The initiation of translation in eukaryotes requires several multisubunit complexes, including eukaryotic translation initiation factor 4F (eIF4F). In higher eukaryotes eIF4F is composed of the cap binding protein eIF4E, the adapter protein eIF4G, and the RNA-stimulated ATPase eIF4A. The association of eIF4A with Saccharomyces cerevisiae eIF4F has not yet been demonstrated, and therefore the degree to which eIF4A’s conserved function relies upon this association has remained unclear. Here we report an interaction between yeast eIF4G and eIF4A. Specifically, we found that the growth arrest phenotype associated with three temperature-sensitive alleles of yeast eIF4G2 was suppressed by excess eIF4A and that this suppression was allele specific. In addition, in vitro translation extracts derived from an eIF4G2 mutant strain could be heat inactivated, and this inactivation could be reversed upon the addition of recombinant eIF4A. Finally, in vitro binding between yeast eIF4G and eIF4A was demonstrated, as was diminished binding between mutant eIF4G2 proteins and eIF4A. In total, these data indicate that yeast eIF4G and eIF4A physically associate and that this association performs an essential function.
The binding of mRNA to the ribosome is mediated by eukaryotic initiation factors eukaryotic initiation factor 4F (eIF4F), eIF4B, eIF4A, and eIF3, eIF4F binds to the mRNA cap structure and, in combination with eIF4B, is believed to unwind the secondary structure in the 5' untranslated region to facilitate ribosome binding. eIF3 associates with the 40S ribosomal subunit prior to mRNA binding. eIF4B copurifies with eIF3 and eIF4F through several purification steps, suggesting the involvement of a multisubunit complex during translation initiation. To understand the mechanism by which eIF4B promotes 40S ribosome binding to the mRNA, we studied its interactions with partner proteins by using a filter overlay (protein-protein [far Western]) assay and the two-hybrid system. In this report, we show that eIF4B self-associates and also interacts directly with the p170 subunit of eIF3. A region rich in aspartic acid, arginine, tyrosine, and glycine, termed the DRYG domain, is sufficient for self-association of eIF4B, both in vitro and in vivo, and for interaction with the p170 subunit of eIF3. These experiments suggest that eIF4B participates in mRNA-ribosome binding by acting as an intermediary between the mRNA and eIF3, via a direct interaction with the p170 subunit of eIF3.
The short-lived proto-oncoprotein c-Fos is a component of the activator protein 1 (AP-1) transcription factor. A large region of c-Fos is intrinsically unstructured and susceptible to a recently characterized proteasomal ubiquitin-independent degradation (UID) pathway. UID is active by a default mechanism that is inhibited by NAD(P)H:quinone oxidoreductase 1 (NQO1), a 20S proteasome gatekeeper. Here, we show that NQO1 binds and induces robust c-Fos accumulation by blocking the UID pathway. c-Jun, a partner of c-Fos, also protects c-Fos from proteasomal degradation by default. Our findings suggest that NQO1 protects monomeric c-Fos from proteasomal UID, a function that is fulfilled later by c-Jun. We show that this process regulates c-Fos homeostasis (proteostasis) in response to serum stimulation, phosphorylation, nuclear translocation, and transcription activity. In addition, we show that NQO1 is important to ensure immediate c-Fos accumulation in response to serum, since a delayed response was observed under low NQO1 expression. These data suggest that in vivo, protein unstructured regions determine the kinetics and the homeostasis of regulatory proteins. Our data provide evidence for another layer of regulation of key regulatory proteins that functions at the level of protein degradation and is designed to ensure optimal formation of functional complexes such as AP-1.
Mammalian eukaryotic translation initiation factor 4F (eIF4F) is a cap-binding protein complex consisting of three subunits: eIF4E, eIF4A, and eIF4G. In yeast and plants, two related eIF4G species are encoded by two different genes. To date, however, only one functional eIF4G polypeptide, referred to here as eIF4GI, has been identified in mammals. Here we describe the discovery and functional characterization of a closely related homolog, referred to as eIF4GII. eIF4GI and eIF4GII share 46% identity at the amino acid level and possess an overall similarity of 56%. The homology is particularly high in certain regions of the central and carboxy portions, while the amino-terminal regions are more divergent. Far-Western analysis and coimmunoprecipitation experiments were used to demonstrate that eIF4GII directly interacts with eIF4E, eIF4A, and eIF3. eIF4GII, like eIF4GI, is also cleaved upon picornavirus infection. eIF4GII restores cap-dependent translation in a reticulocyte lysate which had been pretreated with rhinovirus 2A to cleave endogenous eIF4G. Finally, eIF4GII exists as a complex with eIF4E in HeLa cells, because eIF4GII and eIF4E can be purified together by cap affinity chromatography. Taken together, our findings indicate that eIF4GII is a functional homolog of eIF4GI. These results may have important implications for the understanding of the mechanism of shutoff of host protein synthesis following picornavirus infection.
