Sequence Determination and cDNA Cloning of Eukaryotic Initiation Factor 4D, the Hypusine-containing Protein

Unlocking the role of EIF5A: A potential diagnostic marker regulating the cell cycle and showing negative correlation with immune infiltration in lung adenocarcinoma

BACKGROUND

Despite EIF5A upregulation related to tumor progression in LUAD (lung adenocarcinoma), the underlying mechanisms remain elusive. In addition, there are few comprehensive analyses of EIF5A in LUAD.

METHODS

We investigated the EIF5A expression level in LUAD patients using data from the TCGA and GEO databases. We employed qRT-PCR and western blot to verify EIF5A expression in cell lines, while immunohistochemistry was utilized for clinical sample analysis. We analyzed EIF5A expression in tumor-infiltrating immune cells using the TISCH database and assessed its association with immune infiltration in LUAD using the "ESTIMATE" R package. Bioinformatics approaches were developed to discover the EIF5A-related genes and explore EIF5A potential mechanisms in LUAD. Proliferation ability was verified through CCK-8, clone formation, and EdU assays, while flow cytometry assessed apoptosis and cell cycle. Western blot was used to detect the expression of pathway-related proteins.

RESULTS

EIF5A was significantly upregulated in LUAD. Moreover, we constructed a MAZ-hsa-miR-424-3p-EIF5A transcriptional network. We explored the potential mechanism of EIF5A in LUAD and further investigated the cAMP signaling pathway and the cell cycle. Finally, we proved that EIF5A silencing induced G1/S Cell Cycle arrest, promoted apoptosis, and inhibited proliferation via the cAMP/PKA/CREB signaling pathway.

CONCLUSION

EIF5A serves as a prognostic biomarker with a negative correlation to immune infiltrates in LUAD. It regulated the cell cycle in LUAD by inhibiting the cAMP/PKA/CREB signaling pathway.

eIF5A promotes +1 programmed ribosomal frameshifting in Euplotes octocarinatus

Programmed ribosomal frameshifting (PRF) exists in all branches of life that regulate gene expression at the translational level. The single-celled eukaryote Euplotes exhibit high frequency of PRF. However, the molecular mechanism of modulating Euplotes PRF remains largely unknown. Here, we identified two novel eIF5A genes, eIF5A1 and eIF5A2, in Euplotes octocarinatus and found that the Eo-eIF5A2 gene requires a -1 PRF to produce complete protein product. Although both Eo-eIF5As showed significant structural similarity with yeast eIF5A, neither of them could functionally replace yeast eIF5A. Eo-eIF5A knockdown inhibited +1 PRF of the η-tubulin gene. Using an in vitro reconstituted translation system, we found that hypusinated Eo-eIF5A (Eo-eIF5AH) can promote +1 PRF at the canonical AAA_UAA frameshifting site of Euplotes. The results showed eIF5A is a novel trans-regulator of PRF in Euplotes and has an evolutionary conserved role in regulating +1 PRF in eukaryotes.

Hypusination-induced DHPS/eIF5A pathway as a new therapeutic strategy for human diseases: A mechanistic review and structural classification of DHPS inhibitors

The discovery of new therapeutic strategies for diseases is essential for drug research. Deoxyhypusine synthase (DHPS) is a critical enzyme that modifies the conversion of the eukaryotic translation initiation factor 5A (eIF5A) precursor into physiologically active eIF5A (eIF5A-Hyp). Recent studies have revealed that the hypusine modifying of DHPS on eIF5A has an essential regulatory role in human diseases. The hypusination-induced DHPS/eIF5A pathway has been shown to play an essential role in various cancers, and it could regulate immune-related diseases, glucose metabolism-related diseases, neurological-related diseases, and aging. In addition, DHPS has a more defined substrate and a well-defined structure within the active pocket than eIF5A. More and more researchers are focusing on the prospect of advanced development of DHPS inhibitors. This review summarizes the regulatory mechanisms of the hypusination-induced DHPS/eIF5A pathway in a variety of diseases in addition to the inhibitors related to this pathway; it highlights and analyzes the structural features and mechanisms of action of DHPS inhibitors and expands the prospects of future drug development using DHPS as an anticancer target.

The pleiotropic roles of eIF5A in cellular life and its therapeutic potential in cancer

Protein synthesis is dysregulated in the majority of cancers and this process therefore provides a good therapeutic target. Many novel anti-cancer agents are directed to target the initiation stage of translation, however, translation elongation also holds great potential as a therapeutic target. The elongation factor eIF5A that assists the formation of peptidyl bonds during the elongation process is of considerable interest in this regard. Overexpression of eIF5A has been linked with the development of a variety of cancers and inhibitors of the molecule have been proposed for anti-cancer clinical applications. eIF5A is the only protein in the cell that contains the post-translational modification hypusine. Hypusination is a two-step enzymatic process catalysed by the Deoxyhypusine Synthase (DHPS) and Deoxyhypusine Hydroxylase (DOHH). In addition, eIF5A can be acetylated by p300/CBP-associated factor (PCAF) which leads to translocation of the protein to the nucleus and its deactivation. In addition to the nucleus, eIF5A has been found in the mitochondria and the endoplasmic reticulum (ER) with eIF5A localisation related to function from regulation of mitochondrial activity and apoptosis to maintenance of ER integrity and control of the unfolded protein response (UPR). Given the pleiotropic functions of eIF5A and by extension the hypusination enzymes, this system is being considered as a target for a range of cancers including multiple myeloma, B-Cell lymphoma, and neuroblastoma. In this review, we explore the role of eIF5A and discuss the therapeutic strategies that are currently developing both in the pre- and the clinical stage.

