Immunoblot analysis of the structure of protein synthesis initiation factor eIF3 from HeLa cells

Apoptosis-inducing factor (AIF) inhibits protein synthesis by interacting with the eukaryotic translation initiation factor 3 subunit p44 (eIF3g)

Apoptosis-inducing factor (AIF) is a ubiquitous FAD-binding flavoprotein comprised of 613 amino acids and plays an important role in caspase-independent apoptosis. During apoptotic induction, AIF is translocated from the mitochondrial intermembrane space to the nucleus, where it interacts with DNA and activates a nuclear endonuclease. By performing a yeast two-hybrid screen with mature AIF, we have isolated the eukaryotic translation initiation factor 3 subunit p44 (eIF3g). Our deletion mutant analysis revealed that the eIF3g N-terminus interacts with the C-terminal region of AIF. The direct interaction between AIF and eIF3g was confirmed in a GST pull-down assay and also verified by the results of co-immunoprecipitation and confocal microscopy studies. Using an in vitro TNT coupled transcription-translation system, we found that mature AIF could inhibit newly-translated protein synthesis and this inhibition was significantly blocked by eIF3g competitively. These results were also confirmed in cells. In addition, mature AIF overexpression specifically resulted in the activation of caspase-7, thereby amplifying the inhibition of protein synthesis including eIF3g cleavage. Our data suggest that eIF3g is one of the cytosolic targets that interacts with mature AIF, and provide insight into the AIF's cellular functions of the inhibition of protein synthesis during apoptosis.

Mass spectrometric analysis of the N terminus of translational initiation factor eIF4G-1 reveals novel isoforms

In eukaryotes, translation initiation factor 4G (eIF4G) acts as the central binding protein for an unusually large number of proteins involved in mRNA metabolism. Several gene products homologous to eIF4G have been described, the most studied being eIF4G-1. By its association with other initiation factors, eIF4G-1 effects mRNA cap and poly(A) recognition, unwinding of secondary structure, and binding to the 43S initiation complex. Multiple electrophoretic isoforms of eIF4G-1 are observed, and multiple cDNAs have been reported, yet the relationship between the two is not known. We report here a new cDNA for eIF4G-1, present as a previously unidentified human expressed sequence tag, that extends the long open reading frame, provides a new in-frame initiation codon, and predicts a longer form of eIF4G-1 than reported previously. eIF4G isoforms from human K562 cells were cleaved with recombinant Coxsackievirus 2A protease and the N- terminal domains purified by m(7)GTP-Sepharose chromatography and polyacrylamide gel electrophoresis. Proteins were digested with proteolytic enzymes and peptides masses determined by matrix-assisted laser desorption ionization-time of flight mass spectrometry. In selected cases, peptides were sequenced by electrospray-mass spectrometry fragmentation. This identified the N termini of the three most abundant eIF4G-1 isoforms, two of which had not previously been proposed. These proteins appear to have been initiated from three different AUG codons.

Two subcellular localizations of eIF3 p170 and its interaction with membrane-bound microfilaments: implications for alternative functions of p170

We previously identified a 170-kDa protein (p170) highly expressed in lung cancers as the major subunit of the eukaryotic translation initiation factor 3 (eIF3). p170 was recently cloned and little is known concerning its characteristics and subcellular localization. In this paper, we report our surprising findings that about 20% of p170 is associated with membranes while the remaining portion is located in the cytoplasm presumably in the eIF3 complex. We also find that p170 interacts with both endoplasmic reticulum and plasma membranes. The binding of p170 to membranes is through actin filaments, consistent with the fact that p170 contains a spectrin repeat motif that may be involved in actin binding. Furthermore, the cytoplasmic p170 is phosphorylated at serine and threonine residues and the phosphorylation is stimulated by serum. However, the membrane-actin-bound p170 is not phosphorylated. The results obtained in this study suggest that p170 may have other functions in addition to participating in translation initiation. Phosphorylation may play an important regulatory role in the function of p170 in translation initiation and other alternative functions.

