Initiation of the polypeptide chain by reticulocyte cell-free systems. Survey of different inhibitors of translation

A Quick Guide to Small-Molecule Inhibitors of Eukaryotic Protein Synthesis

Eukaryotic ribosome and cap-dependent translation are attractive targets in the antitumor, antiviral, anti-inflammatory, and antiparasitic therapies. Currently, a broad array of small-molecule drugs is known that specifically inhibit protein synthesis in eukaryotic cells. Many of them are well-studied ribosome-targeting antibiotics that block translocation, the peptidyl transferase center or the polypeptide exit tunnel, modulate the binding of translation machinery components to the ribosome, and induce miscoding, premature termination or stop codon readthrough. Such inhibitors are widely used as anticancer, anthelmintic and antifungal agents in medicine, as well as fungicides in agriculture. Chemicals that affect the accuracy of stop codon recognition are promising drugs for the nonsense suppression therapy of hereditary diseases and restoration of tumor suppressor function in cancer cells. Other compounds inhibit aminoacyl-tRNA synthetases, translation factors, and components of translation-associated signaling pathways, including mTOR kinase. Some of them have antidepressant, immunosuppressive and geroprotective properties. Translation inhibitors are also used in research for gene expression analysis by ribosome profiling, as well as in cell culture techniques. In this article, we review well-studied and less known inhibitors of eukaryotic protein synthesis (with the exception of mitochondrial and plastid translation) classified by their targets and briefly describe the action mechanisms of these compounds. We also present a continuously updated database (http://eupsic.belozersky.msu.ru/) that currently contains information on 370 inhibitors of eukaryotic protein synthesis.

Trichothecin Inhibits Cancer-Related Features in Colorectal Cancer Development by Targeting STAT3

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that contributes to cancer progression through multiple processes of cancer development, which makes it an attractive target for cancer therapy. The IL-6/STAT3 pathway is associated with an advanced stage in colorectal cancer patients. In this study, we identified trichothecin (TCN) as a novel STAT3 inhibitor. TCN was found to bind to the SH2 domain of STAT3 and inhibit STAT3 activation and dimerization, thereby blocking STAT3 nuclear translocation and transcriptional activity. TCN did not affect phosphorylation levels of STAT1. TCN significantly inhibited cell growth, arrested cell cycle at the G0/G1 phase, and induced apoptosis in HCT 116 cells. In addition, the capacities of colony formation, migration, and invasion of HCT 116 cells were impaired upon exposure to TCN with or without IL-6 stimulation. In addition, TCN treatment abolished the tube formation of HUVEC cells in vitro. Taken together, these results highlight that TCN inhibits various cancer-related features in colorectal cancer development in vitro by targeting STAT3, indicating that TCN is a promising STAT3 inhibitor that deserves further exploration in the future.

Tumour-specific proline vulnerability uncovered by differential ribosome codon reading

Tumour growth and metabolic adaptation may restrict the availability of certain amino acids for protein synthesis. It has recently been shown that certain types of cancer cells depend on glycine, glutamine, leucine and serine metabolism to proliferate and survive. In addition, successful therapies using L-asparaginase-induced asparagine deprivation have been developed for acute lymphoblastic leukaemia. However, a tailored detection system for measuring restrictive amino acids in each tumour is currently not available. Here we harness ribosome profiling for sensing restrictive amino acids, and develop diricore, a procedure for differential ribosome measurements of codon reading. We first demonstrate the functionality and constraints of diricore using metabolic inhibitors and nutrient deprivation assays. Notably, treatment with L-asparaginase elicited both specific diricore signals at asparagine codons and high levels of asparagine synthetase (ASNS). We then applied diricore to kidney cancer and discover signals indicating restrictive proline. As for asparagine, this observation was linked to high levels of PYCR1, a key enzyme in proline production, suggesting a compensatory mechanism allowing tumour expansion. Indeed, PYCR1 is induced by shortage of proline precursors, and its suppression attenuated kidney cancer cell proliferation when proline was limiting. High PYCR1 is frequently observed in invasive breast carcinoma. In an in vivo model system of this tumour, we also uncover signals indicating restrictive proline. We further show that CRISPR-mediated knockout of PYCR1 impedes tumorigenic growth in this system. Thus, diricore has the potential to reveal unknown amino acid deficiencies, vulnerabilities that can be used to target key metabolic pathways for cancer treatment.

