mRNA reading frame maintenance during eukaryotic ribosome translocation.

One of the most critical steps of protein synthesis is coupled translocation of messenger RNA (mRNA) and transfer RNAs (tRNAs) required to advance the mRNA reading frame by one codon. In eukaryotes, translocation is accelerated and its fidelity is maintained by elongation factor 2 (eEF2)1,2. At present, only a few snapshots of eukaryotic ribosome translocation have been reported3-5. Here we report ten high-resolution cryogenic-electron microscopy (cryo-EM) structures of the elongating eukaryotic ribosome bound to the full translocation module consisting of mRNA, peptidyl-tRNA and deacylated tRNA, seven of which also contained ribosome-bound, naturally modified eEF2. This study recapitulates mRNA-tRNA2-growing peptide module progression through the ribosome, from the earliest states of eEF2 translocase accommodation until the very late stages of the process, and shows an intricate network of interactions preventing the slippage of the translational reading frame. We demonstrate how the accuracy of eukaryotic translocation relies on eukaryote-specific elements of the 80S ribosome, eEF2 and tRNAs. Our findings shed light on the mechanism of translation arrest by the anti-fungal eEF2-binding inhibitor, sordarin. We also propose that the sterically constrained environment imposed by diphthamide, a conserved eukaryotic posttranslational modification in eEF2, not only stabilizes correct Watson-Crick codon-anticodon interactions but may also uncover erroneous peptidyl-tRNA, and therefore contribute to higher accuracy of protein synthesis in eukaryotes.

A novel multiple-stage antimalarial agent that inhibits protein synthesis.

There is an urgent need for new drugs to treat malaria, with broad therapeutic potential and novel modes of action, to widen the scope of treatment and to overcome emerging drug resistance. Here we describe the discovery of DDD107498, a compound with a potent and novel spectrum of antimalarial activity against multiple life-cycle stages of the Plasmodium parasite, with good pharmacokinetic properties and an acceptable safety profile. DDD107498 demonstrates potential to address a variety of clinical needs, including single-dose treatment, transmission blocking and chemoprotection. DDD107498 was developed from a screening programme against blood-stage malaria parasites; its molecular target has been identified as translation elongation factor 2 (eEF2), which is responsible for the GTP-dependent translocation of the ribosome along messenger RNA, and is essential for protein synthesis. This discovery of eEF2 as a viable antimalarial drug target opens up new possibilities for drug discovery.

Preclinical Antimalarial Combination Study of M5717, a Plasmodium falciparum Elongation Factor 2 Inhibitor, and Pyronaridine, a Hemozoin Formation Inhibitor.

Antimalarial drug resistance in the Plasmodium falciparum parasite poses a constant challenge for drug development. To mitigate this risk, new antimalarial medicines should be developed as fixed-dose combinations. Assessing the pharmacodynamic interactions of potential antimalarial drug combination partners during early phases of development is essential in developing the targeted parasitological and clinical profile of the final drug product. Here, we have studied the combination of M5717, a P. falciparum translation elongation factor 2 inhibitor, and pyronaridine, an inhibitor of hemozoin formation. Our test cascade consisted of in vitro isobolograms as well as in vivo studies in the P. falciparum severe combined immunodeficient (SCID) mouse model. We also analyzed pharmacokinetic and pharmacodynamic parameters, including genomic sequencing of recrudescent parasites. We observed no pharmacokinetic interactions with the combination of M5717 and pyronaridine. M5717 did not negatively impact the rate of kill of the faster-acting pyronaridine, and the latter was able to suppress the selection of M5717-resistant mutants, as well as significantly delay the recrudescence of parasites both with suboptimal and optimal dosing regimens.

Myostatin inhibits eEF2K-eEF2 by regulating AMPK to suppress protein synthesis.

Growth of skeletal muscle is dependent on the protein synthesis, and the rate of protein synthesis is mainly regulated in the stage of translation initiation and elongation. Myostatin, a member of the transforming growth factor-ß (TGF-ß) superfamily, is a negative regulator of protein synthesis. C2C12 myotubes was incubated with 0, 0.01, 0.1, 1, 2, 3 µg/mL myostatin recombinant protein, and then we detected the rates of protein synthesis by the method of SUnSET. We found that high concentrations of myostatin (2 and 3 µg/mL) inhibited protein synthesis by blocking mTOR and eEF2K-eEF2 pathway, while low concentration of myostatin (0.01, 0.1 and 1 µg/mL) regulated eEF2K-eEF2 pathway activity to block protein synthesis without affected mTOR pathway, and myostatin inhibited eEF2K-eEF2 pathway through regulating AMPK pathway to suppress protein synthesis. It provided a new mechanism for myostatin regulating protein synthesis and treating muscle atrophy.

