Cocrystal structure of the messenger RNA 5' cap-binding protein (eIF4E) bound to 7-methyl-GDP

The alveolate translation initiation factor 4E family reveals a custom toolkit for translational control in core dinoflagellates

Background

Dinoflagellates are eukaryotes with unusual cell biology and appear to rely on translational rather than transcriptional control of gene expression. The eukaryotic translation initiation factor 4E (eIF4E) plays an important role in regulating gene expression because eIF4E binding to the mRNA cap is a control point for translation. eIF4E is part of an extended, eukaryote-specific family with different members having specific functions, based on studies of model organisms. Dinoflagellate eIF4E diversity could provide a mechanism for dinoflagellates to regulate gene expression in a post-transcriptional manner. Accordingly, eIF4E family members from eleven core dinoflagellate transcriptomes were surveyed to determine the diversity and phylogeny of the eIF4E family in dinoflagellates and related lineages including apicomplexans, ciliates and heterokonts.

Results

The survey uncovered eight to fifteen (on average eleven) different eIF4E family members in each core dinoflagellate species. The eIF4E family members from heterokonts and dinoflagellates segregated into three clades, suggesting at least three eIF4E cognates were present in their common ancestor. However, these three clades are distinct from the three previously described eIF4E classes, reflecting diverse approaches to a central eukaryotic function. Heterokonts contain four clades, ciliates two and apicomplexans only a single recognizable eIF4E clade. In the core dinoflagellates, the three clades were further divided into nine sub-clades based on the phylogenetic analysis and species representation. Six of the sub-clades included at least one member from all eleven core dinoflagellate species, suggesting duplication in their shared ancestor. Conservation within sub-clades varied, suggesting different selection pressures.

Conclusions

Phylogenetic analysis of eIF4E in core dinoflagellates revealed complex layering of duplication and conservation when compared to other eukaryotes. Our results suggest that the diverse eIF4E family in core dinoflagellates may provide a toolkit to enable selective translation as a strategy for controlling gene expression in these enigmatic eukaryotes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0301-9) contains supplementary material, which is available to authorized users.

Global mRNA selection mechanisms for translation initiation

Background

The selection and regulation of individual mRNAs for translation initiation from a competing pool of mRNA are poorly understood processes. The closed loop complex, comprising eIF4E, eIF4G and PABP, and its regulation by 4E-BPs are perceived to be key players. Using RIP-seq, we aimed to evaluate the role in gene regulation of the closed loop complex and 4E-BP regulation across the entire yeast transcriptome.

Results

We find that there are distinct populations of mRNAs with coherent properties: one mRNA pool contains many ribosomal protein mRNAs and is enriched specifically with all of the closed loop translation initiation components. This class likely represents mRNAs that rely heavily on the closed loop complex for protein synthesis. Other heavily translated mRNAs are apparently under-represented with most closed loop components except Pab1p. Combined with data showing a close correlation between Pab1p interaction and levels of translation, these data suggest that Pab1p is important for the translation of these mRNAs in a closed loop independent manner. We also identify a translational regulatory mechanism for the 4E-BPs; these appear to self-regulate by inhibiting translation initiation of their own mRNAs.

Conclusions

Overall, we show that mRNA selection for translation initiation is not as uniformly regimented as previously anticipated. Components of the closed loop complex are highly relevant for many mRNAs, but some heavily translated mRNAs interact poorly with this machinery. Therefore, alternative, possibly Pab1p-dependent mechanisms likely exist to load ribosomes effectively onto mRNAs. Finally, these studies identify and characterize a complex self-regulatory circuit for the yeast 4E-BPs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0559-z) contains supplementary material, which is available to authorized users.

Translational control by oncogenic signaling pathways

Messenger RNA (mRNA) translation is highly regulated in cells and plays an integral role in the overall process of gene expression. The initiation phase of translation is considered to be the most rate-limiting and is often targeted by oncogenic signaling pathways to promote global protein synthesis and the selective translation of tumor-promoting mRNAs. Translational control is a crucial component of cancer development as it allows cancer cells to adapt to the altered metabolism that is generally associated with the tumor state. The phosphoinositide 3-kinase (PI3K)/Akt and Ras/mitogen-activated protein kinase (MAPK) pathways are strongly implicated in cancer etiology, and they exert their biological effects by modulating both global and specific mRNA translation. In addition to having respective translational targets, these pathways also impinge on the mechanistic/mammalian target of rapamycin (mTOR), which acts as a critical signaling node linking nutrient sensing to the coordinated regulation of cellular metabolism. mTOR is best known as a central regulator of protein synthesis and has been implicated in an increasing number of pathological conditions, including cancer. In this article, we describe the current knowledge on the roles and regulation of mRNA translation by various oncogenic signaling pathways, as well as the relevance of these molecular mechanisms to human malignancies. This article is part of a Special Issue entitled: Translation and cancer.

Five eIF4E isoforms from Arabidopsis thaliana are characterized by distinct features of cap analogs binding

The assembly of the ribosome on majority of eukaryotic mRNAs is initiated by the recruitment of eIF4E protein to the mRNA 5' end cap structure. Flowering plants use two eIF4E isoforms, named eIF4E and eIF(iso)4E, as canonical translation initiation factors and possess a homolog of mammalian 4EHP (or eIF4E-2) termed nCBP. Plants from Brassicaceae family additionally conserve a close paralog of eIF4E which in Arabidopsis thaliana has two copies named eIF4E1b and eIF4E1c. In order to assess the efficiency of plant non-canonical (eIF4E1b/1c and nCBP) and canonical (eIF4E and eIF(iso)4E) eIF4E proteins to bind mRNAs we utilized fluorescence titrations to determine accurate binding affinities of five A.thaliana eIF4E isoforms for a series of cap analogs. We found that eIF4E binds cap analogs from 4-fold to 10-fold stronger than eIF(iso)4E, while binding affinities of nCBP and eIF(iso)4E are comparable. Furthermore, eIF4E1c interacts similarly strongly with the cap as eIF4E, but eIF4E1b binds cap analogs ca. 2-fold weaker than eIF4E1c, regardless of the 95% sequence identity between these two proteins. The use of differentially chemically modified cap analogs in binding studies and a detailed analysis of the obtained homology models gave us insight into the molecular characteristic of varying cap-binding abilities of Arabidopsis eIF4E isoforms.