Anti-angiogenic quassinoid-rich fraction from Eurycoma longifolia modulates endothelial cell function

Targeting angiogenesis could be an excellent strategy to combat angiogenesis-dependent pathophysiological conditions such as cancer, rheumatoid arthritis, obesity, systemic lupus erythematosus, psoriasis, proliferative retinopathy and atherosclerosis. Recently a number of clinical investigations are being undertaken to assess the potential therapeutic application of various anti-angiogenic agents. Many of these angiogenesis inhibitors are directed against the functions of endothelial cells, which are considered as the building blocks of blood vessels. Similarly, roots of a traditional medicinal plant, Eurycoma longifolia, can be used as an alternative treatment to prevent and treat the angiogenesis-related diseases. In the present study, antiangiogenic potential of partially purified quassinoid-rich fraction (TAF273) of E. longifolia root extract was evaluated using ex vivo and in vivo angiogenesis models and the anti-angiogenic efficacy of TAF273 was investigated in human umbilical vein endothelial cells (HUVEC). TAF273 caused significant suppression in sprouting of microvessels in rat aorta with IC50 11.5μg/ml. TAF273 (50μg/ml) showed remarkable inhibition (63.13%) of neovascularization in chorioallantoic membrane of chick embryo. Tumor histology also revealed marked reduction in extent of vascularization. In vitro, TAF273 significantly inhibited the major angiogenesis steps such as proliferation, migration and differentiation of HUVECs. Phytochemical analysis revealed high content of quassinoids in TAF273. Specially, HPLC characterization showed that TAF273 is enriched with eurycomanone, 13α(21)-epoxyeurycomanone and eurycomanol. These results demonstrated that the antiangiogenic activity of TAF273 may be due to its inhibitory effect on endothelial cell proliferation, differentiation and migration which could be attributed to the high content of quassinoids in E. longifolia.

The bacterial translocon SecYEG opens upon ribosome binding

In co-translational translocation, the ribosome funnel and the channel of the protein translocation complex SecYEG are aligned. For the nascent chain to enter the channel immediately after synthesis, a yet unidentified signal triggers displacement of the SecYEG sealing plug from the pore. Here, we show that ribosome binding to the resting SecYEG channel triggers this conformational transition. The purified and reconstituted SecYEG channel opens to form a large ion-conducting channel, which has the conductivity of the plug deletion mutant. The number of ion-conducting channels inserted into the planar bilayer per fusion event roughly equals the number of SecYEG channels counted by fluorescence correlation spectroscopy in a single proteoliposome. Thus, the open probability of the channel must be close to unity. To prevent the otherwise lethal proton leak, a closed post-translational conformation of the SecYEG complex bound to a ribosome must exist.

Anti-proliferative activity of the quassinoid NBT-272 in childhood medulloblastoma cells

BACKGROUND

With current treatment strategies, nearly half of all medulloblastoma (MB) patients die from progressive tumors. Accordingly, the identification of novel therapeutic strategies remains a major goal. Deregulation of c-MYC is evident in numerous human cancers. In MB, over-expression of c-MYC has been shown to correlate with anaplasia and unfavorable prognosis. In neuroblastoma--an embryonal tumor with biological similarities to MB--the quassinoid NBT-272 has been demonstrated to inhibit cellular proliferation and to down-regulate c-MYC protein expression.

METHODS

To study MB cell responses to NBT-272 and their dependence on the level of c-MYC expression, DAOY (wild-type, empty vector transfected or c-MYC transfected), D341 (c-MYC amplification) and D425 (c-MYC amplification) human MB cells were used. The cells were treated with different concentrations of NBT-272 and the impact on cell proliferation, apoptosis and c-MYC expression was analyzed.

