Kinetic studies on the role of elongation factors 1 beta and 1 gamma in protein synthesis

Quaternary organization of the human eEF1B complex reveals unique multi-GEF domain assembly

Protein synthesis in eukaryotic cell is spatially and structurally compartmentalized that ensures high efficiency of this process. One of the distinctive features of higher eukaryotes is the existence of stable multi-protein complexes of aminoacyl-tRNA synthetases and translation elongation factors. Here, we report a quaternary organization of the human guanine-nucleotide exchange factor (GEF) complex, eEF1B, comprising α, β and γ subunits that specifically associate into a heterotrimeric form eEF1B(αβγ)3. As both the eEF1Bα and eEF1Bβ proteins have structurally conserved GEF domains, their total number within the complex is equal to six. Such, so far, unique structural assembly of the guanine-nucleotide exchange factors within a stable complex may be considered as a 'GEF hub' that ensures efficient maintenance of the translationally active GTP-bound conformation of eEF1A in higher eukaryotes.

Expression and regulation of 42Sp50 in spotted scat (Scatophagus argus)

42Sp50 is an isoform of the eukaryotic translation elongation factor 1 A (eEF1A) and is vital for fish ovarian development. Spotted scat (Scatophagus argus) is a popular marine cultured fish species in Southern Asia and China, and its artificial reproduction is complicated, with a relatively low success ratio in practice. In this study, the 42Sp50 gene was cloned from spotted scat. Tissue distribution analysis showed that 42Sp50 was mainly expressed in the ovary. qRT-PCR showed that 42Sp50 expression levels gradually decreased insignificantly in the ovaries from phase II to IV. Western blot analysis showed that 42Sp50 was highly expressed in the ovary, while it was almost undetectable in the testis. Immunohistochemistry analysis stained 42Sp50 mainly in the cytoplasm of the previtellogenic oocytes in ovaries of normal XX-female and sex-reversed XY-female. Aside from fish and amphibians, 42Sp50 was also identified in some reptile species using genomic database searching. Analyses of the transcriptome data from four different fish species (Hainan medaka (Oryzias curvinotus), silver sillago (Sillago sihama), Nile tilapia (Oreochromis niloticus), and Hong Kong catfish (Clarias fuscus)) revealed ovaries biased expression of 42Sp50 in all, similar to spotted scat. While the neighbor genes of 42Sp50 did not show ovary biased expression in the fish species analyzed. Bisulfite Sequencing PCR (BSP) results showed that the DNA methylation level of 42Sp50 promoter was low in ovaries, testes, and muscles. The luciferase reporter assay demonstrated that Dmrt4 activated 42Sp50 expression in the presence of Sf1 or Foxh1. These results suggest that 42Sp50 may be involved in regulating the early phase oocytes development of spotted scat.

Eukaryotic Translation Elongation Factor 1 Delta Inhibits the Nuclear Import of the Nucleoprotein and PA-PB1 Heterodimer of Influenza A Virus

The viral ribonucleoprotein (vRNP) of the influenza A virus (IAV) is responsible for the viral RNA transcription and replication in the nucleus, and its functions rely on host factors. Previous studies have indicated that eukaryotic translation elongation factor 1 delta (eEF1D) may associate with RNP subunits, but its roles in IAV replication are unclear. Herein, we showed that eEF1D was an inhibitor of IAV replication because knockout of eEF1D resulted in a significant increase in virus yield. eEF1D interacted with RNP subunits polymerase acidic protein (PA), polymerase basic 1 (PB1), polymerase basic 2 (PB2), and also with nucleoprotein (NP) in an RNA-dependent manner. Further studies revealed that eEF1D impeded the nuclear import of NP and PA-PB1 heterodimer of IAV, thereby suppressing the vRNP assembly, viral polymerase activity, and viral RNA synthesis. Together, our studies demonstrate eEF1D negatively regulating the IAV replication by inhibition of the nuclear import of RNP subunits, which not only uncovers a novel role of eEF1D in IAV replication but also provides new insights into the mechanisms of nuclear import of vRNP proteins.IMPORTANCE Influenza A virus is the major cause of influenza, a respiratory disease in humans and animals. Different from most other RNA viruses, the transcription and replication of IAV occur in the cell nucleus. Therefore, the vRNPs must be imported into the nucleus for viral transcription and replication, which requires participation of host proteins. However, the mechanisms of the IAV-host interactions involved in nuclear import remain poorly understood. Here, we identified eEF1D as a novel inhibitor for the influenza virus life cycle. Importantly, eEF1D impaired the interaction between NP and importin α5 and the interaction between PB1 and RanBP5, which impeded the nuclear import of vRNP. Our studies not only reveal the molecular mechanisms of the nuclear import of IAV vRNP but also provide potential anti-influenza targets for antiviral development.

Cellular alteration and differential protein profile explain effects of GA3 and ABA and their inhibitor on Trichocline catharinensis (Asteraceae) seed germination

Seed physiology of wild species has not been studied as deeply as that of domesticated crop species. Trichocline catharinensis (Asteraceae) is an endemic wildflower species from the high-altitude fields of southern Brazil. This species is of interest as a source of genes to improve cultivated Asteraceae because of its ornamental features, disease resistance and ability to tolerate drought and poor soil conditions. We studied the effects of abscisic acid (ABA) and gibberellic acid (GA₃ ) and their inhibitors, fluridone (FLU) and paclobutrazol (PAC), on seed germination. We individually assessed ultrastructural changes and differential protein accumulation. The principal component analysis explained 69.66% of differential accumulation for 32 proteins at phase II of seed germination in response to hormone and inhibitor treatment. GA₃ -imbibed seed germination (98.75%) resulted in increased protein accumulation to meet energy demand, redox regulation, and reserve metabolism activation. FLU-imbibed seeds showed a higher germination speed index as a consequence of metabolism activation. ABA-imbibed seeds (58.75%) showed osmotolerance and flattened cells in the hypocotyl-radicular axis, suggesting that ABA inhibits cell expansion. PAC-imbibed seeds remained at phase II for 300 h, and germination was suppressed (7.5%) because of the increased signaling proteins and halted reserve mobilization. Therefore, our findings provide insight into the behavior of Asteraceae non-dormant seed germination, which broadens our knowledge of seed germination in a wild and endemic plant species from a threatened ecosystem.

The role of translation elongation factor eEF1 subunits in neurodevelopmental disorders

The multi-subunit eEF1 complex plays a crucial role in de novo protein synthesis. The central functional component of the complex is eEF1A, which occurs as two independently encoded variants with reciprocal expression patterns: whilst eEF1A1 is widely expressed, eEF1A2 is found only in neurons and muscle. Heterozygous mutations in the gene encoding eEF1A2, EEF1A2, have recently been shown to cause epilepsy, autism, and intellectual disability. The remaining subunits of the eEF1 complex, eEF1Bα, eEF1Bδ, eEF1Bγ, and valyl-tRNA synthetase (VARS), together form the GTP exchange factor for eEF1A and are ubiquitously expressed, in keeping with their housekeeping role. However, mutations in the genes encoding these subunits EEF1B2 (eEF1Bα), EEF1D (eEF1Bδ), and VARS (valyl-tRNA synthetase) have also now been identified as causes of neurodevelopmental disorders. In this review, we describe the mutations identified so far in comparison with the degree of normal variation in each gene, and the predicted consequences of the mutations on the functions of the proteins and their isoforms. We discuss the likely effects of the mutations in the context of the role of protein synthesis in neuronal development.

