Schizosaccharomyces pombe has a novel eukaryotic initiation factor 4F complex containing a cap-binding protein with the human eIF4E C-terminal motif KSGST

Analysis of eIF4E-family members in fungi contributes to their classification in eukaryotes

The kingdom of fungi contains highly diverse species. However, fundamental processes sustaining life such as RNA metabolism are much less comparatively studied in Fungi than in other kingdoms. A key factor in the regulation of mRNA expression is the cap-binding protein eIF4E, which plays roles in mRNA nuclear export, storage, and translation. The advent of massive genomics has unveiled a constellation of eIF4E-family members across eukaryotes. However, how this protein diverged into fungal species remains largely unexplored. Here, we studied the genome of 538 species from six evolutionarily distant phyla and retrieved 1462 eIF4E cognates. The analyzed species contained 1 to 7 paralogs. We sorted all cognates in six phylogenetically coherent clades, that we termed Class I to VII (mammalian Class III was absent in Fungi). Proteins from Classes IV to VII did not match the current eIF4Es classification that is based on variations in the residues equivalent to W43 and W56 of the human protein. eIF4Es from other eukaryotes do not fit into this classification either. Thus, we have updated the eIF4E categorization based on cladistics and the presence of cap-binding amino acids to better fit eIF4E's diversity across eukaryotes. Finally, we predicted the structure of the global protein and the cap-binding pocket and experimentally tested the ability to rescue the lack of endogenous eIF4E in Saccharomyces cerevisiae of representative members of each of the six classes of fungal eIF4E.

eIF4E and Interactors from Unicellular Eukaryotes

eIF4E, the mRNA cap-binding protein, is well known as a general initiation factor allowing for mRNA-ribosome interaction and cap-dependent translation in eukaryotic cells. In this review we focus on eIF4E and its interactors in unicellular organisms such as yeasts and protozoan eukaryotes. In a first part, we describe eIF4Es from yeast species such as Saccharomyces cerevisiae, Candida albicans, and Schizosaccharomyces pombe. In the second part, we will address eIF4E and interactors from parasite unicellular species—trypanosomatids and marine microorganisms—dinoflagellates. We propose that different strategies have evolved during evolution to accommodate cap-dependent translation to differing requirements. These evolutive “adjustments” involve various forms of eIF4E that are not encountered in all microorganismic species. In yeasts, eIF4E interactors, particularly p20 and Eap1 are found exclusively in Saccharomycotina species such as S. cerevisiae and C. albicans. For protozoan parasites of the Trypanosomatidae family beside a unique cap4-structure located at the 5′UTR of all mRNAs, different eIF4Es and eIF4Gs are active depending on the life cycle stage of the parasite. Additionally, an eIF4E-interacting protein has been identified in Leishmania major which is important for switching from promastigote to amastigote stages. For dinoflagellates, little is known about the structure and function of the multiple and diverse eIF4Es that have been identified thanks to widespread sequencing in recent years.

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.

Translational control of eukaryotic gene expression

Translational control mechanisms are, besides transcriptional control and mRNA stability, the most determining for final protein levels. A large number of accessory factors that assist the ribosome during initiation, elongation, and termination of translation are required for protein synthesis. Cap-dependent translational control occurs mainly during the initiation step, involving eukaryotic initiation factors (eIFs) and accessory proteins. Initiation is affected by various stimuli that influence the phosphorylation status of both eIF4E and eIF2 and through binding of 4E-binding proteins to eIF4E, which finally inhibits cap- dependent translation. Under conditions where cap-dependent translation is hampered, translation of transcripts containing an internal ribosome entry site can still be supported in a cap-independent manner. An interesting example of translational control is the switch between cap-independent and cap-dependent translation during the eukaryotic cell cycle. At the G1-to-S transition, translation occurs predominantly in a cap-dependent manner, while during the G2-to-M transition, cap-dependent translation is inhibited and transcripts are predominantly translated through a cap-independent mechanism.