NAD(P)H:quinone oxidoreductase 1 (NQO1) is an FAD containing quinone reductase that catalyzes the 2-electron reduction of a broad range of quinones. The 2-electron reduction of quinones to hydroquinones by NQO1 is believed to be a detoxification process since this reaction bypasses the formation of the highly reactive semiquinone. NQO1 is expressed at high levels in normal epithelium, endothelium and adipocytes as well as in many human solid tumors. In addition to its function as a quinone reductase NQO1 has been shown to reduce superoxide and regulate the 20 S proteasomal degradation of proteins including p53. Biochemical studies have indicated that NQO1 is primarily located in the cytosol, however, lower levels of NQO1 have also been found in the nucleus. In these studies we demonstrate using immunocytochemistry and confocal imaging that NQO1 was found associated with mitotic spindles in cells undergoing division. The association of NQO1 with the mitotic spindles was observed in many different human cell lines including nontransformed cells (astrocytes, HUVEC) immortalized cell lines (HBMEC, 16HBE) and cancer (pancreatic adenocarcinoma, BXPC3). Confocal analysis of double-labeling experiments demonstrated co-localization of NQO1with alpha-tubulin in mitotic spindles. In studies with BxPc-3 human pancreatic cancer cells the association of NQO1 with mitotic spindles appeared to be unchanged in the presence of NQO1 inhibitors ES936 or dicoumarol suggesting that NQO1 can associate with the mitotic spindle and still retain catalytic activity. Analysis of archival human squamous lung carcinoma tissue immunostained for NQO1 demonstrated positive staining for NQO1 in the spindles of mitotic cells. The purpose of this study is to demonstrate for the first time the association of the quinone reductase NQO1 with the mitotic spindle in human cells.
Eukaryotic translation initiation factor 4F (eIF4F), comprising the cap-binding protein eIF4E, the helicase eIF4A, and the central scaffold eIF4G, is a convergence node for a complex signaling network that controls protein synthesis. Together with eIF3 and eIF4A/4B, eIF4G recruits ribosomal subunits to mRNAs and facilitates 5′ untranslated region unwinding. Mammalian eIF4G contains three HEAT domains and unstructured regions involved in protein-protein interactions. Despite detailed eIF4G structure data, the mechanisms controlling initiation scaffold formation remain obscure. We found a new, highly regulated eIF4B/-3 binding site within the HEAT-1/-2 interdomain linker, harboring two phosphorylation sites that we identified as substrates for Erk1/2 and casein kinase 2. Phorbol ester-induced sequential phosphorylation of both sites detached HEAT-2 from the complex with eIF4A/-4B/-3 and stimulated the association of HEAT-3 with the mitogen-activated protein kinase signal integrating kinase Mnk1. Our results provide a mechanistic link between intracellular signal transduction and dynamic initiation complex formation coordinated by flexible eIF4G structure.
Eukaryotic translation initiation factor 5 (eIF5) interacts with the 40S initiation complex (40S–eIF3–AUG–Met-tRNAf–eIF2–GTP) to promote the hydrolysis of ribosome-bound GTP. eIF5 also forms a complex with eIF2 by interacting with the β subunit of eIF2. In this work, we have used a mutational approach to investigate the importance of eIF5-eIF2β interaction in eIF5 function. Binding analyses with recombinant rat eIF5 deletion mutants identified the C terminus of eIF5 as the eIF2β-binding region. Alanine substitution mutagenesis at sites within this region defined several conserved glutamic acid residues in a bipartite motif as critical for eIF5 function. The E346A,E347A and E384A,E385A double-point mutations each caused a severe defect in the binding of eIF5 to eIF2β but not to eIF3-Nip1p, while a eIF5 hexamutant (E345A,E346A,E347A,E384A,E385A,E386A) showed negligible binding to eIF2β. These mutants were also severely defective in eIF5-dependent GTP hydrolysis, in 80S initiation complex formation, and in the ability to stimulate translation of mRNAs in an eIF5-dependent yeast cell-free translation system. Furthermore, unlike wild-type rat eIF5, which can functionally substitute for yeast eIF5 in complementing in vivo a genetic disruption of the chromosomal copy of the TIF5 gene, the eIF5 double-point mutants allowed only slow growth of this ΔTIF5 yeast strain, while the eIF5 hexamutant was unable to support cell growth and viability of this strain. These findings suggest that eIF5-eIF2β interaction plays an essential role in eIF5 function in eukaryotic cells.
