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

Cloning and expression of cDNA for a newly identified isozyme of bovine liver 3-hydroxyacyl-CoA dehydrogenase and its import into mitochondria

cDNA for a heretofore undescribed mitochondrial 3-hydroxyacyl-CoA dehydrogenase, designated as the type II enzyme with different molecular and catalytic properties, compared to those of the classical mitochondrial beta-oxidation enzyme (type I enzyme), was cloned from a bovine liver cDNA library. Nucleotide sequence of the cDNA encoded 261 amino acids with a subunit molecular weight of 27,140. The deduced primary structure of the type II enzyme showed no significant homology to the reported amino acid sequence of the classical 3-hydroxyacyl-CoA dehydrogenases. On SDS-PAGE, no differences in subunit molecular weights were observed among the in vitro translation products, the recombinant type II enzyme produced in Escherichia coli and the purified enzyme. NH2-terminal and COOH-terminal amino acid sequence analysis of the purified type II enzyme revealed that the mature enzyme had not been proteolytically processed. The in vitro translation products of the type II enzyme were efficiently incorporated into isolated rat liver mitochondria, without changes in size, thereby suggesting that unlike other mitochondrial enzymes of fatty acid beta-oxidation, the type II enzyme had no cleavable signal peptide upon import into mitochondria.

The requirement of heat shock cognate 70 protein for mitochondrial import varies among precursor proteins and depends on precursor length

The cytosolic heat shock cognate 70-kDa protein (hsc70) is required for efficient import of ornithine transcarbamylase precursor (pOTC) into rat liver mitochondria (K. Terada, K. Ohtsuka, N. Imamoto, Y. Yoneda, and M. Mori, Mol. Cell. Biol. 15:3708-3713, 1995). The requirement of hsc70 for mitochondrial import of various precursor proteins and truncated pOTCs was studied by using an in vitro translation import system in which hsc70 was completely depleted. hsc70-dependent import of pOTC was about 60% of the total import, while import of the aspartate aminotransferase precursor, the serine:pyruvate aminotransferase precursor, and 3-oxoacyl coenzyme A thiolase was about 50, 30, and 0%, respectively. The subunit sizes of these four precursor proteins were 40 to 47 kDa. When pOTC was serially truncated from the COOH terminal, the hsc70 requirement decreased gradually and was not evident for the shortest truncated pOTCs of 90 and 72 residues. These truncated pOTCs were imported and proteolytically processed rapidly in 0.5 to 2 min at 25 degrees C, and the processed mature portions and the presequence portion were rapidly degraded. Sucrose gradient centrifugation analysis followed by import assay showed that pOTC synthesized in rabbit reticulocyte lysate forms an import-competent complex of about 11S in an hsc70-dependent manner. S values of import-competent forms of aspartate aminotransferase precursor, serine:pyruvate aminotransferase precursor, and 3-oxoacyl coenzyme A thiolase were 9S, 9S, and 4S, respectively. Thus, the S value decreased as the hsc70 dependency decreased. Precursor proteins were coimmunoprecipitated from the reticulocyte lysate containing the newly synthesized precursor proteins with an hsc70 antibody. The amount of coimmunoprecipitated proteins was much larger in the absence of ATP than in its presence. Among the four precursor proteins, the amount of coimmunoprecipitated protein decreased as the hsc70 dependency decreased.

Cloning of a human cDNA for protoporphyrinogen oxidase by complementation in vivo of a hemG mutant of Escherichia coli

