Yeast versus mammalian mitochondrial protein synthesis

Unusual polypeptide synthesis in the kinetoplast-mitochondria from Leishmania tarentolae. Identification of individual de novo translation products

The de novo synthesis of cytochrome c oxidase subunits I, II (COI and COII), and apocytochrome b (Cyb) was investigated in kinetoplast-mitochondria of Leishmania. The organelles were isolated after breaking whole cells with nitrogen cavitation. Individual COI, COII, and Cyb polypeptides were identified by fractionation of the kinetoplast membranes, labeled with [(35)S]methionine and cysteine, using two-dimensional (9 versus 14% and 20 versus 11%) denaturing gel electrophoresis. The reaction did not require exogenous energy sources or amino acids. On the contrary, the presence of amino acids other than methionine somewhat inhibited the labeling reaction probably by competing with the uptake of labeled amino acids. The synthesis reaction was insensitive to 100 microg/ml chloramphenicol, gentamycin, paromomycin, lincomycin, hygromycin, and tetracycline, as well as cycloheximide. The process showed a linear increase in the amount of synthesized polypeptides during the first 2 h of incubation, followed by a slower accumulation of products for up to 4 h. The de novo synthesized polypeptides were stable for several additional hours. Their assembly into respiratory complexes, investigated using two-dimensional Blue Native/N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine-SDS gels, began early during the incubation and continued throughout the course of the synthesis. This work represents the first unequivocal identification of the polypeptide synthesis in kinetoplasts.

Translation rates of isolated liver mitochondria under conditions of hepatic mitochondrial proliferation

The hepatic mitochondrial content is increased in rats by treatment with the hypolipidaemic drug clofibrate and by administration of the cobalamin analogue hydroxycobalamin[c-lactam] (HCCL), an inhibitor of hepatic L-methylmalonyl-CoA mutase activity. As a first step in defining the mechanisms regulating liver mitochondrial contents in these models, the current studies were designed to test the hypothesis that hepatic mitochondrial proliferation is associated with enhanced translation rates of mitochondrial DNA gene products. Incorporation of [35S]methionine and [3H]leucine into protein was quantified in mitochondria isolated from control, clofibrate- and HCCL-treated rats. Use of multiple amino acid substrate concentrations permitted the maximal rate of translation (Vmax.) to be determined independent of endogenous amino acid concentrations. The Vmax. for methionine incorporation was not different in the models evaluated (0.062, 0.057 and 0.061 pmol/min per mg of mitochondrial protein in control, clofibrate- and HCCL-treated rats respectively). Similar results were obtained for leucine incorporation when absolute fractional radiolabel incorporation rates were analysed and when conventional Lineweaver-Burk analysis was employed. These results demonstrate no change in the intrinsic capacity of mitochondrial translation in these two models of hepatic mitochondrial proliferation.

Identification of F0 subunits in the rat liver mitochondrial F0F1-ATP synthase

In order to identify the subunits constituting the rat liver F0F1-ATP synthase, the complex prepared by selective extraction from the mitochondrial membranes with a detergent followed by purification on a sucrose gradient has been compared to that obtained by immunoprecipitation with an anti-F1 serum. The subunits present in both preparations that are assumed to be authentic components of the complex have been identified. The results show that the total rat liver F0F1-ATP synthase contains at least 13 different proteins, seven of which can be attributed to F0. The following F0 subunits have been identified: the subunit b (migrating as a 24 kDa band in SDS-PAGE), the oligomycin-sensitivity-conferring protein (20 kDa), and F6 (9 kDa) that have N-terminal sequences homologous to the beef-heart ones; the mtDNA encoded subunits 6 (20 kDa) and 8 (less than 7 kDa) that can be synthesized in isolated mitochondria; an additional 20 kDa protein that could be equivalent to the beef heart subunit d.

Isolation and incubation conditions to study heart mitochondrial protein synthesis

Although much is now known with regard to the processes of mammalian mitochondrial gene expression, relatively little is known concerning the quantitative regulation of this pathway in response to hormones or other physiological stimuli. This has been caused, in large part, by the lack of adequate assay systems in which such processes can be meaningfully measured. The purpose of this and the companion paper [E. E. McKee, B. L. Grier, G. S. Thompson, A. C. F. Leung, and J. D. McCourt. Am. J. Physiol. 258 [Endocrinol. Metab. 21):E503-E510, 1990] is to describe a system in which the quantitative regulation of mitochondrial protein synthesis in rat heart can be investigated. In this report the conditions for mitochondrial isolation and labeling are described, and the importance of isolating intact, tightly coupled mitochondria in obtaining high and reliable rates of protein synthesis is demonstrated. The highest levels of protein synthesis are obtained in mitochondria isolated from hearts perfused and homogenized in the presence of subtilisin, conditions in which the fastest rates of state 3 respiration and the highest respiratory control ratios are also observed. Analysis of the free amino acid pools indicates that isolated heart mitochondria have a negligible level of endogenous methionine as well as other amino acids. As a result, the concentration and specific radioactivity of the [35S]methionine pool serving protein synthesis could be easily determined. Optimal translation occurred at 30 degrees C at a pH of 7.0-7.2 and required the addition of methionine (20 microM), the other 19 amino acids (0.1 mM each), K+ (60-90 mM), Cl- (30-90 mM), Mg2+ (0.5-5 mM), and bovine serum albumin (1 mg/ml). As shown in the companion paper, adenine nucleotide (0.5-4.0 mM) and oxidizable substrate (10-20 mM glutamate) are also required for isolated heart mitochondrial protein synthesis. Analysis of labeled mitochondrial translation products demonstrated that bona fide mitochondrial peptides were synthesized.

