Human factor H-related protein 5 (FHR-5). A new complement-associated protein

A novel human plasma protein has been identified as a universal component of complement deposits, when complement is detected immunohistochemically in vivo. The protein is homologous to complement factor H and related proteins and has been designated factor H-related protein 5 (FHR-5). FHR-5 was identified by a monoclonal antibody raised using pathologic human glomerular preparations as the immunogen. FHR-5 was purified by affinity chromatography from complement-lysed erythrocytes, and the peptide sequence was obtained. The cDNA was cloned from a human liver library, and FHR-5 was deduced to be a protein containing 551 amino acids organized into nine short consensus repeat motifs. The short consensus repeats of FHR-5 show homology to Factor H and to other Factor H-related proteins, with some unique features demonstrated. Recombinant FHR-5, expressed in insect cells, was shown to bind C3b in vitro. The strong association of FHR-5 with tissue complement deposits in vivo suggests that this additional member of the Factor H family of proteins has a function in complement regulation.     

Crystal structure of hyperthermophilic archaeal initiation factor 5A: a homologue of eukaryotic initiation factor 5A (eIF-5A)

Eukaryotic initiation factor 5A (eIF-5A) is ubiquitous in eukaryotes and archaebacteria and is essential for cell proliferation and survival. The crystal structure of the eIF-5A homologue (PhoIF-5A) from a hyperthermophilic archaebacterium Pyrococcus horikoshii OT3 was determined at 2.0 A resolution by the molecular replacement method. PhoIF-5A is predominantly composed of beta-strands comprising two distinct folding domains, an N-domain (residues 1-69) and a C-domain (residues 72-138), connected by a short linker peptide (residues 70-71). The N-domain has an SH3-like barrel, while the C-domain folds in an (oligonucleotide/oligosaccharide binding) OB fold. Comparison of the structure of PhoIF-5A with those of archaeal homologues from Methanococcus jannaschii and Pyrobaculum aerophilum showed that the N-domains could be superimposed with root mean square deviation (rmsd) values of 0.679 and 0.624 A, while the C-domains gave higher values of 1.824 and 1.329 A, respectively. Several lines of evidence suggest that eIF-5A functions as a biomodular protein capable of interacting with protein and nucleic acid. The surface representation of electrostatic potential shows that PhoIF-5A has a concave surface with positively charged residues between the N- and C-domains. In addition, a flexible long hairpin loop, L1 (residues 33-41), with a hypusine modification site is positively charged, protruding from the N-domain. In contrast, the opposite side of the concave surface at the C-domain is mostly negatively charged. These findings led to the speculation that the concave surface and loop L1 at the N-domain may be involved in RNA binding, while the opposite side of the concave surface in the C-domain may be involved in protein interaction.     

Hepatitis C virus nonstructural protein 5A (NS5A) is an RNA-binding protein

Hepatitis C virus (HCV) nonstructural protein 5A (NS5A) has been shown to antagonize numerous cellular pathways, including the antiviral interferon-alpha response. However, the capacity of this protein to interact with the viral polymerase suggests a more direct role for NS5A in genome replication. In this study, we employed two bacterially expressed, soluble derivatives of NS5A to probe for novel functions of this protein. We find that NS5A has the capacity to bind to the 3'-ends of HCV plus and minus strand RNAs. The high affinity binding site for NS5A in the 3'-end of plus strand RNA maps to the polypyrimidine tract, an element known to be essential for genome replication and infectivity. NS5A has a preference for single-stranded RNA containing stretches of uridine or guanosine. Values for the equilibrium dissociation constants for high affinity binding sites were in the 10 nM range. Two-dimensional gel electrophoresis followed by Western blotting revealed the presence of unphosphorylated NS5A in Huh-7 cells stably expressing the subgenomic replicon. Moreover, RNA immunoprecipitation and NS5A pull-down experiments showed the capacity of replicon-derived NS5A to bind to synthetic RNA and the HCV genome, respectively. Deletion of all of the casein kinase II phosphorylation sites in NS5A supported stable replication of a subgenomic replicon in Huh-7. However, this derivative could not be labeled with inorganic phosphate, suggesting that extensive phosphorylation of NS5A is not required for the replication functions of NS5A. The discovery that NS5A is an RNA-binding protein defines a new functional target for development of agents to treat HCV infection and a new structural class of RNA-binding proteins.     

Homology modelling of the human eukaryotic initiation factor 5A (eIF-5A).

