Transport of the precursor for rat-liver ornithine carbamoyltransferase into mitochondria in vitro

Targeting of proteins to mitochondria

A clear picture has emerged over the past years on how a 'classic' mitochondrial protein, like subunit IV of cytochrome c oxidase, might be targeted to mitochondria. The targeting and subsequent import process involves the commitment of the TOM (translocase in the outer mitochondrial membrane) receptor complex on the mitochondrial surface, a TIM (translocase in the inner mitochondrial membrane) translocation complex in the mitochondrial inner membrane, and assorted chaperones and processing enzymes within the organelle. Recent work suggests that while very many mitochondrial precursor proteins might follow this basic targeting pathway, a large number have further requirements if they are to be successfully imported. These include ribosome-associated factors and soluble factors in the cytosol, soluble factors in the mitochondrial intermembrane space, an additional TIM translocase in the inner membrane and a range of narrow specificity assembly factors in the inner membrane. This review is focused on the targeting of proteins up to the stage at which they enter the TOM complex in the outer membrane.

The mitochondrial protein targeting suppressor (mts1) mutation maps to the mRNA-binding domain of Npl3p and affects translation on cytoplasmic polysomes

In all eukaryotic organisms, messenger RNA (mRNA) is synthesized in the nucleus and then exported to the cytoplasm for translation. The export reaction requires the concerted action of a large number of protein components, including a set of shuttle proteins that can exit and re-enter the nucleus through the nuclear pore complex. Here, we show that, in Saccharomyces cerevisiae, the shuttle protein Npl3p leaves the nuclear pore complex entirely and continues to function in the cytoplasm. A mutation at position 219 in its RNA-binding domain leaves Npl3p lingering in the cytoplasm associated with polysomes. Yeast cells expressing the mutant Npl3(L-219S) protein show alterations in mRNA stability that can affect protein synthesis. As a result, defects in nascent polypeptide targeting to subcellular compartments such as the mitochondria are also suppressed.

In vivo mitochondrial import. A comparison of leader sequence charge and structural relationships with the in vitro model resulting in evidence for co-translational import

The positive charges and structural properties of the mitochondrial leader sequence of aldehyde dehydrogenase have been extensively studied in vitro. The results of these studies showed that increasing the helicity of this leader would compensate for reduced import from positive charge substitutions of arginine with glutamine or the insertion of negative charged residues made in the native leader. In this in vivo study, utilizing the green fluorescent protein (GFP) as a passenger protein, import results showed the opposite effect with respect to helicity, but the results from mutations made within the native leader sequence were consistent between the in vitro and in vivo experiments. Leader mutations that reduced the efficiency of import resulted in a cytosolic accumulation of a truncated GFP chimera that was fluorescent but devoid of a mitochondrial leader. The native leader efficiently imported before GFP could achieve a stable, import-incompetent structure, suggesting that import was coupled with translation. As a test for a co-translational mechanism, a chimera of GFP that contained the native leader of aldehyde dehydrogenase attached at the N terminus and a C-terminal endoplasmic reticulum targeting signal attached to the C terminus of GFP was constructed. This chimera was localized exclusively to mitochondria. The import result with the dual signal chimera provides support for a co-translational mitochondrial import pathway.

