Highly efficient translation of messenger RNA in cell-free extracts prepared from L-cells

Translational extracts active biologically in vitro obtained from eukaryotic monolayer cells: a versatile method for viral RNA studies

Preservation of enzymatic activities in biological samples, especially after freeze/thawing, is a crucial requirement in virological research. Theoretically, this preservation can be achieved with the presence of cryopreservative agents. In contrast to tedious methods, it was found that this might be readily achieved by using well-defined conditions, including sucrose in the samples. Hence, the generation of a translational extract obtained from eukaryotic cells that have grown as monolayers is described below. This versatile method could be used advantageously for the in vitro translation of messenger RNAs, added exogenously, including viral mRNAs. The translational extract can be prepared freshly on a daily basis, or more conveniently it can be frozen and thawed subsequently for further use, without loss of activity. It can replace the Krebs ascites fluid and the commercial rabbit reticulocyte lysate. The procedure employed for the preservation of the biological activity of the translational extract can be extended to various other biological samples.

Modulation of thermoprotection and translational thermotolerance induced by Semliki Forest virus capsid protein

Low amounts of Semliki Forest virus capsid protein transferred into target cells by electroporation-mediated delivery (10(3)-10(4) molecules incorporated/cell) confer thermal resistance resulting in enhanced survival. Furthermore, when exposed to 43 degrees C, these cells display an enhanced expression of heat-shock protein-70 and a translational thermotolerance. Similarly, low amounts of capsid protein transferred into cells in which transcription is blocked by actinomycin D, also protect the translational machinery at 43 degrees C. In a cell-free translation system, added capsid protein appears to modulate translational efficiency of endogenous mRNAs. At approximately 1 molecule/ribosome, capsid protein is able to enhance translation at 30 degrees C and at 43 degrees C. In contrast, high concentrations of capsid protein are responsible for a marked inhibition of protein synthesis at 30 degrees C, but only hamper translational thermotolerance at 43 degrees C. Our results favor the hypothesis that small amounts of capsid protein trigger a chaperone-like activity that is able to protect the translational machinery from thermal damage.

Cell-free translation systems prepared from starfish oocytes faithfully reflect in vivo activity; mRNA and initiation factors stimulate supernatants from immature oocytes

Meiotic maturation stimulates a change in the translation of stored mRNAs: mRNAs encoding proteins needed for growth of oocytes are translated before meiotic maturation, whereas those encoding proteins required for cleavage are translated after meiotic maturation. Studies of translational regulation during meiotic maturation have been limited by the lack of translationally active cell-free supernatants. Starfish oocytes are ideal for preparing cell-free translation systems because experimental application of the hormone 1-methyladenine induces their maturation, synchronizing meiosis. We have prepared such systems from both immature and mature oocytes of starfish. Changes in protein synthesis rates and the specificity of proteins synthesized in these cell-free translation supernatants mimic those seen in vivo. Supernatants both from immature and mature oocytes have a high capacity to initiate new translation because 90% of the proteins made are newly initiated from mRNAs. Cell-free supernatants from mature oocytes have a much higher rate of initiation of translation than those from immature oocytes and use the 43S preinitiation complexes more efficiently in initiation of translation. Similarly, we have shown that mRNAs and initiation factors are rate limiting in cell-free translation systems prepared from immature oocytes. In addition, cell-free translation systems prepared from immature oocytes are only slightly, if at all, inhibitory to cell-free translation systems from mature oocytes. Thus, soluble inhibitors, if they exist, are rapidly converted by cell-free supernatants from mature oocytes. The similarities between translation in our starfish cell-free translation systems and in intact oocytes suggests that the cell-free translation systems will be useful tools for further studies of maturation events and translational control during meiosis.

