Mutations in the elongation factor 2 gene which confer resistance to diphtheria toxin and Pseudomonas exotoxin A. Genetic and biochemical analyses

Diphthamide - a conserved modification of eEF2 with clinical relevance

Diphthamide, a complex modification on eukaryotic translation elongation factor 2 (eEF2), assures reading-frame fidelity during translation. Diphthamide and enzymes for its synthesis are conserved in eukaryotes and archaea. Originally identified as target for diphtheria toxin (DT) in humans, its clinical relevance now proves to be broader than the link to pathogenic bacteria. Diphthamide synthesis enzymes (DPH1 and DPH3) are associated with cancer, and DPH gene mutations can cause diphthamide deficiency syndrome (DDS). Finally, new analyses provide evidence that diphthamide may restrict propagation of viruses including SARS-CoV-2 and HIV-1, and that DPH enzymes are targeted by viruses for degradation to overcome this restriction. This review describes how diphthamide is synthesized and functions in translation, and covers its clinical relevance in human development, cancer, and infectious diseases.

Hosts and Heterologous Expression Strategies of Recombinant Toxins for Therapeutic Purposes

The production of therapeutic recombinant toxins requires careful host cell selection. Bacteria, yeast, and mammalian cells are common choices, but no universal solution exists. Achieving the delicate balance in toxin production is crucial due to potential self-intoxication. Recombinant toxins from various sources find applications in antimicrobials, biotechnology, cancer drugs, and vaccines. "Toxin-based therapy" targets diseased cells using three strategies. Targeted cancer therapy, like antibody-toxin conjugates, fusion toxins, or "suicide gene therapy", can selectively eliminate cancer cells, leaving healthy cells unharmed. Notable toxins from various biological sources may be used as full-length toxins, as plant (saporin) or animal (melittin) toxins, or as isolated domains that are typical of bacterial toxins, including Pseudomonas Exotoxin A (PE) and diphtheria toxin (DT). This paper outlines toxin expression methods and system advantages and disadvantages, emphasizing host cell selection's critical role.

Structural Insights into Pseudomonas aeruginosa Exotoxin A-Elongation Factor 2 Interactions: A Molecular Dynamics Study

Exotoxin A (ETA) is an extracellular secreted toxin and a single-chain polypeptide with A and B fragments that is produced by Pseudomonas aeruginosa. It catalyzes the ADP-ribosylation of a post-translationally modified histidine (diphthamide) on eukaryotic elongation factor 2 (eEF2), which results in the inactivation of the latter and the inhibition of protein biosynthesis. Studies show that the imidazole ring of diphthamide plays an important role in the ADP-ribosylation catalyzed by the toxin. In this work, we employ different in silico molecular dynamics (MD) simulation approaches to understand the role of diphthamide versus unmodified histidine in eEF2 on the interaction with ETA. Crystal structures of the eEF2-ETA complexes with three different ligands NAD⁺, ADP-ribose, and βTAD were selected and compared in the diphthamide and histidine containing systems. The study shows that NAD⁺ bound to ETA remains very stable in comparison with other ligands, enabling the transfer of ADP-ribose to the N3 atom of the diphthamide imidazole ring in eEF2 during ribosylation. We also show that unmodified histidine in eEF2 has a negative impact on ETA binding and is not a suitable target for the attachment of ADP-ribose. Analyzing of radius of gyration and COM distances for NAD⁺, βTAD, and ADP-ribose complexes revealed that unmodified His affects the structure and destabilizes the complex with all different ligands throughout the MD simulations.

NAD metabolism in aging and cancer

IMPACT STATEMENT

NAD is a central metabolite connecting energy balance and organismal growth with genomic integrity and function. It is involved in the development of malignancy and has a regulatory role in the aging process. These processes are mediated by a diverse series of enzymes whose common focus is either NAD's biosynthesis or its utilization as a redox cofactor or enzyme substrate. These enzymes include dehydrogenases, cyclic ADP-ribose hydrolases, mono(ADP-ribosyl)transferases, poly(ADP-ribose) polymerases, and sirtuin deacetylases. This article describes the manifold pathways that comprise NAD metabolism and promotes an increased awareness of how perturbations in these systems may be important in disease prevention and/or progression.

PARPs and ADP-ribosylation in RNA biology: from RNA expression and processing to protein translation and proteostasis

In this review, Kim et al. discuss the importance of PARP family members and ADPRylation in gene regulation, mRNA processing, and protein abundance.

ADP-ribosylation (ADPRylation) is a posttranslational modification of proteins discovered nearly six decades ago, but many important questions remain regarding its molecular functions and biological roles, as well as the activity of the ADP-ribose (ADPR) transferase enzymes (PARP family members) that catalyze it. Growing evidence indicates that PARP-mediated ADPRylation events are key regulators of the protein biosynthetic pathway, leading from rDNA transcription and ribosome biogenesis to mRNA synthesis, processing, and translation. In this review we describe the role of PARP proteins and ADPRylation in all facets of this pathway. PARP-1 and its enzymatic activity are key regulators of rDNA transcription, which is a critical step in ribosome biogenesis. An emerging role of PARPs in alternative splicing of mRNAs, as well as direct ADPRylation of mRNAs, highlight the role of PARP members in RNA processing. Furthermore, PARP activity, stimulated by cellular stresses, such as viral infections and ER stress, leads to the regulation of mRNA stability and protein synthesis through posttranscriptional mechanisms. Dysregulation of PARP activity in these processes can promote disease states. Collectively, these results highlight the importance of PARP family members and ADPRylation in gene regulation, mRNA processing, and protein abundance. Future studies in these areas will yield new insights into the fundamental mechanisms and a broader utility for PARP-targeted therapeutic agents.