Quantitative proteomics identifies Gemin5, a scaffolding protein involved in ribonucleoprotein assembly, as a novel partner for eukaryotic initiation factor 4E

Protein complexes are dynamic entities; identification and quantitation of their components is critical in elucidating functional roles under specific cellular conditions. We report the first quantitative proteomic analysis of the human cap-binding protein complex. Components and proteins associated with the translation initiation eIF4F complex that may affect complex formation were identified and quantitated under distinct growth conditions. Site-specific phosphorylation of eIF4E and eIF4G and elevated levels of eIF4G:eIF4E complexes in phorbol ester treated HEK293 cells, and in serum-starved tumorigenic human mesenchymal stromal cells, attested to their activated translational states. The WD-repeat, scaffolding-protein Gemin5 was identified as a novel eIF4E binding partner, which interacted directly with eIF4E through a motif (YXXXXLPhi) present in a number of eIF4E-interacting partners. Elevated levels of Gemin5:eIF4E complexes were found in phorbol ester treated HEK293 cells. Gemin5 and eIF4E co-localized to cytoplasmic P-bodies in human osteosarcoma U2OS cells. Interaction between eIF4E and Gemin5 and their co-localization to the P-bodies, may serve to recruit capped mRNAs to these RNP complexes, for functions related to RNP assembly, remodeling and/or transition from active translation to mRNA degradation. Our results demonstrate that our quantitative proteomic strategy can be applied to the identification and quantitation of protein complex components in human cells grown under different conditions.

Functional diversity of the eukaryotic translation initiation factors belonging to eIF4 families

Protein synthesis in eukaryotic cells is fundamental for gene expression. This process involves the binding of an mRNA molecule to the small ribosomal subunit in a group of reactions catalyzed by eukaryotic translation initiation factors (eIF) eIF4. To date, the role of each of the four eIF4, i.e. eIF4E, eIF4G, eIF4A and eIF4B, is well established. However, with the advent of genome-wide sequencing projects of various organisms, families of genes for each translation initiation factor have been identified. Intriguingly, recent studies have now established that certain eIF4 proteins can promote or inhibit translation of specific mRNAs, and also that some of them are active in processes other than translation. In addition, there is evidence of tissue- and developmental-stage-specific expression for some of these proteins. These new findings point to an additional level of complexity in the translation initiation process. In this review, we analyze the latest advances concerning the functionality of members of the eIF4 families in eukaryotic organisms and discuss the implications of this in the context of our current understanding of regulation of the translation initiation process.

Functional analysis of seven genes encoding eight translation initiation factor 4E (eIF4E) isoforms in Drosophila

The Drosophila genome-sequencing project has revealed a total of seven genes encoding eight eukaryotic initiation factor 4E (eIF4E) isoforms. Four of them (eIF4E-1,2, eIF4E-3, eIF4E-4 and eIF4E-5) share exon/intron structure in their carboxy-terminal part and form a cluster in the genome. All eIF4E isoforms bind to the cap (m7GpppN) structure. All of them, except eIF4E-6 and eIF4E-8 were able to interact with Drosophila eIF4G or eIF4E-binding protein (4E-BP). eIF4E-1, eIF4E-2, eIF4E-3, eIF4E-4 and eIF4E-7 rescued a yeast eIF4E-deficient mutant in vivo. Only eIF4E-1 mRNAs and, at a significantly lower level, eIF4E3 and eIF4E-8 are expressed in embryos and throughout the life cycle of the fly. The transcripts of the remaining isoforms were detected from the third instar larvae onwards. This indicates the cap-binding activity relies mostly on eIF4E-1 during embryogenesis. This agrees with the proteomic analysis of the eIF4F complex purified from embryos and with the rescue of l(3)67Af, an embryonic lethal mutant for the eIF4E-1,2 gene, by transgenic expression of eIF4E-1. Overexpression of eIF4E-1 in wild-type embryos and eye imaginal discs results in phenotypic defects in a dose-dependent manner.

