Cloning of murine translation initiation factor 6 and functional analysis of the homologous sequence YPR016c in Saccharomyces cerevisiae

Rice OseIF6.1 encodes a eukaryotic translation initiation factor and is essential for the development of grain and anther

The eIF6 proteins are distributed extensively in eukaryotes and play diverse and essential roles. The bona fide eIF6 protein in Arabidopsis, At-eIF6;1, is essential for embryogenesis. However, the role of eIF6 proteins in rice growth and development remains elusive and requires further investigation. Here, we characterized the functions of OseIF6.1, which is homologous to At-eIF6;1. OseIF6.1 encodes an eukaryotic translation initiation factor with a conserved eIF6 domain. The knockdown of OseIF6.1 resulted in a decrease in grain length and pollen sterility, whereas the overexpression of OseIF6.1 displayed opposite phenotypes. Further studies revealed that OseIF6.1 regulates grain shape by influencing cell expansion and proliferation. In addition, OseIF6.1 interacts with OsNMD3, which is a nuclear export adaptor for the 60S ribosomal subunit. The knockdown of OsNMD3 in plants exhibited reduced fertility and seed setting. Therefore, our findings have significantly enriched the current understanding of the role of OseIF6.1 in rice growth and development.

The RNA binding protein OsLa influences grain and anther development in rice

La proteins are found widely in eukaryotes and play a variety of vital roles. AtLa1 has been identified as an La protein that is necessary for embryogenesis in Arabidopsis; however, the existence and biological functions of La proteins in rice (Oryza sativa L.) remain unclear. In this study, we identified and characterized two La proteins in rice that are homologous to AtLa1 and named them OsLa1 and OsLa2. Both the OsLa1 and OsLa2 genes encode RNA-binding proteins with an La domain and two RNA-binding domains. Mutant OsLa1 reduced grain length and pollen fertility, whereas OsLa1 overexpression caused the opposite phenotypes. Further experiments indicated that OsLa1 modulates grain size by influencing cell expansion. Interestingly, mutant OsLa2 resulted in thin grains with decreased weight and a low seed-setting rate. We also found that OsLa1 interacted with OsLa2 and that both OsLa1 and OsLa2 interacted with OseIF6.1, a eukaryotic translation initiation factor involved in ribosome biogenesis. In addition, OsLa1 was able to bind to OseIF6.1 mRNA to modulate its expression. Complete OseIF6.1 knockout caused lethality and OseIF6.1/oseif6.1 heterozygous plants displayed low fertility and low seed setting. Together, our results enrich our knowledge of the role of La proteins in rice growth and development, as well as the relationship between La and eIF6 in rice.

Proteins of the Nucleolus of Dictyostelium discoideum: Nucleolar Compartmentalization, Targeting Sequences, Protein Translocations and Binding Partners

The nucleoli of Dictyostelium discoideum have a comparatively unique, non-canonical, localization adjacent to the inner nuclear membrane. The verified nucleolar proteins of this eukaryotic microbe are detailed while other potential proteins are introduced. Heat shock protein 32 (Hsp32), eukaryotic translation initiation factor 6 (eIF6), and tumour necrosis factor receptor-associated protein 1 (TRAP1) are essential for cell survival. NumA1, a breast cancer type 1 susceptibility protein-C Terminus domain-containing protein linked to cell cycle, functions in the regulation of nuclear number. The cell cycle checkpoint kinase 2 homologue forkhead-associated kinase A (FhkA) and BRG1-associated factor 60a homologue Snf12 are also discussed. While nucleoli appear homogeneous ultrastructurally, evidence for nucleolar subcompartments exists. Nucleolar localization sequences (NoLS) have been defined that target proteins to either the general nucleolar area or to a specific intranucleolar domain. Protein translocations during mitosis are protein-specific and support the multiple functions of the Dictyostelium nucleolus. To enrich the picture, binding partners of NumA1, the most well-characterized nucleolar protein, are examined: nucleolar Ca²⁺-binding protein 4a (CBP4a), nuclear puromycin-sensitive aminopeptidase A (PsaA) and Snf12. The role of Dictyostelium as a model for understanding the contribution of nucleolar proteins to various diseases and cellular stress is discussed throughout the review.

