The binding of aminoacyl transferase II to ribosomes

Elongation factor 2 as a target for selective inhibition of protein synthesis in vitro by the novel aromatic bisamidine

The effect of the novel aromatic bisamidine 1 on protein synthesis in cell-free translational system isolated from rat livers was studied. The bisamidine 1 caused inhibition of [14C]leucine incorporation into proteins proportionally to its concentration. To establish a precise mechanism of inhibition, we evaluated the effect of the bisamidine 1 on the isolated ribosomes and purified to homogeneity elongation factors. Preincubation of the bisamidine 1 with ribosomes resulted in partial inhibition of their activity in whole elongation system. The eucaryotic elongation factor 1 (eEF-1) was not significantly affected by the bisamidine 1. In contrast to eEF-1, the bisamidine 1 preincubated with the eucaryotic elongation factor 2 (eEF-2) caused total inhibition of its activity in the translocation process. The inhibitory effect of the bisamidine 1 on eEF-2 activity was confirmed in diphtheria toxin-dependent ADP-ribosylation reaction. The results suggest a high action specificity of the bisamidine 1 as potential anticancer drug, since the primary target seems to be highly conserved protein-elongation factor 2.

Characterization of the ribosomal properties required for formation of a GTPase active complex with the eukaryotic elongation factor 2

The binding stability of the different nucleotide-dependent and -independent interactions between elongation factor 2 (EF-2) and 80S ribosomes, as well as 60S subunits, was studied and correlated to the kinetics of the EF-2- and ribosome-dependent hydrolysis of GTP. Empty reconstituted 80S ribosomes were found to contain two subpopulations of ribosomes, with approximately 80% capable of binding EF-2.GuoPP[CH2]P with high affinity (Kd less than 10(-9) M) and the rest only capable of binding the factor-nucleotide complex with low affinity (Kd = 3.7 x 10(-7) M). The activity of the EF-2- and 80S-ribosome dependent GTPase did not respond linearly to increasing factor concentrations. At low EF-2/ribosome ratios the number of GTP molecules hydrolyzed/factor molecule was considerably lower than at higher ratios. The low response coincided with the formation of the high-affinity complex. At increasing EF-2/ribosome ratios, the ribosomes capable of forming the high-affinity complex was saturated with EF-2, thus allowing formation of the low-affinity ribosome.EF-2 complex. Simultaneously, the GTPase activity/factor molecule increased, indicating that the low-affinity complex was responsible for activating the GTP hydrolysis. The large ribosomal subunits constituted a homogeneous population that interacted with EF-2 in a low-affinity (Kd = 1.3 x 10(-6) M) GTPase active complex, suggesting that the ribosomal domain responsible for activating the GTPase was located on the 60S subunit. Ricin treatment converted the 80S particles to the type of conformation only capable of interacting with EF-2 in a low-affinity complex. The structural alteration was accompanied by a dramatic increase in the EF-2-dependent GTPase activity. Surprisingly, ricin had no effect on the factor-catalyzed GTP hydrolysis in the presence of 60S subunits alone.

The components of Melissa officinalis L. that influence protein biosynthesis in-vitro

An investigation of an inhibiting activity of a substance(s) in a tanninless extract from Melissa officinalis leaves on protein biosynthesis in-vitro has been made. At least two components which inhibited protein biosynthesis were present in the extract; these were caffeic acid and an unidentified glycoside. Freshly prepared buffered solutions of caffeic acid inhibited protein biosynthesis less than solutions stored for several days at room temperature (20 degrees C). In this case derivatives of caffeic acid were formed, which may be responsible for the increase in the inhibitory effect of stored caffeic acid solution. An inhibitor, in the homogeneous state, was also isolated from the glycoside fraction of M. officinalis. Studies on the mechanism of the action of this inhibitor revealed its effect is to use the result of a direct interaction with elongation factor EF-2, and the blocking of the binding reaction of EF-2 with ribosomes.

