Endogenous ADP-ribosylation of elongation factor 2 in polyribosome fraction of rabbit reticulocytes

In Memory of Lev Ovchinnikov

Lev Ovchinnikov was a true man of Science. Until the end of his life, he retained not only loyalty to strict scientific principles, but also a benevolent attitude towards the people around him. He devoted his scientific career to the study of mRNP and regulation of protein biosynthesis. He created a unique scientific school that received international recognition.

RNA Regulation by Poly(ADP-Ribose) Polymerases

Post-transcriptional regulation of RNA facilitates the fine-tuning of gene expression. It occurs through multiple pathways that include the nuclear processing of mRNA and its precursors, mRNA silencing, regulation of mRNA decay, and regulation of translation. Poly(ADP-ribose) polymerases (PARPs), enzymes that modify target proteins with ADP-ribose, play important roles in many of the RNA regulatory pathways through multiple mechanisms. For example, RNA-binding PARPs can target specific transcripts for regulation, ADP-ribosylation of RNA-regulatory proteins can alter their localization, activity or RNA-binding, and non-covalent interactions of RNA-binding proteins with poly(ADP-ribose) can affect their function. In addition to regulating RNA during non-stress conditions, PARPs mediate RNA regulation during cellular stress conditions that are critical for the proper execution of a stress response. In this review, we summarize the current knowledge regarding PARP-dependent regulation of RNAs, and describe how by modulating RNA processing, translation and decay, PARPs impact multiple processes in the cell.

The diphthamide modification pathway from Saccharomyces cerevisiae--revisited

Diphthamide is a conserved modification in archaeal and eukaryal translation elongation factor 2 (EF2). Its name refers to the target function for diphtheria toxin, the disease-causing agent that, through ADP ribosylation of diphthamide, causes irreversible inactivation of EF2 and cell death. Although this clearly emphasizes a pathobiological role for diphthamide, its physiological function is unclear, and precisely why cells need EF2 to contain diphthamide is hardly understood. Nonetheless, the conservation of diphthamide biosynthesis together with syndromes (i.e. ribosomal frame-shifting, embryonic lethality, neurodegeneration and cancer) typical of mutant cells that cannot make it strongly suggests that diphthamide-modified EF2 occupies an important and translation-related role in cell proliferation and development. Whether this is structural and/or regulatory remains to be seen. However, recent progress in dissecting the diphthamide gene network (DPH1-DPH7) from the budding yeast Saccharomyces cerevisiae has significantly advanced our understanding of the mechanisms required to initiate and complete diphthamide synthesis on EF2. Here, we review recent developments in the field that not only have provided novel, previously overlooked and unexpected insights into the pathway and the biochemical players required for diphthamide synthesis but also are likely to foster innovative studies into the potential regulation of diphthamide, and importantly, its ill-defined biological role.

A guide to taming a toxin--recombinant immunotoxins constructed from Pseudomonas exotoxin A for the treatment of cancer

Pseudomonas exotoxin A (PE) is a highly toxic protein secreted by the opportunistic pathogen Pseudomonas aeruginosa. The modular structure and corresponding mechanism of action of PE make it amenable to extensive modifications that can redirect its potent cytotoxicity from disease to a therapeutic function. In combination with a variety of artificial targeting elements, such as receptor ligands and antibody fragments, PE becomes a selective agent for the elimination of specific cell populations. This review summarizes our current understanding of PE, its intoxication pathway, and the ongoing efforts to convert this toxin into a treatment for cancer.

Endogenous ADP-ribosylation of elongation factor-2 by interleukin-1β

Eukaryotic elongation factor-2 (eEF-2) catalyses the motion of the growing peptide chain relative to the mRNA at the ribosomes during protein synthesis. This highly conserved G-protein is the specific target of two lethal bacterial toxins, Pseudomonas aeruginosa exotoxin A and diphtheria toxin. These toxins exert their detrimental action by ADP-ribosylating a biologically unique posttranslationally modified histidine residue (diphthamide(715)) within eEF-2, thus inactivating the enzyme. Diphthamide(715) is also the target of endogenous (mono) ADP-ribosyl transferase activity. In this article, we report the first known activator of endogenous ADP-ribosylation of eEF-2, interleukin-1β (IL-1β). Thereby, systemic inflammatory processes may link to protein synthesis 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.