Integration of porin synthesized in vitro into outer mitochondrial membranes

Porin, an intrinsic protein of outer mitochondrial membranes of rat liver, was synthesized in vitro in a cell-free in a cell-free translation system with rat liver RNA. The apparent molecular mass of porin synthesized in vitro was the same as that of its mature form (34 kDa). This porin was post-translationally integrated into the outer membrane of rat liver mitochondria when the cell-free translation products were incubated with mitochondria at 30 degrees C even in the presence of a protonophore (carbonyl cyanide m-chlorophenylhydrazone). Therefore, the integration of porin seemed to proceed energy-independently as reported by Freitag et al. [(1982) Eur. J. Biochem. 126, 197-202]. Its integration seemed, however, to require the participation of the inner membrane, since porin was not integrated when isolated outer mitochondrial membranes alone were incubated with the translation products. Porin in the cell-free translation products bound to the outside of the outer mitochondrial membrane when incubated with intact mitochondria at 0 degrees C for 5 min. When the incubation period at 0 degrees C was prolonged to 60 min, this porin was found in the inner membrane fraction, which contained monoamine oxidase, suggesting that porin might bind to a specific site on the outer membrane in contact or fused with the inner membrane (a so-called OM-IM site). This porin bound to the OM-IM site was integrated into the outer membrane when the membrane fraction was incubated at 30 degrees C for 60 min. These observations suggest that porin bound to the outside of the outer mitochondrial membrane is integrated into the outer membrane at the OM-IM site by some temperature-dependent process(es).

Chicken ornithine transcarbamylase: its unexpected expression

Ornithine transcarbamylase (OTC), one of the enzymes of the urea cycle, is detectable in some strains of chickens, although they have no functional urea cycle. The enzyme consists of three identical subunits of 36 kd and is present in mitochondria of the kidney. Using immunoabsorbent column chromatography, we found further evidence that the enzyme is detectable as a precursor form (40 kd) in chicken brain, heart, liver, pancreas, gizzard, small intestine, and breast muscle. When an extract of small intestine containing only precursor OTC was treated with a kidney extract, the precursor was converted into OTC. This suggests that there is a tissue-specific processing protease in the kidney which splits a peptide off the precursor, causing the expression of OTC activity in this organ. However, the reason why the enzyme or its precursor is expressed in these organs is not known. The results of this study suggest that, unlike mammals, chickens are more organ specific with regard to the ability to incorporate precursor OTC into mitochondria.

Import and processing of P-450SCC and P-450(11) beta precursors by corpus luteal mitochondria: a processing pathway recognizing homologous and heterologous precursors

Maturation of the precursor forms of bovine cholesterol side-chain cleavage cytochrome P-450 (P-450SCC) and 11 beta-hydroxylase cytochrome P-450 (P-450(11)beta) was investigated using mitochondria from bovine corpus luteum. The results show that both precursors, whose synthesis was directed by bovine adrenocortical RNA, can be imported and proteolytically processed to their corresponding mature forms by bovine corpus luteal mitochondria, even though P-450(11)beta is not expressed in this tissue. Furthermore, the efficiency of processing of pre-P-450(11)beta by corpus luteal mitochondria is similar to that of pre-P-450SCC, an endogenous enzyme of these mitochondria. However, the P-450(11)beta precursor is not processed by mitochondria from a nonsteroidogenic tissue (heart), a result observed previously for the P-450SCC precursor (M. F. Matocha and M. R. Waterman (1984) J. Biol. Chem. 259, 8672-8678). This discriminatory processing of pre-P-450(11)beta by heterologous mitochondria suggests that the precursor forms of P-450SCC and P-450(11)beta are processed via a common pathway in steroidogenic mitochondria and that this pathway is absent in nonsteroidogenic mitochondria.

Translocation of proteins into rat liver mitochondria. The precursor polypeptides of a large subunit of succinate dehydrogenase and ornithine aminotransferase and their imports into their own locations of mitochondria

