Mechanisms in polypeptide chain elongation on ribosomes

Interleukin-1β effect on the endogenous ADP-ribosylation and phosphorylation of eukaryotic elongation factor 2

Eukaryotic elongation factor 2 (eEF2) plays an important role in eukaryotic polypeptide chain elongation. Adenosine diphosphate (ADP)-ribosylation is a post-translational modification reaction that catalyzes the transfer of ADP-ribose group to eEF2 and this causes the inhibition of protein synthesis. Indeed, in the absence of diptheria toxin, endogenous ADP-ribosylation can occur. eEF2 is phosphorylated by eEF2 kinase which prevents binding to ribosomes thus inhibiting its activity. Increase in endogenous ADP-ribosylation level approximately 70-75 % was observed in IL-1β treated HUVECs. Moreover, a 70 % rise of phosphorylation of eEF2 was measured. Alteration of endogenous ADP-ribosylation of eEF2 activity was related with cellular mono-ADP-ribosyltransferases (ADPrT). Increment of endogenous ADP-ribosylation on eEF2 did not seem to occur as a direct effect of IL-1β; it arises from the activation of ADPrT. This 2.5 fold increase was abolished by ADPrT inhibitors. Due to these post-translational modifications, global protein synthesis is inhibited. After dephosphorylation of phospho-eEF2, around 20 % increase in protein synthesis was observed. In conclusion, systemic IL-1β has an important role in the regulation of global protein synthesis.

Global protein profiling studies of chikungunya virus infection identify different proteins but common biological processes

Chikungunya fever (CHIKF) caused by the mosquito-transmitted chikungunya virus (CHIKV) swept into international prominence from late 2005 as an epidemic of CHIKF spread around countries surrounding the Indian Ocean. Although significant advances have been made in understanding the pathobiology of CHIKF, numerous questions still remain. In the absence of commercially available specific drugs to treat the disease, or a vaccine to prevent the diseases, the questions have particular significance. A number of studies have used global proteome analysis to increase our understanding of the process of CHIKV infection using a number of different experimental techniques and experimental systems. In all, over 700 proteins have been identified in nine different analyses by five different groups as being differentially regulated. Remarkably, only a single protein, eukaryotic elongation factor 2, has been identified by more than two different groups as being differentially regulated during CHIKV infection. This review provides a critical overview of the studies that have used global protein profiling to understand CHIKV infection and shows that while a broad consensus is emerging on which biological processes are altered during CHIKV infection, this consensus is poorly supported in terms of consistent identification of any key proteins mediating those biological processes.

C-terminal Src kinase (Csk)-mediated phosphorylation of eukaryotic elongation factor 2 (eEF2) promotes proteolytic cleavage and nuclear translocation of eEF2

Protein-tyrosine kinase C-terminal Src kinase (Csk) was originally purified as a kinase for phosphorylating Src and other Src family kinases. The phosphorylation of a C-terminal tyrosine residue of Src family kinases suppresses their kinase activity. Therefore, most physiological studies regarding Csk function have been focused on Csk as a negative regulator of Src family tyrosine kinases and as a potential tumor suppressor. Paradoxically, the protein levels of Csk were elevated in some human carcinomas. In this report, we show that eukaryotic elongation factor 2 (eEF2) is a new protein substrate of Csk and could locate in the nucleus. We demonstrate that Csk-mediated phosphorylation of eEF2 has no effect on its cytoplasmic function in regulating protein translation. However, phosphorylation of eEF2 enhances its proteolytic cleavage and the nuclear translocation of the cleaved eEF2 through a SUMOylation-regulated process. Furthermore, we show that cleaved fragments of eEF2 can induce nuclear morphological changes and aneuploidy similar to those in cancer cells, suggesting that there is an additional mechanism for Csk in tumorigenesis through regulation of eEF2 subcellular localization.

Development of ecotoxicoproteomics on the freshwater amphipod Gammarus pulex: identification of PCB biomarkers in glycolysis and glutamate pathways

PCBs are persistent organic pollutants largely distributed in the biosphere. Although their effects on vertebrates are well described, little is known about their action on freshwater invertebrate's metabolism. Gammarus pulex (Linné) was selected as an indicator model to develop a proteomic approach in order to characterize the effects of PCBs on the protein profile of this freshwater crustacean. Sublethal coplanar PCBs exposition and related 2D gel were performed. More than 560 spots were detected and a total of 21 proteins exhibiting significant expression differences in PCB exposed to G. pulex were identified by mass spectrometry. Database searches were conducted to relate the results to well-known metabolic pathways (pentose phosphate, cytoskeleton, energy, etc.). In particular, glyceraldehyde 3-phosphate dehydrogenase and arginine kinase were found to be sensitive to the PCB exposition of G. pulex. The aim of the present study was to assess the biochemical responses and the metabolic changes in G. pulex following intoxication to coplanar PCB congeners CB77 and CB169 by a proteomic approach. This approach allowed us, by the identification of key proteins, to highlight important biochemical mechanisms disturbed by the presence of these contaminants in G. pulex.