Structural analysis of the eukaryotic initiation factor 4E gene controlling potyvirus resistance in pepper: exploitation of a BAC library

The pvr2 locus in pepper codes for a eukaryotic translation initiation factor 4E (eIF4E) gene that confers resistance to viruses belonging to the potyvirus genus. In this work, we describe the isolation and characterisation of the genomic sequence carrying the pvr2 locus. A Bacterial Artificial Chromosome (BAC) library that consisted of 239,232 clones with an average insert size of 123 kilobases (kb) was constructed from a Capsicum annuum line with the pvr2(+) allele for susceptibility to potato virus Y (PVY) and tobacco etch virus (TEV). Based on a polymerase chain reaction (PCR) screen with single-copy markers, three to seven positive BAC clones per markers were identified, indicating that the BAC library is suitable for pepper genome analysis. To determine the genomic organization of the pepper eIF4E gene, the library was screened with primers designed from the cDNA sequence and four positive BAC clones carrying the pvr2 locus were identified. A 7-kb DNA fragment containing the complete eIF4E gene was sub-cloned from the positive BAC clones and analysed. The eIF4E gene is organised into five exons and four introns and showed a strictly conserved exon/intron structure with eIF4E genes from Arabidopsis thaliana and rice. Moreover, the splice sites between plant exons 1/2 and 2/3 are conserved among eukaryotes including human, Drosophila and yeast. Several potential binding sites for MADS box transcription factors within the 5' flanking region of eIF4E genes from the three plant species were also predicted.

Characterization of mammalian eIF4E-family members

The translational factor eukaryotic initiation factor 4E (eIF4E) is a central component in the initiation and regulation of translation in eukaryotic cells. Through its interaction with the 5' cap structure of mRNA, eIF4E functions to recruit mRNAs to the ribosome. The accumulation of expressed sequence tag sequences has allowed the identification of three different eIF4E-family members in mammals termed eIF4E-1, eIF4E-2 (4EHP, 4E-LP) and eIF4E-3, which differ in their structural signatures, functional characteristics and expression patterns. Unlike eIF4E-1, which is found in all eukaryotes, orthologues for eIF4E-2 appear to be restricted to metazoans, while those for eIF4E-3 have been found only in chordates. Like prototypical eIF4E-1, eIF4E-2 was found to be ubiquitously expressed, with the highest levels in the testis. Expression of eIF4E-3 was detected only in heart, skeletal muscle, lung and spleen. Similarly to eIF4E-1, both eIF4E-2 and eIF4E-3 can bind to the mRNA cap-structure. However, in contrast to eIF4E-1 which interacts with both the scaffold protein, eIF4G and the translational repressor proteins, the eIF4E-binding proteins (4E-BPs), eIF4E-2 and eIF4E-3 each possesses a range of partial activities. eIF4E-2 does not interact with eIF4G, but does interact with 4E-BPs. Conversely, eIF4E-3 interacts with eIF4G, but not with 4E-BPs. Neither eIF4E-2 nor eIF4E-3 is able to rescue the lethality of eIF4E gene deletion in yeast. It is hypothesized that each eIF4E-family member fills a specialized niche in the recruitment of mRNAs by the ribosome through differences in their abilities to bind cap and/or to interact with eIF4G and the 4E-BPs.

eIF4E isoform 2 in Schizosaccharomyces pombe is a novel stress-response factor

Cap-binding proteins of the elF4E family are generally involved in mediating ribosome recruitment to capped mRNA via an interaction with the initiation factor elF4G. However, Schizosaccharomyces pombe has two elF4E isoforms, one of which (elF4E2, encoded by tif452) has a relatively low affinity for elF4G. We show that tif452 is required for specific stress responses. An S. pombe, tif452delta mutant manifests slow growth under conditions of nutrient, temperature and salt stress. elF4E2 shows a distinct subcellular distribution to elF4E1, the cap-binding factor that is required for mainstream translation. In response to salt stress, the cellular level of elF4E2 increases, whereas the amount of intact elF4G decreases, leaving elF4E2 as the predominant elF4E isoform in a cell deficient in ElF4G. The presence of elF4E2 modifies the competence of S. pombe ribosomes to translate mRNAs with structured leaders in vivo. The tif452 promoter has putative stress-response (T-rich) motifs, whereas elF4E2 seems to be a new type of stress-response factor.

