Effect of deletions within the leader peptide of pre-ornithine transcarbamylase on mitochondrial import

Multiple 40-kDa heat-shock protein chaperones function in Tom70-dependent mitochondrial import

Mitochondrial preproteins that are imported via the translocase of the mitochondrial outer membrane (Tom)70 receptor are complexed with cytosolic chaperones before targeting to the mitochondrial outer membrane. The adenine nucleotide transporter (ANT) follows this pathway, and its purified mature form is identical to the preprotein. Purified ANT was reconstituted with chaperones in reticulocyte lysate, and bound proteins were identified by mass spectrometry. In addition to 70-kDa heat-shock cognate protein (Hsc70) and 90-kDa heat-shock protein (Hsp90), a specific subset of cochaperones were found, but no mitochondria-specific targeting factors were found. Interestingly, three different Hsp40-related J-domain proteins were identified: DJA1, DJA2, and DJA4. The DJAs bound preproteins to different extents through their C-terminal regions. DJA dominant-negative mutants lacking the N-terminal J-domains impaired mitochondrial import. The mutants blocked the binding of Hsc70 to preprotein, but with varying efficiency. The DJAs also showed significant differences in activation of the Hsc70 ATPase and Hsc70-dependent protein refolding. In HeLa cells, the DJAs increased new protein folding and mitochondrial import, although to different extents. No single DJA was superior to the others in all aspects, but each had a profile of partial specialization. The Hsp90 cochaperones p23 and Aha1 also regulated Hsp90-preprotein interactions. We suggest that multiple cochaperones with similar yet partially specialized properties cooperate in optimal chaperone-preprotein complexes.

Human CIA30 is involved in the early assembly of mitochondrial complex I and mutations in its gene cause disease

In humans, complex I of the respiratory chain is composed of seven mitochondrial DNA (mtDNA)-encoded and 38 nuclear-encoded subunits that assemble together in a process that is poorly defined. To date, only two complex I assembly factors have been identified and how each functions is not clear. Here, we show that the human complex I assembly factor CIA30 (complex I intermediate associated protein) associates with newly translated mtDNA-encoded complex I subunits at early stages in their assembly before dissociating at a later stage. Using antibodies we identified a CIA30-deficient patient who presented with cardioencephalomyopathy and reduced levels and activity of complex I. Genetic analysis revealed the patient had mutations in both alleles of the NDUFAF1 gene that encodes CIA30. Complex I assembly in patient cells was defective at early stages with subunits being degraded. Complementing the deficiency in patient fibroblasts with normal CIA30 using a novel lentiviral system restored steady-state complex I levels. Our results indicate that CIA30 is a crucial component in the early assembly of complex I and mutations in its gene can cause mitochondrial disease.

Hsp90 functions in the targeting and outer membrane translocation steps of Tom70-mediated mitochondrial import

The Tom70 import receptor on the mitochondrial outer membrane specifically recognizes Hsp90 and Hsc70, a critical step for the import of mitochondrial preproteins, the targeting of which depends on these cytosolic chaperones. To analyze the role of Hsp90 in mitochondrial import, the effects of the Hsp90 inhibitors geldanamycin and novobiocin were compared. Geldanamycin occludes the N-terminal ATP-binding site of Hsp90, whereas novobiocin targets the C-terminal region of the chaperone. Here, novobiocin was found to inhibit preprotein import and, in particular, targeting to the purified cytosolic fragment of Tom70. Hsp90 cross-linking to preprotein and coprecipitation of Hsp90 with Tom70 were both impaired by novobiocin. Overall, novobiocin treatment increased preprotein aggregation, contributing to reduced import competence. In contrast, geldanamycin had no apparent effect on preprotein interactions with Hsp90, formation of preprotein-chaperone complexes, Hsp90 docking onto Tom70, or preprotein association with the outer membrane. Instead, geldanamycin impaired formation of preprotein import intermediates at the outer membrane. This suggests a novel active role for Hsp90 in import steps subsequent to Tom70 targeting. Our results outline the mechanisms of Hsp90 function in preprotein targeting and transport.

