Cell-free translation and thyroxine induction of carbamyl phosphate synthetase I messenger RNA in tadpole liver

Cell-free co-expression of functional membrane proteins and apolipoprotein, forming soluble nanolipoprotein particles

Here we demonstrate rapid production of solubilized and functional membrane protein by simultaneous cell-free expression of an apolipoprotein and a membrane protein in the presence of lipids, leading to the self-assembly of membrane protein-containing nanolipoprotein particles (NLPs). NLPs have shown great promise as a biotechnology platform for solubilizing and characterizing membrane proteins. However, current approaches are limited because they require extensive efforts to express, purify, and solubilize the membrane protein prior to insertion into NLPs. By the simple addition of a few constituents to cell-free extracts, we can produce membrane proteins in NLPs with considerably less effort. For this approach an integral membrane protein and an apolipoprotein scaffold are encoded by two DNA plasmids introduced into cell-free extracts along with lipids. For this study reported here we used plasmids encoding the bacteriorhodopsin (bR) membrane apoprotein and scaffold protein Delta1-49 apolipoprotein A-I fragment (Delta49A1). Cell free co-expression of the proteins encoded by these plasmids, in the presence of the cofactor all-trans-retinal and dimyristoylphosphatidylcholine, resulted in production of functional bR as demonstrated by a 5-nm shift in the absorption spectra upon light adaptation and characteristic time-resolved FT infrared difference spectra for the bR --> M transition. Importantly the functional bR was solubilized in discoidal bR.NLPs as determined by atomic force microscopy. A survey study of other membrane proteins co-expressed with Delta49A1 scaffold protein also showed significantly increased solubility of all of the membrane proteins, indicating that this approach may provide a general method for expressing membrane proteins enabling further studies.

Stage-specific polypeptide and villin expression during thyroid-hormone-induced substitution of the amphibian intestinal epithelium

Treatment of anuran tadpoles with 5 nM 3,3',5-triiodo-L-thyronine (T3) results in the complete substitution of the intestinal epithelium. We have examined the developmental pattern of protein synthesis in Alytes obstetricans intestinal epithelium using two-dimensional gel electrophoresis. Four different types of changes have been observed. The group I polypeptides (Mr: 41,500; 44,500; 51,500; 55,000 and 101,000) are only synthesized during the first week of hormonal treatment. They are specific of the primary (larval) epithelium. On the other hand, polypeptides referred to as Group II (Mr: 47,000; 48,000; 58,000; 66,500, pl 5.2; 99,500 and 102,000) are not detected until day 8. They are characteristic of the secondary tissue. Polypeptides of Group III (Mr: 42,000, pl 5.15 and 5.25; 42,500, 47,500, pl 5.25 and 5.55) expressed between the 6th and 8th day of T3 treatment, are specific of growing stem cells. During this critical period, Group IV polypeptides (Mr: 63,500; 66,500, pl 6.35; 105,000, pl 5.5 and 5.55) are not synthesized. The protein of Mr 105,000 (pI 5.5 and 5.55) is immunologically related to villin, a core protein of intestinal microvilli. Expression of this protein has been analyzed by immunoreplica and immunocytochemical procedures during differentiation of basal stem cells into secondary absorptive epithelial cells. The results have been compared to that obtained during spontaneous metamorphosis.

Triiodothyronine increases translatable albumin messenger RNA in Rana catesbeiana tadpole liver

The effects of both 3,5,3'-triiodo-L-thyronine and spontaneous metamorphosis on Rana catesbeiana liver mRNA were studied using in vitro translation of isolated liver poly(A)+ RNA in a rabbit reticulocyte lysate system. Conventional phenol extraction methods yielded degraded RNA due to high levels of endogenous ribonucleases released upon homogenization of Rana catesbeiana liver. Isolation of intact total RNA was achieved using the potent ribonuclease denaturant, guanidinium thiocyanate. Adult bullfrog serum albumin was purified to homogeneity and a monospecific antibody was elicited against it. A serum protein of 23,000 daltons that migrated near serum albumin on a 6% native gel was also purified to homogeneity. A monospecific antibody was also raised against this protein. Both antibodies were used to quantitatively immunoprecipitate the in vitro translation products of poly(A)+ RNA isolated at intervals following a single injection of triiodothyronine or during various stages of spontaneous amphibian metamorphosis. Triiodothyronine caused a sevenfold increase in translatable albumin mRNA and a threefold increase in translatable mRNA for the 23,000 dalton protein. These increases are consistent with a nuclear initiated mechanism for thyroid hormone action during amphibian metamorphosis.

