Synthesis of the mitochondrial inner membrane in cultured Xenopus laevis oocytes.

The purpose of this study was to investigate the contribution of mitochondrial and cytoplasmic protein synthesis to the biogenesis of cytochrome oxidase (ferrocytochrome c:oxygen oxidoreductase EC 1.9.3.1) and rutamycin-sensitive adenosine triphosphatase (ATP phosphohydrolase EC 3.6.1.3) in cultured oocytes of the toad, Xenopus laevis. X. laevis cytochrome oxidase was purified over 23-fold with respect to specific activity and over 29-fold with respect to specific heme a content from oocyte submitochondrial particles. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate separated the enzyme into six subunits with molecular weights of 44,000, 33,000, 23,000, 17,000, 12,000 and 9,500. the synthesis of the three larger subunits is sensitive to chloramphenicol (an inhibitor of mitochondrial protein synthesis), indicating that these subunits are made on mitochondrial ribosomes; the synthesis of the three smaller subunits is sensitive to cycloheximide (an inhibitor of cytoplasmic protein synthesis) and therefore occurs on cytoplasmic ribosomes. X. laevis rutamycin-sensitive ATPase, purified over 19-fold from oocyte submitochondrial pparticles, consists of 10 subunits with molecular weights of 56,000, 53,000, 41,000, 32,000, 29,000, 24,000, 21,000, 17,500 (2), and 11,500 on sodium dodecyl sulfate-polyacrylamide gels. The 29,000, 21,000, and one of the 17,500-dalton polypeptides are synthesized in the presence of cycloheximide and are, therefore, products of mitochondrial protein synthesis; the synthesis of the remaining seven subunits occurs in the presence of chloramphenicol, indicating that these subunits are made on cytoplasmic ribosomes. The synthesis of protein by mitochondria in cultured oocytes appears to be dependent upon cytoplasmic protein synthesis. In the presence of cycloheximide, the mitoribosomal synthesis of the subunits of cytochrome oxidase and rutamycin-sensitive ATPase is detectable only after a prior inhibition of mitochondrial protein synthesis by chloramphenicol. Oocyte mitochondrial ribosomes synthesize at least nine polypeptides after chloramphenicol treatment, three of which are components of neither cytochrome oxidase nor rutamycin-sensitive ATPase.     

Thyroid hormone regulation of nuclear-encoded mitochondrial inner membrane polypeptides of the liver

The effects of thyroid hormone on nuclear-encoded mitochondrial inner membrane proteins were investigated by in vitro translation of the endogenous mRNA present in a postmitochondrial fraction from the livers of rats treated in vivo with hormone. The levels of the mRNAs were estimated by quantitative immunoabsorption of the translation mixture. Total protein synthesis was increased 2.6-fold after 4 days of in vivo hormone treatment, but only 10-15% of the polypeptides were dramatically altered (greater than 5-fold). Among the most highly elevated were cytochrome c1 (greater than 10-fold increase) and the Rieske iron-sulfur protein of the cytochrome bc1 complex. Other inner membrane proteins (core protein 1, beta subunit of F1 ATPase, subunit IV of cytochrome oxidase, 3-hydroxybutyrate dehydrogenase) and non-mitochondrial proteins (rat serum albumin, beta 2-microglobulin) were not altered significantly by hormone treatment. Cytochrome c1 and the Rieske protein increased after 12 h of hormone treatment, a relatively early response in mammalian mitochondrial biogenesis. The possible significance of this response for the regulation of mitochondrial synthesis and assembly is discussed.     