Integrated physiology and proteome analysis of embryo and endosperm highlights complex metabolic networks involved in seed germination in wheat (Triticum aestivum L.)

The aim of this study was to investigate the physiological and proteomic changes in the embryo and endosperm during seed germination in the elite Chinese bread wheat cultivar Zhengmai 366. Upon imbibition, seed size and water content increased rapidly, followed by a series of metabolic changes including increases in soluble sugar content and α-amylase activity, a decrease in starch content, and a rapid increase in plant hormones. In total, 57 and 45 differentially accumulated proteins (DAPs) from the embryo and endosperm, respectively, were identified at five germination stages (0, 6, 12, 18, and 24 h). Principal component analysis revealed a significant proteome difference between embryo and endosperm as well as the different germination stages. The largest proteome changes occurred 24 h after seed imbibition. Embryo DAP spots were mainly involved in energy metabolism, amino acid metabolism, stress/defense, and protein metabolism; those from the endosperm were primarily related to storage protein and carbohydrate metabolism. Protein-protein interaction analysis revealed a complicated interaction network between energy-related proteins and other proteins. Metabolic pathway analysis highlighted complex regulatory networks in the embryo and endosperm that regulate wheat seed germination. These results provide new insights into the molecular mechanisms of seed germination.

Strain-Specific Contribution of Eukaryotic Elongation Factor 1 Gamma to the Translation of Influenza A Virus Proteins

Influenza A virus exploits multiple host proteins during infection. To define the virus–host interactome, our group conducted a proteomics-based screen and identified 299 genes that contributed to virus replication and 24 genes that were antiviral. Of these genes, we focused on the role during virus replication of eukaryotic elongation factor 1 gamma (eEF1G), which is a subunit of the eukaryotic elongation factor-1 complex and known to be a pro-viral host protein. Using the CRISPR/Cas9 system, we obtained two clones that were defective in eEF1G expression. In both of these clones, A/WSN/33 (H1N1) virus growth and protein expression were significantly suppressed, but viral mRNA, vRNA, and cRNA expression were not reduced. However, the replication and protein expression of A/California/04/2009 (H1N1pdm) virus in both clones were similar to those in parental cells. We found that the PB2 and PA proteins of WSN virus were responsible for the eEF1G-dependent replication. Our data show that eEF1G plays a role in the translation of virus proteins in a strain-specific manner. Additional analyses may be needed to further understand the role of strain-specific host proteins during virus replication.

Evolution of the archaeal and mammalian information processing systems: towards an archaeal model for human disease

Current evolutionary models suggest that Eukaryotes originated from within Archaea instead of being a sister lineage. To test this model of ancient evolution, we review recent studies and compare the three major information processing subsystems of replication, transcription and translation in the Archaea and Eukaryotes. Our hypothesis is that if the Eukaryotes arose within the archaeal radiation, their information processing systems will appear to be one of kind and not wholly original. Within the Eukaryotes, the mammalian or human systems are emphasized because of their importance in understanding health. Biochemical as well as genetic studies provide strong evidence for the functional similarity of archaeal homologs to the mammalian information processing system and their dissimilarity to the bacterial systems. In many independent instances, a simple archaeal system is functionally equivalent to more elaborate eukaryotic homologs, suggesting that evolution of complexity is likely an central feature of the eukaryotic information processing system. Because fewer components are often involved, biochemical characterizations of the archaeal systems are often easier to interpret. Similarly, the archaeal cell provides a genetically and metabolically simpler background, enabling convenient studies on the complex information processing system. Therefore, Archaea could serve as a parsimonious and tractable host for studying human diseases that arise in the information processing systems.

A non-catalytic N-terminal domain negatively influences the nucleotide exchange activity of translation elongation factor 1Bα

Eukaryotic translation elongation factor 1Bα (eEF1Bα) is a functional homolog of the bacterial factor EF-Ts, and is a component of the macromolecular eEF1B complex. eEF1Bα functions as a catalyst of guanine nucleotide exchange on translation elongation factor 1A (eEF1A). The C-terminal domain of eEF1Bα is necessary and sufficient for its catalytic activity, whereas the N-terminal domain interacts with eukaryotic translation elongation factor 1Bγ (eEF1Bγ) to form a tight complex. However, eEF1Bγ has been shown to enhance the catalytic activity of eEF1Bα attributed to the C-terminal domain of eEF1Bα. This suggests that the N-terminal domain of eEF1Bα may in some way influence the guanine nucleotide exchange process. We have shown that full-length recombinant eEF1Bα and its truncated forms are non-globular proteins with elongated shapes. Truncation of the N-terminal domain of eEF1Bα, which is dispensable for catalytic activity, resulted in acceleration of the rate of guanine nucleotide exchange on eEF1A compared to full-length eEF1Bα. A similar effect on the catalytic activity of eEF1Bα was observed after its interaction with eEF1Bγ. We suggest that the non-catalytic N-terminal domain of eEF1Bα may interfere with eEF1A binding to the C-terminal catalytic domain, resulting in a decrease in the overall rate of the guanine nucleotide exchange reaction. Formation of a tight complex between the eEF1Bγ and eEF1Bα N-terminal domains abolishes this inhibitory effect.

Mammalian translation elongation factor eEF1A2: X-ray structure and new features of GDP/GTP exchange mechanism in higher eukaryotes

Eukaryotic elongation factor eEF1A transits between the GTP- and GDP-bound conformations during the ribosomal polypeptide chain elongation. eEF1A*GTP establishes a complex with the aminoacyl-tRNA in the A site of the 80S ribosome. Correct codon–anticodon recognition triggers GTP hydrolysis, with subsequent dissociation of eEF1A*GDP from the ribosome. The structures of both the ‘GTP’- and ‘GDP’-bound conformations of eEF1A are unknown. Thus, the eEF1A-related ribosomal mechanisms were anticipated only by analogy with the bacterial homolog EF-Tu. Here, we report the first crystal structure of the mammalian eEF1A2*GDP complex which indicates major differences in the organization of the nucleotide-binding domain and intramolecular movements of eEF1A compared to EF-Tu. Our results explain the nucleotide exchange mechanism in the mammalian eEF1A and suggest that the first step of eEF1A*GDP dissociation from the 80S ribosome is the rotation of the nucleotide-binding domain observed after GTP hydrolysis.