The precursor polypeptides of a large subunit of succinate dehydrogenase and ornithine aminotransferase (the enzymes which are located in the mitochondrial inner membrane and matrix respectively) were synthesized as a larger molecular mass than their mature subunits, when rat liver RNA was translated in vitro. These precursor polypeptides were also detected in vivo in ascites hepatoma cells (AH-130 cells). When the 35S-labeled precursor polypeptides were incubated with isolated rat liver mitochondria at 30 degrees C in the presence of an energy-generating system, these two precursors were converted to their mature size and the 35S-labeled mature-size polypeptides associated with mitochondria. Furthermore, these mature-size polypeptides were recovered from their own locations, the inner mitochondrial membrane and the matrix. The precursor of ornithine aminotransferase incubated with rat liver mitochondria at 0 degree C was specifically and tightly bound to the surface of the mitochondria even in the presence of an uncoupler of oxidative phosphorylation. This precursor, bound to the mitochondria, was imported into the matrix when the mitochondria were reisolated and incubated at 30 degrees C in the presence of an energy-generating system, suggesting that a specific receptor may be involved in the binding of the precursor. The processing enzyme for both precursor polypeptides seemed to be located in the mitochondrial matrix and was partially purified from the mitochondria. A metal-chelating agent strongly inhibited the processing enzyme and the inhibition was recovered by the addition of Mn2+ or Co2+.

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.

Synthesis, intracellular transport and processing of mitochondrial urea cycle enzymes

Carbamyl phosphate synthetase I and ornithine transcarbamylase are matrix enzymes synthesized outside the mitochondria in the form of larger precursors and are transported rapidly into mitochondria, in association with post-translational proteolytic processing to the mature enzymes. Treatment of isolated rat hepatocytes with 40 micrograms/ml of rhodamine 123 resulted both in a potent inhibition of the processing of the enzyme precursors and in accumulation of the precursors. In pulse-chase experiments, the labeled precursor disappeared much more slowly in the presence of the dye. Rhodamine 123 strongly inhibited the uptake and processing of the ornithine transcarbamylase precursor by isolated rat liver mitochondria. Other positively charged rhodamines such as rhodamines 6G and 6GX were also strongly inhibitory. On the other hand, rhodamine B which has no net charge was much less inhibitory. These results suggest that the positively charged rhodamines inhibit the binding of the positively charged enzyme precursors to a negatively charged protein(s) or to phospholipids of the mitochondrial outer membrane. Potassium and magnesium ions, and probably a cytosolic protein(s), were required for the maximal uptake and processing of the ornithine transcarbamylase precursor by the isolated mitochondria. The concentrations of potassium and magnesium ions required for the maximal transport and processing were about 120 mM and 0.8-1.6 mM, respectively. Dialyzed postribosomal supernatant of rabbit reticulocyte lysate (36-72 mg protein/ml), in combination with potassium and magnesium ions, stimulated the precursor transport and processing 3- to 4-fold. The stimulatory activity of the dialyzed lysate was inactivated by trypsin treatment or heat treatment. No significant amount of the enzyme precursor was associated with the mitochondria when incubation was performed in the absence of these compounds. All these results indicate that potassium and magnesium ions, and probably a cytosolic protein(s), are required for the binding of the ornithine transcarbamylase precursor to the mitochondria or its transport into the organelle.

Mechanism of synthesis and localization of mitochondrial and cytosolic fumarases in rat liver