An analysis of the influence of thyroid hormone on the synthesis of proteins in the tail fin of bullfrog tadpoles

By incubation of explants of tail fin from tadpoles of Rana catesbeiana in a solution of 35S-methionine for 4 h, newly synthesized proteins were labeled isotopically. After separation by two-dimensional polyacrylamide gel electrophoresis, those proteins were visualized by fluorography. Exposure of explants to culture medium containing thyroxine (T4) (150 nM) increased the incorporation of 35S-methionine into several proteins with 48 h. Effects of T4 on the relative abundance of two of these newly synthesized proteins were detected after 8 h of hormonal treatment. Very similar patterns of newly synthesized proteins were observed when proteins from explants of tail fin removed from tadpoles at metamorphic climax and immediately incubated with 35S-methionine were compared with proteins produced in fin derived from premetamorphic animals. These results are interpreted to indicate that both treatment of explants with T4 and elevation of endogenous levels of thyroid hormones during spontaneous metamorphosis increased the relative rates of synthesis of several identical proteins. The potential involvement of those proteins in early phases of metamorphic action which eventually lead to cell death and resorption is discussed.

Messenger RNA coding for argininosuccinate synthetase in citrullinemia

Messenger RNA coding for argininosuccinate synthetase (ASS), extracted from the livers of some patients with citrullinemia, was analyzed using a cell-free translation system and dot and Northern blot hybridization with cDNA probe for ASS. In patients with quantitative-type citrullinemia, called type II here, previous studies have demonstrated that the hepatic content of the enzyme was about 10% of the control value, whereas the translatable mRNA level for the enzyme was similar to that of control livers. Here, we confirmed that the type II liver contained an almost normal amount of mRNA coding for ASS, judged by the dot-blot hybridization technique with cDNA. Northern blot hybridization of RNA indicated that there was hybridizable mRNA of approximately normal size (about 1.7 kilobase [kb]) in each, suggesting that large structural gene deletions had not occurred. These results indicate that in type II citrullinemia, the decrease in the enzyme protein is due either to increased degradation of the enzyme or to decreased or inhibited translation in the liver. Another type of citrullinemia was found and classified as type III. It is characterized by no detectable enzyme activity for ASS or translation activity for ASS mRNA. However, a smaller amount of RNA molecule hybridized for ASS cDNA was detected.

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

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

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.

Level of translatable messenger RNA coding for argininosuccinate synthetase in the liver of the patients with quantitative-type citrullinemia

The translation activity of mRNA coding for argininosuccinate synthetase in total RNA extracted from the liver of three patients with quantitative-type citrullinemia was determined using a cell-free translation system. In two patients, the hepatic content of the enzyme was about 20% of the control value, whereas translatable mRNA level for the enzyme was similar to or slightly lower than those of control livers. In the third patient, the enzyme content was about 50% of the control value, and mRNA activity for the enzyme was low normal. These results indicate that at least in the first two patients, the decrease in the enzyme protein is due either to increased degradation of the enzyme or to decreased translation in the patient's liver.

Cell-free synthesis of ornithine decarboxylase. Changes in mRNA activity in the liver of thioacetamide-treated rats

Ornithine decarboxylase (ODC)mRNA associated with free polysomes of rat liver was translated in a reticulocyte lysate cell-free system. Newly synthesized ODC protein was identified by specific immunoprecipitation, molecular size as determined by polyacrylamide gel electrophoresis with sodium dodecyl sulfate, and competition by excess unlabeled ODC in the immunoprecipitation. A single injection of thioacetamide was found to cause several fold increases in both immunotitratable ODC protein and polysomal ODC-mRNA activity, while it provoked a much larger increase in ODC activity in rat liver. The results indicate that the induction of hepatic ODC activity by thioacetamide treatment is due not only to an increase in the activity of polysomal ODC-mRNA but also to a translational and/or posttranslational control.