Molecular chaperones Hsp90 and Hsp70 deliver preproteins to the mitochondrial import receptor Tom70

The role of cytosolic factors in protein targeting to mitochondria is poorly understood. Here, we show that in mammals, the cytosolic chaperones Hsp90 and Hsp70 dock onto a specialized TPR domain in the import receptor Tom70 at the outer mitochondrial membrane. This interaction serves to deliver a set of preproteins to the receptor for subsequent membrane translocation dependent on the Hsp90 ATPase. Disruption of the chaperone/Tom70 recognition inhibits the import of these preproteins into mitochondria. In yeast, Hsp70 rather than Hsp90 is used in import, and Hsp70 docking is required for the formation of a productive preprotein/Tom70 complex. We outline a novel mechanism in which chaperones are recruited for a specific targeting event by a membrane-bound receptor.

Differences in the human and mouse amino-terminal leader peptides of ornithine transcarbamylase affect mitochondrial import and efficacy of adenoviral vectors

Mouse models of ornithine transcarbamylase (OTC) deficiency are being used to test the efficacy of viral vectors as possible vehicles for gene therapy. However, it has been demonstrated that virus containing the human OTC cDNA failed to express functional OTC enzyme in the recipient animals. Because functional OTC is assembled as a homotrimer in the mitochondria, there are at least two possible explanations for these results. Either endogenous mutant protein coassembles with the human OTC and has a "dominant-negative effect," or the human version of the protein is not appropriately imported or processed in the mouse mitochondria. To test the importance of processing, which in rodents is thought to depend on the leader peptide, adenoviral vectors containing chimeric OTC cDNAs were prepared. These vectors were evaluated in the OTC-deficient sparse fur mouse models. Although comparable levels of transgene expression were observed in all groups of mice, the only mice that had high levels of OTC activity and mitochondrial OTC immunoreactivity were those mice injected with the vectors containing the mouse leader peptide (mouse OTC and a mouse-human chimera of OTC). To address possible dominant-negative effects, adenoviruses containing mutant human or mouse OTC cDNAs were prepared and evaluated in cell lines or normal C3H mice, respectively. No inhibition of normal OTC activity was observed in either model system. Together, these studies provide no evidence of a dominant-negative effect and suggest that the human and rodent enzymes responsible for transporting of OTC and possibly other mitochondrial proteins have different specificity.

hTom34: a novel translocase for the import of proteins into human mitochondria

Most mitochondrial proteins are nuclear encoded, synthesized on cytosolic ribosomes, and imported into the mitochondria. We have identified and characterized a 309 amino acid human protein with a molecular weight of 34 kDa that functions as a subunit of the translocase for the import of such proteins. hTom34 (34-kDa Translocase of the Outer Mitochondrial Membrane) is displayed on the surface of mitochondria and is resistant to extraction under alkaline conditions. Antibodies raised against hTom34 specifically inhibit in vitro import of the mitochondrial precursor protein preornithine transcarbamylase into mitochondria isolated from rat liver. Based on trypsin digestion experiments, the receptor has a large (27 kDa) C-terminal domain exposed to the cytosol. This novel component of the protein import machinery possesses a 62 residue motif conserved with the Tom70 family of mitochondrial receptors but otherwise appears to have no counterpart so far characterized in the mitochondria of any other species.

