Phosphorylation of only serine-51 in protein synthesis initiation factor-2 is associated with inhibition of peptide-chain initiation in reticulocyte lysates

Origins of robustness in translational control via eukaryotic translation initiation factor (eIF) 2

Phosphorylation of eukaryotic translation initiation factor 2 (eIF2) is one of the best studied and most widely used means for regulating protein synthesis activity in eukaryotic cells. This pathway regulates protein synthesis in response to stresses, viral infections, and nutrient depletion, among others. We present analyses of an ordinary differential equation-based model of this pathway, which aim to identify its principal robustness-conferring features. Our analyses indicate that robustness is a distributed property, rather than arising from the properties of any one individual pathway species. However, robustness-conferring properties are unevenly distributed between the different species, and we identify a guanine nucleotide dissociation inhibitor (GDI) complex as a species that likely contributes strongly to the robustness of the pathway. Our analyses make further predictions on the dynamic response to different types of kinases that impinge on eIF2.

Enterovirus 71 induces dsRNA/PKR-dependent cytoplasmic redistribution of GRP78/BiP to promote viral replication

GRP78/BiP is an endoplasmic reticulum (ER) chaperone protein with the important function of maintaining ER homeostasis, and the overexpression of GRP78/BiP alleviates ER stress. Our previous studies showed that infection with enterovirus 71 (EV71), a (+)RNA picornavirus, induced GRP78/BiP upregulation; however, ectopic GRP78/BiP overexpression in ER downregulates virus replication and viral particle formation. The fact that a virus infection increases GRP78/BiP expression, which is unfavorable for virus replication, is counterintuitive. In this study, we found that the GRP78/BiP protein level was elevated in the cytoplasm instead of in the ER in EV71-infected cells. Cells transfected with polyinosinic-polycytidylic acid, a synthetic analog of replicative double-stranded RNA (dsRNA), but not with viral proteins, also exhibited upregulation and elevation of GRP78/BiP in the cytosol. Our results further demonstrate that EV71 infections induce the dsRNA/protein kinase R-dependent cytosolic accumulation of GRP78/BiP. The overexpression of a GRP78/BiP mutant lacking a KDEL retention signal failed to inhibit both dithiothreitol-induced eIF2α phosphorylation and viral replication in the context of viral protein synthesis and viral titers. These data revealed that EV71 infection might cause upregulation and aberrant redistribution of GRP78/BiP to the cytosol, thereby facilitating virus replication.

The role of NOS-mediated ROS accumulation in an early phase Cu-induced acute cytotoxicity in MCF-7 cells

Copper (Cu) ion is essential for the biological systems, however, high level of CuCl2 exposure causes detrimental effects, which leads to cell apoptosis. Nitric oxide (NO) is an efficient cell signal messenger, which plays an important role in cell apoptosis. However, the potential mechanism of an early phase Cu-induced acute cytotoxicity through the nitric oxide synthase (NOS) signaling pathway and its interaction has not been studied. In this report, we provide data showing that high level of CuCl2 could rapidly decrease the NO production with the release of Ca(2+) and Zn(2+), and then modulate the transcriptional and translational expression of NOSs in MCF-7 cells. The reactive oxygen species (ROS) level in cells was increased after high level of CuCl2 exposure, which led to the alpha subunit of eukaryotic initiation factor 2 phosphorylation. By using the free radical scavenger N-acetyl-L-cysteine or the NOS substrate L-arginine, it demonstrated that NOS played a critical role on the Cu-induced ROS generation, which further led to the oxidative stress and cell apoptosis. These results suggested that Cu-induced apoptosis was associated with the oxidative stress, and through the NOS-mediated signaling pathway.

