The precursor to ornithine carbamyl transferase is transported to mitochondria as a 5S complex containing an import factor

Interactions of the human mitochondrial protein import receptor, hTom20, with precursor proteins in vitro reveal pleiotropic specificities and different receptor domain requirements

Tom20 is part of a multiple component, dynamic complex that functions to import specific cytosolic proteins into or through the outer membrane of the mitochondrion. To analyze the contribution of Tom20 to precursor protein recognition, the cytosolic domain of the human mitochondrial import receptor, hTom20, has been expressed as a fusion protein with glutathione S-transferase and conditions established to measure specific interactions of the receptor component with precursor proteins in vitro. Reconstitution of receptor binding from purified components revealed that a prototypic matrix-destined precursor protein, pODHFR, interacts with Tom20 by a mechanism that is dependent on an active matrix targeting signal but does not require cytosolic components or ATP. Binding was influenced by both salt concentration and detergent. The effect of salt or detergent, however, varied for different precursor proteins. In particular, detergent selectively enhanced binding of pODHFR to receptor, possibly because of induced changes in the structure of the signal sequence. Finally, mutations were introduced into hTom20 which had a dramatic effect on binding of some precursor proteins but not on others. Taken together, the results suggest that hTom20 recognizes and physically interacts with precursor proteins bearing a diverse array of topogenic sequences and that such pleiotropic specificity for these precursor proteins may involve different domains within the receptor molecule.

Protein sorting between mitochondrial outer and inner membranes. Insertion of an outer membrane protein into the inner membrane

The amino terminal 29 amino acids of the outer mitochondrial membrane protein of yeast, OMM70 (MAS70), consisting of the targeting and membrane anchor domains, has been fused to a reporter protein, dihydrofolate reductase. The hybrid protein, designated pOMD29, was efficiently imported into the outer membrane of rat heart mitochondria by a process dependent on ATP and proteinase-sensitive components on the surface of the organelle, and in which the orientation of the native protein was retained. To determine if the protein translocation machinery of the inner membrane is also capable of recognizing and inserting pOMD29, direct access to the intermembrane space was provided to pOMD29 by selectively rupturing the mitochondrial outer membrane by osmotic shock. In this system, the outer membrane binding site for matrix-destined precursor proteins can be bypassed, and efficient import restored to proteinase-pretreated mitochondria. pOMD29 was imported into the inner membrane of osmotically-shocked mitochondria, mediated by protein components. The outer membrane orientation of pOMD29 was conserved when inserted into the inner membrane but, unlike the outer membrane, import into the inner membrane required delta psi. We conclude that the protein translocation machinery of the mitochondrial inner membrane is capable of recognizing and inserting a protein whose topogenic information otherwise results in insertion of the protein to the outer membrane. The significance of these findings for sorting of proteins between the mitochondrial inner and outer membranes is discussed.

Molecular analysis of a human liver mitochondrial ornithine transcarbamylase deficiency

The liver of a young girl which had been successfully transplanted was investigated at the ornithine transcarbamylase (OTC, EC 2.1.3.3) gene expression level. Northern blot hybridization using a human OTC cDNA probe showed a greater than 80% decrease in specific OTC mRNA although having the same molecular size as a normal control. OTC polypeptide was simultaneously synthesized with a normal molecular size but at a low level (20%) as shown by immunoblotting. The OTC enzyme from the deficient liver exhibited very little catalytic activity (7.2% as compared to the normal subject). These results may support several explanations of this disease such as mutation of the OTC gene promoter leading to a low transcriptional activity or mutation of the encoding sequence which results in a modified translation product but with a normal size. mRNA instability may also occur.

Purification and identification of a cytosolic factor required for import of precursors of mitochondrial proteins into mitochondria

A cytosolic factor required for import of the precursor of mitochondrial protein into mitochondria was purified to homogeneity from a rabbit reticulocyte lysate by affinity column chromatography using a synthetic peptide containing the presequence of ornithine amino-transferase as a ligand. The molecular mass of the purified protein was estimated as 28 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The import of precursors of ornithine aminotransferase and sulfite oxidase into mitochondria was inhibited by anti-28-kDa protein IgG raised in guinea pigs. This antibody also blocked the binding of these precursors to mitochondria. These results suggest that the 28-kDa protein is an essential component of the import machinery in the cytosol and that anti-28-kDa protein IgG blocked the binding of the precursor of ornithine aminotransferase to mitochondria, but not the penetration step. Therefore, the 28-kDa protein may be a factor that should be named the "targeting factor" for import of mitochondrial protein.