Reduced ribosomal binding of eukaryotic elongation factor 2 following ADP-ribosylation. Difference in binding selectivity between polyribosomes and reconstituted monoribosomes

The biological activity of elongation factor 2 (EF-2) following NAD+ - and diphtheria-toxin-dependent ADP-ribosylation was studied (i) in translation experiments using the reticulocyte lysate system and (ii) in ribosomal binding experiments using either reconstituted empty rat liver ribosomes or programmed reticulocyte polysomes. Treatment of the lysates with toxin and NAD+ at a NAD+/ribosome ratio of 4 resulted in a 90% inhibition of the amino acid incorporation rate. The inhibition was overcome by the addition of native EF-2. At this level of inhibition more than 90% of the EF-2 present in the lysates was ADP-ribosylated and the total ribosome association of EF-2 was reduced by approx. 50%. All of the remaining unmodified factor molecules were associated with the ribosomes, whereas only about 3% of the ribosylated factor was ribosome-associated. The nucleotide requirement for the binding of EF-2 to empty reconstituted rat liver ribosomes and programmed reticulocyte polysomes was studied together with the stability of the resulting EF-2 X ribosome complexes using purified 125I-labelled rat liver EF-2. With both types of ribosomes, the complex formation was strictly nucleotide-dependent. Stable, high-affinity complexes were formed in the presence of the non-hydrolysable GTP analogue guanosine 5'-(beta, gamma-methylene)triphosphate (GuoPP[CH2]P). In contrast to the reconstituted ribosomes, GTP stimulated the formation of high-affinity complexes in the presence of polysomes, albeit at a lower efficiency than GuoPP[CH2]P. The formation of high-affinity complexes was restricted to polysomes in the pretranslocation phase of the elongation cycle. Low-affinity post-translocation complexes, demonstrable after fixation, were formed in the presence of GTP, GuoPP[CH2]P and GDP. In polysomes, these complexes involved a different population of particles than did the high-affinity complexes. In the binding experiments using reconstituted or programmed ribosomes, the pretranslocation binding of EF-2 observed in the presence of GuoPP[CH2]P was reduced by approx. 50% after ADP-ribosylation, whereas the post-translocation binding in the presence of GDP was unaltered. The data indicate that the inhibition of translocation caused by diphtheria toxin and NAD+ is mediated through a reduced affinity of the ADP-ribosylated EF-2 for binding to ribosomes in the pretranslocation state.

Chromosomal assignment of the gene for human elongation factor 2

Elongation factor 2 (EF-2), polypeptidyl -tRNA translocase, is an essential factor for protein synthesis in eukaryotic cells and Archebacteria . We isolated diphtheria toxin-resistant human primary embryo cells that contain EF-2 that cannot be ADP-ribosylated by diphtheria toxin and Pseudomonas exotoxin A (PA toxin). Somatic cell hybrids were constructed from mouse L cells and toxin-resistant human embryo cell mutants. Forty-one hybrid clones were isolated, of which 15 clones were resistant to PA toxin. Karyotypic analysis and isozyme studies revealed that there was an absolute correlation between human chromosome 19 and resistance to PA toxin in the hybrids. On subcloning of PA toxin-resistant hybrid cells, we obtained one PA toxin-resistant hybrid subclone containing human chromosome 19 as the only human chromosome. Furthermore, the resistance to PA toxin of hybrid cell strains was lost after infection with poliovirus, for which sensitivity is conferred by human chromosome 19. It was confirmed by using two-dimensional gel electrophoresis that PA toxin resistance in hybrid cells was caused by the presence of EF-2 resistant to ADP-ribosylation by fragment A of diphtheria toxin. These facts suggest that the gene encoding EF-2 is located on human chromosome 19.