IRES-mediated translation of cellular messenger RNA operates in eIF2α- independent manner during stress

Physiological and pathophysiological stress attenuates global translation via phosphorylation of eIF2α. This in turn leads to the reprogramming of gene expression that is required for adaptive stress response. One class of cellular messenger RNAs whose translation was reported to be insensitive to eIF2α phosphorylation-mediated repression of translation is that harboring an Internal Ribosome Entry Site (IRES). IRES-mediated translation of several apoptosis-regulating genes increases in response to hypoxia, serum deprivation or gamma irradiation and promotes tumor cell survival and chemoresistance. However, the molecular mechanism that allows IRES-mediated translation to continue in an eIF2α-independent manner is not known. Here we have used the X-chromosome linked Inhibitor of Apoptosis, XIAP, IRES to address this question. Using toeprinting assay, western blot analysis and polysomal profiling we show that the XIAP IRES supports cap-independent translation when eIF2α is phosphorylated both in vitro and in vivo. During normal growth condition eIF2α-dependent translation on the IRES is preferred. However, IRES-mediated translation switches to eIF5B-dependent mode when eIF2α is phosphorylated as a consequence of cellular stress.

Nitric oxide: a regulator of eukaryotic initiation factor 2 kinases

Generation of nitric oxide (NO(•)) can upstream induce and downstream mediate the kinases that phosphorylate the α subunit of eukaryotic initiation factor 2 (eIF2α), which plays a critical role in regulating gene expression. There are four known eIF2α kinases (EIF2AKs), and NO(•) affects each one uniquely. Whereas NO(•) directly activates EIF2AK1 (HRI), it indirectly activates EIF2AK3 (PERK). EIF2AK4 (GCN2) is activated by depletion of l-arginine, which is used by nitric oxide synthase (NOS) during the production of NO(•). Finally EIF2AK2 (PKR), which can mediate inducible NOS expression and therefore NO(•) production, can also be activated by NO(•). The production of NO(•) and activation of EIF2AKs coordinately regulate physiological and pathological events such as innate immune response and cell apoptosis.

A decrease in cellular energy status stimulates PERK-dependent eIF2alpha phosphorylation and regulates protein synthesis in pancreatic beta-cells

In the present study, we demonstrate that, in pancreatic beta-cells, eIF2alpha (eukaryotic initiation factor 2alpha) phosphorylation in response to a decrease in glucose concentration is primarily mediated by the activation of PERK [PKR (protein kinase RNA activated)-like endoplasmic reticulum kinase]. We provide evidence that this increase in PERK activity is evoked by a decrease in the energy status of the cell via a potentially novel mechanism that is independent of IRE1 (inositol requiring enzyme 1) activation and the accumulation of unfolded nascent proteins within the endoplasmic reticulum. The inhibition of eIF2alpha phosphorylation in glucose-deprived cells by the overexpression of dominant-negative PERK or an N-terminal truncation mutant of GADD34 (growth-arrest and DNA-damage-inducible protein 34) leads to a 53% increase in the rate of total protein synthesis. Polysome analysis revealed that this coincides with an increase in the amplitude but not the number of ribosomes per mRNA, indicating that eIF2alpha dephosphorylation mobilizes hitherto untranslated mRNAs on to polysomes. In summary, we show that PERK is activated at low glucose concentrations in response to a decrease in energy status and that this plays an important role in glucose-regulated protein synthesis in pancreatic beta-cells.

eIF2-dependent and eIF2-independent modes of initiation on the CSFV IRES: a common role of domain II

Specific interactions of the classical swine fever virus internal ribosomal entry site (IRES) with 40S ribosomal subunits and eukaryotic translation initiation factor (eIF)3 enable 43S preinitiation complexes containing eIF3 and eIF2-GTP-Met-tRNA(iMet) to bind directly to the initiation codon, yielding 48S initiation complexes. We report that eIF5B or eIF5B/eIF3 also promote Met-tRNA(iMet) binding to IRES-40S complexes, forming 48S complexes that can assemble elongation-competent ribosomes. Although 48S complexes assembled both by eIF2/eIF3- and eIF5B/eIF3-mediated Met-tRNA(iMet) recruitment were destabilized by eIF1, dissociation of 48S complexes formed with eIF2 could be out-competed by efficient subunit joining. Deletion of IRES domain II, which is responsible for conformational changes induced in 40S subunits by IRES binding, eliminated the sensitivity of 48S complexes assembled by eIF2/eIF3- and eIF5B/eIF3-mediated mechanisms to eIF1-induced destabilization. However, 48S complexes formed by the eIF5B/eIF3-mediated mechanism on the truncated IRES could not undergo efficient subunit joining, as reported previously for analogous complexes assembled with eIF2, indicating that domain II is essential for general conformational changes in 48S complexes, irrespective of how they were assembled, that are required for eIF5-induced hydrolysis of eIF2-bound GTP and/or subunit joining.