Potential protein partners for the human TIMAP revealed by bacterial two-hybrid screening

BacterioMatch Two-Hybrid System (Stratagene) was applied in order to identify potential human TIMAP interaction proteins in the lung. TIMAP highly expressed in endothelial cells and may be involved in endothelial cytoskeletal and barrier regulation. Seven TIMAP interacting partner proteins were identified. Four of identified proteins: cystein and glycine-rich protein 1, eukaryotic translation elongation factor 2, U5 snRNP-specific protein 116 kD, and solute carrier family 3 member 2 are involved in actin cytoskeleton organization, cell adhesion or translation and transcriptional regulation.

Endogenous ADP-ribosylation for eukaryotic elongation factor 2: evidence of two different sites and reactions

Eukaryotic elongation factor 2 can undergo ADP-ribosylation in the absence of diphtheria toxin under the action of an endogenous transferase. The investigation which aimed to gain insight into the nature of endogenous ADP-ribosylation revealed that this reaction may be, in some cases, due to covalent binding of free ADP-ribose to elongation factor 2. Binding of free ADP-ribose, and NAD- and endogenous transferase-dependent ADP-ribosylation were suggested to be distinct reactions by different findings. Free ADP-ribose could bind to elongation factor 2 previously subjected to ADP-ribosylation by diphtheria toxin or endogenous transferase. The binding of free ADP-ribose was inhibited by neutral NH2OH, L-lysine and picrylsulfonate, whereas endogenous ADP-ribosyltransferase was inhibited by NAD glycohydrolase inhibitors and L-arginine. The ADP-ribosyl-elongation factor 2 adduct which formed upon binding of free ADP-ribose was resistant to neutral NH2OH, but decomposed almost completely upon treatment with NaOH. The product of endogenous transferase-dependent ADP- ribosylation was partially resistant to NH2OH and NaOH treatment. Moreover, this reaction was reversed in the presence of diphtheria toxin and nicotinamide. Both types of endogenous ADP-ribosylation gave rise to inhibition of polyphenylalanine synthesis. This study thus provides evidence for the presence of two different types of endogenous ADP-ribosylation of eukaryotic elongation factor 2. The respective sites involved in these reactions are distinct from one another as well as from diphthamide, the site of attack by diphtheria toxin.

Proteomic analysis of differential protein expression in early process of pancreatic regeneration in pancreatectomized rats

AIM

A broad-range proteomic approach was applied to investigate the complexity of the mechanisms involved in pancreatic regeneration for identification of new targets of diabetes treatment and potential markers of pancreatic stem cells.

METHODS

A regeneration pancreatic model was induced by 90% partial pancreatectomy (Px) in rats. Changes in the protein expression in regenerating rat pancreas on the third day after Px, as compared with rats that received sham surgery, were analyzed by using 2-D gel electrophoresis (2-DE), mass spectrometry (MS), and mass fingerprinting.

RESULTS

2-DE revealed 91 spots with at least 1.5-fold increases in expression at 3 d after pancreatectomy and 53 differentially expressed proteins that were identified by peptide mass fingerprinting (PMF). These included cell growth-related, lipid and energy metabolism-related, protein and amino acid metabolism-related proteins, and signal transduction proteins. Vimentin, CK8, L-plastin, hnRNP A2/B1, and AGAT are associated with embryogenesis and cell differentiation, and may be new potential pancreatic stem cells markers.

CONCLUSION

The proteome profiling technique provided a broad-based and effective approach for the rapid assimilation and identification of adaptive protein changes during pancreas regeneration induced by pancreatectomy. Our data clarify the global proteome during the pancreatic proliferation and differentiation processes, which is important for better understanding of pancreatic regeneration and for discovering of protein biomarkers for pancreatic stem cells.