Isolation, subunit composition, and site of synthesis of human cytochrome c oxidase

Cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1), the terminal oxidase of the respiratory chain in eucaryotic cells, has been purified from human placenta mitochondria. Seven polypeptides have been identified reproducibly by high-resolution electrophoresis of the enzyme complex through sodium dodecyl sulfate (Na-DodSO4)--urea polyacrylamide gels; these correspond closely in size to the subunits of beef heart cytochrome c oxidase. When HeLa cells, grown in suspension culture, were pulse-labeled with [35S]methionine in the presence of cycloheximide to inhibit cytoplasmic protein synthesis and chased with an excess of unlabeled methionine in the absence of the drug, the mitochondrially synthesized polypeptides were resolved into at least 17 components by NaDodSO4--urea polyacrylamide gel electrophoresis. After labeled HeLa mitochondria were mixed with human placenta mitochondria and the cytochrome c oxidase was isolated, three of the labeled components were found to copurify with the three largest subunits of the complex. We conclude that human cytochrome c oxidase contains seven subunits, the three largest of which are synthesized on mitochondrial ribosomes, while the other four are synthesized in the cytoplasm.     

Cytochrome oxidase assembly in yeast requires the product of COX11, a homolog of the P. denitrificans protein encoded by ORF3.

The synthesis of cytochrome oxidase in Saccharomyces cerevisiae was recently shown to require a protein encoded by the nuclear gene COX10. This protein was found to be homologous to the putative protein product of the open reading frame ORF1 reported in one of the cytochrome oxidase operons of Paracoccus denitrificans. In the present study we demonstrate the existence in yeast of a second nuclear gene, COX11, whose encoded protein is homologous to another open reading frame (ORF3) present in the same operon of P. denitrificans. Mutations in COX11 elicit a deficiency in cytochrome oxidase. In this and in other respects cox11 and cox10 mutants have very similar phenotypes. An antibody has been obtained against the yeast COX11 protein. The antibody recognizes a 28 kd protein in yeast mitochondria, consistent with the size of the protein predicted from the sequence of COX11. The COX11 protein is tightly associated with the mitochondrial membrane but is not a component of purified cytochrome oxidase. An analysis of cytochrome oxidase subunits in wild type and in a cox11 mutant suggests that the COX11 protein is not required either for synthesis or transport of the subunit polypeptides into mitochondria. It seems more probable that COX11 protein exerts its effect at some terminal stage of enzyme synthesis, perhaps in directing assembly of the subunits.     

DNA-dependent RNA polymerase of thermoacidophilic archaebacteria

Among 979 non-glycerol growers of the yeast Schizosaccharomyces pombe, 40 strains were found to be deficient in the mitochondrial ATPase activity. Three of them exhibited an alteration in either the alpha or beta subunits of the F1ATPase. The alpha subunit was not immunodetected in the A23/13 mutant. The beta subunit was not immuno-detected in the B59/1 mutant. The existence of these two mutants shows that the alpha and beta subunits can be present independently of each other in the inner mitochondrial membrane. The beta subunit of the mutant F25/28 had a slower electrophoretic mobility than that of the wild-type beta subunit. This phenotype indicates abnormal processing or specific modification of the beta subunit. All mutants showed reduced activities of the NADH-cytochrome c reductase and of the cytochrome oxidase and a decreased synthesis of cytochrome aa3 and cytochrome b. This pleiotropic phenotype appears to result from specific modifications in the mitochondrial protein synthesis. The mitochondrial synthesis of four polypeptides (three cytochrome oxidase and one cytochrome b subunits) was markedly decreased or absent while three new polypeptides (Mr = 54000, 20000 and 15000) were detected in all the mutants analysed. This observation suggests that a functional F1ATPase is necessary for the correct synthesis and/or assembly of the mitochondrially made components of the cytochrome oxidase and cytochrome b complexes.     