The many roles of the eukaryotic elongation factor 1 complex

The vast majority of proteins are believed to have one specific function. Throughout the course of evolution, however, some proteins have acquired additional functions to meet the demands of a complex cellular milieu. In some cases, changes in RNA or protein processing allow the cell to make the most of what is already encoded in the genome to produce slightly different forms. The eukaryotic elongation factor 1 (eEF1) complex subunits, however, have acquired such moonlighting functions without alternative forms. In this article, we discuss the canonical functions of the components of the eEF1 complex in translation elongation as well as the secondary interactions they have with other cellular factors outside of the translational apparatus. The eEF1 complex itself changes in composition as the complexity of eukaryotic organisms increases. Members of the complex are also subject to phosphorylation, a potential modulator of both canonical and non-canonical functions. Although alternative functions of the eEF1A subunit have been widely reported, recent studies are shedding light on additional functions of the eEF1B subunits. A thorough understanding of these alternate functions of eEF1 is essential for appreciating their biological relevance.

Global and local depletion of ternary complex limits translational elongation

The translation of genetic information according to the sequence of the mRNA template occurs with high accuracy and fidelity. Critical events in each single step of translation are selection of transfer RNA (tRNA), codon reading and tRNA-regeneration for a new cycle. We developed a model that accurately describes the dynamics of single elongation steps, thus providing a systematic insight into the sensitivity of the mRNA translation rate to dynamic environmental conditions. Alterations in the concentration of the aminoacylated tRNA can transiently stall the ribosomes during translation which results, as suggested by the model, in two outcomes: either stress-induced change in the tRNA availability triggers the premature termination of the translation and ribosomal dissociation, or extensive demand for one tRNA species results in a competition between frameshift to an aberrant open-reading frame and ribosomal drop-off. Using the bacterial Escherichia coli system, we experimentally draw parallels between these two possible mechanisms.

Mapping organelle proteins and protein complexes in Drosophila melanogaster

Many proteins within eukaryotic cells are organized spatially and functionally into membrane bound organelles and complexes. A protein's location thus provides information about its function. Here, we apply LOPIT, a mass-spectrometry based technique that simultaneously maps proteins to specific subcellular compartments, to Drosophila embryos. We determine the subcellular distribution of hundreds of proteins, and protein complexes. Our results reveal the potential of LOPIT to provide average snapshots of cells.

Elongation factor eEF1B modulates functions of the release factors eRF1 and eRF3 and the efficiency of translation termination in yeast

BACKGROUND

Termination of translation in eukaryotes is controlled by two interacting polypeptide chain release factors, eRF1 and eRF3. While eRF1 recognizes nonsense codons, eRF3 facilitates polypeptide chain release from the ribosome in a GTP-dependent manner. Besides termination, both release factors have essential, but poorly characterized functions outside of translation.

RESULTS

To characterize further the functions of yeast eRF1 and eRF3, a genetic screen for their novel partner proteins was performed. As a result, the genes for gamma (TEF4 and TEF3/CAM1) and alpha (TEF5/EFB1) subunits of the translation elongation factor eEF1B, known to catalyze the exchange of bound GDP for GTP on eEF1A, were revealed. These genes act as dosage suppressors of a synthetic growth defect caused by some mutations in the SUP45 and SUP35 genes encoding eRF1 and eRF3, respectively. Extra copies of TEF5 and TEF3 can also suppress the temperature sensitivity of some sup45 and sup35 mutants and reduce nonsense codon readthrough caused by these omnipotent suppressors. Besides, overproduction of eEF1Balpha reduces nonsense codon readthrough in the strain carrying suppressor tRNA. Such effects were not shown for extra copies of TEF2, which encodes eEF1A, thus indicating that they were not due to eEF1A activation.

CONCLUSION

The data obtained demonstrate involvement of the translation elongation factor eEF1B in modulating the functions of translation termination factors and suggest its possible role in GDP for GTP exchange on eRF3.

Generic algorithm to predict the speed of translational elongation: implications for protein biogenesis

Synonymous codon usage and variations in the level of isoaccepting tRNAs exert a powerful selective force on translation fidelity. We have developed an algorithm to evaluate the relative rate of translation which allows large-scale comparisons of the non-uniform translation rate on the protein biogenesis. Using the complete genomes of Escherichia coli and Bacillus subtilis we show that stretches of codons pairing to minor tRNAs form putative sites to locally attenuate translation; thereby the tendency is to cluster in near proximity whereas long contiguous stretches of slow-translating triplets are avoided. The presence of slow-translating segments positively correlates with the protein length irrespective of the protein abundance. The slow-translating clusters are predominantly located down-stream of the domain boundaries presumably to fine-tune translational accuracy with the folding fidelity of multidomain proteins. Translation attenuation patterns at highly structurally and functionally conserved domains are preserved across the species suggesting a concerted selective pressure on the codon selection and species-specific tRNA abundance in these regions.

Coordination of eukaryotic translation elongation factor 1A (eEF1A) function in actin organization and translation elongation by the guanine nucleotide exchange factor eEF1Balpha

Eukaryotic translation elongation factor 1A (eEF1A) both shuttles aminoacyl-tRNA (aa-tRNA) to the ribosome and binds and bundles actin. A single domain of eEF1A is proposed to bind actin, aa-tRNA and the guanine nucleotide exchange factor eEF1Balpha. We show that eEF1Balpha has the ability to disrupt eEF1A-induced actin organization. Mutational analysis of eEF1Balpha F163, which binds in this domain, demonstrates effects on growth, eEF1A binding, nucleotide exchange activity, and cell morphology. These phenotypes can be partially restored by an intragenic W130A mutation. Furthermore, the combination of F163A with the lethal K205A mutation restores viability by drastically reducing eEF1Balpha affinity for eEF1A. This also results in a consistent increase in actin bundling and partially corrected morphology. The consequences of the overlapping functions in this eEF1A domain and its unique differences from the bacterial homologs provide a novel function for eEF1Balpha to balance the dual roles in actin bundling and protein synthesis.

Robust simplifications of multiscale biochemical networks

Background

Cellular processes such as metabolism, decision making in development and differentiation, signalling, etc., can be modeled as large networks of biochemical reactions. In order to understand the functioning of these systems, there is a strong need for general model reduction techniques allowing to simplify models without loosing their main properties. In systems biology we also need to compare models or to couple them as parts of larger models. In these situations reduction to a common level of complexity is needed.

Results

We propose a systematic treatment of model reduction of multiscale biochemical networks. First, we consider linear kinetic models, which appear as "pseudo-monomolecular" subsystems of multiscale nonlinear reaction networks. For such linear models, we propose a reduction algorithm which is based on a generalized theory of the limiting step that we have developed in [1]. Second, for non-linear systems we develop an algorithm based on dominant solutions of quasi-stationarity equations. For oscillating systems, quasi-stationarity and averaging are combined to eliminate time scales much faster and much slower than the period of the oscillations. In all cases, we obtain robust simplifications and also identify the critical parameters of the model. The methods are demonstrated for simple examples and for a more complex model of NF-κB pathway.