Distinct features of cap binding by eIF4E1b proteins

eIF4E1b, closely related to the canonical translation initiation factor 4E (eIF4E1a), cap-binding protein is highly expressed in mouse, Xenopus and zebrafish oocytes. We have previously characterized eIF4E1b as a component of the CPEB mRNP translation repressor complex along with the eIF4E-binding protein 4E-Transporter, the Xp54/DDX6 RNA helicase and additional RNA-binding proteins. eIF4E1b exhibited only very weak interactions with m⁷GTP-Sepharose and, rather than binding eIF4G, interacted with 4E-T. Here we undertook a detailed examination of both Xenopus and human eIF4E1b interactions with cap analogues using fluorescence titration and homology modeling. The predicted structure of eIF4E1b maintains the α + β fold characteristic of eIF4E proteins and its cap-binding pocket is similarly arranged by critical amino acids: Trp56, Trp102, Glu103, Trp166, Arg112, Arg157 and Lys162 and residues of the C-terminal loop. However, we demonstrate that eIF4E1b is 3-fold less well able to bind the cap than eIF4E1a, both proteins being highly stimulated by methylation at N⁷ of guanine. Moreover, eIF4E1b proteins are distinguishable from eIF4E1a by a set of conserved amino acid substitutions, several of which are located near to cap-binding residues. Indeed, eIF4E1b possesses several distinct features, namely, enhancement of cap binding by a benzyl group at N⁷ position of guanine, a reduced response to increasing length of the phosphate chain and increased binding to a cap separated by a linker from Sepharose, suggesting differences in the arrangement of the protein's core. In agreement, mutagenesis of the amino acids differentiating eIF4E1b from eIF4E1a reduces cap binding by eIF4E1a 2-fold, demonstrating their role in modulating cap binding.

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Sequence analysis of vertebrate eIF4E1a and eIF4E1b proteins identified a set of conserved substitutions, including those near to cap-binding residues.

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The fluorescence titration assay revealed that human and Xenopus eIF4E1b have 3-fold lower affinity for m⁷GTP than the eIF4E1a proteins.

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Additional distinct features of cap binding by eIF4E1b suggest differences in the arrangement of the protein's core and its C-terminal loop.

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Mutagenesis of the distinguishing amino acids reduced cap binding by eIF4E1a 2-fold, demonstrating their role in modulating affinity to m⁷GTP.

Mnks, eIF4E phosphorylation and cancer

The MAP kinase signal-integrating kinases or MAP kinase-interacting protein kinases (Mnks) are activated by signaling through the oncogenic MAP kinase (ERK) pathway. The best-known Mnk substrate is eukaryotic initiation factor eIF4E, the protein which binds the 5'-cap structure of eukaryotic mRNAs and helps to recruit ribosomes to them. eIF4E is a well-established proto-oncogene, whose expression or activation is associated with transformation and tumorigenesis. Mnks phosphorylate eIF4E at a single site. Increasing evidence implicates the Mnks and/or phosphorylation of eIF4E in cell transformation, tumorigenesis or tumor progression, in a growing range of settings. Mnks and/or the phosphorylation of eIF4E have been suggested to regulate the expression of proteins involved in cell cycle progression, cell survival and cell motility. Further work is needed to extend our understanding of the impact of the Mnks on gene expression, explore the biochemical mechanisms involved and evaluate the utility of targeting the Mnks in cancer therapy. This article is part of a Special Issue entitled: Translation and Cancer.

Triazole-containing monophosphate mRNA cap analogs as effective translation inhibitors

Synthetic analogs of the 5' end of mRNA (cap structure) are widely used in molecular studies on mechanisms of cellular processes such as translation, intracellular transport, splicing, and turnover. The best-characterized cap binding protein is translation initiation factor 4E (eIF4E). Recognition of the mRNA cap by eIF4E is a critical, rate-limiting step for efficient translation initiation and is considered a major target for anticancer therapy. Here, we report a facile methodology for the preparation of N2-triazole-containing monophosphate cap analogs and present their biological evaluation as inhibitors of protein synthesis. Five analogs possessing this unique hetero-cyclic ring spaced from the m7-guanine of the cap structure at a distance of one or three carbon atoms and/or additionally substituted by various groups containing the benzene ring were synthesized. All obtained compounds turned out to be effective translation inhibitors with IC50 similar to dinucleotide triphosphate m(7)GpppG. As these compounds possess a reduced number of phosphate groups and, thereby, a negative charge, which may support their cell penetration, this type of cap analog might be promising in terms of designing new potential therapeutic molecules. In addition, an exemplary dinucleotide from a corresponding mononucleotide containing benzyl substituted 1,2,3-triazole was prepared and examined. The superior inhibitory properties of this analog (10-fold vs. m(7)GpppG) suggest the usefulness of such compounds for the preparation of mRNA transcripts with high translational activity.

eIF4F-like complexes formed by cap-binding homolog TbEIF4E5 with TbEIF4G1 or TbEIF4G2 are implicated in post-transcriptional regulation in Trypanosoma brucei

Members of the eIF4E mRNA cap-binding family are involved in translation and the modulation of transcript availability in other systems as part of a three-component complex including eIF4G and eIF4A. The kinetoplastids possess four described eIF4E and five eIF4G homologs. We have identified two new eIF4E family proteins in Trypanosoma brucei, and define distinct complexes associated with the fifth member, TbEIF4E5. The cytosolic TbEIF4E5 protein binds cap 0 in vitro. TbEIF4E5 was found in association with two of the five TbEIF4Gs. TbIF4EG1 bound TbEIF4E5, a 47.5-kDa protein with two RNA-binding domains, and either the regulatory protein 14-3-3 II or a 117.5-kDa protein with guanylyltransferase and methyltransferase domains in a potentially dynamic interaction. The TbEIF4G2/TbEIF4E5 complex was associated with a 17.9-kDa hypothetical protein and both 14-3-3 variants I and II. Knockdown of TbEIF4E5 resulted in the loss of productive cell movement, as evidenced by the inability of the cells to remain in suspension in liquid culture and the loss of social motility on semisolid plating medium, as well as a minor reduction of translation. Cells appeared lethargic, as opposed to compromised in flagellar function per se. The minimal use of transcriptional control in kinetoplastids requires these organisms to implement downstream mechanisms to regulate gene expression, and the TbEIF4E5/TbEIF4G1/117.5-kDa complex in particular may be a key player in that process. We suggest that a pathway involved in cell motility is affected, directly or indirectly, by one of the TbEIF4E5 complexes.

Hypermethylated-capped selenoprotein mRNAs in mammals

Mammalian mRNAs are generated by complex and coordinated biogenesis pathways and acquire 5′-end m⁷G caps that play fundamental roles in processing and translation. Here we show that several selenoprotein mRNAs are not recognized efficiently by translation initiation factor eIF4E because they bear a hypermethylated cap. This cap modification is acquired via a 5′-end maturation pathway similar to that of the small nucle(ol)ar RNAs (sn- and snoRNAs). Our findings also establish that the trimethylguanosine synthase 1 (Tgs1) interacts with selenoprotein mRNAs for cap hypermethylation and that assembly chaperones and core proteins devoted to sn- and snoRNP maturation contribute to recruiting Tgs1 to selenoprotein mRNPs. We further demonstrate that the hypermethylated-capped selenoprotein mRNAs localize to the cytoplasm, are associated with polysomes and thus translated. Moreover, we found that the activity of Tgs1, but not of eIF4E, is required for the synthesis of the GPx1 selenoprotein in vivo.

