Eukaryotic Initiation Factor eIF 4G Research Articles

Ferritin Iron Responsive Elements (IREs) mRNA Interacts with eIF4G and Activates In Vitro Translation

Eukaryotic initiation factor (eIF) 4G plays an important role in assembling the initiation complex required for ribosome binding to mRNA and promote translation. Translation of ferritin IRE mRNAs is regulated by iron through iron responsive elements (IREs) and iron regulatory protein (IRP). The noncoding IRE stem-loop (30-nt) structure control synthesis of proteins in iron trafficking, cell cycling, and nervous system function. High cellular iron concentrations promote IRE RNA binding to ribosome and initiation factors, and allow synthesis of ferritin. In vitro translation assay was performed in depleted wheat germ lysate with supplementation of initiation factors. Fluorescence spectroscopy was used to characterize eIF4F/IRE binding. Eukaryotic initiation factor eIF4G increases the translation of ferritin through binding to stem loop structure of iron responsive elements mRNA in the 5'-untranslated region. Our translation experiment demonstrated that exogenous addition of eIF4G selectively enhanced the translation of ferritin IRE RNA in depleted WG lysate. However, eIF4G facilitates capped IRE RNA translation significantly higher than uncapped IRE RNA translation. Addition of iron with eIF4G to depleted WG lysate significantly enhanced translation for both IRE mRNA (capped and uncapped), confirming the contribution of eIF4G and iron as a potent enhancer of ferritin IRE mRNA translation. Fluorescence data revealed that ferritin IRE strongly interacts to eIF4G (Kd = 63 nM), but not eIF4E. Further equilibrium studies showed that iron enhanced (~4-fold) the ferritin IRE binding to eIF4G. The equilibrium binding effects of iron on ferritin IRE RNA/eIFs interaction and the temperature dependence of this reaction were measured and compared. The Kd values for the IRE binding to eIF4G ranging from 18.2 nM to 63.0 nM as temperature elevated from 5 °C to 25 °C, while the presence of iron showed much stronger affinity over the same range of temperatures. Thermodynamic parameter revealed that IRE RNA binds to eIF4G with ΔH = -42.6 ± 3.3 kJ. mole-1, ΔS = -11.5 ± 0.4 J. mole-1K-1, and ΔG = -39.2 ± 2.7 kJ. mole-1, respectively. Furthermore, addition of iron significantly changed the values of thermodynamic parameters, favoring stable complex formation, thus favoring efficient protein synthesis. This study first time demonstrate the participation of eIF4G in ferritin IRE mRNA translation. eIF4G specifically interacts with ferritin IRE RNA and promotes eIF4G-dependent translation.

YelA, a putative Dictyostelium translational regulator, acts as antagonist of DIF-1 signaling to control cell-type proportioning.

DIF-1 (differentiation-inducing factor1) is a polyketide produced by Dictyostelium prespore cells which induces initially uncommitted cells to differentiate as prestalk cells. Exposure of cells to DIF-1 causes transitory hypo-phosphorylation of seven serine residues in YelA, a protein with a region of strong homology to the MIF4G domain of the eukaryotic initiation factor eIF4G. Based upon its domain architecture, which in one important aspect closely resembles that of Death-Associated Protein 5 (DAP5), we predict a role in stimulating internal ribosome entry-driven mRNA translation. The two paradigmatic DIF-1 inducible genes are ecmA (extracellular matrixA) and ecmB. In support of a YelA function in DIF-1 signaling, a YelA null strain showed greatly increased expression of ecmA and ecmB in response to DIF-1. Also, during normal development in the null strain, expression of the two genes is accelerated. This is particularly evident for ecmB, a marker of stalk tube and supporting structure differentiation. Mutants in DIF-1 bio-synthesis or signaling display a rudimentary or no basal disc and, conversely, YelA null mutants produce fruiting bodies with a highly enlarged basal disc that ectopically expresses a stalk tube-specific marker. Thus YelA acts as an antagonist of DIF-1 signaling, with a consequent effect on cell type proportioning and it is predicted to act as a translational regulator.

