Hypusine is required for a sequence-specific interaction of eukaryotic initiation factor 5A with postsystematic evolution of ligands by exponential enrichment RNA

The Many Faces of Hypusinated eIF5A: Cell Context-Specific Effects of the Hypusine Circuit and Implications for Human Health

The unique amino acid hypusine [Nε-(4-amino-2-hydroxybutyl)lysine] is exclusively formed on the translational regulator eukaryotic initiation factor 5A (eIF5A) via a process coined hypusination. Hypusination is mediated by two enzymes, deoxyhypusine synthase (DHPS) and deoxyhypusine hydroxylase (DOHH), and hypusinated eIF5A (eIF5AHyp) promotes translation elongation by alleviating ribosome pauses at amino acid motifs that cause structural constraints, and it also facilitates translation initiation and termination. Accordingly, eIF5AHyp has diverse biological functions that rely on translational control of its targets. Homozygous deletion of Eif5a, Dhps, or Dohh in mice leads to embryonic lethality, and heterozygous germline variants in EIF5A and biallelic variants in DHPS and DOHH are associated with rare inherited neurodevelopmental disorders, underscoring the importance of the hypusine circuit for embryonic and neuronal development. Given the pleiotropic effects of eIF5AHyp, a detailed understanding of the cell context-specific intrinsic roles of eIF5AHyp and of the chronic versus acute effects of eIF5AHyp inhibition is necessary to develop future strategies for eIF5AHyp-targeted therapy to treat various human health problems. Here, we review the most recent studies documenting the intrinsic roles of eIF5AHyp in different tissues/cell types under normal or pathophysiological conditions and discuss these unique aspects of eIF5AHyp-dependent translational control.

Analysis of the effect of hypusination in myeloid cells on colitis and colitis-associated cancer

Hypusine is an amino acid synthesized by the enzyme deoxyhypusine synthase (DHPS). It is critical for the activity of eukaryotic translation initiation factor 5A (EIF5A). We reported that hypusination i) in macrophages supports the innate response towards pathogenic bacteria and ii) in epithelial cells maintains intestinal homeostasis. Herein, we investigated the effect of myeloid hypusination on the outcome of colitis and colitis-associated cancer. We found that patients with Crohn's disease exhibit increased levels of DHPS and EIF5AHyp in cells infiltrating the colon lamina propria. However, the specific deletion of Dhps in myeloid cells had no impact on clinical, histological, or inflammatory parameters in mice treated with dextran sulfate sodium (DSS). Further, tumorigenesis and level of dysplasia were not affected by myeloid deletion of Dhps in the azoxymethane-DSS model. The composition of the fecal and the mucosa-associated microbiome was similar in animals lacking or not DHPS in myeloid cells. Thus, hypusination in myeloid cells does not regulate colitis associated with epithelial injury and colitis-associated cancer. Enhancement of the DHPS/hypusine pathway in patients with inflammatory bowel disease could have therapeutic impact through epithelial effects, but modulation of hypusination in myeloid cells will be unlikely to affect the disease.

Hypusination in intestinal epithelial cells protects mice from infectious colitis

Enteropathogenic Escherichia coli (EPEC) is a bacterium that causes attaching/effacing (A/E) lesions and serious diarrheal disease, a major health issue in developing countries. EPEC pathogenicity results from the effect of virulence factors and dysregulation of host responses. Polyamines, including spermidine, play a major role in intestinal homeostasis. Spermidine is the substrate for deoxyhypusine synthase (DHPS), which catalyzes the conjugation of the amino acid hypusine to eukaryotic translation initiation factor 5A (EIF5A); hypusinated EIF5A (EIF5AHyp) binds specific mRNAs and initiates translation. Our aim was to determine the role of hypusination during infection with A/E pathogens. We found that DHPS and EIF5AHyp levels are induced in i) a colonic epithelial cell line and human-derived colon organoids infected with EPEC, and ii) the colon of mice infected with Citrobacter rodentium, the rodent equivalent of EPEC. Specific deletion of Dhps in intestinal epithelial cells worsened clinical, histological, and pro-inflammatory parameters in C. rodentium-infected mice. These animals also exhibited an exacerbated pathogenic transcriptome in their colon. Furthermore, infected mice with specific Dhps deletion exhibited reduced levels of proteins involved in detoxification of tissue-damaging reactive aldehydes and consequently increased electrophile adducts in the colon. Thus, hypusination in intestinal epithelial cells protects from infectious colitis mediated by A/E pathogens.

Hypusination Maintains Intestinal Homeostasis and Prevents Colitis and Carcinogenesis by Enhancing Aldehyde Detoxification

BACKGROUND & AIMS

The amino acid hypusine, synthesized from the polyamine spermidine by the enzyme deoxyhypusine synthase (DHPS), is essential for the activity of eukaryotic translation initiation factor 5A (EIF5A). The role of hypusinated EIF5A (EIF5AHyp) remains unknown in intestinal homeostasis. Our aim was to investigate EIF5AHyp in the gut epithelium in inflammation and carcinogenesis.

METHODS

We used human colon tissue messenger RNA samples and publicly available transcriptomic datasets, tissue microarrays, and patient-derived colon organoids. Mice with intestinal epithelial-specific deletion of Dhps were investigated at baseline and in models of colitis and colon carcinogenesis.

RESULTS

We found that patients with ulcerative colitis and Crohn's disease exhibit reduced colon levels of DHPS messenger RNA and DHPS protein and reduced levels of EIF5AHyp. Similarly, colonic organoids from colitis patients also show down-regulated DHPS expression. Mice with intestinal epithelial-specific deletion of Dhps develop spontaneous colon hyperplasia, epithelial proliferation, crypt distortion, and inflammation. Furthermore, these mice are highly susceptible to experimental colitis and show exacerbated colon tumorigenesis when treated with a carcinogen. Transcriptomic and proteomic analysis on colonic epithelial cells demonstrated that loss of hypusination induces multiple pathways related to cancer and immune response. Moreover, we found that hypusination enhances translation of numerous enzymes involved in aldehyde detoxification, including glutathione S-transferases and aldehyde dehydrogenases. Accordingly, hypusination-deficient mice exhibit increased levels of aldehyde adducts in the colon, and their treatment with a scavenger of electrophiles reduces colitis.

CONCLUSIONS

Hypusination in intestinal epithelial cells has a key role in the prevention of colitis and colorectal cancer, and enhancement of this pathway via supplementation of spermidine could have a therapeutic impact.

Role of eIF5A in Mitochondrial Function

The eukaryotic translation initiation factor 5A (eIF5A) is an evolutionarily conserved protein that binds ribosomes to facilitate the translation of peptide motifs with consecutive prolines or combinations of prolines with glycine and charged amino acids. It has also been linked to other molecular functions and cellular processes, such as nuclear mRNA export and mRNA decay, proliferation, differentiation, autophagy, and apoptosis. The growing interest in eIF5A relates to its association with the pathogenesis of several diseases, including cancer, viral infection, and diabetes. It has also been proposed as an anti-aging factor: its levels decay in aged cells, whereas increasing levels of active eIF5A result in the rejuvenation of the immune and vascular systems and improved brain cognition. Recent data have linked the role of eIF5A in some pathologies with its function in maintaining healthy mitochondria. The eukaryotic translation initiation factor 5A is upregulated under respiratory metabolism and its deficiency reduces oxygen consumption, ATP production, and the levels of several mitochondrial metabolic enzymes, as well as altering mitochondria dynamics. However, although all the accumulated data strongly link eIF5A to mitochondrial function, the precise molecular role and mechanisms involved are still unknown. In this review, we discuss the findings linking eIF5A and mitochondria, speculate about its role in regulating mitochondrial homeostasis, and highlight its potential as a target in diseases related to energy metabolism.