Suboptimal T-cell receptor signaling compromises protein translation, ribosome biogenesis, and proliferation of mouse CD8 T cells

Global transcriptomic and proteomic analyses of T cells have been rich sources of unbiased data for understanding T-cell activation. Lack of full concordance of these datasets has illustrated that important facets of T-cell activation are controlled at the level of translation. We undertook translatome analysis of CD8 T-cell activation, combining polysome profiling and microarray analysis. We revealed that altering T-cell receptor stimulation influenced recruitment of mRNAs to heavy polysomes and translation of subsets of genes. A major pathway that was compromised, when TCR signaling was suboptimal, was linked to ribosome biogenesis, a rate-limiting factor in both cell growth and proliferation. Defective TCR signaling affected transcription and processing of ribosomal RNA precursors, as well as the translation of specific ribosomal proteins and translation factors. Mechanistically, IL-2 production was compromised in weakly stimulated T cells, affecting the abundance of Myc protein, a known regulator of ribosome biogenesis. Consequently, weakly activated T cells showed impaired production of ribosomes and a failure to maintain proliferative capacity after stimulation. We demonstrate that primary T cells respond to various environmental cues by regulating ribosome biogenesis and mRNA translation at multiple levels to sustain proliferation and differentiation.

The Rice Eukaryotic Translation Initiation Factor 3 Subunit e (OseIF3e) Influences Organ Size and Pollen Maturation

Eukaryotic translation initiation factor 3 (eIF3) is a large protein complex that participates in most translation initiation processes. While eIF3 has been well characterized, less is known about the roles of individual eIF3 subunits, particularly in plants. Here, we identified and characterized OseIF3e in rice (Oryza sativa L.). OseIF3e was constitutively expressed in various tissues, but most strongly in vigorously growing organs. Transgenic OseIF3e-silenced rice plants showed inhibited growth in seedling and vegetative stages. Repression of OseIF3e led to defects in pollen maturation but did not affect pollen mitosis. In rice, eIF3e interacted with eIF3 subunits b, d, e, f, h, and k, and with eIF6, forming homo- and heterodimers to initiate translation. Furthermore, OseIF3e was shown by yeast two-hybrid assay to specifically bind to inhibitors of cyclin-dependent kinases 1, 5, and 6. This interaction was mediated by the sequence of amino acid residues at positions 118-138, which included a conserved motif (IGPEQIETLYQFAKF). These results suggested although OseIF3e is not a "functional core" subunit of eIF3, it still plays crucial roles in rice growth and development, in combination with other factors. We proposed a pathway by which OseIF3e influence organ size and pollen maturation in rice, providing an opportunity to optimize plant architecture for crop breeding.

eIF6 anti-association activity is required for ribosome biogenesis, translational control and tumor progression

Here we discuss the function of eukaryotic initiation factor 6 (eIF6; Tif6 in yeast). eIF6 binds 60S ribosomal subunits and blocks their joining to 40S. In this context, we propose that eIF6 impedes unproductive 80S formation, namely, the formation of 80S subunits without mRNA. Genetic evidence shows that eIF6 has a dual function: in yeast and mammals, nucleolar eIF6 is necessary for the biogenesis of 60S subunits. In mammals, cytoplasmic eIF6 is required for insulin and growth factor-stimulated translation. In contrast to other translation factors, eIF6 activity is not under mTOR control. The physiological significance of eIF6 impacts on cancer and on inherited Shwachman-Bodian-Diamond syndrome. eIF6 is overexpressed in specific human tumors. In a murine model of lymphomagenesis, eIF6 depletion leads to a striking increase of survival, without adverse effects. Shwachman-Bodian-Diamond syndrome is caused by loss of function of SBDS protein. In yeast, point mutations of Tif6, the yeast homolog of eIF6, rescue the quasi-lethal effect due to the loss of the SBDS homolog, Sdo1. We propose that eIF6 is a node regulator of ribosomal function and predict that prioritizing its pharmacological targeting will be of benefit in cancer and Shwachman-Bodian-Diamond syndrome. This article is part of a Special Issue entitled: Translation and Cancer.