Overproduction of a conserved domain of fission yeast and mammalian translation initiation factor eIF4G causes aberrant cell morphology and results in disruption of the localization of F-actin and the organization of microtubules

BACKGROUND

The recruitment of mRNA for translation involves the assembly at the 5'cap of a complex of three initiation factors: the cap binding protein eIF4E, the ATP-dependent RNA helicase eIF4A and the scaffold protein eIF4G. eIF4G mediates the binding of this mRNA-protein complex to the 43S ribosomal preinitiation complex. There is growing recognition that the components of the translational apparatus interact functionally with cytoskeletal components. Here we report specific effects of the over-expression of human and fission yeast eIF4G domains on cell morphology in Schizosaccharomyces pombe.

RESULTS

A single gene encoding fission yeast eIF4G was identified and demonstrated to be essential. We have over-expressed fragments corresponding to the conserved functional domains of eIF4G. At expression levels that did not disrupt rates of overall translation or protein accumulation, a fragment of S. pombe eIF4G, 4G-NOB, corresponding to the minimal region of human eIF4G required to support cap-independent mRNA recruitment, was found to impair cell proliferation in fission yeast. This resulted from defects in cytokinesis, and was associated with the disruption of both microtubules and actin microfilaments. The over-expressed fragment was itself localized to the cell ends, the nuclear periphery and the septum.

CONCLUSIONS

This is the first demonstration of a link between a translation initiation factor and mechanisms controlling cell morphology. The data suggest a direct or indirect interaction between the functional domains of eIF4G and cellular structures involved in cytokinesis.

A nuclear protein in Schizosaccharomyces pombe with homology to the human tumour suppressor Fhit has decapping activity

A number of eukaryotic proteins are already known to orchestrate key steps of mRNA metabolism and translation via interactions with the 5' m7GpppN cap. We have characterized a new type of histidine triad (HIT) motif protein (Nhm1) that co-purifies with the cap-binding complex eIF4F of Schizosaccharomyces pombe. Nhm1 is an RNA-binding protein that binds to m7GTP-Sepharose, albeit with lower specificity and affinity for methylated GTP than is typical for the cap-binding protein known as eukaryotic initiation factor 4E. Sequence searches have revealed that proteins with strong sequence similarity over all regions of the new protein exist in a wide range of eukaryotes, yet none has been characterized up to now. However, other proteins that share specific motifs with Nhm1 include the human Fhit tumour suppressor protein and the diadenosine 5', 5"'-P1, P4-tetraphosphate asymmetrical hydrolase of S. pombe. Our experimental work also reveals that Nhm1 inhibits translation in a cell-free extract prepared from S. pombe, and that it is therefore a putative translational modulator. On the other hand, purified Nhm1 manifests mRNA decapping activity, yet is physically distinct from the Saccharomyces cerevisiae decapping enzyme Dcp1. Moreover, fluorescence and immunofluorescence microscopy show that Nhm1 is predominantly, although not exclusively, nuclear. We conclude that Nhm1 has evolved as a special branch of the HIT motif superfamily that has the potential to influence both the metabolism and the translation of mRNA, and that its presence in S. pombe suggests the utilization of a novel decapping pathway.

Sum1, a component of the fission yeast eIF3 translation initiation complex, is rapidly relocalized during environmental stress and interacts with components of the 26S proteasome

Eukaryotic translation initiation factor 3 (eIF3) is a multisubunit complex that plays a central role in translation initiation. We show that fission yeast Sum1, which is structurally related to known eIF3 subunits in other species, is essential for translation initiation, whereas its overexpression results in reduced global translation. Sum1 is associated with the 40S ribosome and interacts stably with Int6, an eIF3 component, in vivo, suggesting that Sum1 is a component of the eIF3 complex. Sum1 is cytoplasmic under normal growth conditions. Surprisingly, Sum1 is rapidly relocalized to cytoplasmic foci after osmotic and thermal stress. Int6 and p116, another putative eIF3 subunit, behave similarly, suggesting that eIF3 is a dynamic complex. These cytoplasmic foci, which additionally comprise eIF4E and RNA components, may function as translation centers during environmental stress. After heat shock, Sum1 additionally colocalizes stably with the 26S proteasome at the nuclear periphery. The relationship between Sum1 and the 26S proteasome was further investigated, and we find cytoplasmic Sum1 localization to be dependent on the 26S proteasome. Furthermore, Sum1 interacts with the Mts2 and Mts4 components of the 26S proteasome. These data indicate a functional link between components of the structurally related eIF3 translation initiation and 26S proteasome complexes.

