Regulation of protein synthesis in rabbit reticulocyte lysates: purification and initial characterization of the cyclic 3':5'-AMP independent protein kinase of the heme-regulated translational inhibitor

Mechanism of interferon action: eIF-2 alpha phosphatase in interferon-treated mouse fibroblasts is double-stranded RNA independent

The effect of double-stranded RNA on the dephosphorylation of purified, 32P-labeled eIF-2 was examined in cell-free extracts prepared from interferon-treated mouse L929 cells. Dephosphorylation of the alpha subunit of eIF-2 occurred at comparable rates in the presence and in the absence of reovirus dsRNA. By contrast, the beta subunit of eIF-2 was not dephosphorylated to any significant extent in either the presence or the absence of dsRNA. These results indicate that the enhanced phosphorylation of eIF-2 alpha observed in IFN-treated systems in the presence of double-stranded RNA (dsRNA) is indeed caused by an activation of a protein kinase rather than to an inhibition of a phosphoprotein phosphatase by the dsRNA.

Purification and characterization of a specific histone H1 protein kinase from mouse plasmacytoma

A protein kinase with high specificity for histone H1 was purified from a plasmacytoma microsomal fraction by a high-salt wash, ammonium sulfate precipitation, chromatography on DEAE-cellulose, hydroxyapatite and Sephadex G-200 columns, and the main properties of this kinase were studied. A sulfhydryl compound, such as 2-mercaptoethanol or dithiothreitol, was necessary for full activity. The optimum pH was 7.4-7.8. After purification, the histone H1 kinase was not stimulated by cAMP or cGMP. It was not inhibited by the heat-stable cAMP-dependent protein kinase inhibitor from beef heart. It utilized preferentially GTP over ATP as phosphate donor. Km values for ATP and GTP were 58 microM and 1.4 microM respectively; the Km for histone H1 was 14 microgram ml-1. The molecular weight was approximately 90 000 by gel-exclusion chromatography. Analysis of the purified H1-specific protein kinase by polyacrylamide gel electrophoresis in dodecylsulfate showed two bands having molecular weights of approximately 64 000 and 54 000. Many characteristics of this kinase were similar to those of the chromatin-bound protein kinase reported by other workers in rapidly proliferating cells.

Relationship between phosphorylation and activity of heme-regulated eukaryotic initiation factor 2 alpha kinase

In heme-deficient reticulocytes and their lysates, a heme-regulated inhibitor of protein synthesis is activated; this inhibitor is a cyclic AMP-independent protein kinase that specifically phosphorylates the alpha subunit of the eukaryotic initiation factor 2 (eIF-2 alpha). Heme regulates this kinase by inhibiting its activation and activity. The purified heme-regulated kinase (HRI) undergoes autophosphorylation; at least 3 mol of phosphate can be incorporated per HRI subunit (Mr 80,000). The phosphorylation of HRI, its eIF-2 alpha kinase activity, and its ability to inhibit protein synthesis are diminished by hemin (5 microM) and increased by N-ethylmaleimide (MalNEt). Treatment of MalNEt-activated HRI with hemin reduces its autophosphorylation and its ability to inhibit protein synthesis . These findings demonstrate a correlation of the phosphorylation of HRI, its eIF-2 alpha kinase activity, and its inhibition of protein synthesis. The mechanism of hemin regulation of HRI activity was studied by examining the binding of hemin to purified HRI. Significant binding was demonstrable by difference spectroscopy which revealed a pronounced shift in the absorption spectrum of hemin with the appearance of a peak at 418 nm, a shift similar to that observed with proteins known to bind hemin. These findings are consistent with a direct effect of hemin on HRI.

