A supernatant factor involved in initiation complex formation with eukaryotic ribosomes

Direct and high throughput (HT) interactions on the ribosomal surface by iRIA

Ribosomes function as platforms for binding of other molecules, but technologies for studying this process are lacking. Therefore we developed iRIA (in vitro Ribosomes Interaction Assay). In approach I, Artemia salina ribosomes spotted on solid phase are used for binding picomoles of analytes; in approach II, cellular extracts allow the measurement of ribosome activity in different conditions. We apply the method to analyze several features of eIF6 binding to 60S subunits. By approach I, we show that the off-rate of eIF6 from preribosomes is slower than from mature ribosomes and that its binding to mature 60S occurs in the nM affinity range. By approach II we show that eIF6 binding sites on 60S are increased with mild eIF6 depletion and decreased in cells that are devoid of SBDS, a ribosomal factor necessary for 60S maturation and involved in Swachman Diamond syndrome. We show binding conditions to immobilized ribosomes adaptable to HT and quantify free ribosomes in cell extracts. In conclusion, we suggest that iRIA will greatly facilitate the study of interactions on the ribosomal surface.

Comparative analysis of the ribosomal components of the hydrogenosome-containing protist, Trichomonas vaginalis

The ribosomes of the amitochondriate but hydrogenosome-containing protist lineage, the trichomonads, have previously been reported to be prokaryotic or primitive eukaryotic, based on evidence that they have a 70S sedimentation coefficient and a small number of proteins, similar to prokaryotic ribosomes. In order to determine whether the components of the trichomonad ribosome indeed differ from those of typical eukaryotic ribosomes, the ribosome of a representative trichomonad, Trichomonas vaginalis, was characterized. The sedimentation coefficient of the T. vaginalis ribosome was smaller than that of Saccharomyces cerevisiae and larger than that of Escherichia coli. Based on two-dimensional PAGE analysis, the number of different ribosomal proteins was estimated to be approximately 80. This number is the same as those obtained for typical eukaryotes (approximately 80) but larger than that of E. coli (approximately 55). N-Terminal amino acid sequencing of 18 protein spots and the complete sequences of 4 ribosomal proteins as deduced from their genes revealed these sequences to display typical eukaryotic features. Phylogenetic analyses of the five ribosomal proteins currently available also clearly confirmed that the T. vaginalis sequences are positioned within a eukaryotic clade. Comparison of deduced secondary structure models of the small and large subunit rRNAs of T. vaginalis with those of other eukaryotes revealed that all helices commonly found in typical eukaryotes are present and conserved in T. vaginalis, while variable regions are shortened or lost. These lines of evidence demonstrate that the T. vaginalis ribosome has no prokaryotic or primitive eukaryotic features but is clearly a typical eukaryotic type.

Signaling and growth responses of LLC-PK1/Cl4 cells transfected with the rabbit AT1 ANG II receptor

Angiotensin II (ANG II) receptors of the AT1 subtype are present on the apical and basolateral membranes of renal proximal tubule cells. Cells of the proximal tubulelike cell line, LLC-PK1/Cl4, were transfected with an expression plasmid containing cDNA encoding the rabbit AT1 ANG II receptor. In transfected cells, specific binding of 125I-ANG II was detected on both apical and basolateral membranes; wild-type LLC-PK1/Cl4 cells did not express ANG II receptors. In transfected cells, apical or basolateral ANG II increased both S6 kinase activity and incorporation of [3H]leucine. In cells pretreated with pertussis toxin, the stimulatory effect of apical or basolateral ANG II on [3H]leucine incorporation was abolished. In contrast, ANG II did not affect mitogenesis, determined by [3H]thymidine incorporation. Apical or basolateral ANG II (10(-6) M) stimulated phosphoinositide turnover by 13.4 +/- 4.4% (n = 8) and 16.3 +/- 4.2% (n = 9), respectively. The activity of protein kinase C, determined by phosphorylation of a specific protein kinase C peptide substrate, was also stimulated by ANG II in transfected cells. Apical or basolateral ANG II had no significant effect on cellular adenosine 3',5'-cyclic monophosphate levels. In permeabilized transfected cells, apical ANG II (10(-6) M) inhibited the phosphorylation of a specific peptide substrate of protein kinase A; lower apical concentrations or basolateral ANG II were without significant effect. These results indicate that AT1 ANG II receptors sort to both apical and basolateral membranes in renal epithelial cells and are coupled to activation of phospholipase C. ANG II stimulates protein synthesis by binding to either apical or basolateral receptors; this effect requires coupling to G proteins and may be mediated by activation of S6 kinase. Because high concentrations of ANG II exist in proximal tubule, binding to apical and basolateral receptors may regulate proximal tubule cell growth under physiological conditions.