A nonradioactive, cell-free method for measuring protein synthesis inhibition by Pseudomonas exotoxin

Pseudomonas exotoxin A (PE) inhibits protein synthesis by NAD-dependent ADP-ribosylation of eukaryotic elongation factor 2. Traditionally, toxin activity has been characterized, either in living cells or cell-free systems, using radioactive compounds for quantification. The increased costs of radioactive waste disposal together with heightened security concerns have made the use of radioactive isotopes less attractive for routine laboratory assays. We therefore adapted a cell-free rabbit reticulocyte in vitro transcription-translation system that utilizes a reporter (beta-galactosidase) to measure toxin activity. The assay for PE is rapid, scalable, log-linear, NAD dependent and can be used to assess the neutralizing activity of anti-PE antibody preparations.

Proteomics in Myzus persicae: effect of aphid host plant switch

Chemical ecology is the study of how particular chemicals are involved in interactions of organisms with each other and with their surroundings. In order to reduce insect attack, plants have evolved a variety of defence mechanisms, both constitutive and inducible, while insects have evolved strategies to overcome these plant defences (such as detoxification enzymes). A major determinant of the influence of evolutionary arms races is the strategy of the insect: generalist insect herbivores, such as Myzus persicae aphid, need more complex adaptive mechanisms since they need to respond to a large array of different plant defensive chemicals. Here we studied the chemical ecology of M. persicae associated with different plant species, from Brassicaceae and Solanaceae families. To identify the involved adaptation systems to cope with the plant secondary substances and to assess the differential expression of these systems, a proteomic approach was developed. A non-restrictive approach was developed to identify all the potential adaptation systems toward the secondary metabolites from host plants. The complex protein mixtures were separated by two-dimensional electrophoresis methods and the related spots of proteins significantly varying were selected and identified by mass spectrometry (ESI MS/MS) coupled with data bank investigations. Fourteen aphid proteins were found to vary according to host plant switch; ten of them were down regulated (proteins involved in glycolysis, TCA cycle, protein and lipid synthesis) while four others were overexpressed (mainly related to the cytoskeleton). These techniques are very reliable to describe the proteome from organisms such as insects in response to particular environmental change such as host plant species of herbivores.

Effect of oxidative stress on in vivo ADP-ribosylation of eukaryotic elongation factor 2

Different lines of evidence indicate that eukaryotic elongation factor 2 (eEF2) can be ADP-ribosylated endogenously. The physiological significance of this reaction has, however, remained unclarified. In order to address this issue we investigated the in vivo ADP-ribosylation of eEF2 and the effect of oxidative stress thereon. The investigation revealed that the endogenous ADP-ribosylation of eEF2 is complex and can take place in K562 cell lysates either under the action of endogenous transferase from [adenosine-14C]NAD or by direct binding of free [14C]ADP-ribose. These two types of ADP-ribosylation were distinguished by use of different treatments based on the chemical stability of the respective bonds formed. Under standard culture conditions, in vivo labeling of eEF2 in the presence of [14C]adenosine was reversed to about 65% in the presence of diphtheria toxin and nicotinamide. This finding implied that the modification that took place under physiological circumstances was, mainly, of an enzymic nature. On the other hand, H2O2-promoted oxidative stress gave rise to a nearly two-fold increase in the extent of in vivo labeling of eEF2. This was accompanied by a loss of eEF2 activity in polypeptide chain elongation. Oxidative stress specifically inhibited the subsequent binding of free ADP-ribose to eEF2. The results thus provide evidence that endogenous ADP-ribosylation of eEF2 can also take place by the binding of free ADP-ribose. This nonenzymic reaction appears to account primarily for in vivo ADP-ribosylation of eEF2 under oxidative stress.

Actin--an inhibitor of eukaryotic elongation factor activities

An inhibitor of diphtheria toxin- and endogenous transferase-dependent ADP-ribosylation of eukaryotic elongation factor 2 (eEF2) has been found in the cytoplasmic fraction from rat liver. We provide evidence that this cytoplasmic inhibitor corresponds to actin, which gives rise also to inhibition of polyphenylalanine (polyPhe) synthesis. Both globular monomeric (G-actin) and filamentous (F-actin) forms of actin appear to be inhibitory on the action of elongation factors 1 and 2 (eEF1 and eEF2) in polyPhe synthesis with the inhibitory effect of G-actin proving to be stronger. Some component(s) in the postribosomal supernatant (S-130) fraction and also DNase I prevent actin-promoted inhibition of polyPhe synthesis.

