Purification and properties of rabbit reticulocyte protein synthesis elongation factor 2

Purification and characterization of native human elongation factor 2

Human elongation factor 2 is the translocase that is responsible for the movement of tRNA from the A- to P- and P- to E-site on the ribosome during the elongation phase of translation. Being a vital factor of protein biosynthesis, its function is highly controlled and regulated. It has been implicated in numerous diseases and pathologies, and as such it is important to have a source for isolated pure and active protein for biomedical and biochemical studies. Here we report development of a purification protocol for native human elongation factor 2 from HEK-293S cells. The resulting protein is active, pure, has an intact diphtamide and is obtainable in yields suitable for functional and structural studies.

The Regulation of Translation in Alphavirus-Infected Cells

Sindbis virus (SINV) contains an RNA genome of positive polarity with two open reading frames (ORFs). The first ORF is translated from the genomic RNA (gRNA), rendering the viral non-structural proteins, whereas the second ORF is translated from a subgenomic mRNA (sgRNA), which directs the synthesis of viral structural proteins. SINV infection strongly inhibits host cell translation through a variety of different mechanisms, including the phosphorylation of the eukaryotic initiation factor eIF2α and the redistribution of cellular proteins from the nucleus to the cytoplasm. A number of motifs have been identified in SINV sgRNA, including a hairpin downstream of the AUG initiation codon, which is involved in the translatability of the viral sgRNA when eIF2 is inactivated. Moreover, a 3′-UTR motif containing three stem-loop structures is involved in the enhancement of translation in insect cells, but not in mammalian cells. Accordingly, SINV sgRNA has evolved several structures to efficiently compete for the cellular translational machinery. Mechanistically, sgRNA translation involves scanning of the 5′-UTR following a non-canonical mode and without the requirement for several initiation factors. Indeed, sgRNA-directed polypeptide synthesis occurs even after eIF4G cleavage or inactivation of eIF4A by selective inhibitors. Remarkably, eIF2α phosphorylation does not hamper sgRNA translation during the late phase of SINV infection. SINV sgRNA thus constitutes a unique model of a capped viral mRNA that is efficiently translated in the absence of several canonical initiation factors. The present review will mainly focus in the non-canonical mechanism of translation of SINV sgRNA.

A dispensable yeast ribosomal protein optimizes peptidyltransferase activity and affects translocation

Yeast ribosomal protein L41 is dispensable in the yeast. Its absence had no effect on polyphenylalanine synthesis activity, and a limited effect on growth, translational accuracy, or the resistance toward the antibiotic paromomycin. Removal of L41 did not affect the 60:40 S ratio, but it reduced the amount of 80 S, suggesting that L41 is involved in ribosomal subunit association. However, the two most important effects of L41 were on peptidyltransferase activity and translocation. Peptidyltransferase activity was measured as a second-order rate constant (k(cat)/K(s)) corresponding to the rate of peptide bond formation; this k(cat)/K(s) was lowered 3-fold to 1.15 min(-1) mm(-1) in the L41 mutant compared with 3.46 min(-1) mm(-1) in the wild type. Translocation was also affected by L41. Elongation factor 2 (EF2)-dependent (enzymatic) translocation of Ac-Phe-tRNA from the A- to P-site was more efficient in the absence of L41, because 50% translocation was achieved at only 0.004 microm EF2 compared with 0.02 microm for the wild type. Furthermore, the EF2-dependent translocation was inhibited by 50% at 2.5 microm of the translocation inhibitor cycloheximide in the L41 mutant compared with 1.2 microm in the wild type. Finally, the rate of EF2-independent (spontaneous) translocation was increased in the absence of L41.

Phosphorylation of elongation factor 1 and ribosomal protein S6 by multipotential S6 kinase and insulin stimulation of translational elongation