Nucleotide-mediated interactions of eukaryotic elongation factor EF-2 with ribosomes

Eukaryotic elongation factor EF-2 isolated from rat liver microsomal salt-wash showed two types of nucleotide-dependent interactions with reconstituted empty 80S ribosomes when analyzed by gradient centrifugation. Stable EF-2 X ribosome complexes were only formed with GTP analogues with reduced or no ability to serve as substrates for the EF-2 and ribosome-dependent GTPase. GTP-stimulated complex formation was only demonstrable after glutaraldehyde fixation, unless a GTP-regenerating system was included throughout the gradient centrifugation. GDP and to a lesser extent GMP and guanosine also stimulated the less stable type of complex formation as demonstrable after fixation. Under the same conditions some complex formation was also observed with ATP and its analogue adenosine 5'-[beta,gamma-methylene]triphosphate, although with less efficiency than with the corresponding guanosine nucleotides. The results in combination with available data indicate that EF-2 has two binding states with different affinities on the 80S ribosome: a high-affinity pre-translocation state specific for EF-2 X GTP and a low-affinity post-translocation state, in which EF-2 X GDP is bound to the ribosome in a less stable and specific complex.

Amino acid incorporation in a cell-free system derived from rat liver studied with the aid of selenodiglutathione

Selenodiglutathione (GSSeSG), a potent inhibitor of elongation factor 2 (EF2) has been used to study amino acid incorporation in a rat liver cell-free system. While translocation of the ribosomes was inhibited by GSSeSG, ribosomes with a free acceptor site were still capable of incorporating one amino acid residue. From this the average number of amino acids incorporated per ribosomes was calculated to be 2--5. In this respect virtually no difference has been observed between ribosomes present on small or large aggregates. The time required for one translocation by all active ribosomes, and the time required for the incorporation of one amino acid (starting with aminoacyl-tRNA or amino acids) has also been determined. By incubation under conditions for amino acid incorporation, part of the ribosomes were completely inactivated whereas the rest remained as active as at the start of the incubation.

Studies on the effects of chronic ethanol ingestion on the properties of rat brain ribosomes

Previous observations have demonstrated decreased in vivo and in vitro protein synthesis by brain ribosomal systems following long-term ethanol ingestion. For further investigation of the properties of brain ribosomes, the 40S and 60S ribosomal subunits were successfully isolated from control and chronic 10% ethanol-drinking rats. For a successful dissociation of ribosomes into subunits NH4Cl, puromycin and a high-salt treatment at 10 degrees C were essential with a critical concentration of Mg2+ since ribosomes could not be resolved at less than 7 mM Mg2+. Analysis of the A260 profile of the subunits on the sucrose gradients showed no significant differences between the control and ethanol-ingesting groups. Studies on 3H-labeled ribosomes following in vivo RNA labeling showed correspondence of the radioactive profiles from the incorporation of [5(-3) H) orotic acid into RNA with the sucrose gradient absorbance profile of 60S and 40S ribosomal subunits. Furthermore, active reassociation of both subunits occurred at 37 degrees C as demonstrated by the increased [14 C]-phenylalanine incorporation in the presence of poly(U). Results further showed that the poly(U)-dependent [14C]phenylalanine incorporation was significantly reduced by the subunits from the ethanol-ingesting animals. These findings suggest that long-term ingestion of ethanol caused functional changes in the properties of brain ribosomes, specifically on the reassociation process of the two subunits.

Studies on native ribosomal subunits from rat liver. Purification and characterization of a ribosome dissociation factor

A population of free, native ribosomal 40S subunits, that do not react with 60S subunits to form 80S ribosomes, has been identified in the postmicrosomal fraction of rat liver homogenates. A protein (IF-3) has been purified from high salt (0.88 M KCI) extracts of native 40S subunits by gradient centrifugation and by ammonium sulfate fractionation; it prevents the reassociation of subunits and to a limited extent dissociates ribosomes to subunits. The activity is measured by ultracentrifugation of the reaction products on linear sucrose gradients, or with an assay developed in this laboratory that couples dissociation with the 60S-specific peptidyltransferase reaction; the latter procedure measures the amount of 60S subunits released from ribosomes or remaining in incubations in the presence of IF-3. Dissociation factor activity is recovered from most of the particles that are resolved by zonal centrifugation of the total "native subunits" obtained from the postmicrosomal fraction; the highest concentration of IF-3, however, appears to be associated with native 40S subunits. The purified dissociation factor IF-3 is composed of about ten polypeptides and the molecular weight is estimated to be between 500 000 and 700 000, on the basis of glycerol and cesium chloride gradient centrifugation. When purified 40S subunits react with IF-3 or when 80S ribosomes are dissociated by IF-3, a product is formed which is dependent on the concentration of the protein factor and has the characteristics of a 40SIF-3 complex; centrifugation of the complex on sucrose and cesium chloride gradients suggests that the complex consists of 1 equiv of each of the two components. Although dissociation factor IF-3 appears to react in a specific manner with free or ribosome-associated 40S subunits, the reaction with subunits differs in several respects from that with ribosomes. The dissociation factor also appears to interact with 60S subunits but multiple complexes are formed, some with more than 1 IF-3 equiv per 60S particle. The IF-3 converts 40S dimers (55S particles) to the 40S-IF-3 complex and dissociates free, native 80S particles present in the postmicrosomal fraction, but it does not affect polysome-associated ribosomes engaged in protein synthesis.