The life and death of translation elongation factor 2

eEF2 (eukaryotic elongation factor 2) occupies an essential role in protein synthesis where it catalyses the translocation of the two tRNAs and the mRNA after peptidyl transfer on the 80 S ribosome. Recent crystal structures of eEF2 and the cryo-electron microscopy reconstruction of its 80 S complex now provide a substantial structural framework for dissecting the functional properties of this factor. The factor can be modified by either phosphorylation or ADP-ribosylation, which results in cessation of translation. We review the structural and functional properties of eEF2 with particular emphasis on the unique diphthamide residue, which is ADP-ribosylated by diphtheria toxin from Corynebacterium diphtheriae and exotoxin A from Pseudomonas aeruginosa.

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.

Cytoplasmic inhibitor of eEF-2 ADP-ribosylation catalyzed by diphtheria toxin or endogenous transferase in rat liver cells

eEF-2 (100 kDa) isolated from rat liver cells undergo ADP-ribosylation in the presence of diphtheria toxin or endogenous ADP-ribosyltransferase, which was co-purified with the factor. We separated the fraction free of elongation factor and endogenous transferase, which strongly inhibited the ADP-ribosylation of eEF-2. This fraction did not affect the activity of the elongation factor. The lack of endogenous transferase activity (which is potentially lethal for the cell) in the postribosomal supernatant could be the result of its inhibition. eEF-2 (65 kDa) which is probably responsible for the process of translocation (Gajko, A. et al. (1999) Biochem. Biophys. Res. Commun. 255, 535-538) was protected from ADP-ribosylation and its irreversible inactivation in the presence of the rat liver extract fraction.

Nitric oxide regulation of ADP-ribosylation of G proteins in hypertension

Nitric oxide stimulates endogenous ADP-ribosylation of cytosolic and membrane-bound proteins. Endogenous ADP-ribosyltransferases modify several intracellular proteins including the heterotrimeric GTP-binding proteins (G proteins). ADP-ribosylation of G proteins in vascular smooth muscle leads to increased activation of adenylate cyclase and decreased activation of phospholipase C leading to vasodilation. We hypothesize that in hypertension, chronically depressed endothelium-derived nitric oxide levels lead to decreased ADP-ribosylation of G proteins. This reduced ADP-ribosylation leads to vasoconstriction since activation of the G proteins by agonists is unopposed. Thus, disinhibition of G proteins, mediated by nitric oxide deficit, is responsible for the observed increased sensitivity to vasoconstrictor agonists in hypertension. This novel role for nitric oxide in hypertension will provide a new area of research for antihypertensive therapeutic intervention.

ADP-ribosylation reactions in Sulfolobus solfataricus, a thermoacidophilic archaeon

An ADP-ribosylating system was detected in a crude homogenate from Sulfolobus solfataricus, a thermophilic archaeon, optimally growing at 87 degrees C. The archaeal ADP-ribosylation reaction was time-, temperature- and NAD-dependent. It proved to be highly thermostable, with about 30% decrease of 14C incorporation from [14C]NAD on incubation at 80 degrees C for up to 24 h. The main reaction product was found to be mono-ADP-ribose. Testing both [adenine-14C(U)]NAD and [adenine-14C(U)]ADPR as substrates, it was found that acceptor proteins were modified by ADP-ribose both enzymatically, via ADP-ribosylating enzymes, and via chemical attachment of free ADP-ribose, likely produced by NAD glycohydrolase activity. The synthesis of ADP-ribose-protein complexes was shown to involve mainly acceptors with molecular masses in the 40-100 kDa range, as determined by electrophoresis on polyacrylamide gel in the presence of sodium dodecyl sulphate.

