Blocks in elongation and initiation of protein synthesis induced by interferon treatment in mouse L cells

Protein kinase double-stranded RNA-dependent (PKR) in antiviral defence in fish and mammals

The interferon-inducible double-stranded RNA-dependent protein kinase (PKR) is one of the key antiviral arms of the innate immune system. Upon binding of viral double stranded RNA, a viral Pattern Associated Molecular Pattern (PAMP), PKR gets activated and phosphorylates the eukaryotic initiation factor 2α (eIF2α) resulting in a protein shut-down that limits viral replication. Since its discovery in the mid-seventies, PKR has been shown to be involved in multiple important cellular processes including apoptosis, proinflammatory and innate immune responses. Viral subversion mechanisms of PKR underline its importance in the antiviral response of the host. PKR activation pathways and its mechanisms of action were previously identified and characterised mostly in mammalian models. However, fish Pkr and fish-specific paralogue Z-DNA-dependent protein kinase (Pkz) also play key role in antiviral defence. This review gives an update on the current knowledge on fish Pkr/Pkz, their conditions of activation and their implication in the immune responses to viruses, in comparison to their mammalian counterparts.

To translate, or not to translate: viral and host mRNA regulation by interferon-stimulated genes

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Translational regulation of viral and host mRNA is an important function of a subset of interferon-stimulated genes (ISGs).

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These ISGs repress translation through binding to viral RNA, direct interaction with, or perturbation of the translation machinery components.

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Several ISGs localize to cytoplasmic granules such as stress granules (SGs) and processing bodies (PBs), and interfere with the processing or function of microRNAs (miRNAs).

Type I interferon (IFN) is one of the first lines of cellular defense against viral pathogens. As a result of IFN signaling, a wide array of IFN-stimulated gene (ISG) products is upregulated to target different stages of the viral life cycle. We review recent findings implicating a subset of ISGs in translational regulation of viral and host mRNAs. Translation inhibition is mediated either by binding to viral RNA or by disrupting physiological interactions or levels of the translation complex components. In addition, many of these ISGs localize to translationally silent cytoplasmic granules, such as stress granules and processing bodies, and intersect with the microRNA (miRNA)-mediated silencing pathway to regulate translation of cellular mRNAs.

Inhibition of the aminoacylation of selected tRNA molecules by an estrogen-regulated factor on uterine ribosomes. Regulation of aminoacylation of tRNA by estrogens

Administration of estradiol to ovariectomized mature rats for 1 h induces a transient increase in the peptide elongation rate on uterine ribosomes. An inhibitor of the peptide elongation rate, which appears to be regulated by estrogen treatment in vivo, can be extracted from ribosomes of estrogen-deprived rats. The extracted inhibitor or a native inhibitor-ribosome complex affects the rate of the peptide elongation reaction in a uterine cell-free protein synthesis system by inhibiting the ability of selected tRNAs in the assay to be charged with amino acids by their respective aminoacyl-tRNA synthetases. The degree of inhibition of charging of the affected tRNAs ranges from 22% to 78%, the order of inhibition being Pro greater than Val greater than Arg greater than Try greater than Leu greater than Glu greater than Ile greater than Gly greater than His greater than Ser greater than Lys. Inhibition results from a specific dose-dependent, and presumably reversible, effect of the inhibitor on tRNA, but not on the aminoacyl-tRNA synthetase. The effect does not result from removal of A-C-C terminal nucleotides from the 3' end of tRNA, but does inhibit the ability of selected tRNAs to bind to the aminoacyl-tRNA synthetases. We propose that regulation of the peptide elongation rate on uterine ribosomes by estradiol occurs through the estradiol-induced inactivation of a ribosome-associated inhibitor, which causes a reversible alteration to selected tRNAs. The modified tRNAs are unable to bind to their respective aminoacyl-tRNA synthetase to become charged with an amino acid thus causing the availability of selected aminoacyl-tRNAs to become rate-limiting in the sequential elongation of peptides.