Stimulation of protein synthesis in response to insulin is concomitant with increased phosphorylation of initiation factors 4B and 4G and ribosomal protein S6 (Morley, S. J., and Traugh, J. A. (1993) Biochimie 75, 985-989) and is due at least in part to multipotential S6 kinase. When elongation factor 1 (EF-1) from rabbit reticulocytes was examined as substrate for multipotential S6 kinase, up to 1 mol/mol of phosphate was incorporated into the alpha, beta, and delta subunits. Phosphorylation of EF-1 resulted in a 2-2. 6-fold stimulation of EF-1 activity, as measured by poly(U)-directed polyphenylalanine synthesis. The rate of elongation was also stimulated by approximately 2-fold with 80 S ribosomes phosphorylated on S6 by multipotential S6 kinase. When the rates of elongation in extracts from serum-fed 3T3-L1 cells and cells serum-deprived for 1.5 h were compared, a 40% decrease was observed upon serum deprivation. The addition of insulin to serum-deprived cells for 15 min stimulated elongation to a rate equivalent to that of serum-fed cells. Similar results were obtained with partially purified EF-1, with both EF-1 and ribosomes contributing to stimulation of elongation. These data are consistent with a ribosomal transit time of 3.2 min for serum-deprived cells and 1.6 min following the addition of insulin for 15 min. Taken together, the data suggest that insulin stimulation involves coordinate regulation of EF-1 and ribosomes through phosphorylation by multipotential S6 kinase.

Variable aberrant cDNAs in single diphtheria toxin-resistant human fibroblasts

We treated transformed human fibroblasts with diphtheria toxin (DT) and isolated 40 single cells that were toxin resistant but unable to propagate. In 13 of them toxin resistance was associated with the presence of one or more aberrant transcripts of the structural gene for elongation factor 2 (EF-2). cDNA obtained from these transcripts had 164-447 bp-long deletions. Each of these deletions was associated with 2-8 base pairs-long repeats at its breakpoints. Only 10 out of 16 cDNA deletions were associated with presumed exon junctions. A role is suggested for errors in transcription in producing the aberrant transcripts which gave rise to the deletion-bearing cDNA species.

Study of phosphorylation of translation elongation factor 2 (EF-2) from wheat germ

Phosphorylation of elongation factor 2 (EF-2) by specific Ca2+/calmodulin-dependent kinase is considered as a possible mechanism of regulation of protein biosynthesis in animal cells at the level of polypeptide chain elongation. In this report we show that wheat germ EF-2 can be intensively phosphorylated by the rabbit reticulocyte EF-2 kinase. Phosphorylation results in inhibition of the activity of plant EF-2 in poly(U)-dependent cell-free translation system. Thus, the activity of EF-2 in plant cells can be potentially regulated by phosphorylation. However, we could not detect endogenous EF-2 kinase activity in wheat germ either in vitro or in vivo. Furthermore, EF-2 kinase activity is not displayed in different organs of wheat and other higher plants.

Affinity labeling of eukaryotic initiation factor 2 and elongation factor 1 alpha beta gamma with GTP analogs

As part of an attempt to understand the specific function and role of each subunit in multisubunit protein synthesis factors, we have attempted to identify the nucleotide binding peptides of eukaryotic initiation factor 2 (eIF-2). To ensure that the interactions were of a specific nature, two general controls were used: first, other protein factors with characterized GTP binding activity were tested; second, all affinity labeling was checked for nucleotide specificity by protection with the authentic nucleotide at a 10-fold molar excess over the affinity reagent. Results with a number of GTP modifying reagents ([alpha-32P]GTP, [alpha-32P]GDP, oxidized [alpha-32P]GTP, 3'-p-azidobenzoyl-[alpha-32P]GTP, 3'-p-azidobenzoyl-[alpha-32P]GDP, and 5'-p-[8-3H]fluorosulfonylbenzoyl guanosine) indicate that appropriate conditions for both nucleotide and subunit specific labeling have been achieved. Under these conditions all reagents modified the beta subunit of eIF-2. Complementary studies with subunit-deficient forms of eIF-2 also suggest that the beta subunit of eIF-2 is involved with GTP binding. Coupled with other data suggesting that the gamma subunit of eIF-2 might be involved in GTP binding and amino acid sequence data of eIF-2 gamma from which a part of a GTP binding consensus sequence can be localized, support is provided for the concept of alternate GTP binding domains or a GTP binding domain shared between different subunits of eIF-2.