Post-translational formation of hypusine in eIF5A: implications in human neurodevelopment

Hypusine [Nε-(4-amino-2-hydroxybutyl)lysine] is a derivative of lysine that is formed post-translationally in the eukaryotic initiation factor 5A (eIF5A). Its occurrence at a single site in one cellular protein defines hypusine synthesis as one of the most specific post-translational modifications. Synthesis of hypusine involves two enzymatic steps: first, deoxyhypusine synthase (DHPS) cleaves the 4-aminobutyl moiety of spermidine and transfers it to the ε-amino group of a specific lysine residue of the eIF5A precursor protein to form an intermediate, deoxyhypusine [Nε-(4-aminobutyl)lysine]. This intermediate is subsequently hydroxylated by deoxyhypusine hydroxylase (DOHH) to form hypusine in eIF5A. eIF5A, DHPS, and DOHH are highly conserved in all eukaryotes, and both enzymes exhibit a strict specificity toward eIF5A substrates. eIF5A promotes translation elongation globally by alleviating ribosome stalling and it also facilitates translation termination. Hypusine is required for the activity of eIF5A, mammalian cell proliferation, and animal development. Homozygous knockout of any of the three genes, Eif5a, Dhps, or Dohh, leads to embryonic lethality in mice. eIF5A has been implicated in various human pathological conditions. A recent genetic study reveals that heterozygous germline EIF5A variants cause Faundes-Banka syndrome, a craniofacial-neurodevelopmental malformations in humans. Biallelic variants of DHPS were identified as the genetic basis underlying a rare inherited neurodevelopmental disorder. Furthermore, biallelic DOHH variants also appear to be associated with neurodevelopmental disorder. The clinical phenotypes of these patients include intellectual disability, developmental delay, seizures, microcephaly, growth impairment, and/or facial dysmorphisms. Taken together, these findings underscore the importance of eIF5A and the hypusine modification pathway in neurodevelopment in humans.

Recent insights into eukaryotic translation initiation factors 5A1 and 5A2 and their roles in human health and disease

The eukaryotic translation initiation factor 5A1 (eIF5A1) and its homolog eIF5A2 are the only two human proteins containing the unique post-translational modification-hypusination, which is essential for the function of these two proteins. eIF5A1 was initially identified as a translation initiation factor by promoting the first peptide bond formation of protein during translation; however, recent results suggest that eIF5A1 also functions as a translation elongation factor. It has been shown that eIF5A1 is implicated in certain human diseases, including diabetes, several human cancer types, viral infections and diseases of neural system. Meanwhile, eIF5A2 is overexpressed in many cancers, and plays an important role in the development and progression of cancers. As multiple roles of these two factors were observed among these studies, therefore, it remains unclear whether they act as oncogene or tumor suppressor. In this review, the recent literature of eIF5As and their roles in human diseases, especially in human cancers, will be discussed.

Engineered SUMO/protease system identifies Pdr6 as a bidirectional nuclear transport receptor

Cleavage of affinity tags by specific proteases can be exploited for highly selective affinity chromatography. The SUMO/SENP1 system is the most efficient for such application but fails in eukaryotic expression because it cross-reacts with endogenous proteases. Using a novel selection system, we have evolved the SUMOEu/SENP1Eu pair to orthogonality with the yeast and animal enzymes. SUMOEu fusions therefore remain stable in eukaryotic cells. Likewise, overexpressing a SENP1Eu protease is nontoxic in yeast. We have used the SUMOEu system in an affinity-capture-proteolytic-release approach to identify interactors of the yeast importin Pdr6/Kap122. This revealed not only further nuclear import substrates such as Ubc9, but also Pil1, Lsp1, eIF5A, and eEF2 as RanGTP-dependent binders and thus as export cargoes. We confirmed that Pdr6 functions as an exportin in vivo and depletes eIF5A and eEF2 from cell nuclei. Thus, Pdr6 is a bidirectional nuclear transport receptor (i.e., a biportin) that shuttles distinct sets of cargoes in opposite directions.

Targeting deoxyhypusine hydroxylase activity impairs cap-independent translation initiation driven by the 5'untranslated region of the HIV-1, HTLV-1, and MMTV mRNAs

Replication of the human immunodeficiency virus type 1 (HIV-1) is dependent on eIF5A hypusination. Hypusine is formed post-translationally on the eIF5A precursor by two consecutive enzymatic steps; a reversible reaction involving the enzyme deoxyhypusine synthase (DHS) and an irreversible step involving the enzyme deoxyhypusine hydroxylase (DOHH). In this study we explored the effect of inhibiting DOHH activity and therefore eIF5A hypusination, on HIV-1 gene expression. Results show that the expression of proteins from an HIV-1 molecular clone is reduced when DOHH activity is inhibited by Deferiprone (DFP) or Ciclopirox (CPX). Next we evaluated the requirement of DOHH activity for internal ribosome entry site (IRES)-mediated translation initiation driven by the 5'untranslated region (5'UTR) of the full length HIV-1 mRNA. Results show that HIV-1 IRES activity relies on DOHH protein concentration and enzymatic activity. Similar results were obtained for IRES-dependent translation initiation mediated by 5'UTR of the human T-cell lymphotropic virus type 1 (HTLV-1) and the mouse mammary tumor virus (MMTV) mRNAs. Interestingly, activity of the poliovirus IRES, was less sensitive to the targeting of DOHH suggesting that not all viral IRESs are equally dependent on the cellular concentration or the activity of DOHH. In summary we present evidence indicating that the cellular concentration of DOHH and its enzymatic activity play a role in HIV-1, HTLV-1 and MMTV IRES-mediated translation initiation.