RESULTS

NBT-272 treatment resulted in a dose-dependent inhibition of cellular proliferation (IC50 in the range of 1.7-9.6 ng/ml) and in a dose-dependent increase in apoptotic cell death in all human MB cell lines tested. Treatment with NBT-272 resulted in up to 90% down-regulation of c-MYC protein, as demonstrated by Western blot analysis, and in a significant inhibition of c-MYC binding activity. Anti-proliferative effects were slightly more prominent in D341 and D425 human MB cells with c-MYC amplification and slightly more pronounced in c-MYC over-expressing DAOY cells compared to DAOY wild-type cells. Moreover, treatment of synchronized cells by NBT-272 induced a marked cell arrest at the G1/S boundary.

CONCLUSION

In human MB cells, NBT-272 treatment inhibits cellular proliferation at nanomolar concentrations, blocks cell cycle progression, induces apoptosis, and down-regulates the expression of the oncogene c-MYC. Thus, NBT-272 may represent a novel drug candidate to inhibit proliferation of human MB cells in vivo.

Ribosome binding to and dissociation from translocation sites of the endoplasmic reticulum membrane

We have addressed how ribosome-nascent chain complexes (RNCs), associated with the signal recognition particle (SRP), can be targeted to Sec61 translocation channels of the endoplasmic reticulum (ER) membrane when all binding sites are occupied by nontranslating ribosomes. These competing ribosomes are known to be bound with high affinity to tetramers of the Sec61 complex. We found that the membrane binding of RNC-SRP complexes does not require or cause the dissociation of prebound nontranslating ribosomes, a process that is extremely slow. SRP and its receptor target RNCs to a free population of Sec61 complex, which associates with nontranslating ribosomes only weakly and is conformationally different from the population of ribosome-bound Sec61 complex. Taking into account recent structural data, we propose a model in which SRP and its receptor target RNCs to a Sec61 subpopulation of monomeric or dimeric state. This could explain how RNC-SRP complexes can overcome the competition by nontranslating ribosomes.

PERK is responsible for the increased phosphorylation of eIF2alpha and the severe inhibition of protein synthesis after transient global brain ischemia

Reperfusion after global brain ischemia results initially in a widespread suppression of protein synthesis in neurons that is due to inhibition of translation initiation as a result of the phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2). To address the role of the eIF2alpha kinase RNA-dependent protein kinase-like endoplasmic reticulum kinase (PERK) in the reperfused brain, transgenic mice with a targeted disruption of the Perk gene were subjected to 20 min of forebrain ischemia followed by 10 min of reperfusion. In wild-type mice, phosphorylated eIF2alpha was detected in the non-ischemic brain and its levels were elevated threefold after 10 min of reperfusion. Conversely, there was no phosphorylated eIF2alpha detected in the non-ischemic transgenic mice and there was no sizeable rise in phosphorylated eIF2alpha levels in the forebrain after ischemia and reperfusion. Moreover, there was a substantial rescue of protein translation in the reperfused transgenic mice. Neither group showed any change in total eIF2alpha, phosphorylated eukaryotic elongation factor 2 or total eukaryotic elongation factor 2 levels. These data demonstrate that PERK is responsible for the large increase in phosphorylated eIF2alpha and the suppression of translation early in reperfusion after transient global brain ischemia.

Ribosomal crystallography: peptide bond formation and its inhibition

Ribosomes, the universal cellular organelles catalyzing the translation of genetic code into proteins, are protein/RNA assemblies, of a molecular weight 2.5 mega Daltons or higher. They are built of two subunits that associate for performing protein biosynthesis. The large subunit creates the peptide bond and provides the path for emerging proteins. The small has key roles in initiating the process and controlling its fidelity. Crystallographic studies on complexes of the small and the large eubacterial ribosomal subunits with substrate analogs, antibiotics, and inhibitors confirmed that the ribosomal RNA governs most of its activities, and indicated that the main catalytic contribution of the ribosome is the precise positioning and alignment of its substrates, the tRNA molecules. A symmetry-related region of a significant size, containing about two hundred nucleotides, was revealed in all known structures of the large ribosomal subunit, despite the asymmetric nature of the ribosome. The symmetry rotation axis, identified in the middle of the peptide-bond formation site, coincides with the bond connecting the tRNA double-helical features with its single-stranded 3' end, which is the moiety carrying the amino acids. This thus implies sovereign movements of tRNA features and suggests that tRNA translocation involves a rotatory motion within the ribosomal active site. This motion is guided and anchored by ribosomal nucleotides belonging to the active site walls, and results in geometry suitable for peptide-bond formation with no significant rearrangements. The sole geometrical requirement for this proposed mechanism is that the initial P-site tRNA adopts the flipped orientation. The rotatory motion is the major component of unified machinery for peptide-bond formation, translocation, and nascent protein progression, since its spiral nature ensures the entrance of the nascent peptide into the ribosomal exit tunnel. This tunnel, assumed to be a passive path for the growing chains, was found to be involved dynamically in gating and discrimination.