Conclusion

Our approach allows critical parameter identification and produces hierarchies of models. Hierarchical modeling is important in "middle-out" approaches when there is need to zoom in and out several levels of complexity. Critical parameter identification is an important issue in systems biology with potential applications to biological control and therapeutics. Our approach also deals naturally with the presence of multiple time scales, which is a general property of systems biology models.

Cofactor dependent conformational switching of GTPases

This theoretical work covers structural and biochemical aspects of nucleotide binding and GDP/GTP exchange of GTP hydrolases belonging to the family of small GTPases. Current models of GDP/GTP exchange regulation are often based on two specific assumptions. The first is that the conformation of a GTPase is switched by the exchange of the bound nucleotide from GDP to GTP or vice versa. The second is that GDP/GTP exchange is regulated by a guanine nucleotide exchange factor, which stabilizes a GTPase conformation with low nucleotide affinity. Since, however, recent biochemical and structural data seem to contradict this view, we present a generalized scheme for GTPase action. This novel ansatz accounts for those important cases when conformational switching in addition to guanine nucleotide exchange requires the presence of cofactors, and gives a more nuanced picture of how the nucleotide exchange is regulated. The scheme is also used to discuss some problems of interpretation that may arise when guanine nucleotide exchange mechanisms are inferred from experiments with analogs of GTP, like GDPNP, GDPCP, and GDP gamma S.

Phosphorylation of human eukaryotic elongation factor 1Bgamma is regulated by paclitaxel

Paclitaxel (Ptx) is an antitumoural drug that inhibits microtubule dynamics, causes G2/M arrest and induces cell death. 2-D PAGE and MALDI-TOF-MS analysis of HeLa cells extracts revealed that Ptx up-regulates a form of the eukaryotic elongation factor 1Bgamma (eEF1Bgamma) and down-regulates another one. This event, linked to the lack of Ptx effect over eEF1Bgamma mRNA or protein levels suggested a PTM of this elongation factor. Further 2-D PAGE analysis followed by a phosphospecific staining with PRO-Q Diamond showed the staining of the Ptx up-regulated form only. Moreover, this Ptx up-regulated form of eEF1Bgamma disappears upon treatment with protein phosphatase. Thus, we demonstrate that human eEF1Bgamma phosphorylation is regulated by Ptx.

Kinetics of the interactions between yeast elongation factors 1A and 1Balpha, guanine nucleotides, and aminoacyl-tRNA

The interactions of elongation factor 1A (eEF1A) from Saccharomyces cerevisiae with elongation factor 1Balpha (eEF1Balpha), guanine nucleotides, and aminoacyl-tRNA were studied kinetically by fluorescence stopped-flow. eEF1A has similar affinities for GDP and GTP, 0.4 and 1.1 microm, respectively. Dissociation of nucleotides from eEF1A in the absence of the guanine nucleotide exchange factor is slow (about 0.1 s(-1)) and is accelerated by eEF1Balpha by 320-fold and 250-fold for GDP and GTP, respectively. The rate constant of eEF1Balpha binding to eEF1A (10(7)-10(8) M (-1) s(-1)) is independent of guanine nucleotides. At the concentrations of nucleotides and factors prevailing in the cell, the overall exchange rate is expected to be in the range of 6 s(-1), which is compatible with the rate of protein synthesis in the cell. eEF1A.GTP binds Phe-tRNA(Phe) with a K(d) of 3 nm, whereas eEF1A.GDP shows no significant binding, indicating that eEF1A has similar tRNA binding properties as its prokaryotic homolog, EF-Tu.

Structural insights into the interaction of the evolutionarily conserved ZPR1 domain tandem with eukaryotic EF1A, receptors, and SMN complexes

Eukaryotic genomes encode a zinc finger protein (ZPR1) with tandem ZPR1 domains. In response to growth stimuli, ZPR1 assembles into complexes with eukaryotic translation elongation factor 1A (eEF1A) and the survival motor neurons protein. To gain insight into the structural mechanisms underlying the essential function of ZPR1 in diverse organisms, we determined the crystal structure of a ZPR1 domain tandem and characterized the interaction with eEF1A. The ZPR1 domain consists of an elongation initiation factor 2-like zinc finger and a double-stranded beta helix with a helical hairpin insertion. ZPR1 binds preferentially to GDP-bound eEF1A but does not directly influence the kinetics of nucleotide exchange or GTP hydrolysis. However, ZPR1 efficiently displaces the exchange factor eEF1Balpha from preformed nucleotide-free complexes, suggesting that it may function as a negative regulator of eEF1A activation. Structure-based mutational and complementation analyses reveal a conserved binding epitope for eEF1A that is required for normal cell growth, proliferation, and cell cycle progression. Structural differences between the ZPR1 domains contribute to the observed functional divergence and provide evidence for distinct modalities of interaction with eEF1A and survival motor neuron complexes.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

Kinectin-dependent assembly of translation elongation factor-1 complex on endoplasmic reticulum regulates protein synthesis

Kinectin is an integral membrane protein with many isoforms primarily found on the endoplasmic reticulum. It has been found to bind kinesin, Rho GTPase, and translation elongation factor-1delta. None of the existing models for the quaternary organization of the elongation factor-1 complex in higher eukaryotes involves kinectin. We have investigated here the assembly of the elongation factor-1 complex onto endoplasmic reticulum via kinectin using in vitro and in vivo assays. We established that the entire elongation factor-1 complex can be anchored to endoplasmic reticulum via kinectin, and the interacting partners are as follows. Kinectin binds EF-1delta, which in turn binds EF-1gamma but not EF-1beta; EF-1gamma binds EF-1delta and EF-1beta but not kinectin. In vivo splice blocking of the kinectin exons 36 and 37 produced kinectin lacking the EF-1delta binding domain, which disrupted the membrane localization of EF-1delta, EF-1gamma, and EF-1beta on endoplasmic reticulum, similar to the disruptions seen with the overexpression of kinectin fragments containing the EF-1delta binding domain. The disruptions of the EF-1delta/kinectin interaction inhibited expression of membrane proteins but enhanced synthesis of cytosolic proteins in vivo. These findings suggest that anchoring the elongation factor-1 complex onto endoplasmic reticulum via EF-1delta/kinectin interaction is important for regulating protein synthesis in eukaryotic cells.