Structure of the exportin Xpo4 in complex with RanGTP and the hypusine-containing translation factor eIF5A

Xpo4 is a bidirectional nuclear transport receptor that mediates nuclear export of eIF5A and Smad3 as well as import of Sox2 and SRY. How Xpo4 recognizes such a variety of cargoes is as yet unknown. Here we present the crystal structure of the RanGTP·Xpo4·eIF5A export complex at 3.2 Å resolution. Xpo4 has a similar structure as CRM1, but the NES-binding site is occluded, and a new interaction site evolved that recognizes both globular domains of eIF5A. eIF5A contains hypusine, a unique amino acid with two positive charges, which is essential for cell viability and eIF5A function in translation. The hypusine docks into a deep, acidic pocket of Xpo4 and is thus a critical element of eIF5A's complex export signature. This further suggests that Xpo4 recognizes other cargoes differently, and illustrates how Xpo4 suppresses - in a chaperone-like manner - undesired interactions of eIF5A inside nuclei.

EIF5A2 is a novel chemoresistance gene in breast cancer

BACKGROUND

The eIF5A2 gene (encoding the eukaryotic initiation factor 5A2) located at 3q26 is a putative oncogene that is overexpressed in colon and rectal carcinomas, lung cancer and hepatocellular carcinoma. EIF5A2 overexpression correlates significantly with tumor metastasis and is an adverse prognostic marker. However, eIF-5A2 overexpression in breast cancer and its effect on chemotherapy are unknown.

METHODS

We measured eIF-5A2 expression and doxorubicin sensitivity in different human breast cancer cell lines (Bcap-1937, HCC1937, and MCF-7). To investigate a role for eIF-5A2 in chemoresistance, cells were treated with eIF-5A2-siRNA, exposed to various concentrations of doxorubicin, and toxicity was assayed by CCK-8 (cell counting kit).

RESULTS

The eIF-5A2 expression levels varied among breast cancer cells. Higher expression levels correlated with decreased doxorubicin sensitivity. Silencing of eIF-5A2 significantly improved doxorubicin toxicity in all three breast cancer cell lines.

CONCLUSION

This study shows that eIF-5A2 plays an important role in doxorubicin chemoresistance in breast cancer cells.

The translation factor eIF5A and human cancer

The eukaryotic initiation factor eIF5A is a translation factor that, unusually, has been assigned functions in both initiation and elongation. Additionally, it is implicated in transcription, mRNA turnover and nucleocytoplasmic transport. Two eIF5A isoforms are generated from distinct but related genes. The major isoform, eIF5A1, is considered constitutive, is abundantly expressed in most cells, and is essential for cell proliferation. The second isoform, eIF5A2, is expressed in few normal tissues but is highly expressed in many cancers and has been designated a candidate oncogene. Elevated expression of either isoform carries unfavorable prognostic implications for several cancers, and both have been advanced as cancer biomarkers. The amino acid hypusine, a presumptively unique eIF5A post-translational modification, is required for most known eIF5A functions and it renders eIF5A susceptible to inhibitors of the modification pathway as therapeutic targets. eIF5A has been shown to regulate a number of gene products specifically, termed the eIF5A regulon, and its role in translating proline-rich sequences has recently been identified. A model is advanced that accommodates eIF5A in both the initiation and elongation phases of translation. We review here the biochemical functions of eIF5A, the relationship of its isoforms with human cancer, and evolving clinical applications. This article is part of a Special Issue entitled: Translation and Cancer.

The hypusine-containing translation factor eIF5A

In addition to the small and large ribosomal subunits, aminoacyl-tRNAs, and an mRNA, cellular protein synthesis is dependent on translation factors. The eukaryotic translation initiation factor 5A (eIF5A) and its bacterial ortholog elongation factor P (EF-P) were initially characterized based on their ability to stimulate methionyl-puromycin (Met-Pmn) synthesis, a model assay for protein synthesis; however, the function of these factors in cellular protein synthesis has been difficult to resolve. Interestingly, a conserved lysine residue in eIF5A is post-translationally modified to hypusine and the corresponding lysine residue in EF-P from at least some bacteria is modified by the addition of a β-lysine moiety. In this review, we provide a summary of recent data that have identified a novel role for the translation factor eIF5A and its hypusine modification in the elongation phase of protein synthesis and more specifically in stimulating the production of proteins containing runs of consecutive proline residues.

Assay of deoxyhypusine synthase activity

Deoxyhypusine synthase catalyzes an unusual protein modification reaction. A portion of spermidine is covalently added to one specific lysine residue of one eukaryotic protein, eIF5A (eukaryotic initiation factor 5A) to form a deoxyhypusine residue. The assay measures the incorporation of radioactivity from [1,8-(3)H]spermidine into the eIF5A protein. The enzyme is specific for the eIF5A precursor protein and does not work on short peptides (<50 amino acids). Optimum conditions for the reaction and four detection methods for the product, deoxyhypusine-containing eIF5A, are described in this chapter. The first, and most specific, method is the measurement of the amount of [(3)H]deoxyhypusine in the protein hydrolysate after its separation by ion exchange chromatography. However, this method requires some specialized equipment. The second method is counting the radioactivity in TCA-precipitated protein after thorough washing. The third method involves determining the radioactivity in the band of [(3)H]deoxyhypusine-containing eIF5A after separation by SDS-PAGE. The fourth method is a filter-binding assay. It is important to minimize nonspecific binding of [(3)H]spermidine to proteins in the assay mixture, especially for methods 2 and 4, as illustrated in a comparison figure in the chapter.