Decapping Scavenger (DcpS) enzyme: advances in its structure, activity and roles in the cap-dependent mRNA metabolism

Decapping Scavenger (DcpS) enzyme rids eukaryotic cells of short mRNA fragments containing the 5' mRNA cap structure, which appear in the 3'→5' mRNA decay pathway, following deadenylation and exosome-mediated turnover. The unique structural properties of the cap, which consists of 7-methylguanosine attached to the first transcribed nucleoside by a triphosphate chain (m(7)GpppN), guarantee its resistance to non-specific exonucleases. DcpS enzymes are dimers belonging to the Histidine Triad (HIT) superfamily of pyrophosphatases. The specific hydrolysis of m(7)GpppN by DcpS yields m(7)GMP and NDP. By precluding inhibition of other cap-binding proteins by short m(7)GpppN-containing mRNA fragments, DcpS plays an important role in the cap-dependent mRNA metabolism. Over the past decade, lots of new structural, biochemical and biophysical data on DcpS has accumulated. We attempt to integrate these results, referring to DcpS enzymes from different species. Such a synergistic characteristic of the DcpS structure and activity might be useful for better understanding of the DcpS catalytic mechanism, its regulatory role in gene expression, as well as for designing DcpS inhibitors of potential therapeutic application, e.g. in spinal muscular atrophy.

The dynamic epitranscriptome: N6-methyladenosine and gene expression control

N(6)-methyladenosine (m(6)A) is a modified base that has long been known to be present in non-coding RNAs, ribosomal RNA, polyadenylated RNA and at least one mammalian mRNA. However, our understanding of the prevalence of this modification has been fundamentally redefined by transcriptome-wide m(6)A mapping studies, which have shown that m(6)A is present in a large subset of the transcriptome in specific regions of mRNA. This suggests that mRNA may undergo post-transcriptional methylation to regulate its fate and function, which is analogous to methyl modifications in DNA. Thus, the pattern of methylation constitutes an mRNA 'epitranscriptome'. The identification of adenosine methyltransferases ('writers'), m(6)A demethylating enzymes ('erasers') and m(6)A-binding proteins ('readers') is helping to define cellular pathways for the post-transcriptional regulation of mRNAs.

Inhibition of translation by IFIT family members is determined by their ability to interact selectively with the 5'-terminal regions of cap0-, cap1- and 5'ppp- mRNAs

Ribosomal recruitment of cellular mRNAs depends on binding of eIF4F to the mRNA's 5'-terminal 'cap'. The minimal 'cap0' consists of N7-methylguanosine linked to the first nucleotide via a 5'-5' triphosphate (ppp) bridge. Cap0 is further modified by 2'-O-methylation of the next two riboses, yielding 'cap1' (m7GpppNmN) and 'cap2' (m7GpppNmNm). However, some viral RNAs lack 2'-O-methylation, whereas others contain only ppp- at their 5'-end. Interferon-induced proteins with tetratricopeptide repeats (IFITs) are highly expressed effectors of innate immunity that inhibit viral replication by incompletely understood mechanisms. Here, we investigated the ability of IFIT family members to interact with cap1-, cap0- and 5'ppp- mRNAs and inhibit their translation. IFIT1 and IFIT1B showed very high affinity to cap-proximal regions of cap0-mRNAs (K1/2,app ∼9 to 23 nM). The 2'-O-methylation abrogated IFIT1/mRNA interaction, whereas IFIT1B retained the ability to bind cap1-mRNA, albeit with reduced affinity (K1/2,app ∼450 nM). The 5'-terminal regions of 5'ppp-mRNAs were recognized by IFIT5 (K1/2,app ∼400 nM). The activity of individual IFITs in inhibiting initiation on a specific mRNA was determined by their ability to interact with its 5'-terminal region: IFIT1 and IFIT1B efficiently outcompeted eIF4F and abrogated initiation on cap0-mRNAs, whereas inhibition on cap1- and 5'ppp- mRNAs by IFIT1B and IFIT5 was weaker and required higher protein concentrations.

Mammalian 5'-capped microRNA precursors that generate a single microRNA

MicroRNAs (miRNAs) are short RNA gene regulators typically produced from primary transcripts that are cleaved by the nuclear microprocessor complex, with the resulting precursor miRNA hairpins exported by exportin 5 and processed by cytoplasmic Dicer to yield two (5p and 3p) miRNAs. Here, we document microprocessor-independent 7-methylguanosine (m(7)G)-capped pre-miRNAs, whose 5' ends coincide with transcription start sites and 3' ends are most likely generated by transcription termination. By establishing a small RNA Cap-seq method that employs the cap-binding protein eIF4E, we identified a group of murine m(7)G-capped pre-miRNAs genome wide. The m(7)G-capped pre-miRNAs are exported via the PHAX-exportin 1 pathway. After Dicer cleavage, only the 3p-miRNA is efficiently loaded onto Argonaute to form a functional microRNP. This unusual miRNA biogenesis pathway, which differs in pre-miRNA synthesis, nuclear-cytoplasmic transport, and guide strand selection, enables the development of shRNA expression constructs that produce a single 3p-siRNA.

New 7-methylguanine derivatives targeting the influenza polymerase PB2 cap-binding domain

The heterotrimeric influenza virus polymerase performs replication and transcription of viral RNA in the nucleus of infected cells. Transcription by "cap-snatching" requires that host-cell pre-mRNAs are bound via their 5' cap to the PB2 subunit. Thus, the PB2 cap-binding site is potentially a good target for new antiviral drugs that will directly inhibit viral replication. Docking studies using the structure of the PB2 cap-binding domain suggested that 7-alkylguanine derivatives substituted at position N-9 and N-2 could be good candidates. Four series of 7,9-di- and 2,7,9-trialkyl guanine derivatives were synthesized and evaluated by an AlphaScreen assay in competition with a biotinylated cap analogue. Three synthesized compounds display potent in vitro activity with IC50 values lower than 10 μM. High-resolution X-ray structures of three inhibitors in complex with the H5N1 PB2 cap-binding domain confirmed the binding mode and provide detailed information for further compound optimization.

Potential therapeutic applications of RNA cap analogs

Cap analogs are chemically modified derivatives of the unique cap structure present at the 5´ end of all eukaryotic mRNAs and several non-coding RNAs. Until recently, cap analogs have served primarily as tools in the study of RNA metabolism. Continuing advances in our understanding of cap biological functions (including RNA stabilization, pre-mRNA splicing, initiation of mRNA translation, as well as cellular transport of mRNAs and snRNAs) and the consequences of the disruption of these processes - resulting in serious medical disorders - have opened new possibilities for pharmaceutical applications of these compounds. In this review, the medicinal potential of cap analogs in areas, such as cancer treatment (including eIF4E targeting and mRNA-based immunotherapy), spinal muscular atrophy treatment, antiviral therapy and the improvement of the localization of nucleus-targeting drugs, are highlighted. Advances achieved to date, challenges, plausible solutions and prospects for the future development of cap analog-based drug design are described.

