In vitro synthesis and posttranslational uptake of cytochrome c into isolated mitochondria: role of a specific addressing signal in the apocytochrome

Transport of proteins into mitochondrial matrix. Evidence suggesting a common pathway for 3-ketoacyl-CoA thiolase and enzymes having presequences

Rat liver 3-ketoacyl-CoA thiolase, a mitochondrial matrix enzyme which catalyzes a step of fatty acid beta-oxidation, was synthesized in a rabbit reticulocyte lysate cell-free system. The in vitro product was apparently the same in molecular size and charge as the subunit of the mature enzyme. The enzyme synthesized in vitro was transported into isolated rat liver mitochondria in an energy-dependent manner. In pulse experiments with isolated rat hepatocytes at 37 degrees C, the radioactivity of the newly synthesized enzyme in the cytosolic fraction remained essentially unchanged during 5-20 min of incubation, whereas that of the enzyme in the particulate fraction increased with time during the incubation. The pulse-labeled enzyme disappeared with an apparent half-life of less than 3 min from the cytosolic fraction, in pulse-chase experiments. Purified 3-ketoacyl-CoA thiolase inhibited the mitochondrial uptake and processing of the precursors of the other matrix enzymes, ornithine carbamoyltransferase, medium-chain acyl-CoA dehydrogenase and acetoacetyl-CoA thiolase. These results indicate that 3-ketoacyl-CoA thiolase has an internal signal which is recognized by the mitochondria and suggest that this enzyme and the three others are transported into the mitochondria by a common pathway.

The oxidation of exogenous cytochrome c by mitochondria. Resolution of a long-standing controversy

Several reports in the past have dealt with the oxidation of cytochrome c added to suspensions of rat liver mitochondria. Yet, it is generally believed that the cytochrome cannot penetrate the outer membrane. Probably it has been assumed that the permeability of the outer membrane to cytochrome c is very low but finite, and that fast oxidation may be observed if time is allowed for sufficient penetration before initiation of electron flow. Here we show that this view is false. The main fraction of rat liver mitochondria, as isolated by conventional procedures, does not catalyse any significant oxidation of added cytochrome c, even after prolonged incubation. The observed appreciable oxidation of added cytochrome c is catalysed by a very small fraction (5-12%) of the mitochondria that apparently has a damaged outer membrane. Consequently, the turnover of cytochrome oxidase is very high in this fraction during oxidation of added cytochrome c. This finding readily explains why Moyle and Mitchell (e.g., FEBS Lett. 88 (1978) 268-272; 90 (1978) 361-365) have failed to observe proton translocation by cytochrome oxidase during oxidation of ferrocytochrome c added to rat liver mitochondria, which has been their main reason for rejecting the proton-pumping function of cytochrome oxidase.

Developmental expression of nuclear genes that encode mitochondrial proteins: insect cytochromes c

To investigate tissue-specific developmental regulation of mitochondrial biogenesis, we studied the expression of the Manduca sexta (tobacco hornworm moth) thoracic muscle cytochrome c gene during adult eclosion and used this information to obtain a cDNA clone for this gene, which in turn was used to isolate the corresponding Drosophila melanogaster gene. Over the 3 days prior to adult Manduca emergence, mitochondrial inner membranes become progressively more electron dense and lamellar, and, while there is no accumulation of apocytochrome c, the amount of the holoprotein increases 40-fold per insect thorax. As determined by in vitro translation and blot hybridization analysis, the major thoracic muscle cytochrome c gene is primarily regulated at the transcriptional level, with cytochrome c mRNA increasing from less than 0.01% to 0.04% of total poly(A)+ RNA and declining to an undetectable level by day 2 after eclosion. Furthermore, the ratio of cytochrome c to the other cytochromes remains the same at all times, indicating that these components of the respiratory chain follow coordinated developmental programs. By using polysome immunoadsorption, a poly(A)+ RNA population of greater than or equal to 95% cytochrome c mRNA was isolated from thoracic muscle tissue and was used to construct a cDNA library, which was screened by hybrid selection/translation. We report the sequence of one of those clones, pMSc750, and its use to isolate the major thoracic muscle cytochrome c gene of Drosophila.