Mg2+ and a key lysine modulate exchange activity of eukaryotic translation elongation factor 1B alpha

To sustain efficient translation, eukaryotic elongation factor B alpha (eEF1B alpha) functions as the guanine nucleotide exchange factor for eEF1A. Stopped-flow kinetics using 2'-(or 3')-O-N-methylanthraniloyl (mant)-GDP showed spontaneous release of nucleotide from eEF1A is extremely slow and accelerated 700-fold by eEF1B alpha. The eEF1B alpha-stimulated reaction was inhibited by Mg2+ with a K(1/2) of 3.8 mM. Previous structural studies predicted the Lys-205 residue of eEF1B alpha plays an important role in promoting nucleotide exchange by disrupting the Mg2+ binding site. Co-crystal structures of the lethal K205A mutant in the catalytic C terminus of eEF1B alpha with eEF1A and eEF1A.GDP established that the lethality was not due to a structural defect. Instead, the K205A mutant drastically reduced the nucleotide exchange activity even at very low concentrations of Mg2+. A K205R eEF1B alpha mutant on the other hand was functional in vivo and showed nearly wild-type nucleotide dissociation rates but almost no sensitivity to Mg2+. These results indicate the significant role of Mg2+ in the nucleotide exchange reaction by eEF1B alpha and establish the catalytic function of Lys-205 in displacing Mg2+ from its binding site.

eEF1B: At the dawn of the 21st century

Translational regulation of gene expression in eukaryotes can rapidly and accurately control cell activity in response to stimuli or when rapidly dividing. There is increasing evidence for a key role of the elongation step in this process. Elongation factor-1 (eEF1), which is responsible for aminoacyl-tRNA transfer on the ribosome, is comprised of two entities: a G-protein named eEF1A and a nucleotide exchange factor, eEF1B. The multifunctional nature of eEF1A, as well as its oncogenic potential, is currently the subject of a number of studies. Until recently, less work has been done on eEF1B. This review describes the macromolecular complexity of eEF1B, its multiple phosphorylation sites and numerous cellular partners, which lead us to suggest an essential role for the factor in the control of gene expression, particularly during the cell cycle.

Regulation of elongation factor-1 expression by vitamin E in diabetic rat kidneys

Translation elongation factor-1 (EF-1) forms a primary site of regulation of protein synthesis and has been implicated amongst others in tumorigenesis, diabetes and cell death. To investigate whether diabetes-induced oxidative stress affects EF-1 gene expression, we used a free radical scavenger, vitamin E. The following groups of rats (5/group) were studied: control, vitamin E control, diabetic and diabetic treated with vitamin E. Markers of hyperglycemia, kidney function, oxidative stress, and kidney hypertrophy were elevated in diabetic rats. Increased urinary protein excretion indicated early signs of glomerular and tubular dysfunction. The mRNA and protein levels of the three EF-1 subunits (A, Balpha, and Bgamma) were determined in renal cortex extracts using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR), northern blot analysis and western blotting. EF-1A mRNA expression in renal cortex extracts was significantly increased by at least 2-fold (p < 0.002) in diabetic rats; however, there was no change in the mRNA levels of EF-1Balpha and EF-1Bgamma subunits. Similar results were observed at the protein level. Treatment of diabetic rats with vitamin E for 10 days suppressed both glycemic and oxidative stresses in renal cortex and kidney hypertrophy. EF-1A mRNA and protein levels were also reduced to control levels. In conclusion, EF-1A but not EF-1Balpha and EF-1Bgamma gene expression is significantly enhanced in the renal cortex of diabetic rats. Normalization of enhanced EF-1A expression by vitamin E treatment suggests a role for EF-1A during diabetes-induced oxidative stress.

Kinectin anchors the translation elongation factor-1 delta to the endoplasmic reticulum

Kinectin has been proposed to be a membrane anchor for kinesin on intracellular organelles. A kinectin isoform that lacks a major portion of the kinesin-binding domain does not bind kinesin but interacts with another resident of the endoplasmic reticulum, the translation elongation factor-1 delta (EF-1 delta). This was shown by yeast two-hybrid analysis and a number of in vitro and in vivo assays. EF-1 delta provides the guanine nucleotide exchange activities on EF-1 alpha during elongation step of protein synthesis. The minimal EF-1 delta-binding domain on kinectin resides within a conserved region present in all the kinectin isoforms. Overexpression of the kinectin fragments in vivo disrupted the intracellular localization of EF-1 delta proteins. This report provides evidence of an alternative kinectin function as the membrane anchor for EF-1 delta on the endoplasmic reticulum and provides clues to the EF-1 complex assembly and anchorage on the endoplasmic reticulum.

Sequences of elongation factors-1 alpha and -1 gamma and stimulation by juvenile hormone in Locusta migratoria

Two cDNAs encoding the alpha and gamma subunits of translation elongation factor-1 (EF-1) have been cloned and sequenced from the African migratory locust, Locusta migratoria. Southern blotting and real-time PCR analyses indicated that these sequences represent single copy genes. Comparison with sequences from other species indicated greater conservation for EF-1 alpha than for EF-1 gamma. The developmental profiles for EF-1 alpha and -1 gamma mRNA expression in the fat body paralleled reported changes in the hemolymph juvenile hormone (JH) titer in the fifth instar and were elevated during early reproductive maturation in the female adult. In maturing adults, there was a greater accumulation of EF-1 alpha and -1 gamma transcripts in females than in males. The levels of both transcripts were greatly increased by an enriched diet, previously shown to elevate JH titers and accelerate vitellogenin production. Treating JH-deprived adult females with the JH analog, methoprene, resulted in more than doubling of transcript levels of both genes, supporting the hypothesis that JH could stimulate the accumulation of LmEF-1 alpha and -1 gamma transcripts. We suggest that production of elongation factors, increased by JH, may contribute to the massive protein synthesis required for egg production.

Novel complexes of mammalian translation elongation factor eEF1A.GDP with uncharged tRNA and aminoacyl-tRNA synthetase. Implications for tRNA channeling

Multimolecular complexes involving the eukaryotic elongation factor 1A (eEF1A) have been suggested to play an important role in the channeling (vectorial transfer) of tRNA during protein synthesis [Negrutskii, B.S. & El'skaya, A.V. (1998) Prog. Nucleic Acids Res. Mol. Biol. 60, 47-78]. Recently we have demonstrated that besides performing its canonical function of forming a ternary complex with GTP and aminoacyl-tRNA, the mammalian eEF1A can produce a noncanonical ternary complex with GDP and uncharged tRNA [Petrushenko, Z.M., Negrutskii, B.S., Ladokhin, A.S., Budkevich, T.V., Shalak, V.F. & El'skaya, A.V. (1997) FEBS Lett. 407, 13-17]. The [eEF1A.GDP.tRNA] complex has been hypothesized to interact with aminoacyl-tRNA synthetase (ARS) resulting in a quaternary complex where uncharged tRNA is transferred to the enzyme for aminoacylation. Here we present the data on association of the [eEF1A.GDP.tRNA] complex with phenylalanyl-tRNA synthetase (PheRS), e.g. the formation of the above quaternary complex detected by the gel-retardation and surface plasmon resonance techniques. To estimate the stability of the novel ternary and quaternary complexes of eEF1A the fluorescence method and BIAcore analysis were used. The dissociation constants for the [eEF1A.GDP.tRNA] and [eEF1A.GDP.tRNAPhe.PheRS] complexes were found to be 20 nm and 9 nm, respectively. We also revealed a direct interaction of PheRS with eEF1A in the absence of tRNAPhe (Kd = 21 nm). However, the addition of tRNAPhe accelerated eEF1A.GDP binding to the enzyme. A possible role of these stable novel ternary and quaternary complexes of eEF1A.GDP with tRNA and ARS in the channeled elongation cycle is discussed.