15-Deoxyspergualin Primarily Targets the Trafficking of Apicoplast Proteins in Plasmodium falciparum

15-Deoxyspergualin, an immunosuppressant with tumoricidal and antimalarial properties, has been implicated in the inhibition of a diverse array of cellular processes including polyamine synthesis and protein synthesis. Endeavoring to identify the mechanism of antimalarial action of this molecule, we examined its effect on Plasmodium falciparum protein synthesis, polyamine biosynthesis, and transport. 15-Deoxyspergualin stalled protein synthesis in P. falciparum through Hsp70 sequestration and subsequent phosphorylation of the eukaryotic initiation factor eIF2alpha. However, protein synthesis inhibition as well as polyamine depletion were invoked only by high micromolar concentrations of 15-deoxyspergualin, in contrast to the submicromolar concentrations sufficient to inhibit parasite growth. Further investigations demonstrated that 15-deoxyspergualin in the malaria parasite primarily targets the hitherto underexplored process of trafficking of nucleus-encoded proteins to the apicoplast. Our finding that 15-deoxyspergualin kills the malaria parasite by interfering with targeting of nucleus-encoded proteins to the apicoplast not only exposes a chink in the armor of the malaria parasite, but also reveals new realms in our endeavors to study this intriguing biological process.

15-deoxyspergualin inhibits eukaryotic protein synthesis through eIF2alpha phosphorylation

DSG (15-deoxyspergualin), an immunosuppressant with tumoricidal properties, binds potently to the regulatory C-terminal 'EEVD' motif of Hsps (heat-shock proteins). In the present study we demonstrate that DSG inhibits eukaryotic protein synthesis by sequestering Hsp70 which is required for maintaining HRI (haem-regulated inhibitor), a kinase of the eIF2alpha (eukaryotic initiation factor 2alpha), inactive. DSG stalled initiation of protein synthesis through phosphorylation of HRI and eIF2alpha. Addition of a recombinant eIF2alpha (S51A) protein, which lacks the phosphorylation site, lowered the inhibitory potential of DSG in reticulocyte lysate. The inhibitory effect of DSG was also attenuated in HRI knockdown cells. Moreover, exogenous addition of Hsp70 or the peptide 'EEVD' reversed the inhibitory effect of DSG. Interestingly, the inhibitory effect of DSG in different mammalian cancer cells was found to negatively correlate with the amount of Hsp70 expressed in the cells, emphasizing the link with Hsp70 in DSG inhibition of eukaryotic translation.

The roles of translation initiation regulation in ultraviolet light-induced apoptosis

Ultraviolet light (UV) inhibits translation initiation through activation of kinases that phosphorylate the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha). Two eIF2alpha kinases, PERK and GCN2, are known to phosphorylate the Serine-51 of eIF2alpha in response to UV-irradiation. In this report, we present evidence that phosphorylation of eIF2alpha plays a role in UV-induced apoptosis. Our data show that wild-type mouse embryo fibroblasts (MEF(s/s)) are less sensitive to UV-induced apoptosis than MEF(A/A) cells in which the phosphorylation site, Ser51, of eIF2alpha is replaced with a non-phosphorylatable Ala (Ser51Ala). PARP expression in MEF(A/A) cells is reduced without being cleaved after UV-irradiation. In contrast, PARP is cleaved without a significant decrease in parental PARP in MEF(S/S) cells after UV-irradiation. Our data also show that MEF(GCN2-/-) cells, in which GCN2 is knocked out, are more sensitive to UV-irradiation, agreeing with the observation from MEF(A/A) cells. However, MEF(PERK-/-) cells, in which PERK is knocked out, are less sensitive to UV-irradiation. In addition, MCF-7-PERKDeltaC cells, which are stably transfected with a kinase domain deleted mutant of PERK (PERKDeltaC), are more resistant to UV-induced apoptosis than parental MCF-7 cells. Overexpression of wild-type PERK sensitizes MCF-7 cells to UV-induced apoptosis without directly inducing cell death. These results suggest that the level of eIF2alpha phosphorylation impacts PARP expression upon UV-irradiation. The eIF2alpha kinases may mediate UV-induced apoptosis via an eIF2alpha dependent or independent signaling pathway.