Efficiency and fidelity of cell-free protein synthesis by transfer RNA from aged mice

Transfer RNAs (tRNAs) from heart, kidney, liver, and spleen of mature (10-12 months old) and aged (29 months old) C57BL/6 mice were tested for their ability to translate encephalomyocarditis viral RNA in a tRNA-dependent cell-free system derived from mouse ascites tumor cells. The rates of in vitro protein synthesis were compared as a function of tRNA concentration, and the fidelity of translation was examined by sodium dodecyl sulfate gel electrophoresis and isoelectric focusing of the viral polypeptides synthesized in vitro. No significant age-related differences in either the efficiency or fidelity of synthesis were discovered, indicating that alterations in tRNAs are probably not involved in the cellular aging of these tissues.

Differential effects of two interferon-induced translational inhibitors on initiation of protein synthesis

At least two different mechanisms for the inhibition of mRNA translation operate in extracts of interferon-treated L cells. One is mediated by an interferon-induced protein kinase which, when activated by double-stranded RNA and ATP, phosphorylates the small subunit of initiation factor eIF-2. Addition of the purified interferon-induced protein kinase to L cell extracts, strongly reduces the amount of methionyl-tRNA bound to 40-S ribosomal subunits. The second translational inhibition is due to the synthesis of (2'-5')oligo(adenylate) by interferon-induced enzyme E. The oligonucleotide in turn activates a ribonuclease F constitutively present in L cells. Addition of the purified nuclease with its oligonucleotide activator to L cell extracts produces a strong decrease in polyribosome formation and an accumulation of initiation complex. These experiments differentiate the effects of the two interferon-induced inhibitors on mRNA translation.

RNA methylation in vaccinia virus-infected chick embryo fibroblasts treated with homologous interferon

Interferon-pretreatment of vaccinia-infected chick embryo fibroblasts resulted in a greater than 50% decrease in ribose methylation of the penultimate "cap" nucleotide in virus-specific mRNA. However, in contrast to results obtained with cell-free systems, in intact infected cells there was (a) no detectable reduction in methylation of the 5'-ultimate m7G of viral mRNA; (b) a virus specificity of the interferon-induced change in mRNA "CAP"-methylation seems unlikely and (c) analysis of the ribosomal and transfer RNA fractions isolated from interferon-treated and control cells revealed identical patterns of methylated nucleotides. Thus, the interferon-induced change in methylation is specific for mRNA "CAPS".

Synthesis of an oligonucleotide inhibitor of protein synthesis in rabbit reticulocyte lysates analogous to that formed in extracts from interferon-treated cells

A heat-stable, low-molecular-weight inhibitor of protein synthesis is formed on incubation of haemin-supplemented rabbit reticulocyte lysates with ATP and double-stranded RNA (dsRNA). It inhibits the translation of both added encephalomyocarditis virus RNA (EMC RNA) and endogeneous messenger RNA in reticulocyte lysates and mouse L-cell extracts. The enzyme responsible for the synthesis of the inhibitor binds to dsRNA and can be purified on a column of poly(I).poly (C) bound to an inert support. The highly purified enzyme in its stable column-bound state can be conveniently employed to synthesise the inhibitor and to label it with [3H]ATP, or [alpha-32P]ATP or [gamma-32P]ATP as substrate. The radioactive inhibitor synthesised in this way with material from rabbit reticulocyte lysates shows the same spectrum of resistance and sensitivity to alkali and a variety of enzymes as corresponding material similarly synthesised with extracts from interferon-treated mouse L-cells. The inhibitors from the two systems have comparable absorbance spectra, are chromatographically and electrophoretically indistinguishable and are apparently identical in specific activity in the inhibition of protein synthesis in the cell-free system. The inhibitor is also formed on inhibition of protein synthesis by dsRNA in reticulocyte lysates. On comparison of the spectrum of polypeptide products synthesised in response to EMC RNA in the reticulocyte lysate, the effects of the inhibitor or dsRNA were similar: a distinctly different effect was obtained with the haemin-controlled repressor, a known inhibitor of initiation. The significance of these results with respect to the mechanism of action of the inhibitor and its role in the inhibition observed in response to dsRNA is discussed.