CDK11p46 and RPS8 associate with each other and suppress translation in a synergistic manner

CDK11p46, a 46kDa isoform of the PITSLRE kinase family, is a key mediator of cell apoptosis, while the precise mechanism remains to be elucidated. By using His pull-down and mass spectrometry analysis, we identified the ribosomal protein S8 (RPS8), a member of the small subunit ribosome, as an interacting partner of CDK11p46. Further analysis confirmed the association of CDK11p46 and RPS8 in vitro and in vivo, and revealed that RPS8 was not a substrate of CDK11p46. Moreover, RPS8 and CDK11p46 synergize to inhibit the translation process both in cap- and internal ribosomal entry site (IRES)-dependent way, and sensitize cells to Fas ligand-induced apoptosis. Taken together, our results provide evidence for the novel role of CDK11p46 in the regulation of translation and cell apoptosis.

Characterization of plant eukaryotic translation initiation factor 6 (eIF6) genes: The essential role in embryogenesis and their differential expression in Arabidopsis and rice

Eukaryotic translation initiation factor 6 (eIF6) is an essential component of ribosome biogenesis. In our present study, we characterize plant eIF6 genes for the first time. Although a single gene encodes eIF6 in yeast and animals, two genes were found to encode proteins homologous to animal and yeast eIF6 in Arabidopsis and rice, denoted At-eIF6;1 and At-eIF6;2, and Os-eIF6;1 and Os-eIF6;2, respectively. Analysis of the yeast eif6 (tif6) mutant suggested that plant eIF6, at least in the case of At-eIF6;1, can complement the essential function of eIF6 in yeast. Evidence for the essential role of eIF6 in plants was also provided by the embryonic-lethal phenotype of the at-eif6;1 mutant. In contrast, At-eIF6;2 appears not to be essential due to its very low expression level and the normal growth phenotype of the eif6;2 mutants. Consistent with the putative role of plant eIF6 in ribosome biogenesis, At-eIF6;1 is predominately expressed in tissues where cell division actively proceeds under the control of intronic cis-regulatory elements. On the other hand, both Os-eIF6;1 and Os-eIF6;2 are probably active genes because they are expressed at significant expression levels. Interestingly, the supply of ammonium nitrate as a plant nutrient was found to induce specifically the expression of Os-eIF6;2. Our present findings indicate that the eIF6 genes have differently evolved in plant and animal kingdoms and also in distinct plant species.

Eukaryotic initiation factor 6 mediates a continuum between 60S ribosome biogenesis and translation

Eukaryotic ribosome biogenesis and translation are linked processes that limit the rate of cell growth. Although ribosome biogenesis and translation are mainly controlled by distinct factors, eukaryotic initiation factor 6 (eIF6) has been found to regulate both processes. eIF6 is a necessary protein with a unique anti-association activity, which prevents the interaction of 40S ribosomal subunits with 60S subunits through its binding to 60S ribosomes. In the nucleolus, eIF6 is a component of the pre-ribosomal particles and is required for the biogenesis of 60S subunits, whereas in the cytoplasm it mediates translation downstream from growth factors. The translational activity of eIF6 could be due to its anti-association properties, which are regulated by post-translational modifications; whether this anti-association activity is required for the biogenesis and nuclear export of ribosomes is unknown. eIF6 is necessary for tissue-specific growth and oncogene-driven transformation, and could be a new rate-limiting step for the initiation of translation.

Eukaryotic initiation factor 6 is rate-limiting in translation, growth and transformation