CRIF1 Is Essential for the Synthesis and Insertion of Oxidative Phosphorylation Polypeptides in the Mammalian Mitochondrial Membrane

Although substantial progress has been made in understanding the mechanisms underlying the expression of mtDNA-encoded polypeptides, the regulatory factors involved in mitoribosome-mediated synthesis and simultaneous insertion of mitochondrial oxidative phosphorylation (OXPHOS) polypeptides into the inner membrane of mitochondria are still unclear. In the present study, disruption of the mouse Crif1 gene, which encodes a mitochondrial protein, resulted in a profound deficiency in OXPHOS caused by the disappearance of OXPHOS subunits and complexes in?vivo. CRIF1 was associated with large mitoribosomal subunits that were located close to the polypeptide exit tunnel, and the elimination of CRIF1 led to both aberrant synthesis and defective insertion of mtDNA-encoded nascent OXPHOS polypeptides into the inner membrane. CRIF1 interacted with nascent OXPHOS polypeptides and molecular chaperones, e.g., Tid1. Taken together, these results suggest that CRIF1 plays a critical role in the integration of OXPHOS polypeptides into the mitochondrial membrane in mammals.     

The effect of diabetes on protein synthesis and the respiratory chain of rat skeletal muscle and kidney mitochondria

The effects of streptozotocin-induced diabetes mellitus upon mitochondria from rat skeletal muscle and kidney were examined. The rate of amino acid incorporation in vitro by isolated skeletal muscle mitochondria from diabetic animals was decreased by 50–60% from control values. Treatment of diabetic animals with insulin lowered blood glucose levels to control values and restored the rate of muscle mitochondrial protein synthesis in vitro to control levels. The rates of skeletal muscle mitochondrial protein synthesis were also decreased 23–27% by a 2-day fast. Comparison of the translation products synthesized by isolated muscle mitochondria from control and diabetic rats by dodecyl sulfate polyacrylamide-gel electrophoresis revealed a uniform decrease in the synthesis of all polypeptides. Aurintricarboxylic acid and pactamycin, inhibitors of chain initiation, blocked protein synthesis to a greater extent in muscle mitochondria from control as compared to diabetic animals suggesting that mitochondria from diabetics are unable to initiate protein synthesis at a rate comparable to control. Phenotypic changes observed in diabetic muscle mitochondria included a 36% decrease in the content of cytochromes aa₃ and a 27% decrease in cytochrome b, both established as containing mitochondrial translation products in lower eucaryotes. State 3 respiration with glutamate as substrate decreased by 27% and uncoupler-stimulated respiration decreased by 23% in the diabetic mitochondria. By contrast, the specific activities of NADH and succinate dehydrogenases, established as products of cytoplasmic protein synthesis in lower eucaryotes, were not decreased in skeletal muscle mitochondria from the diabetic animals. These results suggest that the considerable muscular atrophy observed in diabetics may involve decreases in both cytoplasmic and mitochondrial protein synthesis, the latter reflected in profound changes in the respiratory chain. By contrast, comparison of kidney mitochondria from control and diabetic rats revealed no differences in the rates of protein synthesis in vitro, nor in the mitochondrial translation products, which corresponded closely to liver and skeletal muscle translation products. Similarly, the mitochondrial content of cytochromes b, c + c₁, and aa₃, the specific activity of succinate dehydrogenase, the rate of state 3 respiration, and the recovery of mitochondria from kidney homogenates did not differ in control and diabetic animals. Kidney mitochondria are thus like liver mitochondria in being relatively unaffected by insulin deprivation.     

Protonmotive Q cycle pathway of electron transfer and energy transduction in the three-subunit ubiquinol-cytochrome c oxidoreductase complex of Paracoccus denitrificans