One of the most regulated steps of translation initiation is the recruitment of mRNA by the translation machinery. In eukaryotes, this step is mediated by the 5′end cap-binding factor eIF4E bound to the bridge protein eIF4G and forming the eIF4F complex. In plants, different isoforms of eIF4E and eIF4G form the antigenically distinct eIF4F and eIF(iso)4F complexes proposed to mediate selective translation. Using a microarray analysis of polyribosome- and non-polyribosome-purified mRNAs from 15 day-old Arabidopsis thaliana wild type [WT] and eIF(iso)4E knockout mutant [(iso)4E-1] seedlings we found 79 transcripts shifted from polyribosomes toward non-polyribosomes, and 47 mRNAs with the opposite behavior in the knockout mutant. The translationally decreased mRNAs were overrepresented in root-preferentially expressed genes and proteins from the endomembrane system, including several transporters such as the phosphate transporter PHOSPHATE1 (PHO1), Sucrose transporter 3 (SUC3), ABC transporter-like with ATPase activity (MRP11) and five electron transporters, as well as signal transduction-, protein modification- and transcription-related proteins. Under normal growth conditions, eIF(iso)4E expression under the constitutive promoter 35 S enhanced the polyribosomal recruitment of PHO1 supporting its translational preference for eIF(iso)4E. Furthermore, under phosphate deficiency, the PHO1 protein increased in the eIF(iso)4E overexpressing plants and decreased in the knockout mutant as compared to wild type. In addition, the knockout mutant had larger root, whereas the 35 S directed expression of eIF(iso)4E caused shorter root under normal growth conditions, but not under phosphate deficiency. These results indicate that selective translation mediated by eIF(iso)4E is relevant for Arabidopsis root development under normal growth conditions.
Protein synthesis in eukaryotes initiates with binding of the multisubunit translation initiation complex eIF4F. This complex contains eIF4E, eIF4A and eIF4G. eIF4E directly interacts with the cap structure, eIF4A is an RNA helicase and eIF4G acts as a scaffold for the complex. eIF4G contains the binding sites for both the subunits i.e., eIF4A and eIF4E and it also interacts with poly(A)-binding protein (PABP). In present study we have identified and characterized the main components of the eIF4F complex i.e., eIF4E, eIF4A and eIF4G and PABP from Plasmodium falciparum. Molecular modeling of PfeIF4E, PfeIF4G and PfPABP confirms that they contain all the characteristic conserved structural features. We have annotated some of the genes of P. falciparum and as a result these studies demonstrate that the components of translation initiation complex are highly conserved. Therefore these studies will contribute to understand the basic biology and components of translation complex in P. falciparum.
mRNA; malaria; poly(A)-binding protein; protein synthesis; RNA binding; translation; translation initiation factor 4A; translation initiation factor 4E; translation initiation factor 4G; RNA unwinding
Ribosome binding to eukaryotic mRNA is a multistep process which is mediated by the cap structure [m7G(5′)ppp(5′)N, where N is any nucleotide] present at the 5′ termini of all cellular (with the exception of organellar) mRNAs. The heterotrimeric complex, eukaryotic initiation factor 4F (eIF4F), interacts directly with the cap structure via the eIF4E subunit and functions to assemble a ribosomal initiation complex on the mRNA. In mammalian cells, eIF4E activity is regulated in part by three related translational repressors (4E-BPs), which bind to eIF4E directly and preclude the assembly of eIF4F. No structural counterpart to 4E-BPs exists in the budding yeast, Saccharomyces cerevisiae. However, a functional homolog (named p20) has been described which blocks cap-dependent translation by a mechanism analogous to that of 4E-BPs. We report here on the characterization of a novel yeast eIF4E-associated protein (Eap1p) which can also regulate translation through binding to eIF4E. Eap1p shares limited homology to p20 in a region which contains the canonical eIF4E-binding motif. Deletion of this domain or point mutation abolishes the interaction of Eap1p with eIF4E. Eap1p competes with eIF4G (the large subunit of the cap-binding complex, eIF4F) and p20 for binding to eIF4E in vivo and inhibits cap-dependent translation in vitro. Targeted disruption of the EAP1 gene results in a temperature-sensitive phenotype and also confers partial resistance to growth inhibition by rapamycin. These data indicate that Eap1p plays a role in cell growth and implicates this protein in the TOR signaling cascade of S. cerevisiae.