Structural insights into the mechanism of translational inhibition by the fungicide sordarin.

The translational machinery has been found to be the target for a number of antibiotics. One such antibiotic sordarin selectively inhibits fungal translation by impairing the function of elongation factor 2 (eEF2) while being ineffective to higher eukaryotes. Surprisingly, sordarin is not even equally effective in impairing translation for all fungal species. The binding cavity of sordarin on eEF2 has been localized by X-ray crystallographic study and its unique specificity towards sordarin has been attributed to the species specific substitutions within a stretch of amino acids (sordarin specificity region, SSR) at the entrance of the cavity. In this study, we have analyzed the sordarin-binding cavity of eEF2 from different species both in isolated and ribosome-bound forms in order to decipher the mechanism of sordarin binding selectivity. Our results reveal that the molecular architecture as well as the microenvironment of the sordarin-binding cavity changes significantly from one species to another depending on the species specific substitutions within the cavity. Moreover, eEF2 binding to ribosome aggravates the effects of these substitutions. Thus, this study, while shedding light on the molecular mechanism underpinning the selective inhibitory effects of sordarin, will also be a helpful guide for future studies aiming at developing novel antifungal drugs with broader spectrum of activity.

Safety, pharmacokinetics, and antimalarial activity of the novel plasmodium eukaryotic translation elongation factor 2 inhibitor M5717: a first-in-human, randomised, placebo-controlled, double-blind, single ascending dose study and volunteer infection study.

BACKGROUND: M5717 is the first plasmodium translation elongation factor 2 inhibitor to reach clinical development as an antimalarial. We aimed to characterise the safety, pharmacokinetics, and antimalarial activity of M5717 in healthy volunteers. METHODS: This first-in-human study was a two-part, single-centre clinical trial done in Brisbane, QLD, Australia. Part one was a double-blind, randomised, placebo-controlled, single ascending dose study in which participants were enrolled into one of nine dose cohorts (50, 100, 200, 400, 600, 1000, 1250, 1800, or 2100 mg) and randomly assigned (3:1) to M5717 or placebo. A sentinel dosing strategy was used for each dose cohort whereby two participants (one assigned to M5717 and one assigned to placebo) were initially randomised and dosed. Randomisation schedules were generated electronically by independent, unblinded statisticians. Part two was an open-label, non-randomised volunteer infection study using the Plasmodium falciparum induced blood-stage malaria model in which participants were enrolled into three dose cohorts. Healthy men and women of non-childbearing potential aged 18-55 years were eligible for inclusion; individuals in the volunteer infection study were required to be malaria naive. Safety and tolerability (primary outcome of the single ascending dose study and secondary outcome of the volunteer infection study) were assessed by frequency and severity of adverse events. The pharmacokinetic profile of M5717 was also characterised (primary outcome of the volunteer infection study and secondary outcome of the single ascending dose study). Parasite clearance kinetics (primary outcome of the volunteer infection study) were assessed by the parasite reduction ratio and the corresponding parasite clearance half-life; the incidence of recrudescence up to day 28 was determined (secondary outcome of the volunteer infection study). Recrudescent parasites were tested for genetic mutations (exploratory outcome). The trial is registered with ClinicalTrials.gov (NCT03261401). FINDINGS: Between Aug 28, 2017, and June 14, 2019, 221 individuals were assessed for eligibility, of whom 66 men were enrolled in the single ascending dose study (eight per cohort for 50-1800 mg cohorts, randomised three M5717 to one placebo, and two in the 2100 mg cohort, randomised one M5717 to one placebo) and 22 men were enrolled in the volunteer infection study (six in the 150 mg cohort and eight each in the 400 mg and 800 mg cohorts). No adverse event was serious; all M5717-related adverse events were mild or moderate in severity and transient, with increased frequency observed at doses above 1250 mg. In the single ascending dose study, treatment-related adverse events occurred in three of 17 individuals in the placebo group; no individual in the 50 mg, 100 mg, or 200 mg groups; one of six individuals in each of the 400 mg, 1000 mg, and 1250 mg groups; two of six individuals in the 600 mg group; and in all individuals in the 1800 mg and 2100 mg groups. In the volunteer infection study, M5717-related adverse events occurred in no participants in the 150 mg or 800 mg groups and in one of eight participants in the 400 mg group. Transient oral hypoesthesia (in three participants) and blurred vision (in four participants) were observed in the 1800 mg or 2100 mg groups and constituted an unknown risk; thus, further dosing was suspended after dosing of the two sentinel individuals in the 2100 mg cohort. Maximum blood concentrations occurred 1-7 h after dosing, and a long half-life was observed (146-193 h at doses ≥200 mg). Parasite clearance occurred in all participants and was biphasic, characterised by initial slow clearance lasting 35-55 h (half-life 231·1 h [95% CI 40·9 to not reached] for 150 mg, 60·4 h [38·6 to 138·6] for 400 mg, and 24·7 h [20·4 to 31·3] for 800 mg), followed by rapid clearance (half-life 3·5 h [3·1 to 4·0] for 150 mg, 3·9 h [3·3 to 4·8] for 400 mg, and 5·5 h [4·8 to 6·4] for 800 mg). Recrudescence occurred in three (50%) of six individuals dosed with 150 mg and two (25%) of eight individuals dosed with 400 mg. Genetic mutations associated with resistance were detected in four cases of parasite recrudescence (two individuals dosed with 150 mg and two dosed with 400 mg). INTERPRETATION: The safety, pharmacokinetics, and antimalarial activity of M5717 support its development as a component of a single-dose antimalarial combination therapy or for malaria prophylaxis. FUNDING: Wellcome Trust and the healthcare business of Merck KGaA, Darmstadt, Germany.