Ubiquinol-cytochrome c oxidoreductase (cytochrome bc1) complex from Paracoccus denitrificans consists of only three polypeptide subunits (Yang, X., and Trumpower, B. L. (1986) J. Biol. Chem. 261, 12282-12289), whereas the analogous complexes of eukaryotic mitochondria consist of nine or more polypeptides (Schagger, H., Link, T. A., Engel, W. D., and von Jagow, G. (1986) Methods Enzymol. 126, 224-237). Using the purified three-subunit Paracoccus complex we have tested whether this simple cytochrome bc1 complex has the same electron transfer pathway and proton translocation activity as the bc1 complexes of mitochondria. Under presteady state conditions, the effects of inhibitors on reduction of cytochromes b and c1 by quinol and oxidant-induced reduction of cytochrome b indicate a cyclic electron transfer pathway and two routes of cytochrome b reduction in the three-subunit Paracoccus cytochrome bc1 complex. A novel method was developed to incorporate the cytochrome bc1 complex into liposomes with the detergent dodecyl maltoside. The enzyme reconstituted into liposomes translocated protons with an H+/2e value of 3.9. Carbonyl cyanide m-chlorophenylhydrazone eliminated proton translocation, while permitting the scalar release of protons from quinol, and thus reduced the H+/2e ratio to 2. These values agree with the predicted stoichiometries for proton translocation by a protonmotive Q cycle pathway. No inhibition of proton translocation by N',N'-dicyclohexylcarbodiimide was detected when the Paracoccus cytochrome bc1 complex was incubated with N',N'-dicyclohexylcarbodiimide before or after reconstitution into liposomes. Electron transfer in the three-subunit complex thus appears to occur by a protonmotive Q cycle pathway identical to that in mitochondrial cytochrome bc1 complexes. Only three polypeptides, cytochromes b, c1, and the Rieske iron-sulfur protein, are required for respiration and energy transduction in the cytochrome bc1 complex. The function of the supernumerary polypeptides in mitochondrial bc1 complexes is thus unclear.     

The effect of long-term inhibition of mitochondrial protein synthesis on the oxidation capacity of mitochondria for NADH-linked substrates

Experiments are presented showing that specific inhibition of mitochondrial protein synthesis by tetracyclines decreases the activity of the NADH-dehydrogenase complex in liver mitochondria, if rats are treated for long periods with these antibiotics. The corresponding inhibition of this complex in tumor cells (Zajdela hepatoma) and tumor mitochondria (Leydig cell tumor) is even more pronounced. It is concluded that the mitochondrial genetic system is involved in the assembly of the NADH-dehydrogenase complex, most likely by coding for one or more subunits. It is argued that this information, contrary to the situation for cytochrome c oxidase, the cytochrome bc1 complex and ATPsynthase, has been missed in previous experiments employing differential inhibition of mitochondrial protein synthesis, because of the circumstance that the inhibition did not reach the level at which it became rate-limiting.     

Immunochemical analysis of the membrane proteins of rat liver and Zajdela hepatoma mitochondria

The contents of mitochondrial inner membrane protein complexes were compared in normal liver and in Zajdela hepatoma mitochondria by the immunotransfer technique. Antibodies against core proteins 1 and 2, cytochrome c1, the iron-sulfur protein of Complex III, subunits I and II of cytochrome oxidase, and the alpha and beta subunits of the F1-ATPase were used. In addition, antibodies against a primary dehydrogenase, beta-hydroxybutyrate dehydrogenase, as well as the outer membrane pore protein were used. The results indicate that the components of the cytochrome chain and porin are greatly enriched in hepatoma mitochondria compared to normal rat liver mitochondria. This enrichment was also reflected in the rates of respiration in tumor mitochondria using a variety of substrates. Enrichment of porin may partially account for increased hexokinase binding to tumor mitochondria. In contrast to the respiratory chain components, the F1-ATPase and F0 (measured by DCCD binding) were not increased in tumor mitochondria. Thus, Zajdela hepatoma mitochondria components are nonstoichiometric, being enriched in oxidative capacity but relatively deficient in ATP synthesizing capacity. Finally, beta-hydroxybutyrate dehydrogenase, which is often decreased in hepatoma mitochondria, was shown here by immunological methods to be decreased by only 40%, whereas enzyme activity was less than 5% of that in normal rat liver.     

Nuclear mutants of Neurospora crassa temperature-sensitive for the synthesis of cytochrome aa3. Mitochondrial protein synthesis and analysis of the polypeptide composition of cytochrome c oxidase.