Diphtheria Toxin A-Resistant Cell Lines Enable Robust Production and Evaluation of DTA-Encoding Lentiviruses

Suicide genes have been widely investigated for their utility as therapeutic agents and as tools for in vitro negative selection strategies. Several methods for delivery of suicide genes have been explored. Two important considerations for delivery are the quantity of delivered cargo and the ability to target the cargo to specific cells. Delivery using a lentiviral vector is particularly attractive due to the ability to encode the gene within the viral genome, as well as the ability to limit off-target effects by using cell type-specific glycoproteins. Here, we present the design and validation of a diphtheria toxin A (DTA)-encoding lentiviral vector expressing DTA under the control of a constituitive promoter to allow for expression of DTA in a variety of cell types, with specificity provided via selection of glycoproteins for pseudotyping of the lentiviral particles. DTA exerts its toxic activity through inhibition of eukaryotic translation elongation factor 2 (eEF2) via adenosine diphosphate (ADP)-ribosylation of a modified histidine residue, diphthamide, at His715, which blocks protein translation and leads to cell death. Thus, we also detail development of DTA-resistant cell lines, engineered through CRISPR/Cas9-mediated knockout of the diphthamide 1 (DPH1) gene, which enable both robust virus production by transfection and evaluation of DTA-expressing virus infectivity.

Use of the XRCC2 promoter for in vivo cancer diagnosis and therapy

The homologous recombination (HR) pathway is a promising target for cancer therapy as it is frequently upregulated in tumors. One such strategy is to target tumors with cancer-specific, hyperactive promoters of HR genes including RAD51 and RAD51C. However, the promoter size and the delivery method have limited its potential clinical applications. Here we identified the ~2.1 kb promoter of XRCC2, similar to ~6.5 kb RAD51 promoter, as also hyperactivated in cancer cells. We found that XRCC2 expression is upregulated in nearly all types of cancers, to a degree comparable to RAD51 while much higher than RAD51C. Further study demonstrated that XRCC2 promoter is hyperactivated in cancer cell lines, and diphtheria toxin A (DTA) gene driven by XRCC2 promoter specifically eliminates cancer cells. Moreover, lentiviral vectors containing XRCC2 promoter driving firefly luciferase or DTA were created and applied to subcutaneous HeLa xenograft mice. We demonstrated that the pXRCC2-luciferase lentivirus is an effective tool for in vivo cancer visualization. Most importantly, pXRCC2-DTA lentivirus significantly inhibited the growth of HeLa xenografts in comparison to the control group. In summary, our results strongly indicate that virus-mediated delivery of constructs built upon the XRCC2 promoter holds great potential for tumor diagnosis and therapy.

Evolution of the archaeal and mammalian information processing systems: towards an archaeal model for human disease

Current evolutionary models suggest that Eukaryotes originated from within Archaea instead of being a sister lineage. To test this model of ancient evolution, we review recent studies and compare the three major information processing subsystems of replication, transcription and translation in the Archaea and Eukaryotes. Our hypothesis is that if the Eukaryotes arose within the archaeal radiation, their information processing systems will appear to be one of kind and not wholly original. Within the Eukaryotes, the mammalian or human systems are emphasized because of their importance in understanding health. Biochemical as well as genetic studies provide strong evidence for the functional similarity of archaeal homologs to the mammalian information processing system and their dissimilarity to the bacterial systems. In many independent instances, a simple archaeal system is functionally equivalent to more elaborate eukaryotic homologs, suggesting that evolution of complexity is likely an central feature of the eukaryotic information processing system. Because fewer components are often involved, biochemical characterizations of the archaeal systems are often easier to interpret. Similarly, the archaeal cell provides a genetically and metabolically simpler background, enabling convenient studies on the complex information processing system. Therefore, Archaea could serve as a parsimonious and tractable host for studying human diseases that arise in the information processing systems.

