Developing a snapshot of the ATP binding domain(s) of aminoglycoside phosphotransferases

The aminoglycoside (AG) 3'-phosphotransferases [APH(3')s] are an important class of modifying enzymes which confer high-level resistance to those AGs actively modified by the enzymes. They catalyze the transfer of the terminal phosphate from ATP to the drug, thus preventing the AG s action at the 70S ribosome. These enzymes, which utilize ATP as a co-substrate, appear from amino acid alignments to be part of a much larger superfamily of kinases and ATP-binding proteins. Structure-function analyses have been initiated in our laboratory for APH(3')-II, whose gene was derived from transposon Tn5. Site-directed mutagenesis of the cloned APH(3')-II gene was used to genetically examine the residues in two highly-conserved motifs proposed to participate in ATP binding. Several of these residues, in fact, were shown to affect the enzyme s affinity for ATP. We have also initiated studies using photoaffinity labelling of APH(3')-II with the ATP analogs, 8-azido-ATP and 2-azido-ATP. We have shown that 8-N3ATP and 2-N3ATP can be substituted for ATP in the APH(3')-II catalyzed phosphorylation of kanamycin; such findings indicate that the interaction of these photoaffinity analogs of ATP with APH(3')-II is biologically relevant. One of the best-characterized of the APH(3') enzymes is APH(3')-IIIa, the first of the group whose structure has been analyzed by x- ray crystallography. Several studies have demonstrated that this enzyme functions by a Theorell-Chance mechanism. Moreover, the architecture of the enzyme, crystallized in the presence of ADP has revealed residues in the ATP-binding pocket which are likely to play important roles in catalysis. Once the results from biochemical analyses can be correlated with those from mutagenesis studies and x-ray crystallography, a clearer picture of the active site will be provided for an important class of AG-modifying enzymes and phosphotransferases. This picture will also allow a better understanding of these enzymes within the greater context of kinases and nucleotide-binding proteins.

Purification and some properties of an oxydative inhibitor in rabbit reticulocyte lysates

Protein synthesis in rabbit reticulocyte lysates in the presence of heme is inhibited by 50% by the addition of 4 mM GSSG (oxidized glutathione). The incubation of the rabbit reticulocyte lysate with 4 mM GSSG at 30 degrees C for 30 min will cause activation of an inhibitor of protein synthesis which could be purified from the lysates through a five-step procedure. The inhibitor results in a 70-80% inhibition after a 1 h incubation. The inhibitor consists of one polypeptide of 23 kDa apparent molecular weight and is 90% pure as judged by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. However, in the presence of cAMP (10 mM) or GEF (guanine nucleotide exchange factor) (0.3 microgram), protein synthesis in the inhibited reticulocyte lysate will be already recovered.

Activation of the double-stranded RNA-regulated protein kinase by depletion of endoplasmic reticular calcium stores

Perturbants of the endoplasmic reticulum (ER), including Ca(2+)-mobilizing agents, provoke a rapid suppression of translational initiation in conjunction with an increased phosphorylation of the alpha-subunit of eukaryotic initiation factor (eIF)-2. Depletion of ER Ca2+ stores was found to signal the activation of a specific eIF-2 alpha kinase. Analysis of extracts derived from cultured cells that had been pretreated with Ca2+ ionophore A23187 or thapsigargin revealed a 2-3-fold increase in eIF-2 alpha kinase activity without detectable changes in eIF-2 alpha phosphatase activity. A peptide of 65-68 kDa, which was phosphorylated concurrently with eIF-2 alpha in extracts of pretreated cells, was identified as the interferon-inducible, double-stranded RNA (dsRNA)-regulated protein kinase (PKR). Depletion of ER Ca2+ stores did not alter the PKR contents of extracts. When incubated with reovirus dsRNA, extracts derived from cells with depleted ER Ca2+ stores displayed greater degrees of phosphorylation of PKR and of eIF-2 alpha than did control extracts. The enhanced dsRNA-dependent phosphorylation of PKR was observed regardless of prior induction of the kinase with interferon. Lower concentrations of dsRNA were required for maximal phosphorylation of PKR in extracts of treated as compared to control preparations. These findings suggest that PKR mediates the translational suppression occurring in response to perturbation of ER Ca2+ homeostasis.

