Effect of cyclic AMP and cyclic GMP on the autophosphorylation of elongation factor 1 from wheat embryos

Evidence for regulation of protein synthesis at the elongation step by CDK1/cyclin B phosphorylation

Eukaryotic elongation factor 1 (eEF-1) contains the guanine nucleotide exchange factor eEF-1B that loads the G protein eEF-1A with GTP after each cycle of elongation during protein synthesis. Two features of eEF-1B have not yet been elucidated: (i) the presence of the unique valyl-tRNA synthetase; (ii) the significance of target sites for the cell cycle protein kinase CDK1/cyclin B. The roles of these two features were addressed by elongation measurements in vitro using cell-free extracts. A poly(GUA) template RNA was generated to support both poly(valine) and poly(serine) synthesis and poly(phenylalanine) synthesis was driven by a poly(uridylic acid) template. Elongation rates were in the order phenylalanine > valine > serine. Addition of CDK1/cyclin B decreased the elongation rate for valine whereas the rate for serine and phenylalanine elongation was increased. This effect was correlated with phosphorylation of the eEF-1delta and eEF-1gamma subunits of eEF-1B. Our results demonstrate specific regulation of elongation by CDK1/cyclin B phosphorylation.

Molecular cloning and characterization of the Arabidopsis thaliana alpha-subunit of elongation factor 1B

Using a PCR-based approach, we have isolated two Arabidopsis thaliana cDNA clones (alpha1 and alpha2) encoding the alpha-subunit of translation elongation factor 1B (eEF1Balpha). They encode open reading frames of 228 and 224 amino acids respectively, with extensive homology to eEF1Balpha subunits from different organisms, particularly in the C-terminal half of the protein. They both lack a conserved phosphorylation site that has been implicated in regulating nucleotide exchange activity. Using a plasmid shuffling experiment, we demonstrated that both alpha1 and alpha2 clones are able to complement a mutant yeast strain deficient for the eEF1Balpha subunit. This provides evidence that Arabidopsis encodes at least two functional isoforms of this subunit, termed eEF1Balpha1 and eEF1Balpha2. A third cDNA clone was isolated that appeared to result from an alternative splicing event of the eEF1Balpha1 gene.

The elongation factor-1delta (EF-1delta) originates from gene duplication of an EF-1beta ancestor and fusion with a protein-binding domain

The molecular evolution of two components of elongation factor-1 (EF-1), EF-1beta and EF-1delta was analysed using the distance matrix, the maximum parsimony and the maximum likelihood methods, after careful alignment of protein and cDNA sequences. The topology of the phylogenetic trees obtained supports monophyly of plant EF-1beta and EF-1beta' sequences, and monophyly of higher eukaryotic animal EF-1beta and EF-1delta sequences. EF-1beta and EF-1delta are homologous in their C-terminal domain. EF-1delta, which emerged before arthropods, originates from a beta-type ancestor gene and fusion with a leucine zipper N-terminal motif. Plant EF-1beta and EF-1beta' correspond to paralogous genes whose ancestor was most likely duplicated before the emergence of monocotyledons and dicotyledons.

A novel variant of translation elongation factor-1beta: isolation and characterization of the rice gene encoding EF-1beta2

A rice gene encoding a novel isoform of translation elongation factor-1beta subunit (termed EF-1beta2) was isolated and characterized. The gene comprises of eight exons, and encodes a 226-amino-acid protein. Expression of EF-1beta2 mRNA is abundant in seeds and cultured cells, but is considerably low in the tissues of the rice seedling. Antiserum raised against an EF-1beta2 synthetic peptide detected a protein with a relative molecular mass of about 32 kDa, indicating the EF-1beta2 gene is actually expressed in rice tissues. EF-1beta2 showed a close similarity to the cognate subunits from plant (beta and beta').

RNA polymerase of vesicular stomatitis virus specifically associates with translation elongation factor-1 alphabetagamma for its activity

An RNA-dependent RNA polymerase is packaged within the virions of purified vesicular stomatitis virus, a nonsegmented negative-strand RNA virus, which carries out transcription of the genome RNA into mRNAs both in vitro and in vivo. The RNA polymerase is composed of two virally encoded polypeptides: a large protein L (240 kDa) and a phosphoprotein P (29 kDa). Recently, we obtained biologically active L protein from insect cells following infection by a recombinant baculovirus expressing L gene. During purification of the L protein from Sf21 cells, we obtained in addition to an active L fraction an inactive fraction that required uninfected insect cell extract to restore its activity. The cellular factors have now been purified, characterized, and shown to be beta and gamma subunits of the protein synthesis elongation factor EF-1. We also demonstrate that the alpha subunit of EF-1 remains tightly bound to the L protein in the inactive fraction and betagamma subunits associate with the L(alpha) complex. Further purification of L(alpha) from the inactive fraction revealed that the complex is partially active and is significantly stimulated by the addition of betagamma subunits purified from Sf21 cells. A putative inhibitor(s) appears to co-elute in the inactive fraction that blocked the L(alpha) activity. The purified virions also package all three subunits of EF-1. These findings have a striking similarity with Qbeta RNA phage, which also associates with the bacterial homologue of EF-1 for its replicase function, implicating a possible evolutionary relationship between these host proteins and the RNA-dependent RNA polymerase of RNA viruses.