Identification of the functional binding domains of eukaryotic initiation factor eIF4G with 3’ Cap‐independent translation element of Barley Yellow Dwarf Virus (LB203)

Different from canonical translation initiation, some plant viral RNAs lack a 5’cap (an unusual 7‐methyl guanosine linked by a 5’to 5’ triphosphate). They utilize a cap independent translation element (CITE) to efficiently start translation. Barley yellow dwarf virus (BYDV) has a translation element (BTE) located in its 3’UTR mRNA. BTE binds with eukaryotic initiation factor eIF4G (one subunit of heterodimer of eIF4F) and recruits other initiation factors to assemble the ribosome preinitiation complex for synthesis of viral proteins. BYDV is one of the world‐wide economically most important viruses causing severe crop loss. How BTE mediate BYDV translation is still unclear. By measuring fluorescence anisotropy of labeled RNA, here we report 3’BTE binds with a truncated fragment of eIF4G, eIF4G601‐1196 (contained amino acid residue from 601 to 1196). The equilibrium dissociation constants (Kd) of this binding is 39.5±3.5 nM. Adding eIF4E, the cap binding protein, can increase the binding ability of eIF4G601‐1196 with 3’BTE. eIF4G601‐1196•4E complex binds with 3’BTE 5.8 folds tighter than eIF4G601‐1196 alone, with the equilibrium dissociation constant 6.78±1.49nM. Further circular dichroism spectra (CD) and thermodynamic studies will characterize the binding of the eIF4G with 3’ BTE. These results suggest eIF4E plays an indirect role in translation initiation, even for an uncapped virus.Grant Funding Source: NSF MCB 1157632

252 INVITED Targeting the protein translation factor eIF4E for cancer therapy

In wheat germ, the interaction between poly(A)-binding protein and eukaryotic initiation factor eIF 4G increases the affinity of eIF4E for the cap by 20–40-fold. Recent findings that wheat germ eIF4G is required for interaction with the IRES, pseudoknot 1 (PK1), of tobacco etch virus to promote cap-independent translation led us to investigate the effects of PABP on the interaction of eIF4F with PK1. The fluorescence anisotropy data showed addition of PABP to eIF4F increased the binding affinity ∼ 2.0-fold for PK1 RNA as compared with eIF4F alone. Addition of both PABP and eIF4B to eIF4F enhance binding affinity to PK1 about 4-fold, showing an additive effect rather than the large increase in affinity shown for cap binding. The van't Hoff analyses showed that PK1 RNA binding to eIF4F, eIF4F·PABP, eIF4F·4B and eIF4F·4B·PABP is enthalpy-driven and entropy-favorable. PABP and eIF4B decreased the entropic contribution 65% for binding of PK1 RNA to eIF4F. The lowering of entropy for the formation of eIF4F·4B·PABP–PK1 complex suggested reduced hydrophobic interactions for complex formation. Overall, these results demonstrate the first direct effect of PABP on the interaction of eIF4F and eIF4F·4B with PK1 RNA.

Effects of poly(A)-binding protein on the interactions of translation initiation factor eIF4F and eIF4F·4B with internal ribosome entry site (IRES) of tobacco etch virus RNA

In wheat germ, the interaction between poly(A)-binding protein and eukaryotic initiation factor eIF 4G increases the affinity of eIF4E for the cap by 20-40-fold. Recent findings that wheat germ eIF4G is required for interaction with the IRES, pseudoknot 1 (PK1), of tobacco etch virus to promote cap-independent translation led us to investigate the effects of PABP on the interaction of eIF4F with PK1. The fluorescence anisotropy data showed addition of PABP to eIF4F increased the binding affinity approximately 2.0-fold for PK1 RNA as compared with eIF4F alone. Addition of both PABP and eIF4B to eIF4F enhance binding affinity to PK1 about 4-fold, showing an additive effect rather than the large increase in affinity shown for cap binding. The van't Hoff analyses showed that PK1 RNA binding to eIF4F, eIF4F.PABP, eIF4F.4B and eIF4F.4B.PABP is enthalpy-driven and entropy-favorable. PABP and eIF4B decreased the entropic contribution 65% for binding of PK1 RNA to eIF4F. The lowering of entropy for the formation of eIF4F.4B.PABP-PK1 complex suggested reduced hydrophobic interactions for complex formation. Overall, these results demonstrate the first direct effect of PABP on the interaction of eIF4F and eIF4F.4B with PK1 RNA.

Cap‐independent translation of maize Hsp101

Maize embryonic axes contain stored mRNAs, some of which are able to undergo cap-independent translation initiation during germination. The Hsp101 mRNA, encoding a heat shock protein, is essential for thermo-tolerance induction and is present among the stored transcripts. This research aimed to investigate whether the Hsp101 transcript is IRES-driven regulated upon heat stress. Hsp101 transcribed either in vitro or in vivo was efficiently translated via a cap-independent mechanism. This was observed either in an animal in vitro translation system containing proteolytically cleaved eukaryotic initiation factor eIF4G or in a plant system lacking both eIF4E and eIFiso4E initiation factors. Deletion of the 5' untranslated region (UTR) from the Hsp101 mRNA abolished its cap-independent translation indicating that this nucleotide sequence is required to confer cap-independent initiation. Bicistronic constructs containing the Hsp101 mRNA 5'UTR in sense and anti-sense directions between two reporter genes were translated in both cap-independent systems. A similar bicistronic construct containing a viral internal ribosome entry site (IRES) element between the reporter genes was used as control. Internal translation of the second reporter gene was observed when the Hsp101 5'UTR was in the sense but not in the anti-sense orientation in the bicistronic construct. Taken together, these data suggest that the 5'UTR of maize Hsp101, a plant cellular mRNA, functions as an IRES-like element accounting for its cap-independent translation during heat stress.