The delta-subunit of murine guanine nucleotide exchange factor eIF-2B. Characterization of cDNAs predicts isoforms differing at the amino-terminal end

Protein synthesis in mammalian cells is regulated at the level of the guanine nucleotide exchange factor, eIF-2B, which catalyzes the exchange of eukaryotic initiation factor 2-bound GDP for GTP. We have isolated and sequenced cDNA clones encoding the delta-subunit of murine eIF-2B. The cDNA sequence encodes a polypeptide of 544 amino acids with molecular mass of 60 kDa. Antibodies against a synthetic polypeptide of 30 amino acids deduced from the cDNA sequence specifically react with the delta-subunit of mammalian eIF-2B. The cDNA-derived amino acid sequence shows significant homology with the yeast translational regulator Gcd2, supporting the hypothesis that Gcd2 may be the yeast homolog of the delta-subunit of mammalian eIF-2B. Primer extension studies and anchor polymerase chain reaction analysis were performed to determine the 5'-end of the transcript for the delta-subunit of eIF-2B. Results of these experiments demonstrate two different mRNAs for the delta-subunit of eIF-2B in murine cells. The isolation and characterization of two different full-length cDNAs also predicts the presence of two alternate forms of the delta-subunit of eIF-2B in murine cells. These differ at their amino-terminal end but have identical nucleotide sequences coding for amino acids 31-544.

The translation efficiency of ovalbumin mRNA is determined in part by a 5'-end hairpin structure

We have investigated the role in translation of the secondary structure in the noncoding leader (NCL) sequence of ovalbumin mRNA. Deletion of a stem-loop structure from the mRNA 5'-end (hairpin-1) produced a 2.5-fold decrease in mRNA translation rate in both rabbit reticulocyte wheat germ cell-free systems. A corresponding 2-fold reduction in mRNA binding affinity for reticulocyte eucaryotic initiation factor 2 (eIF-2) was also observed. These effects were independent of mRNA capping. Both translation rate and eIF-2 binding affinity were restored by addition to the mRNA of a sequence containing a hairpin-1-like structure. The positive correlation between cell free translation rate and mRNA binding to eIF-2 suggests that hairpin-1 is both an initiation signal and part of a specific eIF-2 recognition site. Methylation interference indicated a direct interaction between eIF-2 and hairpin-1 of ovalbumin mRNA.

Internal translation initiation on poliovirus RNA: further characterization of La function in poliovirus translation in vitro

Initiation of poliovirus RNA translation by internal entry of ribosomes is believed to require the participation of trans-acting factors. The mechanism of action of these factors is poorly defined. The limiting amount of one of these factors, La protein, in rabbit reticulocyte lysates (RRL) has been postulated to partially explain the inefficient translation of poliovirus RNA in this system. To further characterize La activity in translation and to identify other potential limiting factors, we assayed the ability of La protein as well as purified initiation factors, eIF-2, guanine nucleotide exchange factor (GEF), eIF-4A, eIF-4B, eIF-4F, and eIF-3, to stimulate the synthesis of P1, the capsid precursor protein, in poliovirus type 1 (Mahoney) RNA-programmed RRL. Of the proteins tested, only La, GEF, and to some extent eIF-2 stimulated the synthesis of P1. The enhanced translation of P1 in response to La occurred concomitantly with the inhibition of synthesis of most aberrant polypeptides, resulting from initiation in the middle of the genome. Deletion of the carboxy-terminal half (214 amino acids) of La did not decrease its binding to the poliovirus 5' untranslated region but abrogated the stimulatory and correcting activity in translation. In contrast to La, GEF and eIF-2 stimulated the overall translation and increased the synthesis of aberrant products as well as P1. Neither La, GEF, nor any other factor stimulated translation of encephalomyocarditis virus RNA in RRL. The implications of these findings for the mechanism of internal translation initiation on picornavirus RNAs are discussed.