Cellular ADP-ribosylation of elongation factor 2

A cellular ADP-ribosyltransferase activity has been found in a variety of animals and tissues. The enzyme transfers ADP-ribose from NAD to elongation factor 2, inactivating the factor and thus inhibiting in vitro protein synthesis. Although, the mechanism of action of the cellular enzyme appears similar to diphtheria toxin and Pseudomonas exotoxin A, it differs from the toxins in that only a fraction of the EF-2 pool is modified. The endogenously ADP-ribosylated EF-2 has been detected by a variety of methods including two-dimensional electrophoresis and immunoprecipitation with elongation factor 2 antibody. The nature of the cellular ADP-ribosyltransferase and its physiological significance are unknown.

Increased phosphorylation of elongation factor 2 in Alzheimer's disease

Elongation factor 2 (EF-2) is a phosphoprotein that mediates the translocation step of elongation during protein synthesis. We investigated its phosphorylation to characterize translational regulation of gene expression in Alzheimer's disease. EF-2 was identified on two-dimensional (2D) gels of brain homogenates by analyzing immunoblots with EF-2-specific antibody (M(r) 96,000; pI 6.8). Four distinct charge variant isoforms were observed. We identified the two most acidic isoforms as being the phosphorylated forms by incorporation of radiolabeled phosphate. The phosphorylation of EF-2 in control and Alzheimer's disease (AD) brain was directly measured as the distribution of the four polypeptides on silver stained 2D gels. The ratio of the phosphorylated forms to unphosphorylated forms was elevated 45% in AD homogenates compared to controls (1.07 +/- 0.18; n = 9 vs 0.73 +/- 0.20; n = 6; P less than 0.004) which indicated an increased phosphorylation of AD EF-2. The phosphorylation exhibited specificity to the disease in that it was observed in affected areas (cortex and hippocampus) but not in an unaffected area (thalamus) of the same brains. Because phosphorylation of EF-2 inhibits protein synthesis, the observed AD-associated phosphorylation of EF-2 is consistent with the reduced in vitro activity of polysomes isolated from AD tissues that we have previously reported.

Mechanism and regulation of eukaryotic protein synthesis

This review presents a description of the numerous eukaryotic protein synthesis factors and their apparent sequential utilization in the processes of initiation, elongation, and termination. Additionally, the rare use of reinitiation and internal initiation is discussed, although little is known biochemically about these processes. Subsequently, control of translation is addressed in two different settings. The first is the global control of translation, which is effected by protein phosphorylation. The second is a series of specific mRNAs for which there is a direct and unique regulation of the synthesis of the gene product under study. Other examples of translational control are cited but not discussed, because the general mechanism for the regulation is unknown. Finally, as is often seen in an active area of investigation, there are several observations that cannot be readily accommodated by the general model presented in the first part of the review. Alternate explanations and various lines of experimentation are proposed to resolve these apparent contradictions.

Characterization of the endogenous ADP-ribosylation of wild-type and mutant elongation factor 2 in eukaryotic cells