High-pressure-liquid-chromatographic and fluorimetric methods for the determination of adenine released from ribosomes by ricin and gelonin

The high fluorescence of adenine-containing compounds after reaction with chloroacetaldehyde was used to measure the adenine released from rat liver and Artemia salina ribosomes by the action of ricin A chain and gelonin, two ribosome-inactivating proteins (RIPs) that share the same mechanism of action, consisting in the hydrolysis of the N-glycosidic bond of A-4324 of 28 S rRNA. Two methods were employed: (i) h.p.l.c. of the chloroacetaldehyde-reactive material released by RIPs; h.p.l.c. associated with a fluorescence detector allows the identification of adenine and its dosage at quantities as low as 2 ng; (ii) the direct fluorimetric measurement of the material that had reacted with chloroacetaldehyde. The amount of adenine released increases when ribosomes are pretreated in conditions that lead to their dissociation into subunits. Adenine protects ribosomes from the inhibition by ricin A-chain. When ribosomes were incubated with ricin A-chain in the presence of [14C]adenine no incorporation of radioisotope in ribosomes was observed, indicating that neither exchange nor reversal reactions occurred. A binding of [14C]adenine to ricin A chain was not detected by equilibrium dialysis.

Elongation factor 1 beta gamma from Artemia. Purification and properties of its subunits

The guanine nucleotide exchange factor, elongation factor 1 beta gamma (EF-1 beta gamma) has been purified from Artemia cysts using an improved method. The protein consists of two distinct polypeptides with relative molecular masses of 26,000 (EF-1 beta) and 46,000 (EF-1 gamma). A nucleoside diphosphate phosphotransferase activity often found in EF-1 beta gamma preparations has been completely separated from the actual guanine nucleotide exchange stimulatory activity of EF-1 beta gamma, thus indicating that nucleotide diphosphate phosphotransferase is not an intrinsic property of EF-1 beta. Both EF-1 beta gamma and EF-1 beta have been shown to stimulate the following three reactions to a comparable degree: (a) exchange of GDP bound to EF-1 alpha with exogenous GDP; (b) EF-1 alpha-dependent binding of Phe-tRNA to ribosomes; (c) poly(U)-dependent poly(phenylalanine) synthesis. However, a significantly higher nucleotide exchange rate was observed in the presence of EF-1 beta gamma compared to EF-1 beta alone. Concerning elongation factor 1 gamma (EF-1 gamma) the following observations were made. In contrast to EF-1 beta, pure EF-1 gamma is rather insoluble in aqueous buffers, but the tendency to precipitate can be partially suppressed by the addition of detergents. In particular, EF-1 gamma partitions solely into the detergent phase of Triton X-114 solutions. EF-1 gamma is also more susceptible to spontaneous, specific fragmentation. It is remarkably that about 5% of the cellular pool of EF-1 beta gamma was found to be present in membrane fractions, under conditions where no EF-1 alpha was detectable in these fractions. Furthermore it was noted that EF-1 beta gamma copurified strongly with tubulin on DEAE-cellulose. Moreover, it was observed that from a mixture of EF-1 beta gamma and tubulin, EF-1 gamma coprecipitates with tubulin using a non-denaturating immunoprecipitation technique. These findings suggest that EF-1 gamma has a hydrophobic domain and interacts with membrane and cytoskeleton structures in the cell.

Characterization of the elongation factors from calf brain. 1. Purification, molecular and immunological properties

This work describes a method for the purification of the elongation factors (EF) from calf brain. The elongation factor responsible for the binding of aminoacyl-tRNA to the ribosome is found in this organ as a light form (EF-1 alpha) and as a component of heavy, polydispersed aggregates (EF-1H). EF-1 beta, the factor enhancing the EF-1 alpha GDP/GTP exchange, is part of EF-1H and of smaller aggregates. The fraction of EF-1 alpha and EF-1 beta not associated with EF-1H, and EF-2 have been purified to homogeneity after several chromatographic steps. EF-1H consists of many proteins; among them, EF-1 alpha, EF-1 beta and an EF-1 gamma-like protein represent three of the major components. This conclusively shows that EF-1H from calf brain is not a polydispersed aggregate of only EF-1 alpha. EF-1 beta has also been purified to homogeneity from EF-1H. The property of EF-1 beta to aggregate with other proteins suggests that this factor plays an important role in the organization of EF-1H. The relative molecular mass of the purified factors have been determined as: EF-1 alpha, 50,000; EF-1 beta, 30,000; the EF-1 gamma-like component, 49,000; EF-2, 85,000. Some cross-reactivity with the antibodies against the prokaryotic counterparts has been shown for EF-1 alpha, EF-1 beta and EF-2 by functional and immuno-precipitation methods, suggesting the existence of structural homologies.