Homology modelling of the human eIF-5A protein has been performed by using a multiple predictions strategy. As the sequence identity between the target and the template proteins is nearly 30%, which is lower than the commonly used threshold to apply with confidence the homology modelling method, we developed a specific predictive scheme by combining different sequence analyses and predictions, as well as model validation by comparison to structural experimental information. The target sequence has been used to find homologues within sequence databases and a multiple alignment has been created. Secondary structure for each single protein has been predicted and compared on the basis of the multiple sequence alignment, in order to evaluate and adjust carefully any gap. Therefore, comparative modelling has been applied to create the model of the protein on the basis of the optimized sequence alignment. The quality of the model has been checked by computational methods and the structural features have been compared to experimental information, giving us a good validation of the reliability of the model and its correspondence to the protein structure in solution. Last, the model was deposited in the Protein Data Bank to be accessible for studies on the structure-function relationships of the human eIF-5A.     

The hypusine-containing translation factor eIF5A

Abstract

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.     

Domain 2 of nonstructural protein 5A (NS5A) of hepatitis C virus is natively unfolded

Nonstructural protein 5A protein (NS5A) of hepatitis C virus (HCV) plays an important role in the regulation of viral replication, interferon resistance, and apoptosis. HCV NS5A comprises three domains. Recently the structure of domain 1 has been determined, revealing a structural scaffold with a novel zinc-binding motif and a disulfide bond. At present, the structures of domains 2 and 3 remain undefined. Domain 2 of HCV NS5A (NS5A-D2) is important for functions of NS5A and involved in molecular interactions with its own NS5B and PKR, a cellular interferon-inducible serine/threonine specific protein kinase. In this study we performed structural analysis of domain 2 by multinuclear nuclear magnetic resonance (NMR) spectroscopy. The analysis of the backbone 1H, 13C, and 15N resonances, 3JHNalpha coupling constants ,and 3D NOE data indicates that NS5A-D2 lacks secondary structural elements and reveals characteristics of unfolded proteins. NMR relaxation parameters confirmed the lack of rigid structure in the domain. The absence of an ordered conformation and the observation of a highly dynamic behavior of NS5A-D2 may provide an underlying molecular basis on its physiological function to allow NS5A-D2 to interact with a variety of biological partners.     

eIF5A interacts functionally with eEF2

eIF5A is highly conserved from archaea to mammals, essential for cell viability and the only protein known to contain the essential amino acid residue hypusine, generated by a unique posttranslational modification. eIF5A was originally identified as a translation initiation factor due to its ability to stimulate the formation of the first peptide bond. However, recent studies have shown that depletion of eIF5A causes a significant decrease in polysome run-off and an increase in the ribosome transit time, suggesting that eIF5A is actually involved in the elongation step of protein synthesis. We have previously shown that the depletion mutant tif51A-3 (eIF5A(C39Y/G118D)) shows a sicker phenotype when combined with the dominant negative mutant eft2 ( H699K ) of the elongation factor eEF2. In this study, we used the eIF5A(K56A) mutant to further investigate the relationship between eIF5A and eEF2. The eIF5A(K56A) mutant is temperature sensitive and has a defect in protein synthesis, but instead of causing depletion of the eIF5A protein, this mutant has a defect in hypusine modification. Like the mutant tif51A-3, the eIF5A(K56A) mutant is synthetic sick with the mutant eft2 ( H699K ) of eEF2. High-copy eEF2 not only improves cell growth of the eIF5A(K56A) mutant, but also corrects its increased cell size defect. Moreover, eEF2 suppression of the eIF5A(K56A) mutant is correlated with the improvement of total protein synthesis and with the increased resistance to the protein synthesis inhibitor hygromycin B. Finally, the polysome profile defect of the eIF5A(K56A) mutant is largely corrected by high-copy eEF2. Therefore, these results demonstrate that eIF5A is closely related to eEF2 function during translation elongation.     

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.     

Synthesis and overproduction of the 5A protein of insertion sequence IS5.

We have demonstrated both the synthesis and overproduction of the 5A protein encoded by the longest open reading frame of the bacterial insertion sequence IS5. Expression was obtained in vitro and in Escherichia coli maxicells from plasmids containing IS5 in either orientation, as well as in vitro from a restriction fragment containing exclusively IS5 DNA. When IS5 was cloned in the appropriate orientation downstream of a strong tac promoter, production of the 5A protein was increased to 10 to 20% of the total protein synthesized in vitro.     

