Protein synthesis in brine shrimp embryos and rabbit reticulocytes. The effect of Mg2+ on binary (eukaryotic initiation factor 2 X GDP) and ternary (eukaryotic initiation factor 2 X GTP X met-tRNAf) complex formation

Characterization of the initiation factor eIF2B and its regulation in Drosophila melanogaster

Eukaryotic initiation factor (eIF) 2B catalyzes a key regulatory step in the initiation of mRNA translation. eIF2B is well characterized in mammals and in yeast, although little is known about it in other eukaryotes. eIF2B is a hetropentamer which mediates the exchange of GDP for GTP on eIF2. In mammals and yeast, its activity is regulated by phosphorylation of eIF2alpha. Here we have cloned Drosophila melanogaster cDNAs encoding polypeptides showing substantial similarity to eIF2B subunits from yeast and mammals. They also exhibit the other conserved features of these proteins. D. melanogaster eIF2Balpha confers regulation of eIF2B function in yeast, while eIF2Bepsilon shows guanine nucleotide exchange activity. In common with mammalian eIF2Bepsilon, D. melanogaster eIF2Bepsilon is phosphorylated by glycogen synthase kinase-3 and casein kinase II. Phosphorylation of partially purified D. melanogaster eIF2B by glycogen synthase kinase-3 inhibits its activity. Extracts of D. melanogaster S2 Schneider cells display eIF2B activity, which is inhibited by phosphorylation of eIF2alpha, showing the insect factor is regulated similarly to eIF2B from other species. In S2 cells, serum starvation increases eIF2alpha phosphorylation, which correlates with inhibition of eIF2B, and both effects are reversed by serum treatment. This shows that eIF2alpha phosphorylation and eIF2B activity are under dynamic regulation by serum. eIF2alpha phosphorylation is also increased by endoplasmic reticulum stress in S2 cells. These are the first data concerning the structure, function or control of eIF2B from D. melanogaster.

Nerve and epidermal growth factor induce protein synthesis and eIF2B activation in PC12 cells

The regulation of protein synthesis and of eukaryotic initiation factor eIF2B was studied in PC12 cells. An increase in protein synthesis was observed after nerve growth factor (NGF) and epidermal growth factor (EGF) treatment of PC12 cells, and this increase coincided with activation of eIF2B. Growth factor addition in the presence of the phosphatidylinositol-3'-OH kinase inhibitor wortmannin showed that both NGF- and EGF-induced protein synthesis and eIF2B activation were phosphatidylinositol-3'-OH kinase dependent. The EGF-induced stimulation of protein synthesis and activation of eIF2B was dependent upon FK506-binding protein-rapamycin-associated protein, as shown with the immunosuppressant rapamycin, whereas NGF induction was partially dependent upon FK506-binding protein-rapamycin-associated protein. The activities of two kinases that act on eIF2B, glycogen synthase kinase-3 and casein kinase II, were measured to assess their potential roles in the activation of eIF2B in PC12 cells. Inactivation of glycogen synthase kinase-3 was seen in response to both NGF and EGF and this coincided with activation of eIF2B. However, inactivation of glycogen synthase kinase-3 was not rapamycin sensitive, in contrast to the activation of eIF2B. This indicates the involvement of another protein kinase or regulatory mechanism in the eIF2B activation. Both growth factors activated casein kinase II. However, the time course of its activation and its insensitivity to wortmannin and rapamycin suggest that casein kinase II does not play a major regulatory role in eIF2B activation under these conditions.

Inactivation of eIF2B and phosphorylation of PHAS-I in heat-shocked rat hepatoma cells

Various factors are involved in the heat shock-induced inhibition of protein synthesis. Changes upon heat shock in phosphorylation, leading to inactivation, of eukaryotic initiation factors (eIFs) eIF2 and eIF4E have been shown for several cell types. However, in mammalian cells these changes occur at temperatures of 43 degrees C or higher while protein synthesis is already affected at milder heat shock temperatures. In searching for the cause for the inhibition of protein synthesis, the regulation of eIF2 and eIF4E by additional factors was analyzed. In this respect, the activity of eIF2B was measured during and after heat shock. A very clear correlation was found between the activity of this guanine exchange factor and the levels of protein synthesis, also at mild heat shock conditions. Changes in the phosphorylation of eIF4E and of the eIF4E-binding protein PHAS-I were also analyzed. Surprisingly, in H35 cells as well as in some other cell lines, PHAS-I phosphorylation was increased by heat shock, whereas in others it was decreased. Therefore, decreasing the eIF4E availability under stressful conditions does not seem to be a general mechanism to inhibit protein synthesis by heat shock. Regulation of eIF2B activity appears to be the main mechanism to control translation initiation after heat shock at mild temperatures.