Fumarases in the mitochondrial and cytosolic fractions of rat liver were separately purified and crystallized. These two fumarases were not distinguishable in physicochemical, catalytic, or immunochemical properties. The sequences of seven amino acids in the C-terminal portions of the two fumarases were shown using carboxypeptidase P to be identical, i.e.-Val-Asp-Glu-Thr-Ala-Leu-Lys-. The amino acid sequence of the N-terminal portion of the mitochondrial fumarase was determined by the Edman method as Ala-Gln-Gln-Asn-Phe-Glu-Ile-Pro-Asp-, but that of the cytosolic fumarase could not be determined by the Edman method, since the N-terminal amino acid was blocked. The N-terminal amino acid of the cytosolic fumarase was identified as N-acetyl-alanine by analysis of the acidic amino acid produced by digestion of the enzyme protein with pronase E, carboxypeptidase A and B. Then the sequence of five amino acids in the N-terminal portion was determined by analyzing the acidic peptide obtained by limited proteolysis of the enzyme protein with carboxypeptidase A as Ac-Ala-Ser-Gln-Asn-Ser-. Peptide mapping of the tryptic peptides obtained from the mitochondrial and cytosolic fumarases showed no difference in the amino acid sequences of the two except in their N-terminal portions. The turnover rates of the mitochondrial and cytosolic fumarases were determined by injecting L-[U-14C]leucine into rat and following the decay of specific radioactivity incorporated into immunoprecipitates from the partially purified enzyme. The half-life of the cytosolic fumarase was estimated as 4.8 days from the decay curve of its specific radioactivity. The decay curve of the specific radioactivity of the mitochondrial fumarase, obtained after a single injection of L-[U-14]leucine, was quite unusual: its specific radioactivity remained constant for about 7 days after pulse labeling, and then decreased exponentially with a half-life of 9.7 days. Similar amounts of cytosolic and mitochondrial fumarase were found in the livers of the rat, mouse, rabbit, dog, chicken, snake, frog, and carp, respectively. Similar subcellular distributions of the enzyme were also found in the kidney, heart, and skeletal muscle of rats, and in hepatoma cells (AH-109A). However, in rat brain no fumarase activity was detected in the cytosolic fraction. Two putative precursor polypeptides of rat liver fumarase were synthesized when rat liver RNA was translated in vitro in a rabbit reticulocyte lysate system.(ABSTRACT TRUNCATED AT 400 WORDS)

Ornithine transcarbamylase deficiency: a case with a truncated enzyme precursor and a case with undetectable mRNA activity

The cell-free translation of ornithine transcarbamylase (OTC) mRNA from the livers of two heterozygous patients (from different families) with OTC deficiency was performed. The enzyme activities and the immunoreactive proteins in both patients were about 5% of those in controls. Immunoblotting assay of liver extracts from both patients showed decreased amounts of the OTC protein. The mRNA from the liver of patient 1 directed the synthesis of a very small amount of OTC precursor of normal subunit size (40,000 Da), whereas that from patient 2 directed the synthesis of small amounts of two distinct in vitro products; one was 40,000 Da and the other was about 30,000 Da. The in vitro product of normal precursor synthesized with mRNA from patient 2 was converted to mature-sized OTC by isolated rat liver mitochondria, whereas the smaller product was degraded during the incubation with the mitochondria. These results indicate that in both patients the translatable level of mRNA for active OTC from liver cells was much lower than that in the controls. The results also suggest that in patient 2, the smaller product presumably derived from an abnormal gene could not be transferred to the mitochondria.

Differential import and processing of the precursors to F1-ATPase beta-subunit and ornithine carbamyltransferase by liver, spleen, heart and kidney mitochondria

The cytoplasmically made subunits 2 (beta) and 3 (gamma) of the H+-ATPase from mammalian mitochondria are synthesized in vitro as larger polypeptides. In contrast, pre-cytochrome c could not, on the basis of its molecular weight, be distinguished from the mature polypeptide. This was shown by programming a reticulocyte lysate with rat heart RNA and immunoprecipitating the labeled translation products with polypeptide-specific antibodies. When a translated lysate containing the precursor to the beta-subunit was incubated with isolated rat spleen mitochondria, it was converted to the mature subunit and was no longer susceptible to externally added trypsin. The conversion to the mature form occurred in the absence of protein synthesis. This post-translational maturation process of the beta-subunit was more efficient when carried out with spleen or liver mitochondria than with heart or kidney mitochondria. The converse relative efficiency was observed when the processing of the precursor to ornithine carbamyltransferase by these mitochondria was examined. These results indicate that mitochondria do not discriminate against tissue-specific mitochondrial proteins. In addition, the observed varying degrees of efficiency of mitochondria from different tissues in importing and processing these two precursors suggest that the activity of precursor(s)-specific translocation-maturation systems varies between different types of mitochondria.

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.