Protoporphyrinogen oxidase (PPO; EC 1.3.3.4) is the enzyme that catalyzes in the penultimate step in the heme biosynthetic pathway. Hemes are essential components of redox enzymes, such as cytochromes. Thus, a hemG mutant strain of Escherichia coli deficient in PPO is defective in aerobic respiration and grows poorly even in rich medium. By complementation with a human placental cDNA library, we were able to isolate a clone that enhanced the poor growth of such a hemG mutant strain. The clone encoded the gene for human PPO. Sequence analysis revealed that PPO consists of 477 amino acids with a calculated molecular mass of 50.8 kilodaltons. The deduced protein exhibited a high degree of homology over its entire length to the amino acid sequence of PPO encoded by the hemY gene of Bacillus subtilis. The NH2-terminal amino acid sequence of the deduced PPO contains a conserved amino acid sequence that forms the dinucleotide-binding site in many flavin-containing proteins. Northern blot analysis revealed the synthesis of a 1.8-kilobase pair mRNA for PPO. A homogenate of the monkey kidney COS-1 cells that had been transfected with the cDNA had much higher PPO activity than an extract of control cells, and this activity was inhibited by acifluorfen, a specific inhibitor of PPO. Furthermore, the cDNA was expressed in vitro as 51-kilodalton protein, and after incubation with isolated mitochondria the protein was found to be located in the mitochondria, having just the same size as before, an indication that PPO is a mitochondrial enzyme and has no apparent transport-specific leader sequence.

Structurally different rat liver medium-chain acyl CoA dehydrogenases directed by complementary DNAs differing in their 5'-region

Different forms of rat liver medium-chain acyl CoA dehydrogenase (MCAD) (EC 1.3.99.3) were produced in Escherichia coli carrying expression plasmids (pRMCADm-1 approximately 9) differing at the 5'-region of the cDNA. The proteins expressed could be readily extracted from the cells. The protein (approximately 44 kDa) directed by pRMCADm-3 showed the highest activity and was readily purified to homogeneity. The purified enzyme contained non-covalently bound FAD and was similar to rat liver mitochondrial enzyme in all respects examined. The purified protein (approximately 45 kDa) directed by pRMCADm-1 did not contain FAD and showed no enzymatic activity. Therefore, the leader peptide disturbs the binding of FAD to the apoprotein. The purified protein (approximately 40 kDa) directed by pRMCADm-6 did not contain FAD. Thus, the deletion of the NH2-terminal portion of the apoprotein to some extent results in its inability to combine with FAD.

Mitochondrial protein import

A dynamic picture of the mitochondrial protein import pathway is emerging, with conformational alteration a critical feature both preceding and following membrane translocation. The mediators of these steps of conformational alteration, as well as steps of recognition, translocation, and proteolytic cleavage, appear to be proteins. Using powerful tools of genetics and biochemistry, in years to come it should be possible to determine the precise molecular function of these proteins in mediating these novel reactions.

A chimeric mitochondrial precursor protein with internal disulfide bridges blocks import of authentic precursors into mitochondria and allows quantitation of import sites

Bovine pancreatic trypsin inhibitor (which contains three intramolecular disulfide bridges) was chemically coupled to the COOH terminus of a purified artificial mitochondrial precursor protein. When the resulting chimeric precursor was presented to energized isolated yeast mitochondria, its trypsin inhibitor moiety prevented the protein from completely entering the organelle; the protein remained stuck across both mitochondrial membranes, with its NH2 terminus in the matrix and its trypsin inhibitor moiety still exposed on the mitochondrial surface. The incompletely imported protein appeared to "jam" mitochondrial protein import sites since it blocked import of three authentic mitochondrial precursor proteins; it did not collapse the potential across the mitochondrial inner membrane. Quantification of the inhibition indicated that each isolated mitochondrial particle contains between 10(2) and 10(3) protein import sites.

Subcellular distribution and properties of carbonyl reductase in guinea pig lung

On subcellular fractionation, carbonyl reductase (EC 1.1.1.184) activity in guinea pig lung was found in the mitochondrial, microsomal, and cytosolic fractions; the specific activity in the mitochondrial fraction was more than five times higher than those in the microsomal and cytosolic fractions. Further separation of the mitochondrial fraction on a sucrose gradient revealed that about half of the reductase activity is localized in mitochondria and one-third in a peroxidase-rich fraction. Although carbonyl reductase in both the mitochondrial and microsomal fractions was solubilized effectively by mixing with 1% Triton X-100 and 1 M KCl, the enzyme activity in the mitochondrial fraction was more highly enhanced by the solubilization than was that in the microsomal fraction. Carbonyl reductases were purified to homogeneity from the mitochondrial, microsomal, and cytosolic fractions. The three enzymes were almost identical in catalytic, structural, and immunological properties. Carbonyl reductase, synthesized in a rabbit reticulocyte lysate cell-free system, was apparently the same in molecular size as the subunit of the mature enzyme purified from cytosol. These results indicate that the same enzyme species is localized in the three different subcellular compartments of lung.