Transient expression of a mitochondrial precursor protein. A new approach to study mitochondrial protein import in cells of higher eukaryotes

In order to study mitochondrial protein import in the context of whole cell metabolism, we have used the transfection technique based on Semliki Forest virus (SFV) to express a mitochondrial precursor protein within BHK21 cells and human fibroblasts. Recombinant SFV particles mediate a highly efficient, transient transfection of higher eukaryotic cells. The mitochondrial precursor protein used is a fusion protein consisting of the mitochondrial targeting sequence of Neurospora crassa ATPase subunit 9 and mouse dihydrofolate (H2folate) reductase. Transfected BHK21 cells synthesized substantial amounts of subunit-9-H2folate-reductase. Immunofluorescence staining revealed that the protein colocalized with the mitochondria. The precursor protein was processed to the intermediate and mature form, implying that is was successfully imported into the mitochondrial matrix. Import was dependent on a proton gradient across the mitochondrial membranes since uncoupling of oxidative phosphorylation inhibited the process. The mature-sized protein was folded into a protease-resistant conformation. These results indicate that, in mammalian cells, transport of the precursor subunit-9-H2folate-reductase into mitochondria and its subsequent maturation occurs in a similar way as in lower eukaryotes. Import and processing of the fusion protein proceeded very rapidly in BHK21 cells but were substantially slower in human fibroblasts. SFV-mediated transfection proved to be excellently suited to study protein import into mitochondria of living cells and is probably applicable to transport studies with other organelles as well. The approach could also be helpful in the diagnosis of hereditary disorders or organelle protein import.

A receptor for the import of proteins into human mitochondria

We have characterised a 16.3-kDa human protein that functions as a receptor for the import of preproteins into mitochondria. Based on amino acid sequence alignments, the protein (hMas20p) is 41% similar to Mas20p (20-kDa mitochondrial assembly protein) from yeast Saccharomyces cerevisiae and 38% similar to MOM19 (19-kDa mitochondrial outer-membrane protein) from Neurospora crassa. hMas20p has a putative N-terminal transmembrane sequence of 29 amino acids and an acidic C-terminus. A 13-kDa fragment [des-(1-29)-hMas20p], which lacks the 29-amino acid putative N-terminal transmembrane domain, is soluble when expressed in Escherichia coli. Antibodies produced against this domain crossreacted with a protein of 16 kDa in outer membranes of mitochondria from rat liver and inhibited import of protein into isolated mitochondria from rat liver. In addition, the recombinant soluble domain folds into a functional structure as it competes with hMas20p on the mitochondrial surface for precursor binding, confirming the functional role of hMas20p in the import of preproteins into mitochondria.

Solution structure of the acetylated and noncleavable mitochondrial targeting signal of rat chaperonin 10

Chaperonin 10 (Cpn10) is one of only a few mitochondrial matrix proteins synthesized without a cleavable targeting signal. Using a truncated form of Cpn10 and synthetic peptides in mitochondrial import assays, we show that the N-terminal region is both necessary and sufficient for organellar targeting in vitro. To elucidate the structural features of this topogenic signal, peptides representing residues 1-25 of rat Cpn10 were synthesized with and without the naturally occurring N-terminal acetylation. 1H NMR spectroscopy in 20% CF3CH2OH,H2O showed that both peptides assume a stable helix-turn-helix motif and are highly amphiphilic in nature. Chemical shift and coupling constant data revealed that the N-terminal helix is stabilized by N-acetylation, whereas NOE and exchange studies were used to derive a three dimensional structure for the acetylated peptide. These findings are discussed with respect to a recent model predicting that targeting sequences forming a continuous alpha-helix of more than 11 residues cannot adopt a conformation necessary for proteolysis by the matrix located signal peptidases (Hammen, P. K., Gorenstein, D. G., and Weiner, H. (1994) Biochemistry 33, 8610-8617).