The role of polyamines in gastric cancer

Advancements in our understanding of polyamine molecular and cellular functions have led to increased interest in targeting polyamine metabolism for anticancer therapeutic benefits. The polyamines putrescine, spermidine, and spermine are polycationic alkylamines commonly found in all living cells and are essential for cellular growth and survival. This review summarizes the existing research on polyamine metabolism and function, specifically the role of polyamines in gastric immune cell and epithelial cell function. Polyamines have been implicated in a multitude of cancers, but in this review, we focus on the role of polyamine dysregulation in the context of Helicobacter pylori-induced gastritis and subsequent progression to gastric cancer. Due to the emerging implication of polyamines in cancer development, there is an increasing number of promising clinical trials using agents to target the polyamine metabolic pathway for potential chemoprevention and anticancer therapy.

Hypusination, a Metabolic Posttranslational Modification of eIF5A in Plants during Development and Environmental Stress Responses

Hypusination is a unique posttranslational modification of eIF5A, a eukaryotic translation factor. Hypusine is a rare amino acid synthesized in this process and is mediated by two enzymes, deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH). Despite the essential participation of this conserved eIF5A protein in plant development and stress responses, our knowledge of its proper function is limited. In this review, we demonstrate the main findings regarding how eIF5A and hypusination could contribute to plant-specific responses in growth and stress-related processes. Our aim is to briefly discuss the plant-specific details of hypusination and decipher those signal pathways which can be effectively modified by this process. The diverse functions of eIF5A isoforms are also discussed in this review.

Life as a Vector of Dengue Virus: The Antioxidant Strategy of Mosquito Cells to Survive Viral Infection

Dengue fever is a mosquito-borne viral disease of increasing global importance. The disease has caused heavy burdens due to frequent outbreaks in tropical and subtropical areas of the world. The dengue virus (DENV) is generally transmitted between human hosts via the bite of a mosquito vector, primarily Aedes aegypti and Ae. albopictus as a minor species. It is known that the virus needs to alternately infect mosquito and human cells. DENV-induced cell death is relevant to the pathogenesis in humans as infected cells undergo apoptosis. In contrast, mosquito cells mostly survive the infection; this allows infected mosquitoes to remain healthy enough to serve as an efficient vector in nature. Overexpression of antioxidant genes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), glutaredoxin (Grx), thioredoxin (Trx), and protein disulfide isomerase (PDI) have been detected in DENV2-infected mosquito cells. Additional antioxidants, including GST, eukaryotic translation initiation factor 5A (eIF5a), and p53 isoform 2 (p53-2), and perhaps some others, are also involved in creating an intracellular environment suitable for cell replication and viral infection. Antiapoptotic effects involving inhibitor of apoptosis (IAP) upregulation and subsequent elevation of caspase-9 and caspase-3 activities also play crucial roles in the ability of mosquito cells to survive DENV infection. This article focused on the effects of intracellular responses in mosquito cells to infection primarily by DENVs. It may provide more information to better understand virus/cell interactions that can possibly elucidate the evolutionary pathway that led to the mosquito becoming a vector.

Hypusination Orchestrates the Antimicrobial Response of Macrophages

Innate responses of myeloid cells defend against pathogenic bacteria via inducible effectors. Deoxyhypusine synthase (DHPS) catalyzes the transfer of the N-moiety of spermidine to the lysine-50 residue of eukaryotic translation initiation factor 5A (EIF5A) to form the amino acid hypusine. Hypusinated EIF5A (EIF5AHyp) transports specific mRNAs to ribosomes for translation. We show that DHPS is induced in macrophages by two gastrointestinal pathogens, Helicobacter pylori and Citrobacter rodentium, resulting in enhanced hypusination of EIF5A. EIF5AHyp was also increased in gastric macrophages from patients with H. pylori gastritis. Furthermore, we identify the bacteria-induced immune effectors regulated by hypusination. This set of proteins includes essential constituents of antimicrobial response and autophagy. Mice with myeloid cell-specific deletion of Dhps exhibit reduced EIF5AHyp in macrophages and increased bacterial burden and inflammation. Thus, regulation of translation through hypusination is a critical hallmark of the defense of eukaryotic hosts against pathogenic bacteria.

Nuclear Translocation of Glutaminase GLS2 in Human Cancer Cells Associates with Proliferation Arrest and Differentiation

Glutaminase (GA) catalyzes the first step in mitochondrial glutaminolysis playing a key role in cancer metabolic reprogramming. Humans express two types of GA isoforms: GLS and GLS2. GLS isozymes have been consistently related to cell proliferation, but the role of GLS2 in cancer remains poorly understood. GLS2 is repressed in many tumor cells and a better understanding of its function in tumorigenesis may further the development of new therapeutic approaches. We analyzed GLS2 expression in HCC, GBM and neuroblastoma cells, as well as in monkey COS-7 cells. We studied GLS2 expression after induction of differentiation with phorbol ester (PMA) and transduction with the full-length cDNA of GLS2. In parallel, we investigated cell cycle progression and levels of p53, p21 and c-Myc proteins. Using the baculovirus system, human GLS2 protein was overexpressed, purified and analyzed for posttranslational modifications employing a proteomics LC-MS/MS platform. We have demonstrated a dual targeting of GLS2 in human cancer cells. Immunocytochemistry and subcellular fractionation gave consistent results demonstrating nuclear and mitochondrial locations, with the latter being predominant. Nuclear targeting was confirmed in cancer cells overexpressing c-Myc- and GFP-tagged GLS2 proteins. We assessed the subnuclear location finding a widespread distribution of GLS2 in the nucleoplasm without clear overlapping with specific nuclear substructures. GLS2 expression and nuclear accrual notably increased by treatment of SH-SY5Y cells with PMA and it correlated with cell cycle arrest at G2/M, upregulation of tumor suppressor p53 and p21 protein. A similar response was obtained by overexpression of GLS2 in T98G glioma cells, including downregulation of oncogene c-Myc. Furthermore, human GLS2 was identified as being hypusinated by MS analysis, a posttranslational modification which may be relevant for its nuclear targeting and/or function. Our studies provide evidence for a tumor suppressor role of GLS2 in certain types of cancer. The data imply that GLS2 can be regarded as a highly mobile and multilocalizing protein translocated to both mitochondria and nuclei. Upregulation of GLS2 in cancer cells induced an antiproliferative response with cell cycle arrest at the G2/M phase.

Indications for a moonlighting function of translation factor aIF5A in the crenarchaeum Sulfolobus solfataricus

Translation factor a/eIF5A is highly conserved in Eukarya and Archaea. The eukaryal eIF5A protein is required for transit of ribosomes across consecutive proline codons, whereas the function of the archaeal orthologue remains unknown. Here, we provide a first hint for an involvement of Sulfolobus solfataricus (Sso) aIF5A in translation. CRISPR-mediated knock down of the aif5A gene resulted in strong growth retardation, underlining a pivotal function. Moreover, in vitro studies revealed that Sso aIF5A is endowed with endoribonucleolytic activity. Thus, aIF5A appears to be a moonlighting protein that might be involved in protein synthesis as well as in RNA metabolism.

