Comparison of phosphorylation of elongation factor 1 from different species by casein kinase II

Proteomic and phosphoproteomic determination of ABA's effects on grain-filling of Oryza sativa L. inferior spikelets

Cultivars of rice (Oryza sativa L.), especially the large-spikelet-type, often fail to achieve the high yield potential due to poor grain-filling of their inferior (late-flowering) spikelets. The superior (early-flowering) spikelets normally contain more abscisic acid (ABA) than the inferior spikelets. It was speculated that ABA might play a pivotal role in the grain-filling of inferior spikelets. To understand the molecular regulation involved in this process, we employed the 2-D gel-based comparative proteomic and phosphoproteomic analyses to search for differentially expressed proteins in the inferior spikelets under exogenous ABA treatment. A total of 111 significantly differential proteins and 31 phosphoproteins were found in the inferior spikelets after treatment. Among them, 100 proteins and 23 phosphoproteins were identified by using MALDI-TOF/TOF MS. In addition, the gene expression patterns of the inferior spikelets were confirmed with RT-PCR. These differentially expressed proteins are active in defense response, carbohydrate, protein, amino acid, energy and secondary metabolisms, as well as cell development and photosynthesis. The results suggest that the grain-filling of rice inferior spikelets is regulated by ABA through some proteins and phosphoproteins participating in carbon, nitrogen and energy metabolisms.

Proteomic analysis of phosphoproteins regulated by abscisic acid in rice leaves

Abscisic acid (ABA) is a hormone that regulates plant development and adaptation to environmental stresses. Protein phosphorylation has been recognized as an important mechanism for ABA signaling. However, the target phosphoproteins regulated by ABA are still largely unknown. Here, we report the identification of ABA-regulated phosphoproteins in rice using proteomic approaches. Six ABA-regulated phosphoproteins were identified as G protein beta subunit-like protein, ascorbate peroxidase, manganese superoxide dismutase, triosephosphate isomerase, putative Ca(2+)/H(+) antiporter regulator protein, and glyoxysomal malate dehydrogenase. These results provide new insight into the regulatory mechanism for some ABA signaling proteins and implicate several previously unrecognized proteins in ABA action.

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.

Recombinant subunits of mammalian elongation factor 1 expressed in Escherichia coli. Subunit interactions, elongation activity, and phosphorylation by protein kinase CKII

The first step in elongation requires two different activities; elongation factor (EF)-1alpha transfers aminoacyl-tRNA to the ribosome and is released upon hydrolysis of GTP, EF-1betagammadelta catalyzes exchange of GDP on EF-1alpha with GTP. To analyze the role of the individual subunits of EF-1 in elongation, the cDNAs for the beta, gamma, and delta subunits of EF-1 from rabbit were cloned, and proteins of 225, 437, and 280 amino acids, respectively, were expressed in Escherichia coli. The purified recombinant beta subunit migrates as a dimer and the gamma subunit as a trimer upon gel filtration, whereas the delta subunit forms a large aggregate. Complexes of betagamma, gammadelta and betagammadelta were formed by self-association and eluted with a molecular mass of approximately 160, 530, and 670 kDa, respectively; no interaction was observed between beta and delta. The activity of the recombinant subunits was determined with native EF-1alpha by measuring stimulation of the rate of elongation by poly(U)-directed polyphenylalanine synthesis. Recombinant beta and delta alone stimulated the rate of elongation by 10-fold, with a ratio of 5alpha:2beta or delta. The betagammadelta complex stimulated EF-1alpha activity up to 10-fold with a ratio of 20alpha to 1betagammadelta. Phosphorylation of the beta and delta subunits alone or in betagammadelta by protein kinase CKII had no effect on the rate of elongation.

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.