Isolation and characterization of Leishmania infantum cDNA encoding a protein homologous to eukaryotic elongation factor 1 gamma

This paper reports the isolation and characterization of a complementary deoxyribonucleic acid clone showing sequence homology with genes coding for the eukaryotic elongation factor 1 gamma (EF-1 gamma). The clone encodes an open reading frame of 404 amino acids corresponding to a deduced molecular mass of 46.2 kDa. Database searches revealed 30-64% sequence identity between the Leishmania infantum EF-1 gamma and several eukaryotic homologues. Southern blot analysis indicated that 2 genes tandemly organized were present in the L. infantum genome. The 3' untranslated regions of these 2 genes differed in size. Southern hybridization and pulsed field gel electrophoresis showed that EF-1 gamma genes are highly conserved among members of the Leishmania genus and must be clustered in a single chromosomal locus.

Plasmodium protein phosphatase 2C dephosphorylates translation elongation factor 1beta and inhibits its PKC-mediated nucleotide exchange activity in vitro

The elongation step of protein synthesis involves binding of aminoacyl-tRNA to the ribosomal A site, formation of a peptide bond and translocation of the newly formed peptidyl-tRNA to the P site. The nucleotide exchange factor EF-1beta plays a major role in the regulation of this process by regenerating a GTP-bound EF-1alpha necessary for each elongation cycle. EF-1beta has been shown to be phosphorylated and its phosphorylation is critical for optimal activity. We have previously identified a serine/threonine protein phosphatase 2C (PP2C) from the human malaria parasite Plasmodium falciparum. In the current work, we performed Far-Western analysis to identify PfPP2C substrates. Several components of the translation and transcription machinery were identified, including translation elongation factor 1-beta (PfEF-1beta). PfEF-1beta is efficiently phosphorylated by protein kinase C and this phosphorylation results in a 400% increase in its nucleotide exchange activity. PKC-phosphorylated PfEF-1beta is readily and selectively dephosphorylated by recombinant and native PfPP2C, which downregulates the nucleotide exchange activity to its basal level. The identification of a translation elongation component as substrate for PP2C suggests an important regulatory function for this enzyme and suggests that it may be a good target for drug design in the fight against malaria.

Structural basis for nucleotide exchange and competition with tRNA in the yeast elongation factor complex eEF1A:eEF1Balpha

The crystal structure of a complex between the protein biosynthesis elongation factor eEF1A (formerly EF-1alpha) and the catalytic C terminus of its exchange factor, eEF1Balpha (formerly EF-1beta), was determined to 1.67 A resolution. One end of the nucleotide exchange factor is buried between the switch 1 and 2 regions of eEF1A and destroys the binding site for the Mg(2+) ion associated with the nucleotide. The second end of eEF1Balpha interacts with domain 2 of eEF1A in the region hypothesized to be involved in the binding of the CCA-aminoacyl end of the tRNA. The competition between eEF1Balpha and aminoacylated tRNA may be a central element in channeling the reactants in eukaryotic protein synthesis. The recognition of eEF1A by eEF1Balpha is very different from that observed in the prokaryotic EF-Tu:EF-Ts complex. Recognition of the switch 2 region in nucleotide exchange is, however, common to the elongation factor complexes and those of Ras:Sos and Arf1:Sec7.

The interaction between the archaeal elongation factor 1alpha and its nucleotide exchange factor 1beta

In Sulfolobus solfataricus the binding of the exchange factor 1beta (SsEF-1beta) to SsEF-1alpha-GDP displaces the nucleotide and the SsEF-1alpha-SsEF-1beta complex is formed. The complex itself is stable, but it dissociates upon the addition of GDP or Gpp(NH)p but not ATP. Since the rate of the formation of the SsEF-1alpha-SsEF-1beta complex is significatively slower than the rate of the nucleotide exchange catalyzed by SsEF-1beta it can be inferred that in vivo the GDP/GTP exchange reaction proceeds via an SsEF-1alpha-SsEF-1beta interaction without involving the formation of a stable binary complex as an intermediate.

Functional interaction of mammalian valyl-tRNA synthetase with elongation factor EF-1alpha in the complex with EF-1H

In mammalian cells valyl-tRNA synthetase (ValRS) forms a high Mr complex with the four subunits of elongation factor EF-1H. The beta, gamma, and delta subunits, that contribute the guanine nucleotide exchange activity of EF-1H, are tightly associated with the NH2-terminal polypeptide extension of valyl-tRNA synthetase. In this study, we have examined the possibility that the functioning of the companion enzyme EF-1alpha could regulate valyl-tRNA synthetase activity. We show here that the addition of EF-1alpha and GTP in excess in the aminoacylation mixture is accompanied by a 2-fold stimulation of valyl-tRNAVal synthesis catalyzed by the valyl-tRNA synthetase component of the ValRS.EF-1H complex. This effect is not observed in the presence of EF-1alpha and GDP or EF-Tu.GTP and requires association of valyl-tRNA synthetase within the ValRS.EF-1H complex. Since valyl-tRNA synthetase and elongation factor EF-1alpha catalyze two consecutive steps of the in vivo tRNA cycle, aminoacylation and formation of the ternary complex EF-1alpha.GTP. Val-tRNAVal that serves as a vector of tRNA from the synthetase to the ribosome, the data suggest a coordinate regulation of these two successive reactions. The EF-1alpha.GTP-dependent stimulation of valyl-tRNA synthetase activity provides further evidence for tRNA channeling during protein synthesis in mammalian cells.

The solution structure of the guanine nucleotide exchange domain of human elongation factor 1beta reveals a striking resemblance to that of EF-Ts from Escherichia coli

BACKGROUND

In eukaryotic protein synthesis, the multi-subunit elongation factor 1 (EF-1) plays an important role in ensuring the fidelity and regulating the rate of translation. EF-1alpha, which transports the aminoacyl tRNA to the ribosome, is a member of the G-protein superfamily. EF-1beta regulates the activity of EF-1alpha by catalyzing the exchange of GDP for GTP and thereby regenerating the active form of EF-1alpha. The structure of the bacterial analog of EF-1alpha, EF-Tu has been solved in complex with its GDP exchange factor, EF-Ts. These structures indicate a mechanism for GDP-GTP exchange in prokaryotes. Although there is good sequence conservation between EF-1alpha and EF-Tu, there is essentially no sequence similarity between EF-1beta and EF-Ts. We wished to explore whether the prokaryotic exchange mechanism could shed any light on the mechanism of eukaryotic translation elongation.