Structures, Properties, and Dynamics of Intermediates in eEF2-Diphthamide Biosynthesis

The eukaryotic translation Elongation Factor 2 (eEF2) is an essential enzyme in protein synthesis. eEF2 contains a unique modification of a histidine (His699 in yeast; HIS) into diphthamide (DTA), obtained via 3-amino-3-carboxypropyl (ACP) and diphthine (DTI) intermediates in the biosynthetic pathway. This essential and unique modification is also vulnerable, in that it can be efficiently targeted by NAD(+)-dependent ADP-ribosylase toxins, such as diphtheria toxin (DT). However, none of the intermediates in the biosynthesis path is equally vulnerable against the toxins. This study aims to address the different susceptibility of DTA and its precursors against bacterial toxins. We have herein undertaken a detailed in silico study of the structural features and dynamic motion of different His699 intermediates along the diphthamide synthesis pathway (HIS, ACP, DTI, DTA). The study points out that DTA forms a strong hydrogen bond with an asparagine which might explain the ADP-ribosylation mechanism caused by the diphtheria toxin (DT). Finally, in silico mutagenesis studies were performed on the DTA modified protein, in order to hamper the formation of such a hydrogen bond. The results indicate that the mutant structure might in fact be less susceptible to attack by DT and thereby behave similarly to DTI in this respect.

Engineering toxin-resistant therapeutic stem cells to treat brain tumors

Pseudomonas exotoxin (PE) potently blocks protein synthesis by catalyzing the inactivation of elongation factor-2 (EF-2). Targeted PE-cytotoxins have been used as antitumor agents, although their effective clinical translation in solid tumors has been confounded by off-target delivery, systemic toxicity, and short chemotherapeutic half-life. To overcome these limitations, we have created toxin-resistant stem cells by modifying endogenous EF-2, and engineered them to secrete PE-cytotoxins that target specifically expressed (interleukin-13 receptor subunit alpha-2) or overexpressed (epidermal growth factor receptor) in glioblastomas (GBM). Molecular analysis correlated efficacy of PE-targeted cytotoxins with levels of cognate receptor expression, and optical imaging was applied to simultaneously track the kinetics of protein synthesis inhibition and GBM cell viability in vivo. The release of IL13-PE from biodegradable synthetic extracellular matrix (sECM) encapsulated stem cells in a clinically relevant GBM resection model led to increased long-term survival of mice compared to IL13-PE protein infusion. Moreover, multiple patient-derived GBM lines responded to treatment, underscoring its clinical relevance. In sum, integrating stem cell-based engineering, multimodal imaging, and delivery of PE-cytotoxins in a clinically relevant GBM model represents a novel strategy and a potential advancement in GBM therapy.

Pseudomonas aeruginosa exotoxin T induces potent cytotoxicity against a variety of murine and human cancer cell lines

In patients with malignancy, the major barrier to achieving complete response is emergence of resistance to current chemotherapeutic agents. One of the major mechanisms by which tumour cells become resistant to therapies is by altering cellular drug targets through mutations and/or deletions. Resistance by this mechanism is achieved more easily if the drug has limited cellular targets and/or processes. We hypothesized that as Pseudomonas aeruginosa exotoxin T (ExoT) targets six proteins that are required for cancer cell survival and proliferation, it is highly unlikely for cancer cells to develop resistance to this toxin. We assessed ExoT's cytotoxicity against multiple invasive and highly resistant tumour cell lines in order to evaluate its potential as a chemotherapeutic agent. Our data demonstrated that ExoT induced potent cytotoxicity in all tumour cell lines that we examined. Collectively, our data highlighted the potential of ExoT as a possible chemotherapeutic candidate for the treatment of cancer.

ADP-ribosylation of translation elongation factor 2 by diphtheria toxin in yeast inhibits translation and cell separation

Eukaryotic translation elongation factor 2 (eEF2) facilitates the movement of the peptidyl tRNA-mRNA complex from the A site of the ribosome to the P site during protein synthesis. ADP-ribosylation (ADP(R)) of eEF2 by bacterial toxins on a unique diphthamide residue inhibits its translocation activity, but the mechanism is unclear. We have employed a hormone-inducible diphtheria toxin (DT) expression system in Saccharomyces cerevisiae which allows for the rapid induction of ADP(R)-eEF2 to examine the effects of DT in vivo. ADP(R) of eEF2 resulted in a decrease in total protein synthesis consistent with a defect in translation elongation. Association of eEF2 with polyribosomes, however, was unchanged upon expression of DT. Upon prolonged exposure to DT, cells with an abnormal morphology and increased DNA content accumulated. This observation was specific to DT expression and was not observed when translation elongation was inhibited by other methods. Examination of these cells by electron microscopy indicated a defect in cell separation following mitosis. These results suggest that expression of proteins late in the cell cycle is particularly sensitive to inhibition by ADP(R)-eEF2.

A conserved eEF2 coding variant in SCA26 leads to loss of translational fidelity and increased susceptibility to proteostatic insult

The autosomal dominant spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of disorders exhibiting cerebellar atrophy and Purkinje cell degeneration whose subtypes arise from 31 distinct genetic loci. Our group previously published the locus for SCA26 on chromosome 19p13.3. In this study, we performed targeted deep sequencing of the critical interval in order to identify candidate causative variants in individuals from the SCA26 family. We identified a single variant that co-segregates with the disease phenotype that produces a single amino acid substitution in eukaryotic elongation factor 2. This substitution, P596H, sits in a domain critical for maintaining reading frame during translation. The yeast equivalent, P580H EF2, demonstrated impaired translocation, detected as an increased rate of -1 programmed ribosomal frameshift read-through in a dual-luciferase assay for observing translational recoding. This substitution also results in a greater susceptibility to proteostatic disruption, as evidenced by a more robust activation of a reporter gene driven by unfolded protein response activation upon challenge with dithiothreitol or heat shock in our yeast model system. Our results present a compelling candidate mutation and mechanism for the pathogenesis of SCA26 and further support the role of proteostatic disruption in neurodegenerative diseases.