Maturation hormone induced an increase in the translational activity of starfish oocytes coincident with the phosphorylation of the mRNA cap binding protein, eIF-4E, and the activation of several kinases

The stimulation of translation in starfish oocytes by the maturation hormone, 1-methyladenine (1-MA), requires the activation or mobilization of both initiation factors and mRNAs [Xu and Hille, Cell Regul. 1:1057, 1990]. We identify here the translational initiation complex, eIF-4F, and the guanine nucleotide exchange factor for eIF-2, eIF-2B, as the rate controlling components of protein synthesis in immature oocytes of the starfish, Pisaster orchraceus. Increased phosphorylation of eIF-4E, the cap binding subunit of the eIF-4F complex, is coincident with the initial increase in translational activity during maturation of these oocytes. Significantly, protein kinase C activity increased during oocyte maturation in parallel with the increase in eIF-4E phosphorylation and protein synthesis. An increase in the activities of cdc2 kinase and mitogen-activated myelin basic protein kinase (MBP kinase) similarly coincide with the increase in eIF-4E phosphorylation. However, neither cdc2 kinase nor MBP kinase phosphorylates eIF-4E in vitro. Casein kinase II activity does not change during oocyte maturation, and therefore, cannot be responsible for the activation of translation. Treatment of oocytes with phorbol 12-myristate 13-acetate, an activator of protein kinase C, for 30 min prior to the addition of 1-MA resulted in the inhibition of 1-MA-induced phosphorylation of eIF-4E, translational activation, and germinal vesicle breakdown. Therefore, protein kinase C may phosphorylate eIF-4E, after very early events of maturation. Another possibility is that eIF-4E is phosphorylated by an unknown kinase that is activated by the cascade of reactions stimulated by 1-MA. In conclusion, our results suggest a role for the phosphorylation of eIF-4E in the activation of translation during maturation, similar to translational regulation during the stimulation of growth in mammalian cells.

Purification and characterization of sea urchin initiation factor 2. The requirement of guanine nucleotide exchange factor for the release of eukaryotic polypeptide chain initiation factor 2-bound GDP

Protein synthesis in sea urchin eggs is stimulated dramatically upon fertilization. We previously demonstrated that this stimulation is primarily due to an increase in the rate of polypeptide chain initiation which in turn may be regulated at the level of recycling of eukaryotic initiation factor 2 (eIF-2) (Colin, A. M., Brown, B. D., Dholakia, J. N., Woodley, C. L., Wahba, A. J., and Hille, M. B. (1987) Dev. Biol. 123, 354-363). We have now purified eIF-2 from sea urchin Strongylocentrotus purpuratus blastulae to apparent homogeneity by chromatography on DEAE-cellulose, phosphocellulose, Mono Q, Mono P, and Mono S columns. The factor, which differs from mammalian eIF-2, is composed of three non-identical subunits with apparent molecular weights of 40,000-alpha; 47,000-beta, and 58,000-gamma as estimated by sodium dodecyl-polyacrylamide gel electrophoresis. Antibodies raised against rabbit reticulocyte eIF-2 do not cross-react with sea urchin eIF-2. The binding of Met-tRNA(f) to sea urchin eIF-2 is totally dependent on GTP. A 4-fold stimulation in the rate of protein synthesis in unfertilized sea urchin egg extracts is observed by the addition of 1 micrograms of purified eIF-2. The factor also binds GDP to form a binary (eIF-2.GDP) complex which is stable in the presence of Mg2+. GDP binding to sea urchin eIF-2 inhibits ternary (eIF-2-GTP.[35S]Met-tRNA(f) complex formation. The rabbit reticulocyte guanine nucleotide exchange factor (GEF) catalyzes the exchange of GDP bound to sea urchin eIF-2 for GTP and stimulates ternary complex formation. The requirement of GEF for the recycling of eIF-2 suggests that protein synthesis in sea urchins is similar to that in mammalian systems and may also be regulated at the level of GEF activity. The reticulocyte heme-controlled repressor phosphorylates the alpha-subunit of eIF-2 from both sea urchins and rabbit reticulocytes. However, casein kinase II which phosphorylates the beta-subunit of the reticulocyte factor specifically phosphorylates the alpha-subunit of sea urchin eIF-2. In this respect, the sea urchin factor is similar to eIF-2 isolated from other nonmammalian sources. Since both heme controlled repressor and casein kinase II phosphorylate the alpha-subunit of sea urchin eIF-2 caution should be exercised when interpreting the significance of eIF-2(alpha) phosphorylation in sea urchins.