Interactions of eukaryotic elongation factor 2 with actin: a possible link between protein synthetic machinery and cytoskeleton

Eukaryotic elongation factor 2 (EF-2) was shown to bind to F-actin as assayed by co-sedimentation. In the presence of guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) binding was increased fourfold. At saturation level a molar ratio of about 0.12 EF-2 per F-actin (subunit) was observed. Our results suggest a single type of binding site with an apparent dissociation constant of 0.85 microM. The stoichiometry was independent of the filament length, and ADP-ribosylation had no effect on the binding. Experimental data indicated the involvement of SH-groups of both EF-2 and actin in the binding. The interaction EF-2 with F-actin appeared to be inhibited competitively by EF-1 alpha and non-competitively by G-actin.

Elongation factor 2 from Artemia salina embryos and its affinity for ribosomes

Crude extracts from Artemia salina undeveloped embryos do not contain detectable elongation-factor-2 (EF2) kinase and endogenous ADP-ribosylating activities. Accordingly, EF2 purified from this source is an enzyme relatively free from phosphorylated and ADP-ribosylated forms. Endogenous ADP-ribosyltransferase activity appears only after purification of EF2. The affinities of EF2 and of ADP-ribosyl-EF2 for ribosomes from A. salina undeveloped embryos have been calculated by measuring the ability of the factors to inhibit the N-glycosidase activity of ricin on ribosomes.

Mutagenesis of the NH2-terminal domain of elongation factor Tu

Mutagenesis was carried out in the N-terminal domain of elongation factor Tu (EF-Tu) to characterize the structure-function relationships of this model GTP binding protein with respect to stability, the interaction with GTP and GDP, and the catalytic activity. The substitutions were introduced in elements around the guanine nucleotide binding site or in the loops defining this site, in the intact molecule or in the isolated N-terminal domain (G domain). The double substitution Val88----Asp and Leu121----Lys, two residues situated on two vicinal alpha-helices, influences the basic activities of the truncated factor to a limited extent, probably via long-range interactions, and induces a destabilisation of the G domain structure. The functional alterations brought about by substitutions on the consensus sequences 18-24 and 80-83 highlight the importance of these residues for the interaction with GTP/GDP and the GTPase activity. Mutations concerning residues interacting with the guanine base lead to proteins in large part insoluble and inactive. In one case, the mutated protein (EF-TuAsn135----Asp) inhibited the growth of the host cell. This demonstrates the crucial role of the base specificity for the active conformation of EF-Tu. The obtained results are discussed in the light of the three-dimensional structure of EF-Tu.

Complete sequence of the coding region of human elongation factor 2 (EF-2) by enzymatic amplification of cDNA from human ovarian granulosa cells

The use of two primers allowed the specific enzymatic amplification of elongation factor 2 starting with total double-stranded cDNA from human ovarian granulosa cells. The amplified DNA fragment with a length of 1765 bp was restricted and sequenced by the shot gun approach. From the sequences obtained from the amplified fragment and the cDNA insert of pHGR81 [Rapp et al. (1988) Biol. Chem. Hoppe-Seyler 369, 247-250] respectively, the DNA sequence containing the complete coding as well as the 3'-untranslated region was assembled.

Elongation factor 2 as the major substrate for Ca2+/calmodulin-dependent protein kinase in rat adrenal glomerulosa cells

We have previously shown the existence of the major substrate protein of Mr 100,000 (substrate 100 K protein) for Ca2+/calmodulin (CaM)-dependent protein kinase in rat adrenal glomerulosa cells. In the present study, the identity of the substrate 100 K protein to elongation factor 2 (EF-2) was investigated. In a 105,000 g-supernatant fraction (cytosol), the protein of Mr 100,000 with the pI (isoelectric point) value of 6.7 was phosphorylated in the presence of calcium and CaM. The optical densities of this phosphorylated band were greatly enhanced in the presence of the EF-2 purified from pig liver (1 microgram) [20-23-fold, n = 5] when compared with those in the absence of the component. In the presence of the purified EF-2, the phosphorylation of Mr 100,000 was detected only in the presence of calcium alone or calcium plus CaM. This phosphorylation in the presence of calcium alone was completely inhibited in the presence of the CaM antagonist pimozide (500 microM), showing the existence of endogenous CaM in the cytosol. In the same fraction, the ADP-ribosylated protein of Mr 100,000 was detected in the presence of diphtheria toxin (fragment A) and (adenylate-32P) NAD, indicating the presence of EF-2 in the cytosol from rat adrenal glomerulosa cells. These results suggest that the substrate 100 K protein may be identical to EF-2 in rat adrenal glomerulosa cells.