Diphthamide modification on eukaryotic elongation factor 2 is needed to assure fidelity of mRNA translation and mouse development

To study the role of the diphthamide modification on eukaryotic elongation factor 2 (eEF2), we generated an eEF2 Gly(717)Arg mutant mouse, in which the first step of diphthamide biosynthesis is prevented. Interestingly, the Gly(717)-to-Arg mutation partially compensates the eEF2 functional loss resulting from diphthamide deficiency, possibly because the added +1 charge compensates for the loss of the +1 charge on diphthamide. Therefore, in contrast to mouse embryonic fibroblasts (MEFs) from OVCA1(-/-) mice, eEF2(G717R/G717R) MEFs retain full activity in polypeptide elongation and have normal growth rates. Furthermore, eEF2(G717R/G717R) mice showed milder phenotypes than OVCA1(-/-) mice (which are 100% embryonic lethal) and a small fraction survived to adulthood without obvious abnormalities. Moreover, eEF2(G717R/G717R)/OVCA1(-/-) double mutant mice displayed the milder phenotypes of the eEF2(G717R/G717R) mice, suggesting that the embryonic lethality of OVCA1(-/-) mice is due to diphthamide deficiency. We confirmed that the diphthamide modification is essential for eEF2 to prevent -1 frameshifting during translation and show that the Gly(717)-to-Arg mutation cannot rescue this defect.

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.

Unique modifications of translation elongation factors

Covalent modifications of proteins often modulate their biological functions or change their subcellular location. Among the many known protein modifications, three are exceptional in that they only occur on single proteins: ethanolamine phosphoglycerol, diphthamide and hypusine. Remarkably, the corresponding proteins carrying these modifications, elongation factor 1A, elongation factor 2 and initiation factor 5A, are all involved in elongation steps of translation. For diphthamide and, in part, hypusine, functional essentiality has been demonstrated, whereas no functional role has been reported so far for ethanolamine phosphoglycerol. We review the biosynthesis, attachment and physiological roles of these unique protein modifications and discuss common and separate features of the target proteins, which represent essential proteins in all organisms.

Pseudomonas exotoxin kills Drosophila S2 cells via apoptosis

Pseudomonas exotoxin A (PE) is cytotoxic for eukaryotic cells because it enters cells by receptor-mediated endocytosis, translocates to the cell cytosol and ADP-ribosylates elongation factor 2 (EF2). However, the interaction of this toxin with eukaryotic cells and the mechanism of PE-mediated cell death have not been extensively characterized. The feasibility of carrying out a genome-wide RNAi screen, makes Drosophila melanogaster S2 cells as a good model system to identify essential genes in PE-mediated cytotoxicity, provided a suitable multi-well assay is developed. Here, using the alamarBlue viability assay, we show that Drosophila S2 cells are sensitive to PE at picomolar concentrations and that toxin treatments provoke an increase in caspase activity. This prompted us to use RNAi to characterize the mechanism of cell death. Results indicated that PE-mediated death of S2 cells was dependent on the presence of diphthamide, the post translational modification of EF2, and on the presence of Drice, the terminal caspase of insect cells. RNAi to drice or chemical inhibition of caspase action by z-VAD-fmk protected cells from PE-mediated death. Protection from death by RNAi or z-VAD-fmk did not interfere with toxin delivery to the cytosol leading to inhibition of protein synthesis. Using a convenient alamarBlue assay, our data confirms the cytotoxicity of PE for S2 cells and establishes apoptosis as the mode of PE-mediated death. This confirms the suitability of Drosophila cells as a convenient and simple model to elucidate the role of specific genes and proteins required for PE action.

Characterization of a Chinese hamster ovary cell mutant having a mutation in elongation factor-2

Retroviral insertional mutagenesis provides an effective forward genetic method for identifying genes involved in essential cellular pathways. A Chinese hamster ovary cell line mutant resistant to several bacterial ADP-ribosylating was obtained by this approach. The toxins used catalyze ADP-ribosylation of eukaryotic elongation factor 2 (eEF-2), block protein synthesis, and cause cell death. Strikingly, in the CHO PR328 mutant cells, the eEF-2 substrate of these ADP-ribosylating toxins was found to be modified, but the cells remained viable. A systematic study of these cells revealed the presence of a structural mutation in one allele of the eEF-2 gene. This mutation, Gly717Arg, is close to His715, the residue that is modified to become diphthamide. This Arg substitution prevents diphthamide biosynthesis at His715, rendering the mutated eEF-2 non-responsive to ADP-ribosylating toxins, while having no apparent effect on protein synthesis. Thus, CHO PR328 cells are heterozygous, having wild type and mutant eEF-2 alleles, with the latter allowing the cells to survive even in the presence of ADP-ribosylating toxins. Here, we report the comprehensive characterization of these cells.