The major mRNA-associated protein YB-1 is a potent 5' cap-dependent mRNA stabilizer

mRNA silencing and storage play an important role in gene expression under diverse circumstances, such as throughout early metazoan development and in response to many types of environmental stress. Here we demonstrate that the major mRNA-associated protein YB-1, also termed p50, is a potent cap-dependent mRNA stabilizer. YB-1 addition or overexpression dramatically increases mRNA stability in vitro and in vivo, whereas YB-1 depletion results in accelerated mRNA decay. The cold shock domain of YB-1 is responsible for the mRNA stabilizing activity, and a blocked mRNA 5' end is required for YB-1-mediated stabilization. Significantly, exogenously added YB-1 destabilizes the interaction of the cap binding protein, eIF4E, with the mRNA cap structure. Conversely, sequestration of eIF4E from the cap increases the association of endogenous YB-1 with mRNA at or near the cap, and significantly enhances mRNA stability. These data support a model whereby down-regulation of eIF4E activity or increasing the YB-1 mRNA binding activity or concentration in cells activates a general default pathway for mRNA stabilization.

Translation is not required To generate virion precursor RNA in human immunodeficiency virus type 1-infected T cells

The retroviral primary transcription product is a multifunctional RNA that is utilized as pre-mRNA, mRNA, and genomic RNA. The relationship between human immunodeficiency virus type 1 (HIV-1) unspliced transcripts used as mRNA for viral protein synthesis and as virion precursor RNA (vpRNA) for encapsidation remains an important question. We developed a biochemical assay to evaluate the hypothesis that prior utilization as mRNA template for protein synthesis is necessary to generate vpRNA. HIV-1-infected T cells were treated with translation inhibitors under conditions that maintain virus production. Immunoprecipitation of newly synthesized HIV-1 Gag protein revealed that de novo translation is not necessary to sustain assembly, release, or processing of Gag structural protein. Both newly synthesized protein and steady-state Gag are competent for assembly, and the extracellular accumulation of Gag is proportional to the intracellular abundance of Gag. As early as 2 h after transcription, newly synthesized RNA is detectable in cell-free virions and encapsidation is sustained upon inhibition of host cell translation. Results of both [(3)H]uridine incorporation assays and HIV-1-specific RNase protection assays (RPAs) indicate that translation inhibition reduces the absolute amounts of both cytoplasmic and virion-associated RNA. Evaluation of encapsidation efficiency by RPA revealed that the cytoplasmic availability of vpRNA is increased, indicating that HIV-1 unspliced mRNA can be rerouted to function as vpRNA. Our data contrast with results from the HIV-2 and murine leukemia virus systems and indicate that HIV-1 unspliced RNA constitutes a single functional pool that can function interchangeably as mRNA and as vpRNA.

Binding sites of the antibiotics pactamycin and celesticetin on ribosomal RNAs

The binding sites of the antibiotics pactamycin and celesticetin on the rRNAs of Escherichia coli ribosomes were investigated by a chemical footprinting procedure. Pactamycin protected residues G-693 and C-795 in 16S RNA which are located in an important functional region of the 30S subunit participating in initiation complex formation and ribosomal subunit interaction. Celesticetin altered the reactivities of 5 residues A-2058, A-2059, A-2062, A-2451 and G-2505 within the central loop of domain V of 23S RNA which has been implicated in peptidyltransferase activity. Inferences are drawn concerning the mode of action of the antibiotics.

