Isolation of a major hydrophobic protein of the mitochondrial inner membrane

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

An electrophoretic analysis of proteolipids from different rat brain subcellular fractions

Proteolipid proteins were extracted from adult rat brain subcellular fractions and purified by chromatography on Sephadex LH-60. Polyacrylamide gel electrophoresis of the delipidized proteins, in the presence or absence of 8 M urea, was carried out with all fractions. The distribution of the various types of proteolipid proteins was studied and their molecular weight calculated by the Ferguson relationship. Several bands of proteolipid proteins were found in the five membrane fractions analyzed. Some of them, such as the 17.5 K and 37 K components were very prominent in mitochondria and synaptosomes. The 30 K component was found in myelin-derived membranes and in microsomes, while the 20 K and 25 K proteolipid proteins were present in all subcellular fractions. The 30 K component (proteolipid protein (PLP)), typical of the purified myelin membranes, showed a similar distribution to that of 2',3'-cyclic-nucleotide 3'-phosphohydrolase (EC 3.1.4.37) activity, while the other major proteolipid protein present in all subcellular fractions (25 K) did not show such parallelism, indicating that it might not be an exclusive component of myelin. The electrophoretic pattern of microsomal proteolipid proteins did not show the high molecular weight components (aggregates of PLP) which are found in myelin. Furthermore, the 30 K component showed a smaller Y0 value than that of the 30 K found in myelin. Thus the presence of 30 K proteolipid protein in microsomes should not be considered as being due to myelin contamination.

Incorporation of N-ethylmaleimide into the membrane-bound ADP/ATP translocator. Isolation of the protein labeled with N-[3H]ethylmaleimide

The incorporation of N-ethylmaleimide into the 30,000-Mr component of beef-heart mitochondria has been studied as a function of various ligands to the ADP/ATP carrier and the isolation of the N-ethylmaleimide-labeled protein is reported. 1. The incorporation of N-ethylmaleimide into the 30,000-Mr component is specifically stimulated by ADP and ATP. Thus by differential incorporation of N-ethylmaleimide, the 30,000-Mr component is preferentially labeled. 2. Addition of carboxyatractylate inhibits, whereas bongkrekate tolerates, the incorporation of N-ethylmaleimide. 3. After solubilization by Triton the purification of N-ethylmaleimide-labeled protein is facilitated in the presence of bongkrekate but not of carboxyatractylate, in agreement with the postulated existence of only a bongkrekate-N-ethylmaleimide-protein complex. The labeled protein was purified to homogeneity on hydroxyapatite in Triton and subsequently, after denaturation in dodecylsulfate, on Sepharose 6B. 4. The identify of the isolated labeled protein with the formerly isolated bongkrekate-protein or carboxyatractylate-protein complexes is confirmed by the isoelectric point and amino acid composition. 5. Two moles of N-ethylmaleimide must be incorporated into the 30,000-Mr component in order to inhibit fully the binding of one mole carboxyatractylate. This corresponds to one -SH group per unit.

Isolation of the ADP, ATP carrier as the carboxyatractylate . protein complex from mitochondria