Characterization of the elongation factors from calf brain. 2. Functional properties of EF-1 alpha, the action of physiological ligands and kirromycin

The properties of EF-1 alpha from calf brain have been investigated and compared with those of EF-Tu. EF-1 alpha binds GDP and GTP in a 1:1 stoichiometry, showing the same affinity for both nucleotides (K'd = 2-4 microM). EF-1 beta strongly enhances the dissociation rate of the EF-1 alpha X GDP complex and to a lesser extent of the EF-1 alpha X GTP complex. Aminoacyl-tRNA (aa-tRNA) stabilized EF-1 alpha X GTP much less efficiently than the EF-Tu X GTP complex. Unlike EF-Tu, EF-1 alpha sustains the binding of aa-tRNA to the ribosome also in the presence of GDP or in the absence of any nucleotide, though to a lesser degree than with GTP. Kirromycin enhances the dissociation rate of both EF-1 alpha X GTP and EF-1 alpha X GDP but especially that of the latter. This effect results in an increase of the exchange rate of the EF-1 alpha-bound nucleotide with free nucleotides. Although in this regard the effect of kirromycin mimics that of EF-1 beta, the antibiotic is incapable of increasing the EF-1 alpha X GDP/GTP exchange rate when aa-tRNA and ribosomes are present. Therefore, unlike EF-1 beta, kirromycin cannot enhance the rate of poly(Phe) synthesis. On the other hand, the failure of kirromycin to induce a GTP-like conformation of EF-1 alpha X GDP, as in the case of EF-Tu X GDP, explains its inability to inhibit peptide bond formation in the eukaryotic system.

Changes in levels of purine and pyrimidine nucleotides during acute hypoxia and recovery in neonatal rat brain

Neonatal rat brains were examined for changes in levels of ATP, ADP, AMP, cyclic AMP, GTP, GDP, UTP, UDP, UMP, and CTP during exposure to 100% nitrogen for 20 min and subsequent recovery in air. During hypoxia, ATP, GTP, UTP, and CTP levels and the GTP/GDP ratio decreased to 38, 50, 26, 21, and 21%, respectively, of control levels. No significant change in cyclic AMP level was observed. The decrease in the total uridine nucleotide pool during hypoxia was markedly greater (to 53% of control levels) than that in the total adenine nucleotide pool (to 92% of control levels). During recovery, ATP and GTP levels were rapidly and almost completely restored. On the other hand, CTP levels returned slowly to control values after a 2-h recovery period. Restoration of the UTP level was slow and incomplete (87% of the control value even after a 3-h recovery period). The GTP/GDP ratio also did not return to normal. These data suggest that hypoxic insult to the neonate may have an effect on the synthesis of nucleotidyl sugars, phospholipids, and proteins in the brain, resulting in significant problems with developmental processes of the brain. The present study also showed that the delayed restorations of the UTP level and the GTP/GDP ratio were not seen in the brains of adult rats subjected to acute severe hypoxic insult.

Conservation of RNA sequence and cross-linking ability in ribosomes from a higher eukaryote: photochemical cross-linking of the anticodon of P site bound tRNA to the penultimate cytidine of the UACACACG sequence in Artemia salina 18S rRNA

The complex of Artemia salina ribosomes and Escherichia coli acetylvalyl-tRNA could be cross-linked by irradiation with near-UV light. Cross-linking required the presence of the codon GUU, GUA being ineffective. The acetylvalyl group could be released from the cross-linked tRNA by treatment with puromycin, demonstrating that cross-linking had occurred at the P site. This was true both for pGUU- and also for poly(U2,G)-dependent cross-linking. All of the cross-linking was to the 18S rRNA of the small ribosomal subunit. Photolysis of the cross-link at 254 nm occurred with the same kinetics as that for the known cyclobutane dimer between this tRNA and Escherichia coli 16S rRNA. T1 RNase digestion of the cross-linked tRNA yielded an oligonucleotide larger in molecular weight than any from un-cross-linked rRNA or tRNA or from a prephotolyzed complex. Extended electrophoresis showed this material to consist of two oligomers of similar mobility, a faster one-third component and a slower two-thirds component. Each oligomer yielded two components on 254-nm photolysis. The slower band from each was the tRNA T1 oligomer CACCUCCCUVACAAGp, which includes the anticodon. The faster band was the rRNA 9-mer UACACACCGp and its derivative UACACACUG. Unexpectedly, the dephosphorylated and slower moving 9-mer was derived from the faster moving dimer. Deamination of the penultimate C to U is probably due to cyclobutane dimer formation and was evidence for that nucleotide being the site of cross-linking. Direct confirmation of the cross-linking site was obtained by "Z"-gel analysis [Ehresmann, C., & Ofengand, J. (1984) Biochemistry 23, 438-445].(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleotide sequence of a crustacean 18S ribosomal RNA gene and secondary structure of eukaryotic small subunit ribosomal RNAs