The plant translational apparatus

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

Expression of recombinant elongation factor 1 beta from rabbit in Escherichia coli. Phosphorylation by casein kinase II

The beta subunit of eukaryotic elongation factor 1 (EF-1) catalyzes the GDP/GTP exchange activity on EF-1 alpha. In these studies, two cDNAs for the beta subunit of EF-1 from rabbit are cloned and sequenced. The cDNAs consist of 808 and 798 bp and are identical except for the 5' leader sequences of 67 and 57 bp. Both cDNAs code for a protein of 225 amino acids. Using the pT7-7 expression vector, EF-1 beta was expressed in Escherichia coli and purified to apparent homogeneity by chromatography on DEAE-cellulose and FPLC on Superose 12 and Mono Q. EF-1 beta was highly phosphorylated by casein kinase II, with up to 1.3 mol of phosphate incorporated per mol protein. From microsequence analysis and manual Edman degradation, the majority of the phosphate was shown to be present in serine 106 in the peptide DLFGS106DDEEES112EEA. Serine 112 was also phosphorylated by casein kinase II, but to a lesser extent. Previously, little phosphorylation of the beta subunit by casein kinase II was observed in native EF-1 unless GDP was bound to the alpha subunit (Palen, E., Venema, R.C., Chang, Y-W.E. and Traugh, J.A. (1994) Biochemistry, 8515-8520). In contrast, purified recombinant EF-1 beta was highly and specifically phosphorylated by casein kinase II; GDP and polylysine had little effect on the rate of phosphorylation of the purified subunit.

Cloning and characterization of the cDNA encoding rice elongation factor 1 beta

We have cloned and sequenced a cDNA coding for rice elongation factor 1 beta (EF-1 beta). The clone was 1420 bp long and contained an open reading frame coding for 229 amino acids. The overall identity between rice EF-1 beta and rice EF-1 beta' [Matsumoto, S., Oizumi, N., Taira, H. and Ejiri, S. (1992) FEBS Lett. 311, 46-48] is 60% at the amino acid sequence level; a higher percent identical residues (81%) were especially observed in the C-terminal region. Rice EF-1 beta has no conserved phosphorylation site for casein kinase II and no leucine zipper motif, although these motifs are well conserved in EF-1 delta (= beta in plants) subunits of animal EF-1.

Cloning and characterization of the elongation factor EF-1 beta homologue of Saccharomyces cerevisiae. EF-1 beta is essential for growth

A Saccharomyces cerevisiae cDNA homologue of the elongation factor EF-1 beta was found among the clones obtained by immunoscreening of a yeast cDNA expression library with an antibody against calmodulin affinity-purified proteins. The cDNA encoded a protein of 206 amino acids which was highly homologous (about 70% homology) with Artemia salina and human EF-1 beta. A protein with an apparent molecular mass of 33,000, significantly larger than that expected from the gene, was identified by Western blotting. Gene disruption experiments indicated that EF-1 beta is essential for growth.

Cloning and sequencing of the cDNA encoding rice elongation factor 1 beta'

A cDNA clone coding for rice elongation factor 1 beta' (EF-1 beta') was isolated from a rice anther cDNA library. The clone, named RB', was 980 bp long and contained a single open reading frame coding for 223 amino acids; the first 31 amino acids, except for the first methionine, which is absent in the mature protein, are identical to those of the purified protein determined with a protein sequencer. The amino acid sequence of rice EF-1 beta' shows homology to the C-terminal half of Artemia salina EF-1 beta (59%) and human EF-1 beta (63%), but might not have a phosphorylation site for casein kinase II which has been conserved in Artemia saline EF-1 beta and EF-1 delta, human EF-1 beta and silkworm EF-1 beta'.

Rapid purification and N-terminal sequencing of a potato tuber cyclic nucleotide binding phosphatase

A high affinity cyclic nucleotide binding phosphatase was purified to homogeneity from potato tubers by a rapid procedure involving batchwise elution from carboxymethylcellulose and gel filtration. The phosphatase has a molecular weight of 28,000 as estimated from both SDS-PAGE and gel filtration. The phosphatase binds to Con A-agarose and is eluted by 0.5 M alpha-methylglucoside. The phosphatase catalyses the hydrolysis of nucleoside monophosphates, p-nitrophenylphosphate and O-phospho-L-tyrosine, but not of O-phospho-L-serine or O-phospho-L-threonine. N-terminal sequencing of the phosphatase has revealed significant homology with two similar-size soybean leaf and stem storage glycoproteins.