Anti-[ADP-ribosylated elongation factor 2 (EF-2)] antiserum has been used to immunoprecipitate the modified form of EF-2 from polyoma-virus-transformed baby hamster kidney (pyBHK) cells [Fendrick, J. L. & Iglewski, W. J. (1989) Proc. Natl Acad. Sci. USA 86, 554-557]. This antiserum also immunoprecipitates a 32P-labelled protein of similar size to EF-2 from a variety of primary and continuous cell lines derived from many species of animals. One of these cell lines, chinese hamster ovary CHO-K1 cells was further characterized. The time course of labelling of ADP-ribosylated EF-2 with [32P]orthophosphate was similar in pyBHK cells and in CHO-K1 cells. The kinetics of labelling were more rapid for cells cultured in 2% serum than 10% serum, with incorporation of 32P reaching a maximum at 6 h and 10 h, respectively. EF-2 mutants of pyBHK and CHO-K1 cells resistant to diphtheria-toxin-catalyzed ADP-ribosylation of EF-2 remain sensitive to cellular ADP-ribosylation of EF-2. The 32P-labelled moiety of ADP-ribosylated EF-2 was digested by snake venom phosphodiesterase and the product was identified as AMP. The same 32P-labelled tryptic peptide was modified by toxin in wild-type EF-2 and by the cellular transferase in mutant EF-2. When purified EF-2 from pyBHK cells was incubated with [carbonyl-14C]nicotinamide and diphtheria toxin fragment A, under conditions for reversal of the ADP-ribosylation reaction, [14C]NAD was generated. The results suggest that cellular ADP-ribosylated EF-2 exists in a variety of cell types, and the ribosylated product is identical to that produced by toxin ADP-ribosylation of EF-2, except in diphthamide mutant cells. Studies with the mutant cell lines indicate that the toxin and the cellular transferase, however, recognize different determinants at the ADP-ribose acceptor site in EF-2. The cellular transferase does not require the diphthamide modification of the histidine ring in the amino acid sequence of EF-2 for the transfer of ADP-ribose to the ring. Therefore, we would expect the cellular transferase active site to be similar to, but not identical to, the critical amino acids demonstrated in the active site of diphtheria toxin and Pseudomonas exotoxin A.

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.

Modification of the reactivity of three amino-acid residues in elongation factor 2 during its binding to ribosomes and translocation

The accessibility of three amino acids of EF-2, located within highly conserved regions near the N- and C-terminal extremities of the molecule (the E region and the ADPR region, respectively) to modifying enzymes has been compared within nucleotide-complexed EF-2 and ribosomal complexes that mimic the pre- and posttranslocational ones: the high-affinity complex (EF-2)-nonhydrolysable GTP analog GuoPP[CH2]P ribosome and the low-affinity (EF-2)-GDP-ribosome complex, EF-2 and ribosomes being from rat liver. We studied the reactivity of two highly conserved residues diphthamide-715 and Arg-66, to diphtheria-toxin-dependent ADP-ribosylation and trypsin attack, and of a threonine that probably lies between residues 51 and 60, to phosphorylation by a Ca2+/calmodulin-dependent protein kinase. Diphthamide 715 and this threonine residue were unreactive within the high-affinity complex but seemed fully reactive in the low-affinity complex. Arg-66 was resistant to trypsin in both complexes. The possible involvement of the E and ADPR regions of EF-2 in the interaction with ribosome in the two complexes is discussed.

Heterogeneity of native rat liver elongation factor 2

The high heterogeneity of native rat liver EF-2 prepared from either 105000 x g supernatant or microsome high-salt extract was detected by two-dimensional equilibrium isoelectric focusing-SDS-polyacrylamide gel electrophoresis in the presence of 9.5 M urea. Five spots were always detected, all of Mr 95,000, which were not artefactual for their amount varied when EF-2 was specifically ADP-ribosylated by diphtheria toxin in the presence of NAD+, and/or phosphorylated on a threonine residue by a Ca2+/calmodulin-dependent protein kinase (most likely Ca2+/calmodulin-dependent protein kinase III described by others [(1987) J. Biol. Chem. 262, 17299-17303; (1988) Nature 334, 170-173]). Results of ADP-ribosylation and/or phosphorylation experiments with either unlabeled or labeled reagents ([14C]NAD and [32P]ATP) strongly suggest that our preparation contained native ADP-ribosylated and native phosphorylated forms which could be estimated at about 20% and 40% of the whole EF-2. Phosphorylated and ADP-ribosylated forms of EF-2 could be ADP-ribosylated and phosphorylated, respectively, but a native form both ADP-ribosylated and phosphorylated was not detected. Our results also suggest the existence of a minor native form of EF-2 and of its phosphorylated and ADP-ribosylated derivatives.