Stm1p alters the ribosome association of eukaryotic elongation factor 3 and affects translation elongation

Stm1p is a Saccharomyces cerevisiae protein that is primarily associated with cytosolic 80S ribosomes and polysomes. Several lines of evidence suggest that Stm1p plays a role in translation under nutrient stress conditions, although its mechanism of action is not yet known. In this study, we show that yeast lacking Stm1p (stm1Delta) are hypersensitive to the translation inhibitor anisomycin, which affects the peptidyl transferase reaction in translation elongation, but show little hypersensitivity to other translation inhibitors such as paromomycin and hygromycin B, which affect translation fidelity. Ribosomes isolated from stm1Delta yeast have intrinsically elevated levels of eukaryotic elongation factor 3 (eEF3) associated with them. Overexpression of eEF3 in cells lacking Stm1p results in a growth defect phenotype and increased anisomycin sensitivity. In addition, ribosomes with increased levels of Stm1p exhibit decreased association with eEF3. Taken together, our data indicate that Stm1p plays a complementary role to eEF3 in translation.

The role of the diphthamide-containing loop within eukaryotic elongation factor 2 in ADP-ribosylation by Pseudomonas aeruginosa exotoxin A

eEF2 (eukaryotic elongation factor 2) contains a post-translationally modified histidine residue, known as diphthamide, which is the specific ADP-ribosylation target of diphtheria toxin, cholix toxin and Pseudomonas aeruginosa exotoxin A. Site-directed mutagenesis was conducted on residues within the diphthamide-containing loop (Leu693-Gly703) of eEF2 by replacement with alanine. The purified yeast eEF2 mutant proteins were then investigated to determine the role of this loop region in ADP-ribose acceptor activity of elongation factor 2 as catalysed by exotoxin A. A number of single alanine substitutions in the diphthamide-containing loop caused a significant reduction in the eEF2 ADP-ribose acceptor activities, including two strictly conserved residues, His694 and Asp696. Analysis by MS revealed that all of these mutant proteins lacked the 2'-modification on the His699 residue and that eEF2 is acetylated at Lys509. Furthermore, it was revealed that the imidazole ring of Diph699 (diphthamide at position 699) still functions as an ADP-ribose acceptor (albeit poorly), even without the diphthamide modification on the His699. Therefore, this diphthamide-containing loop plays an important role in the ADP-ribosylation of eEF2 catalysed by toxin and also for modification of His699 by the endogenous diphthamide modification machinery.

Cloning and expression of translation elongation factor 2 (EF-2) in zebrafish

We have identified a developmentally regulated gene translation elongation factor 2 (EF-2) in zebrafish (GenBank Accession No. AAQ91234). Analysis of DNA sequence of zebrafish EF-2 shows that the 2826 bp cDNA spans an open reading frame from nucleotide 55 to 2631 and encodes a protein of 858 amino acids. It shares an identity of 92, 93, 93, 92, 79 and 80% in amino acid sequence to human, mouse, Chinese hamster, Gallus gullus, C. elegans and Drosophila EF-2, respectively. Zebrafish EF-2 protein has 16 conserved domains, GTP-binding domain is found in the NH2 terminus, and the ADP-ribosylation domain locates at the COOH terminus. Whole mount in situ hybridization on zebrafish embryos shows that the transcripts of EF-2 gene are detected during the early development of zebrafish embryo and constantly change from 5-somite stage to protruding-mouth stage. It expresses strongly throughout envelope at 5-somite stage. Then the stained cells concentrate strongly in the eyes, brain and muscle tissue. From prim-25 stage the stained cells only appear strongly in the lens and the anterior portion of the cerebellum.

Diphtheria toxin mutant CRM197 is an inhibitor of protein synthesis that induces cellular toxicity

Diphtheria toxin (DT) ADP-rybosylates elongation factor-2 on a modified histidine residue called diphthamide, leading to a block in protein synthesis. CRM197 is a nontoxic DT mutant commonly used as a carrier for conjugate vaccines, and may have a potential as an anti-tumor agent. We now report that CRM197 expression is indeed toxic to cells, and inhibits protein synthesis. These results should be considered for future vaccine studies, and to investigate the mechanism of CRM197-mediated anti-tumor effect.

Site-specific mutagenesis of the histidine precursor of diphthamide in the human elongation factor-2 gene confers resistance to diphtheria toxin