Identification of [1-14C]pantothenic-acid-mediated modified mitochondrial proteins

The in vivo administration of [1-14C]pantothenic acid, which is the precursor of coenzyme A, resulted in the radioactive labelling of several mitochondrial proteins in rat liver. The incorporated radioactivity could be released by glutathione or 2-mercaptoethanol. Two mitochondrial matrix proteins acetyl-CoA acetyltransferase (liver and heart), an enzyme involved in the biosynthesis or degradation of ketone bodies, and 3-oxoacyl-CoA thiolase (liver), a protein participating in fatty acid oxidation were identified as modified proteins. The radioactivity was localized exclusively in forms A1 and A2 indicating that these forms represent the modified states of the acetyl-CoA acetyltransferase protein. Kinetics of incorporation of radioactivity revealed an accumulation of the modified forms. The ratio of specific radioactivities of A2 compared to A1 was 2.41 +/- 0.15 (n = 10). After in vivo labelling with [14C]leucine, the specific radioactivity of acetyl-CoA acetyltransferase depended on the state of the enzyme protein. The unmodified enzyme exhibited a lower specific radioactivity than its modified forms suggesting different turnover rates of these proteins.

Uricoteley:its nature and origin during the evolution of tetrapod vertebrates

The hepatic mechanism for detoxication of ammonia formed during amino acid gluconeogenesis in uricotelic vertebrates requires the intramitochondrial synthesis of glutamine by glutamine synthetase. This glutamine then serves as a precursor of uric acid in the cytosol. The evolutionary development of uricoteley thus required the localization of glutamine synthetase in liver mitochondria. The mechanism for the mitochondrial import of glutamine synthetase in uricotelic vertebrate liver is not yet known. Tortoises, extant relatives of the stem reptiles, possess both the ureotelic and uricotelic hepatic systems. It therefore seems likely that the genetic events allowing the mitochondrial localization of glutamine synthetase in liver occurred in the amniote amphibian ancestors of the stem reptiles. The selection of ureoteley by the theropsids and of uricoteley by the sauropsids were major events in the divergence and subsequent evolution of these two lines. Once established in the sauropsid line, uricoteley has persisted through to the higher reptiles, crocodilians, and birds. Uricoteley was in part responsible for the radiation of the archosaurs during the Triassic as a water-conserving mechanism in the adult, thereby allowing them to invade the arid environments of that period. Contrary to dogma, uricoteley was probably of minor significance in the development of the cleidoic egg. Neither mammalian nor avian embryonic liver tissues catabolize amino acids to any great extent, so it is inappropriate to attribute to them a kind of "waste" nitrogen metabolism.

Apocytochrome-c competes with pre-ornithine carbamoyl transferase for transport into mitochondria

Apocytochrome c, the cytosolic precursor of cytochrome c, competes with the precursor of ornithine carbamoyltransferase (OCT) for entry into isolated rat liver mitochondria.

Enzyme activities in the androgenized rat uterus refractory to oestrogenic stimulation

Uterine enzymes involved in the intermediary metabolism of glucose have been measured in the androgenized rat in which there is evidence of diminution of the oestrogenic responses despite raised glycogen and glucose typical of maximal oestrogenic stimulation. Phosphofructokinase and isocitrate dehydrogenase (NADP, cytosolic) activities were significantly decreased in the androgenized rat and were elevated following treatment with natural progesterone and synthetic progestins which partially reverse the uterine abnormalities of the androgenized rat. Mitochondrial protein was decreased in the uterus of the androgenized rat but there was an apparent sparing effect on isocitrate (NAD) and malate (NAD) dehydrogenase. The data suggest that selective effects on specific enzymes involved in intermediary metabolism are a feature of the refractory state associated with constant oestrogenic stimulation. The possible cellular mechanisms underlying these effects are discussed.