The cowpox virus host range gene, CP77, affects phosphorylation of eIF2 alpha and vaccinia viral translation in apoptotic HeLa cells

Host restriction of vaccinia virus has been previously described in CHO and RK13 cells in which a cowpox virus CP77 gene rescues vaccinia virus growth at the viral protein translation level. Here we investigate the restrictive stage of vaccinia virus in HeLa cells using a vaccinia mutant virus (VV-hr) that contains a deletion of 18-kb genome sequences resulting in no growth in HeLa cells. Insertion of CP77 gene into VV-hr generated a recombinant virus (VV-36hr) that multiplied well in HeLa cells. Both viruses could enter cells, initiate viral DNA replication and intermediate gene transcription. However, translation of viral intermediate gene was only detected in cells infected with VV-36hr, indicating that CP77 relieves host restriction at the intermediate gene translation stage in HeLa cells. Caspase-2 and -3 activation was observed in HeLa cells infected with VV-hr coupled with dramatic morphological alterations and cleavage of the translation initiation factor eIF4G. Caspase activation was reduced in HeLa cells infected with VV-36hr, indicating that CP77 acts upstream of caspase activation. Enhanced phosphorylation of PKR and eIF2alpha was also observed in cells infected with VV-hr and was suppressed by CP77. Suppression of eIF4G cleavage with the caspase inhibitor ZVAD did not rescue virus translation, whereas expression of a mutant eIF2alpha protein with an alanine substitution of serine at amino acid position 51 (eIF2alphaS51A) partially restored viral translation and moderately increased virus growth in HeLa cells.

Ultraviolet light activates NFkappaB through translational inhibition of IkappaBalpha synthesis

UV light induces a delayed and prolonged (3-20 h) activation of NFkappaB when compared with the immediate and acute (10-90 min) activation of NFkappaB in response to tumor necrosis factor alpha treatment. In the early phase (3-12 h) of NFkappaB activation, UV light reduces inhibitor of NFkappaB (IkappaB) through an IkappaB kinase-independent, but polyubiquitin-dependent, pathway. However, the mechanism for the UV light-induced reduction of IkappaB and activation of NFkappaB is not known. In this report, we show that UV light down-regulates the total amount of IkappaB through decreasing IkappaB mRNA translation. Our data show that UV light inhibits translation of IkappaB in wild-type mouse embryo fibroblasts (MEF(S/S)) and that this inhibition is prevented in MEF(A/A) cells in which the phosphorylation site, Ser-51 in the eukaryotic translation initiation factor 2 alpha-subunit, is replaced with a non-phosphorylatable Ala (S51A). Our data also show that UV light-induced NFkappaB activation is delayed in MEF(A/A) cells and in an MCF-7 cell line that is stably transfected with a trans-dominant negative mutant protein kinase-like endoplasmic reticulum kinase (PERK). These results suggest that UV light-induced eukaryotic translation initiation factor 2 alpha-subunit phosphorylation translationally inhibits new IkappaB synthesis. Without a continuous supply of newly synthesized IkappaB, the existing IkappaB is degraded through a polyubiquitin-dependent proteasomal pathway leading to NFkappaB activation. Based upon our results, we propose a novel mechanism by which UV light regulates early phase NFkappaB activation by means of an ER-stress-induced translational inhibition pathway.