Enzymatic synthesis of RNAs capped with nucleotide analogues reveals the molecular basis for substrate selectivity of RNA capping enzyme: impacts on RNA metabolism

RNA cap binding proteins have evolved to specifically bind to the N7-methyl guanosine cap structure found at the 5’ ends of eukaryotic mRNAs. The specificity of RNA capping enzymes towards GTP for the synthesis of this structure is therefore crucial for mRNA metabolism. The fact that ribavirin triphosphate was described as a substrate of a viral RNA capping enzyme, raised the possibility that RNAs capped with nucleotide analogues could be generated in cellulo. Owing to the fact that this prospect potentially has wide pharmacological implications, we decided to investigate whether the active site of the model Parameciumbursaria Chlorella virus-1 RNA capping enzyme was flexible enough to accommodate various purine analogues. Using this approach, we identified several key structural determinants at each step of the RNA capping reaction and generated RNAs harboring various different cap analogues. Moreover, we monitored the binding affinity of these novel capped RNAs to the eIF4E protein and evaluated their translational properties in cellulo. Overall, this study establishes a molecular rationale for the specific selection of GTP over other NTPs by RNA capping enzyme It also demonstrates that RNAs can be enzymatically capped with certain purine nucleotide analogs, and it also describes the impacts of modified RNA caps on specific steps involved in mRNA metabolism. For instance, our results indicate that the N7-methyl group of the classical N7-methyl guanosine cap is not always indispensable for binding to eIF4E and subsequently for translation when compensatory modifications are present on the capped residue. Overall, these findings have important implications for our understanding of the molecular determinants involved in both RNA capping and RNA metabolism.

3D Molecular Modelling Study of the H7N9 RNA-Dependent RNA Polymerase as an Emerging Pharmacological Target

Currently not much is known about the H7N9 strain, and this is the major drawback for a scientific strategy to tackle this virus. Herein, the 3D complex structure of the H7N9 RNA-dependent RNA polymerase has been established using a repertoire of molecular modelling techniques including homology modelling, molecular docking, and molecular dynamics simulations. Strikingly, it was found that the oligonucleotide cleft and tunnel in the H7N9 RNA-dependent RNA polymerase are structurally very similar to the corresponding region on the hepatitis C virus RNA-dependent RNA polymerase crystal structure. A direct comparison and a 3D postdynamics analysis of the 3D complex of the H7N9 RNA-dependent RNA polymerase provide invaluable clues and insight regarding the role and mode of action of a series of interacting residues on the latter enzyme. Our study provides a novel and efficiently intergraded platform with structural insights for the H7N9 RNA-dependent RNA Polymerase. We propose that future use and exploitation of these insights may prove invaluable in the fight against this lethal, ongoing epidemic.

TGFβ-induced PI 3 kinase-dependent Mnk-1 activation is necessary for Ser-209 phosphorylation of eIF4E and mesangial cell hypertrophy

Transforming growth factorβ (TGFβ)-induced canonical signal transduction is involved in glomerular mesangial cell hypertrophy; however, the role played by the noncanonical TGFβ signaling remains largely unexplored. TGFβ time-dependently stimulated eIF4E phosphorylation at Ser-209 concomitant with enhanced phosphorylation of Erk1/2 (extracellular signal regulated kinase1/2) and MEK (mitogen-activated and extracellular signal-regulated kinase kinase) in mesangial cells. Inhibition of Erk1/2 by MEK inhibitor or by expression of dominant negative Erk2 blocked eIF4E phosphorylation, resulting in attenuation of TGFβ-induced protein synthesis and mesangial cell hypertrophy. Expression of constitutively active (CA) MEK was sufficient to induce protein synthesis and hypertrophy similar to those induced by TGFβ. Pharmacological or dominant negative inhibition of phosphatidylinositol (PI) 3 kinase decreased MEK/Erk1/2 phosphorylation leading to suppression of eIF4E phosphorylation. Inducible phosphorylation of eIF4E at Ser-209 is mediated by Mnk-1 (mitogen-activated protein kinase signal-integrating kinase-1). Both PI 3 kinase and Erk1/2 promoted phosphorylation of Mnk-1 in response to TGFβ. Dominant negative Mnk-1 significantly inhibited TGFβ-stimulated protein synthesis and hypertrophy. Interestingly, inhibition of mTORC1 activity, which blocks dissociation of eIF4E-4EBP-1 complex, decreased TGFβ-stimulated phosphorylation of eIF4E without any effect on Mnk-1 phosphorylation. Furthermore, mutant eIF4E S209D, which mimics phosphorylated eIF4E, promoted protein synthesis and hypertrophy similar to TGFβ. These results were confirmed using phosphorylation deficient mutant of eIF4E. Together our results highlight a significant role of dissociation of 4EBP-1-eIF4E complex for Mnk-1-mediated phosphorylation of eIF4E. Moreover, we conclude that TGFβ-induced noncanonical signaling circuit involving PI 3 kinase-dependent Mnk-1-mediated phosphorylation of eIF4E at Ser-209 is required to facilitate mesangial cell hypertrophy.

Synthetic mRNAs with superior translation and stability properties

The translational efficiency and stability of synthetic mRNA in both cultured cells and whole animals can be improved by incorporation of modified cap structures at the 5'-end. mRNAs are synthesized in vitro by a phage RNA polymerase transcribing a plasmid containing the mRNA sequence in the presence of all four NTPs plus a cap dinucleotide. Modifications in the cap dinucleotide at the 2'- or 3'-positions of m(7)Guo, or modifications in the polyphosphate chain, can improve both translational efficiency and stability of the mRNA, thereby increasing the amount and duration of protein expression. In the context of RNA-based immunotherapy, the latter is especially important for antigen production and presentation by dendritic cells. Protocols are presented for synthesis of modified mRNAs, their introduction into cells and whole animals, and measurement of their translational efficiency and stability.

Adaptive mutations in the H5N1 polymerase complex

Adaptation of the viral polymerase to host factors plays an important role in interspecies transmission of H5N1 viruses. Several adaptive mutations have been identified that, in general, determine not only host range, but also pathogenicity and transmissibility of the virus. The available evidence indicates that most of these mutations are found in the PB2 subunit of the polymerase. Particularly prominent mutations are located in the C-terminal domain of PB2 involving the amino acid exchanges E627K and D701N. Both mutations, that are also responsible for the adaptation of other avian viruses to mammalian hosts, have been described in human H5N1 isolates. In animal models, it could be demonstrated that they enhance pathogenicity in mice and induce contact transmission in guinea pigs. Mutation E627K has also been identified as a determinant of air-borne H5N1 transmission in ferrets. We are only beginning to understand the underlying mechanisms at the molecular level. Thus, mutation D701N promotes importin-α mediated nuclear transport in mammalian cells. Mutation E627K also enhances the replication rate in an importin-α dependent fashion in mammalian cells, yet without affecting nuclear entry of PB2. Numerous other adaptive mutations, some of which compensate for the lack of PB2 E627K, have been observed in PB2 as well as in the polymerase subunit PB1, the nucleoprotein NP, and the nuclear export protein NEP (NS2).