Protein synthesis elongation factor 2 (EF-2) from eukaryotes contains a conserved post-translationally modified histidine residue known as diphthamide. Diphthamide is a unique site of ADP-ribosylation by diphtheria toxin (DT), which is responsible for cell killing. In this report, we describe the construction of DT-resistant HeLa cell lines by engineering the toxin-resistant form of its specific substrate, protein elongation factor-2. Using site-specific mutagenesis of the histidine precursor of diphthamide, the histidine residue of codon 715 in human EF-2 cDNA was substituted with one of four amino acid residue codons: leucine, methionine, asparagine or glutamine. Mutant EF-2s were subcloned into a pCMVexSVneo expression vector, transfected into HeLa cells, and DT-resistant cell clones were isolated. The protective effect of mutant EF-2s against cell killing by DT, after exposing all four mutant strains derived from HeLa cells to different concentrations of the toxin (5-20 ng/mL) was demonstrated by: (1) the normal morphological appearance of the cells; (2) their unaffected or slightly slower growth rates; (3) their undisturbed electrophoretic DNA profiles whose integrity was virtually preserved. Mutant cell strains showed also considerable levels of resistance to very high concentrations of DT, in that they maintained slower but consistent rates of cell growth. It was hence concluded that despite its strict conservation and unique modification, the diphthamide histidine appears not to be essential to the function of human EF-2 in protein synthesis. In addition, DT-resistant HeLa cell clones should prove valuable hosts for various DT gene-containing vectors that express the toxin intracellularly.

Construction and use of retroviral vectors encoding the toxic gene barnase

Suicide genes for negative selection of cells have been powerful tools in somatic cell genetic studies and in gene therapy. Here we report on the construction, characterization, and utilization of retroviral vectors encoding barnase, a ribonuclease from Bacillus amyloliquefaciens, expression of which results in apoptosis of transduced mammalian cells. High-titer viral vector production was enabled by expression of an inhibitor of barnase (barstar) in transfected cells generating murine leukemia virus (MLV)- and HIV-1-based vectors. To identify cellular genes required for infection we used barnase-encoding vectors in a genetic screen to isolate mutant mammalian cells that are resistant to infection by MLV and HIV-1. We describe one such mutant clone that is inhibited in the infection process after reverse transcription. These results suggest that barnase-encoding vectors should be useful for negative selection strategies examining retroviral infection from entry to integration. Furthermore these vectors could have utility in approaches for gene therapy that require specific cell ablation.

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.

Structure-function analysis of water-soluble inhibitors of the catalytic domain of exotoxin A from Pseudomonas aeruginosa

The mono-ADPRT (mono-ADP-ribosyltransferase), Pseudomonas aeruginosa ETA (exotoxin A), catalyses the transfer of ADP-ribose from NAD+ to its protein substrate. A series of water-soluble compounds that structurally mimic the nicotinamide moiety of NAD+ was investigated for their inhibition of the catalytic domain of ETA. The importance of an amide locked into a hetero-ring structure and a core hetero-ring system that is planar was a trend evident by the IC50 values. Also, the weaker inhibitors have core ring structures that are less planar and thus more flexible. One of the most potent inhibitors, PJ34, was further characterized and shown to exhibit competitive inhibition with an inhibition constant K(i) of 140 nM. We also report the crystal structure of the catalytic domain of ETA in complex with PJ34, the first example of a mono-ADPRT in complex with an inhibitor. The 2.1 A (1 A=0.1 nm) resolution structure revealed that PJ34 is bound within the nicotinamide-binding pocket and forms stabilizing hydrogen bonds with the main chain of Gly-441 and to the side-chain oxygen of Gln-485, a member of a proposed catalytic loop. Structural comparison of this inhibitor complex with diphtheria toxin (a mono-ADPRT) and with PARPs [poly(ADP-ribose) polymerases] shows similarity of the catalytic residues; however, a loop similar to that found in ETA is present in diphtheria toxin but not in PARP. The present study provides insight into the important features required for inhibitors that mimic NAD+ and their binding to the mono-ADPRT family of toxins.

Gene trap mutagenesis-based forward genetic approach reveals that the tumor suppressor OVCA1 is a component of the biosynthetic pathway of diphthamide on elongation factor 2

OVCA1 is a tumor suppressor identified by positional cloning from chromosome 17p13.3, a hot spot for chromosomal aberration in breast and ovarian cancers. It has been shown that expression of OVCA1 is reduced in some tumors and that it regulates cell proliferation, embryonic development, and tumorigenesis. However, the biochemical function of OVCA1 has remained unknown. Recently, we isolated a novel mutant resistant to diphtheria toxin and Pseudomonas exotoxin A from the gene trap insertional mutants library of Chinese hamster ovary cells. In this mutant, the Ovca1 gene was disrupted by gene trap mutagenesis, and this disruption well correlated with the toxin-resistant phenotype. We demonstrated direct evidence that the tumor suppressor OVCA1 is a component of the biosynthetic pathway of diphthamide on elongation factor 2, the target of bacterial ADP-ribosylating toxins. A functional genetic approach utilizing the random gene trap mutants library of mammalian cells should become a useful strategy to identify the genes responsible for specific phenotypes.

Crystal structure of ADP-ribosylated ribosomal translocase from Saccharomyces cerevisiae

The crystal structure of ADP-ribosylated yeast elongation factor 2 in the presence of sordarin and GDP has been determined at 2.6 A resolution. The diphthamide at the tip of domain IV, which is the target for diphtheria toxin and Pseudomonas aeruginosa exotoxin A, contains a covalently attached ADP-ribose that functions as a very potent inhibitor of the factor. We have obtained an electron density map of ADP-ribosylated translation factor 2 revealing both the ADP-ribosylation and the diphthamide. This is the first structure showing the conformation of an ADP-ribosylated residue and confirms the inversion of configuration at the glycosidic linkage. Binding experiments show that the ADP-ribosylation has limited effect on nucleotide binding affinity, on ribosome binding, and on association with exotoxin A. These results provide insight to the inhibitory mechanism and suggest that inhibition may be caused by erroneous interaction of the translation factor with the codon-anticodon area in the P-site of the ribosome.