Cell growth and proliferation require coordinated ribosomal biogenesis and translation. Eukaryotic initiation factors (eIFs) control translation at the rate-limiting step of initiation. So far, only two eIFs connect extracellular stimuli to global translation rates: eIF4E acts in the eIF4F complex and regulates binding of capped messenger RNA to 40S subunits, downstream of growth factors, and eIF2 controls loading of the ternary complex on the 40S subunit and is inhibited on stress stimuli. No eIFs have been found to link extracellular stimuli to the activity of the large 60S ribosomal subunit. eIF6 binds 60S ribosomes precluding ribosome joining in vitro. However, studies in yeasts showed that eIF6 is required for ribosome biogenesis rather than translation. Here we show that mammalian eIF6 is required for efficient initiation of translation, in vivo. eIF6 null embryos are lethal at preimplantation. Heterozygous mice have 50% reduction of eIF6 levels in all tissues, and show reduced mass of hepatic and adipose tissues due to a lower number of cells and to impaired G1/S cell cycle progression. eIF6(+/-) cells retain sufficient nucleolar eIF6 and normal ribosome biogenesis. The liver of eIF6(+/-) mice displays an increase of 80S in polysomal profiles, indicating a defect in initiation of translation. Consistently, isolated hepatocytes have impaired insulin-stimulated translation. Heterozygous mouse embryonic fibroblasts recapitulate the organism phenotype and have normal ribosome biogenesis, reduced insulin-stimulated translation, and delayed G1/S phase progression. Furthermore, eIF6(+/-) cells are resistant to oncogene-induced transformation. Thus, eIF6 is the first eIF associated with the large 60S subunit that regulates translation in response to extracellular signals.

Eukaryotic initiation factor 6 selectively regulates Wnt signaling and beta-catenin protein synthesis

Eukaryotic initiation factor 6 (eIF6), an essential protein important in ribosome biosynthesis and assembly, was identified as an interacting partner of the beta-catenin C terminus in the yeast two-hybrid assay. Independent studies identified Drosophila eIF6 (DeIF6) in a genetic screen designed to detect new genes involved in the regulation of the Wnt/Wg (wingless) pathway. Ectopic expression of DeIF6 in wing discs results in a Wg phenotype. Expression of eIF6 in adenomatous polyposis coli (APC)-mutant colon cancer cells, which express high levels of active beta-catenin, showed that eIF6 selectively inhibits the Wnt pathway at the level of beta-catenin protein independently of proteasomal degradation. Incorporation of radiolabeled amino acids into beta-catenin was selectively decreased in cells that overexpressed eIF6. A similar inverse relationship of the two proteins was observed in the APC(min/+) mouse intestine, in which beta-catenin levels are very high. Taken together these data reveal a link between eIF6 and Wnt signaling, perhaps at the level of ribosome recycling on beta-catenin mRNA.

Nuclear export and cytoplasmic maturation of ribosomal subunits

Based on the characterization of ribosome precursor particles and associated trans-acting factors, a biogenesis pathway for the 40S and 60S subunits has emerged. After nuclear synthesis and assembly steps, pre-ribosomal subunits are exported through the nuclear pore complex in a Crm1- and RanGTP-dependent manner. Subsequent cytoplasmic biogenesis steps of pre-60S particles include the facilitated release of several non-ribosomal proteins, yielding fully functional 60S subunits. Cytoplasmic maturation of 40S subunit precursors includes rRNA dimethylation and pre-rRNA cleavage, allowing 40S subunits to achieve translation competence. We review current knowledge of nuclear export and cytoplasmic maturation of ribosomal subunits.

In vivo functional characterization of the Saccharomyces cerevisiae 60S biogenesis GTPase Nog1

The Saccharomyces cerevisiae Nog1 GTPase is critical for assembly of the large ribosomal subunit. Mutations in conserved residues in the GTP-binding pocket cause defects in cell growth and 60S ribosome assembly but mutant proteins retain their ability to associate with the pre-60S. Association of Nog1 with the pre-60S is independent of guanine nucleotide added to cell extracts. Thus, it appears that nucleotide occupancy does not substantially affect Nog1 association with pre-60S particles. Somewhat surprisingly, neither of the conserved threonines in the G2 motif of the GTPase domain is essential for Nog1 function. Neither the steady-state rRNA levels nor the protein composition (as determined by isobaric labeling and identification by mass spectrometry of peptides) of the pre-60S particles in the nog1P176V mutant are grossly perturbed, although levels of four proteins (Nog1, Nop2, Nop15, and Tif6) are modestly reduced in pre-60S particles isolated from the mutant. Deletion analysis revealed that the C-terminal 168 amino acids are not required for function; however, the N-terminal 126 amino acids are required. Optimal association with pre-60S particles requires sequences between amino acids 347-456. Several conserved charge-to-alanine substitutions outside the GTPase domain display modest growth phenotypes indicating that these residues are not critical for function.