Site-specific phosphorylation of the alpha subunit of eukaryotic initiation factor eIF-2 by the heme-regulated and double-stranded RNA-activated eIF-2 alpha kinases from rabbit reticulocyte lysates

The site specificity of phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2alpha) by the heme-regulated and double-stranded RNA-activated eIF-2alpha kinases were compared by phosphopeptide mapping. eIF-2alpha was maximally phosphorylated in vitro with [gamma-(32)P]ATP and either crude or partially purified preparations of the kinases. (32)P-Labeled eIF-2alpha was isolated by electrophoresis in sodium dodecyl sulfate/polyacrylamide gels. The fixed, stained, and dried polypeptide band was excised and then exhaustively digested directly in the gel slice with one of several proteases (trypsin, chymotrypsin, subtilisin, or thermolysin); the resultant [(32)P]phosphopeptides were analyzed by one-dimensional chromatography or by two-dimensional chromatography and high-voltage electrophoresis. In addition, limited proteolysis of [(32)P]eIF-2alpha contained in fixed, dried, and stained gel slices was achieved with Staphylococcus aureus protease V8, chymotrypsin, or subtilisin, and the partial (32)P-labeled cleavage products were analyzed by gel electrophoresis. Each protease produced distinct and reproducible [(32)P]phosphopeptide profiles after partial or exhaustive proteolysis of [(32)P]eIF-2alpha. With a given protease, identical [(32)P]phosphopeptide patterns were obtained whether eIF-2alpha was phosphorylated by the heme-regulated or the double-stranded RNA-activated kinase. These data indicate that, in vitro, the kinases phosphorylate sites on eIF-2alpha that are identical or proximally located in the primary sequence. In this report we also provide preliminary evidence that the two eIF-2alpha kinases activated in lysates by heme deficiency or double-stranded RNA phosphorylate site(s) of endogenous eIF-2alpha that are similar, if not identical, to the sites phosphorylated in vitro with partially purified eIF-2alpha kinase(s) and eIF-2.

Characterization of double-stranded-RNA-activated kinase that phosphorylates alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha) in reticulocyte lysates

Incubation of reticulocyte lysates with low levels of double-stranded (ds) RNA (1-20 ng/ml) activates a cAMP-independent protein kinase (dsI) that phosphorylates the alpha-subunit (M(r) 38,000) of initiation factor 2 (eIF-2) and produces an inhibition of protein chain initiation similar to that caused by heme deficiency. Activation of dsI from its latent precursor takes place on the ribosomes and requires ATP. dsI can also be activated in ribosomal salt washes and in partially purified preparations of the latent precursor of dsI. In all preparations, activation is accompanied by the ds RNA-dependent phosphorylation of a polypeptide doublet that migrates as bands of 67 and 68.5 kilodaltons (67/68.5) in NaDodSO(4)/acrylamide gels. The rate of phosphorylation of these components in a ribosome salt wash is more rapid than the ds RNA-dependent phosphorylation of eIF-2alpha. Other polypeptides in the salt wash also undergo ds RNA-dependent phosphorylation, but their significance is not clear. All of these phosphorylations are prevented by high concentrations of poly(I).poly(C)(20 mug/ml), but not by an antiserum specific for the heme-regulated eIF-2alpha kinase. Both the latent and activated forms of dsI have been partially purified from a 0.5 M KCl wash of reticulocyte ribosomes. The two species have similar M(r)s ( approximately 120,000) and sedimentation coefficients ( approximately 3.75 S), which suggests that activation of dsI probably does not involve extensive changes. By comparison, the heme-regulated eIF-2alpha kinase has an M(r) of approximately 160,000 and sediments at approximately 6.6 S. However, in vitro, dsI and HRI both phosphorylate the same site(s) of eIF-2alpha. Purified dsI inhibits protein synthesis in hemin-supplemented lysates with the same kinetics induced by the addition of ds RNA; both inhibitions are reversed by eIF-2. dsI that has been activated in the salt wash and then purified does not require ds RNA for expression and no longer displays phosphorylation of the 68.5/67 doublet, which appears to occur only during activation. The data support the view that this component(s) may be the eIF-2alpha kinase activated by ds RNA.