Suppression of the kinase for elongation factor 2 alleviates mGluR-LTD impairments in a mouse model of Alzheimer's disease.

Impaired mRNA translation (protein synthesis) is linked to Alzheimer's disease (AD) pathophysiology. Recent studies revealed the role of increased phosphorylation of eukaryotic elongation factor 2 (eEF2) in AD-associated cognitive deficits. Phosphorylation of eEF2 (at the Thr56 site) by its only known kinase eEF2K leads to inhibition of general protein synthesis. AD is considered as a disease of "synaptic failure" characterized by impairments of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Deficiency of metabotropic glutamate receptor 5-dependent LTD (mGluR-LTD) is indicated in cognitive syndromes associated with various neurological disorders, including AD, but the molecular signaling mechanisms underlying the mGluR-LTD dysregulation in AD remain unclear. In this brief communication, we report genetic repression of eEF2K in aged APP/PS1 AD model mice prevented AD-associated hippocampal mGluR-LTD deficits. Using a pharmacological approach, we further observed that impairments of mGluR-LTD in APP/PS1 mice were rescued by treating hippocampal slices with a small molecule eEF2K antagonist NH125. Our findings, taken together, suggest a critical role of abnormal protein synthesis dysregulation at the elongation phase in AD-associated mGluR-LTD failure, thus providing insights into a mechanistic understanding of synaptic impairments in AD and other related dementia syndromes.

Protocol for interfering peptide injection into adult mouse hippocampus and spatial memory testing.

Metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) occurs in diverse brain regions and contributes to the plasticity of behavior, learning, and memory. mGluR-LTD relies on rapid (in minutes) local protein synthesis. Here, we describe a detailed protocol for delivering an interfering peptide into the adult mouse hippocampus. The delivered peptide disrupts the interaction between polyglutamine binding protein 1 and eukaryotic elongation factor 2, resulting in impaired hippocampal mGluR-LTD and mGluR-LTD-associated behaviors. For complete details on the use and execution of this protocol, please refer to Shen et al. (2021).

Lopinavir impairs protein synthesis and induces eEF2 phosphorylation via the activation of AMP-activated protein kinase.

HIV anti-retroviral drugs decrease protein synthesis, although the underlying regulatory mechanisms of this process are not fully established. Therefore, we investigated the effects of the HIV protease inhibitor lopinavir (LPV) on protein metabolism. We also characterized the mechanisms that mediate the effects of this drug on elongation factor-2 (eEF2), a key component of the translational machinery. Treatment of C2C12 myocytes with LPV produced a dose-dependent inhibitory effect on protein synthesis. This effect was observed at 15 min and was maintained for at least 4 h. Mechanistically, LPV increased the phosphorylation of eEF2 and thereby decreased the activity of this protein. Increased phosphorylation of eEF2 was associated with increased activity of its upstream regulators AMP-activated protein kinase (AMPK) and eEF2 kinase (eEF2K). Both AMPK and eEF2K directly phosphorylated eEF2 in an in vitro kinase assay suggesting two distinct paths lead to eEF2 phosphorylation. To verify this connection, myocytes were treated with the AMPK inhibitor compound C. Compound C blocked eEF2K and eEF2 phosphorylation, demonstrating that LPV affects eEF2 activity via an AMPK-eEF2K dependent pathway. In contrast, incubation of myocytes with rottlerin suppressed eEF2K, but not eEF2 phosphorylation, suggesting that eEF2 can be regulated independent of eEF2K. Finally, LPV did not affect PP2A activity when either eEF2 or peptide was used as the substrate. Collectively, these results indicate that LPV decreases protein synthesis, at least in part, via inhibition of eEF2. This appears regulated by AMPK which can act directly on eEF2 or indirectly via the action of eEF2K.