Molecular cloning and sequencing of cDNA for yeast porin, an outer mitochondrial membrane protein: a search for targeting signal in the primary structure

We have cloned a full-length cDNA for yeast porin, the major outer mitochondrial membrane protein from Saccharomyces cerevisiae, and determined its nucleotide sequence. The primary structure of the protein, deduced from the nucleotide sequence, consisted of 283 amino acid residues and its NH2-terminal sequence, Met-Ser-Pro-Pro-Val-Tyr-Ser, coincided with that determined by Edman degradation for yeast porin, except that the initiator methionine was missing in the mature protein. The deduced sequence had an overall polarity index of 46.3%, a value which falls in the normal range for soluble proteins. An evaluation of hydropathy of the protein indicated that the NH2-terminal one third was relatively hydrophilic and the rest of the molecule was rather hydrophobic. An interesting finding was that the NH2-terminal region of yeast porin (consisting of some 50 amino acid residues) shows structural features that resemble those of the corresponding portion of 70-kd protein, which is also a yeast outer mitochondrial membrane protein. We postulate that this NH2-terminal sequence, like that of 70-kd protein, is required for targeting the porin to the outer mitochondrial membrane.

Import and processing of cytochrome b-c1 complex subunits in isolated hepatoma ascites cells. Inhibition by Rhodamine 6G

The import and processing of cytochrome c1 and the iron sulfur protein of the cytochrome b-c1 complex were studied in Zajdela hepatoma ascites cells. Both peptides were synthesized as larger percursor molecules which were approximately 2-3 kDa and 5-6 kDa larger than the mature forms of apocytochrome c1 and apo-iron sulfur protein, respectively. Comparison of these precursors to those reported for functionally homologous peptides in yeast and Neurospora indicate significant size changes have occurred in mammals. Rhodamine 6G, a specific vital stain for mitochondria, is a potent inhibitor of precursor processing in isolated hepatoma cells. Both precursor to cytochrome c1 and precursor to FeS accumulate in the soluble and particulate fractions obtained by digitonin treatment of tumor cells treated with Rhodamine 6G. Appearance of the mature peptides was abolished. The precursors are unstable, however, and disappear from the cytosolic and membrane fractions during a 10 min chase. Comparison of the effects of Rhodamine 6G and carbonylcyanide m-chlorophenylhydrazone on precursor processing shows that: (a) Rhodamine 6G is a more effective inhibitor of processing, (b) it has less of an inhibitory effect on cellular protein synthesis, and (c) it inhibits processing under conditions in which it appears to have little influence on coupled respiration in whole cells. The data suggest that the most likely mode of action of Rhodamine 6G is on the matrix processing step.

Transport of proteins into mitochondria

There is still much that is obscure concerning the transport of proteins into or through the mitochondrial membrane systems. In addition, as pointed out previously, it is unlikely that the details of the process are the same for proteins destined for different compartments of the organelle. A brief summary of the process for matrix proteins might be as follows: The proteins are synthesized on free polysomes as precursors of higher molecular weight than the native forms. These precursors are liberated into the cell cytosol and subsequently translocated into the mitochondria. This timing might be different in yeast under some circumstances, synthesis being completed in association with the mitochondria. The precursors interact with a receptor in the outer mitochondrial membrane interaction being mediated by the presequences of the precursors. The presequences therefore act as addressing signals as well as possibly playing a role in one or all of (a) solubilization of precursors, (b) prevention of premature assembly into multimeric structures, or (c) maintenance of nonnative configurations required for transport. Interaction occurs with a second receptor, this time in the inner membrane of the mitochondria, interaction being with multiple sites in the polypeptide chain. Transport across the inner membrane then occurs, this transport depending on a transmembrane electrochemical gradient of which the proton component is the essential part. Transport is accompanied or followed by proteolysis of the prepiece, and formation of the native structure. While steps 1 and 2 of this sequence can be considered well established, the remaining steps are still poorly understood or purely hypothetical. Nevertheless, this sequence of events is consistent with known facts about the process and provides a framework for future investigations.

Receptor sites involved in posttranslational transport of apocytochrome c into mitochondria: specificity, affinity, and number of sites