Elongation factor 2 as the target of the reaction product between sodium selenite and glutathione (GSSeSG) in the inhibiting of amino acid incorporation in vitro

The product of the reaction between sodium selenite and glutathione, designated as selenodiglutathione (GSSeSG), nearly completely inhibits amino acid incorporation from [14C]leucyl-tRNA by free polyribosomes isolated from rat liver. The mechanism of this inhibition was studied on the basis of the following three findings. Glutathione decomposes GSSeSG to harmless products; GSSeSG acts instantaneously on some component of the complete incubation system during preparation of the incubation vessels (at 0 degrees C); once GSSeSG has reacted its inhibitory effect cannot be reversed by glutathione. Accordingly, the effect of GSSeSG on the various steps of the amino acid incorporation process was studied by varying the sequence of additions of the reaction components, GSSeSG and GSH. The results of these and other experiments showed elongation factor 2 to be target of GSSeSG. The GSSeSG-B blocked factor could be regenerated by reduction with glutathione reductase and NADPH.

Dietary protein intake and skeletal-muscle protein metabolism in rats. Studies with the ammonium chloride-wash fraction from crude polyribosomes of well-nourished and protein-depleted rats

1. Crude polyribosomes from skeletal muscle of the hind leg of rats fed on a low-protein diet for 10 days are less active in cell-free protein synthesis than are polyribosomes obtained from well-nourished control rats. 2. The polyribosomes were salt-washed (0.5m-NH(4)Cl) and the wash extract was examined for its amino acid incorporating activity and for EF (elongation factor) 1 and EF 2 activities. 3. Compared with preparations from control rats, the salt-wash fraction from protein-depleted rats was less active and showed lower EF 1 and EF 2 activity. 4. The ribosomes were rendered equal in activity by salt-washing, but no inhibitor was detected in the salt wash. 5. Differences in the incorporating activity of crude polyribosomes from the diet groups persisted in the presence of saturating amounts of partially purified EF 1 and EF 2. 6. It is concluded that the lowered protein-synthetic activity of crude polyribosomes caused by restricted protein intake is not causally related to the lower activities of EF 1 and EF 2 in the polyribosome preparations. 7. The possible nature of the change in crude polyribosome activity due to low-protein feeding is discussed.

Dietary protein intake and skeletal-muscle protein metabolism in rats. Studies with salt-washed ribosomes and transfer factors

1. Aspects of skeletal muscle protein synthesis in vitro were studied in young rats given a low-protein diet for up to 10 days and during re-feeding with an adequate diet. 2. Partially purified muscle transfer factors (transferases I and II), crude and purified (NH(4)Cl-washed) ribosomes and a pH5 enzyme fraction were prepared for this purpose. 3. A marked decrease in the capacity of crude ribosomes to carry out cell-free polypeptide synthesis occurred within 4 days of feeding the low-protein diet. 4. The capacity of salt-washed ribosomes to promote amino acid polymerization, in the presence of added transfer factors and aminoacyl-tRNA, was only slightly decreased by the dietary treatment. 5. However, the capacity of salt-washed ribosomes to bind (14)C-labelled aminoacyl-tRNA was decreased by feeding the low-protein diet. 6. The capacity of the pH5 enzyme fraction to promote amino acid incorporation in a complete cell-free system was decreased within 2 days of feeding the low-protein diet. There is no evidence that the change is associated with aminoacyl-tRNA synthetase or binding enzyme activities of the pH5 fractions. 7. These changes are discussed in relation to the diminished rate of protein synthesis in the intact muscle cell when rats are given a low-protein diet.