PhDHS Is Involved in Chloroplast Development in Petunia

Deoxyhypusine synthase (DHS) is encoded by a nuclear gene and is the key enzyme involved in the post-translational activation of the eukaryotic translation initiation factor eIF5A. DHS plays important roles in plant growth and development. To gain a better understanding of DHS, the petunia (Petunia hybrida) PhDHS gene was isolated, and the role of PhDHS in plant growth was analyzed. PhDHS protein was localized to the nucleus and cytoplasm. Virus-mediated PhDHS silencing caused a sectored chlorotic leaf phenotype. Chlorophyll levels and photosystem II activity were reduced, and chloroplast development was abnormal in PhDHS-silenced leaves. In addition, PhDHS silencing resulted in extended leaf longevity and thick leaves. A proteome assay revealed that 308 proteins are upregulated and 266 proteins are downregulated in PhDHS-silenced plants compared with control, among the latter, 21 proteins of photosystem I and photosystem II and 12 thylakoid (thylakoid lumen and thylakoid membrane) proteins. In addition, the mRNA level of PheIF5A-1 significantly decreased in PhDHS-silenced plants, while that of another three PheIF5As were not significantly affected in PhDHS-silenced plants. Thus, silencing of PhDHS affects photosynthesis presumably as an indirect effect due to reduced expression of PheIF5A-1 in petunia. Significance: PhDHS-silenced plants develop yellow leaves and exhibit a reduced level of photosynthetic pigment in mesophyll cells. In addition, arrested development of chloroplasts is observed in the yellow leaves.

Regulation of gene expression by translation factor eIF5A: Hypusine-modified eIF5A enhances nonsense-mediated mRNA decay in human cells

Nonsense-mediated mRNA decay (NMD) couples protein synthesis to mRNA turnover. It eliminates defective transcripts and controls the abundance of certain normal mRNAs. Our study establishes a connection between NMD and the translation factor eIF5A (eukaryotic initiation factor 5A) in human cells. eIF5A modulates the synthesis of groups of proteins (the eIF5A regulon), and undergoes a distinctive two-step post-translational modification (hypusination) catalyzed by deoxyhypusine synthase and deoxyhypusine hydroxylase. We show that expression of NMD-susceptible constructs was increased by depletion of the major eIF5A isoform, eIF5A1. NMD was also attenuated when hypusination was inhibited by RNA interference with either of the two eIF5A modifying enzymes, or by treatment with the drugs ciclopirox or deferiprone which inhibit deoxyhypusine hydroxylase. Transcriptome analysis by RNA-Seq identified human genes whose expression is coordinately regulated by eIF5A1, its modifying enzymes, and the pivotal NMD factor, Upf1. Transcripts encoding components of the translation system were highly represented, including some encoding ribosomal proteins controlled by alternative splicing coupled to NMD (AS-NMD). Our findings extend and strengthen the association of eIF5A with NMD, previously inferred in yeast, and show that hypusination is important for this function of human eIF5A. In addition, they advance drug-mediated NMD suppression as a therapeutic opportunity for nonsense-associated diseases. We propose that regulation of mRNA stability contributes to eIF5A's role in selective gene expression.

Recombinant Trichomonas vaginalis eIF-5A protein expressed from a eukaryotic system binds specifically to mammalian and putative trichomonal eIF-5A response elements (EREs)

Trichomonas vaginalis eIF-5A-like protein (TveIF-5A) belongs to the highly conserved eIF-5A family of proteins that contains a unique polyamine-derived amino acid, hypusine. Recently, we determined that the polyamine putrescine is required for tveif-5a mRNA stability, and it is necessary for stability and maturation of the TveIF-5A protein. Eukaryotic eIF-5A is known to be involved in mRNA turnover and is capable of sequence-specific RNA binding to eIF-5A response elements (EREs). These ERE sequences are present in diverse mammalian mRNAs, including human cyclooxygenase-2 (cox-2). Here, we cloned the complete coding sequence of TveIF-5A and overexpressed it in a eukaryotic system. The recombinant protein (rTveIF-5A) was purified in soluble form using size-exclusion chromatography. Because of the polyamine-dependent regulation of TvCP39 (a protease of T. vaginalis) at the protein and RNA messenger (mRNA) levels, we looked for an ERE-like structure in the 3' region of tvcp39 mRNA. In RNA gel-shift assays, rTveIF-5A bound to transcripts at the EREs of cox-2 or tvcp39 mRNAs. This work shows the eIF-5A/ERE-like interaction in T. vaginalis.

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.

Bifunctional activity of deoxyhypusine synthase/hydroxylase from Trichomonas vaginalis

The Trichomonas vaginalis genome analysis suggested the presence of a putative deoxyhypusine synthase (TvDHS) that catalyzes the posttranslational modification of eIF-5A. Herein, we expressed and purified the recombinant TvDHS (rTvDHS) protein (43 kDa) and the recombinant TveIF-5A (rTveIF-5A) precursor protein (46 kDa). A 41 kDa band of the native TvDHS was recognized by western blot analysis in T. vaginalis total protein extract by a mouse polyclonal anti-rTvDHS antibody. The enzymatic activity of rTvDHS was determined by in vitro rTveIF-5A precursor modification. The modification reaction was performed by using ((3)H)-spermidine, and the biochemical analysis showed that rTvDHS exhibited Km value of 0.6 μM. The rTvDHS activity was inhibited by the spermidine analog, N″-guanyl-1,7-diamino-heptane (GC7). Native gel electrophoresis analysis showed two bands corresponding to an rTvDHS-rTveIF-5A complex and an intermediate form of rTveIF-5A. The two forms were subsequently separated by ion exchange chromatography to identify the hypusine residue by MS/MS analysis. Moreover, mutations in TvDHS showed that the putative HE motif present in this enzyme is involved in the hydroxylation of TveIF-5A. We observed that only hypusine-containing TveIF-5A was bound to an RNA hairpin ERE structure from the cox-2 gene, which contains the AAAUGUCACAC consensus sequence. Interestingly, 2DE-WB assays, using parasites that were grown in DAB-culture conditions and transferred to exogenous putrescine, showed the new isoform of TveIF-5A. In summary, our results indicate that T. vaginalis contains an active TvDHS capable of modifying the precursor TveIF-5A protein, which subsequently exhibits RNA binding activity.

Eukaryotic initiation factor 5A plays an essential role in luteinizing hormone receptor regulation

Down-regulation of LH receptor (LHR) in the ovary by its ligand is mediated by a specific RNA-binding protein, designated LH receptor mRNA-binding protein (LRBP), through translational suppression and mRNA degradation. Using yeast 2-hybrid screens, we previously identified eukaryotic initiation factor 5A (eIF5A) as one of the proteins that interacts with LRBP during LHR mRNA down-regulation. The present study examined the role of eIF5A and its hypusination in the context of LHR mRNA down-regulation. The association of eIF5A with LRBP or LHR mRNA was determined using immunoprecipitation and RNA immunoprecipitation assays. The results showed that the association of eIF5A with the LHR mRNA-LRBP complex increased significantly during down-regulation. Furthermore, gel fractionation and the hypusination activity assay both showed increased hypusination of eIF5A during LHR mRNA down-regulation. Abolishment of hypusination by pretreatment with the chemical inhibitor GC7 prevented the association of eIF5A with LHR mRNA and LRBP. Inhibition of hypusination also reduced the extent of ligand-induced down-regulation of LHR mRNA as well as the expression of functional LHRs assessed by real-time PCR and (125)I-human chorionic gonadotropin (hCG) binding assays, respectively. The loss of human chorionic gonadotropin-mediated downstream signaling during LHR down-regulation was also restored by inhibition of hypusination of eIF5A. Thus, the present study, for the first time, reveals the crucial role of eIF5A and its hypusination in the regulation of LHR expression in the ovary.