Protein synthesis in rabbit reticulocytes: characteristics of a postribosomal supernatant factor that reverses inhibition of protein synthesis in heme-deficient lysates and inhibition of ternary complex (Met-tRNAfMet.eIF-2.GTP) formation by heme-regulated inhibitor

During heme deficiency in reticulocyte lysates, a translational inhibitor (heme-regulated inhibitor, HRI) that blocks polypeptide chain initiation is activated. HRI is a protein kinase that specifically phosphorylates the 38,000-dalton subunit of the Met-tRNAfMet binding factor, eIF-2. Phosphorylation of eIF-2 by HRI prevents its interaction with at least two additional factors, resulting in a net reduction in formation of ternary complex (Met-tRNAfMet.eIF-2.GTP) and AUG-dependent transfer of Met-tRNAfMet to 40S ribosomal subunits. A factor (sRF) that reverses protein synthesis inhibition in heme-deficient lysates has been purified from reticulocyte postribosomal supernatant. sRF also reverses the inhibition of ternary complex formation by HRI in a fractionated system. The ternary complex inhibition reversal activity and the protein synthesis inhibition reversal activity cosediment at 12.5 S upon glycerol density gradient centrifugation, and both activities are sensitive to heat or N-ethylmaleimide. Purified sRF does not dephosphorylate eIF-2 whose phosphorylation has been catalyzed by HRI, nor does the sRF prevent the phosphorylation of eIF-2 by HRI in a fractionated system. sRF stimulates ternary complex formation by both phosphorylated and nonphosphorylated eIF-2. These observations suggest that the sensitivity of protein synthesis to phosphorylation of eIF-2 by HRI may be modulated by the concentration and activity of sRF.

Protein synthesis in rabbit reticulocytes: mechanism of protein synthesis inhibition by heme-regulated inhibitor

Partially purified Met-tRNAf binding factor, eIF-2, was phosphorylated by using heme-regulated inhibitor (HRI). Phosphorylated eIF-2 was freed from HRI by phosphocellulose column chromatography. Analysis by isoelectric focusing showed 100% phosphorylation of the 38,000-dalton subunit of eIF-2. Both eIF-2 and eIF-2(P) formed ternary complexes with Met-tRNAf and GTP with almost the same efficiency, and in both cases the ternary complex formation was drastically inhibited by prior addition of Mg2+. However, whereas the ternary complexes formed with eIF-2 could be stimulated by Co-eIF-2C at 1 mM Mg2+ and dissociated by Co-eIF-2B at 5 mM Mg2+, the ternary complexes formed with eIF-2(P) were unresponsive to both Co-eIF-2B and Co-e-IF-2C. Also, under conditions of eIF-2 phosphorylation, HRI drastically inhibited AUG-dependent Met-tRNAf binding to 40S ribosomes. However, HRI (in the presence of ATP) had no effect on the joining of preformed Met-tRNAf . 40S . AUG complex to the 60S ribosomal subunit to form Met-tRNAf-80S . AUG complex. These studies suggest that HRI inhibits protein synthesis initiation by phosphorylation of the 38,000-dalton subunit of eIF-2. HRI-phosphorylated eIF-2 does not interact with at least two other protein factors, Co-eIF-2B and Co-eIF-2C, and is thus inactive in protein synthesis initiation.

Purification and characterization of a protein factor that reverses the inhibition of protein synthesis by the heme-regulated translational inhibitor in rabbit reticulocyte lysates

We have purified and partially characterized a supernatant factor which reverses the effect of the heme-regulated translational inhibitor on protein synthesis in rabbit reticulocyte lysates. The anti-inhibitor restores protein synthesis activity in heme deficient lysates (and in lysates to which the inhibitor has been added) to the level observed in the presence of heme. The factor has no effect on the phosphorylation of eIF-2 by the inhibitor nor on any reaction carried out with purified initiation factors. The anti-inhibitor probably consists of three subunits with molecular weights of 81000, 60000 and 41000. The factor is isolated from the postribosomal supernatant of rabbit reticulocytes both free and complexed to eIF-2. A possible mechanism of action is discussed.