Aminoacyltransferase I-catalysed binding of phenylalanyl-transfer ribonucleic acid to muscle ribosomes from normal and diabetic rats

The aminoacyltransferase I-catalysed binding of phenylalanyl-tRNA (unfractionated Escherichia coli B tRNA acylated with radioactive phenylalanine and 19 non-radioactive amino acids) to skeletal-muscle ribosomes from diabetic rats was less than that to ribosomes from normal rats when the Mg(2+) concentration was low (7.5mm); whereas just the reverse was true when the concentration of the cation was higher (15mm). Thus the Mg(2+) dependency of aminoacyltransferase I-catalysed binding of phenylalanyl-tRNA to ribosomes from normal and diabetic rats paralleled the effect of Mg(2+) concentration on synthesis of polyphenylalanine reported before. During incubation at 7.5mm-Mg(2+) phenylalanyl-tRNA was bound only to ribosomes bearing nascent peptidyl-tRNA. There are fewer such ribosomes in a preparation from the muscle of diabetic animals because diabetic animals synthesize less protein in vivo. Thus the difference in polyphenylalanine synthesis in vitro is adequately explained by the difference in enzyme-catalysed binding of phenylalanyl-tRNA to ribosomes, however, the basis of the difference in protein synthesis in vivo is still unknown.

The rate-limiting step of protein synthesis in vivo and in vitro and the distribution of growing peptides between the puromycin-labile and puromycin-non-labile sites on polyribosomes

1. At 3 min after an intravenous injection of radioactive amino acids into the rat, the bulk of radioactivity associated with liver polyribosomes can be interpreted as growing peptides. 2. In an attempt to identify the rate-limiting step of protein synthesis in vivo and in vitro, use was made of the action of puromycin at 0 degrees C, in releasing growing peptides only from the donor site, to study the distribution of growing peptides between the donor and acceptor sites. 3. Evidence is presented that all growing peptides in a population of liver polyribosomes labelled in vivo are similarly distributed between the donor and acceptor sites, and that the proportion released by puromycin is not an artifact of methodology. 4. The proportion released by puromycin is about 50% for both liver and muscle polyribosomes labelled in vivo, suggesting that neither the availability nor binding of aminoacyl-tRNA nor peptide bond synthesis nor translocation can limit the rate of protein synthesis in vivo. Attempts to alter this by starvation, hypophysectomy, growth hormone, alloxan, insulin and partial hepatectomy were unsuccessful. 5. Growing peptides on liver polyribosomes labelled in a cell-free system in vitro or by incubating hemidiaphragms in vitro were largely in the donor site, suggesting that either the availability or binding of aminoacyl-tRNA, or peptide bond synthesis, must be rate limiting in vitro and that the rate-limiting step differs from that in vivo. 6. Neither in vivo nor in the hemidiaphragm system in vitro was a correlation found between the proportion of growing peptides in the donor site and changes in the rate of incorporation of radioactivity into protein. This could indicate that the intracellular concentration of amino acids or aminoacyl-tRNA limits the rate of protein synthesis and that the increased incorporation results from a rise to a higher but still suboptimum concentration.

Cytoplasmic particles and aminoacyl transferase I activity

It has been possible to show by electron microscopy of samples selected from sucrose gradients that particles of specific size and shape are present in supernatant fluids derived from nucleated animal and plant cells, but not in extracts from Escherichia coli. Aminoacyl transferase I activity in these same gradients sediments in two peaks representing material of approximately 5-7S and 18-20S. A rectangular particle, 100 x 145 A in size, sediments at 19S and coincides with the second peak of transferase I activity. The possibility that the rectangular particle may be a "carrier" particle associated with transferase I is discussed.