Developmental and nutritional regulation of the messenger RNAs for fatty acid synthase, malic enzyme and albumin in the livers of embryonic and newly-hatched chicks

The mRNAs for fatty acid synthase and malic enzyme were almost undetectable in total RNA extracted from the livers of 16-day old chick embryos. Both mRNAs increased in abundance between the 16th day of incubation and the day of hatching. In neonates, fatty acid synthase mRNA level was dependent on nutritional status, increasing slowly if the chicks were starved and rapidly if they were fed. The abundance of malic enzyme mRNA decreased in starved neonatal chicks and increased in fed ones. When neonates were first fed and then starved, starvation caused a large decrease in the abundance of both mRNAs. Conversely, feeding, after a period of starvation, resulted in a substantial increase in both mRNAs. The relative abundances of fatty acid synthase and malic enzyme mRNAs correlated positively with relative rates of enzyme synthesis. Thus, nutritional and hormonal regulation of the synthesis of these two 'lipogenic' enzymes is exerted primarily at a pre-translational level. The abundance of albumin mRNA decreased significantly between the 16th day of incubation and the day of hatching but did not change thereafter in fed or starved chicks. The relative stability of albumin mRNA levels after hatching attests to the selectivity of the nutritional regulation of fatty acid synthase and malic enzyme mRNAs. The decrease in albumin mRNA which occurred between 16 days of incubation and hatching contrasts with the increase in albumin mRNA sequences which occurred during late gestation in the fetal rat (20). High levels of albumin in the chick embryo may be related to the lack of an analogue of mammalian alpha-fetoprotein in birds.

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.

Transport of proteins into mitochondria

There is still much that is obscure concerning the transport of proteins into or through the mitochondrial membrane systems. In addition, as pointed out previously, it is unlikely that the details of the process are the same for proteins destined for different compartments of the organelle. A brief summary of the process for matrix proteins might be as follows: The proteins are synthesized on free polysomes as precursors of higher molecular weight than the native forms. These precursors are liberated into the cell cytosol and subsequently translocated into the mitochondria. This timing might be different in yeast under some circumstances, synthesis being completed in association with the mitochondria. The precursors interact with a receptor in the outer mitochondrial membrane interaction being mediated by the presequences of the precursors. The presequences therefore act as addressing signals as well as possibly playing a role in one or all of (a) solubilization of precursors, (b) prevention of premature assembly into multimeric structures, or (c) maintenance of nonnative configurations required for transport. Interaction occurs with a second receptor, this time in the inner membrane of the mitochondria, interaction being with multiple sites in the polypeptide chain. Transport across the inner membrane then occurs, this transport depending on a transmembrane electrochemical gradient of which the proton component is the essential part. Transport is accompanied or followed by proteolysis of the prepiece, and formation of the native structure. While steps 1 and 2 of this sequence can be considered well established, the remaining steps are still poorly understood or purely hypothetical. Nevertheless, this sequence of events is consistent with known facts about the process and provides a framework for future investigations.

Submitochondrial localization, cell-free synthesis, and mitochondrial import of 2-isopropylmalate synthase of yeast

2-Isopropylmalate synthase (EC 4.1.3.12) of yeast is a mitochondrial enzyme. We now provide evidence showing that a large part of the 2-isopropylmalate synthase activity that is associated with the mitochondria is located in the mitochondrial matrix. In vitro translation of total yeast RNA followed by immunoprecipitation with anti-2-isopropylmalate synthase antibody yields two polypeptides. The larger of these has an apparent molecular weight identical to that of purified 2-isopropylmalate synthase subunit (ca. 65,000). It is incorporated into isolated yeast mitochondria with no detectable change in molecular weight. The import requires energy. The smaller polypeptide migrates to a position corresponding to a molecular weight of 63,000-64,000. It is not taken up by mitochondria. Both polypeptides, which also can be obtained by immunoprecipitation of crude extracts, become labeled when in vitro translation is performed in the presence of N-formyl[35S]methionyl-tRNAf. Mutants with no detectable 2-isopropylmalate synthase activity are deficient in either one or both synthase-related polypeptides. These results are discussed in the light of recent evidence for two 2-isopropylmalate synthase-encoding genes in yeast.