A cell-penetrating phospholamban-specific RNA aptamer enhances Ca2+ transients and contractile function in cardiomyocytes

The sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a (SERCA2a)-phospholamban (PLN) system of sarcoplasmic reticulum plays a pivotal role in regulation of intracellular Ca(2+) cycling in ventricular cardiomyocytes. Given that Ca(2+) cycling is impaired in heart failure, proteins that contribute to this process are potential targets for the treatment of this condition. We have now isolated PLN-specific aptamers with a phosphorothioate-modified backbone from a library of RNA molecules containing a randomized 40-nucleotide sequence by application of the systematic evolution of ligands by exponential enrichment (SELEX) protocol with a fusion protein containing the cytoplasmic region of human PLN. One of these aptamers was shortened to a 30-nucleotide oligomer (RNA-Apt30) without loss of function. RNA-Apt30 showed a high affinity for the cytoplasmic region of PLN (Kd=11 nM), but it did not bind to the phosphorylated form of PLN or to a phosphomimetic mutant. It also increased SERCA2a activity in isolated cardiac SR vesicles with an EC50 of 18 nM by relieving PLN-mediated inhibition. Conjugation of RNA-Apt30 to a cell-penetrating peptide allowed its delivery into adult rat cardiomyocytes, in which it enhanced both Ca(2+) transients and contractile function. These effects of the aptamer were also apparent in the presence of the β-adrenergic receptor antagonist propranolol. This cell-penetrating PLN aptamer may thus provide a basis for the development of new therapeutic agents for heart failure without the need for gene transfer or a change in endogenous protein expression.

A novel mouse model for inhibition of DOHH-mediated hypusine modification reveals a crucial function in embryonic development, proliferation and oncogenic transformation

The central importance of translational control by post-translational modification has spurred major interest in regulatory pathways that control translation. One such pathway uniquely adds hypusine to eukaryotic initiation factor 5A (eIF5A), and thereby affects protein synthesis and, subsequently, cellular proliferation through an unknown mechanism. Using a novel conditional knockout mouse model and a Caenorhabditis elegans knockout model, we found an evolutionarily conserved role for the DOHH-mediated second step of hypusine synthesis in early embryonic development. At the cellular level, we observed reduced proliferation and induction of senescence in 3T3 Dohh-/- cells as well as reduced capability for malignant transformation. Furthermore, mass spectrometry showed that deletion of DOHH results in an unexpected complete loss of hypusine modification. Our results provide new biological insight into the physiological roles of the second step of the hypusination of eIF5A. Moreover, the conditional mouse model presented here provides a powerful tool for manipulating hypusine modification in a temporal and spatial manner, to analyse both how this unique modification normally functions in vivo as well as how it contributes to different pathological conditions.

Cryptosporidium parvum has an active hypusine biosynthesis pathway

The protozoan parasite Cryptosporidium parvum causes severe enteric infection and diarrheal disease with substantial morbidity and mortality in untreated AIDS patients and children in developing or resource-limited countries. No fully effective treatment is available. Hypusination of eIF5A is an important post-translational modification essential for cell proliferation. This modification occurs in a two step process catalyzed by deoxyhypusine synthase (DHS) followed by deoxyhypusine hydroxylase. An ORF of 1086bp was identified in the C. parvum (Cp) genome which encodes for a putative polypeptide of 362 amino acids. The recombinant CpDHS protein was purified to homogeneity and used to probe the enzyme's mechanism, structure, and inhibition profile in a series of kinetic experiments. Sequence analysis and structural modeling of CpDHS were performed to probe differences with respect to the DHS of other species. Unlike Leishmania, Trypanosomes and Entamoeba, Cryptosporidium contains only a single gene for DHS. Phylogenetic analysis shows that CpDHS is more closely related to apicomplexan DHS than kinetoplastid DHS. Important residues that are essential for the functioning of the enzyme including NAD(+) binding residues, spermidine binding residues and the active site lysine are conserved between CpDHS and human DHS. N(1)-guanyl-1,7-diaminoheptane (GC7), a potent inhibitor of DHS caused an effective inhibition of infection and growth of C. parvum in HCT-8 cells.

Biochemical quantitation of the eIF5A hypusination in Arabidopsis thaliana uncovers ABA-dependent regulation

The eukaryotic translation elongation factor eIF5A is the only protein known to contain the unusual amino acid hypusine which is essential for its biological activity. This post-translational modification is achieved by the sequential action of the enzymes deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH). The crucial molecular function of eIF5A during translation has been recently elucidated in yeast and it is expected to be fully conserved in every eukaryotic cell, however the functional description of this pathway in plants is still sparse. The genetic approaches with transgenic plants for either eIF5A overexpression or antisense have revealed some activities related to the control of cell death processes but the molecular details remain to be characterized. One important aspect of fully understanding this pathway is the biochemical description of the hypusine modification system. Here we have used recombinant eIF5A proteins either modified by hypusination or non-modified to establish a bi-dimensional electrophoresis (2D-E) profile for the three eIF5A protein isoforms and their hypusinated or unmodified proteoforms present in Arabidopsis thaliana. The combined use of the recombinant 2D-E profile together with 2D-E/western blot analysis from whole plant extracts has provided a quantitative approach to measure the hypusination status of eIF5A. We have used this information to demonstrate that treatment with the hormone abscisic acid produces an alteration of the hypusine modification system in Arabidopsis thaliana. Overall this study presents the first biochemical description of the post-translational modification of eIF5A by hypusination which will be functionally relevant for future studies related to the characterization of this pathway in Arabidopsis thaliana.

Characterization of a novel polyclonal anti-hypusine antibody

The translation factor eIF5A is the only protein known to contain the amino acid hypusine, which is formed posttranslationally. Hypusinated eIF5A is necessary for cellular proliferation and responses to extracellular stressors, and has been proposed as a target for pharmacologic therapy. Here, we provide the first comprehensive characterization of a novel polyclonal antibody (IU-88) that specifically recognizes the hypusinated eIF5A. IU-88 will be useful for the investigation of eIF5A biology and for the development of assays recognizing hypusinated eIF5A.

eIF5A dimerizes not only in vitro but also in vivo and its molecular envelope is similar to the EF-P monomer

The protein eukaryotic initiation factor 5A (eIF5A) is highly conserved among archaea and eukaryotes, but not in bacteria. Bacteria have the elongation factor P (EF-P), which is structurally and functionally related to eIF5A. eIF5A is essential for cell viability and the only protein known to contain the amino acid residue hypusine, formed by post-translational modification of a specific lysine residue. Although eIF5A was initially identified as a translation initiation factor, recent studies strongly support a function for eIF5A in the elongation step of translation. However, the mode of action of eIF5A is still unknown. Here, we analyzed the oligomeric state of yeast eIF5A. First, by using size-exclusion chromatography, we showed that this protein exists as a dimer in vitro, independent of the hypusine residue or electrostatic interactions. Protein-protein interaction assays demonstrated that eIF5A can form oligomers in vitro and in vivo, in an RNA-dependent manner, but independent of the hypusine residue or the ribosome. Finally, small-angle X-ray scattering (SAXS) experiments confirmed that eIF5A behaves as a stable dimer in solution. Moreover, the molecular envelope determined from the SAXS data shows that the eIF5A dimer is L-shaped and superimposable on the tRNA(Phe) tertiary structure, analogously to the EF-P monomer.