The weak immunosuppressant cyclosporine D as well as the immunologically inactive cyclosporine H are potent inhibitors in vivo of phorbol ester TPA-induced biological effects in mouse skin and of Ca2+/calmodulin dependent EF-2 phosphorylation in vitro

Various biological effects induced by the tumor promoting phorbol ester TPA in mouse skin are comparably suppressed by the immunologically inactive cyclosporine H (CsH) and by the strongly immunosuppressive cyclosporine A (CsA). These effects inhibited include the development of edema, stimulation of alkaline phosphatase activity, DNA and protein synthesis, as well as tumor promotion. Furthermore, CsH, like CsA, inhibits the Ca2+/calmodulin-dependent phosphorylation of the elongation factor 2 (EF-2) in vitro and the TPA-induced increases in the amount of EF-2 in vivo. Similar observations were made using the weak immunosuppressant CsD. We conclude from these results that the ability of cyclosporines to act as immunosuppressants and their ability to inhibit TPA-effects are based on two different mechanisms of action. Inhibition of TPA-effects may involve suppression of calmodulin-dependent processes, such as augmentation and phosphorylation of EF-2.

Specific purification of elongation factor 2 and isolation of its antibody

Elongation factor 2 (EF-2) was purified from rat liver extracts by affinity chromatography using fragment A of diphtheria toxin as the ligand. Purified EF-2 has a molecular weight of 96,000 and isoelectric point of 6.6-6.8. The sequence of the nineteen N-terminal amino acid is Val-Asn-Phe-Thr-Val-Asp-Gln-Ile-Arg-Ala Ile-Met-Asp-Lys-Lys-Ala-Asn and the C-terminal amino acid is leucine. Purified rat EF-2 modified with ADP-ribose was injected into rabbits to prepare antibodies against EF-2. The anti-EF-2 antibodies can immunoprecipitate with EF-2 from various eukaryotic cells.

Purification and characterization of the mitochondrial translocase from Euglena gracilis

The Euglena gracilis mitochondrial protein biosynthetic elongation factor G (EF-Gmt) has been purified in four steps to greater than 50% homogeneity by use of a fusidic acid affinity procedure and conventional chromatographic techniques. The purification scheme results in 1100-fold purification with about 3% recovery of the total EF-G activity present in the postribosomal supernatant prepared from whole cell extracts. E. gracilis EF-Gmt has an approximate molecular weight of 76,000, comparable to that observed for procaryotic translocases. As is the case for other translocases which have been examined, pretreatment of E. gracilis EF-Gmt with N-ethylmaleimide results in a loss of polymerization activity, indicating a role for an essential cysteine residue in catalytic activity. GDP partially protects EF-Gmt from N-ethylmaleimide inactivation. E. gracilis EF-Gmt functions well on both Escherichia coli and E. gracilis chloroplast ribosomes, but has negligible activity on wheat germ cytoplasmic ribosomes. In this respect, it differs significantly from the mitochondrial translocase of yeast which has very little activity on chloroplast ribosomes. When assayed on E. coli ribosomes, E. gracilis EF-Gmt is sensitive to the steroid antibiotic, fusidic acid, at levels similar to that required for inactivation of E. coli EF-G. It is less sensitive than E. gracilis chloroplast EF-G, and is more sensitive than Bacillus subtilis EF-G. When assayed on E. gracilis chloroplast ribosomes, the same trends in sensitivities are observed, although the exact level of fusidic acid required for inactivation is slightly altered.

Biological characterization of various forms of elongation factor 1 from rabbit reticulocytes

Two forms of elongation factor 1 (EF-1) have been tested for a variety of biological functions. One form, EF-1H, is a high-molecular-weight aggregate (Mr greater than 500,000) containing four distinct polypeptides (alpha, beta, gamma, delta). The other form, EF-1 alpha, consists of a single polypeptide which is the same as the alpha subunit of EF-1H. Both EF-1 alpha and EF-1H function catalytically in binding Phe-tRNA to ribosomes, and in poly(U)-directed polyphenylalanine synthesis. The activity of EF-1 alpha is enhanced in polyphenylalanine synthesis by a complementary component, EF-1 beta delta. It is also shown that EF-1 beta delta can facilitate an exchange of EF-1 alpha-bound GDP for GTP. The EF-1 alpha dissociation constants for GDP and GTP were 0.47 and 0.55 microM respectively, while the EF-1H dissociation constants for GDP and GTP were 2.0 and 1.6 microM, respectively. Thus, while EF-1 alpha and EF-1H had approximately the same affinities for GDP and GTP, the EF-1 alpha dissociation constants were about fourfold lower than the EF-1H dissociation constants. Attempts to isolate complexes of EF-1 alpha or EF-1H with GTP and Phe-tRNA or with GTP, Phe-tRNA, and ribosomes were unsuccessful using either Millipore filters, gel filtration, or sucrose density gradients. The results presented in this report, along with studies from other laboratories, strengthen the hypothesis that the general mechanism of the elongation cycle is similar in eucaryotes and procaryotes.