Translation initiation factor eIF-4E, which binds to the 5' cap structure of eukaryotic mRNAs, is believed to play an important role in the control of cell growth. Consistent with this, overexpression of eIF-4E in fibroblasts results in their malignant transformation. The activity of eIF-4E is thought to be regulated by phosphorylation on a single serine residue (Ser-53). Treatment of rat pheochromocytoma (PC12) cells with nerve growth factor (NGF) strongly curtails their growth and causes their differentiation into cells that resemble sympathetic neurons. The present study shows that eIF-4E is rapidly phosphorylated in PC12 cells upon NGF treatment, resulting in a significant increase in the steady-state levels of the phosphorylated protein. In contrast, epidermal growth factor, a factor which elicits a weak mitogenic response in PC12 cells, did not significantly enhance eIF-4E phosphorylation. We also show that although the mitogen and tumor promoter, phorbol 12-myristate-13-acetate, is able to induce phosphorylation of eIF-4E in PC12 cells, the NGF-mediated increase is primarily a protein kinase C-independent response. The NGF-induced enhancement of eIF-4E phosphorylation is abrogated in PC12 cells expressing a dominant inhibitory ras mutant (Ser-17 replaced by Asn), indicating that eIF-4E phosphorylation is dependent on a ras signalling pathway. As phosphorylation of eIF-4E effects translation initiation, these results suggest that NGF-mediated and ras-dependent eIF-4E phosphorylation may play a role in switching the pattern of gene expression during the differentiation of PC12 cells.
The eukaryotic initiation factor (eIF) 4GI gene locus (eIF4GI) contains three identified promoters, generating alternately spliced mRNAs, yielding a total of five eIF4GI protein isoforms. Although eIF4GI plays a critical role in mRNA recruitment to the ribosomes, little is known about the functions of the different isoforms, their partner binding capacities, or the role of the homolog, eIF4GII, in translation initiation. To directly address this, we have used short interfering RNAs (siRNAs) expressed from DNA vectors to silence the expression of eIF4GI in HeLa cells. Here we show that reduced levels of specific mRNA and eIF4GI isoforms in HeLa cells promoted aberrant morphology and a partial inhibition of translation. The latter reflected dephosphorylation of 4E-BP1 and decreased eIF4F complex levels, with no change in eIF2α phosphorylation. Expression of siRNA-resistant Myc-tagged eIF4GI isoforms has allowed us to show that the different isoforms exhibit significant differences in their ability to restore translation rates. Here we quantify the efficiency of eIF4GI promoter usage in mammalian cells and demonstrate that even though the longest isoform of eIF4GI (eIF4GIf) was relatively poorly expressed when reintroduced, it was more efficient at promoting the translation of cellular mRNAs than the more highly expressed shorter isoforms used in previous functional studies.
During translation initiation in Saccharomyces cerevisiae, an Arg- and Ser-rich segment (RS1 domain) of eukaryotic translation initiation factor 4G (eIF4G) and the Lys-rich segment (K-boxes) of eIF2β bind three common partners, eIF5, eIF1, and mRNA. Here, we report that both of these segments are involved in mRNA recruitment and AUG recognition by distinct mechanisms. First, the eIF4G-RS1 interaction with the eIF5 C-terminal domain (eIF5-CTD) directly links eIF4G to the preinitiation complex (PIC) and enhances mRNA binding. Second, eIF2β-K-boxes increase mRNA binding to the 40S subunit in vitro in a manner reversed by the eIF5-CTD. Third, mutations altering eIF4G-RS1, eIF2β-K-boxes, and eIF5-CTD restore the accuracy of start codon selection impaired by an eIF2β mutation in vivo, suggesting that the mutual interactions of the eIF segments within the PIC prime the ribosome for initiation in response to start codon selection. We propose that the rearrangement of interactions involving the eIF5-CTD promotes mRNA recruitment through mRNA binding by eIF4G and eIF2β and assists the start codon-induced release of eIF1, the major antagonist of establishing tRNAiMet:mRNA binding to the P site.