Plant initiation factor 3 subunit composition resembles mammalian initiation factor 3 and has a novel subunit

Eukaryotic initiation factor 3 (eIF3) is a multisubunit complex that is required for binding of mRNA to 40 S ribosomal subunits, stabilization of ternary complex binding to 40 S subunits, and dissociation of 40 and 60 S subunits. These functions and the complex nature of eIF3 suggest multiple interactions with many components of the translational machinery. Recently, the subunits of mammalian and Saccharomyces cerevisiae eIF3 were identified, and substantial differences in the subunit composition of mammalian and S. cerevisiae were observed. Mammalian eIF3 consists of 11 nonidentical subunits, whereas S. cerevisiae eIF3 consists of up to eight nonidentical subunits. Only five of the subunits of mammalian and S. cerevisiae are shared in common, and these five subunits comprise a "core" complex in S. cerevisiae. eIF3 from wheat consists of at least 10 subunits, but their relationship to either the mammalian or S. cerevisiae eIF3 subunits is unknown. Peptide sequences derived from purified wheat eIF3 subunits were used to correlate each subunit with mammalian and/or S. cerevisiae subunits. The peptide sequences were also used to identify Arabidopsis thaliana cDNAs for each of the eIF3 subunits. We report seven new cDNAs for A. thaliana eIF3 subunits. A. thaliana eIF3 was purified and characterized to confirm that the subunit composition and activity of wheat and A. thaliana eIF3 were similar. We report that plant eIF3 closely resembles the subunit composition of mammalian eIF3, having 10 out of 11 subunits in common. Further, we find a novel subunit in the plant eIF3 complex not present in either mammalian or S. cerevisiae eIF3. These results suggest that plant and mammalian eIF3 evolved similarly, whereas S. cerevisiae has diverged.

Structure of cDNAs encoding human eukaryotic initiation factor 3 subunits. Possible roles in RNA binding and macromolecular assembly

The mammalian translation initiation factor 3 (eIF3), is a multiprotein complex of approximately 600 kDa that binds to the 40 S ribosome and promotes the binding of methionyl-tRNAi and mRNA. cDNAs encoding 5 of the 10 subunits, namely eIF3-p170, -p116, -p110, -p48, and -p36, have been isolated previously. Here we report the cloning and characterization of human cDNAs encoding the major RNA binding subunit, eIF3-p66, and two additional subunits, eIF3-p47 and eIF3-p40. Each of these proteins is present in immunoprecipitates formed with affinity-purified anti-eIF3-p170 antibodies. Human eIF3-p66 shares 64% sequence identity with a hypothetical Caenorhabditis elegans protein, presumably the p66 homolog. Deletion analyses of recombinant derivatives of eIF3-p66 show that the RNA-binding domain lies within an N-terminal 71-amino acid region rich in lysine and arginine. The N-terminal regions of human eIF3-p40 and eIF3-p47 are related to each other and to 17 other eukaryotic proteins, including murine Mov-34, a subunit of the 26 S proteasome. Phylogenetic analyses of the 19 related protein sequences, called the Mov-34 family, distinguish five major subgroups, where eIF3-p40, eIF3-p47, and Mov-34 are each found in a different subgroup. The subunit composition of eIF3 appears to be highly conserved in Drosophila melanogaster, C. elegans, and Arabidopsis thaliana, whereas only 5 homologs of the 10 subunits of mammalian eIF3 are encoded in S. cerevisiae.

The translation initiation factor eIF3-p48 subunit is encoded by int-6, a site of frequent integration by the mouse mammary tumor virus genome

Translation initiation factor eIF3 is a large, multisubunit protein complex that plays a central role in the pathway of initiation by promoting the binding of both methionyl-tRNAi and mRNA to the 40S ribosomal subunit. As part of a broad effort to elucidate the structure of eIF3, we have cloned and sequenced the human cDNA encoding the 48-kDa subunit, eIF3-p48. The recombinant protein comigrates with the authentic p48 subunit in purified eIF3 and coprecipitates with affinity-purified antibodies to the p170 subunit of eIF3. A search of the data base indicates that the mouse gene encoding eIF3-p48 had previously been identified and characterized by others as int-6. The int-6 gene is the site of frequent integration of mouse mammary tumor virus DNA into chromosomes, implicating the gene in the regulation of cell proliferation. In addition, it was shown elsewhere that the homologous human int-6 gene product binds to the human T-cell leukemia virus type I Tax protein, leading to the translocation of Int-6 to the cytoplasm. We discuss how the cytosolic function of eIF3-p48 (Int-6) in protein synthesis may account for oncogenesis caused by these two viruses.

