Cell-free synthesis and processing of a putative precursor for mitochondrial carbamyl phosphate synthetase I of rat liver

Biosynthetic pathways of two polypeptide subunits of the light-harvesting chlorophyll a/b protein complex

We have used an in vitro reconstitution system, consisting of cell-free translation products and intact chloroplasts, to investigate the pathway from synthesis to assembly of two polypeptide subunits of the light-harvesting chlorophyll-protein complex. These polypeptides, designated 15 and 16, are integral components of the thylakoid membranes, but they are products of cytoplasmic protein synthesis. Double immunodiffusion experiments reveal that the two polypeptides share common antigenic determinants and therefore are structurally related. Nevertheless, they are synthesized in vitro from distinct mRNAs to yield separate precursors, p15 and p16, each of which is 4,000 to 5,000 daltons larger than its mature form. In contrast to the hydrophobic mature polypeptides, the precursors are soluble in aqueous solutions. Along with other cytoplasmically synthesized precursors, p15 and p16 are imported into purified intact chloroplasts by a post-translational mechanism. The imported precursors are processed to the mature membrane polypeptides which are recovered exclusively in the thylakoids. The newly imported polypeptides are assembled correctly in the thylakoid lipid bilayer and they bind chlorophylls. Thus, these soluble membrane polypeptide precursors must move from the cytoplasm through the two chloroplast envelope membranes, the stroma, and finally insert into the thylakoid membranes, where they assemble with chlorophyll to form the light-harvesting chlorophyll protein complex.

Synthesis, intracellular transport, and processing of the precursors for mitochondrial ornithine transcarbamylase and carbamoyl-phosphate synthetase I in isolated hepatocytes

The synthesis and intracellular transport of the mitochondrial matrix enzymes ornithine transcarbamylase (carbamoylphosphate: L-ornithine carbamoyltransferase, EC 2.1.3.3.) and carbamoyl-phosphate synthetase (ammonia) I [carbon-dioxide:ammonia ligase (ADP-forming, carbamate-phosphorylating), EC 6.3.4.16] were studied in isolated rat hepatocytes. In pulse experiments at 37 degrees C, the larger precursors of the two enzymes appeared in the cytosol of the liver cells, where radioactivity levels of the precursors reached a plateau in 10-20 min after the pulse. The pulse-labeled mature enzymes appeared in the particulate fraction (containing mitochondria) after a time lag and increased almost linearly with time up to 40 min. The specific radioactivities of the precursors in the cytosol were much higher than those of the mature enzymes in the particulate fraction. In pulse--chase experiments, the labeled precursors disappeared from the cytosol with estimated half-lives of about 1-2 min. These results indicate that ornithine transcarbamylase and carbamoyl-phosphate synthetase I are initially synthesized as larger precursors and exist in a cytosolic pool from which they are transported into mitochondria and processed there to the mature enzymes concomitantly with or immediately after transport. Although the rates of synthesis, transport, and processing were decreased about 3-fold at 25 degrees C (as compared to incubation at 37 degrees C), the pool size of the precursors in the cytosol were somewhat larger at this temperature.

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

Administration of the thyroid hormone 3,3,5'-triiodo-L-thyronine (T3) to rats leads to a marked increase in hepatic levels of mRNA for cytochrome c. Messenger RNA prepared from the free polysomes of T3-treated rats directed the in vitro synthesis of a polypeptide which only differed in amino acid sequence from mature cytochrome c in that it contained an NH2-terminal methionine. The in vitro product was incorporated specifically into purified rat liver mitochondria and became inaccessible to added trypsin when the mitochondria were added after translation was completed. Horse heart apocytochrome c, but not the holocytochrome, could compete with the in vitro synthesized polypeptide for its uptake into mitochondria. This suggests that the primary structural features of apocytochrome c, which serve as an addressing signal for mitochondria, are masked after the acquisition of heme and that this process occurs in the mitochondria. The addressing signal seems to be contained in a specific segment of the cytochrome polypeptide because only one fragment generated by CNBr cleavage of horse apocytochrome c, extending from residue 66 to the carboxy end of the molecule, could compete with the in vitro product for its transfer into mitochondria.