NAD(P)H:quinone oxidoreductase 1 (NQO1) is a cytosolic enzyme that protects cells against chemical and radiation-induced oxidative stress and skin cancer. Disruption of NQO1 gene in mice showed thinning of skin epithelium and loss of cytokeratin 14, an early marker of skin differentiation. Immunohistochemistry and western analysis demonstrated downregulation of p63 in NQO1−/− mouse skin, as compared with wild-type (WT) mouse. Further analysis including modulation of NQO1 expression revealed a direct correlation between the levels of NQO1 and p63 in skin-derived keratinocytes and dermal fibroblasts. Modulation of proteasomal activity revealed that p63 is degraded by 20S proteasome and that this degradation is significantly rescued by NQO1. Coimmunoprecipitation studies showed that NQO1 interacts directly with p63 but not 20S to protect against this degradation. In addition, benzo[a]pyrene treatment led to induction of NQO1 and stabilization of p63 inWT but not in NQO1−/− mouse skin and keratinocytes. These data suggest that NQO1 controls stabilization of p63 and progression towards keratinocyte differentiation leading to normal skin development and presumably skin carcinogenesis.
NQO1; p63; 20S proteasome; skin differentiation; carcinogenicity
The herpes simplex virus Vhs endonuclease degrades host and viral mRNAs. Isolated Vhs cuts any RNA at many sites. Yet, within cells, it targets mRNAs and cuts at preferred sites, including regions of translation initiation. Previous studies have shown that Vhs binds the translation factors eIF4A and eIF4H. Here, we show that Vhs binds the cap-binding complex eIF4F. Association with eIF4F correlated with the ability of Vhs to bind eIF4A but not eIF4H. All Vhs proteins that degrade mRNAs associated with eIF4F. However, simply tethering an active endonuclease to eIF4F is not sufficient to degrade mRNAs. Binding to eIF4H may also be required.
Cell growth and proliferation require coordinated ribosomal biogenesis and translation. Eukaryotic Initiation Factors (eIF) control translation at the rate-limiting step of initiation1,2. So far, only two eIFs connect extracellular stimuli to global translation rates3; eIF4E acts in the eIF4F complex and regulates binding of capped mRNA to 40S subunits, downstream of growth factors4; eIF2 controls loading of the ternary complex on the 40S subunit and is inhibited upon stress stimuli5–6. No eIFs have been found to link extracellular stimuli to the activity of the large 60S ribosomal subunit. eIF6 binds 60S ribosomes precluding ribosome joining in vitro7–9. However studies in yeasts showed that eIF6 is required for ribosome biogenesis rather than translation10–13. We show that mammalian eIF6 is required for efficient initiation of translation, in vivo. eIF6 null embryos are lethal at preimplantation. Heterozygous mice have 50% reduction of eIF6 levels in all tissues, and show reduced mass of hepatic and adipose tissues due to a lower number of cells and to impaired G1/S cell cycle progression. eIF6+/− cells retain sufficient nucleolar eIF6 and normal ribosome biogenesis. The liver of eIF6+/− mice displays an increase of 80S in polysomal profiles, indicating a defect in initiation of translation. Consistently, isolated hepatocytes have impaired insulin-stimulated translation. Heterozygous mouse embryonic fibroblasts (MEFs) recapitulate the organism phenotype and have normal ribosome biogenesis, reduced insulin-stimulated translation, and delayed G1/S phase progression. Furthermore, eIF6+/− cells resist to oncogene-induced transformation. Thus, eIF6 is the first eIF associated with the large 60S subunit that regulates translation in response to extracellular signals.
