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

Single-molecule analysis of the rotation of F₁-ATPase under high hydrostatic pressure

F1-ATPase is the water-soluble part of ATP synthase and is an ATP-driven rotary molecular motor that rotates the rotary shaft against the surrounding stator ring, hydrolyzing ATP. Although the mechanochemical coupling mechanism of F1-ATPase has been well studied, the molecular details of individual reaction steps remain unclear. In this study, we conducted a single-molecule rotation assay of F1 from thermophilic bacteria under various pressures from 0.1 to 140 MPa. Even at 140 MPa, F1 actively rotated with regular 120° steps in a counterclockwise direction, showing high conformational stability and retention of native properties. Rotational torque was also not affected. However, high hydrostatic pressure induced a distinct intervening pause at the ATP-binding angles during continuous rotation. The pause was observed under both ATP-limiting and ATP-saturating conditions, suggesting that F1 has two pressure-sensitive reactions, one of which is evidently ATP binding. The rotation assay using a mutant F1(βE190D) suggested that the other pressure-sensitive reaction occurs at the same angle at which ATP binding occurs. The activation volumes were determined from the pressure dependence of the rate constants to be +100 Å(3) and +88 Å(3) for ATP binding and the other pressure-sensitive reaction, respectively. These results are discussed in relation to recent single-molecule studies of F1 and pressure-induced protein unfolding.

Regulation of the F1F0-ATP synthase rotary nanomotor in its monomeric-bacterial and dimeric-mitochondrial forms

The F(1)F(0)-adenosine triphosphate (ATP) synthase rotational motor synthesizes most of the ATP required for living from adenosine diphosphate, Pi, and a proton electrochemical gradient across energy-transducing membranes of bacteria, chloroplasts, and mitochondria. However, as a reversible nanomotor, it also hydrolyzes ATP during de-energized conditions in all energy-transducing systems. Thus, different subunits and mechanisms have emerged in nature to control the intrinsic rotation of the enzyme to favor the ATP synthase activity over its opposite and commonly wasteful ATPase turnover. Recent advances in the structural analysis of the bacterial and mitochondrial ATP synthases are summarized to review the distribution and mechanism of the subunits that are part of the central rotor and regulate its gyration. In eubacteria, the epsilon subunit works as a ratchet to favor the rotation of the central stalk in the ATP synthase direction by extending and contracting two alpha-helixes of its C-terminal side and also by binding ATP with low affinity in thermophilic bacteria. On the other hand, in bovine heart mitochondria, the so-called inhibitor protein (IF(1)) interferes with the intrinsic rotational mechanism of the central gamma subunit and with the opening and closing of the catalytic beta-subunits to inhibit its ATPase activity. Besides its inhibitory role, the IF(1) protein also promotes the dimerization of the bovine and rat mitochondrial enzymes, albeit it is not essential for dimerization of the yeast F(1)F(0) mitochondrial complex. High-resolution electron microscopy of the dimeric enzyme in its bovine and yeast forms shows a conical shape that is compatible with the role of the ATP synthase dimer in the formation of tubular the cristae membrane of mitochondria after further oligomerization. Dimerization of the mitochondrial ATP synthase diminishes the rotational drag of the central rotor that would decrease the coupling efficiency between rotation of the central stalk and ATP synthesis taking place at the F(1) portion. In addition, F(1)F(0) dimerization and its further oligomerization also increase the stability of the enzyme to natural or experimentally induced destabilizing conditions.

High hydrostatic pressure perturbs the interactions between CF(0)F(1) subunits and induces a dual effect on activity

Chloroplast ATP-synthase is an H(+)/ATP-driven rotary motor in which a hydrophobic multi-subunit assemblage rotates within a hydrophilic stator, and subunit interactions dictate alternate-site catalysis. To explore the relevance of these interactions for catalysis we use hydrostatic pressure to induce conformational changes and/or subunit dissociation, and the resulting changes in the ATPase activity and oligomer structure are evaluated. Under moderate hydrostatic pressure (up to 60-80 MPa), ATPase activity is increased by 1.5-fold. This is not related to an increase in the affinity for ATP, but seems to correlate with an enhanced turnover induced by pressure, and an activation volume for the ATPase reaction of -23.7 ml/mol. Higher pressure (up to 200 MPa) leads to dissociation of the enzyme, as shown by enzyme inactivation, increased binding of 8-anilinonaphthalene-1-sulfonate (ANS) to hydrophobic regions, and labeling of specific Cys residues on the beta and alpha subunits by N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylene-4-diamine (IAEDANS). Compression-decompression cycles (between 0.1 and 200 MPa) inactivate CF(0)F(1) in a concentration-dependent manner, although after decompression no enzyme subunit is retained on a Sephadex-G-50 centrifuge column or is further labeled by IAEDANS. It is proposed that moderate hydrostatic pressures induce elastic compression of CF(0)F(1), leading to enhanced turnover. High pressure dissociation impairs the contacts needed for rotational catalysis.