Assay of deoxyhypusine hydroxylase activity

The eukaryotic translation initiation factor 5A (eIF5A) is the only cellular protein that contains an unusual amino acid, hypusine [N (ε)-(4-amino-2-hydroxybutyl)-lysine]. eIF5A and its hypusine/deoxyhypusine modification are vital for eukaryotic cell proliferation. Hypusine is formed posttranslationally by two enzymatic steps catalyzed by deoxyhypusine synthase and deoxyhypusine hydroxylase. Deoxyhypusine hydroxylase catalyzes a stereo-specific hydroxylation of the deoxyhypusine residue in the eIF5A intermediate protein, eIF5A(Dhp). The enzyme is totally specific for this protein and does not act on short peptides (<50 amino acids). The assay measures the conversion of the radiolabeled deoxyhypusine residue to a hypusine residue in eIF5A. Optimum conditions for the reaction and two detection methods for the product, hypusine-containing eIF5A, are described in this chapter. The first, and most reliable, method is the measurement of the amount of [(3)H]hypusine in the protein hydrolysate after its separation from [(3)H]deoxyhypusine, by ion exchange chromatography. This method does require specialized equipment. The second method is based on counting the total TCA soluble radioactivity after sodium periodate oxidation of the reaction mixture, since the radiolabeled 4-amino-2-hydroxy butyl moiety of the hypusine residue is cleaved and is released from protein as radiolabeled β-propionaldehyde and formaldehyde by periodate oxidation.

Apoptosis induction by eIF5A1 involves activation of the intrinsic mitochondrial pathway

The regulatory role of eukaryotic translation initiation factor 5A1 (eIF5A1) in apoptosis was examined using HT-29 and HeLa S3 cells. eIF5A is the only known protein to contain the unusual amino acid, hypusine, and eIF5A1 is one of two human eIF5A family members. Two observations indicated that eIF5A1 is involved in apoptosis. First, siRNA-mediated suppression of eIF5A1 resulted in inhibition of apoptosis induced by various apoptotic stimuli, and second, adenovirus-mediated over-expression of eIF5A1 strongly induced apoptotic cell death. A mutant of eIF5A1 incapable of being hypusinated also induced apoptosis when over-expressed indicating that unhypusinated eIF5A1 is the pro-apoptotic form of the protein. Over-expression of eIF5A1 or of the mutant resulted in loss of mitochondrial transmembrane potential, translocation of Bax to the mitochondria, release of cytochrome c, caspase activation, up-regulation of p53, and up-regulation of Bim, a pro-apoptotic BH3-only Bcl-2 family protein. In addition, Bim(L) and Bim(S), the pro-apoptotic alternative spliced forms of Bim, were induced in response to over-expression of eIF5A1. Thus eIF5A1 appears to induce apoptosis by activating the mitochondrial apoptotic pathway. Proteomic analyses indicated that, of 1,899 proteins detected, 131 showed significant changes in expression (P or=1.5) within 72 h of eIF5A1 up-regulation. Among these are proteins involved in translation and protein folding, transcription factors, proteins mediating proteolysis, and a variety of proteins known to be directly involved in apoptosis. These observations collectively indicate that unhypusinated eIF5A1 plays a central role in the regulation of apoptosis.

The Drosophila deoxyhypusine hydroxylase homologue nero and its target eIF5A are required for cell growth and the regulation of autophagy

Hypusination is a unique posttranslational modification by which lysine is transformed into the atypical amino acid hypusine. eIF5A (eukaryotic initiation factor 5A) is the only known protein to contain hypusine. In this study, we describe the identification and characterization of nero, the Drosophila melanogaster deoxyhypusine hydroxylase (DOHH) homologue. nero mutations affect cell and organ size, bromodeoxyuridine incorporation, and autophagy. Knockdown of the hypusination target eIF5A via RNA interference causes phenotypes similar to nero mutations. However, loss of nero appears to cause milder phenotypes than loss of eIF5A. This is partially explained through a potential compensatory mechanism by which nero mutant cells up-regulate eIF5A levels. The failure of eIF5A up-regulation to rescue nero mutant phenotypes suggests that hypusination is required for eIF5A function. Furthermore, expression of enzymatically impaired forms of DOHH fails to rescue nero clones, indicating that hypusination activity is important for nero function. Our data also indicate that nero and eIF5A are required for cell growth and affect autophagy and protein synthesis.

Interactions of elongation factor EF-P with the Escherichia coli ribosome

EF-P (eubacterial elongation factor P) is a highly conserved protein essential for protein synthesis. We report that EF-P protects 16S rRNA near the G526 streptomycin and the S12 and mRNA binding sites (30S T-site). EF-P also protects domain V of the 23S rRNA proximal to the A-site (50S T-site) and more strongly the A-site of 70S ribosomes. We suggest that EF-P: (a) may play a role in translational fidelity and (b) prevents entry of fMet-tRNA into the A-site enabling it to bind to the 50S P-site. We also report that EF-P promotes a ribosome-dependent accommodation of fMet-tRNA into the 70S P-site.