The primary structure of the gene for 18 S rRNA of the crustacean Artemia salina was determined. The sequence has been aligned with 13 other small ribosomal subunit RNA sequences of eukaryotic, archaebacterial, eubacterial, chloroplastic and plant mitochondrial origin. Secondary structure models for these RNAs were derived on the basis of previously proposed models and additional comparative evidence found in the alignment. Although there is a general similarity in the secondary structure models for eukaryotes and prokaryotes, the evidence seems to indicate a different topology in a central area of the structures.

Simplified in vitro system for study of eukaryotic mRNA translation by measuring di- and tripeptide formation

An in vitro system for measurement of rabbit globin mRNA translation has been developed based on the formation of the NH2-terminal dipeptide, fMet-Val. The basic components include a partially purified initiation factor preparation from rabbit reticulocytes supplemented with eukaryotic initiation factor 4A, purified and formylated yeast Met-tRNAi, and rabbit liver or Escherichia coli Val-tRNA1Val. Picomole quantities of fMet-Val are synthesized, dependent on mRNA, and the dipeptide is readily assayed by a simple extraction procedure. In the presence of Leu-tRNA or His-tRNA, the tripeptides fMet-Val-Leu and fMet-Val-His are synthesized, corresponding to the NH2-terminal sequence of alpha- and beta-globin, respectively. Therefore, tripeptide synthesis provides a simple means to distinguish between the expression of the alpha- and beta-globin mRNA species.

The sequence of the 5.8 S ribosomal RNA of the crustacean Artemia salina. With a proposal for a general secondary structure model for 5.8 S ribosomal RNA

We report the primary structure of 5.8 S rRNA from the crustacean Artemia salina. The preparation shows length heterogeneity at the 5'-terminus, but consists of uninterrupted RNA chains, in contrast to some insect 5.8 S rRNAs, which consist of two chains of unequal length separated in the gene by a short spacer. The sequence was aligned with those of 11 other 5.8 S rRNAs and a general secondary structure model derived. It has four helical regions in common with the model of Nazar et al. (J. Biol. Chem. 250, 8591-8597 (1975)), but for a fifth helix a different base pairing scheme was found preferable, and the terminal sequences are presumed to bind to 28 S rRNA instead of binding to each other. In the case of yeast, where both the 5.8 S and 26 S rRNA sequences are known, the existence of five helices in 5.8 S rRNA is shown to be compatible with a 5.8 S - 26 S rRNA interaction model.

The sequence of 5S ribosomal RNA of the crustacean Artemia salina

The primary structure of the 5 S rRNA isolated from the cryptobiotic cysts of the brine shrimp Artemia salina is pACCAACGGCCAUACCACGUUGAAAGUACCCAGUCUCGUCAGAUCCUGGAAGUCACACAACGUCGGGCCCGGUCAGUACUUGGAUGGGUGACCGCCUGGGAACACCGGGUGCUGUUGGCAU (OH).

Partial purification and characterization of the proteins from the 40-S ribosomes of Artemia salina and wheat germ

The proteins were extracted from purified 40-S ribosomes derived from wheat germ and Artemia salina and separated by carboxymethylcellulose ion-exchange chromatography. Approximately four proteins from Artemia and four proteins from wheat germ were separated in a state of high purity. All proteins were identified by co-electrophoresis using a two-dimensional polyacrylamide gel system. A total of 30 unique proteins were found for Artemia and 32 proteins for wheat. The molecular weights of all proteins were estimated by sodium dodecylsulfate gel electrophoresis. Assuming each protein to be present in one copy per 40-S ribosome, the total protein molecular weight was estimated to be 560,000 associated with Artemia 40-S particles and 550,000 associated with wheat germ 40-S ribosomes.

Ricin and modeccin do not inhibit the elongation factor 1-dependent binding of aminoacyl-tRNA to ribosomes

Ricin and modeccin do not affect the total number of ribosomes to which phenylalanyl-tRNA becomes bound in the EF 1-dependent reaction. Previous inconsistencies resulted from the use of the nitrocellulose-filter technique, which overestimates the number of control ribosomes engaged in the binding reaction if trace amounts of EF 2 contaminate the ribosomal preparations.

