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

Temporal alterations of the nascent proteome in response to mitochondrial stress

Under stress, protein synthesis is attenuated to preserve energy and mitigate challenges to protein homeostasis. Here, we describe, with high temporal resolution, the dynamic landscape of changes in the abundance of proteins synthesized upon stress from transient mitochondrial inner membrane depolarization. This nascent proteome was altered when global translation was attenuated by stress and began to normalize as translation was recovering. This transition was associated with a transient desynchronization of cytosolic and mitochondrial translation and recovery of cytosolic and mitochondrial ribosomal proteins. Further, the elongation factor EEF1A1 was downregulated upon mitochondrial stress, and its silencing mimicked the stress-induced nascent proteome remodeling, including alterations in the nascent respiratory chain proteins. Unexpectedly, the stress-induced alterations in the nascent proteome were independent of physiological protein abundance and turnover. In summary, we provide insights into the physiological and pathological consequences of mitochondrial function and dysfunction.

Lost in Translation: Evidence for Protein Synthesis Deficits in ALS/FTD and Related Neurodegenerative Diseases

Cells utilize a complex network of proteins to regulate translation, involving post-transcriptional processing of RNA and assembly of the ribosomal unit. Although the complexity provides robust regulation of proteostasis, it also offers several opportunities for translational dysregulation, as has been observed in many neurodegenerative disorders. Defective mRNA localization, mRNA sequatration, inhibited ribogenesis, mutant tRNA synthetases, and translation of hexanucleotide expansions have all been associated with neurodegenerative disease. Here, we review dysregulation of translation in the context of age-related neurodegeneration and discuss novel methods to interrogate translation. This review primarily focuses on amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), a spectrum disorder heavily associated with RNA metabolism, while also analyzing translational inhibition in the context of related neurodegenerative disorders such as Alzheimer's disease and Huntington's disease and the translation-related pathomechanisms common in neurodegenerative disease.

eIF2α phosphorylation as a biomarker of immunogenic cell death

Cancer cells exposed to some forms of chemotherapy and radiotherapy die while eliciting an adaptive immune response. Such a functionally peculiar variant of apoptosis has been dubbed immunogenic cell death (ICD). One of the central events in the course of ICD is the activation of an endoplasmic reticulum (ER) stress response. This is instrumental for cells undergoing ICD to emit all the signals that are required for their demise to be perceived as immunogenic by the host, and culminates with the phosphorylation of eukaryotic translation initiation factor 2α (eIF2α). In particular, eIF2α phosphorylation is required for the pre-apoptotic exposure of the ER chaperone calreticulin (CALR) on the cell surface, which is a central determinant of ICD. Importantly, phosphorylated eIF2α can be quantified in both preclinical and clinical samples by immunoblotting or immunohistochemistry using phosphoneoepitope-specific monoclonal antibodies. Of note, the phosphorylation of eIF2α and CALR exposure do not necessarily correlate with each other, and neither of these parameters is sufficient for cell death to be perceived as immunogenic. Nonetheless, accumulating data indicate that assessing the degree of phosphorylation of eIF2α provides a convenient parameter to monitor ICD. Here, we discuss the role of the ER stress response in ICD and the potential value of eIF2α phosphorylation as a biomarker for this clinically relevant variant of apoptosis.

The effects of pyrroloquinoline quinone on heme-regulated eIF-2alpha kinase and eIF-2B activities in eukaryotic protein synthesis