Presence of the cytosolic factor stimulating the import of precursor of mitochondrial proteins in rabbit reticulocytes and rat liver cells

Previously we purified a cytosolic factor that stimulates the import of the extrapeptide (the synthetic peptide of the presequence of ornithine aminotransferase) into the mitochondrial matrix (Ono, H., and Tuboi, S., 1988, J. Biol. Chem. 263, 3188-3193). In this work this cytosolic factor was shown also to stimulate the import of the precursors of ornithine aminotransferase, a large subunit of succinate dehydrogenase, and sulfite oxidase. The amounts of these precursors bound to the outer mitochondrial membrane were increased by this cytosolic factor, suggesting that the cytosolic factor participates in the recognition step in the import process of the precursor protein. When the cytosolic factor was applied to an ATP-agarose column, the import-stimulating activity was recovered entirely in the unadsorbed fraction. Immunochemical studies showed that in these conditions the 70-kDa heat shock-related protein (Hsp 70) was present exclusively in the fraction adsorbed to the ATP-agarose column. The cytosolic factor is thus different from the 70-kDa heat shock-related protein, which was identified as a factor required for the import of mitochondrial proteins in yeast. The cytosolic factor was also detected in the cytosol of rat liver cells, and a considerable amount of this factor was recovered from rat liver mitochondria by washing them with high salt buffer, suggesting that the cytosolic factor has affinity to the outer mitochondrial membrane and binds to its receptor on the membrane. From these results, we conclude that the cytosolic factor forms a complex with the precursor of mitochondrial protein and then this complex binds to the outer mitochondrial membrane, probably via the receptor of the cytosolic factor.

Protein sorting to mitochondria: evolutionary conservations of folding and assembly

According to the endosymbiont hypothesis, mitochondria have lost the autonomy of their prokaryotic ancestors. They have to import most of their proteins from the cytosol because the mitochondrial genome codes for only a small percentage of the polypeptides that reside in the organelle. Recent findings show that the sorting of proteins into the mitochondrial subcompartments and their folding and assembly follow principles already developed in prokaryotes. The components involved may have structural and functional equivalents in bacteria.

In vitro transport of ornithine carbamoyltransferase precursor into rat liver mitochondria using a more homologous medium

The precursor of ornithine carbamoyltransferase can be transported in vitro into rat liver mitochondria using the postmitochondrial supernatant from rat liver, a more homologous medium than the commonly used rabbit reticulocyte lysate. The transport of the precursor in the case of reticulocyte lysate requires a standard translation mixture. In the presence of the postmitochondrial supernatant the same is true. However, when the components of the translation mixture were added individually to the postmitochondrial supernatant, it was found that spermidine or spermine, at physiological concentrations, sufficed for the transport of the precursor of ornithine carbamoyltransferase. The activity of the postmitochondrial supernatant was inactivated by trypsin and slightly decreased by RNase treatment; it was not lost by dialysis or by heating at 100 degrees C.

Import of a mutant mitochondrial precursor fails to respond to stimulation by a cytosolic factor

Import of the precursor to ornithine carbamyltransferase is stimulated by a partially-purified, NEM-sensitive soluble factor from rabbit reticulocyte lysate. A mutant in which the carboxy-terminal 73 amino acids were deleted, had a sharply reduced response to this factor. The NEM-sensitive, import-stimulating factor interacts with the surface of mitochondria in the absence of precursor protein. Thus reticulocyte lysate contains an NEM-sensitive, import stimulating factor which interacts both with the surface of mitochondria and whose activity appears to be dependent upon the structure of the mature portion of the precursor.

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.

70-kD heat shock-related protein is one of at least two distinct cytosolic factors stimulating protein import into mitochondria

We have developed an in vitro system in which the posttranslational import of Put2 (delta-pyrroline-5-carboxylate dehydrogenase), into yeast mitochondria is dependent on the addition of yeast postribosomal supernatant (PRS). When mRNA for a nuclear-encoded yeast mitochondrial matrix protein, Put2, was translated in a wheat germ cell-free system, import into posttranslationally added yeast mitochondria was negligible. However, when a yeast PRS was added, significant import was observed. The import stimulating activity of the yeast PRS was shown to consist of at least two distinct factors. One of these is the recently purified 70-kD heat shock-related protein Ssalp/Ssa2p, two proteins that are 98% homologous. The other factor is an N-ethylmaleimide-sensitive protein(s). Both factors act synergistically.

Spermidine and spermine stimulate the transport of the precursor of ornithine carbamoyltransferase into rat liver mitochondria

We have examined the effect of low molecular weight components of the transport mixture generally used for the import of rat liver pre-ornithine carbamoyltransferase by isolated rat liver mitochondria. These studies revealed that spermidine and spermine, at physiological concentrations, stimulate the transport of the precursor of ornithine carbamoyltransferase into mitochondria. This stimulatory effect of spermidine and spermine is concentration-dependent and is completely inhibited at higher than physiological concentrations (20 mM for spermidine and 4 mM for spermine). Magnesium ions, which also have a stimulatory effect, inhibit the stimulatory effect of spermidine.