The post-translational synthesis of a polyamine-derived amino acid, hypusine, in the eukaryotic translation initiation factor 5A (eIF5A)

The eukaryotic translation initiation factor 5A (eIF5A) is the only cellular protein that contains the unique polyamine-derived amino acid, hypusine [Nepsilon-(4-amino-2-hydroxybutyl)lysine]. Hypusine is formed in eIF5A by a novel post-translational modification reaction that involves two enzymatic steps. In the first step, deoxyhypusine synthase catalyzes the cleavage of the polyamine spermidine and transfer of its 4-aminobutyl moiety to the epsilon-amino group of one specific lysine residue of the eIF5A precursor to form a deoxyhypusine intermediate. In the second step, deoxyhypusine hydroxylase converts the deoxyhypusine-containing intermediate to the hypusine-containing mature eIF5A. The structure and mechanism of deoxyhypusine synthase have been extensively characterized. Deoxyhypusine hydroxylase is a HEAT-repeat protein with a symmetrical superhelical structure consisting of 8 helical hairpins (HEAT motifs). It is a novel metalloenzyme containing tightly bound iron at the active sites. Four strictly conserved His-Glu pairs were identified as iron coordination sites. The structural fold of deoxyhypusine hydroxylase is entirely different from those of the other known protein hydroxylases such as prolyl 4-hydroxylase and lysyl hydroxylases. The eIF5A protein and deoxyhypusine/hypusine modification are essential for eukaryotic cell proliferation. Thus, hypusine synthesis represents the most specific protein modification known to date, and presents a novel target for intervention in mammalian cell proliferation.     

A 5S rRNA/L5 complex is a precursor to ribosome assembly in mammalian cells

A novel 5S RNA-protein (RNP) complex in human and mouse cells has been analyzed using patient autoantibodies. The RNP is small (approximately 7S) and contains most of the nonribosome-associated 5S RNA molecules in HeLa cells. The 5S RNA in the particle is matured at its 3' end, consistent with the results of in vivo pulse-chase experiments which indicate that this RNP represents a later step in 5S biogenesis than a previously described 5S*/La protein complex. The protein moiety of the 5S RNP has been identified as ribosomal protein L5, which is known to be released from ribosomes in a complex with 5S after various treatments of the 60S subunit. Indirect immunofluorescence indicates that the L5/5S complex is concentrated in the nucleolus. L5 may therefore play a role in delivering 5S rRNA to the nucleolus for assembly into ribosomes.     

A dinoflagellate CDK5-like cyclin-dependent kinase

Background information. Mitosis during the dinoflagellate cell cycle is unusual in that the nuclear envelope remains intact and segregation of the permanently condensed chromosomes uses a cytoplasmic mitotic spindle. To examine regulation of the dinoflagellate cell cycle in the context of these unusual nuclear features, it is necessary to isolate and characterize cell cycle regulators such as CDK (cyclin‐dependent kinase). Results. We report the characterization of a CDK from the dinoflagellate Lingulodinium polyedrum. This CDK reacts with an anti‐PSTAIRE antibody and was identified by protein microsequencing after partial purification. The protein microsequence shows homology toward the Pho85/CDK5 clade of CDKs. Neither the amount nor the phosphorylation state changed over the course of the cell cycle, in agreement with results reported for CDK5 family members in other systems. Conclusions. We conclude we have probably isolated a dinoflagellate CDK5‐like protein. The data reported here support the identification of this protein as a CDK5 homologue, and suggest that dinoflagellates may contain several CDK families.     

Acetylation regulates subcellular localization of eukaryotic translation initiation factor 5A (eIF5A)

Eukaryotic translation initiation factor 5A (eIF5A) is a protein subject to hypusination, which is essential for its function. eIF5A is also acetylated, but the role of that modification is unknown. Here, we report that acetylation regulates the subcellular localization of eIF5A. We identified PCAF as the major cellular acetyltransferase of eIF5A, and HDAC6 and SIRT2 as its major deacetylases. Inhibition of the deacetylases or impaired hypusination increased acetylation of eIF5A, leading to nuclear accumulation. As eIF5A is constitutively hypusinated under physiological conditions, we suggest that reversible acetylation plays a major role in controlling the subcellular localization of eIF5A.     

Development of the system ensuring a high-level expression of hepatitis C virus nonstructural NS5B and NS5A proteins

The plasmid pET-21d-2c-5BDelta55 effectively expressing a C-terminally truncated form (NS5BDelta55) of the hepatitis C virus (HCV) RNA-dependent RNA polymerase (RdRp) was constructed. It was derived from pET-21d-5BDelta55 plasmid and contained six mutations in the ATG-start codon region and an additional cistron upstream the target gene. The C-terminally His-tagged NS5BDelta55 protein was expressed in Rosetta(DE3) Escherichia coli strain bearing an additional pRARE plasmid encoding extra copies of rare tRNAs. The yield of the target enzyme exceeded by a factor of 29 the yield of NS5BDelta55 protein expressed from the parental pET-21d-5BDelta55 plasmid (5 mg/L). The increase in the protein yield could be explained by facilitated protein translation initiation, resulted from disruption of the stable secondary mRNA structure. The pET-21d-2c-5BDelta55 plasmid yielded one third amount of the protein when expressed in BL-21(DE3) strain, indicating that the pRARE plasmid is required for a high-level expression of NS5BDelta55 protein. The 29-fold enhancement of the protein yield was accompanied by only a 2.5-fold increase of the corresponding mRNA level. The expression of another HCV NS5A protein His-tagged at the C-terminus in the developed system yielded a similar amount of the protein (4 mg/L), whereas its N-terminally His-tagged counterpart was obtained in a 30 mg/L yield. The NS5A protein purified under denaturing conditions and renatured in solution inhibited the HCV RdRp and was a substrate for human casein kinase II.     