Molecular cloning and functional expression of bovine deoxyhypusine hydroxylase cDNA and homologs

Deoxyhypusine hydroxylase is the second of the two enzymes that catalyzes the maturation of eukaryotic initiation factor 5A (eIF5A). The mature eIF5A is the only known protein in eukaryotic cells that contains the unusual amino acid hypusine (N(epsilon)-(4-amino-2(R)-hydroxybutyl)lysine). Synthesis of hypusine is essential for the function of eIF5A in eukaryotic cell proliferation and survival. Here, we describe the cloning and characterization of bovine deoxyhypusine hydroxylase cDNA and its homologs. The deduced bovine deoxyhypusine hydroxylase protein is 87% identical to human enzyme and 45% identical to yeast enzyme. The overexpressed enzyme showed activity in catalyzing the hydroxylation of the deoxyhypusine residue in the eIF5A intermediate. An amino acid substitution from Glu 57 to Gly located at one of the four conserved His-Glu (HE) pairs, the potential metal coordination sites, resulted in severe reduction of deoxyhypusine hydroxylase activity. A deletion at the HEAT-repeats 1-3 resulted in complete losses of deoxyhypusine hydroxylase activity.

Differential expression of eIF5A-1 and eIF5A-2 in human cancer cells

Eukaryotic translation initiation factor 5A (eIF5A) is the only cellular protein that contains the unusual amino acid hypusine [N(epsilon)-(4-amino-2-hydroxybutyl)lysine]. Vertebrates carry two genes that encode two eIF5A isoforms, eIF5A-1 and eIF5A-2, which, in humans, are 84% identical. eIF5A-1 mRNA (1.3 kb) and protein (18 kDa) are constitutively expressed in human cells. In contrast, expression of eIF5A-2 mRNA (0.7-5.6 kb) and eIF5A-2 protein (20 kDa) varies widely. Whereas eIF5A-2 mRNA was demonstrable in most cells, eIF5A-2 protein was detectable only in the colorectal and ovarian cancer-derived cell lines SW-480 and UACC-1598, which showed high overexpression of eIF5A-2 mRNA. Multiple forms of eIF5A-2 mRNA (5.6, 3.8, 1.6 and 0.7 kb) were identified as the products of one gene with various lengths of 3'-UTR, resulting from the use of different polyadenylation (AAUAAA) signals. The eIF5A-1 and eIF5A-2 precursor proteins were modified comparably in UACC-1598 cells and both were similarly stable. When eIF5A-1 and eIF5A-2 coding sequences were expressed from mammalian vectors in 293T cells, eIF5A-2 precursor was synthesized at a level comparable to that of eIF5A-1 precursor, indicating that the elements causing inefficient translation of eIF5A-2 mRNA reside outside of the open reading frame. On sucrose gradient separation of cytoplasmic RNA, only a small portion of total eIF5A-2 mRNA was associated with the polysomal fraction, compared with a much larger portion of eIF5A-1 mRNA in the polysomes. These findings suggest that the failure to detect eIF5A-2 protein even in eIF5A-2 mRNA positive cells is, at least in part, due to inefficient translation.

Translation initiation factor eIF-5A from Plasmodium falciparum

Eukaryotic translation initiation factor (eIF-5A) is a highly conserved and essential protein that contains the unique amino acid hypusine. The first step in the post-translational biosynthesis of hypusine, the transfer of an aminobutyl moiety from the polyamine substrate spermidine to the -amino group of a specific lysine residue in the eIF-5A precursor, is catalyzed by the enzyme deoxyhypusine synthase. A cDNA encoding a protein homologous to eIF-5A was isolated by plaque hybridization from a cDNA library of Plasmodium falciparum. The cloned cDNA contains an open reading frame encoding a protein of 161 amino acids, which shares a high sequence identity with other eukaryotic eIF-5A sequences. A phylogenetic tree constructed with eIF-5A from P. falciparum and 16 other eIF-5A sequences of eukaryotic and archaeal origin reveals that plasmodial eIF-5A together with other apicomplexan eIF-5A show a higher degree of homology to plant proteins than to animal and fungal sequences. The plasmodial eIF-5A gene was expressed as a six-histidine tagged fusion protein in Escherichia coli. Radioactive incorporation studies with [1,8-3H] spermidine indicated that this protein can serve as a substrate for human deoxyhypusine synthase. Results of quantitative real-time PCR studies with synchronized erythrocytic stages of P. falciparum revealed no significant induction or downregulation but only some variation in the expression level of plasmodial eIF-5A in ring, trophozoite and schizont stage.

Higher activity of recombinant bovine deoxyhypusine synthase vs. human deoxyhypusine synthase

Mature eukaryotic initiation factor 5A (eIF5A) is the only known protein in eukaryotic cells that contains the unusual amino acid hypusine (Nepsilon-(4-amino-2(R)-hydroxybutyl)lysine). The synthesis of hypusine is essential for the function of eIF5A in eukaryotic cell proliferation and survival. Deoxyhypusine synthase is the first of the two enzymes that catalyzes the maturation of eIF5A. We have subcloned the cDNA encoding bovine and human deoxyhypusine synthase into a pET-11a expression vector, separately. T7-tagged bovine and human deoxyhypusine synthase have been overexpressed in Escherichia coli and purified to homogeneity using T7 antibody affinity chromatography. Activities of the enzyme from both human and bovine have been measured by their ability to convert the eIF5A precursor protein to the intermediate, deoxyhypusine form of eIF5A. Our results have shown that bovine deoxyhypusine synthase has considerably higher activity than human deoxyhypusine synthase in catalyzing the synthesis of deoxyhypusine.