Photoaffinity labeling of the rabbit reticulocyte guanine nucleotide exchange factor and eukaryotic initiation factor 2 with 8-azidopurine nucleotides. Identification of GTP- and ATP-binding domains

We have covalently modified rabbit reticulocyte polypeptide chain initiation factor 2 (eIF-2) and the guanine nucleotide exchange factor (GEF) with the 8-azido analogs of GTP (8-N3GTP) and ATP (8-N3ATP). Of the five subunits of GEF, the Mr 40,000 polypeptide binds 8-[gamma-32P]N3GTP, and the Mr 55,000 and 65,000 polypeptides bind 8-[gamma-32P]N3ATP. Both 8-N3GTP and 8-N3ATP specifically label the beta-subunit of eIF-2. Covalent binding of 8-azidopurine analogs to the eukaryotic initiation factors is dependent on UV irradiation. Binding of 8-N3GTP and 8-N3ATP is specific for the guanine- and adenine-binding sites on the protein, respectively. GDP and GTP, but not ATP, inhibit the photoinsertion of 8-N3GTP to the protein. Similarly, ATP, but not GTP, inhibits the photoinsertion of 8-N3ATP. The inclusion of NADP+ in the reaction mixtures also interferes with the binding of 8-N3ATP to GEF. Mg2+ inhibits the binding of the 8-azido analogs of GTP and ATP to both eIF-2 and GEF, whereas EDTA stimulates the photoinsertion of these nucleotides. Identical results are obtained when the binding of GTP and ATP to these proteins, in the presence of Mg2+ or EDTA, is estimated by nitrocellulose membranes. In enzymatic assays, 8-N3GTP supports the activity of eIF-2 and GEF, indicating that the interaction of 8-N3GTP is catalytically relevant.

Frog virus 3-induced translational shut-off: activation of an eIF-2 kinase in virus-infected cells

Infection of susceptible fathead minnow or Friend erythroleukemia cells with either infectious or heat-inactivated frog virus 3 led to the rapid inhibition of cellular protein synthesis. As seen in other cells, translational shut-off was accompanied by the dissociation of polysomes, but not the degradation of irreversible inactivation of cellular mRNAs. In addition, lysates from cells infected with heat-inactivated FV3 showed a reduced capacity to synthesize protein and to form 43S pre-initiation complexes in vitro. These results indicate that the in vitro systems accurately reflected in vivo events, and suggest that translational shut-off occurred prior to the union of the 40S ribosomal subunit and the [eIF-2.GTP.Met tRNAi] ternary complex. To determine the basis for the translational block, lysates from mock- and FV3-infected cells were assayed in vitro for their ability to phosphorylate the alpha subunit of eIF-2. In contrast to lysates from mock-infected cells, lysates from cells infected with heat-inactivated or infectious FV3 readily phosphorylated the alpha subunit of eIF-2. Since phosphorylation of the alpha subunit of eIF-2 inhibits its catalytic utilization during polypeptide chain initiation, these findings suggest that translational shut-off mediated by FV3 may be due to activation of a kinase that selectively phosphorylates this key initiation factor.