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.

Osmolytes protect mitochondrial F(0)F(1)-ATPase complex against pressure inactivation

We have previously reported that carbohydrates and polyols protect different enzymes against thermal inactivation and deleterious effects promoted by guanidinium chloride and urea. Here, we show that these osmolytes (carbohydrates, polyols and methylamines) protect mitochondrial F(0)F(1)-ATPase against pressure inactivation. Pressure stability of mitochondrial F(0)F(1)-ATPase complex by osmolytes was studied using preparations of membrane-bound submitochondrial particles depleted or containing inhibitor protein (IP). Hydrostatic pressure in the range from 0.5 to 2.0 kbar causes inactivation of submitochondrial particles depleted of IP (AS particles). However, the osmolytes prevent pressure inactivation of the complex in a dose-dependent manner, remaining up to 80% of hydrolytic activity at the highest osmolyte concentration. Submitochondrial particles containing IP (MgATP-SMP) exhibit low ATPase activity and dissociation of IP increases the hydrolytic activity of the enzyme. MgATP-SMP subjected to pressure (2.2 kbar, for 1 h) and then preincubated at 42 degrees C to undergo activation did not have an increase in activity. However, particles pressurized in the presence of 1.5 M of sucrose or 3.0 M of glucose were protected and after preincubation at 42 degrees C, showed an activation very similarly to those kept at 1 bar. In accordance with the preferential hydration theory, we believe that osmolytes reduce to a minimum the surface of the macromolecule to be hydrated and oppose pressure-induced alterations of the native fold that are driven by hydration forces.

Effects of high hydrostatic pressures on living cells: a consequence of the properties of macromolecules and macromolecule-associated water

Sixty percent of the Earth's biomass is found in the sea, at depths greater than 1000 m, i.e., at hydrostatic pressures higher than 100 atm. Still more surprising is the fact that living cells can reversibly withstand pressure shifts of 1000 atm. One explanation lies in the properties of cellular water. Water forms a very thin film around macromolecules, with a heterogeneous structure that is an image of the heterogeneity of the macromolecular surface. The density of water in contact with macromolecules reflects the physical properties of their different domains. Therefore, any macromolecular shape variations involving the reorganization of water and concomitant density changes are sensitive to pressure (Le Chatelier's principle). Most of the pressure-induced changes to macromolecules are reversible up to 2000 atm. Both the effects of pressure shifts on living cells and the characteristics of pressure-adapted species are opening new perspectives on fundamental problems such as regulation and adaptation.

Structure, functioning, and assembly of the ATP synthase in cells from patients with the T8993G mitochondrial DNA mutation. Comparison with the enzyme in Rho(0) cells completely lacking mtdna

The structure and functioning of the ATP synthase of human fibroblast cell lines with 91 and 100%, respectively, of the T8993G mutation have been studied, with MRC5 human fibroblasts and Rho(0) cells derived from this cell line as controls. ATP hydrolysis was normal but ATP synthesis was reduced by 60% in the 100% mutants. Both activities were highly oligomycin-sensitive. The levels of F(1)F(0) were close to normal, and the enzyme was stable. It is concluded that the loss of ATP synthesis is because of disruption of the proton translocation step within the F(0) part. This is supported by membrane potential measurements using the dye JC-1. Cells with a 91% mutation load grew well and showed only a 25% loss in ATP synthesis. This much reduced effect for only a 9% difference in mutation load mirrors the reduced pathogenicity in patients. F(1)F(0) has been purified for the first time from human cell lines. A partial complex was obtained from Rho(0) cells containing the F(1) subunits associated with several stalk, as well as F(0) subunits, including oligomycin sensitivity conferring protein, b, and c subunits. This partial complex no longer binds inhibitor protein.