Farnesyltransferase inhibitor SCH66336 induces rapid phosphorylation of eukaryotic translation elongation factor 2 in head and neck squamous cell carcinoma cells.

Farnesyltransferase inhibitors (FTI) are a class of therapeutic agents designed to target tumors with mutations of the ras oncogene. However, the biological effect of FTIs is often independent of ras mutation status, which suggests the existence of additional mechanisms. In this study, we investigated the molecular effects of SCH66336, an FTI, in head and neck squamous cell carcinoma cells using proteomic approaches. We showed that SCH66336 induced phosphorylation (inactivation) of eukaryotic translation elongation factor 2 (eEF2), an important molecule for protein synthesis, as early as 3 hours after SCH66336 administration. Protein synthesis was subsequently reduced in the cells. Paradoxically, activation of eEF2 kinase (eEF2K), the only known kinase that regulates eEF2, was observed only at 12 hours after SCH66336 treatment. Consistent with this observation, the inhibition of phosphorylated-MEK and phosphorylated-p70S6K, the two key signaling molecules responsible for activation of eEF2K, also occurred at least 12 hours after SCH66336 administration. Our data suggest that inhibition of protein synthesis through inactivation of eEF2 is a novel mechanism of SCH66336-mediated growth inhibition and that this effect is independent of ras-MEK/p70S6K-eEF2K signaling cascades.

Structure-based designing of sordarin derivative as potential fungicide with pan-fungal activity.

Fungal infections have become a significant problem for immunosuppressed patients. Sordarin, a promising fungicidal agent, inhibits fungal protein synthesis by impairing elongation factor-2 (eEF2) function. Intriguingly, despite high sequence similarity among eEF2s from different species, sordarin has been shown to inhibit translation specifically in certain fungi while unable to do so in some other fungal species (e.g. Candida parapsilosis and Candida lusitaniae). The sordarin binding site on eEF2 as well as its mechanism of action is known. In a previous study, we have detailed the interactions between sordarin and eEF2 cavities from different fungal species at the molecular level and predicted the probable cause of sordarin sensitivity. Guided by our previous analysis, we aimed for computer-aided designing of sordarin derivatives as potential fungicidal agents that still remain ineffective against human eEF2. We have performed structural knowledge-based designing of several sordarin derivatives and evaluated predicted interactions of those derivatives with the sordarin-binding cavities of different eEF2s, against which sordarin shows no inhibitory action. Our analyses identify an amino-pyrrole derivative as a good template for further designing of promising broad-spectrum antifungal agents. The drug likeness and ADMET prediction on this derivative also supports its suitability as a drug candidate.

Immunoconjugates in the management of hairy cell leukemia.

Hairy cell leukemia (HCL) is an indolent B-cell malignancy effectively treated but not often cured by purine analog therapy; after multiple courses of purine analogs, patients can become purine analog resistant and in need of alternative therapies. Complete remission to single-agent purine analog is often accompanied by minimal residual disease (MRD), residual HCL cells detectable by immunologic methods, considered a risk factor for eventual relapse. Several different non-chemotherapy approaches are being used to target relapsed and refractory HCL, including inhibitors of BRAF, but so far only monoclonal antibody (MAb)-based approaches have been reported to eliminate MRD in a high percentage of patients. One of the MAb-based options for HCL currently under clinical investigation involves recombinant immunotoxins, containing a fragment of a MAb and a bacterial toxin. The bacterial toxin, a highly potent fragment from Pseudomonas exotoxin, catalytically ADP-ribosylates elongation factor 2 (EF2), resulting in protein synthesis inhibition and apoptotic cell death. Recombinant immunotoxins tested in HCL patients include LMB-2, targeting CD25, and BL22, targeting CD22. An affinity matured version of BL22, termed moxetumomab pasudotox (formerly HA22 or CAT-8015) achieved high CR rates in phase I, and is currently undergoing multicenter Phase 3 testing. Phase I testing was without dose-limiting toxicity, although 2 patients had grade 2 hemolytic uremic syndrome (HUS) with transient grade 1 abnormalities in platelets and creatinine. Preclinical work is underway to identify residues on moxetumomab pasudotox leading to immunogenicity. Moxetumomab pasudotox is undergoing pivotal testing for relapsed and refractory HCL.