Three previously isolated mutants of Neurospora crassa, temperature-sensitive for the production of cytochrome aa3, have been further analyzed. These mutants have a slightly reduced capacity for mitochondrial protein synthesis when grown at 41 degrees C, although this relative deficiency appeared to be no greater than the deficiency in other cytochrome-aa3-deficient mutants. Thermolability studies revealed that the cytochrome c oxidase purified from each of the mutants grown at 23 degrees C is no more sensitive to heat inactivation than the enzyme isolated from wild-type cells. Sodium dodecylsulfate gel electrophoresis of immunoprecipitates obtained from the mitochondria of each of the mutants grown at 23 degrees C, using antiserum directed against holocytochrome c oxidase, indicated that all the subunits of cytochrome c oxidase were present in relative amounts similar to those found in mitochondria from wild-type cultures. However, when the mitochondria from mutant cultures grown at 41 degrees C were examined in the above fashion, only subunits 5 and 6 of the oxidase were detected. Nonetheless, the mitochondrially synthesized subunit 1, 2 and 3 polypeptides could be immunoprecipitated from mitochondria isolated from mutant cells grown at 41 degrees C and labelled with [3H]leucine in medium containing cycloheximide. Although subunits 4 and 7 could not be detected, because a suitable antibody was not available, the fact that five of the seven subunits were present, but not associated with each other, suggested that the genetic defects in these mutants may affect the process of cytochrome c oxidase assembly.     

Appearance of alternative respiration in cucumber cotyledon mitochondria after treatment with cycloheximide

Treatment of cotyledons of 4-day-old cucumber (Cucumis sativus L. cv. Tokiwa-jibae) seedlings with cycloheximide (0.3 m M ) inhibited protein synthesis in the cotyledons by 80%. In spite of this inhibition, the cycloheximide treatment induced a marked increase in the capacity of the alternative respiration in mitochondria, accompanied by an increase in the contribution of the pathway to the total respiration. In contrast, the activity of the cytochrome pathway was reduced by cycloheximide treatment. As a result, the total respiration was almost the same in mitochondria from cycloheximide-treated cotyledons and untreated cotyledons. Activities of some mitochondrial enzymes examined were also similar. Mitochondrial proteins synthesized during the treatment were separated by two-dimensional gel-electrophoresis and examined by fluorography. No new spots appeared and no spots disappeared on the fluorograms, but the labeling intensity of some polypeptides showed a relative increase or decrease as the result of cycloheximide treatment.     

The general mitochondrial processing peptidase from potato is an integral part of cytochrome c reductase of the respiratory chain.

The major mitochondrial processing activity removing presequences from nuclear encoded precursor proteins is present in the soluble fraction of fungal and mammalian mitochondria. We found that in potato, this activity resides in the inner mitochondrial membrane. Surprisingly, the proteolytic activity co-purifies with cytochrome c reductase, a protein complex of the respiratory chain. The purified complex is bifunctional, as it has the ability to transfer electrons from ubiquinol to cytochrome c and to cleave off the presequences of mitochondrial precursor proteins. In contrast to the nine subunit fungal complex, cytochrome c reductase from potato comprises 10 polypeptides. Protein sequencing of peptides from individual subunits and analysis of corresponding cDNA clones reveals that subunit III of cytochrome c reductase (51 kDa) represents the general mitochondrial processing peptidase.     

Regulation of biosynthesis of the rat liver inner mitochondrial membrane by thyroid hormone

Regulation of mitochondrial protein synthesis by thyroid hormone has been studied in isolated rat hepatocytes and liver mitochondria. Small doses (5 micrograms/100 g body wt) of triiodothyronine (T3) injected into hypothyroid rats increased both state 3 and 4 respiration by approximately 100%, while the ADP:O ratio remained constant. This suggests that T3 increases the numbers of functional respiratory chain units. T3 also induces mitochondrial protein synthesis by 50-100%. Analysis of the mitochondrial translation products show that all of the products were induced. No differential translation of the peptides involved in the respiratory chain was found. Regulation of the cytoplasmically made inner membrane peptides was also investigated in isolated hepatocytes. The majority of these peptides were not influenced by T3, in contrast to the finding with mitochondrial translation products. Those found to be regulated by T3 belong to two subsets, which were either induced or repressed by hormone. Thus, T3 stimulated a general increase in the synthesis of mitochondrially translated inner membrane peptides, but regulates selectively those inner membrane peptides translated on cytoplasmic ribosomes. The findings suggest that hormone regulation of the respiratory chain is exerted through a few selective proteins, perhaps those which require subunits made from both nuclear and mitochondrial genes.     