Cloning of Dictyostelium eIF6 (p27BBP) and mapping its nucle(ol)ar localization subdomains

Eukaryotic translation initiation factor 6 (eIF6), also termed p27BBP, is an evolutionary conserved regulator of ribosomal function. The protein is involved in maturation and/or export from the nucleus of the 60S ribosomal subunit. Regulated binding to and release from the 60S subunit also regulates formation of 80S ribosomes, and thus translation. The protein is also found in hemidesmosomes of epithelial cells expressing beta4 integrin and is assumed to regulate cross-talk between beta4 integrin, intermediate filaments and ribosomes. In the present study we show that the Dictyostelium eIF6 (also called p27BBP) gene is expressed during growth, down-regulated during the first hours of starvation, and up-regulated again at the end of aggregation. Phagocytosis, and to a lesser extent pinocytic uptake of axenic medium, stimulate gene expression in starving cells. The eIF6 gene is present in single copy and its ablation is lethal. We utilized the green fluorescent protein (GFT) as fusion protein marker to investigate sequences responsible for eIF6 subcellular localization. The protein is found both in cytoplasm and nucleus, and is enriched in nucleoli. Deletion sequence analysis shows that nucle(ol)ar localization sequences are located within the N- and C-terminal subdomains of the protein.

GABP complex regulates transcription of eIF6 (p27BBP), an essential trans-acting factor in ribosome biogenesis

Eukaryotic initiation factor 6 (eIF6, alias p27BBP) is required for the biogenesis of 60S ribosomal subunits. eIF6 expression levels are tightly regulated in vivo, where they correlate with cellular growth. We analyzed how transcriptional regulation of eIF6 is achieved. We show that the human eIF6 promoter contains consensus sites for the GABP (GA-binding protein) transcription factor complex. Functional analysis of GABP consensus sequences by point mutations, EMSA (electrophoretic mobility shift assay) and a dominant negative mutant indicates that GABP is essential for eIF6 promoter activity. These data strengthen the hypothesis that GABP is a global regulator of ribosome synthesis.

Deletion of EFL1 results in heterogeneity of the 60 S GTPase-associated rRNA conformation

Previous work suggested that the release of the nucleolar Tif6 from nascent 60 S subunits occurs in the cytoplasm and requires the cytoplasmic EF-2-like GTPase, Efl1. To check whether this release involves an rRNA structural rearrangement mediated by Efl1, we analyzed the rRNA conformation of the GTPase center of 80 S ribosomes in three contexts: wild-type, Deltaefl1 and a dominant suppressor R1 of Deltaefl1. This analysis was restricted to domain II and VI of 25 S rRNA. The rRNA analysis of R1 ribosomes allows us to distinguish the effects due to depletion of Efl1 from the resulting nucleolar deficit of Tif6. Efl1 inhibits the EF-2 GTPase activity, suggesting that the two proteins share a similar ribosome-binding site. The 80 S ribosomes from either type failed to show any difference of conformation in the two rRNA domains analyzed. However, the same analysis performed on the pool of free 60 S subunits reveals several rRNA conformational differences between wild-type and Deltaefl1 subunits, whereas that from the suppressor strain is similar to wild-type. This suggests that the nucleolar deficit of Tif6 during assembly of the 60 S preribosomes is responsible for the changes in rRNA conformation observed in Deltaefl1 60 S subunits. We also purified 60 S preribosomes from the three genetic contexts by TAP-tagging Tif6. The protein content of 60 S preribosomes associated with Tif6p in a Deltaefl1 strain are obtained at a lower yield but have, surprisingly, a protein composition that is a priori similar to that of wild-type and the suppressor strain.