Translation elongation factor-1 alpha interacts with the 3' stem-loop region of West Nile virus genomic RNA

The conserved 3'-terminal stem-loop (3' SL) of the West Nile virus (WNV) genomic RNA was previously used to probe for cellular proteins that may be involved in flavivirus replication and three cellular proteins were detected that specifically interact with the WNV 3' SL RNA (J. L. Blackwell and M. A. Brinton, J. Virol. 69:5650-5658, 1995). In this study, one of these cellular proteins was purified to apparent homogeneity by ammonium sulfate precipitation and liquid chromatography. Amino acid sequence Western blotting, and supershift analyses identified the cellular protein as translation elongation factor-1 alpha (EF-1 alpha). Competition gel mobility shift assays demonstrated that the interaction between EF-1 alpha and WNV 3' SL RNA was specific. Dephosphorylation of EF-1 alpha by calf intestinal alkaline phosphatase inhibited its binding to WNV 3' SL RNA. The apparent equilibrium dissociation constant for the interaction between EF-1 alpha and WNV 3' SL RNA was calculated to be 1.1 x 10(-9) M. Calculation of the stoichiometry of the interaction indicated that one molecule of EF-1 alpha binds to each molecule of WNV 3' SL RNA. Using RNase footprinting and nitrocellulose filter binding assays, we detected a high-activity binding site on the main stem of the WNV 3' SL RNA. Interaction with EF-1 alpha at the high-activity binding site was sequence specific, since nucleotide substitution in this region reduced the binding activity of the WNV 3' SL RNA for EF-1 alpha by approximately 60%. Two low-activity binding sites were also detected, and each accounted for approximately 15 to 20% of the binding activity. Intracellular association between the host protein and the viral RNA was suggested by coimmunoprecipitation of WNV genomic RNA and EF-1 alpha, using an anti-EF-1 alpha antibody.

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.

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.

Casein kinase 2 down-regulation and activation by polybasic peptides are mediated by acidic residues in the 55-64 region of the beta-subunit. A study with calmodulin as phosphorylatable substrate

The noncatalytic beta-subunit is responsible for the latency of casein kinase 2 (CK2) activity toward calmodulin. Twenty-one mutants of the beta-subunit bearing either deletions or Ala substitutions for charged residues in the 5-6, 55-70, and 171-178 sequences have been assayed for their ability to substitute for wild-type beta-subunit as a suppressor of activity toward calmodulin. The only mutations that reduced the ability of the beta-subunit to suppress calmodulin phosphorylation activity, though being compatible with normal reconstitution of CK2 holoenzyme, were those affecting Asp55, Glu57 and the whole triplet Glu59-Asp-Glu61. The activity of CK2 holoenzyme, either native or reconstituted, toward calmodulin can be elicited by a variety of polybasic effectors, including polylysine, polyarginine, salmine, and histones H4, H3, and, to a lesser extent, H2a and H2b. Histone H1 and polyamines are conversely ineffective. The latent "calmodulin kinase" activity of CK2 can also be specifically unmasked by a peptide (alpha[66-86]) reproducing a basic insert of the catalytic subunit. This effect is reversed by equimolar addition of a peptide (beta[55-71]) including the 55-64 acidic stretch of the beta-subunit. Comparable polylysine stimulation was observed with the holoenzymes reconstituted with either beta wt or the beta mutants capable of assembling with the alpha-subunit, with the notable exception of those bearing Ala substitutions for acidic residues at positions 55, 57, and 59-61. These were nearly insensitive to 42 nM polylysine, which conversely promotes a more than 10-fold increase of calmodulin phosphorylation with wild-type beta.(ABSTRACT TRUNCATED AT 250 WORDS)

Spermine and polylysine enhanced phosphorylation of calmodulin and tubulin in an insect endocrine gland