Isolation of a cDNA clone specifying rat chaperonin 10, a stress-inducible mitochondrial matrix protein synthesised without a cleavable presequence

We have isolated a cDNA clone encoding chaperonin 10 from rat liver. The cDNA specifies a protein of 102 amino acids which, when transcribed and translated in vitro, yields a single basic product (pI > 9) that co-migrates exactly with the heat shock inducible cpn10 of rat hepatoma cells during 2D gel-electrophoresis. It is concluded that cpn10, unlike the majority of nuclear-encoded proteins of the mitochondrial matrix, is synthesised without a cleavable targeting signal and that, following removal of the initiating methionine, it becomes acetylated prior to mitochondrial import. Incubation of 3H- or 35S-labelled cpn10 with mitochondria confirms these conclusions and shows that cpn10 is imported into mitochondria in an energy-dependent process which is inhibited by the presence of 2,4-dinitrophenol.

Prechaperonin 60 and preornithine transcarbamylase share components of the import apparatus but have distinct maturation pathways in rat liver mitochondria

Mitochondrial preornithine transcarbamylase (p-OTC) and premalate dehydrogenase (p-MDH) are the only two matrix-located preproteins so far identified for which the proteolytic processing in vitro requires the formation of genuine processing intermediates, i-OTC and i-MDH, respectively. To establish the processing of other preproteins during import with respect to the two-step processing of p-OTC and p-MDH, the chelators EDTA and 1,10-phenanthroline were used to study the import and processing of rat prechaperonin 60 (p-cpn60) and p-OTC by mitochondria from four cpn60-containing organs. We found no evidence for a secondary processing step in the maturation of p-cpn60, but a clear requirement for two-step processing of p-OTC, even in three organs which do not contain ornithine transcarbamylase. The metal-ion requirement of the p-OTC processing activities in the organelle is consistent with the proposition that the mitochondrial processing protease (MPP) and mitochondrial intermediate peptidase (MIP) activities defined in vitro [Kalousek, F., Hendrick, J.P. & Rosenberg, L. E. (1988) Proc. Natl Acad. Sci. USA 85, 7536-7540] are responsible for precursor processing in vivo. The authenticity of two-step processing in vivo was, furthermore, established by demonstrating that i-OTC accumulates to high levels in Spodoptora frugiperda insect cells supplemented with MnCl2. The inability of the insect cells to process p-OTC fully is not a characteristic of cells grown in culture since cultured rat hepatoma cells process p-OTC to the fully processed m-OTC. Finally, we find that the import and processing of p-cpn60 and p-OTC is inhibited in an identical fashion by presequence-bovine-serum-albumin conjugates. The differences in proteolytic maturation between p-cpn60 and p-OTC are therefore not likely to result from different import pathways as the two precursors compete for common components of the import apparatus.

Identification of a mammalian 10-kDa heat shock protein, a mitochondrial chaperonin 10 homologue essential for assisted folding of trimeric ornithine transcarbamoylase in vitro

We have identified a 10-kDa stress-inducible mitochondrial protein. The protein is synthesized at elevated rates in cultured rat hepatoma cells challenged with heat shock or amino acid analogues and, therefore, designated heat shock protein 10 (Hsp10). Hsp10 was purified to homogeneity from rat liver and found to exhibit a native molecular mass of 65 kDa, as opposed to a monomeric molecular mass of 10,813.4 +/- 0.41 Da. The amino acid sequence of rat Hsp10 disclosed extensive sequence similarity with bacterial chaperonin (Cpn) 10. Rat Hsp10 and Escherichia coli Cpn60 were used to reconstitute functional trimeric rat ornithine transcarbamoylase from a chemically denatured state with high efficiency. This process depended completely upon rat Hsp10 and was abolished in the presence of a nonhydrolyzable ATP analogue. We conclude that Hsp10 is a eukaryotic Cpn10 homologue and, therefore, together with Cpn60 essential for mitochondrial protein biogenesis. The Cpn-mediated protein-folding apparatus, thus, exhibits a high degree of conservation between prokaryotes and mitochondria of higher eukaryotes.