Identification and characterization of a third complementation group of emetine-resistant Chinese hamster cell mutants

We have isolated emetine-resistant cell lines from Chinese hamster peritoneal fibroblasts and have shown that they represent a third distinct class or complementation group of emetine-resistant mutants, as determined by three different criteria. These mutants, like those belonging to the two other complementation groups we have previously defined, which were isolated from Chinese hamster lung and Chinese hamster ovary cells, have alterations that directly affect the protein biosynthetic machinery. So far, there is absolute cell line specificity with respect to the three complementation groups, in that all the emetine-resistant mutants we have isolated from Chinese hamster lung cells belong to one complementation group, all those we have isolated from Chinese hamster ovary cells belong to a second complementation group, and all those isolated from Chinese hamster peritoneal cells belong to a third complementation group. Thus, in cultured Chinese hamster cells, mutations in at least three different loci, designated emtA, emtB, and emtC, encoding for different components of the protein biosynthetic machinery, can give rise to the emetine-resistant phenotype.

The viral protein sigma 3 participates in translation of late viral mRNA in reovirus-infected L cells

Reovirus late (uncapped) mRNA was previously shown to be efficiently translated in vitro extracts prepared from infected cells but not from uninfected cells. We demonstrated that different fractions from infected cells can stimulate translation of late viral mRNA when added to uninfected extracts. The activity of the different fractions correlated with their relative content of the sigma 3 capsid protein; the fraction prepared by high-salt wash of the ribosomes had the highest specific activity. The activity present in this fraction was abolished by preincubation with an anti-sigma 3 serum. Purified sigma 3 protein also stimulated the translation of late viral mRNA, confirming that it was the factor involved. Altogether, these results suggest that this protein plays the role of a late-viral-mRNA-specific initiation factor. The absence of an inhibitory effect of sigma 3 on the translation of other mRNAs indicates that this protein is not directly involved in the inhibition of host and early viral mRNA translation that occurs in infected cells but that a second mechanism is probably operative.

Multiple forms of the sigma 3 protein of reovirus: occurrence and binding properties

The sigma 3 protein of reovirus serotype 3 is present in different forms differing in isoelectric point, as already described by other authors. Proteolytic digestion of each of these forms generates similar peptides. The different forms are present in the mature virus and have affinity for double-stranded RNA and for ribosomes. The observations that the different forms have the same functional properties and are absent in reovirus serotype 1 suggest that the presence of multiple forms is dispensable. The different forms can be synthesized by in vitro translation of mRNA extracted from infected cells. However, only one form is clearly made when mRNA synthesized in vitro, using the cloned S4 gene encoding sigma 3 as a template, is translated in vitro. Similarly the cloned S4 gene expressed in vivo produces only one form of the protein. These results indicate that the other forms are not derived by post-translational modification. The different forms are possibly due to the presence of mutant viruses.

A (re)initiation-dependent cell-free protein-synthesis system from mouse erythroleukemia cells

Cultured mouse erythroleukemia cells (MEL cells) can be induced in vivo to erythroid differentiation which is marked by the onset of globin mRNA and haemoglobin synthesis. When these cells are briefly exposed to hypertonic growth medium prior to lysis, the resulting post-mitochondrial supernatants show a high in vitro protein-synthesis activity. Amino acid incorporation is linear up to 60 min; more than 80% of this is due to (re)initiation, as shown by the inhibition with edeine. Extracts from induced cells reach only a third of overall incorporation as compared to extracts from uninduced cells. This reduction of the protein-synthesizing capacity is also observed in vivo. Polyacrylamide gel electrophoresis shows that extracts from uninduced cells faithfully translate their endogenous mRNA, whereas in extracts from induced cells, non-globin protein synthesis is reduced and globin is preferentially synthesized. Haemin (40 microM) as well as purified eukaryotic initiation factor 2 (eIF-2) from rabbit reticulocytes enhance amino acid incorporation in both kinds of extracts, which suggests that both uninduced and induced MEL cells contain a haemin-controlled eIF-2 alpha kinase. This system should be useful for studying the mechanisms controlling protein synthesis in a nucleated differentiating cell.

Reovirus guanylyltransferase is L2 gene product lambda 2

Reovirus guanylyltransferase, studied as a covalent enzyme-GMP intermediate, was used to guanylate appropriate acceptor molecules in vitro to produce authentic cap structures. Guanylyltransferase activity was associated with lambda 2, the 140-kilodalton product of the L2 gene segment of reovirus serotypes 1 and 3.