Structure of functional A salina -- E coli hybrid ribosome by electron microscopy

Small 40S Artemia salina and large 50S Escherichia coli ribosomal subunits can be assembled into 73S hybrid monosomes active in model assays for protein synthesis. The reciprocal combination -- small 30S E coli and large 60S A salina -- fails to form hybrids. The 73S hybrid particles strongly resemble homologous 70S E coli and 80S A salina monosomes. The morphologic differences between the corresponding eukaryotic and prokaryotic ribosomal particles, established by electron microscopy, do not significantly affect the assembly and mutual orientation of 40S A slina and 50S E coli subunits in the heterologous monosome. The fact that the structure of the interface, the supposed site of protein synthesis, is preserved in the active hybrid implies that retention or loss of biologic activity of hybrid ribosomes is determined by the extent of conformational changes in the interface.

Effect of modeccin on the steps of peptide-chain elongation

Modeccin inhibits polypeptide-chain elongation catalysed by Artemia salina (brine shrimp) ribosomes by inactivating the 60 S ribosomal subunit. Among the individual steps of elongation, peptide-bond formation, catalysed by 60 S peptidyltransferase, is unaffected by the toxin, whereas the binding of EF 2 (elongation factor 2) to ribosomes is strongly inhibited. Modeccin does not affect the poly(U)-dependent non-enzymic binding of either deacylated tRNAPhe or phenylalanyl-tRNA to ribosomes. The inhibitory effect of modeccin on the EF 1 (elongation factor 1)-dependent binding of phenylalanyl-tRNA is discussed, since it is decreased by tRNAPhe, which stimulates the binding reaction. The analysis of the distribution of ribosome-bound radioactivity during protein synthesis shows that modeccin consistently inhibits the radioactivity bound as long-chain peptides, but depending on the experimental conditions, can leave unchanged or even greatly stimulates the radioactivity bound as phenylalanyl-tRNA and/or short-chain peptides. It is concluded that, during the complete elongation cycle, modeccin does not affect the binding of the first aminoacyl-tRNA to ribosomes, but inhibits some step in the subsequent repetitive activity of either EF 1 or EF 2. The results obtained indicate that the mechanism of action of modeccin is very similar to that of ricin and related plant toxins such as abrin and crotin.

Isolation and characterization of the acidic phosphoproteins of 60-S ribosomes from Artemia salina and rat liver

Eucaryotic L7/L12-type proteins are present in ethanol/salt extracts (P1 protein) of ribosomes from Artemia salina and rat liver. These proteins are partially phosphorylated and occur in two forms of closely related structure: a major form eL12 having methionine at the N-terminal position and a minor form of eL12 (eL12') which seems slightly elongated and contains a blocked N terminus. Purification of the four different forms of this protein, eL12, eL12-P, eL12' and eL12'-P, was performed by ion-exchange chromatography on carboxymethyl-cellulose and DEAE-cellulose. Using a radioimmuno assay, 1.8 copies of eL12 and 0.9 of eL12' were found on the 80-S A. salina ribosome. In ribosomes of both rat liver and A. salina, eL12 is present in a larger quantity than eL12'. 40-S and 60-S ribosomal subunits extracted with ethanol/salt were essentially free of eL12 proteins. A large pool of eL12 was found in the cytosol after removal of the ribosomes by centrifugation or molecular sieving. The proteins of rat liver and A. salina are similar with regard to their isoelectric points and molecular weights. Sedimentation equilibrium studies indicated that the isolated protein eL12 occurs as a dimer.

The role of guanine nucleotides in the interaction between aminoacyl-tRNA and elongation factor 1 of Artemia salina

The low-molecular-weight form of elongation factor 1 (EF-1L) of the cysts of the brine shrimp Artemia salina and [3H]phenylalanyl-tRNA are able to form a stable complex which can be isolated on a Sephacryl S200 column. The formation of this complex is inhibited by increasing concentrations of magnesium acetate and KCl. Furthermore, the formation of this complex is independent of the presence of guanine nucleotides. Complex formation between EF-1L and phenylalanyl-tRNA appears to be specific, since acylation of the tRNA is a necessity for this interaction. Although EF-1L alone binds GDP somewhat more strongly than GTP, the complex between EF-1L and phenylalanyl-tRNA binds GTP exclusively. Our results support the idea that complex formation between EF-1L and aminoacyl-tRNA precedes the enzymatic binding of aminoacyl-tRNA to the 80-S ribosome. Subsequently to this binding, release of EF-1L from the ribosome occurs.