High-level expression of a mitochondrial enzyme, ornithine transcarbamylase from rat liver, in a baculovirus expression system

The mitochondrial enzyme, ornithine transcarbamylase (OTC) from rat liver was expressed in Spodoptera frugiperda (Sf) insect cells using a baculovirus vector. When insect cells were infected with recombinant Autographica californica nuclear polyhedrosis virus (AcNPV) containing a cDNA encoding the precursor form of OTC (pOTC) inserted into the polyhedrin gene, they expressed catalytically active enzyme at levels of approximately 2.5 micrograms/10(6) cells. About 25% of the active enzyme was a novel, partially processed product of pOTC containing four extra amino acids at the amino terminus of OTC. The most abundant protein found in mitochondria from infected insect cells was the normal processing intermediate iOTC, which contains 8 extra amino acids at the amino terminus of OTC. Whereas this species, present at 20 micrograms/10(6) cells, was not active and did not bind the transition-state analog inhibitor of OTC, delta-PALO, the novel processing product did bind and was affinity-purified, along with mature OTC, on a PALO-affinity column. The OTC expressed in insect cells was located in the same compartment of the mitochondrion as in rat liver. The incomplete processing occurred in vitro in both noninfected and infected insect cells. The high level of expression of iOTC using the baculoviral expression system provides a means of overproducing an obligatory intermediate in the mitochondrial import process.

Complementation of the Aspergillus nidulans arg B1 mutation by ornithine transcarbamylase cDNA from rat liver

An Aspergillus nidulans strain which is deficient in ornithine transcarbamylase due to the arg B1 mutation was transformed with a plasmid containing the ornithine transcarbamylase cDNA from rat liver under the control of the amd S promoter. Stable transformants were obtained by selection on arginine free medium indicating complementation of the arg B mutation. Proof of expression of the rat enzyme in transformants was obtained by immunoprecipitation of all ornithine transcarbamylase activity from cell extracts with antibodies specific for the rat enzyme. The presence of catalytically active rat ornithine transcarbamylase in the transformants indicated that it is capable of being imported into mitochondria in A. nidulans, proteolytically processed and assembled into its homotrimeric form. In vitro uptake experiments using isolated A. nidulans mitochondria demonstrate that processing of the precursor of rat ornithine transcarbamylase occurs in two temporally separated steps as it does in rat liver mitochondria suggesting evolutionary conservation of the processing machinery. Up to 560 ng of active rat enzyme was produced per gm wet weight mycelia. Use of beta-D-alanine, an inducer of amd S, as sole N-source resulted in increased levels of active rat ornithine transcarbamylase relative to uninduced cultures.

Mitochondrial import and processing of mutant human ornithine transcarbamylase precursors in cultured cells

We have investigated mitochondrial import and processing of the precursor for human ornithine transcarbamylase (OTC; carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3) in HeLa cells stably transformed with cDNA sequences encoding OTC precursors carrying mutations in their leader peptides. The mutant precursors studied included two with amino acid substitutions in the 32-amino-acid leader peptide (glycine for arginine at position 23, designated gly23; glycines for arginines at positions 15, 23, and 26, designated gly15,23,26) and two with deletions (deletion of residues 8 to 22, designated d8-22; deletion of residues 17 to 32, designated N16). Specific immunoprecipitation with anti-OTC antiserum of extracts of L-[35S]methionine-labeled cells expressing these mutations yielded only precursor species; neither mature nor intermediate-size OTC subunits were observed. Fractionation of radiolabeled cells, however, revealed important differences among the various mutants: the gly23 precursor was associated with mitochondria and was not detected in the cytosol; the d8-22 and N16 precursors were found with both the mitochondrial fraction and the cytosol; only the gly15,23,26 precursor was detected exclusively in the cytosol. A large fraction of each of the mitochondrially associated OTC species was in a trypsin-protected compartment. In particular, the gly23 precursor behaved in trypsin protection and mitochondrial fractionation studies in a manner consistent with its translocation into the mitochondrial matrix. On the other hand, the lack of binding of the gly23 protein to a delta-N-phosphonoacetyl-L-ornithine affinity column, which specifically recognizes active OTC enzyme, indicated that, despite its intramitochondrial location, the mutant protein did not assemble into the normal, active trimer. Further, the gly23 mutant precursor was unstable within the mitochondria and was degraded with a t1/2 of less further than 4 h. Thus, we have shown that, in intact HeLa cells, cleavage of the OTC leader peptide is not required for translocation into mitochondria, but is required for assembly into active enzyme.