Selective translation of mengovirus RNA over Host mRNA in homologous, fractionated, cell-free translational systems from Ehrlich-ascites-tumor cells

The selective translation of viral RNA in mengovirus-infected Ehrlich ascites tumor cells was investigated using fractionated translational systems whose macromolecular components were derived entirely from uninfected or virus-infected cells. Both systems translate host mRNA from uninfected cells, host mRNA from virus-infected cells, and mengovirus RNA. In competition experiments, where viral RNA and host mRNA were translated together in systems from uninfected cells, the relative amounts of virus-specific and host-specific proteins synthesized were proportional to the relative concentrations of the RNA templates. In systems whose components were obtained from virus-infected cells, mengovirus RNA was preferentially translated. 70% of the selectivity found in the translational systems derived from infected cells was due to the initiation factor fraction, the remaining 30% to components of the pH 5 enzyme fraction. In addition, host mRNA isolated after virus infection is translated in vitro to a lower extent in the presence of mengovirus RNA than is host mRNA from uninfected cells.

Inhibition of viral protein synthesis in monkey cells treated with interferon late in simian virus 40 lytic cycle

We have investigated the effect of interferon on SV40 gene expression late in the lytic cycle, after early functions have been expressed and viral DNA replication has been initiated. Whereas pretreatment with interferon prior to infection reduces the amount of early SV40 RNA, post-infection treatment does not inhibit viral RNA synthesis. Viral 19S and 16S RNA species are found undiminished in quantity and poly(A) content. Despite the apparent normalcy of viral RNA classes, however, there is a marked reduction in the synthesis of their protein products, both T antigen and capsid polypeptides. The association of viral RNA with heavy polyribosomes is strongly reduced. On the other hand, there is no degradation of nonviral polyribosomes and the synthesis of most cellular proteins continues. These experiments demonstrate that late in infection, interferon treatment results in an inhibition of viral mRNA translation.

Limitation of reticulocyte transfer RNA in the translation of heterologous messenger RNAs

The effect of various tRNAs on protein synthesis was investigated using a tRNA-dependent cell-free system from Ehrlich ascites cells. Ascites cell tRNA and rabbit liver tRNA were found to promote efficient translation of globin mRNA, oviduct mRNA, and encephalomycarditis (EMC) viral RNA. In contrast, reticulocyte tRNA participated efficiently only in the translation of globin mRNA; the translation of oviduct mRNA AND EMC viral RNA in the presence of reticulocyte tRNA resulted in the synthesis of relatively few large mature proteins and the accumulation of discrete, smaller polypeptides. These results suggest that isoaccepting tRNA species required for the synthesis of ovalbumin and EMC viral protein (but not hemoglobin) are probably functionally absent in reticulocyte tRNA, causing a premature, nonrandom termination of synthesis of these proteins. This provides preliminary evidence that variations in tRNA populations, frequently observed between different cell types, are large enough to define and perhaps regulate the proteins that the cell is capable of synthesizing.

A protein factor enhancing activation of inactive 30-S ribosomal subunits of Escherichia coli

A protein factor partially purified from the S-100 fraction (supernatant from 100 000 x g spin) of Escherichia coli stimulates the conversion of 30-S ribosomal subunits from an inactive to an active form. The subunits are normally inactivated at low concentrations of Mg2+ and reactivated when heated under appropriate ionic conditions. The factor, which differs from the known soluble factors required for protein biosynthesis as well as from proteins of the 30-S subunit, enhances activation under conditions which by themselves allow for only a low level of activity to reappear. Fully active ribosomes and the assay reactions are not affected by the factor, indicating that its effect must be restricted to the activation process. The factor is likely to function by facilitating a conformational transition in the ribosome, and might act in vivo in a similar manner.