An inhibitor of protein synthesis initiation from Alhagi kirgisorum S

Polyproanthocyanidin--a plant phenolic compound from Alhagi kirgisorum S. effectively inhibited protein synthesis in rabbit reticulocyte and wheat germ cell-free systems. Poly-proanthocyanidin inhibited translation only at the level of initiation and not at the elongation level and aminoacylation of tRNA. The inhibitory effect of the phenolic compound is due to the blockage of the ternary complex formation of eIF-2 with GTP and initiator Met-tRNA.

A single mutation in 16S rRNA that affects mRNA binding and translation-termination

A single base change in 16S rRNA (C726 to G) has previously been shown to have a dramatic effect on protein synthesis in E. coli (1). This paper more specifically details the effects of the mutation on mRNA binding and translation-termination. The in vitro technique of toeprinting (2) was used to demonstrate that 30S subunits containing the mutation 726G had an altered binding affinity for mRNA by comparison to the wild type. In addition, expression of the mutant ribosomes in vivo resulted in exclusive suppression of the UGA nonsense codon. This effect was supported by in vitro studies that showed the mutant ribosomes to have an altered binding affinity for Release Factor-2.

Alkaloid homoharringtonine inhibits polypeptide chain elongation on human ribosomes on the step of peptide bond formation

The aim of the present study was to investigate homoharringtonine alkaloid effect on: (i) the nonenzymatic and eEF-1-dependent Phe-tRNAPhe binding to poly(U)-programmed human placenta 80 S ribosomes; (ii) diphenylalanine synthesis accompanying nonenzymatic Phe-tRNAPhe binding; and (iii) acetylphenylalanyl-puromycin formation. Neither nonenzymatic nor eEF-1-dependent Phe-tRNAPhe binding were noticeably affected by the alkaloid, whereas diphenylalanine synthesis and puromycin reaction were strongly inhibited by homoharringtonine. It has been proposed that the site of homoharringtonine binding on 80 S ribosomes should overlap or coincide with the acceptor site of the ribosome.

In vitro localization of the protein synthesis defect associated with experimental phenylketonuria

We have used a cell-free system derived from hamster brain to investigate protein synthesis during experimental phenylketonuria. In such a system the elongation inhibitor emetine impeded translation in extracts derived from both treated and control animals. On the other hand the initiation inhibitor aurintricarboxylic acid showed no effects on protein synthesis activity of treated hamsters, although it was severely inhibiting in controls. This suggests that initiation is the altered step in brain protein synthesis failure consecutive to phenylketonuria.

Photolabeling of protein components in the pactamycin binding site of rat liver ribosomes

The antitumoral and antibacterial drug pactamycin can be radioactively labeled by iodination without loss of biological activity. Using the labeled pactamycin, the ribosomal binding site of the drug on rat liver ribosomes has been studied by affinity labeling techniques taking advantage of the photoreactive acetophenone group present in the molecule. When 40 S ribosomal subunits are labeled, one major spot of radioactivity is found associated to protein S25. In addition, weaker spots related to proteins S14/15, S10, S17 and S7 can also be detected in the autoradiogram of the two-dimensional gel slab. Since pactamycin inhibits protein synthesis initiation, the proteins forming its binding site must be related to some step of this process. By comparison with results from pactamycin affinity labeling of Escherichia coli ribosomes (Tejedor, F., Amils, R. and Ballesta, J.P.G. (1985) Biochemistry 24, 3667-3672) these proteins could lie in the mRNA and initiation factors binding region of the rat liver ribosome.