Stimulation of transcription and translation by aurin tricarboxylic acid in mitochondrial lysates from Ehrlich ascites cells

We have prepared a submitochondrial fraction from Ehrlich ascites tumor cell mitochondria which shows transcription and translation activities. The antibiotic aurin tricarboxylic acid (ATA) at low concentrations induces both RNA and protein synthetic activities of the mitochondrial lysate by several fold. At high concentrations, however, ATA inhibits the translation activity but continues to stimulate the transcription activity in a dose dependent manner up to 0.5 mM concentration tested. The lysate system transcribes endogenous DNA yielding RNA species resembling control mitochondrial RNA and synthesizes authentic cytochrome oxydase I and cytochrome oxidase II subunits.

Effect of administration of 2-methyl-4-dimethylaminoazobenzene on the half-lives of rat liver mitochondria and cytochrome oxidase

The turnover of total mitochondrial proteins and cytochrome oxidase in the livers of rats administered with 2-methyl-4-dimethylaminoazobenzene (2-Me-DAB) has been determined. The incorporation of [14C]bicarbonate revealed a half-life of 3.1 days in control and 6 to 9 days in azodye administered animals for whole mitochondrial proteins. The incorporation of [35S]methionine yielded t1/2 values of 8.5 days and 15.4 days, respectively. The t1/2 of cytochrome oxidase, 10.8 days for control and 19.3 days for 2-Me-DAB-treated animals, indicated that the delay in the decay of the enzyme was of the same order as that of whole mitochondria. Short term incorporation revealed that the administration of the azodye stimulated the synthesis of the enzyme. Mitochondria isolated from azodye-administered animals appeared less susceptible to lysosomal proteolysis. Also, azodye administration seemed to impair the ability of lysosomes to degrade mitochondria.

Protein synthesis by hepatic mitochondria isolated from carbon tetrachloride-exposed rats

Hepatic mitochondria isolated from rats 40 h after dosage with 1.1 ml/kg CCl4 are uncoupled and display structural damage. Mitochondrial function returns during hepatic recovery. Because the products of mitochondrial protein synthesis are essential to mitochondrial structure and function, the effects of CCl4 on the rate of mitochondrial protein synthesis, and on the products, was studied using mitochondria from CCl4-exposed rats during the early, maximum development and resolution stages of CCl4-induced mitochondrial damage. Rates of mitochondrial protein synthesis (incorporation of [35S]methionine) were elevated 300% over that of mitochondria from non-exposed rats 17 h after exposure; depressed by 50% at 40 h and above control at 113 h. When the radiolabeled products of incorporation were separated and examined by autoradiography, a novel, low-molecular-weight band, of approx. 9700, was apparent 40 h after CCl4 exposure. A band of similar molecular weight appeared when control mitochondria were incubated without an exogenous supply of ATP. Mitochondria from exposed rats which displayed rates of protein synthesis greater than control consistently had a relative increase in a band that corresponded in size to that of cytochrome oxidase subunit I. It was concluded that the loss of mitochondrial function induced by CCl4 could not be attributed to inhibition of mitochondrial protein synthesis, and that the mitochondria may not always synthesize protein in constant proportions.

Does more than one mitochondrially synthesized protein in yeast have larger precursors?

Yeast cells undergoing derepression (the phase of mitochondriogenesis) were exposed to [14C]formate in the presence of cycloheximide, the cytosolic protein synthesis inhibitor, and of 1,10-phenanthroline, a metallo-protease inhibitor. Extensive labelling was obtained under such conditions. Incubation of these labelled products with mitochondrial lysates released small peptides (mol. wt. 500-1000). These results indicate that mitochondria probably synthesize some of the proteins in the precursor form and they are processed by a specific matrix-located protease before proper integration.

Liver mitochondria contain an ATP-dependent, vanadate-sensitive pathway for the degradation of proteins

A large fraction (30-50%) of the various proteins synthesized within isolated rat liver mitochondria were degraded to amino acids within 60 min after synthesis. Incomplete mitochondrial polypeptides resulting from the incorporation of puromycin were degraded even more extensively (80% per hr). Protein breakdown was measured by the appearance of acid-soluble radioactivity and by the disappearance of labeled polypeptides detected on NaDodSO4/polyacrylamide gel electrophoresis. The amino acids generated by proteolysis were transported rapidly out of the mitochondria and no peptide intermediates accumulated in the organelle. This degradative process did not involve lysosomes or lysosomal enzymes and was markedly stimulated by ATP either generated within the mitochondria or supplied exogenously. An inhibitor of respiration (cyanide) or uncouplers of oxidative phosphorylation (oligomycin, dinitrophenol) reduced proteolysis when mitochondria were provided substrates for ATP generation. When exogenous ATP was provided, these agents did not affect proteolysis, but degradation was then sensitive to atractyloside, an inhibitor of adenine nucleotide transport. Vanadate, an inhibitor of various ATPases, blocked proteolysis even in the presence of ATP and caused a marked stabilization of nearly all polypeptide bands. Thus, mitochondria--like bacteria or the cytosol of animal cells--contain a pathway for complete degradation of proteins which seems to selectively remove polypeptides with abnormal structures. Within this organelle, ATP hydrolysis appears necessary for an initial step in this degradative process.