Comparison of Artemia salina and Escherichia coli ribosome structure by electron microscopy

The structure of eukaryotic Artemia salina and prokaryotic Escherichia coli ribosomes has been compared by electron microscopy. Despite the established differences in size and in the amount and proportion of the protein and RNA moieties, both types of ribosomes appear to have substantial similarity in the overall shape and in the mutual orientation of the subunits on the monosome. The small subunit is located in the "crown" region of the large subunit lengthwise between the two side crests. However, high-resolution electron microscopy reveals distinct differences in the fine structure of both small and large subunits. The 40S A. salina subunit with three structural domains is more complex than the corresponding E. coli subunit. The 60S A. salina subunit has a less expressed "crown" region and shows a knob-like protrusion in the base. Structural asymmetry is a characteristic feature common to subunits and monosomes from both A. salina and E. coli.

Mode of action of the hemin-controlled inhibitor of protein synthesis

Despite the finding that the hemin-controlled translational inhibitor in reticulocyte lysates is a cyclic AMP-independent protein kinase that phosphorylates the small subunit of the initiation factor eIF-2, the mechanism of inhibition of translation remained unexplained. Whereas treatment of hemin-containing lysates with inhibitor in the presence of ATP inhibited translation, the same treatment of highly purified eIF-2 did not affect its ability to form a ternary complex with initiator Met-tRNA and GTP or a 40S initiation complex. We have isolated from ribosomal salt washes a protein (eIF-2 stimulating protein) that enhances the capacity of unphosphorylated eIF-2 to form ternary or 40S initiation complexes but has no effect on the phosphorylated factor. At low concentrations, eIF-2 is virtually inactive without this stimulating protein. Therefore, the translational inhibitor acts by converting eIF-2 to a form that is not stimulated by the stimulating protein.

Initiation of protein synthesis in eukaryotes. Binding to Sepharose-heparin and partial purification of initiation factors from Krebs II ascites cells

By means of affinity chromatography of lysates from Krebs II ascites cells and rabbit reticulocytes on Sepharose-heparin an active fraction of initiation factors has been obtained. The fraction is eluted from the column at 350 mM KCl using a linear gradient and displays a number of activities, i.e. binding of Met-tRNAfMet to form a ternary complex with GTP; transferring this complex to 40-S subunits in an A-U-G-independent step and finally coupling of the 40-S initiation complex to the 60-S subunit, a reaction which is completely A-U-G-dependent. Moreover, MettRNA is bound into the P-site as is indicated by its puromycin sensitivity. The method is very suitable for large-scale preparation. Further purification and characterization of the factors have been carried out on DEAE-cellulose and phosphocellulose columns. Evidence is presented that the polysomes present in a lysate that has been passed through the Sepharose-heparin column can only complete their nascent chains, initiation of new polypeptides is completely dependent on addition of initiation factors.

Role of GTP in eukaryotic polypeptide-chain initiation. Purification and properties of a factor from Artemia salina embryos which interacts with initiator transfer RNA and guanine nucleotides

1. A factor, which makes a ternary complex with GTP and eukaryotic initiator tRNA (Met-tRNAi), has been purified 100-fold from developed cysts of Artemia salina. Some of the properties of the purified factor have been studied. 2. Mg2+ appears to inhibit ternary complex formation. 3. Little or no ternary complex is formed when 5 muM GTP is replaced by an identical concentration of UTP, CTP or ATP. The analog, guanosine 5'-(beta, gamma-imino)triphosphate [GMP-P(NHP)] seems to be a much better substitute for GTP than guanosine 5'-(beta, gamma-methylene)triphosphate [GMP-P(CH2)P]. Since GMP-P(NH)P is as effective as GTP in ternary complex formation, it would appear that GTP plays the role of an allosteric effector in this step of eukaryotic polypeptide chain initiation. 4. GDP inhibits both the rate and extent of ternary complex formation. The inhibition is largely reversed by adding a 5-fold molar excess of GTP over GDP. DGDP is slightly less inhibitory than GDP. UDP and CDP are much less inhibitory than GDP and very little inhibition is obtained with ADP. 5. The preformed ternary complex is rapidly and completely destroyed in the presence of N-ethylmaleimide. The results suggest that free--SH groups of the factor may be essential for maintaining the integrity of the ternary complex.