T-cell activation leads to rapid stimulation of translation initiation factor eIF2B and inactivation of glycogen synthase kinase-3

Mitogenic stimulation of T-lymphocytes causes a rapid activation or protein synthesis, which reflects in part increased expression of many translation components. Their levels, however, rise more slowly than the rate of protein synthesis, indicating an enhancement of the efficiency of their utilization. Initiation factor eIF2B catalyzes a key regulatory step in the initiation of translation, and we have therefore studied its activity following T-cell activation. eIF2B activity rises quickly, increasing as early as 5 min after cell stimulation. This initial phase is followed by an additional slow but substantial increase in eIF2B activity. The level of eIF2B subunits did not change over the initial rapid phase but did increase at later time points. Northern analysis revealed that levels of eIF2B mRNA only rose during the later phase. The rapid activation of EIF2B following mitogenic stimulation of T-cells is therefore mediated by factors other than its own concentration. The largest (epsilon) subunit of eIF2B is a substrate for glycogen synthase kinase-3 (GSK-3), the activity of which rapidly decreases following T-cell activation. Since phosphorylation of eIF2B by GSK-3 appears to inhibit nucleotide exchange in vitro, this provides a potential mechanism by which eIF2B may be activated.

Glucose stimulates the activity of the guanine nucleotide-exchange factor eIF-2B in isolated rat islets of Langerhans

Over short time periods glucose controls insulin biosynthesis predominantly through effects on preexisting mRNA. However, the mechanisms underlying the translational control of insulin synthesis are unknown. The present study was carried out to determine the effect of glucose on the activity and/or phosphorylation status of eukaryotic initiation and elongation factors in islets. Glucose was found to increase the activity of the guanine nucleotide-exchange factor eIF-2B over a rapid time course (within 15 min) and over the same range of glucose concentrations as those that stimulate insulin synthesis (3-20 mM). A nonmetabolizable analogue of glucose (mannoheptulose), which does not stimulate insulin synthesis, failed to activate eIF-2B. The best characterized mechanism for modulating eIF-2B activity involves changes in the phosphorylation of the alpha-subunit of its substrate eIF-2. However, in islets, no change in eIF-2 alpha phosphorylation was seen under conditions where eIF-2B activity was increased, implying that glucose regulates eIF-2B via an alternative pathway. Glucose also did not affect the phosphorylation states of three other regulatory translation factors. These are the cap-binding factor eIF-4E, 4E-binding protein-1, and elongation factor eEF-2, which do not therefore seem likely to be involved modulating the translation of the preproinsulin mRNA under these conditions.

Phosphorylation of the guanine nucleotide exchange factor and eukaryotic initiation factor 2 by casein kinase II regulates guanine nucleotide binding and GDP/GTP exchange

In mammalian cells, chain initiation factor (eIF) 2 and guanine nucleotide exchange factor (GEF) play a major role in the regulation of polypeptide chain initiation. Since guanine nucleotide exchange is the rate-limiting step in the recycling of eIF-2, we examined the effects of phosphorylation of GEF and eIF-2 on guanine nucleotide binding and the rate of GDP/GTP exchange. Phosphorylation of the 82-kDa subunit of GEF in vitro by casein kinase (CK) II results in the stimulation of guanine nucleotide exchange [Dholakia, J. N., & Wahba, A. J. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 51-54]. CK-II also phosphorylates the beta-subunit of eIF2, but the significance of this phosphorylation has not previously been investigated. In this study we demonstrate that treatment of CK-II-phosphorylated GEF or eIF-2 with alkaline phosphatase specifically removes more than 85% of the phosphate incorporated into the factors and alters guanine nucleotide binding to these proteins. In the presence of 1 mM Mg2+, the amount of GTP bound to dephosphorylated GEF is reduced 3.8-fold as compared to that of the CK-II-phosphorylated GEF. Rephosphorylation with CK-II restores GTP binding and increases 4-5-fold the activity of GEF in the exchange of eIF-2-bound GDP for free GTP. On the other hand, the extent of GDP binding to dephosphorylated eIF-2 is increased 2.3-fold as compared to that to the isolated eIF-2. The rate of GEF-catalyzed exchange of dephosphorylated eIF-2-bound GDP for GTP is approximately 2-fold slower than that with the isolated eIF-2.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of dansyl ribonucleotides and their use in steady-state fluorescence anisotropy studies of nucleotide binding by initiation factor-2 (eIF-2) and histone H1