An aptamer-siRNA chimera silences the eukaryotic elongation factor 2 gene and induces apoptosis in cancers expressing αvß3 integrin.

Small interfering RNAs (siRNAs) silence gene expression by triggering the sequence-specific degradation of mRNAs, but the targeted delivery of such reagents remains challenging and a significant obstacle to therapeutic applications. One promising approach is the use of RNA aptamers that bind tumor-associated antigens to achieve the delivery of siRNAs to tumor cells displaying specific antigens. Wholly RNA-based constructs are advantageous because they are inexpensive to synthesize and their immunogenicity is low. We therefore joined an aptamer-recognizing alpha V and integrin beta 3 (αvß3) integrin to a siRNA that targets eukaryotic elongation factor 2 and achieved for the first time the targeted delivery of a siRNA to tumor cells expressing αvß3 integrin, causing the inhibition of cell proliferation and the induction of apoptosis specifically in tumor cells. The impact of our results on the development of therapeutic aptamer-siRNA constructs is discussed.

Translation regulation after taxol treatment in NIH3T3 cells involves the elongation factor (eEF)2.

Changes to the translational machinery that occur during apoptosis have been described in the last few years. The two principal ways in which translational factors are modified during apoptosis are: (i) changes in protein phosphorylation and (ii) specific proteolytic cleavages. Taxol, a member of a new class of anti-tubulin drugs, is currently used in chemotherapeutic treatments of different types of cancers. We have previously demonstrated that taxol induces calpain-mediated apoptosis in NIH3T3 cells [Piñeiro et al., Exp. Cell Res., 2007, 313:369-379]. In this study we found that translation was significantly inhibited during taxol-induced apoptosis in these cells. We have studied the phosphorylation status and expression levels of eIF2a, eIF4E, eIF4G and the regulatory protein 4E-BP1, all of which are implicated in translation regulation. We found that taxol treatment did not induce changes in eIF2alpha phosphorylation, but strongly decreased eIF4G, eIF4E and 4E-BP1 expression levels. MDL28170, a specific inhibitor of calpain, prevented reduction of eIF4G, but not of eIF4E or 4E-BP1 levels. Moreover, the calpain inhibitor did not block taxol-induced translation inhibition. All together these findings demonstrated that none of these factors are responsible for the taxol-induced protein synthesis inhibition. On the contrary, taxol treatment increased elongation factor eEF2 phosphorylation in a calpain-independent manner, supporting a role for eEF2 in taxol-induced translation inhibition.

Hypoxia-induced energy stress regulates mRNA translation and cell growth.

Oxygen (O2) deprivation, or hypoxia, has profound effects on cell metabolism and growth. Cells can adapt to low O2 in part through activation of hypoxia-inducible factor (HIF). We report here that hypoxia inhibits mRNA translation by suppressing multiple key regulators, including eIF2alpha, eEF2, and the mammalian target of rapamycin (mTOR) effectors 4EBP1, p70S6K, and rpS6, independent of HIF. Hypoxia results in energy starvation and activation of the AMPK/TSC2/Rheb/mTOR pathway. Hypoxic AMP-activated protein kinase (AMPK) activation also leads to eEF2 inhibition. Moreover, hypoxic effects on cellular bioenergetics and mTOR inhibition increase over time. Mutation of the TSC2 tumor suppressor gene confers a growth advantage to cells by repressing hypoxic mTOR inhibition and hypoxia-induced G1 arrest. Together, eIF2alpha, eEF2, and mTOR inhibition represent important HIF-independent mechanisms of energy conservation that promote survival under low O2 conditions.

Synthesis and evaluation of N-substituted 1,4-oxazepanyl Sordaricins as selective fungal EF-2 inhibitors.

Sordaricin analogues possessing 6-methoxy-7-methyl-1,4-oxazepane moiety instead of the sugar part were synthesized and evaluated. It was found that N-substituents on the oxazepane ring had influence on biological activity. In particular, N-(2-methylpropenyl) derivative 12p exhibited potent in vitro antifungal activity. Furthermore, 12p maintained significant activity (MIC 0.25 microg/mL) against Candida albicans SANK51486 even in the presence of 20% horse serum.