In situ phosphorylation of the alpha subunit of eukaryotic initiation factor 2 in reticulocyte lysates inhibited by heme deficiency, double-stranded RNA, oxidized glutathione, or the heme-regulated protein kinase

Protein synthesis initiation in reticulocyte lysates is inhibited by heme deficiency, low levels of double-stranded RNA (dsRNA), oxidized glutathione (GSSG), or the purified kinase (HRI) that acts on the alpha polypeptide of eukaryotic initiation factor 2 (eIF-2alpha). The phosphoprotein profiles produced in lysates in response to these various conditions have been monitored directly in lysates after labeling for brief periods with pulses of [gamma-(32)P]ATP. The [(32)P]phosphoprotein profiles were analyzed by electrophoresis in sodium dodecyl sulfate/polyacrylamide slab gels under conditions in which the HRI and eIF-2alpha polypeptides were clearly distinguished. All four modes of inhibition produced a rapid phosphorylation of eIF-2alpha compared to control lysates, which displayed little or no phosphorylation of eIF-2alpha. In heme-deficient lysates, phosphorylation of eIF-2alpha occurred rapidly both before and after the shut-off of protein synthesis; the delayed addition of hemin to these lysates resulted in a decrease in the phosphorylation of eIF-2alpha and the subsequent restoration of protein synthesis. These data suggest that rapid turnover of phosphate occurs at the site(s) of eIF-2alpha phosphorylation. In lysates inhibited by heme deficiency, GSSG, or added HRI, the phosphorylation of eIF-2alpha was accompanied by the rapid in situ phosphorylation of HRI. The inhibition of initiation induced by dsRNA was accompanied by the phosphorylation of eIF-2alpha and a 67,000-dalton polypeptide but not HRI. These observations in situ indicate that (i) the phosphorylation of eIF-2alpha is the critical event in these inhibitions of protein chain initiation, and (ii) the phosphorylation of HRI is associated with its activation in heme deficiency.

Regulation of protein synthesis in reticulocyte lysates: immune serum inhibits heme-regulated protein kinase activity and differentiates heme-regulated protein kinase from double-stranded RNA-induced protein kinase

A specific immune serum to the heme-regulated inhibitor (HRI) has been prepared by immunizing chickens with highly purified reversible HRI prepared from rabbit reticulocyte lysates. Studies with this immune serum demonstrate that the behavior of purified reversible HRI is similar to that of the inhibitor activated in rabbit reticulocyte lysates: the immune serum (i) inhibits the phosphorylation of the small subunit (38,000 daltons) of the eukaryotic initiation factor eIF-2 by both crude and purified inhibitor preparations; (ii) prevents the concomitant inhibition of protein synthesis by both crude and purified inhibitor preparations; and (iii) prevents the autophosphorylation of the 95,000-dalton polypeptide in purified and crude HRI preparations. The protein kinase and inhibitory activities of crude and partially purified preparations of the double-stranded RNA-induced inhibitor of protein synthesis are not affected by the immune serum prepared to reversible HRI. These results indicate that the inhibitor induced by double-stranded RNA is antigenically distinct from the reversible HRI.

Control of eIF-2 phosphatase activity in rabbit reticulocyte lysate

A sensitive assay requiring picomole amounts of [32P]eIF-2 to measure eIF-2 phosphatase activity has been developed. Dephosphorylation of [32P]eIF-2 alpha (38,000-dalton subunit) is extremely rapid (t 1/2 = 20 sec) and occurs at the same rate in both hemin-supplemented and hemin-depleted lysates. In contrast, [32P]eIF-2 beta phosphate is stable under all conditions studied. At concentrations required to produce a transient inhibition of protein synthesis, GDP prevents dephosphorylation of half of the phosphate introduced on eIF-2 alpha by the hemin-controlled repressor. Equimolar GTP is without effect. The concept that the energy charge of the guanylate pool may regulate accessibility of a phosphorylated site on eIF-2 alpha to its phosphatase and the implication of this to the mechanism of hemin-regulated translational control are discussed.