1. Fluorescent analogs of GDP and ATP were prepared with DANSYL-beta-alanine (D beta A) coupled to the (2')3' hydroxyl of the ribose. 2. Observation of changes in both total fluorescence and anisotropy accompanying the binding of D beta A-GDP to eIF-2 allowed determination of Kd (33 nM). 3. When D beta A-ATP bound to H1 histone, the fluorescence quantum yield increased and the emission was blue shifted. Analysis yielded a Kd of 3.4 microM and 20 binding sites per histone. At high levels of ATP, fluorescence anisotropy values and light scattering intensities pointed to significant aggregation of H1 that is strongly dependent on ATP concentration.

Purification, phosphorylation and control of the guanine-nucleotide-exchange factor from rabbit reticulocyte lysates

A simple, improved procedure for the isolation of guanine-nucleotide-exchange factor (GEF) and for eukaryotic initiation factor 2 (eIF-2) from rabbit reticulocyte lysates has been developed using ion-exchange chromatography on S-Sepharose, Q-Sepharose, Mono Q and Mono S. The majority of the eIF-2 is separated from GEF at an early stage in the procedure and the remaining small amount of eIF-2.GEF complex is separated from the bulk of the GEF by FPLC on Mono S. The procedure yields approximately 2 mg each of eIF-2 and GEF, of 90% and greater than 80% purity, respectively, from the blood of ten rabbits. All fractions of purified GEF contain four subunits of molecular masses 84, 66, 54 and 39 kDa, with various amounts of a fifth, 30-kDa subunit. The modulation of GEF activity was investigated using the highly purified factor in a guanine-nucleotide-exchange assay. The activity of GEF was stimulated by physiological concentrations of the polyamines, spermine and spermidine, but was unaffected by another polycationic compound, polylysine. Activity was also found to be inhibited by 1 mM NADP+ or NAD+, and this inhibition was overcome by the presence of 1 mM NADPH. Stoichiometric amounts of GEF were unable to release GDP from eIF-2.GDP complexes in the absence of free guanine nucleotides, suggesting that GEF operates by a ternary-complex mechanism. Casein kinase 1 or casein kinase 2 can each phosphorylate the largest subunit (84 kDa) of GEF. These enzymes both phosphorylate serine residues in GEF but they phosphorylate distinct sites, as demonstrated by phosphopeptide mapping following proteolytic or cyanogen bromide digestion. Neither of these kinases phosphorylated any of the other subunits of GEF to any significant extent and several other kinases were inactive against GEF. No effect of phosphorylation on activity could be demonstrated.

Phosphorylation and guanine nucleotide exchange on polypeptide chain initiation factor-2 from Artemia embryos

Eukaryotic initiation factor-2 (eIF-2) from Artemia embryos is able to exchange guanine nucleotides at the same rate in the presence or absence of Mg2+ when the reaction is carried out with either purified eIF-2 at 30 degrees C or less purified preparations at any temperature (10-30 degrees C). No exchange factor appears to catalyze this reaction. However, with purified eIF-2 at lower temperatures (10 degrees C) the exchange is clearly impaired by Mg2+ and this impairment is overcome by the guanine nucleotide exchange factor (GEF) of rabbit reticulocytes. Thus, Artemia eIF-2 is able to exchange guanine nucleotides by two alternative mechanisms that may reflect two states of the protein. Phosphorylation of the eIF-2 alpha subunit by the heme-controlled inhibitor (HCI) of rabbit reticulocytes abolishes the GEF-dependent reaction, but has no effect on the factor-independent one. The search for eIF-2 alpha kinases in Artemia embryo led to the detection of only one such enzyme, which was identified as a casein kinase type II. None of the exchange reactions is affected by the phosphorylation of the eIF-2 alpha subunit by this kinase, suggesting that, irrespective of the kind of mechanism for guanine nucleotide exchange that is actually operating in Artemia, it might not be a target for regulation by eIF-2 alpha phosphorylation.