Conservation and diversity of eukaryotic translation initiation factor eIF3

The largest of the mammalian translation initiation factors, eIF3, consists of at least eight subunits ranging in mass from 35 to 170 kDa. eIF3 binds to the 40 S ribosome in an early step of translation initiation and promotes the binding of methionyl-tRNAi and mRNA. We report the cloning and characterization of human cDNAs encoding two of its subunits, p110 and p36. It was found that the second slowest band during polyacrylamide gel electrophresis of eIF3 subunits in sodium dodecyl sulfate contains two proteins: p110 and p116. Analysis of the cloned cDNA encoding p110 indicates that its amino acid sequence is 31% identical to that of the yeast protein, Nip1. The p116 cDNA was cloned and characterized as a human homolog of yeast Prt1, as described elsewhere (Methot, N., Rom, E., Olsen, H., and Sonenberg, N. (1997) J. Biol. Chem. 272, 1110-1116). p36 is a WD40 repeat protein, which is 46% identical to the p39 subunit of yeast eIF3 and is identical to TRIP-1, a phosphorylation substrate of the TGF-beta type II receptor. The p116, p110, and p36 subunits localize on 40 S ribosomes in cells active in translation and co-immunoprecipitate with affinity-purified antibodies against the p170 subunit, showing that these proteins are integral components of eIF3. Although p36 and p116 have homologous protein subunits in yeast eIF3, the p110 homolog, Nip1, is not detected in yeast eIF3 preparations. The results indicate both conservation and diversity in eIF3 between yeast and humans.

The human homologue of the yeast Prt1 protein is an integral part of the eukaryotic initiation factor 3 complex and interacts with p170

Eukaryotic initiation factor 3 (eIF3) is a large multisubunit complex that stabilizes the ternary complex, eIF2 x GTP x tRNA(Met)i and promotes mRNA binding to the 40 S ribosomal subunit. eIF3 also functions as a ribosome subunit anti-association factor. The molecular mechanisms by which eIF3 exerts these functions are poorly understood. We describe here the cloning of the cDNA encoding the human homologue of the yeast eIF3 subunit Prt1. The human PRT1 cDNA encodes a protein of predicted molecular mass of 98.9 kDa that migrates at 116 kDa on SDS-polyacrylamide gels. Human and yeast Prt1 share 31% identity and 50% similarity at the amino acid level. The homology is distributed throughout the entire protein, except for the amino terminus, and is particularly high in the central portion of the protein, which contains a putative RNA recognition motif. hPrt1 is recognized by an antibody raised against eIF3, and an affinity-purified antibody to recombinant hPrt1 recognizes a protein migrating at 116 kDa in a purified eIF3 preparation. Far Western analysis shows that hPrt1 interacts directly with the p170 subunit of eIF3. Mapping studies identify the RNA recognition motif as the region required for association with p170. Taken together, these experiments demonstrate that hPrt1 is a component of eIF3. Our data, combined with those of Hershey and co-workers, suggest that mammalian eIF3 is composed of at least 10 subunits: p170, p116 (hPrt1), p110, p66, p48, p47, p44, p40, p36, and p35.

Initiation of protein synthesis in eukaryotes

The study of the regulation of initiation of protein synthesis has recently gained momentum because of the established relationship between translation initiation, cell growth and tumorigenesis. Therefore much effort is devoted to the role of protein kinases which are activated in signal transduction cascades and which are responsible for the phosphorylation of a number of initiation factors. These specific factors are mainly involved in the binding of messenger RNA to the 40S ribosome, a process that makes the unwinding of the 5' untranslated region necessary. It appears that the phosphorylation of these factors increases their ability for cap recognition and helicase activity. The enhanced phosphorylation of the messenger binding factors results not only in an overall stimulation of translation, but especially weak messengers are positively discriminated. The above mechanisms mainly deal with qualitative control of translation, i.e., messenger selection, but phosphorylation also plays a role in quantitative regulation of protein synthesis. The generation of active eIF-2, the initiation factor that binds the Met-tRNA(i) and GTP, is dependent on a factor involved in the GDP-GTP exchange. Phosphorylation of eIF-2 results in sequestration of the exchange factor and a slowing down of the rate of initiation.