Evaluation of RNA Amplification Methods to Improve DC Immunotherapy Antigen Presentation and Immune Response

Dendritic cells (DCs) transfected with total amplified tumor cell RNA have the potential to induce broad antitumor immune responses. However, analytical methods required for quantitatively assessing the integrity, fidelity, and functionality of the amplified RNA are lacking. We have developed a series of assays including gel electrophoresis, northern blot, capping efficiency, and microarray analysis to determine integrity and fidelity and a model system to assess functionality after transfection into human DCs. We employed these tools to demonstrate that modifications to our previously reported total cellular RNA amplification process including the use of the Fast Start High Fidelity (FSHF) PCR enzyme, T7 Powerswitch primer, post-transcriptional capping and incorporation of a type 1 cap result in amplification of longer transcripts, greater translational competence, and a higher fidelity representation of the starting total RNA population. To study the properties of amplified RNA after transfection into human DCs, we measured protein expression levels of defined antigens coamplified with the starting total RNA populations and measured antigen-specific T cell expansion in autologous DC-T cell co-cultured in vitro. We conclude from these analyses that the improved RNA amplification process results in superior protein expression levels and a greater capacity of the transfected DCs to induce multifunctional antigen-specific memory T cells.Molecular Therapy-Nucleic Acids (2013) 2, e91; doi:10.1038/mtna.2013.18; published online 7 May 2013.

Conformational changes induced in the eukaryotic translation initiation factor eIF4E by a clinically relevant inhibitor, ribavirin triphosphate

The eukaryotic translation initiation factor eIF4E is highly elevated in human cancers including acute myeloid leukemia (AML). A potential anticancer agent, ribavirin, targets eIF4E activity in AML patients corresponding to clinical responses. To date, ribavirin is the only direct inhibitor of eIF4E to reach clinical trials. We showed that ribavirin acts as a competitive inhibitor of the methyl 7-guanosine (m(7)G) cap, the natural ligand of eIF4E. Here we examine the conformational changes occurring in human eIF4E upon binding the active metabolite of ribavirin, ribavirin triphosphate (RTP). Our NMR data revealed an unexpected concentration dependence on RTP affinity for eIF4E. We observed NMR spectra characteristic of tight binding at low micromolar concentrations (2-5 μM eIF4E) but much weaker affinity at more typical NMR concentrations (50- ). Comparison of chemical shift perturbation and line broadening suggest that the two eIF4E-RTP complexes differ in the precise positioning of RTP within the cap binding pocket, with the high affinity complex showing more extensive changes to the central β-sheet and dorsal surface of eIF4E, similar to m(7)G cap. The differences between high and low affinity complexes arise due to concentration dependent aggregation of eIF4E and RTP. Given the intracellular concentrations of eIF4E and RTP and the differential binding toward the W56A eIF4E mutant the high affinity complex is the most physiologically relevant. In summary, these findings demonstrate that RTP binds in the cap-binding site but also suggests new features of this pocket that should be considered in drug design efforts and reveal new insights into ligand eIF4E recognition.

Synthesis of biotin labelled cap analogue--incorporable into mRNA transcripts and promoting cap-dependent translation

Analogues of the eukaryotic messenger RNA 5' end (m(7)G cap) are useful tools for studying mRNA fate and serve as reagents for in vitro preparation of 5' capped mRNAs. We designed a biotin-labeled dinucleotide cap analogue that can be incorporated into transcripts to produce 5'-capped and biotinylated mRNAs which retain their biological functionality and may be employed for biotin-(strept)avidin technologies.

A mammalian pre-mRNA 5' end capping quality control mechanism and an unexpected link of capping to pre-mRNA processing

Recently, we reported that two homologous yeast proteins, Rai1 and Dxo1, function in a quality control mechanism to clear cells of incompletely 5' end-capped messenger RNAs (mRNAs). Here, we report that their mammalian homolog, Dom3Z (referred to as DXO), possesses pyrophosphohydrolase, decapping, and 5'-to-3' exoribonuclease activities. Surprisingly, we found that DXO preferentially degrades defectively capped pre-mRNAs in cells. Additional studies show that incompletely capped pre-mRNAs are inefficiently spliced at all introns, a fact that contrasts with current understanding, and are also poorly cleaved for polyadenylation. Crystal structures of DXO in complex with substrate mimic and products at a resolution of up to 1.5Å provide elegant insights into the catalytic mechanism and molecular basis for their three apparently distinct activities. Our data reveal a pre-mRNA 5' end capping quality control mechanism in mammalian cells, indicating DXO as the central player for this mechanism, and demonstrate an unexpected intimate link between proper 5' end capping and subsequent pre-mRNA processing.

'Ribozoomin'--translation initiation from the perspective of the ribosome-bound eukaryotic initiation factors (eIFs)

Protein synthesis is a fundamental biological mechanism bringing the DNA-encoded genetic information into life by its translation into molecular effectors - proteins. The initiation phase of translation is one of the key points of gene regulation in eukaryotes, playing a role in processes from neuronal function to development. Indeed, the importance of the study of protein synthesis is increasing with the growing list of genetic diseases caused by mutations that affect mRNA translation. To grasp how this regulation is achieved or altered in the latter case, we must first understand the molecular details of all underlying processes of the translational cycle with the main focus put on its initiation. In this review I discuss recent advances in our comprehension of the molecular basis of particular initiation reactions set into the context of how and where individual eIFs bind to the small ribosomal subunit in the pre-initiation complex. I also summarize our current knowledge on how eukaryotic initiation factor eIF3 controls gene expression in the gene-specific manner via reinitiation.

Poly(A)-specific ribonuclease (PARN): an allosterically regulated, processive and mRNA cap-interacting deadenylase

Deadenylation of eukaryotic mRNA is a mechanism critical for mRNA function by influencing mRNA turnover and efficiency of protein synthesis. Here, we review poly(A)-specific ribonuclease (PARN), which is one of the biochemically best characterized deadenylases. PARN is unique among the currently known eukaryotic poly(A) degrading nucleases, being the only deadenylase that has the capacity to directly interact during poly(A) hydrolysis with both the m(7)G-cap structure and the poly(A) tail of the mRNA. In short, PARN is a divalent metal-ion dependent poly(A)-specific, processive and cap-interacting 3'-5' exoribonuclease that efficiently degrades poly(A) tails of eukaryotic mRNAs. We discuss in detail the mechanisms of its substrate recognition, catalysis, allostery and processive mode of action. On the basis of biochemical and structural evidence, we present and discuss a working model for PARN action. Models of regulation of PARN activity by trans-acting factors are discussed as well as the physiological relevance of PARN.

The oncogene eIF4E: using biochemical insights to target cancer

The eukaryotic translation initiation factor eIF4E is overexpressed in many human malignancies where it is typically a harbinger of poor prognosis. eIF4E is positioned as a nexus in post-transcriptional gene expression. To carry out these functions, eIF4E needs to bind the m(7)G cap moiety on mRNAs. It plays critical roles in mRNA translation, mRNA export, and most likely in mRNA stability as well. Through these activities, eIF4E coordinately modulates the expression of many transcripts involved in proliferation and survival. eIF4E function is controlled by interactions with protein cofactors in concert with many signaling pathways, including Ras, Mnk, Erk, MAPK, PI3K, mTOR, and Akt. This review describes the eIF4E activity and provides several examples of cellular control mechanisms. Further, we describe some therapeutic strategies in preclinical and clinical development.