The procedure for the isolation from mitochondria of the undenatured ADP, ATP carrier is described. The condition of retaining the nativity are elaborated. 1. As indicator for the ADP, ATP carrier (35S)- or (3H) carboxyatractylate were used. By preloading the mitochondria with carboxyatractylate, a stable carboxyatractylate . protein complex could be retained after solubilization with Triton X-100. Among the polyoxyethylene detergents emulphogen is also solubilizing, whereas Brij and Lubrol fail to solubilize. 2. When unloaded mitochondria are solubilized the capacity for binding carboxyatractylate disappears rapidly, particularly at 20 degrees C. 3. When mitochondria are preloaded with atractylate, the binding after solubilization with Triton X-100 is considerably lower than with carboxyatractylate, indicating that the high affinity of carboxyatractylate is required for effectively protecting the protein. 4. For purification hydroxyapatite is most effective. The carboxyatractylate-protein complex appears in the pass-through whereas the bulk of other mitochondrial proteins are retained such that a 7-fold purification is obtained. The nonadsorptivity to hydroxyapatite is dependent on the undenatured state maintained in the carboxyatractylate . protein complex. 5. Subsequent gel filtration on Sepharose results in a 1.5-fold further enrichment of specific carboxyatractylate binding up to 17 mumol/g protein, corresponding to a 10-fold purification from mitochondria. This value cannot be increased with further measures. 6. At the last purification step, in sodium dodecyl sulfate polyacrylamide gel electrophoresis virtually a single band of 30 000 molecular weight is found, confirming the purity at this stage. A molecular weight of 60 000 is calculated from the carboxyatractylate binding, indicating that the carboxyatractylate protein complex consists of two 30 000 subunits. From this the protein share of the ADP, ATP carrier in beef heart mitochondria can be calculated to amount to 9.5%9 7. The intact carboxyatractylate . protein complex is protected against proteolytic degradation. The release of carboxyatractylate ensues a conformational change of protein as assayed by conformation specific antibodies, concomitant with unmasking of proteolytic site as assayed by tryptic digestion. 8. The amino acid composition indicates hydrophobicity (39% polarity) and a high content of basic amino acid such as lysine and arginine. There is 1.5 mol percent cysteine and a blocked N-terminal. 9. From the solubilized complex (35S) carboxyatractylate can be removed by carboxyatractylate, ADP and ATP but not by ITP, etc., indicating the presence of recognizing sites specific fof ADP, ATP and therefore, identity with the ADP, ATP carrier. 10. Other reported procedures for isolating the ADP, ATP carrier are shown to either fail or have lower yield than the present, original procedure.

Selective solubilization of the components of the mitochondrial inner membrane by lysolecithin

1. Of various phospholipids tested, lysolecithin was the most efficient in the solubilization of the components of beef heart submitochondrial particles. Lysolecithin solubilized selectively nicotinamide nucleotide transhydrogenase, succinate dehydrogenase, NADH dehydrogenase and oligomycin-sensitive ATPase. Various cytochromes other than cytochrome c were only slightly solubilized. 2. The effect of various parameters, e.g. ionic strength, pH, time of centrifugation, and concentrations of lysolecithin and protein was investigated. Increasing times of centrifugation led to a partial sedimentation of NADH dehydrogenase, and a complete sedimentation of oligomycin-sensitive ATPase and cytochrome oxidase. 3. Further fractionation of the lysolecithin extract by centrifugation in the presence of low concentrations of cholate gave a complete separation of NADH dehydrogenase and transhydrogenase, indicating that these enzymes are not related functionally. 4. With the lysolecithin fractionation procedure a more than 10-fold purification of transhydrogenase was achieved. Polyacrylamide gel electrophoresis of the partially purified transhydrogenase in the presence of sodium dodecyl sulphate showed major increases in protein-stained bands corresponding to between 70 000 and 54 000 daltons. 5. A possible mechanism for the detergent action of lysolecithin involving a specific exchange of bound phospholipids for lysolecithin is discussed.

Preparation of the proteolipid apoprotein from bovine heart, liver and kidney

Proteolipid apoproteins have been prepared from heart, kidney, and liver by dialysis in chloroform/methanol against chloroform/methanol, acidified chloroform/methanol, and chloroform/methanol in succession. They are free of lipids (less than 0.05% P; less than 0.1% carbohydrate). They show a high content of non-polar amino acids, methionine, and tryptophan and contain little or no half-cystine. The differ from neural proteolipid apoproteins by absence of half-cystine, and of covalently bound fatty acids. As recovered from chloroform/methanol solutions, they are soluble in chloroform/methanol and insoluble in water, but a water-soluble form can be prepared by changing the solvent from chloroform/methanol to water in a stream of nitrogen. The chloroform-methanol-soluble form and the water-soluble form are interconvertible. ORD and CD spectra of all proteolipid apoproteins indicate 60-70% alpha-helix content in chloroform/methanol solution and 20-30% alpha-helix in water solution. Sodium dodecyl sulfate gel electrophoresis resolves proteolipid apoprotein into two major components corresponding to ca. 12 000 and 34 000 daltons. With sodium dodecyl sulfate/urea numerous bands appear, with a major one at 30 000 daltons and 8 to 10, ranging downward to 2500. For comparison, neural proteolipid apoproteins also show numerous bands with a major one at 25 000. The marked chemical and physical similarities among all proteolipid apoproteins studied suggest a common role in membrane structures.