Domain movements of elongation factor eEF2 and the eukaryotic 80S ribosome facilitate tRNA translocation

An 11.7-A-resolution cryo-EM map of the yeast 80S.eEF2 complex in the presence of the antibiotic sordarin was interpreted in molecular terms, revealing large conformational changes within eEF2 and the 80S ribosome, including a rearrangement of the functionally important ribosomal intersubunit bridges. Sordarin positions domain III of eEF2 so that it can interact with the sarcin-ricin loop of 25S rRNA and protein rpS23 (S12p). This particular conformation explains the inhibitory action of sordarin and suggests that eEF2 is stalled on the 80S ribosome in a conformation that has similarities with the GTPase activation state. A ratchet-like subunit rearrangement (RSR) occurs in the 80S.eEF2.sordarin complex that, in contrast to Escherichia coli 70S ribosomes, is also present in vacant 80S ribosomes. A model is suggested, according to which the RSR is part of a mechanism for moving the tRNAs during the translocation reaction.

Morphological and immunological evidence of a unique selective production and endoplasmic reticular accumulation of interleukin-1alpha in rat peritoneal macrophages induced by Pseudomonas aeruginosa exotoxin A

The immunotoxicity of Pseudomonas aeruginosa exotoxin A (ETA) on macrophages was evaluated by incubating rat peritoneal macrophages (RPM) with 1-100 ng/ml ETA for 3-60 h. Although the overall changes in cell viability and DNA, RNA, and protein synthesis of the ETA-treated RPM (E-RPM) were reduced in a dose- and time-dependent manner, there was a transient but evident rebound in RNA and/or protein synthesis at 24-36 h post-incubation (HPI) at 1-50 ng/ml ETA. However, a more apparent enhancement appeared in RNA and protein synthesis at 36-48 HPI in 10 and 50 ng/ml E-RPM after normalized on the basis of viable cell. Most 50-100 ng/ml E-RPM underwent necrosis/apoptosis before 24 HPI. By 36 HPI, 41% of 10 ng/ml E-RPM remained viable but were full of cytoplasmic granules due to the accumulation of glycoprotein in segmentally dilated endoplasmic reticulum. Immunological staining of the granules revealed strong IL-1alpha but weak or no signals for IL-1beta, IL-1 receptor antagonist, IL-6, and TNF-alpha. A time-dependent increase in IL-1alpha but no IL-1beta was detected in cell lysate of 10 ng/ml E-RPM; however, neither IL-1alpha nor IL-1beta was detected in culture supernatant. Thus, besides cytopathic and functional effects, ETA could induce a unique selective production and endoplasmic reticular accumulation of IL-1alpha in RPM.

Identification of peptide inhibitors of Pseudomonas aeruginosa exotoxin A function using a yeast two-hybrid approach

The yeast two-hybrid system was used to identify peptide inhibitors of exotoxin A of Pseudomonas aeruginosa with the goal of using these to design peptide-based drugs against the toxin. A random peptide library consisting of 10(7) peptides ranging in length from 16 to 63 residues was screened for peptides that interact with the C-domain of exotoxin A. From the 10(7) transformants screened, three unique peptides of 63, 61 and 25 amino acids in length were found to specifically interact with the enzyme domain. The genes encoding these peptides were cloned and expressed as fusion proteins with the maltose-binding protein. In vitro inhibition measurements indicated that two of the peptides were modest inhibitors of toxin enzyme activity. These peptides now provide the basis for the development of more potent inhibitors, which will serve as lead inhibitors for evolution of potent peptide-based therapeutics.

PG13 packaging cells produce recombinant retroviruses carrying a diphtheria toxin mutant which kills cancer cells

The development of suicide gene therapy with gene products that are directly toxic to cells, such as the A subunit of diphtheria toxin (DT-A), has been hampered by the difficulty of engineering recombinant viruses. DT-A is a strong inhibitor of protein synthesis that acts by ADP-ribosylating elongation factor 2, and a low level of DT-A expression in virus producer cells prevents the production of recombinant virus. We analyzed here the natural resistance of packaging cells to DT-A toxicity, and we report that PG13 and PA317 packaging cell lines are resistant to H21G, a DT-A mutant. PG13 cells produce recombinant H21G virus that efficiently kills a variety of human tumor cells. Our finding indicates that PG13 packaging cells provide a new potential for the development of DT-A-based suicide gene therapy.