In vitro synthesis of pig kidney general acyl CoA dehydrogenase

In vitro synthesis of general acyl CoA dehydrogenase [EC 1.3.99.3], one of the mitochondrial flavoenzymes, was carried out to elucidate its biosynthetic mechanism. Poly(A)+ RNA isolated from pig kidney was translated in vitro using wheat germ lysate system and the synthesized enzyme was immunoprecipitated by the antibody against purified pig kidney general acyl CoA dehydrogenase. The apparent molecular weight of the synthesized protein was estimated to be approximately 1,000 daltons larger than that of the mature enzyme, indicating that general acyl CoA dehydrogenase in pig kidney is synthesized as a precursor with a larger molecular weight.

Complete nucleotide and derived amino acid sequence of cDNA encoding the mitochondrial uncoupling protein of rat brown adipose tissue: lack of a mitochondrial targeting presequence

A cDNA clone spanning the entire amino acid sequence of the nuclear-encoded uncoupling protein of rat brown adipose tissue mitochondria has been isolated and sequenced. With the exception of the N-terminal methionine the deduced N-terminus of the newly synthesized uncoupling protein is identical to the N-terminal 30 amino acids of the native uncoupling protein as determined by protein sequencing. This proves that the protein contains no N-terminal mitochondrial targeting prepiece and that a targeting region must reside within the amino acid sequence of the mature protein.

How proteins get into microbodies (peroxisomes, glyoxysomes, glycosomes)

All microbody proteins studies, including one microbody membrane protein, are made on free polysomes and imported post-translationally. This holds for animal tissues, plants, and fungi. The majority of microbody protein sub-units are synthesized in a form not detectably different from mature sub-units. In five cases a larger precursor protein has been found. The position of the extra piece in this precursor is not known. In two of the five cases, processing of the precursor is not coupled to import; in the other three this remains to be determined. It is not even known whether information in the prepiece contributes to topogenesis, or serves other purposes. Microbody preparations from Neurospora, plant tissue and rat liver can take up some newly synthesized microbody proteins in vitro. In most cases uptake is inefficient. No special requirements for uptake have been established and whether a receptor is involved is not yet known. Several examples have been reported of peroxisomal enzymes with a counterpart in another cell compartment. With the exception of catalase, no direct evidence is available in any of these cases for two isoenzymes specified by the same gene. In the Zellweger syndrome, a lethal hereditary disease of man, characterized by a lack of peroxisomes, the levels of several enzymes of lipid metabolism are strongly decreased. In contrast, D-amino-acid oxidase, L-alpha-hydroxyacid oxidase and catalase levels are normal. The catalase resides in the cytosol. Since there is no separate gene for cytosolic catalase, the normal catalase levels in Zellweger cells show that some peroxisomal enzymes can mature and survive stably in the cytosol. It is possible that maturation of the peroxisomal enzyme in the cytoplasm can account for the finding of cytosolic catalase in some normal mammalian cells. The glycosomes of trypanosomes are microbodies that contain a glycolytic system. Comparison of the glycosomal phosphoglycerate kinase with its cytosolic counterpart has shown that these isoenzymes are 93% homologous in amino-acid sequence, but less than 50% homologous to the corresponding enzymes of yeast and mammals. This implies that few alterations are required to direct a protein into microbodies. This interpretation is supported by the evidence for homology between some microbody and mitochondrial isoenzymes in other organisms mentioned under point 4. The major changes of the glycosomal phosphoglycerate kinase relative to the cytosolic enzyme are a large increase in positive charge and a C-terminal extension of 20 amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

A metalloprotease involved in the processing of mitochondrial precursor proteins

A protease that cleaves the precursor of ornithine carbamoyltransferase (EC 2.1.3.3), a mitochondrial matrix enzyme, has been partially purified from the matrix fraction of rat liver mitochondria. The protease cleaved the precursors of several other matrix proteins at apparently correct sites. The protease was inhibited by 1,10-phenanthroline and EDTA, was reactivated by excess Mn2+ or Co2+, and did not cleave the alkali-denatured precursor proteins. These and other results indicate that this protease is responsible for the processing of at least several matrix protein precursors, and that the enzyme recognizes some three-dimensional conformation of the precursors as well as the amino acid sequences around the cleavage sites.