Preventing Fusarium head blight of wheat and cob rot of maize by inhibition of fungal deoxyhypusine synthase

Upon posttranslational activation, the eukaryotic initiation factor-5A (eIF-5A) transports a subset of mRNAs out of the nucleus to the ribosomes for translation. Activation of the protein is an evolutionary highly conserved process that is unique to eIF-5A, the conversion of a lysine to a hypusine. Instrumental for the synthesis of hypusine is the first of two enzymatic reactions mediated by deoxyhypusine synthase (DHS). We show that DHS of wheat and the pathogenic fungus Fusarium graminearum, which causes one of the most destructive crop diseases worldwide, are transcriptionally upregulated during their pathogenic interaction. Although DHS of wheat, fungus, and human can be equally inhibited by the inhibitor CNI-1493 in vitro, application during infection of wheat and maize flowers results in strong inhibition of the pathogen without interference with kernel development. Our studies provide a novel strategy to selectively inhibit fungal growth without affecting plant growth. We identified fungal DHS as a target for the development of new inhibitors, for which CNI-1493 may serve as a lead substance.

Inhibition of deoxyhypusine synthase enhances islet {beta} cell function and survival in the setting of endoplasmic reticulum stress and type 2 diabetes

Islet β cell dysfunction resulting from inflammation, ER stress, and oxidative stress is a key determinant in the progression from insulin resistance to type 2 diabetes mellitus. It was recently shown that the enzyme deoxyhypusine synthase (DHS) promotes early cytokine-induced inflammation in the β cell. DHS catalyzes the conversion of lysine to hypusine, an amino acid that is unique to the translational elongation factor eIF5A. Here, we sought to determine whether DHS activity contributes to β cell dysfunction in models of type 2 diabetes in mice and β cell lines. A 2-week treatment of obese diabetic C57BLKS/J-db/db mice with the DHS inhibitor GC7 resulted in improved glucose tolerance, increased insulin release, and enhanced β cell mass. Thapsigargin treatment of β cells in vitro induces a picture of ER stress and apoptosis similar to that seen in db/db mice; in this setting, DHS inhibition led to a block in CHOP (CAAT/enhancer binding protein homologous protein) production despite >30-fold activation of Chop gene transcription. Blockage of CHOP translation resulted in reduction of downstream caspase-3 cleavage and near-complete protection of cells from apoptotic death. DHS inhibition appeared to prevent the cytoplasmic co-localization of eIF5A with the ER, possibly precluding the participation of eIF5A in translational elongation at ER-based ribosomes. We conclude that hypusination by DHS is required for the ongoing production of proteins, particularly CHOP, in response to ER stress in the β cell.

Pumpkin eIF5A isoforms interact with components of the translational machinery in the cucurbit sieve tube system

In yeast, eIF5A, in combination with eEF2, functions at the translation step, during the protein elongation cycle. This result is of significance with respect to functioning of the enucleate sieve tube system, as eIF5A was recently detected in Cucurbita maxima (pumpkin) phloem sap. In the present study, we further characterized four CmeIF5A isoforms, encoding three proteins, all of which were present in the phloem sap. Although hypusination of CmeIF5A was not necessary for entry into the sieve elements, this unique post-translational modification was necessary for RNA binding. The two enzymes required for hypusination were detected in pumpkin phloem sap, where presumably this modification takes place. A combination of gel-filtration chromatography and protein overlay assays demonstrated that, as in yeast, CmeIF5A interacts with phloem proteins, like eEF2, known to be involved in protein synthesis. These findings are discussed in terms of a potential role for eIF5A in regulating protein synthesis within the enucleate sieve tube system of the angiosperms.

Arabidopsis eIF5A3 influences growth and the response to osmotic and nutrient stress

AteIF5A3, one of three genes encoding eukaryotic translation initiation factor 5A (eIF5A) in Arabidopsis thaliana, and corresponding genes PdeIF5A3 from Populus deltoides (eastern cottonwood) and SleIF5A4 from Solanum lycopersicum (tomato) were constitutively over-expressed in A. thaliana. The resultant transgenic plants exhibited enhanced vegetative and reproductive growth. Indeed, the increase in seed yield relative to empty vector controls for the PdeIF5A3 over-expressing plants ranged from 50% to 300% depending on the line. The PdeIF5A3 over-expressing plants also exhibited enhanced fitness when exposed to osmotic and nutrient (N, P and K) stress. The spatial localization of AteIF5A3 was visualized by confocal microscopy using transgenic plants expressing P(AteIF5A3) :GFP-AteIF5A3. GFP fluorescence reflecting expression of AteIF5A3 was detectable in the phloem, particularly companion cells, of roots, stems and leaves, in the epidermal cells of the root tip, in the columella cells of the root cap and in the chalazal tissue of fertilized ovules, which all play a pivotal role in nutrient or hormone translocation. Thus, AteIF5A3 appears to be involved in supporting growth and to play a regulatory role in the response of plants to sub-lethal osmotic and nutrient stress.

Upregulation of a novel eukaryotic translation initiation factor 5A (eIF5A) in dengue 2 virus-infected mosquito cells

BACKGROUND

Dengue virus, a mosquito-borne flavivirus, is the etiological agent of dengue fever, dengue hemorrhagic fever, and dengue shock syndrome. It generally induces apoptosis in mammalian cells, but frequently results in persistent infection in mosquito cells. That mechanism remains to be explored. In turn, a genomic survey through subtractive hybridization (PCR-select cDNA subtraction) was conducted in order to find gene(s) that may play a role in interactions between the virus and its host cells.

RESULTS

Through this technique, we identified a novel eukaryotic translation initiation factor 5A (eIF5A) which is upregulated in Aedes albopictus-derived C6/36 cells infected by the type 2 dengue (Den-2) virus. The full-length of the identified eIF5A gene consisted of 1498 bp of nucleotides with a 41.39% G+C content, and it possessed a higher similarity and shorter evolutionary distance with insects than with other organisms. Upregulation of eIF5A in response to Den-2 virus infection was validated at both the RNA and protein levels. This phenomenon was also observed by confocal microscopy. In addition, cell death obviously occurred when eIF5A activity was inhibited in C6/36 cells even when they were infected by the virus. However, viral multiplication was not obviously affected in infected C6/36 cells when eIF5A activity was reduced.

CONCLUSIONS

Taken together, we postulated that eIF5A plays a role in preventing mosquito cells from death in response to Den-2 viral infection, thus facilitating continued viral growth and potential persistent infection in mosquito cells. It would be worthwhile to further investigate how its downstream factors or cofactors contribute to this phenomenon of dengue infection.

Revisiting an old acquaintance: role for eIF5A in diabetes

Dysfunction of pancreatic islet beta cells underlies both type 1 and type 2 diabetes and appears to result in part from the local release of proinflammatory cytokines. An improved understanding of the mechanisms that mediate islet responsiveness to proinflammatory cytokines may therefore expand our knowledge of the role of cytokine signaling in the development of diabetes, providing potential new targets for the development of therapeutics to protect pancreatic islets from inflammation. In this issue of the JCI, Maier and colleagues identify eukaryotic translation initiation factor 5A (eIF5A) as a critical regulator of the inflammatory response in mouse pancreatic islets. I believe these data provide new and important insights into the regulatory pathways that contribute to the development of diabetes and deepen our understanding of the function of the, so far, rather enigmatic cellular protein eIF5A.