Immunocytochemical localization of protein p27BBP in human skin and invertebrate (Sepia officinalis) integument

The protein p27BBP (alias eIF6) occurs in yeast and mammalian epithelial cells. It is essential for ribosome genesis and has also been implicated in the functionality of integrins and intermediate filaments. By immunoblot, we show that homogenized integument from Sepia officinalis (Cephalopoda, Mollusca) contains a protein with immunological properties that closely resemble those of p27BBP. We also demonstrate, by immunogold electron microscopy with an indirect immunoreaction technique on ultrathin sections of human skin and Sepia integument, that p27BBP is constantly present in both species in epithelial cells, fibroblasts, and muscle fibers. It is found in the vicinity of intermediate filaments, in nucleoli, along the internal wall of the nuclear membrane, and in association with desmosomes and hemidesmosomes and occasionally occurs extracellularly. Thus, the structure and function of p27BBP seem to have been highly conserved throughout evolution; the protein appears to be essential in eukaryotic cells in which it interacts with several ultrastructural components of diverse function.

Overexpression of p27BBP in head and neck carcinomas and their lymph node metastases

BACKGROUND

p27(BBP) is a regulator of ribosome assembly and an essential nuclear and cytoplasmic component of eukaryotes.

METHODS

We investigated the immunochemical distribution of p27(BBP) in head and neck carcinomas, in the associated normal mucosa, and in regional lymph nodes.

RESULTS

p27(BBP) is detectable in mucosal cells but is overexpressed in carcinomas, highly concentrated in large polymorphous nucleoli, and even larger and more evident in lymph node metastatic foci. Western blotting confirms increased p27(BBP) in carcinomas versus normal mucosa and also in metastatic versus normal lymph nodes. The overexpression of p27(BBP) corresponds to mRNA upregulation in carcinomas. Unexpectedly, a 52-kDa band specifically reacting with antibodies to p27(BBP) was observed in several carcinomas.

CONCLUSIONS

p27(BBP) alterations are common events in the transition to malignancy and are probably involved in squamous carcinoma progression. Immune reagents raised to p27(BBP) may provide additional diagnostic tools for surgical pathology of tumor boundaries and lymph nodes. The 52-kDa band may represent an abnormal form of p27(BBP) expressed by transformed airway epithelia.

Release of eIF6 (p27BBP) from the 60S subunit allows 80S ribosome assembly

The assembly of 80S ribosomes requires joining of the 40S and 60S subunits, which is triggered by the formation of an initiation complex on the 40S subunit. This event is rate-limiting for translation, and depends on external stimuli and the status of the cell. Here we show that 60S subunits are activated by release of eIF6 (also termed p27BBP). In the cytoplasm, eIF6 is bound to free 60S but not to 80S. Furthermore, eIF6 interacts in the cytoplasm with RACK1, a receptor for activated protein kinase C (PKC). RACK1 is a major component of translating ribosomes, which harbour significant amounts of PKC. Loading 60S subunits with eIF6 caused a dose-dependent translational block and impairment of 80S formation, which were reversed by expression of RACK1 and stimulation of PKC in vivo and in vitro. PKC stimulation led to eIF6 phosphorylation, and mutation of a serine residue in the carboxy terminus of eIF6 impaired RACK1/PKC-mediated translational rescue. We propose that eIF6 release regulates subunit joining, and that RACK1 provides a physical and functional link between PKC signalling and ribosome activation.

Phosphorylation of mammalian eukaryotic translation initiation factor 6 and its Saccharomyces cerevisiae homologue Tif6p: evidence that phosphorylation of Tif6p regulates its nucleocytoplasmic distribution and is required for yeast cell growth

The synthesis of 60S ribosomal subunits in Saccharomyces cerevisiae requires Tif6p, the yeast homologue of mammalian eukaryotic translation initiation factor 6 (eIF6). In the present work, we have isolated a protein kinase from rabbit reticulocyte lysates on the basis of its ability to phosphorylate recombinant human eIF6. Mass spectrometric analysis as well as antigenic properties of the purified kinase identified it as casein kinase I. The site of in vitro phosphorylation, which is highly conserved from yeast to mammals, was identified as the serine residues at positions 174 (major site) and 175 (minor site). The homologous yeast protein Tif6p was also phosphorylated in vivo in yeast cells. Mutation of Tif6p at serine-174 to alanine reduced phosphorylation drastically and caused loss of cell growth and viability. When both Ser-174 and Ser-175 were mutated to alanine, phosphorylation of Tif6p was completely abolished. Furthermore, while wild-type Tif6p was distributed both in nuclei and the cytoplasm of yeast cells, the mutant Tif6p (with Ser174Ala and Ser175Ala) became a constitutively nuclear protein. These results suggest that phosphorylatable Ser-174 and Ser-175 play a critical role in the nuclear export of Tif6p.