Identification of domains within the epsilon-subunit of the translation initiation factor eIF2B that are necessary for guanine nucleotide exchange activity and eIF2B holoprotein formation

Eukaryotic translation initiation factor, eIF2B, is a guanine nucleotide exchange factor (GEF) composed of five dissimilar subunits. eIF2B is important for regenerating GTP-bound eIF2 during the initiation process. This event is obligatory for eIF2 to bind initiator methionyl-tRNA, forming the ternary initiation complex. In the current investigation, deletion mutants of the catalytic subunit, eIF2B epsilon, were constructed to identify regions that are necessary for eIF2B catalytic activity and formation of the holoprotein. We used the baculovirus expression system to coexpress wild-type and truncated forms of the epsilon-subunit of mammalian eIF2B (eIF2B epsilon) with the other four subunits (alpha, beta, gamma, delta) of the protein in Sf9 cells. Removal of either the N- or the C-terminal conserved domains of eIF2B epsilon resulted in a significant loss of GEF activity and reduced or abolished interaction with the alpha-, gamma- and delta-subunits of eIF2B. Removal of the C-terminal 552 amino acids of eIF2B epsilon markedly reduced its interaction with the beta-subunit of eIF2 whereas loss of the N-terminal 431 amino acids did not. The results suggest that intact eIF2B epsilon is required for full catalytic activity and formation of the eIF2B holoprotein. In contrast, the C-terminal domain of eIF2B epsilon is sufficient alone for binding the beta-subunit of its substrate, eIF2, in vitro.

Regulation of eukaryotic initiation factor eIF2B: glycogen synthase kinase-3 phosphorylates a conserved serine which undergoes dephosphorylation in response to insulin

Eukaryotic initiation factor eIF2B catalyses a key regulatory step in mRNA translation. eIF2B and total protein synthesis are acutely activated by insulin, and this requires phosphatidylinositol 3-kinase (PI 3-kinase). The epsilon-subunit of eIF2B is phosphorylated by glycogen synthase kinase-3 (GSK-3), which is inactivated by insulin in a PI 3-kinase-dependent manner. Here we identify the phosphorylation site in eIF2Bepsilon as Ser540 and show that treatment of eIF2B with GSK-3 inhibits its activity. Ser540 is phosphorylated in intact cells and undergoes dephosphorylation in response to insulin. This is blocked by PI 3-kinase inhibitors. Insulin-induced dephosphorylation of this inhibitory site in eIF2B seems likely to be important in the overall activation of translation by this hormone.

Regulation of protein synthesis in Swiss 3T3 fibroblasts. Rapid activation of the guanine-nucleotide-exchange factor by insulin and growth factors

Insulin, whole serum, phorbol esters and epidermal growth factor each rapidly stimulate protein synthesis in serum-depleted Swiss 3T3 fibroblasts. The activation of protein synthesis by each of these agents is associated with stimulation of the activity of the guanine-nucleotide-exchange factor (GEF). This protein recycles the initiation factor eIF-2 by promoting exchange of GDP bound to eIF-2 for GTP. Activation of GEF is rapid, becoming maximal within 15 min. The degree of activation of GEF by these stimuli (to greater than 170% of control for insulin, serum or epidermal growth factor; 120% for phorbol dibutyrate) is more than enough to account for their effects on the overall rate of translation. Stimulation of protein synthesis and GEF activity occurs at low nanomolar insulin concentrations, indicating they are mediated through the insulin receptor. The best-characterized mechanism for regulating GEF activity is through changes in the phosphorylation of the smallest subunit of eIF-2 (eIF-2 alpha); however, none of the stimuli studied altered the level of phosphorylation of eIF-2 alpha in Swiss fibroblasts. It seems that direct regulation of GEF activity may be occurring here, and possible mechanisms for this are discussed.

Protein synthesis and the components of protein synthetic machinery during cellular aging

The slowing down of protein synthesis is a change widely observed during the aging of organisms. It has also been claimed that a decline in the rate of protein synthesis occurs during cellular aging. However, the evidence in favour of this view is not clear-cut, and reliable estimates of rates of protein synthesis during cellular aging have yet to be made. Studies on various components of the protein synthetic machinery during cellular aging have revealed a decline in the efficiency and accuracy of ribosomes, an increase in the levels of rRNA and tRNA, and a decrease in the amounts and activities of elongation factors. Detailed studies on the structure and function of ribosomes, tRNA isoacceptor profiles, activities of aminoacyl-tRNA synthetases, levels and activities of initiation factors, rates of protein elongation, and the accuracy of protein synthesis will be needed before the molecular mechanisms of the regulation of protein synthesis during cellular aging can be understood.