ADP-ribosylatable content of elongation factor-2 changes during cell cycle of normal and cancerous human cells

The amount of protein elongation factor EF-2 that can be inactivated by diphtheria toxin-mediated ADP-ribosylation, a measure of its active content, decreases by 45% and 66% in G1-arrested normal human fibroblasts and in HeLa cells respectively. On restimulation of cells with fresh serum, the amounts of ADP-ribosylatable EF-2 begin to increase within 4 h. Whereas the level of active EF-2 returns to normal (exponential phase of growth) in 20 h in the case of fibroblasts, only 47% recovery was observed for HeLa cells during this period. The apparent long half-lives of EF-2 mRNA and protein indicate possibilities of posttranslational ADP-ribosylation and de-ADP-ribosylation as the regulators of the amounts of active EF-2 during human cell cycle.

Endogenous ADP-ribosylation of elongation factor 2 in polyoma virus-transformed baby hamster kidney cells

Polyoma virus-transformed baby hamster kidney (pyBHK) cells were cultured in medium containing [32P]orthophosphate and 10% (vol/vol) fetal bovine serum. A 32P-labeled protein with an apparent molecular mass of 97 kDa was immunoprecipitated from cell lysates with antiserum to ADP-ribosylated elongation factor 2 (EF-2). The 32P labeling of the protein was enhanced by culturing cells in medium containing 2% serum instead of 10% serum. The 32P label was completely removed from the protein by treatment with snake venom phosphodiesterase and the digestion product was identified as [32P]AMP, indicating the protein was mono-ADP-ribosylated. HPLC analysis of tryptic peptides of the 32P-labeled 97-kDa protein and purified EF-2, which was ADP-ribosylated in vitro with diphtheria toxin fragment A and [32P]NAD, demonstrated an identical labeled peptide in the two proteins. The data strongly suggest that EF-2 was endogenously ADP-ribosylated in pyBHK cells. Maximum incorporation of radioactivity in EF-2 occurred by 12 hr and remained constant over the subsequent 12 hr. It was estimated that 30-35% of the EF-2 was ADP-ribosylated in cells cultured in medium containing 2% serum. When 32P-labeled cultures were incubated in medium containing unlabeled phosphate, the 32P label was lost from the EF-2 within 30 min.

Evidence of a yeast proteinase specific for elongation factor 2

Two proteinases active on elongation factor 2 have been found in yeast. The former hydrolyzes the factor producing a single ADP-ribosylatable fragment, whereas it does not produce any fragment when incubated with different proteins. The latter, less specific, is active in cleaving both EF-2 and other proteins giving rise to a noticeable number of fragments. Moreover, when native EF-2 is incubated with the most specific of the two proteinases, the amount of the ADP-ribosylatable fragment increases with time, while no fragments are evident when ADP-ribosylation of EF-2 comes before its incubation with the proteolytic enzyme. A possible regulatory role of this proteinase on EF-2 turnover is hypothesized.

cAMP-dependent activation of protein synthesis correlates with dephosphorylation of elongation factor 2

The addition of 5 mM cAMP to a cell-free translation system from rabbit reticulocytes increases the rate of protein synthesis 3 5-fold. Lower concentrations of cAMP (0.005, 0.05 and 0.5 mM) have no effect on translation in this system. cAMP at all the concentrations tested stimulates the phosphorylation of the same pattern of polypeptides, while 5 mM cAMP additionally stimulates dephosphorylation of the 95 kDa polypeptide identified as elongation factor 2 (EF-2). Testing of the preparations of EF-2 with a different content of the phosphorylated form in poly(U)-directed poly(Phe) synthesis reveals that the EF-2 activity correlates with the fraction of non-phosphorylated EF-2. Thus cAMP-dependent activation of protein synthesis seems to be due to dephosphorylation of EF-2.