Translational initiation factor and ribosome association with the cytoskeletal framework fraction from HeLa cells

The association of mRNA and ribosomes with the cytoskeleton of eucaryotic cells may be important for protein synthesis and its regulation. HeLa cells were gently lysed with detergent, and soluble and cytoskeletal framework subfractions were prepared by centrifugation. We analyzed these fractions for ribosomes and confirmed earlier findings that polysomes are preferentially associated with the cytoskeletal fraction. The levels of initiation factors eIF-2, eIF-3, eIF-4A, and eIF-4B were quantitated by immunoblotting; all are enriched in the cytoskeletal fraction relative to the soluble fraction. Heat shock, fluoride, pactamycin , and cytochalasin caused the release of both ribosomes and initiation factors into the soluble fraction. However, treatment of the cytoskeletal fraction with EDTA or low levels of ribonuclease resulted in polysome degradation but no release. Therefore initiation factor association with the cytoskeletal framework correlates with the presence of ribosomes, whereas ribosome association does not require intact mRNA.

Selective inactivation of rabbit reticulocyte initiation factor eIF-3 by helenalin

Helenalin, a sesquiterpene lactone which reacts primarily with exposed sulfhydryl groups, was shown to be an effective inhibitor of protein synthesis in rabbit reticulocyte lysates. Optimal inhibition required a 30 min preincubation in the absence of any added thiol compound. Beta-Mercaptoethanol was more effective than reduced glutathione in protecting enzyme sulfhydryl groups from inactivation by helenalin. Using partially fractionated systems, it was possible to show that helenalin had no effect on the elongation reactions or on the formation of the ternary initiation complex. However, the conversion of the ternary complex to the 48 S initiation complex was strongly inhibited. In this assay, only the initiation factor(s) were sensitive to helenalin. Using an assay system which requires all the initiation factors for optimal activity it was possible to show that the 0-40% ammonium sulfate cut of initiation factors (containing eIF-3 and eIF-4B) was sensitive to helenalin, while the 40-50% ammonium sulfate cut (containing eIF-2 and eIF-5) was not. Both ammonium sulfate cuts were equally sensitive to inhibition by the sulfhydryl reagent N-ethylmaleimide. Three purified rabbit reticulocyte initiation factors were then tested in the same assay system. Only eIF-3 showed appreciable sensitivity to helenalin, while eIF-2, eIF-3 and eIF-4B were all sensitive to inactivation by N-ethylmaleimide. These data suggest that helenalin may possess a relatively high degree of specificity as a sulfhydryl reagent.

Protein synthesis in lysates of Aedes albopictus cells infected with vesicular stomatitis virus

Aedes albopictus cells (clone LT-C7) showed a marked cytopathic effect and inhibition of protein synthesis (both host and viral) after infection with vesicular stomatitis virus (VSV), but only if (i) cultures were incubated at 34 degrees C rather than 28 degrees C and (ii) serum was present in the medium (S. Gillies and V. Stollar, Mol. Cell. Biol. 2:66-75, 1982). To learn more about how protein synthesis is shut off in VSV-infected A. albopictus cells, we have compared cell-free protein synthesis in extracts prepared from VSV-infected cells and control cells. Extracts prepared 6 h after infection from VSV-infected cells maintained at 34 degrees C in the presence of serum reflected what was observed with intact cells in at least two respects: (i) they showed a markedly diminished capacity to carry out protein synthesis (whether directed by endogenous or exogenously added mRNA), and (ii) there was decreased phosphorylation in vitro by [gamma-32P]ATP of a specific ribosomal protein (Gillies and Stollar, Mol. Cell. Biol. 2:66-75, 1982). In addition, and consistent with a block at the level of initiation, the formation of 80S initiation complexes, as measured by binding of VSV 12 to 18S mRNA, was reduced in the inactive extracts. Addition of an S-100 fraction from uninfected cells to the inactive extract reversed each of the aforementioned changes; i.e., it restored protein synthetic activity, it stimulated the formation of 80S initiation complexes, and it increased phosphorylation of the specific ribosomal protein referred to above. The active component in the S-100 fraction was heat labile and non-dialyzable and, upon ammonium sulfate fractionation of the S-100 fraction, was found in the 40 to 70% saturation fraction. Our findings suggest that VSV infection of A. albopictus cells inhibits protein synthesis by inactivating a macromolecular component, probably a protein, in the S-100 fraction which may be involved in the initiation of protein synthesis. More specifically, we suggest that this component is involved in the joining of the ribosomal subunits to form 80S initiation complexes.