The unique hypusine modification of eIF5A promotes islet beta cell inflammation and dysfunction in mice

In both type 1 and type 2 diabetes, pancreatic islet dysfunction results in part from cytokine-mediated inflammation. The ubiquitous eukaryotic translation initiation factor 5A (eIF5A), which is the only protein to contain the amino acid hypusine, contributes to the production of proinflammatory cytokines. We therefore investigated whether eIF5A participates in the inflammatory cascade leading to islet dysfunction during the development of diabetes. As described herein, we found that eIF5A regulates iNOS levels and that eIF5A depletion as well as the inhibition of hypusination protects against glucose intolerance in inflammatory mouse models of diabetes. We observed that following knockdown of eIF5A expression, mice were resistant to beta cell loss and the development of hyperglycemia in the low-dose streptozotocin model of diabetes. The depletion of eIF5A led to impaired translation of iNOS-encoding mRNA within the islet. A role for the hypusine residue of eIF5A in islet inflammatory responses was suggested by the observation that inhibition of hypusine synthesis reduced translation of iNOS-encoding mRNA in rodent beta cells and human islets and protected mice against the development of glucose intolerance the low-dose streptozotocin model of diabetes. Further analysis revealed that hypusine is required in part for nuclear export of iNOS-encoding mRNA, a process that involved the export protein exportin1. These observations identify the hypusine modification of eIF5A as a potential therapeutic target for preserving islet function under inflammatory conditions.

Inhibition of HIV-1 gene expression by Ciclopirox and Deferiprone, drugs that prevent hypusination of eukaryotic initiation factor 5A

BACKGROUND

Eukaryotic translation initiation factor eIF5A has been implicated in HIV-1 replication. This protein contains the apparently unique amino acid hypusine that is formed by the post-translational modification of a lysine residue catalyzed by deoxyhypusine synthase and deoxyhypusine hydroxylase (DOHH). DOHH activity is inhibited by two clinically used drugs, the topical fungicide ciclopirox and the systemic medicinal iron chelator deferiprone. Deferiprone has been reported to inhibit HIV-1 replication in tissue culture.

RESULTS

Ciclopirox and deferiprone blocked HIV-1 replication in PBMCs. To examine the underlying mechanisms, we investigated the action of the drugs on eIF5A modification and HIV-1 gene expression in model systems. At early times after drug exposure, both drugs inhibited substrate binding to DOHH and prevented the formation of mature eIF5A. Viral gene expression from HIV-1 molecular clones was suppressed at the RNA level independently of all viral genes. The inhibition was specific for the viral promoter and occurred at the level of HIV-1 transcription initiation. Partial knockdown of eIF5A-1 by siRNA led to inhibition of HIV-1 gene expression that was non-additive with drug action. These data support the importance of eIF5A and hypusine formation in HIV-1 gene expression.

CONCLUSION

At clinically relevant concentrations, two widely used drugs blocked HIV-1 replication ex vivo. They specifically inhibited expression from the HIV-1 promoter at the level of transcription initiation. Both drugs interfered with the hydroxylation step in the hypusine modification of eIF5A. These results have profound implications for the potential therapeutic use of these drugs as antiretrovirals and for the development of optimized analogs.

Thermal killing of human colon cancer cells is associated with the loss of eukaryotic initiation factor 5A

Heat-induced cell death appears to be a cell-specific event. Chronic heat stress was lethal to human colon cancer cells (Caco-2, HT29, and HCT116), but not to normal diploid fibroblasts and other cancer cells (BJ-T, WI38, HeLa, ovarian 2008, WI38VA). Acute heat stress (45-51 degrees C, 30 min) caused cell death of colon cancer cells during recovery at physiological temperature. Thermal killing of Caco-2 cells was not mediated via oxidative stress since Caco-2 cells were much more resistant than HeLa and other cancer cells to H(2)O(2)-induced cell death. Acute heat stress caused a striking loss of eukaryotic initiation factor 5A (eIF5A) in colon cancer cells, but not in HeLa and other normal or transformed human fibroblasts. The heat-induced loss of eIF5A is likely to be due to changes in the protein stability. The half-life of eIF5A was changed from >20 h to less than 30 min during the acute heat stress. Sequence analysis of the eIF5A gene from Caco-2 and HeLa cells did not reveal any difference, suggesting that the change in stability in Caco-2 cells was not due to any eIF5A mutation. Pretreatment of cells with protease inhibitors such as phenylmethyl sulfonyl fluoride (PMSF) partially blocked the heat-induced loss of eIF5A and prevented heat-induced cell death. In light of the essential role of eIF5A in cell survival and proliferation, our results suggest that the stability of eIF5A may have an important role in determining the fate of the particular cell type after severe heat stress.

Dimerization of the yeast eukaryotic translation initiation factor 5A requires hypusine and is RNA dependent

Post-translational modification of the highly conserved K51 residue of the Saccharomyces cerevisiae eukaryotic translation initiation factor 5A (eIF5A) to form hypusine, is essential for its many functions including the binding of specific mRNAs. We characterized hypusinated yeast eIF5A by size-exclusion chromatography and native PAGE, showing that the protein exists as a homodimer. A K51R mutant, which was not functional in vivo eluted as a monomer and inhibition of hypusination abolished dimerization. Furthermore, treatment of dimeric eIF5A with RNase A resulted in disruption of the dimer, leading us to conclude that RNA binding is also required for dimerization of eIF5A. We present a model of dimerization, based on the Neurospora crassa structural analogue, HEX-1.

Structural modeling and mutational analysis of yeast eukaryotic translation initiation factor 5A reveal new critical residues and reinforce its involvement in protein synthesis

Eukaryotic translation initiation factor 5A (eIF5A) is a protein that is highly conserved and essential for cell viability. This factor is the only protein known to contain the unique and essential amino acid residue hypusine. This work focused on the structural and functional characterization of Saccharomyces cerevisiae eIF5A. The tertiary structure of yeast eIF5A was modeled based on the structure of its Leishmania mexicana homologue and this model was used to predict the structural localization of new site-directed and randomly generated mutations. Most of the 40 new mutants exhibited phenotypes that resulted from eIF-5A protein-folding defects. Our data provided evidence that the C-terminal alpha-helix present in yeast eIF5A is an essential structural element, whereas the eIF5A N-terminal 10 amino acid extension not present in archaeal eIF5A homologs, is not. Moreover, the mutants containing substitutions at or in the vicinity of the hypusine modification site displayed nonviable or temperature-sensitive phenotypes and were defective in hypusine modification. Interestingly, two of the temperature-sensitive strains produced stable mutant eIF5A proteins--eIF5A(K56A) and eIF5A(Q22H,L93F)--and showed defects in protein synthesis at the restrictive temperature. Our data revealed important structural features of eIF5A that are required for its vital role in cell viability and underscored an essential function of eIF5A in the translation step of gene expression.