Crystallization and X-ray crystallographic analysis of the cap-binding domain of influenza A virus H1N1 polymerase subunit PB2

PB2 is one of the subunits of the influenza virus heterotrimeric polymerase. By its cap-binding domain (PB2cap), PB2 captures the 5' cap of the host pre-mRNA to generate a capped 5' oligonucleotide primer for virus transcription. The crystal structure of influenza A virus H3N2 PB2cap with bound cap analogue m7GTP has been reported previously. To show the substrate-free structural details of PB2cap and clarify whether obvious conformational changes exist between the substrate-free and substrate-bound cap-binding domain, we have successfully obtained the crystal of substrate-free H1N1 PB2cap. The crystal of H1N1 PB2cap diffracted to a high resolution of 1.32 Å. The crystal symmetry belongs to space group P1 with unit-cell parameters a=29.49, b=37.04, c=38.33 Å, α=71.10, β=69.84, γ=75.85°. There is one molecule in the asymmetric unit.

First structural evidence of sequestration of mRNA cap structures by type 1 ribosome inactivating protein from Momordica balsamina

This is the first structural evidence of recognition of mRNA cap structures by a ribosome inactivating protein. It is well known that a unique cap structure is formed at the 5' end of mRNA for carrying out various processes including mRNA maturation, translation initiation, and RNA turnover. The binding studies and crystal structure determinations of type 1 ribosome inactivating protein (RIP-1) from Momordica balsamina (MbRIP-1) were carried out with mRNA cap structures including (i) N7-methyl guanine (m7G), (ii) N7-methyl guanosine diphosphate (m7GDP), and (iii) N7-methyl guanosine triphosphate (m7GTP). These compounds showed affinities to MbRIP-1 at nanomolar concentrations. The structure determinations of the complexes of MbRIP-1 with m7G, m7GDP, and m7GTP at 2.65, 1.77, and 1.75 Å resolutions revealed that all the three compounds bound to MbRIP-1 in the substrate binding site at the positions which are slightly shifted towards Glu85 as compared to those of rRNA substrates. In this position, Glu85 forms several hydrogen bonds with guanine moiety while N-7 methyl group forms van der Waals contacts. However, the guanine rings are poorly stacked in these complexes. Thus, the mode of binding by MbRIP-1 to mRNA cap structures is different which results in the inhibition of depurination. Since some viruses are known to exploit the capping property of the host, this action of MbRIP-1 may have implications for the antiviral activity of this protein in vivo. The understanding of the mode of binding of MbRIP-1 to cap structures may also assist in the design of anti-viral agents.

eIF4E3 acts as a tumor suppressor by utilizing an atypical mode of methyl-7-guanosine cap recognition

Recognition of the methyl-7-guanosine (m(7)G) cap structure on mRNA is an essential feature of mRNA metabolism and thus gene expression. Eukaryotic translation initiation factor 4E (eIF4E) promotes translation, mRNA export, proliferation, and oncogenic transformation dependent on this cap-binding activity. eIF4E-cap recognition is mediated via complementary charge interactions of the positively charged m(7)G cap between the negative π-electron clouds from two aromatic residues. Here, we demonstrate that a variant subfamily, eIF4E3, specifically binds the m(7)G cap in the absence of an aromatic sandwich, using instead a different spatial arrangement of residues to provide the necessary electrostatic and van der Waals contacts. Contacts are much more extensive between eIF4E3-cap than other family members. Structural analyses of other cap-binding proteins indicate this recognition mode is atypical. We demonstrate that eIF4E3 relies on this cap-binding activity to act as a tumor suppressor, competing with the growth-promoting functions of eIF4E. In fact, reduced eIF4E3 in high eIF4E cancers suggests that eIF4E3 underlies a clinically relevant inhibitory mechanism that is lost in some malignancies. Taken together, there is more structural plasticity in cap recognition than previously thought, and this is physiologically relevant.

The immunosuppressive agent mizoribine monophosphate is an inhibitor of the human RNA capping enzyme

Mizoribine monophosphate (MZP) is a specific inhibitor of the cellular inosine-5′-monophosphate dehydrogenase (IMPDH), the enzyme catalyzing the rate-limiting step of de novo guanine nucleotide biosynthesis. MZP is a highly potent antagonistic inhibitor of IMPDH that blocks the proliferation of T and B lymphocytes that use the de novo pathway of guanine nucleotide synthesis almost exclusively. In the present study, we investigated the ability of MZP to directly inhibit the human RNA capping enzyme (HCE), a protein harboring both RNA 5′-triphosphatase and RNA guanylyltransferase activities. HCE is involved in the synthesis of the cap structure found at the 5′ end of eukaryotic mRNAs, which is critical for the splicing of the cap-proximal intron, the transport of mRNAs from the nucleus to the cytoplasm, and for both the stability and translation of mRNAs. Our biochemical studies provide the first insight that MZP can inhibit the formation of the RNA cap structure catalyzed by HCE. In the presence of MZP, the RNA 5′-triphosphatase activity appears to be relatively unaffected while the RNA guanylyltransferase activity is inhibited, indicating that the RNA guanylyltransferase activity is the main target of MZP inhibition. Kinetic studies reveal that MZP is a non-competitive inhibitor that likely targets an allosteric site on HCE. Mizoribine also impairs mRNA capping in living cells, which could account for the global mechanism of action of this therapeutic agent. Together, our study clearly demonstrates that mizoribine monophosphate inhibits the human RNA guanylyltransferase in vitro and impair mRNA capping in cellulo.

Solution-based approach to study binding to the eIF4E cap-binding site using CD spectroscopy

The eukaryotic initiation factor 4E (eIF4E) is the key component of the translational initiation complex that recruits mRNA by binding to a unique "cap" structure located at the 5' end of the mRNA. Overexpression of eIF4E has been implicated in the development of cancer, potentially as a result of increasing the cellular levels of proteins involved in processes that include proliferation and regulation of apoptosis. As a result, the cap-binding site of eIF4E has become a target for the development of anti-cancer therapeutics. The structure of eIF4E bound to the cap mimic 7-methyl-GDP revealed that two tryptophans from different loops in eIF4E sandwiched the 7-methylguanine group between them. This interaction gives rise to a strong exciton coupling signal between the two tryptophans that can be visualized by CD spectroscopy. eIF4E is a challenging protein to work with because of a propensity to aggregate under conditions used in biophysical techniques. CD spectroscopy provides a gentle, solution-based approach to study binding to the cap-binding site of eIF4E. Evidence is provided that the exciton coupling signal can be used to both qualitatively and quantitatively analyze the binding of cap analogs to eIF4E.