Regulation of protein synthesis in rabbit reticulocyte lysates: additional initiation factor required for formation of ternary complex (eIF-2.GTP.Met-tRNAf) and demonstration of inhibitory effect of heme-regulated protein kinase

Heme deficiency in rabbit reticulocytes and their lysates leads to the activation of a heme-regulated translational inhibitor (HRI) which causes the cessation of polypeptide initiation. HRI is a protein kinase that specifically phosphorylates the 38,000-dalton subunit of eukaryotic initiation factor 2 (eIF-2). eIF-2 binds Met-tRNA(f) and GTP in ternary complex. As a continuation of the studies on the molecular basis of the inhibition of the formation of 40S ribosomal subunit-Met-tRNA(f) complexes by HRI [Ranu, R. S., London, I. M., Das, A., Dasgupta, A., Majumdar, A., Ralston, R., Roy, R. & Gupta, N. K. (1978) Proc. Natl. Acad. Sci. USA 75, 745-749], we describe here the isolation and some characteristics of a factor that is required for the HRI-catalyzed inhibition of eIF-2-promoted ternary complex formation. In the presence of 1 mM Mg(2+), ternary complex formation by eIF-2 is dependent on the presence of this stabilization factor (SF). Under these conditions, SF increases the rate and the extent of ternary complex formation. This finding suggests that the interaction of SF with eIF-2 causes a conformational change that stabilizes eIF-2 and promotes efficient ternary complex formation by increasing the affinity of eIF-2 for GTP and Met-tRNA(f). In the absence of Mg(2+), however, eIF-2 efficiently forms the ternary complex and SF has little effect on its ternary complex formation capacity-hence, the name eIF-2 stabilization factor (SF). In the presence of SF, HRI markedly inhibits (70-80%) the ternary complex formation capacity of eIF-2. The inhibitory effect requires both HRI and ATP. Under these conditions, HRI phosphorylates only the 38,000-dalton subunit of eIF-2. Both the rate and the extent of the SF-dependent ternary complex formation are inhibited. These findings are consistent with the idea that phosphorylation causes a conformational change in eIF-2 such that its interactions with other initiation factors in the formation and the binding of ternary complex to 40S ribosomal subunits are inhibited.

Protein synthesis in rabbit reticulocytes: characteristics of a ribosomal factor that reverses inhibition of protein synthesis in heme-deficient lysates

A ribosomal salt (0.5 M KCl) wash factor (RF) that reverses inhibition of protein synthesis in heme-deficient reticulocyte lysates has been resolved from the bulk of Met-tRNAfMet-binding factor (EIF-1), Co-EIF-1, and EIF-2 (ternary complex dissociation factor, TDF). The purified RF restores protein synthesis activity of heme-deficient lysates to the level observed in the presence of hemin. No direct correlation exists between amount of EIF-1 activity and ability to reverse inhibition of protein synthesis in heme-deficient lysates. Homogeneous preparations of EIF-1 are completely inactive in reversal of protein synthesis inhibition in heme-deficient lysates. These findings suggest that RF activity is not due to EIF-1 alone but may or may not require EIF-1 as a component of a complex factor.

