Site in cell-free protein synthesis sensitive to diphtheria toxin

Multiple Gene Expression in Cell-Free Protein Synthesis Systems for Reconstructing Bacteriophages and Metabolic Pathways

As a fast and reliable technology with applications in diverse biological studies, cell-free protein synthesis has become popular in recent decades. The cell-free protein synthesis system can be considered a complex chemical reaction system that is also open to exogenous manipulation, including that which could otherwise potentially harm the cell's viability. On the other hand, since the technology depends on the cell lysates by which genetic information is transformed into active proteins, the whole system resembles the cell to some extent. These features make cell-free protein synthesis a valuable addition to synthetic biology technologies, expediting the design-build-test-learn cycle of synthetic biology routines. While the system has traditionally been used to synthesize one protein product from one gene addition, recent studies have employed multiple gene products in order to, for example, develop novel bacteriophages, viral particles, or synthetic metabolisms. Thus, we would like to review recent advancements in applying cell-free protein synthesis technology to synthetic biology, with an emphasis on multiple gene expressions.

Receptor-mediated endocytosis of diphtheria toxin by cells in culture

The binding and uptake of fluorescently labeled diphtheria toxin by cells in culture has been examined by using epifluorescence video intensification microscopy. Rhodamine-labeled diphtheria toxin retained significant toxicity on bioassay and in cell culture and was tested for uptake by human WI-38 and mouse 3T3 fibroblasts grown in culture. When added to cells at 37 degrees C, toxin was observed to become concentrated and internalized in discrete vesicles in both cell lines. The appearance of fluorescent clusters could be prevented by addition of excess unlabeled diphtheria toxin to the medium or by addition of ATP (which has been shown to block toxin binding to cells), indicating that the rhodamine-labeled toxin was binding to diphtheria toxin-specific cell surface binding sites. When the simultaneous uptake of rhodamine-labeled diphtheria toxin and fluorescein-labeled alpha 2-macroglobulin was monitored, the two proteins appeared in the same clusters indicating that the toxin undergoes receptor-mediated endocytosis. Despite the difference in susceptibility to diphtheria toxin of cells derived from sensitive (human) and resistant (mouse) tissues, the behavior of the rhodamine-labeled derivative in both cell lines was indistinguishable in terms of toxin required for formation of clusters or inhibition by unlabeled toxin or by ATP. These results demonstrate that diphtheria toxin-specific cell surface binding sites occur on both insensitive and sensitive cells and suggest that toxin is processed similarly by both cell types during its initial cell surface binding and internalization by this pathway. The possible involvement of this uptake system in the mechanism of action of diphtheria toxin in cells is discussed.

Archaebacterial elongation factor is ADP-ribosylated by diphtheria toxin

Archaebacteria have been defined as a 'third primary kingdom' of cells in addition to the urkaryotes and the eubacteria. While the latter two correspond approximately to the conventional categories eukaryotes and prokaryotes respectively, the Archaebacteria have up to now comprised four groups of microorganisms: the methanogenic bacteria, the extremely halophilic bacteria and the two thermoacidophilic genera Sulfolobus and Thermoplasma. Based on ribosomal RNA sequence homologies and lipid composition, they apparently form a distinct group. Furthermore they possess or lack typical biochemical markers of both the eukaryotes and the prokaryotes, as well as having unique properties not found elsewhere. Altogether, this indicates that they are not closer to either one of the classical categories. One clear-cut difference between prokaryotes and eukaryotes is the diphtheria toxin reaction, which catalyses the covalent binding of adenosine diphosphate-ribose (ADPR) to the eukaryotic peptide elongation factor EF2 in contrast to the homologous prokaryotic factor EF-G. We report here that diphtheria toxin also catalyses the ADP-riboslation of archaebacterial elongation factors. In this respect, these factors have to be assigned to the EF2 type; we suppose that the ADP-ribosylatable structure arising so early in evolution is of fundamental importance for the elongation process.