Regulation of mRNA translation by signaling pathways

mRNA translation is the most energy consuming process in the cell. In addition, it plays a pivotal role in the control of gene expression and is therefore tightly regulated. In response to various extracellular stimuli and intracellular cues, signaling pathways induce quantitative and qualitative changes in mRNA translation by modulating the phosphorylation status and thus the activity of components of the translational machinery. In this work we focus on the phosphoinositide 3-kinase (PI3K)/AKT and the mitogen-activated protein kinase (MAPK) pathways, as they are strongly implicated in the regulation of translation in homeostasis, whereas their malfunction has been linked to aberrant translation in human diseases, including cancer.

eIF4E/4E-BP ratio predicts the efficacy of mTOR targeted therapies

Active-site mTOR inhibitors (asTORi) hold great promise for targeting dysregulated mTOR signaling in cancer. Because of the multifaceted nature of mTORC1 signaling, identification of reliable biomarkers for the sensitivity of tumors to asTORi is imperative for their clinical implementation. Here, we show that cancer cells acquire resistance to asTORi by downregulating eukaryotic translation initiation factor (eIF4E)-binding proteins (4E-BPs-EIF4EBP1, EIF4EBP2). Loss of 4E-BPs or overexpression of eIF4E renders neoplastic growth and translation of tumor-promoting mRNAs refractory to mTOR inhibition. Conversely, moderate depletion of eIF4E augments the anti-neoplastic effects of asTORi. The anti-proliferative effect of asTORi in vitro and in vivo is therefore significantly influenced by perturbations in eIF4E/4E-BP stoichiometry, whereby an increase in the eIF4E/4E-BP ratio dramatically limits the sensitivity of cancer cells to asTORi. We propose that the eIF4E/4E-BP ratio, rather than their individual protein levels or solely their phosphorylation status, should be considered as a paramount predictive marker for forecasting the clinical therapeutic response to mTOR inhibitors.

SLiMPrints: conservation-based discovery of functional motif fingerprints in intrinsically disordered protein regions

Large portions of higher eukaryotic proteomes are intrinsically disordered, and abundant evidence suggests that these unstructured regions of proteins are rich in regulatory interaction interfaces. A major class of disordered interaction interfaces are the compact and degenerate modules known as short linear motifs (SLiMs). As a result of the difficulties associated with the experimental identification and validation of SLiMs, our understanding of these modules is limited, advocating the use of computational methods to focus experimental discovery. This article evaluates the use of evolutionary conservation as a discriminatory technique for motif discovery. A statistical framework is introduced to assess the significance of relatively conserved residues, quantifying the likelihood a residue will have a particular level of conservation given the conservation of the surrounding residues. The framework is expanded to assess the significance of groupings of conserved residues, a metric that forms the basis of SLiMPrints (short linear motif fingerprints), a de novo motif discovery tool. SLiMPrints identifies relatively overconstrained proximal groupings of residues within intrinsically disordered regions, indicative of putatively functional motifs. Finally, the human proteome is analysed to create a set of highly conserved putative motif instances, including a novel site on translation initiation factor eIF2A that may regulate translation through binding of eIF4E.

Computational studies on the substrate interactions of influenza A virus PB2 subunit

Influenza virus, which spreads around the world in seasonal epidemics and leads to large numbers of deaths every year, has several ribonucleoproteins in the central core of the viral particle. These viral ribonucleoproteins can specifically bind the conserved 3' and 5' caps of the viral RNAs with responsibility for replication and transcription of the viral RNA in the nucleus of infected cells. A fundamental question of most importance is that how the cap-binding proteins in the influenza virus discriminates between capped RNAs and non-capped ones. To get an answer, we performed molecular dynamics simulations and free energy calculations on the influenza A virus PB2 subunit, an important component of the RNP complexes, with a cap analog m7GTP. Our calculations showed that some key residues in the active site, such as Arg355, His357, Glu361 as well as Gln406, could offer significant hydrogen bonding and hydrophobic interactions with the guanine ring of the cap analog m7GTP to form an aromatic sandwich mechanism for the cap recognition and positioning in the active site. Subsequently, we applied this idea to a virtual screening procedure and identified 5 potential candidates that might be inhibitors against the PB2 subunit. Interestingly, 2 candidates Cpd1 and Cpd2 have been already reported to have inhibitory activities to the influenza virus cap-binding proteins. Further calculation also showed that they had comparatively higher binding affinities to the PB2 subunit than that of m7GTP. We believed that our findings could give an atomic insight into the deeper understanding of the cap recognition and binding mechanism, providing useful information for searching or designing novel drugs against influenza viruses.

Crystal structure of a minimal eIF4E-Cup complex reveals a general mechanism of eIF4E regulation in translational repression

Cup is an eIF4E-binding protein (4E-BP) that plays a central role in translational regulation of localized mRNAs during early Drosophila development. In particular, Cup is required for repressing translation of the maternally contributed oskar, nanos, and gurken mRNAs, all of which are essential for embryonic body axis determination. Here, we present the 2.8 Å resolution crystal structure of a minimal eIF4E-Cup assembly, consisting of the interacting regions of the two proteins. In the structure, two separate segments of Cup contact two orthogonal faces of eIF4E. The eIF4E-binding consensus motif of Cup (YXXXXLΦ) binds the convex side of eIF4E similarly to the consensus of other eIF4E-binding proteins, such as 4E-BPs and eIF4G. The second, noncanonical, eIF4E-binding site of Cup binds laterally and perpendicularly to the eIF4E β-sheet. Mutations of Cup at this binding site were shown to reduce binding to eIF4E and to promote the destabilization of the associated mRNA. Comparison with the binding mode of eIF4G to eIF4E suggests that Cup and eIF4G binding would be mutually exclusive at both binding sites. This shows how a common molecular surface of eIF4E might recognize different proteins acting at different times in the same pathway. The structure provides insight into the mechanism by which Cup disrupts eIF4E-eIF4G interaction and has broader implications for understanding the role of 4E-BPs in translational regulation.

Evolutionary conservation and diversification of the translation initiation apparatus in trypanosomatids

Trypanosomatids are ancient eukaryotic parasites that migrate between insect vectors and mammalian hosts, causing a range of diseases in humans and domestic animals. Trypanosomatids feature a multitude of unusual molecular features, including polycistronic transcription and subsequent processing by trans-splicing and polyadenylation. Regulation of protein coding genes is posttranscriptional and thus, translation regulation is fundamental for activating the developmental program of gene expression. The spliced-leader RNA is attached to all mRNAs. It contains an unusual hypermethylated cap-4 structure in its 5′ end. The cap-binding complex, eIF4F, has gone through evolutionary changes in accordance with the requirement to bind cap-4. The eIF4F components in trypanosomatids are highly diverged from their orthologs in higher eukaryotes, and their potential functions are discussed. The cap-binding activity in all eukaryotes is a target for regulation and plays a similar role in trypanosomatids. Recent studies revealed a novel eIF4E-interacting protein, involved in directing stage-specific and stress-induced translation pathways. Translation regulation during stress also follows unusual regulatory cues, as the increased translation of Hsp83 following heat stress is driven by a defined element in the 3′ UTR, unlike higher eukaryotes. Overall, the environmental switches experienced by trypanosomatids during their life cycle seem to affect their translational machinery in unique ways.