Assembly of cytochrome c involves a series of steps: synthesis of apocytochrome c on free ribosomes, specific binding of apocytochrome c to the mitochondrial surface, transfer across the outer membrane, covalent addition of protoheme, refolding of the polypeptide chain, and association of holocytochrome c with its functional sites at the inner membrane. The binding step of apocytochrome c to Neurospora crassa mitochondria was studied by inhibiting the subsequent transfer steps with the heme analogue deuterohemin. The binding sites are highly specific for mitochondrial apocytochromes c. Bound labeled Neurospora apocytochrome c was competitively displaced by unlabeled apocytochrome c from various species. These exhibited different abilities for displacement. Apocytochrome c from Paracoccus denitrificans, the amino-terminal (heme-binding) fragment of Neurospora apocytochrome c, and Neurospora holocytochrome c did not recognize the binding sites. Polylysine did not interfere with apocytochrome c binding. Apocytochrome c is reversibly bound. The binding sites are present in limited number. High-affinity binding sites were present at about 90 pmol/mg of mitochondrial protein. They displayed an association constant of 2.2 X 10(7) M-1. Apocytochrome c was imported into mitochondria and converted to holocytochrome c directly from the binding sites when inhibition by deuterohemin was relieved. We conclude that the apocytochrome c binding sites on mitochondria represent receptors that function in the recognition and import of this precursor by mitochondria.

Purification of 5-aminolaevulinate synthase from liver mitochondria of chick embryo

5-Aminolaevulinate synthase from chick-embryo liver mitochondria has, for the first time, been purified to homogeneity in its native non-degraded form by molecular sieve chromatography, chromatofocusing and affinity chromatography. The enzyme has a minimum molecular weight of 68000 as determined by sodium dodecylsulphate/polyacrylamide gel electrophoresis and a specific activity of 35000 units/mg of protein. This result conflicts with the previous report of Whiting, M.J. and Granick, G. [(1976) J. Biol. Chem. 251, 1340-1346] that the chick embryo enzyme has a molecular weight of 49000. We show here that the purified form can be degraded proteolytically to a smaller form of molecular weight around 50000 while retaining full enzymatic activity. It seem evident, therefore, that the enzyme isolated by Whiting & Granick (1976) was degraded. We have further established by pulse-labelling studies and immunoprecipitation that the enzyme isolated by our new and rapid procedure has the same minimum molecular weight as that which exists in vivo.

Yeast mitochondrial outer membrane specifically binds cytoplasmically-synthesized precursors of mitochondrial proteins

The precursor of cytochrome b(2) (a cytoplasmically-synthesized mitochondrial protein) binds to isolated mitochondria or to isolated outer membrane vesicles. Binding does not require an energized inner membrane, is diminished by trypsin treatment of the membranes and is not observed with the partially processed (intermediate) form of the cytochrome b(2) precursor or with non-mitochondrial proteins. Upon energization of the mitochondria, the bound precursor is imported and cleaved to the mature form. Similar results were obtained with the precursor of citrate synthase. This receptor-like binding activity was present in isolated outer, but not inner membrane. It was solubilized from outer membrane with non-ionic detergent and reconstituted into liposomes.

Import of proteins into mitochondria: a 70 kilodalton outer membrane protein with a large carboxy-terminal deletion is still transported to the outer membrane

The yeast mitochondrial outer membrane contains a major 70-kd protein which is coded by a nuclear gene. Two forms of this gene were isolated from a yeast genomic clone bank: the intact gene, and a truncated gene which had lost a large part of its 3' end during the cloning procedure. Upon transformation into yeast, both the intact and the truncated gene are expressed; the truncated gene generates a shortened protein missing 203 amino acids from the carboxy-terminus. This truncated polypeptide reacts with a monoclonal antibody against the authentic 70-kd protein and is transported to the mitochondrial outer membrane. By integrative transformation, we have constructed a yeast mutant which lacks the 70-kd protein and is unable to adapt to growth on a nonfermentable carbon source at 37 degrees C. This phenotypic lesion can be corrected by transforming the mutant with the intact, but not the truncated gene. The carboxy-terminal sequence of 203 amino acids is thus necessary for the function of the protein, but not for its targeting to the mitochondrion.

cDNA cloning and analysis of chick-embryo-liver cytochrome P-450 mRNA induced by porphyrinogenic drugs

Hepatic microsomal cytochrome P-450 was elevated in 17-day chick embryos by combined administration of the porphyrinogenic drugs 2-allyl-2-isopropylacetamide and 3,5-diethoxycarbonyl-1,4-dihydrocollidine. Increased apoprotein levels were the result of de novo protein synthesis; in vitro obtained translation data suggested that cytochrome P-450 mRNA levels were elevated. A 1000-base cDNA sequence for the drug-induced cytochrome P-450 mRNA was isolated from a chick embryo cDNA 'library' and this was used as a specific probe to investigate drug-mediated induction of cytochrome P-450 mRNA. RNA-DNA 'dot' hybridisation studies demonstrated that drug treatment led to a 3-5-fold increase in the level of this mRNA and that the mRNA was predominantly associated with membrane-bound polyribosomes. Treatment of embryos with the drugs individually demonstrated that both of them induced synthesis of the same mRNA. These studies show directly that treatment of chick embryos with 2-allyl-2-isopropylacetamide or 3,5-diethoxycarbonyl-1,4-dihydrocollidine caused increased levels of cytochrome P-450 mRNA and suggest that this involved increased transcription of the gene.