Molecular cloning of bovine eIF5A and deoxyhypusine synthase cDNA

Deoxyhypusine synthase is the first of the two enzymes that catalyzes the maturation of eukaryotic initiation factor 5A (eIF5A). The mature eIF5A is the only known protein in eukaryotic cells that contains the unusual amino acid hypusine (N(epsilon)-(4-amino-2(R)-hydroxybutyl)-lysine). Synthesis of hypusine is essential for the function of eIF5A in eukaryotic cell proliferation and survival. Here we describe the cloning and characterization of bovine eIF5A and bovine deoxyhypusine synthase. The deduced bovine eIF5A protein is 100% identical to human eIF5A-1, and the deduced bovine deoxyhypusine synthase protein showed a 93% identity to the human protein.

Identification and characterization of eukaryotic initiation factor 5A-2

The phylogenetically conserved eukaryotic translation initiation factor 5A (eIF5A) is the only known cellular protein to contain the post-translationally derived amino acid hypusine [Nepsilon-(4-amino-2-hydroxybutyl)lysine]. Both eIF5A and its hypusine modification are essential for sustained cell proliferation. Normally only one eIF5A protein is expressed in human cells. Recently, we identified a second human EIF5A gene that would encode an isoform (eIF5A-2) of 84% sequence identity. Overexpression of eIF5A-2 mRNA in certain human cancer cells, in contrast to weak normal expression limited to human testis and brain, suggests EIF5A2 as a potential oncogene. However, eIF5A-2 protein has not been described in human or mammalian cells heretofore. Here, we describe the identification of eIF5A-2 protein in human colorectal and ovarian cancer lines, SW-480 and UACC-1598, that overexpress eIF5A-2 mRNAs. Functional characterization of the human isoforms revealed that either human EIF5A gene can complement growth of a yeast strain in which the yeast EIF5A genes were disrupted. This indicates functional similarity of the human isoforms in yeast and suggests that eIF5A-2 has an important role in eukaryotic cell survival similar to that of the ubiquitous eIF5A-1. Detectable structural differences were also noted, including lack of immunological cross-reactivity, formation of different complexes with deoxyhypusine synthase, and Km values (1.5 +/- 0.2 vs. 8.3 +/- 1.4 microm for eIF5A-1 and -2, respectively) as substrates for deoxyhypusine synthase in vitro. These physical characteristics and distinct amino acid sequences in the C-terminal domain together with differences in gene expression patterns imply differentiated, tissue-specific functions of the eIF5A-2 isoform in the mammalian organism and in cancer.

Evolutionary conservation of reactions in translation

Current X-ray diffraction and cryoelectron microscopic data of ribosomes of eubacteria have shed considerable light on the molecular mechanisms of translation. Structural studies of the protein factors that activate ribosomes also point to many common features in the primary sequence and tertiary structure of these proteins. The reconstitution of the complex apparatus of translation has also revealed new information important to the mechanisms. Surprisingly, the latter approach has uncovered a number of proteins whose sequence and/or structure and function are conserved in all cells, indicating that the mechanisms are indeed conserved. The possible mechanisms of a new initiation factor and two elongation factors are discussed in this context.

Human eIF5A2 on chromosome 3q25-q27 is a phylogenetically conserved vertebrate variant of eukaryotic translation initiation factor 5A with tissue-specific expression

Eukaryotic translation initiation factor 5A (eIF5A) is an essential protein tightly linked to cellular polyamine homeostasis. It receives the unique spermidine-derived posttranslational modification hypusine that is necessary for eIF5A's biochemical activity and cellular proliferation. The eIF5A protein stimulates ribosomal peptidyl-transferase and may be involved in nucleocytoplasmic mRNA transport. Little is known about the molecular genetics of eIF5A. Here we report on the sequence and molecular characterization of human EIF5A2, a novel phylogenetically conserved gene for eIF5A. EIF5A2 stretches over 17 kb and consists of five exons and four introns. It is localized at 3q25-q27, often noted for chromosomal instability in cancers. EIF5A2 is highly expressed in testis and colorectal adenocarcinoma and at moderate levels in the brain, in contrast to the ubiquitously expressed EIF5A1 gene. Two EIF5A2 mRNAs share a 129-nt 5' UTR and a coding sequence for the 153-amino-acid eIF5AII protein, but possess two alternative 3' UTRs of 46 and 890 nt that arise through differential polyadenylation. The protein is 84% identical and 94% similar to eIF5AI. Both EIF5A genes are conserved in vertebrates. Our findings lend further support for a specialized gene expression program of polyamine metabolic proteins and regulators that function to maintain polyamine homeostasis at elevated levels during spermatogenesis.

Modulation of molecular mechanisms involved in protein synthesis machinery as a new tool for the control of cell proliferation

In the past years, the attention of scientists has focused mainly on the study of the genetic information and alterations that regulate eukaryotic cell proliferation and that lead to neoplastic transformation. All therapeutic strategies against cancer are, to date, directed at DNA either with cytotoxic drugs or gene therapy. Little or no interest has been aroused by protein synthesis mechanisms. However, an increasing body of data is emerging about the involvement of translational processes and factors in control of cell proliferation, indicating that protein synthesis can be an additional target for anticancer strategies. In this paper we review the novel insights on the biochemical and molecular events leading to protein biosynthesis and we describe their involvement in cell proliferation and tumorigenesis. A possible mechanistic explanation is given by the interactions that occur between protein synthesis machinery and the proliferative signal transduction pathways and that are therefore suitable targets for indirect modulation of protein synthesis. We briefly describe the molecular tools used to block protein synthesis and the attempts made at increasing their efficacy. Finally, we propose a new multimodal strategy against cancer based on the simultaneous intervention on protein synthesis and signal transduction.