MTOR as Therapeutic Target for T-Cell Leukemia: Synergism with Conventional Cytotoxic Drugs and Signaling Inhibitors.

Targeted disruption of signaling pathways essential for tumor cell survival and proliferation offer great promise for the design of more efficient and specific therapies. mTOR plays a central role in sensing mitogenic, nutrient and energy signals, and in the integration of different signaling pathways. Moreover, increasing evidence implicates mTOR in tumorigenesis and in the maintenance of transformed phenotypes. Therefore, we hypothesized that the mTOR pathway may play a significant role in T-cell acute lymphoblastic leukemia (ALL). BM and peripheral blood specimens from 15 pediatric patients were analyzed for the phosphorylation status of two downstream substrates of mTOR, S6 Ribosomal Protein and eIF4G. In all cases, primary leukemia T cells showed marked phosphorylation of S6 Ribosomal Protein (Ser235/236) and phosphorylation of the eukaryotic initiation factor eIF4G (Ser1108), showing that the mTOR pathway is constitutively active in T-ALL cells. Since leukemia T cells did not show significant basal phosphorylation of Akt/PKB, this observation suggests that the mTOR activation observed in T-ALL is Akt-independent. Specific blockade of mTOR by Rapamycin (100nM) reduced or abrogated phosphorylation of S6 ribosomal protein. Moreover, the mTOR blockers Rapamycin (10–100nM) and CCI-779 (10–100nM) significantly inhibited cell survival, and IL-7-promoted proliferation (40% to 85% inhibition) and cell cycle progression of T-ALL cells. Since we have shown previously that PI3K/Akt is critically involved in T-ALL responses to exogenous stimuli, we next evaluated whether inhibition of PI3K/Akt signals would synergize or potentiate the effects of mTOR blockade. We observed that the PI3K-inhibitor LY294002 (10μM) synergized with Rapamycin to abrogate IL-7-promoted leukemia T-cell proliferation. Finally, we explored the possibility that mTOR blockade can enhance the cytotoxic effects on T-ALL of the conventional drugs Dexamethasone (100nM) and Doxorubicin (100nM). Rapamycin potentiated the cytotoxic effects of both Dexamethasone (1.3 to 10 fold) and Doxorubicin (1.7 to 7.3 fold), suggesting that the combination of mTOR blockade with chemotherapy can improve the anti-leukemia efficacy of conventional regimens. In conclusion, we observed that the mTOR pathway is constitutively activated in primary leukemia T cells, and that the mTOR blockers Rapamycin and CCI-779 significantly inhibit T-ALL survival and proliferation. In addition, Rapamycin potentiates the inhibitory activity of other signaling inhibitors and the cytotoxic effects of the conventional drugs Dexamethasone and Doxorubicin. This study suggests that the mTOR pathway is a valid target for the treatment of T-ALL and lay the groundwork for the inclusion of mTOR blockade in the current therapeutic regimens.

Cleavage of eukaryotic initiation factor eIF4G and inhibition of host-cell protein synthesis during feline calicivirus infection.

Caliciviruses are small, non-enveloped, positive-stranded RNA viruses that are pathogenic for both animals and man. Although their capsid structure and genomic organization are distinct from picornaviruses, they have similarities to these viruses in their non-structural proteins. Picornaviruses induce a rapid inhibition of host-cell cap-dependent protein synthesis and this is mainly achieved through cleavage of eIF4G and/or dephosphorylation of 4E-BP1. In this study, the effect of calicivirus infection was examined on host-cell protein synthesis in order to determine whether they also induce host shut-off. We report that infection of cells with feline calicivirus (FCV) leads to the inhibition of cellular protein synthesis. This is accompanied by the cleavage of the eukaryotic translation initiation factors eIF4GI and eIF4GII in a manner reminiscent of that induced by picornaviruses. However, the cleavages occur at different sites. The potential mechanisms of these cleavage events and the implications for the translation of calicivirus mRNA are discussed.