Partial characterization of the enzymatic activity associated with the binary toxin (type C2) produced by Clostridium botulinum

Clostridium botulinum produces a binary toxin that possesses a heavy chain (approximately 100,000 daltons) and a light chain (approximately 50,000 daltons). The heavy chain is a binding component that directs the toxin to vulnerable cells, and the light chain is an enzyme that has mono(ADP-ribosyl)ating activity. A number of experiments have been done to help characterize the enzymatic activity of the toxin. The data reveal that the enzyme has a pH optimum within the range of 7.0 to 8.0. It is not inhibited or stimulated by physiological concentrations of sodium, potassium, calcium, or magnesium. The enzyme is inhibited by high concentrations of salt, however, as well as high concentrations of nicotinamide, thymidine, theophylline, and histamine; and it is stimulated by histone and lysolecithin. Boiling irreversibly denatures the light chain of the toxin, but denaturation caused by guanidine and urea is substantially reversible. Enzymatic activity is not altered by short exposure to lysosomal proteases, including cathepsin B, cathepsin H, dipeptidyl aminopeptidase, and catheptic carboxypeptidase B.

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.

Development of structural organization of protein-synthesizing machinery from prokaryotes to eukaryotes

Though the mechanisms of protein biosynthesis are similar in the cells of prokaryotes and eukaryotes, the eukaryotic translational machinery in the cell is arranged more intricately. One of the most striking characteristic features of the eukaryotic translational machinery is that the eukaryotic proteins involved in the translational process, such as initiation factors, elongation factors and aminoacyl-tRNA synthetases, in contrast to their prokaryotic analogs, possess a non-specific affinity for RNA. Due to the RNA-binding ability, these eukaryotic proteins can be compartmentalized on polyribosomes. In addition to the proteins of the translational apparatus, several other eukaryotic RNA-binding proteins can be also compartmentalized on polyribosomes; these proteins include glycolytic enzymes, steroid hormone receptors and intermediate filament proteins. Thus, the eukaryotic polyribosome is an element of the cytoplasmic labile structure on which various proteins can be compartmentalized and, consequently, different biochemical pathways can be integrated.

On the nature of cellular ADP-ribosyltransferase from rat liver specific for elongation factor 2

A cellular ADP-ribosyltransferase, specific for elongation factor 2 (EF-2), is found in extracts from rat liver. Co-migrating with EF-2 throughout purification, this activity is, moreover, located in the protein bands corresponding to EF-2 after native or sodium dodecyl sulfate polyacrylamide gel electrophoresis. The observed activity is thus implicated to be an inherent property of EF-2. Preincubation of EF-2 with GuoPPCH2Pox inhibits endogenous, but not diphtheria toxin catalyzed ADP-ribosylation.

ADP-ribosylation in cultured cells treated with Clostridium difficile toxin B

In cultured fibroblasts intoxicated with Clostridium difficile toxin B, a radioactive moiety was transferred from [14C-adenosine]NAD, but not from [14C-nicotinamide] NAD, into a cellular protein (MW 90,000). No labeling was detected in toxin-treated cultures not yet showing any toxin-induced cytopathogenic effect, whereas maximal labeling was obtained in cultures with about half of the cells showing a cytopathogenic effect. The radioactivity was removed from the substrate by treatment with snake venom phosphodiesterase. The results suggest that ADP-ribosylation of a cellular protein occurs in toxin B-treated cells and that this reaction may be responsible for development of the cytopathogenic effect.

Characterization of the poly(ADP-ribose) polymerase associated with free cytoplasmic mRNA-protein particles

Poly(ADP-ribose) polymerase associated with free cytoplasmic messenger ribonucleoprotein particles (mRNP) has been characterized in mouse plasmacytoma. This cytoplasmic enzyme undergoes auto-ADP-ribosylation and has a similar molecular weight and common antigenic sites with the chromatin bound poly(ADP-ribose) polymerase in spite of its DNA independency. The free mRNP poly(ADP-ribose) polymerase is released from the particle only by high saline concentrations (0.7 M KCl) and the dissociated enzyme expresses a higher activity. The treatment of free mRNP by RNase A stimulates the poly(ADP-ribose) polymerase activity. Partial destruction of mRNP by high saline concentration or mRNA digestion unmasks new protein sites for ADP-ribosylation. In view of the changes that occur in the free mRNP structure to permit mRNA translation, a possible role of poly(ADP-ribosylation) as an important post-synthetic modification of some of the mRNP proteins is discussed.