In vivo and in vitro studies of RNA degrading activities in Archaea

Controlled degradation of RNA is important for the regulation of gene expression in Bacteria and Eukarya, but information about these processes is limited in the domain of Archaea. To address this, we studied the half-life of different mRNAs in halophilic Archaea after blocking transcription with actinomycin D. We found that the stability of mRNAs of the gvp operons in Haloferax mediterranei varies under different growth conditions. To understand regulated mRNA decay in Archaea, we need to identify stability determinants within mRNAs and proteins, mainly ribonucleases (RNases), which recognize these determinants. First, we wanted to identify archaeal RNases independently of their sequence similarity to known RNases from Bacteria and Eukarya. To this end we performed fractionation of proteins from Halobacterium salinarum and tested the fractions for RNase activity with an internally labeled in vitro-synthesized mRNA. After three purification steps, we isolated an endoribonucleolytically active protein with similarities to the eukaryotic initiation factor 5A. Further characterization was performed with recombinant halobacterial IF-5A, which was purified from H. salinarum or Escherichia coli. Mutational analysis confirmed unambiguously its RNase activity. In another study, we aimed to purify a double-strand-specific endoribonuclease from Sulfolobus solfataricus. Seven purification steps led to the isolation of two different dehydrogenases with RNase properties. Interestingly, their RNase activity resembled that of aIF-5A and of highly diluted RNase A. RNA was cleaved preferentially between C and A nucleotides in single-stranded regions, and the activity was inhibited at MgCl(2) concentrations >5 mM and at KCl concentrations >200 mM. However, it was possible to distinguish the activity of the archaeal proteins from the activity of RNase A. In a different approach, we used a bioinformatics prediction of the archaeal exosome to purify this protein complex from S. solfataricus. Isolation by coimmunoprecipitation revealed the presence of four orthologs of eukaryotic exosomal subunits and at least one archaea-specific subunit. We characterized the S. solfataricus exosome as a major enzyme involved in phosphorolytic RNA degradation and in RNA polyadenylation. Here we describe in detail the techniques used to achieve these results.

Modification of eukaryotic initiation factor 5A from Plasmodium vivax by a truncated deoxyhypusine synthase from Plasmodium falciparum: An enzyme with dual enzymatic properties

The increasing resistance of the malaria parasites enforces alternative directions in finding new drug targets. Present findings from the malaria parasite Plasmodium vivax, causing tertiary malaria, suggest eukaryotic initiation factor 5A (eIF-5A) to be a promising target for the treatment of malaria. Previously we presented the 162 amino acid sequence of eukaryotic initiation factor 5A (eIF-5A) from Plasmodium vivax. In the present study, we have expressed and purified the 20kDa protein performed by one-step Nickel chelate chromatography. In Western blot experiments eIF-5A from P. vivax crossreacts with a polyclonal anti-eIF-5A antiserum from the plant Nicotiana plumbaginifolia (Solanaceae). Transcription of eIF-5A can be observed in both different developmental stages of the parasite being prominent in trophozoites. We recently published the nucleic acid sequence from a genomic clone of P. falciparum strain NF54 encoding a putative deoxyhypusine synthase (DHS), an enzyme that catalyzes the post-translational modification of eIF-5A. After removal of 22 amino acids DHS was expressed as a Histidin fusion protein and purified by Nickel affinity chromatography. Truncated DHS from P. falciparum modifies eIF-5A from P. vivax. DHS from P. falciparum NF54 is a bi-functional protein with dual enzymatic specificities, that is, DHS activity and homospermidine synthase activity (HSS) (0.047 pkatal/mg protein) like in other eukaryotes. Inhibition of DHS from P. falciparum resulted in a K(i) of 0.1 microM for the inhibitor GC7 being 2000-fold less than the nonguanylated derivative 1,7-diaminoheptane. Dhs transcription occurs in both develomental stages suggesting its necessity in cell proliferation.

Is there a role for eIF5A in translation?

The putative translation factor eIF5A is essential for cell viability and is highly conserved from archaebacteria to mammals. This factor is the only cellular protein that undergoes an essential posttranslational modification dependent on the polyamine spermidine, called hypusination. This review focuses on the functional characterization of eIF5A. Although this protein was originally identified as a translation initiation factor, subsequent studies did not support a role for eIF5A in general translation initiation. eIF5A has also been implicated in nuclear export of HIV-1 Rev and mRNA decay, but these findings are controversial in the literature and may reflect secondary effects of eIF-5A function. Next, the involvement of eIF5A and hypusination in the control of the cell cycle and proliferation in various organisms is reviewed. Finally, recent evidence in favor of reconsidering the role of eIF5A as a translation factor is discussed. Future studies may reveal the specific mechanism by which eIF5A affects protein synthesis.

An archaeal protein with homology to the eukaryotic translation initiation factor 5A shows ribonucleolytic activity

To identify proteins that are involved in RNA degradation and processing in Halobacterium sp. NRC-1, we purified proteins with RNA-degrading activity by classical biochemical techniques. One of these proteins showed strong homology to the eukaryotic initiation factor 5A (eIF-5A) and was accordingly named archaeal initiation factor 5A (aIF-5A). Eukaryotic IF-5A is known to be involved in mRNA turnover and to bind RNA. Hypusination of eIF-5A is required for sequence-specific binding of RNA. This unique post-translational modification is restricted to Eukarya and Archaea. The exact function of eIF-5A in RNA turnover remained obscure. Here we show for the first time that aIF-5A from Halobacterium sp. NRC-1 exhibits RNA cleavage activity, preferentially cleaving adjacent to A nucleotides. Detectable RNA binding could be shown for aIF-5A purified from Halobacterium sp. NRC-1 but not from Escherichia coli, while both proteins possess RNA cleavage activity, indicating that hypusination of aIF-5A is required for RNA binding but not for its RNA cleavage activity. Furthermore, we show that the hypusinated form of eIF-5A also shows RNase activity while the unmodified protein does not. Charged amino acids in the N-terminal domain of aIF-5A as well as in the C-terminal domain, which is highly similar to the cold shock protein A (CspA), an RNA chaperone of E. coli, are important for RNA cleavage activity. Moreover our results reveal that activity of aIF-5A depends on its oligomeric state.

Eukaryotic translation initiation factor 5A induces apoptosis in colon cancer cells and associates with the nucleus in response to tumour necrosis factor alpha signalling

Eukaryotic translation initiation factor 5A (eIF5A) is thought to function as a nucleocytoplasmic shuttle protein. There are reports of its involvement in cell proliferation, and more recently it has also been implicated in the regulation of apoptosis. In the present study, we examined the effects of eIF5A over-expression on apoptosis and of siRNA-mediated suppression of eIF5A on expression of the tumour suppressor protein, p53. Over-expression of either eIF5A or a mutant of eIF5A incapable of being hypusinated was found to induce apoptosis in colon carcinoma cells. Our results also indicate that eIF5A is required for expression of p53 following the induction of apoptosis by treatment with Actinomycin D. Depiction of eIF5A localization by indirect immunofluorescence has indicated, for the first time, that the protein is rapidly translocated from the cytoplasm to the nucleus by death receptor activation or following treatment with Actinomycin D. These findings collectively indicate that unhypusinated eIF5A may have pro-apoptotic functions and that eIF5A is rapidly translocated to the nucleus following the induction of apoptotic cell death.

Regulation and execution of molecular disassembly and catabolism during senescence

Senescence is a highly orchestrated developmental stage in the life cycle of plants. The onset of senescence is tightly controlled by signaling cascades that initiate changes in gene expression and the synthesis of new proteins. This complement of new proteins includes hydrolytic enzymes capable of executing catabolism of macromolecules, which in turn sets in motion disassembly of membrane molecular matrices, leading to loss of cell function and, ultimately, complete breakdown of cellular ultrastructure. A distinguishing feature of senescence that sets it apart from other types of programmed cell death is the recovery of carbon and nitrogen from the dying tissue and their translocation to growing parts of the plant such as developing seeds. For this to be accomplished, the initiation of senescence and its execution have to be meticulously regulated. For example, the initiation of membrane disassembly has to be intricately linked with the recruitment of nutrients because their ensuing translocation out of the senescing tissue requires functional membranes. Molecular mechanisms underlying this linkage and its integration with the catabolism of macromolecules in senescing tissues are addressed.