Cordycepin analogs of ppp5'A2'p5'A2'p5'A (2-5A) inhibit protein synthesis through activation of the 2-5A-dependent endonuclease

Analogs of the triphosphate 2'-5'-linked adenylate trimer (ppp5'A2'p5'A2'p5'A, called 2-5A) which contain 3'-deoxyadenosine (cordycepin) instead of adenosine either in positions one and two, or in all three positions, are 10-100-fold less potent than is parent 2-5A in inhibition of protein synthesis in intact cells, when utilizing calcium co-precipitation techniques to introduce the 5'-triphosphate oligonucleotides into the cells. That the inhibition of protein synthesis was a consequence of activation of the 2-5A-dependent endonuclease by the 3'-deoxyadenosine analogs of 2-5A was demonstrated in obtaining the ribosomal RNA cleavage pattern that is characteristic of endonuclease activation by parent 2-5A. Additional results (i.e. lack of activity by the dimer species ppp5'(3'dA)2'p5'-(3'dA) or the monomer 3'dA) as well as kinetic analysis both in intact cells and in cell-free extracts provided further evidence that the inhibition of protein synthesis observed with these 3'-deoxyadenosine 2-5A analogs was not due to their degradation to the antimetabolite monomer unit 3'-deoxyadenosine.     

Identification and functional significance of N-glycosylation of the 5-ht5A receptor

The presence and roles of N-glycosylation of the human (h) 5-ht(5A) receptor were investigated using a heterologous expression system. Following transient transfection of COS-7 cells with h5-ht(5A) receptor cDNA, SDS-PAGE/Western blot analysis of immunoreactivity demonstrated two protein species; a predominant species with a molecular weight of approximately 35-45 kDa and a minor species of approximately 45-55 kDa. Transfected cells grown in the presence of the N-glycosylation inhibitor tunicamycin, failed to express the minor immunoreactive species indicating this represented the N-glycosylated form of the h5-ht(5A) receptor. Comparison of the molecular weights of immunoreactive bands arising from the wild-type and each of the mutant 5-ht(5A) receptors with disruption of the predicted N-glycosylation sites (N6S and N21S) demonstrated that both identified asparagines were N-glycosylated. Immunocytochemical and ELISA studies demonstrated that the [N6S]h5-ht(5A) receptor mutation, but not the [N21S]h5-ht(5A) receptor mutation, reduced protein expression in the cell membrane, indicating that N-glycosylation of the N6 residue is important for the membrane expression of this neurotransmitter receptor; a requirement for receptor function.     

CXXC5多克隆抗体的制备及表达研究

CXXC5基因是从人类胚胎心脏的cDNA文库中克隆出来的一个人类锌指基因,包含zf-CXXC5结构域,该基因编码322个氨基酸,在物种进化上高度保守.为了进一步研究该基因的功能,需要获得CXXC5蛋白并制备其抗体.通过PCR扩增方法扩增得到了CXXC5部分编码区序列,然后将其连接到PGEX4T-1上,转化到大肠杆菌BL...     

Mapping eIF5A binding sites for Dys1 and Lia1: in vivo evidence for regulation of eIF5A hypusination

The evolutionarily conserved factor eIF5A is the only protein known to undergo hypusination, a unique posttranslational modification triggered by deoxyhypusine synthase (Dys1). Although eIF5A is essential for cell viability, the function of this putative translation initiation factor is still obscure. To identify eIF5A-binding proteins that could clarify its function, we screened a two-hybrid library and identified two eIF-5A partners in S. cerevisiae: Dys1 and the protein encoded by the gene YJR070C, named Lia1 (Ligand of eIF5A). The interactions were confirmed by GST pulldown. Mapping binding sites for these proteins revealed that both eIF5A domains can bind to Dys1, whereas the C-terminal domain is sufficient to bind Lia1. We demonstrate for the first time in vivo that the N-terminal alpha-helix of Dys1 can modulate enzyme activity by inhibiting eIF5A interaction. We suggest that this inhibition be abrogated in the cell when hypusinated and functional eIF5A is required.     

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.