Molecular cloning and nucleotide sequence of cDNA for rat ornithine carbamoyltransferase precursor

Messenger RNA of rat ornithine carbamoyltransferase (EC 2.1.3.3), a mitochondrial matrix enzyme, was enriched by immunoprecipitation of rat liver free polysomes, and recombinant plasmids were prepared from the enriched mRNA by a vector-primer method. The cDNA clones for ornithine carbamoyltransferase were identified by hybrid-arrested translation and hybrid-selected translation. One of the clones, designated pOTC-1, contained a 1.6-kilobase insert and hybridized to a mRNA of approximately equal to 1.8 kilobases in rat liver. The cDNA clone was subjected to nucleotide sequence analysis. The deduced amino acid sequence indicates that the ornithine carbamoyltransferase precursor consists of the mature enzyme of 322 amino acid residues and an NH2-terminal peptide extension (presequence) of 32 amino acid residues. The presequence contains 8 basic amino acid residues, no acidic residues, and no hydrophobic amino acid stretch. The amino acid sequence of the rat ornithine carbamoyltransferase was compared with the recently reported sequence of the human enzyme [Horwich, A. L., Fenton, W. A., Williams, K. R., Kalousek, F., Kraus, J. P., Doolittle, R. F., Konigsberg, W. & Rosenberg, L. E. (1984) Science 224, 1068-1074]. The sequences of the mature enzyme portion are 93% identical, whereas those of the presequences are 69% identical. There are two highly conserved segments in the presequences of the rat and human enzymes. One of the two conserved segments is significantly similar to a segment of the presequence of yeast mitochondrial elongation factor EF-Tu. These results suggest that the homologous segments are important for the proteins that are synthesized in the cytosol to be transported into the mitochondrial matrix.

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.

Isolation and characterization of a cDNA clone for bovine cytochrome c oxidase subunit IV

We have isolated a cDNA clone for the precursor to subunit IV of bovine cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1). A cDNA library was constructed from poly(A)+ RNA of adult beef liver by insertion of cDNA into the plasmid vector pBR322. Transformants were screened by colony hybridization with two mixtures of [32P]-labeled synthetic oligodeoxyribonucleotides. We screened 20,000 transformants with a mixture of heptadecamers complementary to all 16 possible sequences encoding amino acids 98-103 and obtained two cDNA clones encoding subunit IV amino acid sequences. We determined the DNA sequence of the larger (416 base-pair) insert, which contains the coding sequence for amino acids 1-107 of the mature protein and an NH2-terminal extension (presequence). The deduced amino acid sequence of the mature protein is identical with the previously determined protein sequence: the sequence of the NH2-terminal extension contains a potential initiator methionine at amino acid -22 from the NH2-terminus of the processed protein. The presequence is quite basic and contains several arginines, including one at the processing site. No hydrophobic region analogous to that found in bacterial and eukaryotic signal peptides is present, but there are homologies with other mitochondrial protein presequences, which may include a common signal for their destination and processing.

Molecular basis of ornithine transcarbamylase deficiency lacking enzyme protein

We report an ornithine transcarbamylase(OTC)-deficient male patient who had no detectable immunoreactive materials but did have active mRNA for OTC-related protein. The total absence of OTC activity in the liver of the patient was caused by a complete lack of immunoreactive material, as determined by Ouchterlony double immunodiffusion, single radial immunodiffusion, and sodium dodecylsulphate-polyacrylamide gel electrophoresis of immunoprecipitate and of liver homogenate. However, mRNA coding for the precursor of OTC was clearly detected in autopsy specimens of the patient's liver as well as of controls in a cell-free translation system consisting of rabbit reticulocyte lysates and [35S]methionine. The labelled precursor of OTC synthesized in vitro with mRNA from the patient could be transported into rat liver and kidney mitochondria and processed to form a protein with a molecular weight indistinguishable from mature OTC, suggesting that there was no defect in the protein structure necessary for its transport into mitochondria. These results suggest that the primary defect of the OTC deficiency was located in the structural gene and that the labile OTC-related protein, after being synthesized with its mRNA, was degraded too rapidly to be detected by the method used.

