inhibitory binding site for IF1 on the membrane. (3) In a second set of experiments, also using IF1 supplemented vesicles, the induction of the ATP hydrolytic capacity after energisation is correlated with the induction of the ATP synthetic capacity. Initial rates of both processes are monitored using firefly luciferase, keeping the assay systems as similar as possible. It is shown that the induction of each capacity follows an exponential time-course, with a half-time of around 40 s. This is in good agreement with the half-times obtained for the induction of ATP hydrolytic capacity and the rate of IF1 release, using the quench-stop method. (4) If the induction of ATP hydrolytic and synthetic capacities is followed in untreated submitochondrial vesicles, i.e., vesicles not supplemented with IF1, the extent and time-course of the change in both hydrolytic and synthetic capacities remain correlated, but the half-time of the transient falls to around 10 s. It is suggested that the length of the transient, observed in IF1 supplemented vesicles, results from partial loss of coupling during repeated centrifugations. (5) These results demonstrate that energy-dependent release of IF1 from F1 into solution results in a concomitant increase in both ATP synthetic and hydrolytic capacities of the ATP synthase complex, and that the time-course of this process is sensitive to the degree of coupling of the vesicles.

The binding and release of the inhibitor protein are governed independently by ATP and membrane potential in ox-heart submitochondrial vesicles

(1) The effects of membrane potential (delta psi) and nucleotides on the interaction between the F1-ATP synthase and its natural inhibitor protein (IF1) are studied in ox-heart submitochondrial vesicles. (2) Membrane potential causes displacement of IF1 from submitochondrial vesicles, as shown by measuring both delta psi-dependent stimulation of ATPase capacity and release of 125I-labelled IF1 from the vesicles. These effects are abolished if ATP is included in the incubation. (3) There is a linear increase in the steady-state ATPase capacity of oxidising vesicles as delta psi is increased from 100 mV to 135 mV. Increasing delta psi above 140 mV leads to no further change. (4) At a constant membrane potential, ATP suppresses the increase in ATPase capacity, with a concentration for half maximal effect of 140 microM. This value is close to the Km for ATP hydrolysis by membrane-bound F1. This suppression is related to ATP concentration rather than to delta Gp or ATP/ADP ratio. (5) The unidirectional on- and off-rates of IF1 were measured separately. The off-rate of IF1 is increased by membrane potential but unaffected by ATP. The on-rate, conversely, is increased by ATP. Thus, the suppression of the potential-dependent net release of IF1 from submitochondrial vesicles by ATP results from an increase of the IF1 on-rate above the off-rate.

The mitochondrial ATP synthase inhibitor protein binds near the C-terminus of the F1 beta-subunit

The specific, mitochondrial ATP synthase protein (IF1) was covalently cross-linked to its binding site on the catalytic sector of the enzyme (F1-ATPase). The cross-linked complex was selectively cleaved, leaving IF1 intact to facilitate the subsequent purification of the F1 fragment to which IF1 was cross-linked. This fragment was identified by sequence analysis as comprising residues 394-459 on the F1 beta-subunit, near the C-terminus. This finding is discussed in the light of secondary structure predictions for both IF1 and the F1 beta-subunit, and sequence homologies between mitochondrial and other ATP synthases.

Properties and regulation of the H+-ATP synthase of mitochondria

A brief survey is made of the function of the H+-ATP synthase of mitochondria with emphasis on how it is regulated. A main regulatory factor is a low molecular weight protein whose binding to the enzyme appears to be essential for optimal accumulation of ATP as driven by electron transport. The ATP synthase is also controlled by ADP that, by binding to a site in the enzyme, inhibits ATP hydrolysis. Data on the spontaneous synthesis of a tightly bound ATP are discussed. Apparently, this requires proper subunit interactions to yield a competent catalytic site.

Interaction of F1-ATPase and its inhibitor peptide. Effect of pH

1. The inhibition of F1-ATPase by its natural peptide inhibitor is mixed non-competitive with two pH optimum values (5.5 and 8.2). 2. A two-step model for the interaction is suggested in which two enzyme conformations would exhibit different affinities for the peptide. 3. At low pH, interaction would be favoured. At high pH, a conformation (not susceptible to inhibition) changes into another (susceptible to inhibition) through the hydrolytic reaction stimulation, due to high pH.

Interaction of F1-ATPase and its inhibitor peptide. Effect of dinitrophenol, nucleotides and anions

1. ATPase natural inhibitor interacted in a mixed non-competitive manner with compounds affecting hydrolytic activity. 2. Ka's for DNP, HCO3- and free ATP, and Ki's for SCN- and ADP became smaller as inhibitor peptide concentration increased, reflecting an increase in affinity of F1-ATPase for these compounds induced by the peptide. 3. Activators increased the peptide inhibitory effect, whereas inhibitors decreased it. 4. A two-step model for the peptide-enzyme interaction is suggested in which ATP hydrolysis is a key factor.

Regulatory role of the ATPase inhibitor protein on proton conduction by mitochondrial H+-ATPase complex

This study shows that the natural inhibitor protein of mitochondrial H+-ATPase complex (IF1) inhibits, in addition to the catalytic activity, the proton conductivity of the complex. The inhibition of ATPase activity by IF1 is less effective in the purified F1 than in submitochondrial particles where F1 is bound to F0. No inhibition of H+ conductivity by F0 is observed in F1-depleted particles.