Eukaryotic translation initiation factor 2B (eIF2B) is the guanine nucleotide exchange factor for protein synthesis initiation factor 2 (eIF2). Composed of five subunits, it converts eIF2 from a GDP-bound form to the active eIF2-GTP complex. This is a regulatory step of translation initiation. In vitro, eIF2B catalytic function can be provided by the largest (epsilon) subunit alone (eIF2Bɛ). This activity is stimulated by complex formation with the other eIF2B subunits. We have analyzed the roles of different regions of eIF2Bɛ in catalysis, in eIF2B complex formation, and in binding to eIF2 by characterizing mutations in the Saccharomyces cerevisiae gene encoding eIF2Bɛ (GCD6) that impair the essential function of eIF2B. Our analysis of nonsense mutations indicates that the C terminus of eIF2Bɛ (residues 518 to 712) is required for both catalytic activity and interaction with eIF2. In addition, missense mutations within this region impair the catalytic activity of eIF2Bɛ without affecting its ability to bind eIF2. Internal, in-frame deletions within the N-terminal half of eIF2Bɛ disrupt eIF2B complex formation without affecting the nucleotide exchange activity of eIF2Bɛ alone. Finally, missense mutations identified within this region do not affect the catalytic activity of eIF2Bɛ alone or its interactions with the other eIF2B subunits or with eIF2. Instead, these missense mutations act indirectly by impairing the enhancement of the rate of nucleotide exchange that results from complex formation between eIF2Bɛ and the other eIF2B subunits. This suggests that the N-terminal region of eIF2Bɛ is an activation domain that responds to eIF2B complex formation.
For many members of the Picornaviridae family, infection of cells results in a shutoff of host protein synthesis. For rhinoviruses and enteroviruses, the shutoff has been explained in part by the cleavage of eukaryotic initiation factor 4GI (eIF4GI), a component of the cap-binding protein complex eIF4F. The cleavage of eIF4GI is mediated by the virus-specific proteinase 2Apro and results in inhibition of cap-dependent, but not cap-independent, translation. The inhibition of host protein synthesis after infection with human rhinovirus 14 (HRV-14) lags behind the cleavage of eIF4GI. Recently, we discovered a functional homolog of eIF4GI, termed eIF4GII, and showed that cleavage of eIF4GII coincides with the shutoff of host cell protein synthesis after poliovirus infection (Gradi et al., Proc. Natl. Acad. Sci. USA 95:11089–11094, 1998). We wished to determine whether eIF4GII cleavage kinetics could also explain the lack of correlation between the kinetics of eIF4GI cleavage and the shutoff of host protein synthesis after rhinovirus infection. In this study, we examined the correlation between human rhinovirus-induced shutoff of host protein synthesis and cleavage of eIF4GI and eIF4GII. In HRV-14-infected HeLa cells, almost no intact eIF4GI could be detected by 4 h postinfection, while only 4% of eIF4GII was cleaved at this time. By 6 h, however, 67% of eIF4GII was cleaved, and this cleavage coincided with a significant (60%) decline of host translation. These results suggest that cleavage of both eIF4GI and eIF4GII is required for HRV-mediated inhibition of host cell protein synthesis and that the cleavage of eIF4GII is the rate-limiting step in the shutoff of host cell protein synthesis after rhinovirus infection.
Eukaryotic mRNAs possess a 5′-terminal cap structure (cap), m7GpppN, which facilitates ribosome binding. The cap is bound by eukaryotic translation initiation factor 4F (eIF4F), which is composed of eIF4E, eIF4G, and eIF4A. eIF4E is the cap-binding subunit, eIF4A is an RNA helicase, and eIF4G is a scaffolding protein that bridges between the mRNA and ribosome. eIF4G contains an RNA-binding domain, which was suggested to stimulate eIF4E interaction with the cap in mammals. In Saccharomyces cerevisiae, however, such an effect was not observed. Here, we used recombinant proteins to reconstitute the cap binding of the mammalian eIF4E-eIF4GI complex to investigate the importance of the RNA-binding region of eIF4GI for cap interaction with eIF4E. We demonstrate that chemical cross-linking of eIF4E to the cap structure is dramatically enhanced by eIF4GI fragments possessing RNA-binding activity. Furthermore, the fusion of RNA recognition motif 1 (RRM1) of the La autoantigen to the N terminus of eIF4GI confers enhanced association between the cap structure and eIF4E. These results demonstrate that eIF4GI serves to anchor eIF4E to the mRNA and enhance its interaction with the cap structure.