Different transport pathways of individual precursor proteins in mitochondria

Transport of mitochondrial precursor proteins into mitochondria of Neurospora crassa was studied in a cell-free reconstituted system. Precursors were synthesized in a reticulocyte lysate programmed with Neurospora mRNA and transported into isolated mitochondria in the absence of protein synthesis. Uptake of the following precursors was investigated: apocytochrome c, ADP/ATP carrier and subunit 9 of the oligomycin-sensitive ATPase. Addition of high concentrations of unlabelled chemically prepared apocytochrome c (1-10 microM) inhibited the appearance in the mitochondrial of labelled cytochrome c synthesized in vitro because the unlabelled protein dilutes the labelled one and because the translocation system has a limited capacity [apparent V is 1-3 pmol X min-1 X (mg mitochondrial protein)-1]. Concentrations of added apocytochrome c exceeding the concentrations of precursor proteins synthesized in vitro by a factor of about 10(4) did not inhibit the transfer of ADP/ATP carrier or ATPase subunit 9 into mitochondria. Carbonylcyanide m-chlorophenylhydrazone, an uncoupler of oxidate phosphorylation, inhibited transfer in vitro of ADP/ATP carrier and of ATPase subunit 9, but not of cytochrome c. These findings suggest that cytochrome c and the other two proteins have different import pathways into mitochondria. It can be inferred from the data presented that different 'receptors' on the mitochondria. It can be inferred from the data presented that different 'receptors' on the mitochondrial surface mediate the specific recognition of precursor proteins by mitochondria by mitochondria as a first step in the transport process.

Posttranslational uptake and processing of in vitro synthesized ornithine transcarbamoylase precursor by isolated rat liver mitochondria

The mitochondrial matrix enzyme ornithine transcarbamoylase (OTCase; ornithine carbamoyltransferase; carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3) is encoded by a nuclear gene on the X chromosome, synthesized on cytoplasmic ribosomes, and translocated across both mitochondrial membranes. Using specific immunoprecipitation, we presented evidence previously that the primary in vitro translation product of OTCase in rat liver is a polypeptide about 4000 daltons larger than the "mature" OTCase augment subunit purified from homologous mitochondria. In this report we augment the immunological identification of this cell-free translation product (pOTCase) with structural information and show, by electrophoresis of proteolysis products, that pOTCase is structurally similar to mitochondrial OTCase. Moreover, we now demonstrate that, when pOTCase is incubated posttranslationally with isolated rat liver mitochondria, it is converted to the size of mature OTCase and is sequestered within the mitochondria in such a way that it becomes resistant to externally added proteases. Such posttranslational processing is catalyzed specifically by the mitochondrial fraction of rat liver cells and is dependent both on the duration of incubation with mitochondria and on the amount of mitochondrial protein added. We conclude that pOTCase is indeed the bona fide precursor of mitochondrial OTCase and that use of this simplified cell-free system will facilitate analysis of OTCase biogenesis at both the cellular and the molecular level.

Precursor Forms of Pea Vicilin Subunits: MODIFICATION BY MICROSOMAL MEMBRANES DURING CELL-FREE TRANSLATION

Polyribosomal RNA isolated from pea cotyledons at various developmental stages programmed the cell-free synthesis of polypeptides which were recognized by antibodies specific for pea storage proteins. There were quantitative and qualitative changes in the template activity during seed maturation. Most of the polysomal RNA was associated with the membrane fraction, and all of the template for storage protein occurred in this fraction. Using RNA from a stage of seed maturation at which the synthesis of the high-molecular weight vicilin polypeptides predominate, it was found that the major translation products, although antigenically recognizable as storage protein, did not coincide with the authentic vicillin polypeptides on denaturing polyacrylamide gels. The addition during translation of microsomal membranes from dog pancreas or pea cotyledons resulted in the appearance of new polypeptides which did coincide with some of the authentic vicilin polypeptides (in the apparent molecular weight regions of 75,000 and 50,000) and were antigenically recognizable as storage protein. Other translation products related to storage protein were not visibly altered in their electrophoretic mobility by the addition of membranes. Microsomal membranes treated with Triton X-100 were not effective in modifying the cell-free products. The modified vicilin polypeptides and at least two other translation products were protected from proteolytic degradation, suggesting that they were sequestered within microsomal vesicles. Thus, these storage protein components may be synthesized by a mechanism analogous to that described for membrane and secretory proteins (Blobel G, B Dobberstein 1975 J Cell Biol 67: 835-851).