Molecular modeling of the human eukaryotic translation initiation factor 5A (eIF5A) based on spectroscopic and computational analyses

The eukaryotic translation initiation factor 5A (eIF5A) is a protein ubiquitously present in archaea and eukarya, which undergoes a unique two-step post-translational modification called hypusination. Several studies have shown that hypusination is essential for a variety of functional roles for eIF5A, including cell proliferation and synthesis of proteins involved in cell cycle control. Up to now neither a totally selective inhibitor of hypusination nor an inhibitor capable of directly binding to eIF5A has been reported in the literature. The discovery of such an inhibitor might be achieved by computer-aided drug design based on the 3D structure of the human eIF5A. In this study, we present a molecular model for the human eIF5A protein based on the crystal structure of the eIF5A from Leishmania brasiliensis, and compare the modeled conformation of the loop bearing the hypusination site with circular dichroism data obtained with a synthetic peptide of this loop. Furthermore, analysis of amino acid variability between different human eIF5A isoforms revealed peculiar structural characteristics that are of functional relevance.

Rapid depletion of mutant eukaryotic initiation factor 5A at restrictive temperature reveals connections to actin cytoskeleton and cell cycle progression

Eukaryotic initiation factor 5A (eIF5A) is the only protein in nature that contains hypusine, an unusual amino acid derived from the modification of lysine by spermidine. Two genes, TIF51A and TIF51B, encode eIF5A in the yeast Saccharomyces cerevisiae. In an effort to understand the structure-function relationship of eIF5A, we have generated yeast mutants by introducing plasmid-borne tif51A into a double null strain where both TIF51A and TIF51B have been disrupted. One of the mutants, tsL102A strain (tif51A L102A tif51aDelta tif51bDelta) exhibits a strong temperature-sensitive growth phenotype. At the restrictive temperature, tsL102A strain also exhibits a cell shape change, a lack of volume change in response to temperature increase and becomes more sensitive to ethanol, a hallmark of defects in the PKC/WSC cell wall integrity pathway. In addition, a striking change in actin dynamics and a complete cell cycle arrest at G1 phase occur in tsL102A cells at restrictive temperature. The temperature-sensitivity of tsL102A strain is due to a rapid loss of mutant eIF5A with the half-life reduced from 6 h at permissive temperature to 20 min at restrictive temperature. Phenylmethyl sulfonylfluoride (PMSF), an irreversible inhibitor of serine protease, inhibited the degradation of mutant eIF5A and suppressed the temperature-sensitive growth arrest. Sorbitol, an osmotic stabilizer that complement defects in PKC/WSC pathways, stabilizes the mutant eIF5A and suppresses all the observed temperature-sensitive phenotypes.

Tandem affinity purification revealed the hypusine-dependent binding of eukaryotic initiation factor 5A to the translating 80S ribosomal complex

Eukaryotic initiation factor 5A (eIF5A) is the only protein in nature that contains hypusine, an unusual amino acid formed post-translationally in two steps by deoxyhypusine synthase and deoxyhypusine hydroxylase. Genes encoding eIF5A or deoxyhypusine synthase are essential for cell survival and proliferation. To determine the physiological function of eIF5A, we have employed the tandem affinity purification (TAP) method and mass spectrometry to search for and identify the potential eIF5A-interacting proteins. The TAP-tag was fused in-frame to chromosomal TIF51A gene and eIF5A-TAP fusion protein expressed at its natural level was used as the bait to fish out its interacting partners. At salt concentrations of 150 mM, deoxyhypusine synthase was the only protein bound to eIF5A. As salt concentrations were lowered to 125 mM or less, eIF5A interacted with a set of proteins, which were identified as the components of the 80S ribosome complex. The eIF5A-ribosome interaction was sensitive to RNase and EDTA treatments, indicating the requirement of RNA and the joining of 40S and 60S ribosomal subunits for the interaction. Importantly, a single mutation of hypusine to arginine completely abolished the eIF5A-ribosome interaction. Sucrose gradient sedimentation analysis of log versus stationary phase cells and eIF3 mutant strain showed that the endogenous eIF5A co-sedimented with the actively translating 80S ribosomes and polyribosomes in an RNase- and EDTA-sensitive manner. Our study demonstrates for the first time that eIF5A interacts in a hypusine-dependent manner with a molecular complex rather than a single protein, suggesting that the essential function of eIF5A is mostly likely mediated through its interaction with the actively translating ribosomes.

Aptamers that preferentially bind type IVB pili and inhibit human monocytic-cell invasion by Salmonella enterica serovar typhi

Salmonella enterica serovar Typhi is an important pathogen exclusively for humans and causes typhoid or enteric fever. It has been shown that type IVB pili, encoded by the S. enterica serovar Typhi pil operon located in Salmonella pathogenicity island 7, are important in the pathogenic process. In this study, by using both an adhesion-invasion assay and fluorescence quantitative PCR analysis, we demonstrated that the entry of type IVB piliated S. enterica serovar Typhi A21-6 (pil(+) Km(r)) into human THP-1 monocytic cells was greater than that of a nonpiliated S. enterica serovar Typhi pilS::Km(r) (pil mutant) strain. We have applied a systematic evolution of ligands by exponential enrichment approach to select oligonucleotides (aptamers) as ligands that specifically bind to type IVB pili. Using this approach, we identified a high-affinity single-stranded RNA aptamer (S-PS(8.4)) as a type IVB pilus-specific ligand and further found that the selected aptamer (S-PS(8.4)) could significantly inhibit the entry of the piliated strain (but not that of the nonpiliated strain) into human THP-1 cells. The binding affinities between aptamers and pre-PilS (structural protein of type IVB pili) were determined by nitrocellulose filter-binding assays, and the K(d) value was determined to be 8.56 nM for the S-PS(8.4) aptamer alone. As an example of an aptamer against type IVB pili of S. enterica serovar Typhi, the aptamer S-PS(8.4) can serve as a tool for analysis of bacterial type IVB pilus-host cell interactions and may yield information for the development of putative new drugs against S. enterica serovar Typhi bacterial infections, useful both in prevention of infection and in therapeutic treatment.

Destination-selective long-distance movement of phloem proteins

The phloem macromolecular transport system plays a pivotal role in plant growth and development. However, little information is available regarding whether the long-distance trafficking of macromolecules is a controlled process or passive movement. Here, we demonstrate the destination-selective long-distance trafficking of phloem proteins. Direct introduction, into rice (Oryza sativa), of phloem proteins from pumpkin (Cucurbita maxima) was used to screen for the capacity of specific proteins to move long distance in rice sieve tubes. In our system, shoot-ward translocation appeared to be passively carried by bulk flow. By contrast, root-ward movement of the phloem RNA binding proteins 16-kD C. maxima phloem protein 1 (CmPP16-1) and CmPP16-2 was selectively controlled. When CmPP16 proteins were purified, the root-ward movement of CmPP16-1 became inefficient, suggesting the presence of pumpkin phloem factors that are responsible for determining protein destination. Gel-filtration chromatography and immunoprecipitation showed that CmPP16-1 formed a complex with other phloem sap proteins. These interacting proteins positively regulated the root-ward movement of CmPP16-1. The same proteins interacted with CmPP16-2 as well and did not positively regulate its root-ward movement. Our data demonstrate that, in addition to passive bulk flow transport, a destination-selective process is involved in long-distance movement control, and the selective movement is regulated by protein-protein interaction in the phloem sap.