Ribosome assembly in eukaryotes

Ribosome synthesis is a highly complex and coordinated process that occurs not only in the nucleolus but also in the nucleoplasm and the cytoplasm of eukaryotic cells. Based on the protein composition of several ribosomal subunit precursors recently characterized in yeast, a total of more than 170 factors are predicted to participate in ribosome biogenesis and the list is still growing. So far the majority of ribosomal factors have been implicated in RNA maturation (nucleotide modification and processing). Recent advances gave insight into the process of ribosome export and assembly. Proteomic approaches have provided the first indications for a ribosome assembly pathway in eukaryotes and confirmed the dynamic character of the whole process.

Formation of nuclear matrix filaments by p27(BBP)/eIF6

p27(BBP)/eIF6 is an evolutionarily conserved protein necessary for ribosome biogenesis which was cloned in mammals for its ability to bind the cytodomain of beta 4 integrin. In cultured cells, a conspicuous fraction of p27(BBP)/eIF6 is associated with the intermediate filaments/nuclear matrix (IF/NM) cytoskeleton. The mechanism of this association is not known. Here we show that in epidermis p27(BBP)/eIF6 is naturally associated with IF/NM. To analyze the intrinsic capability of p27(BBP)/eIF6 to generate cytoskeletal networks, the properties of the pure, recombinant, untagged protein were studied. Recombinant p27(BBP)/eIF6 binds beta 4 integrin. Upon dialysis against IF buffer, p27(BBP)/eIF6 forms polymers which, strikingly, have a morphology identical to NM filaments. Cross-linking experiments suggested that polymerization is favored by the formation of disulphide bridges. These data suggest that p27(BBP)/eIF6 is associated with the cytoskeleton, and contributes to formation of NM filaments. These findings help to settle the controversy on nuclear matrix.

Yph1p, an ORC-interacting protein: potential links between cell proliferation control, DNA replication, and ribosome biogenesis

Immunoprecipitation of the origin recognition complex (ORC) from yeast extracts identified Yph1p, an essential protein containing a BRCT domain. Two Yph1p complexes were characterized. Besides ORC, MCM proteins, cell-cycle regulatory proteins, checkpoint proteins, 60S ribosomal proteins, and preribosome particle proteins were found to be associated with Yph1p. Yph1p is predominantly nucleolar and is required for 60S ribosomal subunit biogenesis and possibly for translation on polysomes. Proliferating cells depleted of Yph1p arrest in G(1) or G(2), with no cells in S phase, or significantly delay S phase progression after release from a hydroxyurea arrest. Yph1p levels decline as cells commit to exit the cell cycle, and levels vary depending on energy source. Yph1p may link cell proliferation control to DNA replication, ribosome biogenesis, and translation on polysomes.

The emerging roles of translation factor eIF4E in the nucleus

The emerging field of nuclear eIF research has yielded many surprises and led to the dissolution of some dogmatic/ideological viewpoints of the place of translation in the regulation of gene expression. Eukaryotic initiation factors (eIFs) are classically defined by their cytoplasmic location and ability to regulate the initiation phase of protein synthesis. For instance, in the cytoplasm, the m7G cap-binding protein eIF4E plays a distinct role in cap-dependent translation initiation. Disruption of eIF4E's regulatory function drastically effects cell growth and may lead to oncogenic transformation. A growing number of studies indicate that many eIFs, including a substantial fraction of eIF4E, are found in the nucleus. Indeed, nuclear eIF4E participates in a variety of important RNA-processing events including the nucleocytoplasmic transport of specific, growth regulatory mRNAs. Although unexpected, it is possible that some eIFs regulate protein synthesis within the nucleus. This review will focus on the novel, nuclear functions of eIF4E and how they contribute to eIF4E's growth-activating and oncogenic properties. Both the cytoplasmic and nuclear functions of eIF4E appear to be dependent on its intrinsic ability to bind to the 5' m7G cap of mRNA. For example, Promyelocytic Leukemia Protein (PML) potentially acts as a negative regulator of nuclear eIF4E function by decreasing eIF4E's affinity for the m7G cap. Therefore, eIF4E protein is flexible enough to utilize a common biochemical activity, such as m7G cap binding, to participate in divergent processes in different cellular compartments.