Inhibition of protein synthesis initiation by oxidized glutathione: activation of a protein kinase that phosphorylates the alpha subunit of eukaryotic initiation factor 2

Oxidized glutathione (GSSG) (0.02-0.5 mM) inhibits reticulocyte lysates by a mechanism similar to that observed in heme deficiency. Incubation of hemin-supplemented postribosomal supernates with GSSG results in the activation of a translational inhibitor [I(GSSG)]. The activation of I(GSSG) is enhanced by the presence of an energy-regenerating system. The simultaneous addition of 1 mM dithiothreitol blocks the activation of the GSSG-induced inhibitor; however, once inhibitor is formed, its activity is not affected by 1 mM dithiothreitol. GSSG-treated postribosomal supernates and partially purified preparations of I(GSSG) inhibit protein synthesis in hemin-supplemented lysates with biphasic kinetics. Inhibition by I(GSSG) is blocked by cyclic AMP (2-10 mM) and is potentiated by ATP (2 mM). The inhibition is also blocked or reversed by eukaryotic initiation factor eIF-2. The activation of I(GSSG) is accompanied by an increased cyclic AMP-independent protein kinase activity which phosphorylates the 38,000-dalton component (alpha subunit) of eIF-2; however, GSSG treatment of supernates does not alter the activity of the cyclic AMP-independent protein kinase activity that phosphorylates the 49,000-dalton polypeptide component (beta subunit) of eIF-2. These data indicate that GSSG treatment of reticulocyte lysates results in the activation of a protein kinase with inhibitory and phosphorylation properties similar to those of the heme-regulated cyclic AMP-independent protein kinase which is activated in heme deficiency.

Effect of starvation on initiation of protein synthesis in skeletal muscle and heart

Psoas muscle of rats starved for 2 or 4 days contained increased levels of ribosomal subunits and exhibited reduced rates of protein synthesis in vitro, demonstrating a starvation-induced inhibition of peptide-chain initiation. The activity of an eIF-2-like initiation factor, assayed in postribosomal supernatants, decreased in psoas during starvation, parallel to a 25% reduction in the RNA level. Reduced eIF-2 activity did not result from nucleotide depletion or increased deacylation of initiator tRNA, nor was it abolished by extensive dialysis. Perfusion of psoas muscle in the presence of insulin reversed the starvation-induced block in peptide-chain initiation, but did not alter the activity of eIF-2 or level of RNA. Furthermore, heart muscle did not manifest a starvation-induced block in peptide-chain initiation even though the activity of eIF-2 and the level of RNA decreased as a result of food deprivation. Thus loss of eIF 2 activity in psoas and heart did not parallel changes in peptide-chain initiation but was associated with a reduction in tissue RNA. These results indicate that the level of eIF-2 is not rate-limiting for peptide-chain initiation under the conditions tested in this study.

Regulation of protein synthesis in rabbit reticulocyte lysates: purification and characterization of heme-reversible translational inhibitor

To define the mechanism of regulation of the protein kinase that is activated in heme deficiency and that inhibits initiation of protein synthesis, we have isolated and purified the heme-reversible form of the protein kinase from rabbit reticulocytes. The inhibitory activity is found in a single band after polyacrylamide gel electrophoresis under nondenaturing conditions. It migrates as a 95,000-dalton polypeptide in 15% sodium dodecyl sulfate/polyacrylamide gels. This purified inhibitor becomes self-phosphorylated in the presence of ATP; the phosphorylated protein and the inhibitory activity copurify. The inhibitor produces characteristic biphasic kinetics of inhibition in reticulocyte lysates and phosphorylates the 38,000-dalton subunit of eukaryotic initiation factor 2 (eIF-2); the inhibition is reversed by added eIF-2. In contrast to the heme-irreversible inhibitor, this heme-reversible inhibitor is no longer inhibitory after incubation with 20 micron hemin. Incubation with hemin also inhibits self-phosphorylation. Preincubation of the heme-reversible inhibitor in the presence of ATP potentiates the inhibition of protein synthesis in the subsequent incubation, as does treatment with N-ethylmaleimide. Phosphorylation of the heme-reversible inhibitor and inhibition of protein synthesis in the lysate due to phosphorylation of eIF-2 appear to be related. These findings suggest that hemin acts directly on the heme-reversible inhibitor.