Inhibition of translation in L-cell lysates by free polyadenylic acid: differences in sensitivity among different mRNAs and possible involvement of an initiation factor

Free polyadenylic acid specifically inhibits in vitro translation of naturally polyadenylated mRNAs in L-cell lysates. The polynucleotide affects the initiation of protein synthesis but has no apparent effect on elongation of polypeptide chains. Reovirus mRNA, naturally devoid of a poly(A) tail, is much less sensitive to this inhibition than are naturally polyadenylated mRNAs. Reovirus mRNA that was polyadenylated in vitro is not more sensitive than normal reovirus mRNA. The degree of inhibition of translation varies for the different reovirus mRNA species. The addition of proteins contained in a high salt wash of ribosomes can mitigate the inhibition of translation of naturally polyadenylated mRNAs by free polyadenylic acid. Altogether these results suggest that the inhibition by polyadenylic acid may be mediated by its interaction with a cellular (initiation) factor. The various sensitivities exhibited by different mRNAs may indicate differences in requirement for this factor.

Regulation of protein synthesis in eukaryotes. Eukaryotic initiation factor eIF-2 and eukaryotic recycling factor eRF from neuroblastoma cells

eIF-2 purified from neuroblastoma cells consists of three subunits, which appear to be of molecular weight identical to those of the subunits of rabbit reticulocyte eIF-2. A protein fraction has been isolated from neuroblastoma cells with characteristics similar to eRF from reticulocytes: stimulation of amino acid incorporation in a hemin-deprived reticulocyte lysate, the removal of GDP from eIF-2-GDP complexes, a 4-5-fold stimulatory effect in a two-step reaction measuring 40 S preinitiation complex formation and a 3-3.5-fold stimulation in the methionyl-puromycin synthesis. In the methionyl-puromycin-synthesizing system phosphorylated eIF-2 is not responsive to the addition of this fraction from neuroblastoma cells. The protein fraction contains eRF which seems to be similar to the eRF isolated from Ehrlich ascites tumor cells and somewhat distinct from the reticulocyte factor. Incubation of neuroblastoma cell lysate in the presence of [gamma-32P]ATP results in the phosphorylation of a protein of Mr 36 000, migrating on SDS-polyacrylamide gels to the position of eIF-2 alpha. This protein is also phosphorylated in vitro by HRI from reticulocytes. These results may reflect a common underlying principle for the quantitative regulation of protein synthesis in eukaryotic cells.

The effect of serum deprivation on the initiation of protein synthesis in mouse neuroblastoma cells

Growth of mouse neuroblastoma cells becomes stationary when cultured in serum-free medium. Within 60 h, the protein-synthesizing capacity of the cells declines to 25% as compared to that of exponentially growing cells. The transitional activity of the crude ribosomal salt washes from serum-deprived and control cells was compared in in vitro protein-synthesizing pH 5 systems. It appears that the ribosomal salt wash from serum-deprived cells has significantly (70%) lost its ability to support the translation of neuroblastoma poly(A)+ RNA. This activity of the ribosomal wash from serum-deprived cells can be restored to control level with rabbit reticulocyte initiation factor eIF-4B only. The ability of the ribosomal wash from serum-deprived cells to support the translation of encephalomyocarditis virus (EMC) and Semliki Forest virus (SFV) 42 S mRNA was tested. We found that EMC-mRNA is efficiently translated with the ribosomal salt wash from serum-deprived cells, whereas on the other hand the translation of SFV 42 S mRNA is severely impaired. Therefore, we conclude that in serum-deprived neuroblastoma cells protein synthesis is regulated in both a quantitative and a qualitative way. Modulation of the activity of initiation factor of protein synthesis eIF-4B is at least partly responsible for the observed (selective) blockade of protein synthesis in serum-deprived cells.