A second eIF4E protein in Schizosaccharomyces pombe has distinct eIF4G-binding properties

The eukaryotic cap-binding proteins belonging to the eIF4E family are generally involved in mediating the recruitment of ribosomes to capped mRNA. We described previously a cap-binding protein (now called eIF4E1) in Schizosaccharomyces pombe that appears to have all of the usual structural and functional attributes of an eIF4E. We have now characterised a new type of cap-binding protein (eIF4E2) from this organism, which at the amino acid sequence level, is 52% identical and 59% similar to eIF4E1. eIF4E2 is not essential in S.pombe but has some novel properties that may be related to a special function in the cell. The ratio of eIF4E2:eIF4E1 in the cell shifts in favour of eIF4E2 at higher temperatures. Despite having all of the dorsal face amino acids that have so far been associated with eIF4G binding to eIF4E1, eIF4E2 binds the eIF4E-binding domain of S.pombe eIF4G >10(2)-times weaker than eIF4E1 in vitro. The eIF4E2 cap-binding affinity is in the typical micromolar range. The results suggest that eIF4E2 is not active on the main pathway of translation initiation in fission yeast but might play a role in the adaptation strategy of this organism under specific growth conditions. Moreover, they provide insight into the molecular characteristics required for tight binding to eIF4G.

Effect of baculovirus infection on the mRNA and protein levels of the Spodoptera frugiperda eukaryotic initiation factor 4E

The cDNA sequence of eukaryotic translation initiation factor eIF4E was derived from a Spodoptera frugiperda cDNA library. Eight tryptophan residues, typical for eIF4E, are strictly conserved in the encoded 210 amino acid protein. A polyclonal antiserum detected a 26 kDa protein in lepidopteran cell lines, but not in dipteran cells. Sf21 cells have a single eIF4E gene copy, which is transcribed into a 1500 nt transcript. Infection with AcMNPV resulted in a decrease in eIF4E mRNA starting between 12 and 24 h postinfection (p.i.), while reduced eIF4E protein levels were observed at 48 h p.i. Two forms of eIF4E were recognized that differed in their iso-electric point, of which the relative abundance did not change during infection. Mutagenesis experiments using recombinant baculoviruses revealed that the variation in mobility between these two forms did not result from a difference in the phosphorylation state of Ser-202, the serine residue that corresponds with the eIF4E phosphorylation site in mammalian eIF4E.

A systematic nomenclature for new translation initiation factor genes from S. pombe and other fungi

Eukaryotic translation initiation factors and their corresponding genes have been characterized using biochemical and genetic methods from a variety of different organisms. The designations of the factors relate to their apparent roles in the biochemical process. Many gene names indicate genetic interactions with other genes or the functional attributes used to identify them. On the other hand, progress in systematic sequencing of the genomes of organisms like Saccharomyces cerevisiae and Schizosaccharomyces pombe has revealed many genes homologous to known translation initiation factor genes. The genes defined by the systematic sequencing approach are assigned numerical designations completely unrelated to their biological function. So far there have been publications on only three genes encoding translation initiation factors from Schizosaccharomyces pombe. We therefore see this an an ideal opportunity to propose a systematic and logical nomenclature for genes encoding translation initiation factor genes that can be applied to all further genes of this type that are characterized in this fission yeast.

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.