Fusidic and helvolic acid inhibition of elongation factor 2 from the archaeon Sulfolobus solfataricus.

Fusidic acid (FA) and helvolic acid (HA) belong to a small family of naturally occurring steroidal antibiotics known as fusidanes. FA was studied for its ability to alter the biochemical properties supported by elongation factor 2 isolated from the archaeon Sulfolobus solfataricus (SsEF-2). Both poly(Phe) synthesis and ribosome-dependent GTPase (GTPase(r)) were progressively impaired by increasing concentrations of FA up to 1 mM, whereas no effect was measured in the intrinsic GTPase of SsEF-2 triggered by ethylene glycol in the presence of barium chloride (GTPase(g)). The highest antibiotic concentration caused inhibition of either poly(Phe) synthesis or GTPase(r) only slightly above 50%. A greater response of SsEF-2 was observed when HA was used instead of FA. HA caused even a weak impairment of GTPase(g). A mutated form of SsEF-2 carrying the L452R substitution exhibited an increased sensitivity to fusidane inhibition in either poly(Phe) synthesis or GTPase(r). Furthermore, both FA and HA were able to cause impairment of GTPase(g). The antibiotic concentrations leading to 50% inhibition (IC(50)) indicate that increased fusidane responsiveness due to the use of HA or the L452R amino acid replacement is mutually independent. However, their combined effect decreased the IC(50) up to 0.1 mM. Despite the difficulties in reaching complete inhibition of the translocation process in S. solfataricus, these findings suggest that fusidane sensibility is partially maintained in the archaeon S. solfataricus. Therefore, it is likely that SsEF-2 harbors the structural requirements for forming complexes with fusidane antibiotics. This hypothesis is further evidenced by the observed low level of impairment of GTPase(g), a finding suggesting a weak direct interaction between the archaeal factor and fusidanes even in the absence of the ribosome. However, the ribosome remains essential for the sensitivity of SsEF-2 toward fusidane antibiotics.

Elongation factor 2 as a target for selective inhibition of protein synthesis in vitro by the novel aromatic bisamidine.

The effect of the novel aromatic bisamidine 1 on protein synthesis in cell-free translational system isolated from rat livers was studied. The bisamidine 1 caused inhibition of [14C]leucine incorporation into proteins proportionally to its concentration. To establish a precise mechanism of inhibition, we evaluated the effect of the bisamidine 1 on the isolated ribosomes and purified to homogeneity elongation factors. Preincubation of the bisamidine 1 with ribosomes resulted in partial inhibition of their activity in whole elongation system. The eucaryotic elongation factor 1 (eEF-1) was not significantly affected by the bisamidine 1. In contrast to eEF-1, the bisamidine 1 preincubated with the eucaryotic elongation factor 2 (eEF-2) caused total inhibition of its activity in the translocation process. The inhibitory effect of the bisamidine 1 on eEF-2 activity was confirmed in diphtheria toxin-dependent ADP-ribosylation reaction. The results suggest a high action specificity of the bisamidine 1 as potential anticancer drug, since the primary target seems to be highly conserved protein-elongation factor 2.

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.

Species-specific inhibition of fungal protein synthesis by sordarin: identification of a sordarin-specificity region in eukaryotic elongation factor 2.

The sordarin class of natural products selectively inhibits fungal protein synthesis by impairing the function of eukaryotic elongation factor 2 (eEF2). Mutations in Saccharomyces cerevisiae eEF2 or the ribosomal stalk protein rpP0 can confer resistance to sordarin, although eEF2 is the major determinant of sordarin specificity. It has been shown previously that sordarin specifically binds S. cerevisiae eEF2 while there is no detectable binding to eEF2 from plants or mammals, despite the high level of amino acid sequence conservation among these proteins. In both whole-cell assays and in vitro translation assays, the efficacy of sordarin varies among different species of pathogenic fungi. To investigate the basis of sordarin's fungal selectivity, eEF2 has been cloned and characterized from several sordarin-sensitive and -insensitive fungal species. Results from in vivo expression of Candida species eEF2s in S. cerevisiae and in vitro translation and growth inhibition assays using hybrid S. cerevisiae eEF2 proteins demonstrate that three amino acid residues within eEF2 account for the selectivity of this class of compounds. It is also shown that the corresponding residues at these positions in human eEF2 are sufficient to confer sordarin insensitivity to S. cerevisiae identical to that observed with mammalian eEF2.