Protein synthesis in lysates of Aedes albopictus cells infected with vesicular stomatitis virus

Aedes albopictus cells (clone LT-C7) showed a marked cytopathic effect and inhibition of protein synthesis (both host and viral) after infection with vesicular stomatitis virus (VSV), but only if (i) cultures were incubated at 34 degrees C rather than 28 degrees C and (ii) serum was present in the medium (S. Gillies and V. Stollar, Mol. Cell. Biol. 2:66-75, 1982). To learn more about how protein synthesis is shut off in VSV-infected A. albopictus cells, we have compared cell-free protein synthesis in extracts prepared from VSV-infected cells and control cells. Extracts prepared 6 h after infection from VSV-infected cells maintained at 34 degrees C in the presence of serum reflected what was observed with intact cells in at least two respects: (i) they showed a markedly diminished capacity to carry out protein synthesis (whether directed by endogenous or exogenously added mRNA), and (ii) there was decreased phosphorylation in vitro by [gamma-32P]ATP of a specific ribosomal protein (Gillies and Stollar, Mol. Cell. Biol. 2:66-75, 1982). In addition, and consistent with a block at the level of initiation, the formation of 80S initiation complexes, as measured by binding of VSV 12 to 18S mRNA, was reduced in the inactive extracts. Addition of an S-100 fraction from uninfected cells to the inactive extract reversed each of the aforementioned changes; i.e., it restored protein synthetic activity, it stimulated the formation of 80S initiation complexes, and it increased phosphorylation of the specific ribosomal protein referred to above. The active component in the S-100 fraction was heat labile and non-dialyzable and, upon ammonium sulfate fractionation of the S-100 fraction, was found in the 40 to 70% saturation fraction. Our findings suggest that VSV infection of A. albopictus cells inhibits protein synthesis by inactivating a macromolecular component, probably a protein, in the S-100 fraction which may be involved in the initiation of protein synthesis. More specifically, we suggest that this component is involved in the joining of the ribosomal subunits to form 80S initiation complexes.

Effect of phosphorylation on the affinity of acidic proteins from Saccharomyces cerevisiae for the ribosomes

Electrofocusing of the acidic proteins extracted from Saccharomyces cerevisiae ribosomes shows the presence of eight bands in the gels, which upon treatment with alkaline phosphatase are reduced to three. Two of them, proteins L44 and L45, correspond to the proteins equivalent to the bacterial L7 and L12 and the third, protein Ax, behaves like a supernatant factor. In the ribosome, proteins L44 and L45 are found unphosphorylated and monophosphorylated while protein Ax is detected mostly in a modified state, showing from one to three phosphate groups per molecule. In the cytoplasm where protein Ax is abundant and proteins L44 and L45 are present in small quantities, the three proteins are unphosphorylated. Protein Ax, having one or two phosphate groups, can be removed from the ribosomes in conditions that release the initiation factors, while the triphosphorylated molecules are tightly bound to the particles. The data indicate a relationship between the degree of phosphorylation of protein Az and its affinity for the ribosome.

Ribosomal function and its inhibition by antibiotics in prokaryotes

Most of the known antibiotics act at the level of protein biosynthesis probably due to the extraordinary complexity of the translation machinery which can be interfered with at many points. At first a survey is given of our present knowledge covering the structure and function of the prokaryotic ribosome. The most important antibiotics acting at the translational level are integrated into this network of data. The binding sites and the inhibition mechanisms of the drugs, together with the ribosomal components altered in resistant mutants are described. Finally, the points of interference with the translational machinery are indicated in an extended scheme of ribosomal functions.

Inhibition of cell-free protein synthesis by low-molecular-weight nuclear polyadenylate-containing ribonucleic acid species isolated from the lactating guinea pig

1. Poly(A)-containing RNA was isolated from the nuclei of mammary gland, liver and brain of lactating guinea pigs. 2. Total nuclear poly(A)-containing RNA from mammary gland inhibited mRNA-directed protein synthesis by a wheat-germ cell-free system. It also inhibited the endogenous activity of the wheat-germ and other cell-free systems. It did not inhibit a wheat-germ cell-free system directed by poly(U). 3. Total nuclear poly(A)-containing RNA from liver and brain did not inhibit the mRNA-directed wheat-germ system. 4. Fractionation of the nuclear poly(A)-containing RNA revealed inhibitory activity in the less than 10 S fraction from mammary gland as well as that from liver and brain. 5. The mechanism of protein-synthesis inhibition appeared to be at the level of elongation. 6. The inhibitory activity could be reversed in a wheat-germ system by increasing the amount of S-30 supernatant. 7. The mechanism of inhibition of protein synthesis is discussed in relation to other RNA species known to inhibit such systems.