Ornithine transcarbamylase in liver mitochondria

Ornithine transcarbamylase (ornithine carbamoyltransferase, EC 2.1.3.3), the second enzyme of urea synthesis, is localized in the matrix of liver mitochondria of ureotelic animals. The enzyme is encoded by a nuclear gene, synthesized outside the mitochondria, and must then be transported into the organelle. The rat liver enzyme is initially synthesized on membrane-free polysomes in the form of a larger precursor with an amino-terminal extension of 3 400-4 000 daltons. In rat liver slices and isolated rat hepatocytes, the pulse-labeled precursor is first released into the cytosol and is then transported with a half life of 1-2 min into the mitochondria where it is proteolytically processed to the mature form of the enzyme. The precursor synthesized in vitro exists in a highly aggregated form and has a conformation different from that of the mature enzyme. The precursor has an isoelectric point (pI = 7.9) higher than that of the mature enzyme (pI = 7.2). The precursor synthesized in vitro can be taken up and processed to the mature enzyme by isolated rat liver mitochondria. The mitochondrial transport and processing system requires membrane potential and a high integrity of the mitochondria. The transport and processing activities are conserved between mammals and birds or amphibians and is presumably common to more than one precursor. Potassium ion, magnesium ion, and probably a cytosolic protein(s), in addition to the transcarbamylase precursor and the mitochondria, are required for the maximal transport and processing of the precursor. A mitochondrial matrix protease which converts the precursor to a product intermediate in size between the precursor and the mature subunit has been highly purified. The protease has an estimated molecular weight of 108 000 and an optimal pH of 7.5-8.0, and appears to be a metal protease. The protease does not cleave several of the protein and peptide substrates tested. The role of this protease in the precursor processing remains to be elucidated. Rats subjected to different levels of protein intake and to fasting show significant changes in the level of enzyme protein and activity of ornithine transcarbamylase. The dietary-dependent changes in the enzyme level are due mainly to an altered level of functional mRNA for the enzyme. In contrast, during fasting, the increase in the enzyme level is associated with a decreased level of translatable mRNA for the enzyme. Pathological aspects of ornithine transcarbamylase including the enzyme deficiency and reduced activities of the enzyme in Reye's syndrome are also described. A possibility that impaired transport of the enzyme precursor into the mitochondria leads to a reduced enzyme activity, is proposed.

Biosynthesis of mitochondrial porin and insertion into the outer mitochondrial membrane of Neurospora crassa

Mitochondrial porin, the major protein of the outer mitochondrial membrane is synthesized by free cytoplasmic polysomes. The apparent molecular weight of the porin synthesized in homologous or heterologous cell-free systems is the same as that of the mature porin. Transfer in vitro of mitochondrial porin from the cytosolic fraction into the outer membrane of mitochondria could be demonstrated. Before membrane insertion, mitochondrial porin is highly sensitive to added proteinase; afterwards it is strongly protected. Binding of the precursor form to mitochondria occurs at 4 degrees C and appears to precede insertion into the membrane. Unlike transfer of many precursor proteins into or across the inner mitochondrial membrane, assembly of the porin is not dependent on an electrical potential across the inner membrane.

Requirement of a membrane potential for the posttranslational transfer of proteins into mitochondria

Posttranslational transfer of most precursor proteins into mitochondria is dependent on energization of the mitochondria. Experiments were carried out to determine whether the membrane potential or the intramitochondrial ATP is the immediate energy source. Transfer in vitro of precursors to the ADP/ATP carrier and to ATPase subunit 9 into isolated Neurospora mitochondria was investigated. Under conditions where the level of intramitochondrial ATP was high and the membrane potential was dissipated, import and processing of these precursor proteins did not take place. On the other hand, precursors were taken up and processed when the intramitochondrial ATP level was low, but the membrane potential was not dissipated. We conclude that a membrane potential is involved in the import of those mitochondrial precursor proteins which require energy for intracellular translocation.