Immunoprecipitation and partial characterization of diphtheria toxin-binding glycoproteins from surface of guinea pig cells

125I-Labeled membrane glycoproteins that specifically interact with diphtheria toxin and CRM197 protein--but not with diphtheria toxoid, fragment A of diphtheria toxin, or cholera toxin--were detected by use of the lactoperoxidase labeling technique followed by an immunoprecipitation system. These glycoproteins, which adhere to lentil lectin-Sepharose columns, are present on the surface of diphtheria toxin-sensitive guinea pig lymph node cells but are completely lacking on the surface of diphtheria toxin-resistant mouse L cells. The major 125I-labeled glycoprotein that interacts with diphtheria toxin exhibits anomalous behavior, characteristic of glycoproteins, when analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. This demonstration of the biochemical nature of specific diphtheria toxin binding membrane components raises the possibility that the detected components are diphtheria toxin receptors.

Uptake of diphtheria toxin and its fragment A moiety by mammalian cells in culture

Evidence suggesting that diphtheria toxin reaches the cytoplasm of susceptible mammalian cells by two independent mechanisms is presented. A schematic model describing the two processes of toxin entry into the cell is developed. One process of toxin uptake considered to by physiologically significant is passage of the protein toxin through the plasma membrane. This most likely happens by binding of fragment B to receptors on the membrane and by subsequent toxin-membrane interaction so that ultimately fragment A, the enzymatically active moiety, is transported tothe cell interior. This process, which ultimately leads to cessation of protein synthesis and cell death, involves a comparatively small number of toxin molecules. A second mechanism of toxin uptake is by classical pinocytosis. The majority of toxin taken into the cell is accomplished by this process. The fate of toxin taken into HEp-2 cells via pinocytosis is proteolysis by lysosomal enzymes. Thus, such vesicle-bound toxin is ordinarily not expressed biologically. Evidence suggesting that ammonium chloride provides total protection to diphtheria toxin-susceptible cells by preventing entry of toxin by the specific receptor-associated process is also provided; data showing that the ammonium salt immobilizes bound toxin on the plasma membrane of HEp-2 cells are presented. Finally, it is suggested that actively endocytic cells such as guinea pig macrophages interact with toxin in a significantly different manner than do nonphagocytic cells.

Interaction of cultured mammalian cells with [125I] diphtheria toxin

The characteristics of cell adsorption and pinocytotic uptake of diphtheria toxin by several mammalian cell types were studied. Purified toxin iodinated by a solid-state lactoperoxidase method provided preparations of high specific activity and unaltered biological activity. Dephtheria toxin-sensitive HEp-2 cells and guinea pig macrophage cultures were compared with resistant mouse L-929 cells. At 37 C the resistant cells in monolayer adsorbed and internalized [125I] toxin to a greater extent than did the HEp-2 cell cultures; no significant differences were observed at 5 C. Ammonium chloride protection levels did not alter uptake of toxin by either L-929 OR HEp-2 cells. Biological activity of the iodinated toxin, however, was negated provided the presence of ammonium chloride was maintained. The ammonium salt appears to maintain toxin in a state amenable to antitoxin neutralization. Guinea pig macrophages internalized iodinated toxin to a level 10 times greater than the established cell lines. In spite of the increased uptake of toxin by the endocytic cells, ammonium chloride prevented expression of toxicity. In an artificial system, toxin adsorbed to polystyrene latex spheres and internalized by guinea pig macrophages during phagocytosis did express biological activity. Ammonium chloride afforded some but not total protection against toxin present in the phagocytic vacuoles. The data suggest that two mechanisms of toxin uptake by susceptible cells may be operative. Toxin taken into the cell by a pinocytotic process probably is not ordinarily of physiological significance since it is usually degraded by lysosomal enzymes before it can reach cytoplasmic constituents on which it acts. When large quantities of toxin are pinocytized, toxicity may be expressed before enzymatic degradation is complete. A more specific uptake involving direct passage of the toxin through the plasma membrane may be the mechanism leading to cell death in the majority of instances.