Homospermidine synthase, the first pathway-specific enzyme of pyrrolizidine alkaloid biosynthesis, evolved from deoxyhypusine synthase

Pyrrolizidine alkaloids are preformed plant defense compounds with sporadic phylogenetic distribution. They are thought to have evolved in response to the selective pressure of herbivory. The first pathway-specific intermediate of these alkaloids is the rare polyamine homospermidine, which is synthesized by homospermidine synthase (HSS). The HSS gene from Senecio vernalis was cloned and shown to be derived from the deoxyhypusine synthase (DHS) gene, which is highly conserved among all eukaryotes and archaebacteria. DHS catalyzes the first step in the activation of translation initiation factor 5A (eIF5A), which is essential for eukaryotic cell proliferation and which acts as a cofactor of the HIV-1 Rev regulatory protein. Sequence comparison provides direct evidence for the evolutionary recruitment of an essential gene of primary metabolism (DHS) for the origin of the committing step (HSS) in the biosynthesis of pyrrolizidine alkaloids.

Cloning and analysis of cDNAs encoding the hypusine-containing protein eIF5A of two lepidopteran insect species

Eukaryotic initiation factor eIF5A is essential for cell viability and contains a characteristic post-translational modification of a specific lysine residue into a hypusine. cDNAs with similarity to eIF5A sequences were derived from Spodoptera exigua and S. frugiperda cDNA libraries. The deduced amino acid sequences are identical for both species and predict a protein with a molecular mass of 17.5 kDa. The Drosophila melanogaster eIF5A cDNA sequence was retrieved from the Drosophila EST Project. The predicted protein is 80% similar to Spodoptera eIF5A. A single eIF5A gene copy is present in the S. frugiperda genome, which is transcribed into four different transcripts. Infection of S. frugiperda cells with a baculovirus resulted in a strong decline of all four transcripts already at 12 h after infection. In contrast, the eIF5A protein was fairly stable up to 48 h post infection.

Nuclear pore localization and nucleocytoplasmic transport of eIF-5A: evidence for direct interaction with the export receptor CRM1

Eukaryotic initiation factor 5A (eIF-5A) is the only cellular protein known to contain the unusual amino acid hypusine. The exact in vivo function of eIF-5A, however, is to date unknown. The finding that eIF-5A is an essential cofactor of the human immunodeficiency virus type 1 (HIV-1) Rev RNA transport factor suggested that eIF-5A is part of a specific nuclear export pathway. In this study we used indirect immunofluorescence and immunogold electron microscopy to demonstrate that eIF-5A accumulates at nuclear pore-associated intranuclear filaments in mammalian cells and Xenopus oocytes. We are able to show that eIF-5A interacts with the general nuclear export receptor, CRM1. Furthermore, microinjection studies in somatic cells revealed that eIF-5A is transported from the nucleus to the cytoplasm, and that this nuclear export is blocked by leptomycin B. Our data demonstrate that eIF-5A is a nucleocytoplasmic shuttle protein.

Structural organization of the human eukaryotic initiation factor 5A precursor and its site-directed variant Lys50-->Arg

The molecular properties of the human eukaryotic initiation factor 5A precursor and its site directed Lys50-->Arg variant have been investigated and compared. Structure perturbation methods were used to gain information about the protein architecture in solution. Intrinsic and extrinsic spectroscopic probes strategically located in the protein matrix detected the independent unfolding of two molecular regions. Three cysteines out of four were titrated in the native protein and the peculiar presence of a tyrosinate band at neutral pH was detected. At alkaline pH only two tyrosines out of three were titratable in the native protein, with an apparent pK of about 9.9. Native protein and its Lys50-->Arg variant reacted in a similar fashion to guanidine and to pH variation, but differently to thermal stress. The complex thermal unfolding of both proteins indicated the presence of intermediates. Spectroscopic data showed that these intermediates are differently structured. Consequently, the two proteins seem to have different unfolding pathways.

Universally conserved translation initiation factors

The process by which translation is initiated has long been considered similar in Bacteria and Eukarya but accomplished by a different unrelated set of factors in the two cases. This not only implies separate evolutionary histories for the two but also implies that at the universal ancestor stage, a translation initiation mechanism either did not exist or was of a different nature than the extant processes. We demonstrate herein that (i) the "analogous" translation initiation factors IF-1 and eIF-1A are actually related in sequence, (ii) the "eukaryotic" translation factor SUI1 is universal in distribution, and (iii) the eukaryotic/archaeal translation factor eIF-5A is homologous to the bacterial translation factor EF-P. Thus, the rudiments of translation initiation would seem to have been present in the universal ancestor stage. However, significant development and refinement subsequently occurred independently on both the bacterial lineage and on the archaeal/eukaryotic line.

The gene encoding the elongation factor P protein is essential for viability and is required for protein synthesis

Elongation factor P (EFP) is a protein that stimulates the peptidyltransferase activity of fully assembled 70 S prokaryotic ribosomes and enhances the synthesis of certain dipeptides initiated by N-formylmethionine. This reaction appears conserved throughout species and is promoted in eukaryotic cells by a homologous protein, eIF5A. Here we ask whether the Escherichia coli gene encoding EFP is essential for cell viability. A kanamycin resistance (KanR) gene was inserted near the N-terminal end of the efp gene and was cloned into a plasmid, pMAK705, that has a temperature-sensitive origin of replication. After transformation into a recA+ E. coli strain, temperature-sensitive mutants were isolated, and their chromosomal DNA was sequenced. Mutants containing the efp-KanR gene in the chromosome grew at 33 degrees C only in the presence of the wild-type copy of the efp gene in the pMAK705 plasmid and were unable to grow at 44 degrees C. Incorporation of various isotopes in vivo suggests that translation is impaired in the efp mutant at 44 degrees C. At 44 degrees C, mutant cells are severely defective in peptide-bond formation. We conclude that the efp gene is essential for cell viability and is required for protein synthesis.