Pyrroloquinoline quinone (PQQ), a novel cofactor of biological redox processes, is ubiquitous in animal cells. We have examined the effects of PQQ on protein synthesis. PQQ inhibits protein synthesis in hemin-supplemented rabbit reticulocyte lysates. This inhibition is characterized by increased phosphorylation of eIF-2alpha and by diminished guanine nucleotide exchange activity of eIF-2B. The increased eIF-2alpha phosphorylation is the result of activation by PQQ of the heme-regulated eIF-2alpha kinase (HRI). The addition of 10 microM PQQ completely inhibits the increase in protein synthesis that occurs on the addition of hemin (20 microM) to heme-deficient lysates, whereas a lower concentration of PQQ (100 nM) causes a very slight stimulation of protein synthesis. The increased eIF-2alpha phosphorylation that occurs at high concentrations of PQQ inhibits eIF-2B activity, presumably due to formation of a 15S complex [eIF-2(alphaP).eIF-2B] in which eIF-2B becomes non-functional. Low concentrations of PQQ (0.1-1 microM) do not affect eIF-2alpha phosphorylation, but rather enhance the guanine nucleotide exchange activity of eIF-2B in reticulocyte lysates. In Chinese hamster ovary cell extract which is devoid of significant eIF-2alpha kinase activity, addition of both low and high concentrations of PQQ results in an increase in eIF-2B activity. The addition of PQQ to reticulocyte lysates activates HRI whereas addition of PQQ to purified HRI in vitro inhibits the autokinase and eIF-2alpha kinase activity of the HRI; the inhibition of purified HRI by PQQ is observed both in the presence and absence of hemin. These findings suggest that PQQ inhibits purified HRI by acting as an oxidant whereas in lysates in which PQQ is readily reduced, the PQQ acts as a reductant and increases the activities of both HRI and eIF-2B.

Erythroid expression of the heme-regulated eIF-2 alpha kinase

The role of heme-regulated eIF-2 alpha kinase (HRI) in the regulation of protein synthesis in rabbit reticulocytes is well documented. Inhibitors of protein synthesis with properties similar to those of HRI have been described in some nonerythroid cell types, but it has not yet been determined whether these eIF-2 alpha kinase activities are mediated by HRI or one or more as yet uncharacterized kinases. We have studied the expression of mRNA, polypeptide, and kinase activities of HRI in various tissues from both nonanemic and anemic rabbits. Our results indicate that HRI is expressed in an erythroid cell-specific manner. HRI is present in the bone marrow and peripheral blood of both nonanemic and anemic rabbits but not in any of the other tissues tested. HRI mRNA is present at low levels in uninduced mouse erythroleukemic (MEL) cells and human K562 cells and accumulates to higher levels upon induction. The accumulation of HRI mRNA in differentiating MEL cells is dependent upon the presence of heme. The addition of 3-amino-1,2,4-triazole (AT), an inhibitor of heme biosynthesis, to the induction medium markedly reduced HRI mRNA accumulation. Simultaneous addition of hemin and AT to the dimethyl sulfoxide induction medium largely prevented the inhibition of HRI mRNA induction by AT. These findings indicate that HRI is expressed in an erythroid cell-specific manner and that the major physiologic role of HRI is in adjusting the synthesis of globins to the availability of heme.

Erythroid expression of the heme-regulated eIF-2 alpha kinase

The role of heme-regulated eIF-2 alpha kinase (HRI) in the regulation of protein synthesis in rabbit reticulocytes is well documented. Inhibitors of protein synthesis with properties similar to those of HRI have been described in some nonerythroid cell types, but it has not yet been determined whether these eIF-2 alpha kinase activities are mediated by HRI or one or more as yet uncharacterized kinases. We have studied the expression of mRNA, polypeptide, and kinase activities of HRI in various tissues from both nonanemic and anemic rabbits. Our results indicate that HRI is expressed in an erythroid cell-specific manner. HRI is present in the bone marrow and peripheral blood of both nonanemic and anemic rabbits but not in any of the other tissues tested. HRI mRNA is present at low levels in uninduced mouse erythroleukemic (MEL) cells and human K562 cells and accumulates to higher levels upon induction. The accumulation of HRI mRNA in differentiating MEL cells is dependent upon the presence of heme. The addition of 3-amino-1,2,4-triazole (AT), an inhibitor of heme biosynthesis, to the induction medium markedly reduced HRI mRNA accumulation. Simultaneous addition of hemin and AT to the dimethyl sulfoxide induction medium largely prevented the inhibition of HRI mRNA induction by AT. These findings indicate that HRI is expressed in an erythroid cell-specific manner and that the major physiologic role of HRI is in adjusting the synthesis of globins to the availability of heme.