Translation of m7G-capped cellular mRNAs is initiated by recruitment of ribosomes to the 5′ end of mRNAs via eukaryotic translation initiation factor 4F (eIF4F), a heterotrimeric complex comprised of a cap-binding subunit (eIF4E) and an RNA helicase (eIF4A) bridged by a scaffolding molecule (eIF4G). Internal translation initiation bypasses the requirement for the cap and eIF4E and occurs on viral and cellular mRNAs containing internal ribosomal entry sites (IRESs). Here we demonstrate that eIF4E availability plays a critical role in the switch from cap-dependent to IRES-mediated translation in picornavirus-infected cells. When both capped and IRES-containing mRNAs are present (as in intact cells or in vitro translation extracts), a decrease in the amount of eIF4E associated with the eIF4F complex elicits a striking increase in IRES-mediated viral mRNA translation. This effect is not observed in translation extracts depleted of capped mRNAs, indicating that capped mRNAs compete with IRES-containing mRNAs for translation. These data explain numerous reported observations where viral mRNAs are preferentially translated during infection.
Messenger RNA translation or protein synthesis, is a fundamental biological process affecting cell growth, survival and proliferation. Initiation is the rate limiting and hence the most regulated step of translation. In eukaryotes, translation initiation is facilitated by multiple protein factors collectively called eIFs (for eukaryotic translation initiation factors). The complex consisting of the eIF4 group factors including the mRNA cap-binding eIF4E protein, large scaffolding protein eIF4G and RNA helicase eIF4A is assisted by the eIF4B co-factor to unwind local secondary structures and create a ribosome landing pad on mRNA. Recruitment of the ribosome and augmentation in the mRNA scanning process culminates in the positioning of the ribosome over the start codon. Deregulated translational control is believed to play an important role in oncogenic transformation. Indeed, many eIFs are bona fide proto-oncogenes. In many types of human cancers, eIFs are either overexpressed or ectopically activated by Ras-MAPK and PI3K-mTOR signaling cascades, resulting in increased survival and accelerated proliferation. In this review we will analyze the bulk of data describing eIF4B and its role in cell survival and proliferation. Recent studies have shown that eIF4B is phosphorylated and activated by Ras-MAPK and PI3K-mTOR signaling cascades. In addition, eIF4B regulates translation of proliferative and pro-survival mRNAs. Moreover, eIF4B depletion in cancer cells attenuates proliferation, sensitizes them to genotoxic stress-driven apoptosis. Taken together, these findings identify eIF4B as a potential target for development of anti-cancer therapies.
eIF4B; translation; signaling; structured 5′UTR; helicase activity; survival; proliferation; apoptosis
In eukaryotes, a rate-limiting step of translation initiation is recognition of the mRNA 5′ m7GpppN cap structure by the eukaryotic initiation factor 4F (eIF4F), a heterotrimeric complex consisting of the cap-binding protein, eIF4E, along with eIF4G, and eIF4A. The eIF4E-binding proteins (4E-BPs) repress translation by disrupting eIF4F formation, thereby preventing ribosome recruitment to the mRNA. Of the three 4E-BPs, 4E-BP2 is the predominant paralog expressed in the mammalian brain and plays an important role in synaptic plasticity and learning and memory. 4E-BP2 undergoes asparagine deamidation, solely in the brain, during early postnatal development. Deamidation spontaneously converts asparagines into a mixture of aspartates or isoaspartates, the latter of which may be destabilizing to proteins. The enzyme protein l-isoaspartyl methyltransferase (PIMT) prevents isoaspartate accumulation by catalyzing the conversion of isoaspartates to aspartates. PIMT exhibits high activity in the brain, relative to other tissues. We report here that 4E-BP2 is a substrate for PIMT. In vitro deamidated 4E-BP2 accrues isoapartyl residues and is methylated by recombinant PIMT. Using an antibody that recognizes 4E-BP2, which harbors isoaspartates at the deamidation sites, Asn99 and Asn102, we demonstrate that 4E-BP2 in PIMT−/− brain lysates contains isoaspartate residues. Further, we show that 4E-BP2 containing isoaspartates lacks the augmented association with raptor that is a feature of deamidated 4E-BP2.