Molecular characterization of the prokaryotic efp gene product involved in a peptidyltransferase reaction

The translation factor EF-P is required for efficient prokaryotic peptide bond synthesis on 70S ribosomes from fMet-tRNAfMet. This protein has been purified from Escherichia coli cells and the gene, efp, encoding it has been cloned and sequenced. We have isolated recombinant clones which overexpress a protein that co-migrates with purified EF-P upon SDS-PAGE analysis. Using these clones, we report the purification, crystallization and initial characterization of the efp gene product. The mechanism by which EF-P stimulates peptide-bond synthesis was studied using several antibiotics that inhibit translocation, peptide-bond synthesis and decoding. The stimulation of peptidyltransferase by EF-P was not inhibited by antibiotics that affect translocation and occupation of the A site (in the elongation state), ie thiostrepton, viomycin, neomycin and fusidic acid but was inhibited by streptomycin as well as by inhibitors of peptidyltransferase, chloramphenicol and lincomycin. This observation and the requirement for L16 but not for the L7/L12 nor L6 or L11 r-proteins suggest that the binding site for EF-P may overlap the peptidyltransferase center of the ribosome.

The plant translational apparatus

Protein synthesis in both eukaryotic and prokaryotic cells is a complex process requiring a large number of macromolecules: initiation factors, elongation factors, termination factors, ribosomes, mRNA, amino-acylsynthetases and tRNAs. This review focuses on our current knowledge of protein synthesis in higher plants.

Identification of a new member of the human eIF-5A gene family

Using an oligodeoxynucleotide generated by rapid PCR amplification of 5'-cDNA ends (5'-RACE) as a detection probe, we have isolated a new genomic clone encoding the human eukaryotic initiation factor 5A (eIF-5A). Sequence analysis revealed that the eIF-5A coding region is identical to the corresponding cDNA but interrupted by three introns. In a plasmid shuffle experiment we show functional replacement of the essential homologous gene in Saccharomyces cerevisiae by this human eIF-5A.

The genomic structure encoding human initiation factor eIF-5A

A genomic clone encoding the human eukaryotic initiation factor 5A (eIF-5A) was isolated and its entire nucleotide sequence was determined. The whole eIF-5A coding region is not interrupted by introns. The functional eIF-5A gene is highly homologous to the corresponding complementary DNA. One single 1.4-kb transcript thereof is expressed in human cell lines. Furthermore, we also isolated and sequenced two additional eIF-5A-related sequences which are, by expression and sequence analyses, identified as pseudogenes of the functional eIF-5A. The sequence homology between these pseudogenes and the functional eIF-5A is 71 and 87%.

Inhibition of the G1-S transition of the cell cycle by inhibitors of deoxyhypusine hydroxylation

The formation of the unusual amino-acid hypusine in eIF-5A (eukaryotic initiation factor 5A) is associated with cellular proliferation. We used a panel of compounds, including mimosine, to probe the relationship between the exit from the G1 phase of the cell cycle, i.e., the onset of DNA replication, and the formation of hypusine by the enzyme deoxyhypusyl hydroxylase (DOHH). These two parameters displayed the same dose dependency and structure-activity relationship. Only compounds that inhibited DOHH also suppressed proliferation. This effect was observed: (i) in spontaneously proliferating, virally transformed, and mitogen-stimulated cells; (ii) for both anchorage-dependent and anchorage-independent proliferation; and (iii) with normal and malignant cell lines. DOHH reactivation occurred rapidly after inhibitor withdrawal and correlated with synchronized entry into S. The changes in the expression of specific genes during the G1-to-S transition mimicked the physiological pattern. These findings suggest that hypusine formation in eIF-5A which occurs in a specific, invariant sequence motif acquired early in evolution, may be involved in the G1-to-S transition in the eukaryotic cells tested.

Cloning and sequencing of a chick embryo cDNA encoding the 20-kDa hypusine-containing protein, eIF-5A

Chick embryo contains 18- and 20-kDa isoforms of eukaryotic translation initiation factor 5A (eIF-5A). cDNA clones corresponding to the 20-kDa eIF-5A were isolated and sequenced. A full-length cDNA clone encodes a 153-amino-acid (aa) protein. The deduced aa sequence exactly matches with the partial aa sequence determined for this protein and shows high identity to that of human or rabbit eIF-5A. The results of Southern and Northern hybridization provide evidence for multiple transcripts for chick embryo eIF-5A or an eIF-5A-like protein that presumably derive from more than one gene.

Is hypusine essential for eukaryotic cell proliferation?

Hypusine [N epsilon-(4-amino-2-hydroxybutyl)-L-lysine] is a most remarkable amino acid, occurring in all eukaryotic cells, yet occupying only a single position in one protein, eukaryotic protein synthesis initiation factor 5A (eIF-5A). The unusual structure of hypusine, its derivation from the polyamine spermidine, and its increased formation in response to growth stimulation, as well as its limited occurrence in the highly conserved amino acid sequence of eIF-5A, have aroused keen interest in the biological significance of its existence and in its relationship to eIF-5A function.