Interaction of pseudomonas aeruginosa with epithelial cells: identification of differentially regulated genes by expression microarray analysis of human cDNAs

Pseudomonas aeruginosa is an opportunistic pathogen that plays a major role in lung function deterioration in cystic fibrosis patients. To identify critical host responses during infection, we have used high-density DNA microarrays, consisting of 1,506 human cDNA clones, to monitor gene expression in the A549 lung pneumocyte cell line during exposure to P. aeruginosa. We have identified host genes that are differentially expressed upon infection, several of which require interaction with P. aeruginosa and the expression of the major subunit of type IV pili, PilA. Differential expression of genes involved in various cellular functions was identified, and we selected the gene encoding the transcription factor interferon regulatory factor 1 (IRF-1) for further analysis. The levels of the IRF-1 transcript increased 3- to 4-fold in A549 cells after adherence by P. aeruginosa. A similar increase of IRF-1 mRNA was observed in A549 cells exposed to wild-type P. aeruginosa when compared to an isogenic, nonpiliated strain. However, this difference was abolished when serum was present during the incubation of bacteria. Exposure of A549 cells to purified P. aeruginosa lipopolysaccharide did not result in a significant increase in IRF-1 mRNA. Although the P. aeruginosa-induced increased IRF-1 expression depends on the presence of bacterial adhesin, our findings do not preclude the possibility that other bacterial products are responsible for IRF-1 activation, which is enhanced by bacterial adherence to cells. These data show that microarray technology can be an important tool for studying the complex interplay between bacterial pathogens and host.

Role of domains 4 and 5 in elongation factor G functions on the ribosome

Elongation factor G (EF-G) is a large, five domain GTPase that catalyses the translocation of the tRNAs on the bacterial ribosome at the expense of GTP. In the crystal structure of GDP-bound EF-G, domain 1 (G domain) makes direct contacts with domains 2 and 5, whereas domain 4 protrudes from the body of the molecule. Here, we show that the presence of both domains 4 and 5 is essential for tRNA translocation and for the turnover of the factor on the ribosome, but not for rapid single-round GTP hydrolysis by EF-G. Replacement of a highly conserved histidine residue at the tip of domain 4, His583, with lysine or arginine decreases the rate of tRNA translocation at least 100-fold, whereas the binding of the factor to the ribosome, GTP hydrolysis and P(i) release are not affected by the mutations. Various small deletions in the tip region of domain 4 decrease the translocation activity of EF-G even further, but do not block the turnover of the factor. Unlike native EF-G, the mutants of EF-G lacking domains 4/5 do not interact with the alpha-sarcin stem-loop of 23 S rRNA. These mutants are not released from the ribosome after GTP hydrolysis or translocation, indicating that the contact with, or a conformational change of, the alpha-sarcin stem-loop is required for EF-G release from the ribosome.

Three-dimensional cryo-electron microscopy localization of EF2 in the Saccharomyces cerevisiae 80S ribosome at 17.5 A resolution

Using a sordarin derivative, an antifungal drug, it was possible to determine the structure of a eukaryotic ribosome small middle dotEF2 complex at 17.5 A resolution by three-dimensional (3D) cryo-electron microscopy. EF2 is directly visible in the 3D map and the overall arrangement of the complex from Saccharomyces cerevisiae corresponds to that previously seen in Escherichia coli. However, pronounced differences were found in two prominent regions. First, in the yeast system the interaction between the elongation factor and the stalk region of the large subunit is much more extensive. Secondly, domain IV of EF2 contains additional mass that appears to interact with the head of the 40S subunit and the region of the main bridge of the 60S subunit. The shape and position of domain IV of EF2 suggest that it might interact directly with P-site-bound tRNA.

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.

Biology and molecular epidemiology of diphtheria toxin and the tox gene

Diphtheria toxin (DT) is an extracellular protein of Corynebacterium diphtheriae that inhibits protein synthesis and kills susceptible cells. The gene that encodes DT (tox) is present in some corynephages, and DT is only produced by C. diphtheriae isolates that harbor tox+ phages. The diphtheria toxin repressor (DtxR) is a global regulatory protein that uses Fe2+ as co-repressor. Holo-DtxR represses production of DT, corynebacterial siderophore, heme oxygenase, and several other proteins. Diagnostic tests for toxinogenicity of C. diphtheriae are based either on immunoassays or on bioassays for DT. Molecular analysis of tox and dtxR genes in recent clinical isolates of C. diphtheriae revealed several tox alleles that encode identical DT proteins and multiple dtxR alleles that encode five variants of DtxR protein. Therefore, recent clinical isolates of C. diphtheriae produce a single antigenic type of DT, and diphtheria toxoid continues to be an effective vaccine for immunization against diphtheria.

Dynamics of translation on the ribosome: molecular mechanics of translocation

The translocation step of protein elongation entails a large-scale rearrangement of the tRNA-mRNA-ribosome complex. Recent years have seen major advances in unraveling the mechanism of the process on the molecular level. A number of intermediate states have been defined and, in part, characterized structurally. The article reviews the recent evidence that suggests a dynamic role of the ribosome and its ligands during translocation. The focus is on dynamic aspects of tRNA movement and on the role of elongation factor G and GTP hydrolysis in translocation catalysis. The significance of structural changes of the ribosome induced by elongation factor G as well the role of ribosomal RNA are addressed. A functional model of elongation factor G as a motor protein driven by GTP hydrolysis is discussed.