Regulation of protein synthesis: activation by double-stranded RNA of a protein kinase that phosphorylates eukaryotic initiation factor 2

Incubation of reticulocyte lysates or isolated crude ribosomes with low levels of double-stranded RNA (0.1-10 ng/ml) induces the formation of an inhibitor of protein synthesis initiation similar to that observed in heme deficiency. The inhibitor is associated with a cyclic AMP-independent protein kinase activity (ATP:protein phosphotransferase, EC 2.7.1.37) that phosphorylates the small polypeptide (38,000 daltons) of the eukaryotic initiation factor eIF-2. Activation of the inhibitor requires ATP in addition to double-stranded RNA and is accompanied by the phosphorylation of a 67,000-dalton polypeptide of unknown function. The inhibitor remains associated with the ribosomes during high-speed sedimentation. Once formed, the ribosome-associated inhibitor phosphorylates eIF-2 and inhibits protein synthesis in the absence of double-stranded RNA. Inhibition is prevented by exogenous eIF-2. The bound inhibitor can be solubilized by extraction with 0.5 M KCl. The soluble inhibitor preparation retains the ability to phosphorylate the small polypeptide of eIF-2 and to inhibit protein synthesis. Untreated crude ribosomes also contain cyclic AMP-independent protein kinase activities that phosphorylate the middle polypeptide (49,000 daltons) of eIF-2 and several polypeptide subunits of eIF-3 (160,000, 125,000, and 65,000 daltons); these kinase activities are not affected by double-stranded RNA and do not inhibit protein synthesis.

Regulation of protein synthesis in rabbit reticulocyte lysates by the heme-regulated protein kinase: inhibition of interaction of Met-tRNAfMet binding factor with another initiation factor in formation of Met-tRNAfMet.40S ribosomal subunit complexes

Protein synthesis in reticulocytes and their lysates is regulated by heme. In heme deficiency a heme-regulated translational inhibitor (HRI) that blocks initiation of polypeptide chains is activated. HRI is a protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) that specifically phosphorylates the 38,000-dalton subunit of the Met-tRNA(f) (Met) binding factor (IF), which forms a ternary complex with Met-tRNA(f) (Met) and GTP, a finding that suggests that the inhibition by HRI involves the phosphorylation of IF. We have investigated the effect of HRI in the partial reactions of protein chain initiation in which the IF-promoted binding of Met-tRNA(f) (Met) to 40S ribosomal subunits is enhanced by another initiation factor [ternary complex dissociation factor (TDF)] and AUG. The results show that HRI at very low concentrations markedly inhibits the binding of Met-tRNA(f) (Met) to 40S subunits. The inhibitory effect of HRI requires ATP. Under these conditions HRI phosphorylates only the 38,000-dalton subunit of IF. The TDF preparations not only promote the binding of the ternary complex to 40S subunits but also promote the dissociation of the ternary complex in the presence of 5 mM Mg(2+) at 0 degrees . The preincubation of purified IF alone with low concentrations of HRI and ATP does not significantly affect its capacity to form the ternary complex; however, the TDF-promoted dissociation of the ternary complex is inhibited. The nonhydrolyzable analog adenosine 5'-[beta,gamma-imido]triphosphate does not substitute for ATP. These findings suggest that phosphorylation causes a conformational modification in IF, which results in inhibition of the interaction between the ternary complex and TDF that is required for the binding of the ternary complex to 40S subunits.

Effect of hemin on site-specific phosphorylation of eukaryotic initiation factor 2

Initiation factor 2 (eIF-2) is phosphorylated in vitro by two different cyclic nucleotide-independent protein kinases. As previously shown, a protein kinase activity that comigrates with the major casein kinase activity from rabbit reticulocytes phosphorylates eIF-2beta. In addition, a second protein kinase that specifically phosphorylates eIF-2alpha has been identified. Both protein kinase activities demonstrate cyclic nucleotide-independent activity and are not inhibited by the inhibitor protein diagnostic for cyclic AMP-regulated protein kinase activities. Phosphorylation of eIF-2alpha is almost completely inhibited by 20--35 muM hemin, whereas phosphorylation of eIF-2beta is only partially inhibited. Hemin acts by decreasing the rate of incorporation of phosphate into eIF-2alpha. The protein kinase activity that modifies eIF-2alpha has been shown to have inhibitory activity in the cell-free protein-synthesizing system, whereas the protein kinase for eIF-2beta has no effect. The identity of the former enzyme with the hemin-controlled repressor and role of hemin in the control of initiation are discussed.