Cloning and sequence analysis of a cDNA from human ovarian granulosa cells encoding the C-terminal part of human elongation factor 2

A cDNA clone, pHGR81, encoding 358 amino-acid residues of the C-terminal region of human elongation factor 2 (EF-2), was isolated from a human ovarian granulosa cell cDNA library. The deduced amino-acid sequence of pHGR81, when compared with the known identical amino-acid sequences of hamster as well as rat EF-2 revealed a substitution of a glutamine by an alanine residue in the partially determined human sequence. The 15 amino-acid-residue sequence comprising the histidine-715, supposed to be of importance for the biological function of EF-2, is preserved in human EF-2. The coding region of the cDNA insert of pHGR81 displays a homology of 87% to hamster and of 88% to rat EF-2 cDNA. In Northern-transfer analysis, pHGR81 specifically hybridizes with an mRNA species of 3.1 kb.

Polypeptide elongation factor Tu from Halobacterium marismortui

A GDP-binding protein of 60 kDa from Halobacterium marismortui has been purified to homogeneity. The purification has been carried out in high-salt buffers or in 50% glycerol buffers to protect the halophilic protein from denaturation. Evidence that this protein is the halophilic elongation factor Tu (hEF-Tu) is provided by the high homology of its N terminus with the corresponding sequences of other EF-Tus, and by immunological studies. Like some other EF-Tus the native protein can be cleaved with trypsin without concomitant loss of GDP-binding ability. The molecular mass of this hEF-Tu is higher than that for the corresponding factors from other sources including the halobacterium Halobacterium cutirubrum. The protein possesses typical halophilic characteristics, in that it is stable and active in 3 M KCl or 2 M (NH4)2SO4. Some other properties, like autofragmentation under sample treatment before SDS-PAGE, are described.

Purification of elongation factor 2 from human placenta and evidence of its fragmentation patterns in various eukaryotic sources

While preparing human placenta elongation factor 2 (EF-2), whose purification and some molecular properties are reported, we noticed the presence of numerous protein fractions which did not have EF-2 activity, but were ADP-ribosylated by diphtheria toxin in the presence of NAD+. All these proteins, like EF-2, were selectively retained by a heparin-Sepharose column, which we used as an affinity-chromatography step. This was also observed when EF-2 was prepared, by this purification step, from other sources, i.e. ox liver and two species of yeasts. In order to assess whether these proteins were a degradation product of EF-2, independent proteins or a mixture of both, they were analysed by subjecting them, after [14C]ADP-ribosylation, to exhaustive trypsinolysis. Only one radioactive peptide was found, thus suggesting that those proteins originate from EF-2 by some proteolytic process. Our findings indicate that this proteolysis does not occur after cell disruption, but is more or less active in the intact cell, depending on the system considered.

Regulation of the activity of eukaryotic peptide elongation factor 1 by autocatalytic phosphorylation

Highly purified peptide elongation factor 1 from rabbit reticulocytes liberates the terminal phosphate from [gamma-32P]GTP and incorporates it into its own protein. Approximately one phosphate residue becomes bound by one molecule of the factor. Only the eEF-1 alpha subunit of the factor (Mr 53 000) becomes phosphorylated as revealed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate followed by autoradiography and by the incubation of [gamma-32P]GTP with individual subunits of the elongation factor separated by chromatofocusing in the presence of 5 M urea. The phosphorylation also takes place, though to a lesser extent, if the factor is incubated with Na2H32PO4, probably due to the presence of endogenous GTP bound in the molecule of the factor. The content of endogenous GTP in various factor preparations was 0.21-0.43 mol/mol factor. Phosphorylation of the peptide elongation factor is ribosome-independent, acid-labile and apparently autocatalytic since no other proteins are required for this reaction. Preincubation of the factor with GTP or with inorganic phosphate results in the phosphorylation of the factor and is followed by an enhanced binding of phenylalanyl-tRNA to 80S ribosomes in the presence of poly(U). This is accompanied by a dephosphorylation of the factor protein and thus the reversible autophosphorylation of the factor apparently activates its binding site for aminoacyl-tRNA. This is supported by the observation that sodium fluoride, which inhibits the dephosphorylation of the factor, blocks the factor-catalyzed binding of aminoacyl-tRNA to ribosomes. The incorporation of phosphate into factor protein also inhibits the formation of an eEF-1 X GDP complex, which is inactive in protein synthesis. Thus GDP liberated by the GTPase activity of the factor cannot affect its binding site for aminoacyl-tRNA. This may be the other reason for the enhanced activity of the phosphorylated factor. The autocatalytic GTP-dependent phosphorylation of the peptide elongation factor 1 apparently modifies its function and may thus play a regulatory role in protein synthesis.