Purification of various forms of elongation factor 1 from rabbit reticulocytes

Previous studies have indicated that the high-molecular-weight form of elongation factor 1 (EF-1H) contained four subunits (alpha, beta, gamma, and delta). Using the conventional methods of gel-filtration and ion-exchange chromatography, various forms of elongation factor 1 (EF-1 alpha, EF-1 beta delta, EF-1 beta gamma delta) have been purified from rabbit reticulocyte lysate. The procedure described allows one to purify these factors from a single batch of lysate in sufficient amounts for physical and biochemical studies. EF-1 alpha is a single polypeptide of Mr 52,000, and has an isoelectric point of 9.1. EF-1 beta delta and EF-1 beta gamma delta are composed of two and three nonidentical polypeptides, respectively, as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Both proteins can form stable aggregates in native conditions that can reach more than 2,000,000 Da. The isoelectric point for each polypeptide was determined; 5.8 for EF-1 beta, 5.5 for EF-1 gamma, and 4.8 for EF-1 delta. The activity of both proteins was compared on a molecular basis by their ability to stimulate EF-1 alpha in the poly(U)-directed synthesis of polyphenylalanine. On the basis of this assay EF-1 beta gamma delta is slightly more active than EF-1 beta delta. The similarity of the amino acid composition of EF-1 gamma and EF-1 delta and the molar ratio of alpha: beta: gamma: delta in EF-1H of approximately 1:1:0.5:0.5 have led to the conclusion that EF-1 delta is probably a breakdown product of EF-1 gamma, and that the native form of EF-1H probably contains only the alpha, beta, and gamma subunits.

The role of eucaryotic factor Tu in protein synthesis. The measurement of the elongation factor Tu content of rabbit reticulocytes and other mammalian cells by a sensitive radioimmunoassay

A sensitive radioimmunoassay for eucaryotic elongation factor Tu (eEF-TU) was developed using radioiodinated elongation factor T (eEF-T) and goat anti-(rabbit eEF-T) immunoglobulins coupled to a solid support. eEF-T was iodinated with 125I to a specific activity of 7 x 10(3) counts min-1 ng-1 using a system employing lactoperoxidase and glucose oxidase coupled to a solid support. The assay exhibits a limit of detection of about 1 ng eEF-TU and an intraassay variability of < 10%. By using the radioimmunoassay, it was found that eEF-Tu is a major non-hemoglobin protein of rabbit reticulocyte postribosomal supernatant proteins, comprising about 3% of the total hemoglobin and 10--13% of the non-hemoglobin proteins. Similar results were found for a number of different tissues and cells, including rabbit heart, brain, liver and kidneys, as well as both induced and non induced Friend erythroleukemia cells. Values of eEF-Tu ranged from 1% of supernatant proteins in heart to about 11% in noninduced erythroleukemic cells. The levels of eEF-Tu in these mammalian tissues were comparable to the level of the homologous factor EF-Tu in Escherichia coli. It has previously been found that EF-Tu constitutes about 6--8% of the supernatant proteins of E. coli [Furano, A. V. (1975) Proc. Natl Acad. Sci. USA, 72, 4780--4784]. The level of eEF-Tu in reticulocytes was compared to the abundance of other components of protein synthesis in reticulocytes, such as translocase (eEF-G), tRNA, ribosomes and eIF-2. In all cases eEF-Tu was present in large excess over these other components.

Dependence of the activity of the poliovirus replicase on the host cell protein

Two poliovirus-specific RNA polymerase activities have been identified: a poly(U) polymerase that copies poly(A).oligo(U) and a replicase that copies natural heteropolymers with some preference for poliovirus RNA. Both activities purified together until a step of a salt gradient elution from poly(U)-agarose, when poly(U) polymerase but no replicase was recovered. Addition of a salt wash fraction from the ribisomes of uninfected cells to this poly(U) polymerase fraction reconstituted replicase activity, and a host factor was purified 50 fold from the ribosomal salt wash. None of the available initiation factors or elongation factors for protein synthesis were able to reconstitute replicase activity. Host factor activity could be supplied by adding oligo(U), suggesting that the factor acts at the initiation step of RNA replication. With the purified replicase-host factor combination, only poly(A)-containing RNAs were copied, and a preference for poliovirus RNA was shown.