RESULTS

Here, we report the structure of the guanine-nucleotide exchange factor (GEF) domain of human EF-1beta (hEF-1beta, residues 135-224); hEF-1beta[135-224], determined by nuclear magnetic resonance spectroscopy. Sequence conservation analysis of the GEF domains of EF-1 subunits beta and delta from widely divergent organisms indicates that the most highly conserved residues are in two loop regions. Intriguingly, hEF-1beta[135-224] shares structural homology with the GEF domain of EF-Ts despite their different primary sequences.

CONCLUSIONS

On the basis of both the structural homology between EF-Ts and hEF-1beta[135-224] and the sequence conservation analysis, we propose that the mechanism of guanine-nucleotide exchange in protein synthesis has been conserved in prokaryotes and eukaryotes. In particular, Tyr181 of hEF-1beta[135-224] appears to be analogous to Phe81 of Escherichia coli EF-Ts.

Expression, purification, and spectroscopic studies of the guanine nucleotide exchange domain of human elongation factor, EF-1beta

Two guanine nucleotide exchange domains, corresponding to the C-terminal region of the human translational elongation factor EF-1beta (which consists of 225 amino acids), were produced by DNA recombinant overexpression techniques in Escherichia coli. We describe here a fast and efficient method for purifying these two protein fragments and for concentrating their solutions rapidly to a level as high as 25 mg/ml. This technique permitted the isolation of 20-30 mg of pure, native protein per liter of bacterial culture. Both fragments were able to form a complex with their natural substrate, elongation factor EF-1alpha, as detected by gel filtration experiments. The domain of 110 residues was slightly more active than the 91-amino-acid domain in guanine nucleotide exchange assays. Folding and stability of the two C-terminal domains were explored by circular dichroism (CD) and NMR spectroscopy. In spite of optimal conditions concerning NaCl concentration, temperature, and pH, during the NMR experiments both proteins showed signs of aggregation after approximately 7 days at 303 degreesK, a time period and temperature required for future heteronuclear NMR experiments. Also, the longer fragment suffered from proteolysis in the N-terminal region, suggestive of flexibility in that part of the structure. The secondary structure content for these two EF-1beta fragments was estimated, using data from both CD and NMR. The results of both methods agree very well and indicate for each fragment the presence of approximately 20% alpha-helix and approximately 50% beta-sheet. Elucidation of the three-dimensional structure of the exchange domain of EF-1beta by NMR spectroscopy appears therefore feasible.

Eukaryotic translation elongation factor 1 alpha: structure, expression, functions, and possible role in aminoacyl-tRNA channeling

This review offers a comprehensive analysis of eukaryotic translation elongation factor 1 (eEF-1 alpha) in comparison with its bacterial counterpart EF-Tu. Altogether, the data presented indicate some variances in the elongation process in prokaryotes and eukaryotes. The differences may be attributed to translational channeling and compartmentalization of protein synthesis in higher eukaryotic cells. The functional importance of the EF-1 multisubunit complex and expression of its subunits under miscellaneous cellular conditions are reviewed. A number of novel functions of EF-1 alpha, which may contribute to the coordinate regulation of multiple cellular processes including growth, division, and transformation, are characterized.

The effect of F-actin on the binding and hydrolysis of guanine nucleotide by Dictyostelium elongation factor 1A

Indirect evidence implicates actin as a cofactor in eukaryotic protein synthesis. The present study directly examines the effects of F-actin on the biochemical properties of eukaryotic elongation factor 1A (eEF1A, formerly EF1alpha), a major actin-binding protein. The basal mechanism of eEF1A alone is determined under physiological conditions with the critical finding that glycerol and guanine nucleotide are required to prevent protein aggregation and loss of enzymatic activity. The dissociation constants (Kd) for GDP and GTP are 2.5 microM and 0.6 microM, respectively, and the kcat of GTP hydrolysis is 1.0 x 10(-3) s-1. When eEF1A binds to F-actin, there is a 7-fold decrease in the affinity for guanine nucleotide and an increase of 35% in the rate of GTP hydrolysis. Based upon our results and the relevant cellular concentrations, the predominant form of cellular eEF1A is calculated to be GTP.eEF1A.F-actin. We conclude that F-actin does not significantly modulate the basal enzymatic properties of eEF1A; however, actin may still influence protein synthesis by sequestering GTP.eEF1A away from interactions with its known translational ligands, e.g. aminoacyl-tRNA and ribosomes.

Phosphorylation of elongation factor 1 and ribosomal protein S6 by multipotential S6 kinase and insulin stimulation of translational elongation

Stimulation of protein synthesis in response to insulin is concomitant with increased phosphorylation of initiation factors 4B and 4G and ribosomal protein S6 (Morley, S. J., and Traugh, J. A. (1993) Biochimie 75, 985-989) and is due at least in part to multipotential S6 kinase. When elongation factor 1 (EF-1) from rabbit reticulocytes was examined as substrate for multipotential S6 kinase, up to 1 mol/mol of phosphate was incorporated into the alpha, beta, and delta subunits. Phosphorylation of EF-1 resulted in a 2-2. 6-fold stimulation of EF-1 activity, as measured by poly(U)-directed polyphenylalanine synthesis. The rate of elongation was also stimulated by approximately 2-fold with 80 S ribosomes phosphorylated on S6 by multipotential S6 kinase. When the rates of elongation in extracts from serum-fed 3T3-L1 cells and cells serum-deprived for 1.5 h were compared, a 40% decrease was observed upon serum deprivation. The addition of insulin to serum-deprived cells for 15 min stimulated elongation to a rate equivalent to that of serum-fed cells. Similar results were obtained with partially purified EF-1, with both EF-1 and ribosomes contributing to stimulation of elongation. These data are consistent with a ribosomal transit time of 3.2 min for serum-deprived cells and 1.6 min following the addition of insulin for 15 min. Taken together, the data suggest that insulin stimulation involves coordinate regulation of EF-1 and ribosomes through phosphorylation by multipotential S6 kinase.

Phenotype of recombinant Trypanosoma cruzi which overexpress elongation factor 1-gamma: possible involvement of EF-1gamma GST-like domain in the resistance to clomipramine

In previous studies, molecular and immunological approaches have been used to characterize the Trypansosoma cruzi elongation factor 1gamma (TcEF-1gamma). A primary sequence homology search revealed that the TcEF-1gamma N-terminal domain showed significant homology to glutathione S-transferases (GSTs). Although studies have suggested the involvement of EF-1gamma in the protein synthesis machinery, the exact function of this protein, particularly the role of its GST-like domain, is not fully understood. Therefore, we have used the protozoan parasite T. cruzi, as a recipient for a shuttle vector which allows overexpression of TcEF-1gamma in order to gain insight into its biological function. The growth of parasites which overexpress TcEF-1gamma and control cells was equally sensitive to inhibition by nifurtimox and benznidazole, which exert a trypanocidal activity through the production of free radicals. In contrast, a strong resistance of transformed organisms to the tricyclic antidepressant drug, clomipramine, a lipophilic compound, was observed, whereas control cells were highly sensitive. Our findings suggest that TcEF-1gamma participates in the detoxification of lipophilic compounds probably by conjugation with glutathione through its GST-like domain. To our knowledge, this is the first report showing that the eukaryotic EF-1gamma GST conserved enzymatic model could play a role in drug resistance. Furthermore, these results reinforce the notion that the aggressiveness of certain tumours could in part be linked to overexpression of EF-1gamma. They also raise a central question regarding the GST as target for chemotherapeutic drugs in cancer research.