Ornithine transcarbamylase in liver mitochondria

Ornithine transcarbamylase (ornithine carbamoyltransferase, EC 2.1.3.3), the second enzyme of urea synthesis, is localized in the matrix of liver mitochondria of ureotelic animals. The enzyme is encoded by a nuclear gene, synthesized outside the mitochondria, and must then be transported into the organelle. The rat liver enzyme is initially synthesized on membrane-free polysomes in the form of a larger precursor with an amino-terminal extension of 3 400-4 000 daltons. In rat liver slices and isolated rat hepatocytes, the pulse-labeled precursor is first released into the cytosol and is then transported with a half life of 1-2 min into the mitochondria where it is proteolytically processed to the mature form of the enzyme. The precursor synthesized in vitro exists in a highly aggregated form and has a conformation different from that of the mature enzyme. The precursor has an isoelectric point (pI = 7.9) higher than that of the mature enzyme (pI = 7.2). The precursor synthesized in vitro can be taken up and processed to the mature enzyme by isolated rat liver mitochondria. The mitochondrial transport and processing system requires membrane potential and a high integrity of the mitochondria. The transport and processing activities are conserved between mammals and birds or amphibians and is presumably common to more than one precursor. Potassium ion, magnesium ion, and probably a cytosolic protein(s), in addition to the transcarbamylase precursor and the mitochondria, are required for the maximal transport and processing of the precursor. A mitochondrial matrix protease which converts the precursor to a product intermediate in size between the precursor and the mature subunit has been highly purified. The protease has an estimated molecular weight of 108 000 and an optimal pH of 7.5-8.0, and appears to be a metal protease. The protease does not cleave several of the protein and peptide substrates tested. The role of this protease in the precursor processing remains to be elucidated. Rats subjected to different levels of protein intake and to fasting show significant changes in the level of enzyme protein and activity of ornithine transcarbamylase. The dietary-dependent changes in the enzyme level are due mainly to an altered level of functional mRNA for the enzyme. In contrast, during fasting, the increase in the enzyme level is associated with a decreased level of translatable mRNA for the enzyme. Pathological aspects of ornithine transcarbamylase including the enzyme deficiency and reduced activities of the enzyme in Reye's syndrome are also described. A possibility that impaired transport of the enzyme precursor into the mitochondria leads to a reduced enzyme activity, is proposed.

Effect of DNA on thyroid-hormone binding by specific receptor proteins from rat-liver nuclei

Influence of double-stranded native DNA on the binding of thyroid hormone, 3,5,3'-triiodo-L-thyronine, by the isolated nuclear receptors was studied and the following results were obtained. (1) The receptor-triiodothyronine complexes bound to DNA with moderate affinities. (2) DNA enhanced the hormone binding of the receptors. (3) The stimulatory DNA effect on triiodothyronine binding of the receptors was dependent on DNA concentration, showing its maximum at 30 microgram/ml. (4) The increase in triiodothyronine binding was observed not only in the initial velocity but also in the plateau level which was attained after sufficient incubation time. (5) There were two types of specific receptors in the rat liver nuclear extract. The dissociation constants and the maximal binding capacities for triiodothyronine, which were determined by Scatchard plot analysis in the presence and absence of DNA, suggested that DNA exerted its effect through increasing binding capacity on one class of the receptors and through enhancing affinity for the hormone on the other class of the receptors. (6) Among various polynucleotides examined, the double-stranded eukaryotic DNA was most effective in enhancing the hormone binding by the receptors. These results indicate that the nuclear thyroid hormone receptors interact with double-stranded DNA in a specific manner and are induced to bind more thyroid hormone. We interpret these results as suggesting that a ternary complex of triiodothyronine, the receptor and DNA is formed in the cell nucleus in vivo, probably representing an intrinsic step in the hormone action. Possible physiological significance of this effect of DNA on the receptors is discussed.

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

Total RNA or poly(A)(+) RNA of rat liver was translated in a rabbit reticulocyte or wheat germ protein-synthesizing system and the carbamyl phosphate synthetase I [carbamoyl-phosphate synthetase (ammonia); carbon dioxide: ammonia ligase (ADP-forming, carbamate-phosphorylating), EC 6.3.4.16] synthesized was isolated by indirect immunoprecipitation by using antibody purified on enzyme-bound Sepharose and Staphylococcus aureus cells. The in vitro product moved on sodium dodecyl sulfate/polyacrylamide gels as a polypeptide that was about 5000 daltons larger than the subunit of the mature enzyme (160,000 daltons). The same polypeptide was also obtained by direct immunoprecipitation or by a double-antibody precipitation method. The mature enzyme competed effectively with the in vitro product for interaction with anti-carbamyl phosphate synthetase I antibody. Digestion of the in vitro product by S. aureus protease gave a pattern of peptide fragments similar to that of the mature enzyme. A mitochondrial membrane preparation from rat liver converted the in vitro product into a polypeptide that comigrated with the mature subunit on sodium dodecyl sulfate gel electrophoresis. Similar proteolytic activity was not detected in either a cytosol or a microsomal fraction of rat liver. These results indicate that the enzyme is synthesized as a larger precursor which is converted to the mature form of enzyme by posttranslational processing.