Phosphorylated proteins as physiological effectors

A variety of neurotransmitters, hormones, and other regulatory agents affect the phosphorylation of specific proteins in their target tissues. The types of stimuli that share this common effect on protein phosphorylation include numerous substances that do not act through cyclic AMP. These and other observations suggest that many different classes of regulatory substances achieve certain of their biological effects by altering the phosphorylation of specific proteins.

Mode of action of the hemin-controlled inhibitor of protein synthesis

Despite the finding that the hemin-controlled translational inhibitor in reticulocyte lysates is a cyclic AMP-independent protein kinase that phosphorylates the small subunit of the initiation factor eIF-2, the mechanism of inhibition of translation remained unexplained. Whereas treatment of hemin-containing lysates with inhibitor in the presence of ATP inhibited translation, the same treatment of highly purified eIF-2 did not affect its ability to form a ternary complex with initiator Met-tRNA and GTP or a 40S initiation complex. We have isolated from ribosomal salt washes a protein (eIF-2 stimulating protein) that enhances the capacity of unphosphorylated eIF-2 to form ternary or 40S initiation complexes but has no effect on the phosphorylated factor. At low concentrations, eIF-2 is virtually inactive without this stimulating protein. Therefore, the translational inhibitor acts by converting eIF-2 to a form that is not stimulated by the stimulating protein.

Binding and release of eukaryotic initiation factor eIF-2 and GTP during protein synthesis initiation

The eukaryotic initiation factor eIF-2 forms a ternary complex with Met-tRNAf and GTP. This complex binds to the 40S ribosomal subunit in the absence of mRNA and mRNA binding factors. Highly purified eIF-2 from rabbit reticulocytes was labeled with 125I by using the Bolton-Hunter reagent or with [gamma-32P]ATP by using the heme-regulated translational inhibitor protein kinase. The labeled eIF-2 was bound, together with equimolar amounts of Met-tRNAf and GTP, to the 40S subunit. In the presence of mRNA, mRNA binding factors, and 60S ribosomal subunits (complete initiation assay), eIF-2 was released from the 40S initiation complex in the subunit joining reaction. GTP also was released in this step and probably was hydrolyzed in the reaction that is dependent upon eIF-5 and the 60S subunit. The function of phosphorylated eIF-2 in initiation of protein synthesis is discussed.

Purification and properties of a ribosomal casein kinase from rabbit reticulocytes

A casein kinase was isolated and purifed from rabbit reticulocytes. About 90% of the enzyme activity co-sedimented with the ribosomal fraction, whereas about 10% of the enzyme activity was found in the ribosome-free supernatant. Both casein kinases (the ribosome-bound enzyme as well as the free enzyme) showed identical activity and the same molecular weight. On sodium dodecyl sulphate/polyacrylamide-gel electrophoresis a single band of about 70 000 mol.wt. was observed. Sucrose-gradient analysis, however, showed that the enzyme activity sedimented with a s20,w of approx. 7.5S. This observation suggested that the casein kinase is a dimer composed of subunits of identical molecular weight. The enzyme utilizes GTP as well as ATP as a phosphoryl donor. It preferentially phosphorylates acidic proteins, in particular the model substrates casein and phosvitin. Casein kinase is cyclic AMP-indepenoent. The Km values for ATP and GTP with phosvitin as a substrate were determined as 1.2 and 8.8 micrometer respectively.