Studies on the biogenesis of the mammalian ATP synthase complex: isolation and characterization of a full-length cDNA encoding the rat F1-beta-subunit

Screening a rat cDNA library with a fragment of the gene encoding the corresponding bovine protein, we have isolated cDNA clones encoding the complete precursor to the rat F1-beta-subunit. Comparison with other mammalian beta-subunit sequences reveals a 5' untranslated sequence of 27 bp, a targeting sequence of 48 amino acid residues, a 480 amino acid long mature polypeptide and a 3' untranslated region of approximately 150 bp. In vitro translation of F1-beta mRNA reveals the synthesis of multiple fragments of the protein. Preliminary characterization of intron-containing cDNA isolates indicates that the gene encoding the rat F1-beta subunit contains at least two introns.

Mitochondrial protein import

Most mitochondrial proteins are synthesized as precursor proteins on cytosolic polysomes and are subsequently imported into mitochondria. Many precursors carry amino-terminal presequences which contain information for their targeting to mitochondria. In several cases, targeting and sorting information is also contained in non-amino-terminal portions of the precursor protein. Nucleoside triphosphates are required to keep precursors in an import-competent (unfolded) conformation. The precursors bind to specific receptor proteins on the mitochondrial surface and interact with a general insertion protein (GIP) in the outer membrane. The initial interaction of the precursor with the inner membrane requires the mitochondrial membrane potential (delta psi) and occurs at contact sites between outer and inner membranes. Completion of translocation into the inner membrane or matrix is independent of delta psi. The presequences are cleaved off by the processing peptidase in the mitochondrial matrix. In several cases, a second proteolytic processing event is performed in either the matrix or in the intermembrane space. Other modifications can occur such as the addition of prosthetic groups (e.g., heme or Fe/S clusters). Some precursors of proteins of the intermembrane space or the outer surface of the inner membrane are retranslocated from the matrix space across the inner membrane to their functional destination ('conservative sorting'). Finally, many proteins are assembled in multi-subunit complexes. Exceptions to this general import pathway are known. Precursors of outer membrane proteins are transported directly into the outer membrane in a receptor-dependent manner. The precursor of cytochrome c is directly translocated across the outer membrane and thereby reaches the intermembrane space. In addition to the general sequence of events which occurs during mitochondrial protein import, current research focuses on the molecules themselves that are involved in these processes.

Mitochondrial import and processing of mutant human ornithine transcarbamylase precursors in cultured cells

We have investigated mitochondrial import and processing of the precursor for human ornithine transcarbamylase (OTC; carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3) in HeLa cells stably transformed with cDNA sequences encoding OTC precursors carrying mutations in their leader peptides. The mutant precursors studied included two with amino acid substitutions in the 32-amino-acid leader peptide (glycine for arginine at position 23, designated gly23; glycines for arginines at positions 15, 23, and 26, designated gly15,23,26) and two with deletions (deletion of residues 8 to 22, designated d8-22; deletion of residues 17 to 32, designated N16). Specific immunoprecipitation with anti-OTC antiserum of extracts of L-[35S]methionine-labeled cells expressing these mutations yielded only precursor species; neither mature nor intermediate-size OTC subunits were observed. Fractionation of radiolabeled cells, however, revealed important differences among the various mutants: the gly23 precursor was associated with mitochondria and was not detected in the cytosol; the d8-22 and N16 precursors were found with both the mitochondrial fraction and the cytosol; only the gly15,23,26 precursor was detected exclusively in the cytosol. A large fraction of each of the mitochondrially associated OTC species was in a trypsin-protected compartment. In particular, the gly23 precursor behaved in trypsin protection and mitochondrial fractionation studies in a manner consistent with its translocation into the mitochondrial matrix. On the other hand, the lack of binding of the gly23 protein to a delta-N-phosphonoacetyl-L-ornithine affinity column, which specifically recognizes active OTC enzyme, indicated that, despite its intramitochondrial location, the mutant protein did not assemble into the normal, active trimer. Further, the gly23 mutant precursor was unstable within the mitochondria and was degraded with a t1/2 of less further than 4 h. Thus, we have shown that, in intact HeLa cells, cleavage of the OTC leader peptide is not required for translocation into mitochondria, but is required for assembly into active enzyme.