Regulation of polar morphogenesis in Caulobacter crescentus

Deoxyribonucleic acid (DNA) phage phi CbK-resistant nonmotile mutants of Caulobacter crescentus CB15 were examined for their formation of polar surface structures (a stalk, a single flagellum, pili, and DNA phage receptors). These mutants were devoid of pili and DNA phage receptors and simultaneously defective either in both stalk formation and flagellar activity (stalk-defective type) or in the formation of normal flagella (flagella-defective type). DNA phage phi Cr30-mediated transductions revealed that stalk-defective mutants were of a single genetic type, whereas flagella-defective mutants were grouped into two different genetic types, I and II. To investigate how membrane proteins change in the above morphology mutants, cell envelopes pulse-labeled with L-[35S]methionine were analyzed by two-dimensional gel electrophoresis. No gross change of membrane proteins was observed in the stalk-defective mutant CB15 pdr-803, except a 49,000-molecular-weight (49K) protein which was found reduced. However, a 27K, two 28.5K, and a 70.5K protein were missing from the membrane of the flagella-defective type I mutant CB15 pdr-813. These proteins are most likely to be flagella-related protein, flagellins A and B, and hook protein, respectively. In another flagella-defective type II mutant, CB15 pdr-816, the 27K and two 28.5K proteins were similarly absent but the 70.5K protein was consistently present in the membrane. The synthesis of flagellin was next assayed radioimmunologically in the above 35S-labeled mutants. Stalk-defective CB15 pdr-803 synthesized flagellin normally, compared to the wild-type strain. Flagellins A (26K) and B (28K) formed multiple spots in isoelectric focusing. A 29K protein was also detected in the flagellin-specific radioactivity from the cytoplasm. Flagella-defective type I CB15 pdr-813 synthesized flagellin only at a basal level. Thus transcription or translation of flagellin appeared to be repressed in this mutant. Another flagella-defective type II strain, CB15 pdr-816, however, synthesized flagellin at an apparently enhanced rate compared with the wild type. Flagellin synthesized in CB15 pdr-816 was flagellin A and a smaller 22K flagellin. Flagellin B was not synthesized in the mutant. It then follows that flagellin B is not a precursor of flagellin A and the 22K flagellin. Flagella-defective type II CB15 pdr-816, without flagellin B, formed a stub structure with a hook attached to one end instead of normal flagella. In the wild-type membrane, flagellin B was the major flagellin, whereas flagellin A was major in the cytoplasm and the flagellar filament. It is suggested from these results that flagellin B is important in the assembly of normal flagella.

Characterization of a protease apparently involved in processing of pre-ornithine transcarbamylase of rat liver

The precursor of rat liver ornithine transcarbamylase (ornithine carbamoyltransferase; carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3) (pre-ornithine transcarbamylase), which was synthesized in a reticulocyte lysate cell-free system, was converted to an apparently mature form of the enzyme by isolated rat liver mitochondria. The proteolytic processing involved two steps: (i) conversion of pre-ornithine transcarbamylase (39,400 daltons) to a product of about 37,000 daltons and (ii) further conversion to the apparently mature form of the enzyme (36,00 daltons). When mitochondria were subfractionated by digitonin treatment followed by sonication of a mitoplast fraction, the proteolytic activity catalyzing the first step was recovered mainly in a matrix fraction. Some activity was found in an intermembrane space fraction. The enzyme activity in the matrix fraction has an optimal pH at about 7.5. The activity was inhibited almost completely by 2 mM leupeptin and partly by 2 mM antipain but not significantly by other microbial protease inhibitors or serine protease inhibitors. It was inhibited strongly by 2 mM EDTA, 2 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetate, 2 mM p-chloromercuriphenylsulfonate, and 2 mM Hg(CH3COO)2 but not by N-ethylmaleimide or iodoacetamide. These results suggest that pre-ornithine transcarbamylase is first transported into the mitochondrial matrix and converted there to the mature form of the enzyme by a novel neutral protease(s).

Assembly of proteins into membranes

Two pathways for protein assembly into biological membranes have been proposed. The "signal hypothesis" emphasizes the role of specific membrane proteins in binding the growing polypeptide and conducting it into the bilayer during its synthesis. The "membrane-triggered folding" hypothesis emphasizes self-assembly and the role of changing protein conformation during transfer from an aqueous compartment into a membrane. These ideas provide a framework for reviewing recent data on the biogenesis of membrane proteins.