A method for analyzing transcription using permeabilized cells

Brief exposure of Friend cells to a buffered hypotonic solution containing 1% Tween 80 caused permeabilization and allowed incorporation of [3H]UTP into RNA. The incorporation was inhibited 85-97% by 20 micrograms/ml actinomycin D and the reaction product was completely hydrolyzed by 0.1 M KOH. UMP incorporation was nearly linear for 60 min at 23 degrees C; however, at 37 degrees C it ceased after 15-20 min of rapid incorporation. The inhibition of UMP incorporation by 2 micrograms/ml alpha-amanitin was much greater at 23 degrees C than at 37 degrees C. The molecular weight of the RNA synthesized in permeabilized cells is broadly distributed with about 83% larger than 18 S. In vitro transcription of the mouse beta-major globin gene was studied by hybridizing 32P-labeled nascent RNA to filter-bound DNA sequences representing this gene and its flanking regions. After induction by hexamethylene-bisacetamide, Friend cells exhibited more than fivefold increases in the rate of transcription for the beta-major globin gene as compared to the uninduced control cells. Induction also caused an increase in the transcription rate of the 3'-flanking region located downstream from the poly(A) addition site. Thus, the primary transcription unit of beta-major globin gene is essentially the same in permeabilized cells as that previously reported for nuclei isolated from the same cell line. In addition, permeabilized cells actively initiate RNA synthesis as determined by the incorporation of a thiol group at the 5' initiating nucleotide, when synthesis was in the presence of [gamma-S]-labeled nucleoside triphosphates. Permeabilized cells are about 7-11 times more active than isolated nuclei in the synthesis of both in vitro-initiated and total RNA.

mRNA discrimination in extracts from uninfected and reovirus-infected L-cells

Uncapped reovirus mRNA extracted at late times from infected L-cells is preferentially translated in extracts from infected L-cells. However, translation of this uncapped, late, reovirus mRNA in extracts from infected cells is sensitive to inhibition by the cap analog m7GTP . These results imply that reovirus infection does not induce a transition from cap-dependent to cap-independent translation. Nevertheless, the results of in vitro translational competition experiments between L-cell mRNA and late viral mRNA were consistent with the view that reovirus does induce an alteration in the cap-dependent translational apparatus of L-cells. The reduced efficiency of translation of a variety of capped mRNAs in extracts from infected cells is also consistent with this notion. We further conclude that a factor exists in reovirus-infected L-cells that specifically stimulates translation of uncapped reovirus mRNAs.

Infection of neuroblastoma cells by Semliki Forest virus. The interference of viral capsid protein with the binding of host messenger RNAs into initiation complexes is the cause of the shut-off of host protein synthesis

From ribosomal washes of neuroblastoma cells infected with Semliki Forest virus (SFV) a protein of Mr 33000 was purified, which comigrated with the viral capsid protein on sodium dodecyl sulfate/polyacrylamide gels and was recognized by antibodies against the capsid protein of SFV. This protein selectively inhibits the translation of host and early viral 42S mRNA in vitro, but has no effect on late viral 26S and encephalomyocarditis virus mRNA translation. Eukaryotic initiation factor 4B and cap-binding protein restore the translation of host and 42S mRNA to control levels. The capsid protein specifically prevents the binding of host mRNA into 80S initiation complexes, but has no effect on that of late viral mRNA. We propose that the capsid protein is the component responsible for the shut-off of host protein synthesis in SFV-infected cells and for the decreased translational activity of the crude ribosomal washes from these cells.

Shutoff of neuroblastoma cell protein synthesis by Semliki Forest virus: loss of ability of crude initiation factors to recognize early Semliki Forest virus and host mRNA's