Cell-free synthesis and transport of precursors of mutant ornithine carbamoyltransferases into mitochondria

Synthesis, mitochondrial transport and processing of ornithine carbamoyltransferase (EC 2.1.3.3) were studied in mutant mice strains (sparse-fur, spf, and sparse-fur with abnormal skin and hair, spf-ash) which exhibit a deficiency in this enzyme. Spf mice have an increased amount (about 150% of control) of the enzyme with abnormal kinetic properties, whereas spf-ash mice have a decreased amount (about 10% of control) of the enzyme with apparently normal kinetic properties. Precursors of the mutant enzymes were synthesized in a reticulocyte lysate cell-free system. The hepatic level of translatable mRNA coding for the enzyme and the rate of the enzyme synthesis in liver slices of spf mice were 58 and 60% of the controls, respectively. In the case of spf-ash mice the activity of translatable mRNA for the enzyme was 10% of the controls. These results indicate that the decreased amount of ornithine carbamoyltransferase protein in spf-ash mice is due mainly to a decreased level of translatable mRNA for the enzyme, whereas the increase in the enzyme amount in spf mice is presumably the result of a decreased rate of enzyme degradation. The subunit molecular weight of the spf enzyme precursor was practically the same as that of the normal enzyme precursor (Mr 40 000). Both precursors synthesized in vitro could be taken up and processed similarly to an apparently mature form (Mr 37 000). In the case of spf-ash enzyme, two discrete in vitro products were observed on sodium dodecyl sulfate polyacrylamide gel; one comigrated with the normal enzyme precursor and the other moved slightly slower. Both products appeared to be taken up and processed to the mature form of the enzyme.

Transport of proteins into mitochondria: a high conservation of precursor uptake and processing system

Ornithine transcarbamylase (EC 2.1.3.3) of rat (Rattus norvegicus var. albus) liver, a urea cycle enzyme, is synthesized extramitochondrially as a larger precursor which is transported posttranslationally into mitochondria and processed to the mature enzyme. The precursor synthesized in vitro was taken up and processed to the mature enzyme by isolated pigeon (Columba livia var. domestica) liver and frog (Rana catesbeiana) liver mitochondria. Carp (Cyprinus carpio) liver mitochondria could also process the precursor. These results indicate that the mitochondrial transport and processing activities are conserved between mammalian and bird, amphibian or fish systems. However, attempts to demonstrate the precursor uptake and processing by Saccharomyces cerevisiae mitochondria were unsuccessful.

Immunological evidence for an ornithine transcarbamylase lesion resulting in the formation of enzyme with smaller protein subunits

An unusual form of ornithine transcarbamylase deficiency was found in a male child who became unconscious at 8 months. Two maternal uncles had died during similar illnesses at 6 years and 11 years, respectively. Detailed studies of the enzyme showed 10% residual activity, a very low Km for carbamyl phosphate (0.015 mmol/l) and near normal amounts of immunoreactive protein with a smaller than normal subunit (molecular weight 37 800 instead of 39 700). This information was obtained from a 10 mg liver biopsy core using protein separation on SDS-polyacrylamide gel, electrophoretic transfer to nitrocellulose filters and probing with antibody to the enzyme. Resolution of the exact mutation causing this change will be of interest to those who are studying the processing of mitochondrial enzymes during transport from the cytoplasm.

Membrane potential (delta psi) depolarizing agents inhibit maturation

Precursor forms of exported proteins were first accumulated in the envelope of phenethyl alcohol (PEA)-treated cells. After removal of PEA, a complete processing could be obtained in a few minutes. In this work, we demonstrate that colicins A and E1, that act on the electrical gradient in the cytoplasmic membrane, prevent the processing of precursor forms previously accumulated. Concentrations of colicins accounting for approximately 1 killing unit (50--3000 molecules/cell) were found to be sufficient for inhibition of processing. Therefore our results strongly suggest that in intact cells the electrical gradient across the cytoplasmic membrane is required for maturation of exported proteins.