Selective permeability of rat liver mitochondria to purified malate dehydrogenase isoenzymes in vitro

1. The mitochondrial malate dehydrogenase from rat liver has been purified to a state of homogeneity as judged by starch-gel electrophoresis and the cytoplasmic isoenzyme has been obtained in a partically purified state. 2. Inhibition of the isoenzymes by sulphite has been studied. 3. In mitochondria loaded with sulphite, the catalytic activity of the (partially inhibited) internal malate dehydrogenase has been measured by addition of oxaloacetate to the suspension medium and observation of the consequent decrease in fluorescence of NADH. 4. Addition of mitochondrial malate dehydrogenase to suspensions of mitochondria loaded with sulphite resulted in an increase in the level of intramitochondrial enzymic activity as measured by the above technique. Addition of the cytoplasmic isoenzyme did not result in such an increase. 5. These results show that mitochondria in suspension are permeable to the mitochondrial malate dehydrogenase but not to the cytoplasmic isoenzyme. 6. This conclusion has been confirmed by direct measurement of a decrease of enzyme activity in solution and an increase inside the mitochondria after incubation of organelles in solutions containing mitochondrial malate dehydrogenase. No such effect was observed with the cytoplasmic isoenzyme. 7. Some features of the permeation process have been studied.

Transport of proteins into mitochondria. Posttranslational transfer of ADP/ATP carrier into mitochondria in vitro

The mitochondrial ADP/ATP carrier is an integral transmembrane protein of the inner membrane. It is synthesized on cytoplasmic ribosomes. Kinetic data suggested that this protein is transferred into mitochondria in a posttranslational manner. The following results provide further evidence for such a mechanism and provide information on its details. 1. In homologous and heterologous translation systems th newly synthesized ADP/ATP carrier protein is present in the postribosomal supernatant. 2. Analysis by density gradient centrifugation and gel filtration shows, that the ADP/ATP carrier molecules in the postribosomal fraction are present as soluble complexes with apparent molecular weights of about 120 000 and 500 000 or larger. The carrier binds detergents such as Triton X-100 and deoxycholate forming mixed micelles with molecular weights of about 200 000-400 000. 3. Incubation of a postribosomal supernatant of a reticulocyte lysate containing newly synthesized ADP/ATP carrier with mitochondria isolated from Neurospora spheroplasts results in efficient transfer of the carrier into mitochondria. About 20-30% of the transferred carrier are resistant to proteinase in whole mitochondria. The authentic mature protein is also largley resistant to proteinase in whole mitochondria and sensitive after lysis of mitochondria with detergent. Integrity of mitochondria is a preprequisite for translocation into proteinase resistant position. 4. The transfer in vitro into a proteinase-resistant form is inhibited by the uncoupler carbonyl-cyanide m-chlorophenylhydrazone but not the proteinase-sensitive binding. These observations suggest that the posttranslational transfer of ADP/ATP carrier occurs via the cytosolic space through a soluble oligomeric precursor form. This precursor is taken up by intact mitochondria into an integral position in the membrane. These findings are considered to be of general importance for the intracellular transfer of insoluble membrane proteins. They support the view that such proteins can exist in a water-soluble form as precursors and upon integration into the membrane undergo a conformational change. Uptake into the membrane may involve the cleavage of an additional sequence in some proteins, but this appears not to be a prerequisite as demonstrated by the ADP/ATP carrier protein.

The four cytoplasmically made subunits of yeast mitochondrial cytochrome c oxidase are synthesized individually and not as a polyprotein

Subunit-specific antisera prepared against each of the four cytoplasmically made subunits (IV, V, VI, and VII) of yeast mitochondrial cytochrome c oxidase (EC 1.9.3.1) were used to precipitate immunoreactive polypeptides that were synthesized either in vitro, in a cell-free protein-synthesizing system programmed with total yeast mRNA, or in vivo, in intact cells and in spheroplasts, under conditions of pulse labeling, pulse-chase labeling, and continuous labeling. Using N-formyl-[35S]Met-rTNA as the only radioactively labeled component in the cell-free system, we demonstrated (i) that each of the four cytoplasmically made subunits is synthesized as a separate entity and not as part of a polyprotein as was claimed by others; (ii) that subunits IV, V, and VI are synthesized as precursors, larger by 1500-3000 daltons than their mature counterparts; in contrast, subunit VII is not synthesized as a larger precursor. Precursor forms of subunits IV, V, and VI identical to those synthesized in vitro were also detected in vivo by pulse-labeling of spheroplasts. The observed disappearance of these larger forms after a chase is compatible with the notion that they represent short-lived precursors that are rapidly converted to their mature counterparts during or shortly after import into mitochondria. Furthermore, using N-formyl-[35S]Met-tRNA, we provide definitive evidence that two of the cytoplasmically made subunits (beta and gamma) of another oligomeric inner mitochondrial membrane protein (F1-ATPase, EC 3.6.1.3) are not synthesized as part of a polyprotein but as individual precursors.