Regulation of protein synthesis at the elongation stage. New insights into the control of gene expression in eukaryotes

There are many reports which demonstrate that the rate of protein biosynthesis at the elongation stage is actively regulated in eukaryotic cells. Possible physiological roles for this type of regulation are: the coordination of translation of mRNA with different initiation rate constants; regulation of transition between different physiological states of a cell, such as transition between stages of the cell cycle; and in general, any situation where the maintenance of a particular physiological state is dependent on continuous protein synthesis. A number of covalent modifications of elongation factors offer potential mechanisms for such regulation. Among the various modifications of elongation factors, phosphorylation of eEF-2 by the specific Ca2+calmodulin-dependent eEF-2 kinase is the best studied and perhaps the most important mechanism of regulation of elongation rate. Since this phosphorylation is strictly Ca(2+)-dependent, and makes eEF-2 inactive in translation, this mechanism could explain how changes in the intracellular free Ca2+ concentration may regulate elongation rate. We also discuss some recent findings concerning elongation factors, such as the discovery of developmental stage-specific elongation factors and the regulated binding of eEF-1 alpha to cytoskeletal elements. Together, these observations underline the importance of the elongation stage of translation in the regulation of the cellular processes essential for normal cell life.

Elongation factor 1 beta gamma from Artemia. Purification and properties of its subunits

The guanine nucleotide exchange factor, elongation factor 1 beta gamma (EF-1 beta gamma) has been purified from Artemia cysts using an improved method. The protein consists of two distinct polypeptides with relative molecular masses of 26,000 (EF-1 beta) and 46,000 (EF-1 gamma). A nucleoside diphosphate phosphotransferase activity often found in EF-1 beta gamma preparations has been completely separated from the actual guanine nucleotide exchange stimulatory activity of EF-1 beta gamma, thus indicating that nucleotide diphosphate phosphotransferase is not an intrinsic property of EF-1 beta. Both EF-1 beta gamma and EF-1 beta have been shown to stimulate the following three reactions to a comparable degree: (a) exchange of GDP bound to EF-1 alpha with exogenous GDP; (b) EF-1 alpha-dependent binding of Phe-tRNA to ribosomes; (c) poly(U)-dependent poly(phenylalanine) synthesis. However, a significantly higher nucleotide exchange rate was observed in the presence of EF-1 beta gamma compared to EF-1 beta alone. Concerning elongation factor 1 gamma (EF-1 gamma) the following observations were made. In contrast to EF-1 beta, pure EF-1 gamma is rather insoluble in aqueous buffers, but the tendency to precipitate can be partially suppressed by the addition of detergents. In particular, EF-1 gamma partitions solely into the detergent phase of Triton X-114 solutions. EF-1 gamma is also more susceptible to spontaneous, specific fragmentation. It is remarkably that about 5% of the cellular pool of EF-1 beta gamma was found to be present in membrane fractions, under conditions where no EF-1 alpha was detectable in these fractions. Furthermore it was noted that EF-1 beta gamma copurified strongly with tubulin on DEAE-cellulose. Moreover, it was observed that from a mixture of EF-1 beta gamma and tubulin, EF-1 gamma coprecipitates with tubulin using a non-denaturating immunoprecipitation technique. These findings suggest that EF-1 gamma has a hydrophobic domain and interacts with membrane and cytoskeleton structures in the cell.

Primary structure of elongation factor 1 gamma from Artemia

Complementary DNA corresponding to elongation factor 1 gamma, which forms a complex with EF-1 beta, has been cloned. A lambda gt11 cDNA library has been screened with an antiserum against EF-1 beta gamma. The derived amino acid sequence of EF-1 gamma corresponds to 429 amino acids excluding the initiator methionine, which is absent in the mature protein. About half of the protein was sequenced by direct protein sequence analysis. No clear homology with any other protein was found.

Ca2+/calmodulin-dependent phosphorylation of elongation factor 2

Incubation of a ribosome-free extract of rabbit reticulocytes or rat liver with [gamma-32P]ATP and Ca2+ results in incorporation of 32P predominantly into a single polypeptide of Mr approximately 100,000. This polypeptide is identified as elongation factor 2 (EF-2). Phosphorylation of EF-2 is strictly Ca2+-dependent and can be inhibited by the calmodulin antagonist trifluoperazine. It is suggested that the Ca2+/calmodulin-dependent phosphorylation of EF-2 is involved in regulation of protein biosynthesis.