Spermine-stimulated and heparin-inhibited phosphorylation of both exogenous casein and endogenous protein substrates of the prothoracic gland were measured in prothoracic gland cytosolic fractions from fifth instar larvae and early pupae of the tobacco hornworm, Manduca sexta. The results reveal a striking increase in casein kinase II (CKII) activity, i.e. approximately 3-fold above basal level in the presence of 5 mM spermine, with the highest activity exhibited by gland fractions from day 0-2 larvae, newly pupated animals and day 1 pupae. These results were verified by the results from Western blot analysis using a CKII alpha-subunit specific antibody and a 10 a.a. synthetic peptide that is a specific substrate for CKII. Several endogenous proteins were found to be substrates for CKII when assayed in the presence of spermine or polylysine. A 19 kDa peptide was shown to be calmodulin (CaM) by using the purified Manduca brain CaM as an indicator, and was only phosphorylated in the presence of polylysine. A 52 kDa protein was identified as tubulin by immunoprecipitation with a tubulin-specific monoclonal antibody, and was shown to be phosphorylated in the presence of spermine and polylysine. The possible roles of phosphocalmodulin and phosphotubulin are discussed in the context of prothoracic gland function.

Mechanism and regulation of eukaryotic protein synthesis

This review presents a description of the numerous eukaryotic protein synthesis factors and their apparent sequential utilization in the processes of initiation, elongation, and termination. Additionally, the rare use of reinitiation and internal initiation is discussed, although little is known biochemically about these processes. Subsequently, control of translation is addressed in two different settings. The first is the global control of translation, which is effected by protein phosphorylation. The second is a series of specific mRNAs for which there is a direct and unique regulation of the synthesis of the gene product under study. Other examples of translational control are cited but not discussed, because the general mechanism for the regulation is unknown. Finally, as is often seen in an active area of investigation, there are several observations that cannot be readily accommodated by the general model presented in the first part of the review. Alternate explanations and various lines of experimentation are proposed to resolve these apparent contradictions.

The effect of polylysine on casein-kinase-2 activity is influenced by both the structure of the protein/peptide substrates and the subunit composition of the enzyme

The mechanism by which polybasic peptides stimulate the activity of casein kinase 2 (CK2) has been studied by comparing the effect of polylysine on the phosphorylation of a variety of protein and peptide substrates by the native CK2 holoenzyme and by its recombinant catalytic alpha subunit, either alone or in combination with the recombinant non-catalytic beta subunit. Calmodulin is not phosphorylated by the CK2 holoenzyme, in either the native or the reconstituted form, unless polylysine is added. In the presence of polylysine, it becomes a good substrate for CK2 (Km 14.2 microM, Kcat 4.6 mol.min-1.mol CK2-1). The recombinant alpha subunit, however, spontaneously phosphorylates calmodulin, this phosphorylation being actually inhibited rather than stimulated by polylysine. The calmodulin tridecapeptide, RKMKDTDSEEEIR, reproducing the phosphorylation site for CK2, is spontaneously phosphorylated by either CK2 holoenzyme or the recombinant alpha subunit with 5.8-fold and 2.8-fold stimulation by polylysine, respectively. The recombinant beta subunit of CK2 is itself a good exogenous substrate for the enzyme, its phosphorylation, however, is inhibited rather than enhanced by polylysine. On the contrary, the phosphorylation of the nonapeptide, MSSSEEVSW, reproducing the beta-subunit phosphoacceptor site, is dramatically stimulated by polylysine. Using a variety of small peptide substrates, it was shown that phosphorylation rate is diversely stimulated by polylysine. The observed stimulation, moreover, is variably accounted for by changes in Vmax and/or Km, depending on the structure of the peptide substrate. Maximum stimulation with all protein/peptide substrates tested requires the presence of the beta subunit, since the recombinant alpha subunit is much less responsive than CK2 holoenzyme, either native or reconstituted. While the phosphorylation of the peptide RRRDDDSDDD by CK2 is stimulated 2.8-fold, with 15 nM polylysine being required for half-maximal stimulation, a stimulation of only 1.9-fold, with 80 nM polylysine required for half-maximal stimulation, is attained with recombinant alpha subunit. The concentration of polylysine required for half-maximal stimulation is comparable to CK2 concentration and increases by increasing CK2 concentration, suggesting that polylysine primarily interacts with the enzyme, rather than with the peptide substrate.