A crude ribosomal wash containing the initiation factors of protein synthesis was isolated from mouse neuroblastoma cells 8 h after infection with Semliki Forest virus (SFV). The activity of this wash was compared with that of a wash from control cells in a cell-free protein-synthesizing "pH5" system, with early SFV mRNA (42S), late SFV mRNA (26S), encephalomyocarditis virus (EMC) mRNA, or neuroblastoma polyadenylated mRNA templates. A pronounced loss of activity (+/-80%) of the crude ribosomal wash from infected cells was observed with host mRNA (neuroblastoma polyadenylated mRNA) and early SFV mRNA, messengers which contain a cap structure at the 5' terminus. However, these washes were only slightly less active in systems programmed with (noncapped) EMC mRNA and late SFV mRNA. Although late SFV mRNA (26S) is capped, the synthesis of late (= structural) proteins in infected lysates was insensitive to inhibition by cap analogs. Purified initiation factors eIF-4B (M(r), 80,000) and cap-binding protein (M(r), 24,000) from reticulocytes (but none of the others) were able to restore the activity of infected factors to about 90% of control levels in systems programmed with early SFV mRNA and host mRNA. These observations indicate that infection-exposed crude initiation factors have a decreased level of eIF-4B and cap-binding protein activity. However, after partial purification of these and other initiation factors from infected and control cells, we found no significant difference in activity when model assay systems were used. Furthermore, both eIF-4B and cap-binding protein from infected cells were able to restore the activity of these infection-exposed factors to the same level obtained when these factors isolated from control cells or reticulocytes were added. A possible mechanism for the shutoff of host cell protein synthesis is discussed.

Identification and characterization of a third complementation group of emetine-resistant Chinese hamster cell mutants

We have isolated emetine-resistant cell lines from Chinese hamster peritoneal fibroblasts and have shown that they represent a third distinct class or complementation group of emetine-resistant mutants, as determined by three different criteria. These mutants, like those belonging to the two other complementation groups we have previously defined, which were isolated from Chinese hamster lung and Chinese hamster ovary cells, have alterations that directly affect the protein biosynthetic machinery. So far, there is absolute cell line specificity with respect to the three complementation groups, in that all the emetine-resistant mutants we have isolated from Chinese hamster lung cells belong to one complementation group, all those we have isolated from Chinese hamster ovary cells belong to a second complementation group, and all those isolated from Chinese hamster peritoneal cells belong to a third complementation group. Thus, in cultured Chinese hamster cells, mutations in at least three different loci, designated emtA, emtB, and emtC, encoding for different components of the protein biosynthetic machinery, can give rise to the emetine-resistant phenotype.

Reovirus progeny subviral particles synthesize uncapped mRNA

Reovirus progeny subviral particles were isolated from L-cells at late times postinfection. It has been shown (D. Skup and S. Millward, J. Virol. 34: 490--496, 1980) that these progeny subviral particles have masked capping enzymes, indicating that mRNA synthesized by these particles should be uncapped. When progeny subviral particles were used for mRNA synthesis in vitro, they failed to incorporate the beta-phosphate of [beta-32P]GTP into the 5' terminal. Direct analysis of reovirus mRNA synthesized by progeny subviral particles in the presence of either [alpha-32P]GTP or [alpha-32P]CTP indicated that the 5' terminal was uncapped, having the structure pGpC... The implications of this finding to the reovirus replicative cycle are discussed.

mRNA capping enzymes are masked in reovirus progeny subviral particles

We examined the enzyme activities associated with progeny subviral particles isolated from L-cells infected with reovirus at 12 h postinfection. Activities normally present in reovirus cores were also found to be present in the progeny subviral particles, with the exception of the capping enzymes. The methylase and guanyl transferase activities, which constitute the capping system, were present in a masked form that could be activated by chymotrypsin digestion. The appearance of these progeny subviral particles in infected cells coincided with the time when mRNA synthesis was maximal, suggesting that viral mRNA synthesized at later times is uncapped.

Reovirus-induced modification of cap-dependent translation in infected L cells

The translational apparatus in cell-free extracts prepared from L cells infected with reovirus undergoes a time-dependent transition from cap dependence to cap independence. Extracts from uninfected L cells translate capped reovirus mRNA at high efficiency and synthesize the expected three size classes of reovirus polypeptides, and the translation is sensitive to m7G(5')ppp. This same extract translates uncapped mRNA at a much lower efficiency. In contrast, extracts from infected L cells translate uncapped reovirus mRNA at high efficiency and synthesize the correct three size classes of polypeptides, and the translation is not sensitive to inhibition by m7G(5')ppp. Infected cell extracts translate capped mRNA at reduced efficiency (a,proximately 25%), the translation is not sensitive to inhibition by m7G(5')ppp, and the correct three size classes of viral polypeptides are not synthesized. These observations may explain how reovirus takes over the host translational apparatus.