The presequence of yeast 5-aminolevulinate synthase is not required for targeting to mitochondria

A truncated form of the yeast mitochondrial 5-aminolevulinate (ALA) synthase was constructed by deletion of the first 75 amino acid residues of its precursor form. This truncated ALA synthase which lost its entire presequence and 40 residues of the mature part possesses a new amino terminus quite different from a typical mitochondrial presequence. This modified protein expressed in vivo is found entirely located within mitochondria. Although it was now unable to reach the matrix space, it was internalized as shown by its resistance to protease in isolated mitochondria. Pulse-chase radiolabeling in the presence of an uncoupler suggests that a membrane potential is not required for the targeting of this truncated ALA synthase. Thus, the amino-terminal signal, if indispensable as a matrix targeting signal, could be replaced by an internal sequence or a particular folding for recognition by the import machinery.

Transcending the impenetrable: how proteins come to terms with membranes

In the living cell, proteins are efficiently sorted to a whole range of subcellular compartments. In many cases, sorting specificity is mediated by short 'sorting signals' attached either permanently or transiently to the protein. At long last, a fairly coherent picture of the design and function of many such sorting signals is beginning to emerge.

Import of rat ornithine transcarbamylase precursor into mitochondria: two-step processing of the leader peptide

The mitochondrial matrix enzyme ornithine transcarbamylase (OTC) is synthesized on cytoplasmic polyribosomes as a precursor (pOTC) with an NH2-terminal extension of 32 amino acids. We report here that rat pOTC synthesized in vitro is internalized and cleaved by isolated rat liver mitochondria in two, temporally separate steps. In the first step, which is dependent upon an intact mitochondrial membrane potential, pOTC is translocated into mitochondria and cleaved by a matrix protease to a product designated iOTC, intermediate in size between pOTC and mature OTC. This product is in a trypsin-protected mitochondrial location. The same intermediate-sized OTC is produced in vivo in frog oocytes injected with in vitro-synthesized pOTC. The proteolytic processing of pOTC to iOTC involves the removal of 24 amino acids from the NH2 terminus of the precursor and utilizes a cleavage site two residues away from a critical arginine residue at position 23. In a second cleavage step, also catalyzed by a matrix protease, iOTC is converted to mature OTC by removal of the remaining eight residues of leader sequence. To define the critical regions in the OTC leader peptide required for these events, we have synthesized OTC precursors with alterations in the leader. Substitution of either an acidic (aspartate) or a "helix-breaking" (glycine) amino acid residue for arginine 23 of the leader inhibits formation of both iOTC and OTC, without affecting translocation. These mutant precursors are cleaved at an otherwise cryptic cleavage site between residues 16 and 17 of the leader. Interestingly, this cleavage occurs at a site two residues away from an arginine at position 15. The data indicate that conversion of pOTC to mature OTC proceeds via the formation of a third discrete species: an intermediate-sized OTC. The data suggest further that, in the rat pOTC leader, the essential elements required for translocation differ from those necessary for correct cleavage to either iOTC or mature OTC.