Protein synthesis in Drosophila melanogaster embryos. Two mechanisms for guanine nucleotide exchange on eukaryotic initiation factor 2

The mechanism for guanine nucleotide exchange with eukaryotic initiation factor-2 (eIF-2) from Drosophila melanogaster embryos was studied using the reaction eIF-2 X [3H]GDP + GDP (GTP) in equilibrium eIF-2 X GDP (GTP) + [3H]GDP. When highly purified eIF-2 is used the rate of nucleotide exchange is greatly reduced by Mg2+ and this reduction is overcome by the guanine-nucleotide-exchange factor (GEF) of rabbit reticulocytes. This GEF-dependent exchange is inhibited when Drosophila eIF-2 is either phosphorylated by the hemin-controlled inhibitor (HCI) of rabbit reticulocytes or treated with phosphatidylserine or a rabbit eIF-2 X phosphatidylserine complex. The Mg2+ impairment of guanine nucleotide exchange is less severe when highly purified eIF-2 is incubated at a higher temperature (37 degrees C) and is not observed at any temperature if partially purified eIF-2 is used instead of the highly purified factor. In the latter two cases the exchange is not inhibited by either phosphorylation with HCI or phospholipid treatment of Drosophila eIF-2, possibly suggesting that the observed exchange is not mediated by a GEF-like factor. Our data support two possible mechanisms for GDP/GTP exchange with Drosophila embryos eIF-2: a GEF-dependent exchange, similar to that described in rabbit reticulocytes, which may be regulated by phosphorylation of eIF-2, and a factor-independent exchange which appears to be insensitive to this type of control.

A fluorescence study of the binding of eucaryotic initiation factors to messenger RNA and messenger RNA analogues

The binding of the eucaryotic polypeptide chain initiation factors (eIFs) 4A, 4B, and 4F to poly(1,N6-ethenoadenylic acid) [poly(epsilon A)] was investigated by fluorescence spectroscopy. Competition experiments allowed us to determine the relative affinity of these proteins for mRNA cap analogues and the triplets AUG, GUG, UUU, UAA, and UGA. The salt dependence of eIF-4A binding to poly(epsilon A) and mRNA suggested that the binding was largely electrostatic and was enhanced in the presence of Mg2+ and ATP. The size of the binding site of eIF-4A, eIF-4B, and eIF-4F on poly(epsilon A) was approximately 13, 25, and 35 nucleotides, respectively. Fluorescence studies with the cap analogue 7-methylguanosine triphosphate as well as competition studies with poly(epsilon A) provide further evidence for a direct interaction of eIF-4F with the cap region. There was no evidence that either eIF-4B or eIF-4A bound the mRNA cap directly. In contrast to the other two factors, eIF-4B was found to bind preferentially to AUG, and of all the triplets tested, AUG was the most effective competitor for poly(epsilon A) binding.

Protein synthesis in Drosophila melanogaster embryos. Purification and characterization of polypeptide chain-initiation factor 2

Eukaryotic initiation factor 2 (elF-2) was purified from the high-salt wash fraction of Drosophila melanogaster embryos. This factor, with a molecular mass of about 90 kDa, consists of two subunits of 47 kDa and 39 kDa on dodecylsulfate/polyacrylamide gel electrophoresis. The 39-kDa subunit is phosphorylated by the hemin-controlled inhibitor of rabbit reticulocytes in a terminal fragment which can be cleaved by mild treatment with trypsin. Drosophila elF-2 is not a substrate for protein kinases capable of phosphorylating the beta subunit of elF-2 from rabbit reticulocytes. It is also shown that Drosophila elF-2 can form a ternary complex with GTP and Met-tRNAi, which can be efficiently transferred to 40S ribosomes in the presence of AUG and Mg2+. This factor is able to form a binary complex with GDP. Furthermore, purified elF-2 contains about 0.3 mol bound GDP/mol suggesting a high affinity of the factor for this nucleotide. Data supporting the notion that this affinity is increased in the presence of Mg2+, which impairs the GDP/GTP exchange on elF-2, are presented. The properties of Drosophila elF-2 suggest that this factor may be susceptible to regulation by a mechanism like that operating on rabbit reticulocyte elF-2.