Inhibitory effects of 'cap' analogues on globin mRNA and encephalomyocarditis RNA translation in a reticulocyte cell-free system

The cap analogues 7-methylguanosine 5'-phosphate [m7G(5')p], 7-methylguanosine 5'-triphosphate [m7G(5')ppp] and 2'-O-methylguanosine 5'-triphosphate [Gm(5')ppp] inhibit the translation of capped globin mRNA and encephalomyocarditis (EMC) RNA (a naturally uncapped mRNA) in a reticulocyte cell-free system. This inhibition occurs at the level of protein synthesis initiation and is of a competitive type since it can be overcome by increasing the mRNA concentration. However, the translation of globin mRNA is more sensitive to the inhibitory effects of the cap analogues m7G(5')p and m7G(5')ppp than translation of EMC RNA. The same spectra of specific inhibition is also observed with some other initiation inhibitors such as aurintricarboxylic acid, which inhibits mRNA binding, but not with pactamycin which does not affect mRNA interaction. A model is presented suggesting that this preferential inhibition by cap analogues could be explained mainly by the different affinities of globin mRNA and EMC RNA for the initiation complexes between 40-S subunits and Met-tRNAf. Moreover Gm(5')ppp cannot be considered simply as a cap analogue since it also affects some step prior to mRNA binding.

Acidic ribosomal proteins from eukaryotic cells. Effect on ribosomal functions

Precipitation of Saccharomyces cerevisiae ribosomes by ethanol under experimental conditions that do not release the ribosomal proteins can affect the activity of the particles. In the presence of 0.4 M NH4Cl and 50% ethanol only the most acidic proteins from yeast and rat liver ribosomes are released. At 1 M NH4Cl two more non-acidic proteins are lost from the ribosomes. The release of the acidic proteins causes a small inactivation of the polymerizing activity of the particles, additional to that caused by the precipitation itself. The elongation-factor-2-dependent GTP hydrolysis of the ribosomes is, however, more affected by the loss of acidic proteins. These proteins can stimulate the GTPase but not the polymerising activity when added back to the treated particles. Eukaryotic proteins cannot be substituted for bacterial acidic proteins L7 and L12. We have not detected immunological cross-reaction between acidic proteins from Escherichia coli and those from yeast, Artemia salina and rat liver or between acidic proteins from these eukaryotic ribosomes among themselves.

Inhibition of translation in eukaryotic systems by harringtonine

The Cephalotaxus alkaloids harringtonine, homoharringtonine and isoharringtonine inhibit protein synthesis in eukaryotic cells. The alkaloids do not inhibit, in model systems, any of the steps of the initiation process but block poly(U)-directed polyphenylalanine synthesis as well as peptide bond formation in the fragment reaction assay, the sparsomycin-induced binding of (C)U-A-C-C-A-[3H]Leu-Ac, and the enzymic and the non-enzymic binding of Phe-tRNA to ribosomes. These results suggest that the Cephalotaxus alkaloids inhibit the elongation phase of translation by preventing substrate binding to the acceptor site on the 60-S ribosome subunit and therefore block aminoacyl-tRNA binding and peptide bond formation. However, the Cephalotaxus alkaloids do not inhibit polypeptide synthesis and peptidyl[3H]puromycin formation in polysomes. Furthermore, these alkaloids strongly inhibit [14C]trichlodermin binding to free ribosomes but hardly affect the interaction of the antibiotic with yeast polysomot interact with polysomes and therefore only inhibit cycles of elongation. This explains the polysome run off that has been observed by some workers in the presence of harringtonine.