Cooperative modulation by eIF4G of eIF4E-binding to the mRNA 5' cap in yeast involves a site partially shared by p20

Interaction between the mRNA 5'-cap-binding protein eIF4E and the multiadaptor protein eIF4G has been demonstrated in all eukaryotic translation assemblies examined so far. This study uses immunological, genetic and biochemical methods to map the surface amino acids on eIF4E that contribute to eIF4G binding. Cap-analogue chromatography and surface plasmon resonance (SPR) analyses demonstrate that one class of mutations in these surface regions disrupts eIF4E-eIF4G association, and thereby polysome formation and growth. The residues at these positions in wild-type eIF4E mediate positive cooperativity between the binding of eIF4G to eIF4E and the latter's cap-affinity. Moreover, two of the mutations confer temperature sensitivity in eIF4G binding to eIF4E which correlates with the formation of large numbers of inactive ribosome 80S couples in vivo and the loss of cellular protein synthesis activity. The yeast 4E-binding protein p20 is estimated by SPR to have a ten times lower binding affinity than eIF4G for eIF4E. Investigation of a second class of eIF4E mutations reveals that p20 shares only part of eIF4G's binding site on the cap-binding protein. The results presented provide a basis for understanding how cycling of eIF4E and eIF4G occurs in yeast translation and explains how p20 can act as a fine, but not as a coarse, regulator of protein synthesis.

Multiple isoforms of eukaryotic protein synthesis initiation factor 4E in Caenorhabditis elegans can distinguish between mono- and trimethylated mRNA cap structures

The rate-limiting step for cap-dependent translation initiation in eukaryotes is recruitment of mRNA to the ribosome. An early event in this process is recognition of the m7GTP-containing cap structure at the 5'-end of the mRNA by initiation factor eIF4E. In the nematode Caenorhabditis elegans, mRNAs from 70% of the genes contain a different cap structure, m32,2,7GTP. This cap structure is poorly recognized by mammalian elF4E, suggesting that C. elegans may possess a specialized form of elF4E that can recognize m32,2,7GTP. Analysis of the C. elegans genomic sequence data base revealed the presence of three elF4E-like genes, here named ife-1, ife-2, and ife-3. cDNAs for these three eIF4E isoforms were cloned and sequenced. Isoform-specific antibodies were prepared from synthetic peptides based on nonhomologous regions of the three proteins. All three eIF4E isoforms were detected in extracts of C. elegans and were retained on m7GTP-Sepharose. One eIF4E isoform, IFE-1, was also retained on m32,2,7GTP-Sepharose. Furthermore, binding of IFE-1 and IFE-2 to m7GTP-Sepharose was inhibited by m32,2,7GTP. These results suggest that IFE-1 and IFE-2 bind both m7GTP- and m32,2, 7GTP-containing mRNA cap structures, although with different affinities. In conjunction with IFE-3, these eIF4E isoforms would permit cap-dependent recruitment of all C. elegans mRNAs to the ribosome.

Sce3, a suppressor of the Schizosaccharomyces pombe septation mutant cdc11, encodes a putative RNA-binding protein

In the fission yeast Schizosaccharomyces pombe, the cdc11 gene is required for the initiation of septum formation at the end of mitosis. The sce3 gene was cloned as a multi-copy suppressor of the heat-sensitive mutant cdc11-136. When over-expressed, it rescues all mutants of cdc11 and also a heat-sensitive allele of cdc14, but not the cdc14 null mutant. Deletion shows that sce3 is not essential for cell proliferation. It encodes a putative RNA-binding protein which shows homology to human eIF4B. Immunolocalisation indicates that Sce3p is located predominantly in the cytoplasm. Elevated expression of sce3 increases the steady-state level of cdc14 mRNA. Possible mechanisms of its action are discussed.

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

The X-ray structure of the eukaryotic translation initiation factor 4E (eIF4E), bound to 7-methyl-GDP, has been determined at 2.2 A resolution. eIF4E recognizes 5' 7-methyl-G(5')ppp(5')N mRNA caps during the rate-limiting initiation step of translation. The protein resembles a cupped hand and consists of a curved, 8-stranded antiparallel beta sheet, backed by three long alpha helices. 7-methyl-GDP binds in a narrow cap-binding slot on the molecule's concave surface, where 7-methyl-guanine recognition is mediated by base sandwiching between two conserved tryptophans, plus formation of three hydrogen bonds and a van der Waals contact between its N7-methyl group and a third conserved tryptophan. The convex dorsal surface of the molecule displays a phylogenetically conserved hydrophobic/acidic portion, which may interact with other translation initiation factors and regulatory proteins.