Mechanism of the nucleotide exchange reaction in eukaryotic polypeptide chain initiation. Characterization of the guanine nucleotide exchange factor as a GTP-binding protein

Polypeptide chain initiation in mammalian systems is regulated at the level of the guanine nucleotide exchange factor (GEF). This multisubunit protein catalyzes the exchange of GDP bound to eukaryotic initiation factor 2 (eIF-2) for GTP. Although various models have been proposed for its mode of action, the exact sequence of events involved in nucleotide exchange is still uncertain. We have studied this reaction by three different experimental techniques: (a) membrane filtration assays to measure the release of [3H]GDP from the eIF-2.[3H]GDP binary complex, (b) changes in the steady-state polarization of fluorescamine-GDP during the nucleotide exchange reaction, and (c) sucrose gradient analysis of the total reaction. The results obtained do not support the reaction as written: eIF-2.GDP + GEF in equilibrium eIF-2.GEF + GDP. The addition of GEF alone does not result in the displacement of eIF-2-bound GDP. The release of bound GDP is dependent on the presence of both GTP and GEF, and this argues against the possibility of a substituted enzyme (ping-pong) mechanism for the guanine nucleotide exchange reaction. An important finding of the present study is the observation that GTP binds to GEF. The Kd value of 4 microM for GTP was estimated (a) by the extent of quenching of tryptophan fluorescence of GEF in the presence of GTP and (b) by the binding of [3H]GTP to GEF as measured on nitrocellulose membranes. The GEF-dependent release of eIF-2-bound GDP was studied at several constant concentrations of one substrate (GTP or eIF-2.GDP) while varying the second substrate concentration, and the results were then plotted according to the Lineweaver-Burk method. Taken together, the results of GTP and eIF-2.GDP binding to GEF and the pattern of the double-reciprocal plots strongly suggest that the guanine nucleotide exchange reaction follows a sequential mechanism.

Phosphorylation of the guanine nucleotide exchange factor from rabbit reticulocytes regulates its activity in polypeptide chain initiation

We have demonstrated that the purified guanine nine nucleotide exchange factor (GEF) may be isolated as a complex with NADPH. Complete inhibition of the GEF-catalyzed exchange of eukaryotic initiation factor 2-bound GDP for GTP was observed in the presence of either 0.5-0.75 mM NAD+ or NADP+. Incubation of GEF with ATP results in the phosphorylation of its Mr 82,000 polypeptide. This phosphorylation is strongly inhibited by heparin but is not affected by heme or H8 (N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride), an inhibitor of cAMP- and cGMP-dependent protein kinases and protein kinase C. The purification of GEF was modified to eliminate any contaminating kinase activity and the isolated protein appears to be homogeneous as judged by NaDodSO4/polyacrylamide gel electrophoresis and silver staining. The Mr 82,000 subunit of GEF is phosphorylated only upon addition of ATP and casein kinase II. The extent of phosphorylation is approximately equal to 0.55 mol of phosphate per mol of GEF, and this results in a 2.3-fold increase in the guanine nucleotide exchange activity. Following treatment of the phosphorylated GEF with alkaline phosphatase, the activity of the protein is reduced by a factor of 5. Rephosphorylation of GEF increases its specific activity to that of the phosphorylated protein. The results of this study suggest that phosphorylation/dephosphorylation of GEF plays a role in regulating polypeptide chain initiation.

Expression of a gene for mouse eucaryotic elongation factor Tu during murine erythroleukemic cell differentiation

The eucaryotic elongation factor Tu (eEF-Tu) is a single polypeptide with an approximate Mr of 53,000. During protein synthesis eEF-Tu promotes the binding of aminoacyl-tRNA to the ribosome. To study the expression of the gene(s) for this factor, a genomic clone was isolated that contains a mouse eEF-Tu gene. We screened a phage genomic library with a synthetic oligonucleotide probe complementary to a region of the Saccharomyces cerevisiae and Artemia sp. eEF-Tu genes which codes for an area that is highly conserved between both yeast and Artemia sp. eEF-Tu. From approximately 75,000 phage plaques we obtained five isolates with apparently identical inserts. All five clones contained a 3.8-kilobase EcoRI fragment that hybridized to additional oligonucleotide probes corresponding to different conserved regions of eEF-Tu. We sequenced the 5' end of one genomic clone and determined the length of the cloned fragment that was protected by eEF-Tu mRNA in S1 nuclease protection assays. A quantitative S1 nuclease protection assay was used to compare the relative steady-state levels of eEF-Tu mRNA in total mRNA in total RNA isolated from hexamethylene-bisacetamide-induced murine erythroleukemia cells. The results show a dramatic reduction in the steady-state level of eEF-Tu mRNA as differentiation proceeds. A similar reduction in transcription of eEF-Tu mRNA was observed in isolated nuclei. Finally, we examined the in vivo synthesis of eEF-Tu during differentiation and found that it declined in a manner parallel to the decline in the steady-state level of eEF-Tu mRNA. In addition, we have isolated and sequenced a cDNA clone for mouse eEF-Tu. The derived amino acid sequence is compared with sequences from other eucaryotes.