Identification of a stable ubisemiquinone and characterization of the effects of ubiquinone oxidation-reduction status on the Rieske iron-sulfur protein in the three-subunit ubiquinol-cytochrome c oxidoreductase complex of Paracoccus denitrificans

The ubiquinol-cytochrome c oxidoreductase (cytochrome bc1) complex from Paracoccus denitrificans exhibits a thermodynamically stable ubisemiquinone radical detectable by EPR spectroscopy. The radical is centered at g = 2.004, is sensitive to antimycin, and has a midpoint potential at pH 8.5 of +42 mV. These properties are very similar to those of the stable ubisemiquinone (Qi) previously characterized in the cytochrome bc1 complexes of mitochondria. The micro-environment of the Rieske iron-sulfur cluster in the Paracoccus cytochrome bc1 complex changes in parallel with the redox state of the ubiquinone pool. This change is manifested as shifts in the gx, gy, and gz values of the iron-sulfur cluster EPR signal from 1.80, 1.89, and 2.02 to 1.76, 1.90, and 2.03, respectively, as ubiquinone is reduced to ubiquinol. The spectral shift is accompanied by a broadening of the signal and follows a two electron reduction curve, with a midpoint potential at pH 8.5 of +30 mV. A hydroxy analogue of ubiquinone, UHDBT, which inhibits respiration in the cytochrome bc1 complex, shifts the gx, gy, and gz values of the iron-sulfur cluster EPR signal to 1.78, 1.89, and 2.03, respectively, and raises the midpoint potential of the iron-sulfur cluster at pH 7.5 from +265 to +320 mV. These changes in the micro-environment of the Paracoccus Rieske iron-sulfur cluster are like those elicited in mitochondria. These results indicate that the cytochrome bc1 complex of P. denitrificans has a binding site for ubisemiquinone and that this site confers properties on the bound ubisemiquinone similar to those in mitochondria. In addition, the line shape of the Rieske iron-sulfur cluster changes in response to the oxidation-reduction status of ubiquinone, and the midpoint of the iron-sulfur cluster increases in the presence of a hydroxyquinone analogue of ubiquinone. The latter results are also similar to those observed in the mitochondrial cytochrome bc1 complex. However, unlike the mitochondrial complexes, which contain eight to 11 polypeptides and are thought to contain distinct quinone binding proteins, the Paracoccus cytochrome bc1 complex contains only three polypeptide subunits, cytochromes b, c1, and iron-sulfur protein. The ubisemiquinone binding site and the site at which ubiquinone and/or ubiquinol bind to affect the Rieske iron-sulfur cluster in Paracoccus thus exist in the absence of any distinct quinone binding proteins and must be composed of domains contributed by the cytochromes and/or iron-sulfur protein.     

The biogenesis of rat mitochondrial ATPase. Two subunits are synthesized inside the mitochondria

Isolated mitochondria from regenerating rat liver synthesize at least five different polypeptides with molecular weights ranging from 19 000 to 43 000. Among these, two polypeptides with molecular weights of 22 000 and 25 ooo could be identified as ATPase subunits. It has previously been shown that these subunits, designated 6 and 7, are lacking in the ATPase complex that is formed in vivo when mitochondrial protein synthesis is blocked by thiamphenicol treatment. The 22 000 Mr protein is enriched in the fraction containing the fully assembled ATPase complex, whereas the 25 000 Mr protein is not.