Synthesis of ¹³C- and ¹⁴C-labeled dinucleotide mRNA cap analogues for structural and biochemical studies

Herein we describe the first simple and short method for specific labeling of mono- and trimethylated dinucleotide mRNA cap analogues with (13)C and (14)C isotopes. The labels were introduced within the cap structures either at the N7 for monomethylguanosine cap or N7 and N2 position for trimethylguanosine cap. The compounds designed for structural and biochemical studies will be useful tools for better understanding the role of the mRNA cap structures in pre-mRNA splicing, nucleocytoplasmic transport, translation initiation and mRNA degradation.

Affinity resins containing enzymatically resistant mRNA cap analogs--a new tool for the analysis of cap-binding proteins

Cap-binding proteins have been routinely isolated using m⁷GTP-Sepharose; however, this resin is inefficient for proteins such as DcpS (scavenger decapping enzyme), which interacts not only with the 7-methylguanosine, but also with the second cap base. In addition, DcpS purification may be hindered by the reduced resin capacity due to the ability of DcpS to hydrolyze m⁷GTP. Here, we report the synthesis of new affinity resins, m⁷GpCH₂pp- and m⁷GpCH₂ppA-Sepharoses, with attached cap analogs resistant to hydrolysis by DcpS. Biochemical tests showed that these matrices, as well as a hydrolyzable m⁷GpppA-Sepharose, bind recombinant mouse eIF4E²⁸⁻²¹⁷ specifically and at high capacity. In addition, purification of cap-binding proteins from yeast extracts confirmed the presence of all expected cap-binding proteins, including DcpS in the case of m⁷GpCH₂pp- and m⁷GpCH₂ppA-Sepharoses. In contrast, binding studies in vitro demonstrated that recombinant human DcpS efficiently bound only m⁷GpCH₂ppA-Sepharose. Our data prove the applicability of these novel resins, especially m⁷GpCH₂ppA-Sepharose, in biochemical studies such as the isolation and identification of cap-binding proteins from different organisms.

Diversity of Eukaryotic Translational Initiation Factor eIF4E in Protists

The greatest diversity of eukaryotic species is within the microbial eukaryotes, the protists, with plants and fungi/metazoa representing just two of the estimated seventy five lineages of eukaryotes. Protists are a diverse group characterized by unusual genome features and a wide range of genome sizes from 8.2 Mb in the apicomplexan parasite Babesia bovis to 112,000-220,050 Mb in the dinoflagellate Prorocentrum micans. Protists possess numerous cellular, molecular and biochemical traits not observed in “text-book” model organisms. These features challenge some of the concepts and assumptions about the regulation of gene expression in eukaryotes. Like multicellular eukaryotes, many protists encode multiple eIF4Es, but few functional studies have been undertaken except in parasitic species. An earlier phylogenetic analysis of protist eIF4Es indicated that they cannot be grouped within the three classes that describe eIF4E family members from multicellular organisms. Many more protist sequences are now available from which three clades can be recognized that are distinct from the plant/fungi/metazoan classes. Understanding of the protist eIF4Es will be facilitated as more sequences become available particularly for the under-represented opisthokonts and amoebozoa. Similarly, a better understanding of eIF4Es within each clade will develop as more functional studies of protist eIF4Es are completed.

The Distribution of eIF4E-Family Members across Insecta

Insects are part of the earliest faunas that invaded terrestrial environments and are the first organisms that evolved controlled flight. Nowadays, insects are the most diverse animal group on the planet and comprise the majority of extant animal species described. Moreover, they have a huge impact in the biosphere as well as in all aspects of human life and economy; therefore understanding all aspects of insect biology is of great importance. In insects, as in all cells, translation is a fundamental process for gene expression. However, translation in insects has been mostly studied only in the model organism Drosophila melanogaster. We used all publicly available genomic sequences to investigate in insects the distribution of the genes encoding the cap-binding protein eIF4E, a protein that plays a crucial role in eukaryotic translation. We found that there is a diversity of multiple ortholog genes encoding eIF4E isoforms within the genus Drosophila. In striking contrast, insects outside this genus contain only a single eIF4E gene, related to D. melanogaster eIF4E-1. We also found that all insect species here analyzed contain only one Class II gene, termed 4E-HP. We discuss the possible evolutionary causes originating the multiplicity of eIF4E genes within the genus Drosophila.

Ribosomal RACK1 promotes chemoresistance and growth in human hepatocellular carcinoma

Coordinated translation initiation is coupled with cell cycle progression and cell growth, whereas excessive ribosome biogenesis and translation initiation often lead to tumor transformation and survival. Hepatocellular carcinoma (HCC) is among the most common and aggressive cancers worldwide and generally displays inherently high resistance to chemotherapeutic drugs. We found that RACK1, the receptor for activated C-kinase 1, was highly expressed in normal liver and frequently upregulated in HCC. Aberrant expression of RACK1 contributed to in vitro chemoresistance as well as in vivo tumor growth of HCC. These effects depended on ribosome localization of RACK1. Ribosomal RACK1 coupled with PKCβII to promote the phosphorylation of eukaryotic initiation factor 4E (eIF4E), which led to preferential translation of the potent factors involved in growth and survival. Inhibition of PKCβII or depletion of eIF4E abolished RACK1-mediated chemotherapy resistance of HCC in vitro. Our results imply that RACK1 may function as an internal factor involved in the growth and survival of HCC and suggest that targeting RACK1 may be an efficacious strategy for HCC treatment.

Versatility of RNA-Binding Proteins in Cancer

Posttranscriptional gene regulation is a rapid and efficient process to adjust the proteome of a cell to a changing environment. RNA-binding proteins (RBPs) are the master regulators of mRNA processing and translation and are often aberrantly expressed in cancer. In addition to well-studied transcription factors, RBPs are emerging as fundamental players in tumor development. RBPs and their mRNA targets form a complex network that plays a crucial role in tumorigenesis. This paper describes mechanisms by which RBPs influence the expression of well-known oncogenes, focusing on precise examples that illustrate the versatility of RBPs in posttranscriptional control of cancer development. RBPs appeared very early in evolution, and new RNA-binding domains and combinations of them were generated in more complex organisms. The identification of RBPs, their mRNA targets, and their mechanism of action have provided novel potential targets for cancer therapy.

Structure-guided design, synthesis, and evaluation of guanine-derived inhibitors of the eIF4E mRNA-cap interaction

The eukaryotic initiation factor 4E (eIF4E) plays a central role in the initiation of gene translation and subsequent protein synthesis by binding the 5' terminal mRNA cap structure. We designed and synthesized a series of novel compounds that display potent binding affinity against eIF4E despite their lack of a ribose moiety, phosphate, and positive charge as present in m7-GMP. The biochemical activity of compound 33 is 95 nM for eIF4E in an SPA binding assay. More importantly, the compound has an IC(50) of 2.5 μM for inhibiting cap-dependent mRNA translation in a rabbit reticular cell extract assay (RRL-IVT). This series of potent, truncated analogues could serve as a promising new starting point toward the design of neutral eIF4E inhibitors with physicochemical properties suitable for cellular activity assessment.