Evidence for simultaneous derepression of messenger RNA and the guanine nucleotide exchange factor in fertilized sea urchin eggs

Translational control was studied in extracts of Lytechinus pictus eggs and zygotes. We showed that neither mRNA nor initiation factors alone limit translation in these lysates; rather they are together rate limiting. Added globin mRNA was translated in egg and zygote lysates but overall protein synthesis did not increase significantly as the added RNA competed with the endogenous message. The lysates mimicked the in vivo response, since microinjection of globin mRNA into L. pictus eggs similarly competed with endogenous mRNAs. A number of translational components were used to determine if they would stimulate protein synthesis in these lysates. The addition of globin polyribosomes increased the level of protein synthesis. The majority of this increase was due to reinitiation of the globin mRNA, and under these conditions the level of endogenous protein synthesis in both egg and zygote extracts did not change. The addition of crude initiation factors alone did not appreciably alter the rate of protein synthesis in the egg lysates. However, in the presence of added mRNA, these initiation factors stimulated translation two- to fourfold. Of all the initiation factors tested, only the guanine nucleotide exchange factor (GEF, eIF-2B, RF) significantly increased protein synthesis when globin mRNA was present. The addition of an unfractionated initiation factor preparation further stimulated protein synthesis in the presence of added GEF and mRNA, suggesting that a component other than mRNA and GEF was also limiting in these egg lysates. Other initiation factors, including eIF-2, eIF-4A, eIF-4B, and eIF-4F, did not substitute for the component in the unfractionated initiation factor preparation. We propose that alkalinization of the cytoplasm and the subsequent activation of initiation factors and mRNAs contribute to the large stimulation of protein synthesis in echinoid eggs after fertilization. Furthermore, we discuss the possibility that the increase in NADPH at the expense of NAD+, which occurs within 3 min after fertilization, may lead to the activation of GEF.

The isolation and characterization from rabbit reticulocytes of two forms of eukaryotic initiation factor 2 having different beta-polypeptides

We have isolated from the high salt wash of rabbit reticulocyte ribosomes two forms of the polypeptide chain initiation factor 2 (eIF-2) which differ with respect to their beta-subunit, GDP content, and sensitivity to Mg2+ in ternary (eIF-2 X GTP X Met-tRNAf) and binary (eIF-2 X GDP) complex formation. The form of eIF-2 eluting first from a cation exchange (Mono S, Pharmacia) column has a beta-subunit of lower molecular weight (eIF-2(beta L] and a more acidic pI value than the form eluting at a higher salt concentration (eIF-2(beta H]. These two forms of eIF-2 beta-polypeptides are also detected in reticulocyte lysates when the proteins are resolved by two-dimensional isoelectric focusing-dodecyl sulfate polyacrylamide gel electrophoresis followed by immunoblotting. The peptide mapping of the isolated beta-subunits after limited proteolysis by papain, pancreatic protease, alpha-chymotrypsin, or Staphylococcus aureus V8 protease further demonstrates that the two forms of beta-subunits are not the product of a non-specific proteolytic action that occurred during the purification procedure, but rather reflects the existence in vivo of both forms of eIF-2. The GDP content of eIF-2(beta L) and eIF-2(beta H) is approximately 0.85 and 0.22 mol of GDP/mol of eIF-2, respectively. The KD for GDP of eIF-2(beta L) was lower (2.2 X 10(-9) M) than that of eIF-2(beta H) (6.0 X 10(-8) M). In the presence of 1 mM Mg2+, the activities of eIF-2(beta L) and eIF-2(beta H) in forming a binary and a ternary complex are inhibited 90 and 25%, respectively. The extent of Mg2+ inhibition and its reversal by the guanine nucleotide exchange factor is directly proportional to the amount of GDP bound to eIF-2. No inhibition by Mg2+ is observed when eIF-2-bound GDP is removed by alkaline phosphatase. In the presence of the guanine nucleotide exchange factor, both forms of eIF-2 are equally active in ternary complex formation, and the complex formed is quantitatively transferred to 40 S ribosomal subunits.