The structure and subunit composition of the particulate NADH-ubiquinone reductase of bovine heart mitochondria

Preparations of NADH-ubiquinone reductase from bovine heart mitochondria (Complex I) were shown to contain at least 16 polypeptides by gel electrophoresis in the presence of sodium dodecyl sulphate. 2. High-molecular-weight soluble NADH dehydrogenase prepared from Triton X-100 extracts of submitochondrial particles [Baugh & King (1972) Biochem. Biophys. Res. Commun. 49, 1165-1173] was similar to Complex I in its polypeptide composition. 3. Solubilization of Complex I by phospholipase A treatment and subsequent sucrose-density-gradient centrifugation did not alter the polypeptide composition. 4. Lysophosphatidylcholine treatment of Complex I caused some selective solubilization of a polypeptide of mol.wt. 33000 previosuly postulated to be the transmembrane component of Complex I in the mitochondrial membrane [Ragan (1975) in Energy Transducing Membranes: Structure, Function and Reconstitution (Bennun, Bacila & Najjar, eds.), Junk, The Hague, in the press]. 5. Chaotropic resolution of Complex I caused solubilization of polypeptides of molecular weights 75000, 53000, 29000, 26000 and 15500 and traces of others in the 10000-20000-mol.wt.range. 6. The major components of the iron-protein fraction from chaotropic resolution had molecular weights of 75000, 53000 and 29000, whereas the flavoprotein contained polypeptides of molecular weights 53000 and 26000 in a 1:1 molar ratio. 7. Iodination of Complex I by lactoperoxidase indicated that the water-soluble polypeptides released by chaotropic resolution, in particular those of the flavoprotein fraction, were largely buried in the intact Complex. 8. The polypeptides of molecular weights 75000, 53000, 42000, 39000, 33000, 29000 and 26000 were present in 1:2:1:1:1:1:1 molar proportions. The two subunits of molecular weight 53000 are probably non-identical.

The influence of adenine nucleotides and oxidizable substrates on triethyltin-mediated chloride uptake by rat liver mitochondria in potassium chloride media

In a 100 mM-KCl medium, pH 6.8, containing ATP increasing concentrations of triethyltin cause an uptake of Cl- into mitochondria with a maximum at 1 muM. This can be inhibited by atractylate or oligomycin, but is virtually unaffected by the presence of rotenone. When the medium contains substrate (pyruvate, beta-hydroxybutyrate or succinate), both in the presence and absence of adenine nucleotides, Cl- uptake is greater with a maximum at 1-10 muM-triethyltin. If substrate oxidation is blocked by respiratory-chain inhibitors the Cl- uptake mediated by triethyltin is inhibited except in the media containing ATP, when the characteristics of Cl- uptake similar to that found in the medium containing ATP alone are observed. Under all conditions tested Cl- uptake is decreased by the presence of 2,4-dinitrophenol. It is concluded that energy from either the oxidation of substrate or the hydrolysis of ATP is associated with the generation of sufficient OH- to enable the triethyltin-mediated Cl-/OH- exchange to occur under the metabolic conditions relevant to this action of triethyltin.

Role of ionophores in energy coupling

If, as we deem inevitable, the principles of energy coupling are universal, then the necessity for charge-separating devices will apply across the board to all bioenergetic systems. Since, apart from the elctron transfer chain, ionophores are the only charge-separating devices available in bioenergetic systems, model of energy coupling that does not feature this central role of ionophores can be taken seriously. The ionophore approach thus opens the royal highway to the ultimate solution of all bioenergetic problems.

Mesosomes: membranous bacterial organelles

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