Phosphorylation of elongation factor 1 (EF-1) by protein kinase C stimulates GDP/GTP-exchange activity

Phosphorylation of the alpha, beta and delta subunits of elongation factor (EF) 1 by protein kinase C results in stimulation of elongation activity up to threefold both in vivo and in vitro [Venema, R. C., Peters, H. I. & Traugh, J. A. (1991) J. Biol. Chem. 266, 11,993-11,998, Venema, R. C., Peters, H. I. & Traugh, J. A. (1991) J. Biol. Chem. 266, 12,574-12,580]. The alpha subunit catalyzes the GTP-dependent binding of amino-acyl-tRNA to the ribosome, while the beta gamma and delta subunits of EF-1 catalyze exchange of the residual GDP on EF-1 alpha for GTP. To determine whether the change in elongation rate following phosphorylation by protein kinase C is due to stimulation of GDP/GTP exchange activity, EF-1 and EF-1.valyl-tRNA-synthetase have been purified from rabbit reticulocytes, phosphorylated in vitro by protein kinase C and the effect of phosphorylation on nucleotide-exchange activity analyzed. The alpha, beta and delta subunits are phosphorylated only on serine, and phosphopeptide maps show distinct phosphopeptides for each subunit. Following quantitative phosphorylation of EF-1 by protein kinase C on the alpha, beta, and delta subunits, a twofold enhancement of the rate of nucleotide exchange over the non-phosphorylated controls is observed with EF-1 and EF-1.valyl-tRNA synthetase. Stimulation of nucleotide exchange results in a two-fold increase in the formation of EF-1 alpha.GTP.Phe-tRNA, leading to an increased rate of binding of Phe-tRNA to ribosomes. The magnitude of stimulation of the exchange rate is similar to that reported previously for the rate of elongation following phosphorylation of EF-1 by protein kinase C. Thus, the enhancement of EF-1 activity in response to 4 beta-phorbol 12-myristate 13-acetate appears to be due to stimulation of the rate of GDP/GTP exchange following phosphorylation of EF-1 by protein kinase C.

Peptide-chain elongation in eukaryotes

The elongation phase of translation leads to the decoding of the mRNA and the synthesis of the corresponding polypeptide chain. In most eukaryotes, two distinct protein elongation factors (eEF-1 and eEF-2) are required for elongation. Each is active as a complex with GTP. eEF-1 is a multimer and mediates the binding of the cognate aminoacyl-tRNA to the ribosome, while eEF-2, a monomer, catalyses the movement of the ribosome relative to the mRNA. Recent work showing that bacterial ribosomes possess three sites for tRNA binding and that during elongation tRNAs may occupy 'hybrid' sites is incorporated into a model of eukaryotic elongation. In fungi, elongation also requires a third factor, eEF-3. A number of mechanisms exist to promote the accuracy or 'fidelity' of elongation: eEF-3 may play a role here. cDNAs for this and the other elongation factors have been cloned and sequenced, and the structural and functional properties of the elongation factors are discussed. eEF-1 and eEF-2 can be regulated by phosphorylation, and this may serve to control rates of elongation in vivo.

The guanine nucleotide-exchange factor, eIF-2B

Eukaryotic initiation factor eIF-2B catalyses the exchange of guanine nucleotides on another translation initiation factor, eIF-2, which itself mediates the binding of the initiator Met-tRNA to the 40S ribosomal subunit during translation initiation. eIF-2B promotes the release of GDP from inactive [eIF-2.GDP] complexes, thus allowing formation of the active [eIF-2.GTP] species which subsequently binds the Met-tRNA. This guanine nucleotide-exchange step, and thus eIF-2B activity, are known to be an important control point for translation initiation. The activity of eIF-2B can be modulated in several ways. The best characterised of these involves the phosphorylation of the alpha-subunit of eIF-2 by specific protein kinases regulated by particular ligands. Phosphorylation of eIF-2 alpha leads to inhibition of eIF-2B. This mechanism is involved in the control of translation under a variety of conditions, including amino acid deprivation in yeast (Saccharomyces cerevisiae) where it causes translational upregulation of the transcription factor GCN4, and in virus-infected animal cells, where it involves a protein kinase activated by double-stranded RNA. There is now also growing evidence for direct regulation of eIF-2B. This appears likely to involve the phosphorylation of its largest subunit. Under certain circumstances eIF-2B may also be regulated by allosteric mechanisms. eIF-2B is a heteropentamer (subunits termed alpha, beta, gamma, delta and epsilon) and is thus more complex than most other guanine nucleotide-exchange factors. The genes encoding all five subunits have been cloned in yeast (exploiting the GCN4 regulatory system): all but the alpha appear to be essential for eIF-2B activity. However, this subunit may confer sensitivity to eIF-2 alpha phosphorylation. cDNAs encoding the alpha, beta, delta and epsilon subunits have been cloned from mammalian sources. There is substantial homology between the yeast and mammalian sequences. Attention is now directed towards understanding the roles of individual subunits in the function and regulation of eIF-2B.

Cloning and genetic characterization of a calcium- and phospholipid-binding protein from Saccharomyces cerevisiae that is homologous to translation elongation factor-1 gamma

We have isolated a gene (CAM1) from the yeast Saccharomyces cerevisiae that encodes a protein homologous to the translational cofactor elongation factor-1 gamma (EF-1 gamma) first identified in the brine shrimp Artemia salina. The predicted Cam1 amino acid sequence consists of 415 residues that share 32% identity with the Artemia protein, increasing to 72% when conservative substitutions are included. The calculated M(r) of Cam1p (47,092 Da) is in close agreement with that of EF-1 gamma (M(r) = 49,200 Da), and hydropathy plots of each protein exhibit strikingly similar profiles. Disruption of the CAM1 locus yields four viable meiotic progeny, indicating that under normal growth conditions the Cam1 protein is non-essential. Attempts to elicit a translational phenotype have been unsuccessful. Since EF-1 gamma participates in the regulation of a GTP-binding protein (EF-1 alpha), double mutants with cam1 disruptions and various mutant alleles of known GTP-binding proteins were constructed and examined. No evidence was found for an interaction of CAM1 with TEF1, TEF2, SEC4, YPT1, RAS1, RAS2, CDC6, ARF1, ARF2 or CIN4. The possibility that Cam1p may play a redundant role in the regulation of protein synthesis or another GTP-dependent process is discussed.