Brain; mTOR Complex (mTORC); Protein Methylation; S-Adenosylmethionine (SAM); Translation; 4E-BP2; Deamidation; Isoaspartate; Protein l-Isoaspartyl Methyltransferase (PIMT)
Eukaryotic translation initiation factor 4E (eIF4E) binds to the mRNA 5′ cap and brings the mRNA into a complex with other protein synthesis initiation factors and ribosomes. The activity of mammalian eIF4E is important for the translation of capped mRNAs and is thought to be regulated by two mechanisms. First, eIF4E is sequestered by binding proteins, such as 4EBP1, in quiescent cells. Mitogens induce the release of eIF4E by stimulating the phosphorylation of 4EBP1. Second, mitogens and stresses induce the phosphorylation of eIF4E at Ser 209, increasing the affinity of eIF4E for capped mRNA and for an associated scaffolding protein, eIF4G. We previously showed that a mitogen- and stress-activated kinase, Mnk1, phosphorylates eIF4E in vitro at the physiological site. Here we show that Mnk1 regulates eIF4E phosphorylation in vivo. Mnk1 binds directly to eIF4G and copurifies with eIF4G and eIF4E. We identified activating phosphorylation sites in Mnk1 and developed dominant-negative and activated mutants. Expression of dominant-negative Mnk1 reduces mitogen-induced eIF4E phosphorylation, while expression of activated Mnk1 increases basal eIF4E phosphorylation. Activated mutant Mnk1 also induces extensive phosphorylation of eIF4E in cells overexpressing 4EBP1. This suggests that phosphorylation of eIF4E is catalyzed by Mnk1 or a very similar kinase in cells and is independent of other mitogenic signals that release eIF4E from 4EBP1.
Increased eukaryotic translation initiation factor 4E (eIF4E) expression occurs in many cancers, and makes fundamental contributions to carcinogenesis by stimulating the expression of cancer-related genes at post-transcriptional levels. This key role is highlighted by the facts that eIF4E levels can predict prognosis, and that eIF4E is an established therapeutic target. However, eIF4E activity is a complex function of expression levels and phosphorylation statuses of eIF4E and eIF4E-binding proteins (4E-BPs). Our hypothesis was that the combined analyses of these pathway components would allow insights into eIF4E activity and its influence on cancer. We have determined expression levels of eIF4E, 4E-BP1, 4E-BP2 and phosphorylated 4E-BP1 within 424 breast tumours, and have carried out analyses to combine these and relate the product to patient survival, in order to estimate eIF4E activity. We show that this analysis gives greater prognostic insights than that of eIF4E alone. We show that eIF4E and 4E-BP expression are positively associated, and that 4E-BP2 has a stronger influence on cancer behaviour than 4E-BP1. Finally, we examine eIF4E, estimated eIF4E activity, and phosphorylated 4E-BP1 as potential predictive biomarkers for eIF4E-targeted therapies, and show that each determines selection of different patient groups. We conclude that eIF4E's influence on cancer survival is modulated substantially by 4E-BPs, and that combined pathway analyses can estimate functional eIF4E.
translation factors; predictive biomarkers; Cox regression; pathway biomarker
The translation initiation GTPase eukaryotic translation initiation factor 5B (eIF5B) binds to the factor eIF1A and catalyzes ribosomal subunit joining in vitro. We show that rapid depletion of eIF5B in Saccharomyces cerevisiae results in the accumulation of eIF1A and mRNA on 40S subunits in vivo, consistent with a defect in subunit joining. Substituting Ala for the last five residues in eIF1A (eIF1A-5A) impairs eIF5B binding to eIF1A in cell extracts and to 40S complexes in vivo. Consistently, overexpression of eIF5B suppresses the growth and translation initiation defects in yeast expressing eIF1A-5A, indicating that eIF1A helps recruit eIF5B to the 40S subunit prior to subunit joining. The GTPase-deficient eIF5B-T439A mutant accumulated on 80S complexes in vivo and was retained along with eIF1A on 80S complexes formed in vitro. Likewise, eIF5B and eIF1A remained associated with 80S complexes formed in the presence of nonhydrolyzable GDPNP, whereas these factors were released from the 80S complexes in assays containing GTP. We propose that eIF1A facilitates the binding of eIF5B to the 40S subunit to promote subunit joining. Following 80S complex formation, GTP hydrolysis by eIF5B enables the release of both eIF5B and eIF1A, and the ribosome enters the elongation phase of protein synthesis.