Autophosphorylation: a salient feature of protein kinases

Most protein kinases catalyze autophosphorylation, a process which is generally intramolecular and is modulated by regulatory ligands. Either serine/threonine or tyrosine serves as the phosphoacceptor, and several sites on the same kinase subunit are usually autophosphorylated. Autophosphorylation affects the functional properties of most protein kinases. Members of the protein kinase family exhibit diversity in the characteristics and functions of autophosphorylation, but certain common themes are emerging.

Content of low-molecular-weight thiols during the imbibition of Pea seeds

The metabolism of low-molecular-weight thiols was investigated in seeds of Pisum sativum L. cv. Kleine Rheinländerin during imbibition in water for 14 h. The amount of oxidized glutathione (GSSG) decreased from 319 nmol (g dry weight)^-1 in dry seeds to 38 nmol (g dry weight)^-1 within the first 14 h of imbibition. The decrease may have been due to the reduction of GSSG to reduced glutathione (GSH), catalyzed by the enzyme glutathione reductase (GR; EC 1.6.4.2). The enzyme activity was high in dry seeds [25 nkat (g dry weight)^-1 ] and decreased to 20 nkat (g dry weight)^-1 within 14 h of imbibition. The activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) decreased from 100 nkat (g dry weight)^-1 in dry seeds to 67 nkat (g dry weight)^-1 after 14 h of imbibition. Within 14 h the amount of γ-glutamyl-cysteine (γ-GC) decreased from 135 to 38 nmol (g dry weight)^-1 , whereas the cysteine content rose from 81 nmol (g dry weight)^-1 in dry seeds to a maximum of 170 nmol (g dry weight)^-1 after 12 h of imbibition, which may be due to the degradation of γ-GC into cysteine.

The substrate specificity of protein kinases which phosphorylate the alpha subunit of eukaryotic initiation factor 2

The alpha subunit of eukaryotic protein synthesis initiation factor (eIF-2 alpha) is phosphorylated at a single serine residue (Ser51) by two distinct and well-characterized protein kinase, the haem-controlled repressor (HCR) and the double-stranded RNA-activated inhibitor (dsI). The sequence adjacent to Ser51 is rich in basic residues (Ser51-Arg-Arg-Arg-Ile-Arg) suggesting that they may be important in the substrate specificity of the two kinases, as is the case for several other protein kinases. A number of proteins and synthetic peptides containing clusters of basic residues were tested as substrates for HCR and dsI. Both kinases were able to phosphorylate histones and protamines ar multiple sites as judged by two-dimensional mapping of the tryptic phosphopeptides. These data also showed that the specificities of the two kinases were different from one another and from the specificities of two other protein kinases which recognise basic residues, cAMP-dependent protein kinase and protein kinase C. In histones, HCR phosphorylated only serine residues while dsI phosphorylated serine and threonine. Based on phosphoamino acid analyses and gel filtration of tryptic fragments, dsI was capable of phosphorylating both 'sites' in clupeine Y1 and salmine A1, whereas HCR acted only on the N-terminal cluster of serines in these protamines. The specificities of HCR and dsI were further studied using synthetic peptides with differing configurations of basic residues. Both kinases phosphorylated peptides containing C-terminal clusters of arginines on the 'target' serine residue, provided that they were present at positions +3 and/or +4 relative to Ser51. However, peptides containing only N-terminal basic residues were poor and very poor substrates for dsI and HCR, respectively. These findings are consistent with the disposition of basic residues near the phosphorylation site in eIF-2 alpha and show that the specificities of HCR and dsI differ from other protein kinases whose specificities have been studied.

Differential effect of hemin-controlled eIF-2 alpha kinases from mouse erythroleukemia cells on protein synthesis

Cultured mouse erythroleukemia (MEL) cells can be induced to erythroid differentiation by a variety of chemical agents. This differentiation process is marked by the onset of globin mRNA and hemoglobin synthesis. In rabbit reticulocytes, globin synthesis is regulated by a hemin-controlled translational inhibitor (HCI) which acts via phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2). From both uninduced and induced MEL cells, hemin-controlled eIF-2 alpha kinases have been partially purified. They resemble HCI with respect to their chromatographic behaviour and their sensitivity towards physiological concentrations of hemin (5-10 microM). Further purification on phosphocellulose, however, reveals that the eIF-2 alpha kinase from uninduced MEL cells is chromatographically distinct from HCI, whilst the eIF-2 alpha kinase activity from induced MEL cells represents a mixture of the former and the HCI-type eIF-2 alpha kinase. The latter inhibits protein synthesis in a fractionated system from rabbit reticulocytes which is free of, but sensitive to, HCI, whereas the eIF-2 alpha kinase from uninduced MEL cells does not show any inhibitory activity. This observation is supported by the finding that induced MEL cells respond in vivo to iron depletion with a shut-off of protein synthesis (as do rabbit reticulocytes), whilst uninduced MEL cells do not.