The nucle(ol)ar Tif6p and Efl1p are required for a late cytoplasmic step of ribosome synthesis

Deletion of elongation factor-like 1 (Efl1p), a cytoplasmic GTPase homologous to the ribosomal translocases EF-G/EF-2, results in nucle(ol)ar pre-rRNA processing and pre-60S subunits export defects. Efl1p interacts genetically with Tif6p, a nucle(ol)ar protein stably associated with pre-60S subunits and required for their synthesis and nuclear exit. In the absence of Efl1p, 50% of Tif6p is relocated to the cytoplasm. In vitro, the GTPase activity of Efl1p is stimulated by 60S, and Efl1p promotes the dissociation of Tif6p-60S complexes. We propose that Tif6p binds to the pre-60S subunits in the nucle(ol)us and escorts them to the cytoplasm where the GTPase activity of Efl1p triggers a late structural rearrangement, which facilitates the release of Tif6p and its recycling to the nucle(ol)us.

Eukaryotic translation initiation factor-6 enhances histamine and IL-2 production in mast cells

Eukaryotic translation initiation factor (eIF)-6 is known to be important in ribosome biogenesis. Previously, we have discovered that eIF-6 mRNA is induced in lung in a murine model of asthma. We also found that there was enhanced eIF-6 expression in mast cells stimulated with PMA plus calcium ionophore. Therefore, we hypothesized that the induction of eIF-6 enhances the production of bioactive mediators by mast cells upon allergic stimulation. In the current study, we found that eIF-6 mRNA was rapidly induced in murine mast cells stimulated by Fc epsilon RI cross-linking, which is a major physiologic stimulant for mast cells. eIF-6 was also induced in human mast cells upon stimulation. The increase in eIF-6 gene expression in murine mast cells was blocked by therapeutic agents such as dexamethasone and cyclosporin A. To determine the location and function of eIF-6, murine mast cells were transfected with a construct that overexpressed enhanced green fluorescent protein-tagged eIF-6. These experiments demonstrated that eIF-6 was localized predominantly in the nucleolus of the mast cells. Also, overexpression of enhanced green fluorescent protein/eIF-6 enhanced the production of histamine and IL-2, but not IL-4 by stimulated murine mast cells. These results suggest that eIF-6 regulates the production of selected bioactive mediators in allergic diseases. This is the first demonstration of a biologic function of eIF-6 in mammalian cells.

Analysis of gene expression in mouse 2-cell embryos using fluorescein differential display: comparison of culture environments

The effect of the oviductal environment on gene expression in 2-cell mouse embryos was examined with mRNA differential display. Embryos used for experiments were cultured in modified Whitten medium with or without oviductal tissue until late 2-cell stage. The results of sequencing indicated that the genes for ATP synthase (ATPase 6), S:-adenosylmethionine decarboxylase (S:-AMDC) and nuclear autoantigenic sperm protein (NASP) were differentially expressed in embryos cultured in the oviductal environment (nonblocking culture condition). The ATPase 6 gene is encoded by mitochondrial DNA and is essential for the production of ATP. This indicates that the expression of ATP synthesis-related genes at the 2-cell stage may be required to maintain normal development in vitro. S:-Adenosylmethionine decarboxylase decarboxylates adenosylmethionine, which is a substrate of DNA methylation. The expression of S:-AMDC may be responsible for the low level of methylation of preimplantation development. As NASP is a histone-binding protein that is thought to be testis and sperm specific, its function in embryos remains unclear. On the other hand, the Tcl1 gene and a novel gene, the c-1 gene, were strongly expressed in embryos cultured without oviductal tissue (blocking culture condition). The expression patterns of these genes are quite similar. However, the detailed functions of these genes in embryos remain to be determined.