Enterovirus 71 contains a type I IRES element that functions when eukaryotic initiation factor eIF4G is cleaved

Human enterovirus 71 (EV71) is a member of the Enterovirus genus of the Picornaviridae family. Other members of this family utilize an unusual mechanism of translation initiation whereby ribosomes are recruited internally to the viral RNA by an internal ribosome site (IRES) located in their 5′ noncoding regions (5′ NCR). Using dicistronic reporter constructs, we demonstrate that the 5′ NCRs of the 7423/MS/87 and BrCr strains of EV71 function as an IRES both in extracts and in cultured cells. Preincubation of translation extracts with purified coxsackievirus 2A protease cleaved elF4G, a component of the cap binding complex, resulting in a significant decrease in translation of capped mRNAs. In contrast, the translational efficiency of the EV71 IRES was enhanced under this condition, demonstrating that the EV71 IRES functions similar to other enterovirus IRES elements when components of the cap binding protein complex are cleaved. Finally, insertion of an upstream, out-of-frame start codon in the 5′ NCR of the EV71 genome inhibited IRES activity, suggesting that EV71 can be classified as a type I IRES, in which ribosomes first bind upstream of the initiation codon and then scan the mRNA until an appropriate downstream AUG start codon is encountered and protein synthesis commences.

The mRNA-binding Protein YB-1 (p50) Prevents Association of the Eukaryotic Initiation Factor eIF4G with mRNA and Inhibits Protein Synthesis at the Initiation Stage

The cytoplasmic messenger ribonucleoprotein particles of mammalian somatic cells contain the protein YB-1, also called p50, as a major core component. YB-1 is multifunctional and involved in regulation of mRNA transcription and translation. Our previous studies demonstrated that YB-1 stimulates initiation of translation in vitro at a low YB-1/mRNA ratio, whereas an increase of YB-1 bound to mRNA resulted in inhibition of protein synthesis in vitro and in vivo. Here we show that YB-1-mediated translation inhibition in a rabbit reticulocyte cell-free system is followed by a decay of polysomes, which is not a result of mRNA degradation or its functional inactivation. The inhibition does not change the ribosome transit time, and therefore, it affects neither elongation nor termination of polypeptide chains and only occurs at the stage of initiation. YB-1 induces accumulation of mRNA in the form of free messenger ribonucleoprotein particles, i.e. it blocks mRNA association with the small ribosomal subunit. The accumulation is accompanied by eukaryotic initiation factor eIF4G dissociation from mRNA. The C-terminal domain of YB-1 is responsible for inhibition of translation as well as the disruption of mRNA interaction with eIF4G.

Invited Review: Role of insulin in translational control of protein synthesis in skeletal muscle by amino acids or exercise.

Protein synthesis in skeletal muscle is modulated in response to a variety of stimuli. Two stimuli receiving a great deal of recent attention are increased amino acid availability and exercise. Both of these effectors stimulate protein synthesis in part through activation of translation initiation. However, the full response of translation initiation and protein synthesis to either effector is not observed in the absence of a minimal concentration of insulin. The combination of insulin and either increased amino acid availability or endurance exercise stimulates translation initiation and protein synthesis in part through activation of the ribosomal protein S6 protein kinase S6K1 as well as through enhanced association of eukaryotic initiation factor eIF4G with eIF4E, an event that promotes binding of mRNA to the ribosome. In contrast, insulin in combination with resistance exercise stimulates translation initiation and protein synthesis through enhanced activity of a guanine nucleotide exchange protein referred to as eIF2B. In both cases, the amount of insulin required for the effects is low, and a concentration of the hormone that approximates that observed in fasting animals is sufficient for maximal stimulation. This review summarizes the results of a number of recent studies that have helped to establish our present understanding of the interactions of insulin, amino acids, and exercise in the regulation of protein synthesis in skeletal muscle.

Insulin stimulates protein synthesis in skeletal muscle by enhancing the association of eIF-4E and eIF-4G.

Insulin stimulated protein synthesis in gastrocnemius muscle of perfused rat hindlimb preparations by approximately twofold. The stimulation of protein synthesis was associated with a 12-fold increase in the amount of eukaryotic initiation factor eIF-4G bound to the mRNA cap-binding protein eIF-4E. In part, the increased binding of eIF-4G to eIF-4E was a result of release of eIF-4E bound to the translational regulator, PHAS-I, through a mechanism involving enhanced phosphorylation of PHAS-I. However, the insulin-induced association of eIF-4E and eIF-4G was not due to increased net phosphorylation of eIF-4E because insulin decreased the amount present in the phosphorylated form from 86 to 59% of total eIF-4E. Overall, the results suggest that insulin stimulates protein synthesis in gastrocnemius muscle through a mechanism involving increased binding of eIF-4G to eIF-4E, which is in part due to phosphorylation of PHAS-I, resulting in a release of eIF-4E from the inactive PHAS-I x eIF-4E complex.