Molecular cloning of cDNA for rat mitochondrial 3-oxoacyl-CoA thiolase

Messenger RNA of rat 3-oxoacyl-CoA thiolase (acetyl-CoA acyltransferase), a mitochondrial matrix enzyme involved in fatty acid beta-oxidation, was enriched by immunoprecipitation of rat liver free polysomes and recombinant plasmids were prepared from the enriched mRNA by a modification of the vector-primer method of Okayama and Berg. The transformants were initially screened for 3-oxoacyl-CoA thiolase cDNA sequences by differential colony hybridization with [32P]cDNAs, synthesized from the immunopurified and unpurified mRNAs. The cDNA clones for 3-oxoacyl-CoA thiolase were identified by hybrid-arrested translation and hybrid-selected translation. One of the clones, designated pT1-1, contained a 700-base insert and hybridized to a mRNA species of 1.6 X 10(3) bases in rat liver. The transformants were rescreened using the cDNA insert of pT1-1 as a hybridization probe and a clone (pT1-19) with a 1.5 X 10(3)-base insert was obtained. Activity and concentration of 3-oxoacyl-CoA thiolase mRNA were quantified by in vitro translation and dot-blot analysis using the cDNA insert as a hybridization probe. The level of translatable and hybridizable mRNA in rat liver was increased about 5.1-fold and 4.6-fold, respectively, after administration of di-(2-ethylhexyl)phthalate, a potent inducer of the enzyme. The 3-oxoacyl-CoA thiolase mRNA levels thus determined correlated closely with levels of the activity and amount of this enzyme.

Control of 5-aminolevulinate synthase in animals

The proposed mechanism by which hepatic ALV-synthase mitochondrial levels are regulated is outlined in Fig. 2. ALV-synthase catalyzes the first and rate-limiting step in the heme pathway and is normally present in low amounts. A cytosolic, regulatory free heme pool tightly controls the amount of ALV-synthase in two ways. In the primary mechanism of regulation, heme is proposed to inhibit the synthesis of ALV-synthase mRNA. Most likely this would be mediated through the action of specific heme-binding protein(s) which recognize regulatory control regions of the ALV-synthase gene. Gene activity therefore is significantly repressed most of the time. When there is an increased demand for heme by newly synthesized cellular hemoproteins, the free heme pool is reduced, leading to a derepression of ALV-synthase mRNA synthesis. Once the need for increased heme synthesis is satisfied, inhibitory heme levels build up again. When drugs such as phenobarbital are administered to animals, there is a rapid induction in the liver of both cytochrome P-450 and ALV-synthase. It is proposed that the heme pool governing ALV-synthase levels is lowered by the increased heme demand due to cytochrome P-450 apoprotein formation. The primary event in the drug induction of ALV-synthase is therefore the increased synthesis of cytochrome P-450 apoprotein. However, the mechanism by which this occurs is unknown, although drugs do increase the synthesis of mRNA for cytochrome P-450 (Fig. 2). (There is evidence that for the aromatic hydrocarbons a specific cytosolic receptor exists.) In the acute hepatic porphyria diseases, uncontrolled synthesis of hepatic ALV-synthase occurs. The various forms are characterized by reduced levels of one of the heme pathway enzymes other than ALV-synthase. Attacks of the disease are commonly precipitated by drugs which induce cytochrome P-450, and the uncontrolled accumulation of ALV-synthase which accompanies these attacks results from the combined action of the block in the heme pathway and the increased cytochrome P-450 levels. A major challenge which now exists is to understand at the molecular level how the genes for ALV-synthase and cytochrome P-450 are regulated in the liver and other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)