Eucaryotic elongation factors Ts is an integral component of rabbit reticulocyte elongation factor 1

Elongation factor 1 (EF-1) was purified from rabbit reticulocytes and found to contain at least two distinct polypeptides: one of Mr 53 000 and one of Mr 30 000. The 30 000-Mr polypeptide was purified from EF-1 by treatment of the factor with 5.4 M guanidine . HCl and subsequent chromatography on DEAE-BioGel A in the presence of 5 M urea. By a number of functional criteria, the 30 000-Mr polypeptide was found to be the eucaryotic elongation factor Ts (eEF-Ts). These criteria include the ability of the polypeptide to stimulate Artemia salina eEF-Tu-dependent binding of aminoacyl-tRNA to 80-S ribosomes as well as eEF-Tu + EF-2-dependent polyphenylalanine synthesis. The reticulocyte factor also markedly increased the rate of exchange of eEF-Tu . gdp complexes with free GTP. Furthermore, rabbit antibodies to EF-1 from A. salina which was previously shown to contain eEF-Ts [Slobin, L. I. and Möller, W. (1978) Eur. J. Biochem. 84, 69--77] were found to cross-react with reticulocyte eEF-Ts, suggesting extensive structural homology between brine shrimp and rabbit eEF-Ts. The demonstration that eEF-Ts is and integral component of EF-1 from such diverse sources as brine shrimp and rabbit reticulocytes supports the conclusion that the factor is universally present in eucaryotic EF-1.

The isolation and characterization of elongation factor eEF-Ts from Krebs-II mouse-ascites-tumor cells and its role in the elongation process

A factor having activity similar to that described in other systems for the eukaryotic elongation factor eEF-Ts was isolated from the heavy, aggregate form of eEF-TH (formally named EF-1H). This protein has a molecular weight of 52000 under native conditions and of 25500 under denaturing conditions. It has been shown to stimulate eEF-Tu-dependent aminoacyl-tRNA binding to ribosomes and therefore eEF-Tu/eEF-G-dependent polyphenylalanine synthesis by ribosomes and was found to stimulate GDP-GTP exchange in eEF-Tu . GDP complexes. In the course of this work, it was also demonstrated that the removal of deacylated tRNA from the ribosome is a GTP-dependent process. This report, therefore, adds further support to the concept that a third elongation factor, eEF-Ts, may be common to all systems in the eukaryotic domain.

Immobilization of reticulocyte elongation factor EF-2

Conditions are described whereby the ADP-ribosylation (from NAD+) of reticulocyte elongation factor EF-2, catalyzed by diphtheria toxin, is essentially complete and whereby the reverse of this process may be carried out with recovery of 60--70% of the original EF-2 activity. Both reactions proceed well at room temperature. The reverse reaction is much slower than the ADP-ribosylation process and requires high nicotinamide concentrations. For the reverse reaction to occur at a significant rate it is necessary to lower the pH to 6.5 (from the 7.5 used for the forward reaction). NAD+ covalently linked to agarose may replace NAD+ in the diphtheria toxin reaction. The characteristics of this reaction are similar to those of the reaction employing free NAD+ except that the velocity is reduced and the concentration of NAD+ moieties greatly increased. NAD+ immobilized on agarose through the C-8 of the adenine ring is a superior substrate compared with NAD+ linked to agarose via its periodate-oxidized ribose moieties. Preliminary experiments indicate that reversal of this latter reaction with recovery of biological activity may be possible.

Binding of reticulocyte elongation factor 1 to ribosomes and nucleic acids

The present study has examined the requirements for the binding of rabbit reticulocyte elongation factor 1 (EF-1) to ribosomes under different assay conditions. When a centrifugation procedure was used to separate the ribosome EF-1 complex, the binding of EF-1 to ribosomes required GTP and Phe-tRNA, but not poly(U). The results suggested that undr these conditions a ternary complex, EF-1 . GTP . aminoacyl-tRNA, is necessary for the formation of a ribosome . EF-1 complex. However, when gel filtration was used to isolate the ribosome . EF-1 complex, only template and tRNA were required. These studie emphasize the fact that the procedure used to isolate the ribosome . EF-1 complex determines the requirements for stable complex formation. EF-1 can also interact with nucleic acids such as 28 S and 18 S rRNA, messenger RNA and DNA. In contrast to the binding to ribosomes, EF-1 binding to nucleic acids requires only Mg2+.