Relationship of eukaryotic initiation factor 3 to poliovirus-induced p220 cleavage activity

The cleavage of the p220 subunit of eukaryotic initiation factor 4F (eIF-4F) that is induced by the poliovirus protease 2A has been shown previously to require another translation initiation factor, eIF-3. The role of eIF-3 in this cleavage reaction, however, is not known. An antiserum was raised against human eIF-3 and used to analyze the eIF-3 subunit composition in poliovirus-infected and uninfected HeLa cells and after incubation of eIF-3 in vitro with viral 2A protease. No evidence for 2Apro-dependent cleavage of any eIF-3 subunit was detected. Infected cells contain an activity that catalyzes the cleavage of p220 to a specific set of cleavage products. This activity is thought to be an activated form of a latent cellular protease. The p220-specific cleavage activity was partially purified. It was resolved from eIF-3 by both gel filtration and anion-exchange chromatography. Neither intact eIF-3 nor any detectable subunits of eIF-3 were found to copurify with the p220-specific cleavage activity. The latter activity behaves as a protein of 55,000 to 60,000 molecular weight and is inhibited by alkylating agents and metals, which indicates the presence of essential thiol groups. When this activity was incubated with partially purified p220, cleavage occurred only in the presence of eIF-3. Thus, eIF-3 appears to play a role in the p220 cleavage cascade which is subsequent to the 2Apro-induced activation of the p220-specific protease.

A consideration of alternative models for the initiation of translation in eukaryotes

Although recent biochemical and genetic investigations have produced some insights into the mechanism of initiation of translation in eukaryotic cells, two aspects of the initiation process remain controversial. One unsettled issue concerns a variety of functions that have been proposed for mRNA binding proteins, including some initiation factors. The need to distinguish between specific and nonspecific binding of proteins to mRNA is discussed herein. The possibility that certain initiation factors might act as RNA helicases is evaluated along with other ideas about the functions of mRNA- and ATP-binding factors. A second controversial issue concerns the universality of the scanning mechanism for initiation of translation. According to the conventional scanning model, the initial contact between eukaryotic ribosomes and mRNA occurs exclusively at the 5' terminus of the message, which is usually capped. The existence of uncapped mRNAs among a few plant and animal viruses has prompted a vigorous search for other modes of initiation. An "internal initiation" mechanism, first proposed for picornaviruses, has received considerable attention. Although a large body of evidence has been adduced in support of such a mechanism, many of the experiments appear flawed or inconclusive. Some suggestions are given for improving experiments designed to test the internal initiation hypothesis.

Eukaryotic initiation factors eIF-2 and eIF-3: interactions, structure and localization in ribosomal initiation complexes

More than ten different protein factors are involved in initiation of protein synthesis in eukaryotes. For binding of initiator tRNA and mRNA to the 40S ribosomal subunit, the initiation factors eIF-2 and eIF-3 are particularly important. They consist of several different subunits and form stable complexes with the 40S ribosomal subunit. The location of eIF-2 and eIF-3 in these complexes as well as the interactions of the individual components have been analyzed by biochemical methods and electron microscopy. The results obtained are summarized in this article, and a model is derived describing the spatial arrangement of eIF-2 and eIF-3 together with initiator tRNA and mRNA on the 40S subunit. Conclusions on the location of functionally important sites of eukaryotic small ribosomal subunits are discussed with regard to the respective location of these sites in the prokaryotic counterpart.

Eukaryotic initiation factor 3 is required for poliovirus 2A protease-induced cleavage of the p220 component of eukaryotic initiation factor 4F

After cultured cells are infected with poliovirus, cellular mRNA fails to bind to ribosomes, and synthesis of the majority of cellular proteins ceases. The defective step has been localized to the cap-dependent activity of the eukaryotic translation initiation factor 4F. Inactivation of this factor correlates with the cleavage of its largest subunit, p220, into characteristic products observed in infected cells. This cleavage is mediated by the poliovirus protease 2Apro. Previous work suggests that 2Apro does not catalyze the reaction directly, suggesting that one or more cellular proteins is required for the degradation of p220. To identify such a protein, we have developed an assay in which cleavage of a p220 substrate in the presence of poliovirus 2Apro is dependent upon the addition of HeLa cell proteins. By using this assay, we show that another factor, eukaryotic translation initiation factor 3, is required for 2Apro-dependent cleavage of p220.