Mitochondrial protein import: nucleoside triphosphates are involved in conferring import-competence to precursors

The role of nucleoside triphosphates (NTPs) in mitochondrial protein import was investigated with the precursors of N. crassa ADP/ATP carrier, F1-ATPase subunit beta, F0-ATPase subunit 9, and fusion proteins between subunit 9 and mouse dihydrofolate reductase. NTPs were necessary for the initial interaction of precursors with the mitochondria and for the completion of translocation of precursors from the mitochondrial surface into the mitochondria. Higher levels of NTPs were required for the latter reactions as compared with the early stages of import. Import of precursors having identical presequences but different mature protein parts required different levels of NTPs. The sensitivity of precursors in reticulocyte lysate to proteases was decreased by removal of NTPs and increased by their readdition. We suggest that the hydrolysis of NTPs is involved in modulating the folding state of precursors in the cytosol, thereby conferring import competence.

A signal sequence domain essential for processing, but not import, of mitochondrial pre-ornithine carbamyl transferase

Studies using deletion mutagenesis indicate that a processing recognition site lies proximal to the normal cleavage position between gln32 and ser33 of pre-ornithine carbamyl transferase (pOCT). pOCT cDNA was manipulated to delete codons specifying amino acids 22-30 of the signal sequence. The mutant precursor, designated pOCT delta 22-30, was imported to the matrix compartment by purified mitochondria, but remained largely unprocessed; the low level of processing that was observed did not involve the normal cleavage site. Several manipulations performed downstream of the normal pOCT processing site (deletion, substitution, and hybrid protein constructions) affected neither import nor correct processing. Our data suggest that domains specifying import and processing site recognition may be functionally segregated within the signal peptide; that processing is not requisite for import of pOCT; and that a proximal region, not involving the normal signal peptide cleavage site, is required for processing site recognition.

Biosynthesis of four rat liver mitochondrial acyl-CoA dehydrogenases: in vitro synthesis, import into mitochondria, and processing of their precursors in a cell-free system and in cultured cells

The synthesis, translocation, processing, and assembly of rat liver short chain acyl-CoA, medium chain acyl-CoA, long chain acyl-CoA, and isovaleryl-CoA dehydrogenases were studied. These four acyl-CoA dehydrogenases are homotetrameric flavoproteins which are located in the mitochondrial matrix. They were synthesized in a cell-free rabbit reticulocyte lysate system, programmed by rat liver polysomal RNA, as precursor polypeptides which are 2-4 kDa larger than their corresponding mature subunits (Mr 41,000-45,000). When the radiolabeled precursors were incubated with intact rat liver mitochondria, they appeared to bind tightly to the mitochondrial outer membrane. At this stage they were completely susceptible to the action of exogenous trypsin. The precursors bound to mitochondria at 0 degrees C were translocated into the mitochondria and processed when the temperature was raised to 30 degrees C. No reaction occurred when the temperature was kept at 0 degrees C, however, suggesting that the binding of the precursors is temperature independent while the subsequent steps of the pathway are energy dependent. Indeed, the translocation reaction was inhibited by compounds such as dinitrophenol and rhodamine 6G which inhibit mitochondrial energy metabolism. The newly imported (mature) enzymes were inaccessible to the proteolytic action of added trypsin. The processing of the precursors to mature subunits was proteolytically carried out in the mitochondrial matrix, and the processed mature subunits mostly assembled to their respective tetrameric forms. Newly synthesized larger precursors of each of the four acyl-CoA dehydrogenases were recovered from intact, cultured Buffalo rat liver cells in the presence of dinitrophenol. When dinitrophenol was removed in a pulse-chase protocol, the accumulated precursors were rapidly (t1/2 3-5 min) converted to their corresponding mature subunits. On the other hand, when the chase was performed in the presence of the inhibitor, the labeled precursors disappeared with t1/2 of greater than 4 h for long chain acyl-CoA dehydrogenase and 1-2 h for the other three enzyme precursors.

Expression in Escherichia coli of functional precursor to the rat liver mitochondrial enzyme, ornithine carbamyl transferase. Precursor import and processing in vitro

cDNA encoding the full-length cytosolic precursor of rat liver ornithine carbamyl transferase, a mitochondrial matrix enzyme, was inserted into pKK223-3 and expressed under the control of the tac promoter. Following transformation of E. coli strain JM105 and induction by isopropylthiogalactoside, a polypeptide was synthesized which reacted with antibody against ornithine carbamyl transferase and co-migrated in SDS-polyacrylamide gels with authentic enzyme precursor (pOCT, Mr = 40,000 kDa); it constituted approximately 0.1% of total E. coli protein. pOCT was synthesized in vitro by coupled transcription-translation of the recombinant plasmid in an E. coli S30 extract; upon subsequent addition of purified rat liver or heart mitochondria, the precursor was imported and processed to mature form. Synthesis of pOCT in a bacterial system, therefore, results in production of a functional precursor polypeptide which does not require additional cytosolic factors from eukaryotic cells to support its uptake and processing by mitochondria in vitro.