Specificity of the protein kinase activity associated with the hemin-controlled repressor of rabbit reticulocyte

Highly purified preparations of hemin-controlled repressor of rabbit reticulocyte contain a 3':5'-cyclic AMP-indenpendent protein kinase activity that phosphorylates the low-molecular-weight (about 38,000) polypeptide chain of the initiation factor that forms a ternary complex with GTP and Met-tRNAf. These preparations also phosphorylate several polypeptide components of reticulocyte 40S ribosomal subunits. However, no significant levels of phosphorylation are observed when casein, histones, Artemia salina 40S ribosomal subunits, or other initiation factor fractions are used as substrates although high levels of phosphorylation are obtained with cruder preparations of the repressor. An antibody to these highly purified preparations of repressor has been obtained from the serum of immunized goats. Preincubation with immune goat IgG results in the neutralization of the inhibitory activity of the repressor, while normal IgG has no effect. Preincubation with immune IgG also abolishes the protein kinase activity responsible for the phosphorylation of the initiation factor and reticulocyte 40S subunits. Histone phosphorylation by crude repressor preparations, on the other hand, is unaffected by preincubation with immune IgG.

Elongation factor 1 from Artemia salina: properties and disaggregation of the enzyme

Elongation factor 1 has been purified from undeveloped embryos of Artemia salina. The purified enzyme appears to be an aggregate (molecular weight approximately to 200 000) which on sodium dodecylsulfate gels shows the presence of two major protein bands whose estimated molecular weights are 52 000 and 47 000. Lipid material appears to be associated with the purified protein. In aminoacyl-tRNA binding to ribosomes, there is only a limited turnover of the enzyme, but the protein acts catalytically in amino acid polymerization. The enzyme is disaggregated by a partially purified phospholipase C preparation, elastase and under certain conditions, by guanosine nucleotides. The significance of these results is discussed with respect to the overall role of elongation faction 1 in aminoacyl-tRNA binding to ribosomes.

On the mode of action of eucaryotic initiation factor 1

A factor isolated from the human tonsillar ribosomal wash specifically stimulated the poly (U)-dependent binding of Phe-tRNA to 40S subunits at low Mg2+ concentration and without any requirement for GTP. The stimulated binding of Phe-tRNA to 40S particles was inhibited in proportion to added deacylated tRNA. The factor was inactivated by N-ethylmaleimide, but in the presence of 40S subunits a considerable protection was observed. 40S subunits, incubated with the factor and isolated by centrifugation, carried significant factor activity. The results imply that the human tonsillar factor, which shows a great functional analogy to the eucaryotic initiation factor 1 from other sources, exerts its effect by an initial interaction with the 40S subunit.

Inhibition of protein synthesis in vitro by crotins and ricin. Effect on the steps of peptide chain elongation

The effects of crotin I and crotin II on the partial reactions of polypeptide chain elongation were investigated and compared with the known effects of ricin. Crotin II was a more powerful inhibitor than crotin I, but no qualitative differences between the two crotins were found. Rat liver ribosomes, preincubated with crotins and washed through sucrose gradients, remained inactive in protein synthesis. Among the individual steps of elongation, the peptidyltransferase reaction was unaffected by crotins, but some of the reactions that involve the interaction of elongation factors 1 and 2 with ribosomes were modified. A strong inhibition of the binding of elongation factor 2 to ribosomes and a stimulation of the elongation factor2-dependent GTP hydrolysis were observed; this indicates the formation of a very unstable elongation factor 2--GDP--ribosome complex, which, however, allows a single round of translocation to take place in the presence ofelongation factor 2 and added GTP. The elongation factor 1-dependent GTP hydrolysis was inhibited by crotins, whereas the enzymic binding of aminoacyl-tRNA, to both rat liver and Artemia salina ribosomes, was scarcely affected. In a protein-synthesizing system the inhibition by crotins and by ricin leads to a block of the nascent peptides on the ribosomal aminoacyl-tRNA site, an effect consistent with inhibition at the level of translocation. The mechanism of action of crotins appears to be very similar to that of ricin.

Polypeptide chain initiation in eukaryotes: mechanism of formation of initiation complex

Artemia salina ribosomal subunits and highly purified reticulocyte initiation factors (IF) are used to study the mechanism of formation of the puromycin-sensitive initiation complex Met-tRNAi-80S ribosome-AUG. A complex with equimolar amounts of 40S subunit, GTP, and Met-tRNAi is formed at low Mg2+ concentration with a requirement for IF-MP (homogeneous) but not AUG or other factors. An 80S complex is formed only upon the further addition of AUG, IF-M2A, and IF-M2B, but not of either factor alone. This complex contains no GTP or GDP. A 40S complex, which cannot be converted to an 80S one, is formed when the nonhydrolyzable analog GMPPCP is substituted for GTP. IF-M2A has no effect on the formation of this complex, but IF-M2B enhances its formation.