The association of NADPH with the guanine nucleotide exchange factor from rabbit reticulocytes: a role of pyridine dinucleotides in eukaryotic polypeptide chain initiation

The guanine nucleotide exchange factor (GEF) was purified to apparent homogeneity from postribosomal supernatants of rabbit reticulocytes by chromatography on DEAE-cellulose and phosphocellulose, fractionation by glycerol gradients, and chromatography on Mono S and Mono Q (Pharmacia). At the Mono S step GEF is isolated as a complex with the eukaryotic polypeptide chain initiation factor 2 (eIF-2) and is separated from this factor by column chromatography on Mono Q. An emission spectrum characteristic of a reduced pyridine dinucleotide was observed when GEF was subjected to fluorescence analysis. By both coupled enzymatic analysis and chromatography on reverse-phase or Mono Q columns, the bound dinucleotide associated with GEF was determined to be NADPH. The GEF-catalyzed exchange of eIF-2-bound GDP for GTP was markedly inhibited by NAD+ and NADP+. This inhibition was not observed in the presence of equimolar concentrations of NADPH. Similarly, the stimulation of ternary complex (eIF-2 X GTP X Met-tRNAf) formation by GEF in the presence of 1 mM Mg2+ was abolished in the presence of oxidized pyridine dinucleotide. These results demonstrate that pyridine dinucleotides may be directly involved in the regulation of polypeptide chain initiation by acting as allosteric regulators of GEF activity.

Structural studies on the eukaryotic chain initiation factor 2 from rabbit reticulocytes and brine shrimp Artemia embryos. Phosphorylation by the heme-controlled repressor and casein kinase II

In contrast to reticulocyte polypeptide chain initiation factor 2 (eIF-2), the Artemia factor retains activity in the presence of Mg2+ or after phosphorylation of its alpha-subunit by rabbit reticulocyte heme-controlled repressor (Mehta, H. B., Woodley, C. L., and Wahba, A. J. (1983) J. Biol. Chem. 258, 3438-3441). Furthermore, we have so far been unable to demonstrate a requirement for a GDP/GTP nucleotide exchange factor with Artemia eIF-2. In order to explain these differences we compared the structure of eIF-2 from Artemia and rabbit reticulocytes by using one- and two-dimensional phosphopeptide and iodopeptide maps. Partial trypsin digestion of the alpha-subunit of Artemia eIF-2 after phosphorylation by the heme-controlled repressor generates a 4000 Mr phosphopeptide. Upon extensive trypsin digestion, the two-dimensional phosphopeptide maps of the alpha-subunits for the reticulocyte and Artemia factors are indistinguishable, whereas the iodopeptide maps are different. In addition, immunoblotting indicates that there is no consistent cross-reactivity of the reticulocyte subunits with antibodies prepared in rabbits against the Artemia eIF-2 subunits. A casein kinase II activity was isolated from Artemia embryos that phosphorylates the beta-subunit of reticulocyte eIF-2, but specifically phosphorylates the alpha-subunit of eIF-2 preparations from several non-mammalian sources, including Artemia, yeast, and wheat germ embryos. Since this kinase phosphorylates a site distinct from that recognized by the heme-controlled repressor, and this phosphorylation does not alter the ability of Artemia eIF-2 to undergo nucleotide exchange, caution must be exercised when interpreting the significance of eIF-2(alpha) phosphorylation in non-mammalian cells.