Two tuf genes in the cyanobacterium Spirulina platensis

Probes derived from the tufA gene of Escherichia coli have been utilized to detect homologous sequences on Spirulina platensis DNA. A 6-kilobase-pair fragment of S. platensis DNA appears to contain two sequences homologous to the E. coli gene. Thus, as reported for gram-negative bacteria, the cyanobacterium presumably contains two tuf genes.

Purification of the elongation factor Tu (EF-Tu) from the cyanobacterium Spirulina platensis

Elongation factor Tu (EF-Tu) has been purified from the cyanobacterium Spirulina platensis. By gel electrophoresis the Mr of the purified protein appears to be 49 000, a value close to that reported for the EF-Tu isolated from a number of bacteria but higher than that reported for the protein isolated from Escherichia coli (43 000). Functionally, however, S. platensis EF-Tu may replace the E. coli protein in a protein-synthesizing system in vitro. In addition, its activity is affected by kirromycin, an antibiotic that specifically interacts with eubacterial EF-Tu.

Isolation and characterization of Streptomyces aureofaciens protein-synthesis elongation factor Tu in an aggregated state

The ability of EF-Tu to aggregate spontaneously was employed for the purification of homogeneous EF-Tu . GDP from Streptomyces aureofaciens. The formation of filamentous structures in the aggregated EF-Tu was demonstrated in a light microscope. The purified factor, with a specific activity of 19,100 +/- 1,000 units/mg in [3H]GDP exchange, was shown to be active in the translation of poly(U). Aggregated EF-Tu . GDP exhibited almost eight-times lower GDP-exchange capacity at 2 degrees C than at 30 degrees C. This suggests that GDP-binding sites are not freely accessible at lower temperatures in the aggregated factor, in contrast to Escherichia coli polymerized EF-Tu. Turbidimetric assays revealed that the solubilization of diluted aggregated S. aureofaciens EF-Tu is strongly dependent on temperature and causes an increase in the number of accessible GDP-binding sites.

Properties and regulation of the GTPase activities of elongation factors Tu and G, and of initiation factor 2

During protein synthesis the interaction with ribosomes of elongation factors Tu (EF-Tu), G (EF-G) and initiation factor 2 (IF-2) is associated with the hydrolysis of GTP which is directly related to the functions of the factors. In this article we review systematically the properties of these GTPase activities in the presence and absence of protein synthesis, and by examining the characteristics of the different minimal systems for the expression of these activities we point to the role of the various effectors and to the enzymological aspects of the systems. For EF-Tu, it has been possible to eliminate any requirement for macromolecular effectors, showing that the factor itself is a GTPase. For EF-G, the presence of at least the 50S ribosomal subunit has remained a requirement, whereas IF-2 needs both the 50S and 30S subunits to exhibit GTPase activity. Between the GTPase activities of the three factors there are some striking similarities, but important differences prevail as a consequence of the specificity of the different functions. This can also be seen by examining the respective ribosomal regions implicated in these reactions. When coupled with protein synthesis, the three GTPase activities reveal characteristics differing from those observed in partial systems.

Two elongation factors from the extremely halophilic archaebacterium Halobacterium cutirubrum. Assay systems and purification at high salt concentrations

Two peptide chain elongation factors from Halobacterium cutirubrum were purified nearly to homogeneity. They were identified and characterized by three assay systems working at very high salt concentrations: poly(Phe) synthesis, GDP binding and ADP-ribosylation. The purification procedure consisted of Sepharose 4B chromatography with a decreasing ammonium sulfate gradient, gel filtration on Sephadex G-100 in the presence of (NH4)2SO4 and, for each of the two separated factors, an independent adsorption chromatography on hydroxyapatite. The factors were absolutely dependent on high salt concentrations for stability and activity. Both factors (I and II) complement each other to give fully active poly(Phe) synthesis, which is totally inhibited by puromycin and anisomycin. Factor I (Mr 51 000) is a major protein of the cell-free extract. It binds GDP, which can be displaced by GTP only to a small extent. The function of factor I in poly(Phe) synthesis is not impaired by high concentrations of kirromycin. The observed characteristics resemble partly those of prokaryotic EF-Tu and partly those of eukaryotic EF-1. Factor II (Mr 111 000) can be ADP-ribosylated by diphtheria toxin, and thus was identified as an EF-2-type elongation factor.