Diphtheria toxin: mode of action and structure

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Biological activity of heated diphtheria toxin

Diphtheria toxin splits into two fragments when heated at 100 C for 10 min in a phosphate buffer. The separated fragments have molecular weights of 24,000 and 39,000, respectively. These molecular weights are similar to those of the A and B fragments found in diphtheria toxin preparations after thiol reduction. Since the separation of toxin into fragments is not complete, it is likely that only nicked toxin molecules having a cleaved peptide bond are split by heating. When toxin is suspended in phosphate buffer at pH 6.4, the B-like fragment precipitates, but at pH 7.8 it does not. Heated toxin is unable to intoxicate sensitive cells or cause a necrodermal response in animals. Fragment A produced by heating is active in inhibiting cell-free protein synthesis. It is able to intoxicate both HeLa and L cells when the uptake of the fragment is facilitated by addition of polyornithine to the cultures. Fragment B produced by heating is involved with binding to the cell surface. It is able to delay the action of toxin on KB cell cultures preincubated with fragment B.

Studies on the mode of action of diphtheria toxin. V. Protein metabolism in a guinea pig model simulating chronic diphtheritic toxemia

An experimental model of "chronic" diphtheria intoxication in the guinea pig was developed. Adult guinea pigs, subjected to a regimen of multiple sublethal doses of purified diphtheria toxin (total of 1.4 minimum lethal doses divided equally in four daily doses), developed a toxemia which terminated in death between 6 and 8 days. During an advanced stage of illness, de novo protein synthesis was assessed by in vivo incorporation of tritiated leucine into tissue proteins. With the exception of pancreas and skeletal muscle, protein synthesis in healthy, control and toxin-treated guinea pigs was comparable. The absence of notable impairment of protein synthesis in this experimental situation is discussed in terms of the mode of action of the diphtheria toxin and the pathophysiology of the disease.

Inhibitory effect of diphtheria toxin on amino acid incorporation in Escherichia coli cell-free system

The mechanism of action of diphtheria toxin in an Escherichia coli cell-free protein-synthesizing system was examined. When the washed ribosomes were incubated with toxin before addition of messenger ribonucleic acid (RNA), peptide syntheses of (14)C-phenylalanine directed by polyuridylic acid or phage R17 RNA were strongly inhibited by a small amount of toxin. Whereas, if the soluble fraction (105,000 x g supernatant fraction) was preincubated with toxin, no effect of toxin occurred either on the induced protein synthesis or on the activity of guanosine triphosphatase even in the presence of nicotinamide adenine dinucleotide. The binding of (3)H-formylmethionyl-transfer RNA to E. coli ribosomes directed by either R17 RNA or trinucleotide AUG was also decreased by toxin. These findings suggest that diphtheria toxin may prevent the binding of messenger RNA by successfully competing with the AUG for ribosomal binding sites. Sucrose-density gradient studies support this concept by showing the decrease in binding of (3)H-labeled R17 RNA to E. coli ribosomes exposed to toxin.

Studies on the mode of action of diphtheria toxin. III. Effect on subcellular components of protein synthesis from the tissues of intoxicated guinea pigs and rats

The effect of diphtheria toxin on subcellular components of protein synthesis was determined. Polyribosomes prepared from intoxicated guinea pigs functioned normally in an in vitro assay system, while the activity of soluble enzymes (transferases) from toxin-treated animals was significantly reduced. At high toxin dosages, this reduction was widespread, but when levels of toxin comparable to those which might be generated in a natural infection were given, inhibition of soluble enzyme activity was found only in extracts from heart and skeletal muscle. Possible nonspecific inhibition in the assay system due to interference by free toxin or by a serum component was eliminated. Since it was possible to demonstrate reactivation of soluble enzyme activity with nicotinamide and toxin, it was suggested that diphtheria toxin acts in the intact sensitive animal in a manner analogous to its action in tissue culture or in cell-free systems. It was hypothesized that the lethal biochemical lesion of the toxin in sensitive animals was the inactivation of transferase enzymes, principally in the heart. It was also suggested that the lethal lesion induced in diphtheria-sensitive and resistant species may not be identical.

Fusidic acid: inhibition of factor T2 in reticulocyte protein synthesis

The steroid antibiotic fusidic acid inhibits reticulocyte protein synthesis. This inhibition appears to be due to interference with the activity of the T(2) supernatant fraction, and strengthens the proposition that T(2) is functionally analogous to the G-factor of bacterial protein synthesis, which is also specifically inhibited by this antibiotic.