Polypeptide chain initiation in eukaryotes: initiation factor requirements for translation of natural messengers

A protein-synthesizing system consisting of ribosomes and supernatant of undeveloped Artemia saline embryos is used for assay of mRNA translation and initiation factors. This system contains the components needed for chain elongation but has low levels of mRNA and initiation factors. Exogenous mRNA is readily translated upon addition of high-speed supernatant or ribosomal salt wash of developing embryos as a source of initiation factors (IF). This requirement can be largely satisfied by a mixture of the reticulocyte factors IF-MP, IF-M3, IF-M2A, and IF-M2B. A. salina IF-M1, which is present in undeveloped embryo supernatant, can be inactivated by A. salina IF-M1 antibody without affecting translation.

Isolation and characterization of two acidic proteins of 60s ribosomes from Artemia salina cysts

60S ribosomes from encysted gastrulae of the brine shrimp Artemia salina contain two acidic proteins, which are homologous to the Escherichia coli proteins L7 and L12. The proteins were purified and characterized with respect to molecular weight, amino-acid composition, peptide maps, and their functional requirement in the elongation factor dependent binding of aminoacyl transfer RNA to the ribosome.

Sites of action of fusidic acid in eukaryotes. Inhibition by fusidic acid of a ribosome-independent GTPase from Artemia salina embryos

1. A ribosome-independent GTPase activity has been isolated from the high-speed supernatant fraction of Artemia salina embryos, and some of its properties have been studied. This activity is inhibited by fusidic acid, an antibiotic generally thought to inhibit only EF-2 in eukaryotes. However, several lines of evidence indicate that the GTPase activity, described here, is distinct from EF-2. The results suggest, therefore, that the inhibitory effect of fusidic acid in eukaryotic systems is not restricted to EF-2 (and ribosome)-dependent functions only. 2. The results of other experiments have revealed that, despite its ability to inhibit the GTPase activity mentioned above, fusidic acid is not a non-specific inhibitor of all ribosome-independent GTPase and ATPase activities present in eukaryotic cells.

Polypeptide chain initiation in eukaryotes: initiation factor MP in Artemia salina embryos

The activity of IF-MP, a polypeptide chain initiation factor that forms a ternary complex with eukaryotic initiator Met-tRNA and GTP and promotes binding of the initiator to 40S ribosomes, is very low in undeveloped Artemia salina embryos but increases over 20-fold following resumption of development upon hydration of the cysts. The factor is present in both the ribosomal salt wash and high-speed-supernatant. Its specific activity is 50 times higher in the wash but its total activity is only about twice as high in the wash as in the supernatant. As is true of IF-MP from other eukaryotic sources, the A. salina factor is specific for eukaryotic Met-tRNAi and sensitive to SH-reagents, and its activity is GTP dependent.

Nonribosomal proteins associated with eukaryotic native small ribosomal subunits

The native small ribosomal subunit (Sn) from rabbit reticulocytes which is able to initiate translation of globin mRNA in a cell-free system carries additional protein components. The latter can be separated from the subunit in a high salt sucrose gradient yielding a top fraction (T) and a complex fraction (C), sedimenting at about 4 and 15 S, respectively. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate revealed that fraction T contained four dominant polypeptides, while fraction C represents a large protein complex consisting of at least 10 polypeptides. Sn isolated from other sources showed similar patterns of their nonribosomal proteins. Reconstitution experiments revealed that fraction C is absolutely required for protein synthesis, while fraction T enhances protein synthesis only in the presence of C. The adherence of these protein factors to the subunit is not mediated by magnesium ions. Treatment of Sn with EDTA and centrifugation in a magnesium-free sucrose gradient caused unfolding of the subunits and dissociation of several ribosomal proteins, but not of the factors. The unfolded ribosomal subunits sedimented as two distinct peaks. The more slowly sedimenting peak contained proteins of fraction T and the faster sedimenting one contained the 15S complex, indicating heterogeneity of the Sn population with respect to the factors attached to them.

The binding sites for tRNA on eukaryotic ribosomes

We have studied the non-enzymic binding of phe-tRNA to ribosomes from rat liver using deacylated tRNA to inhibit binding to the P-site and puromycin (5 x 10-minus3M) to inhibit binding to the A-site. We conclude that at a low concentration of magnesium ions (10mM) phe-tRNA is bound only at the A-site of 80S irbosomes, whereas at a high concentration of magnesium ions (40mM) phe-tRNA is also bound at the P-site. Studies with edeine indicate that, during non-enzymic binding of phe-tRNA, eukaryotic ribosomes (in contrast to prokarotic ribosomes) have the A-site of the 60S subunit and the initiation site of the 40S subunit juxtaposed. This may account for the differences observed, in formation of diphenylalanyl-tRNA and phenylalanyl-puromycin, between phe-tRNA bound non-enzymically to the P-sites of eukaryotic and prokaryotic ribosomes.