Effect of vasopressin on the regulation of protein synthesis initiation in liver cells

Vasopressin was found to be an effective inhibitor of protein labelling in isolated liver cells. Its effect shows the following distinct characteristics: (1) in contrast with alpha-adrenergic agonists, its effect is observable under a wide range of cellular Ca2+-loading conditions; (2) it is not influenced by the nutritional state of the animal. The lack of vasopressin effect on valine production, and its ability to decrease protein labelling from near-saturation concentrations of [3H]valine, indicate that the observed variations in protein labelling reflect actual changes in the rate of protein synthesis. The action of vasopressin is primarily exerted on the initiation step of protein synthesis and this effect is accompanied by a decreased activity of eukaryotic initiation factor 2. Activators of protein kinase C showed similar but not additive effects on protein synthesis, as did vasopressin. It seems plausible to conclude that protein kinase C activation may play an important regulatory role in hepatic protein synthesis as a transducer of hormonal and perhaps other type of signals.

Correlation between the distribution of the reversing factor and eukaryotic initiation factor 2 in heme-deficient or double-stranded RNA-inhibited reticulocyte lysates

The recycling of eukaryotic initiation factor eIF-2 requires the exchange of GDP for GTP, in a reaction catalyzed by the reversing factor (RF). Recent studies have suggested that a 60 S ribosomal subunit-bound eIF-2.GDP complex is an intermediate in protein chain initiation. We have monitored the distribution of RF in heme-deficient and dsRNA-inhibited lysates by immunoblot analysis of sucrose gradient fractions and have compared the distribution with that of eIF-2(alpha-32P). RF and eIF-2(alpha P) were both found to be tightly associated with 60 S and 80 S ribosomes, as their distribution did not change in gradients containing up to 0.1 M K+. The association of eIF-2(alpha-32P) and RF with 60 S and 80 S ribosomes was enhanced in the presence of F-, indicating the presence of an endogenous ribosome-associated phosphatase activity which is capable of dephosphorylating eIF-2(alpha P) in the absence of F-. These observations are consistent with the hypothesis that under physiologic conditions, RF interacts with the 60 S-bound eIF-2.GDP complex to promote the dissociation of GDP from eIF-2 and the release of eIF-2 from the 60 S subunit as a complex with RF.

Physiological stresses inhibit guanine-nucleotide-exchange factor in Ehrlich cells

Previously, we have shown that phosphorylation of the eukaryotic initiation factor eIF-2 alpha increases under several physiological stresses in which protein synthesis is inhibited in Ehrlich ascites tumor cells. As phosphorylated eIF-2 [eIF-2(alpha P)] is a potent inhibitor of guanine nucleotide exchange factor (GEF), it seemed likely that it was responsible for the inhibition. We have assayed GEF activity levels in extracts prepared from Ehrlich cells exposed to three such stresses, namely heat shock, serum deprivation and glutamine deprivation. Activity was estimated by the ability of GEF to enhance the release of [alpha-32P]GDP from purified eIF-2 [a modification of the reticulocyte lysate assay of Matts, R. L. & London, I. M. (1984) J. Biol. Chem. 259, 6708]. GEF activity was reduced from control values in extracts of heat-shocked cells and serum-deprived cells, concomitant with increased eIF-2 alpha phosphorylation. Inhibition of GEF activity in heat-shocked and serum-deprived cells was reversed to control levels by increasing the concentration of purified eIF-2.GDP added as substrate in the GEF assay. Since we have shown elsewhere that eIF-2(alpha P).GDP inhibits GEF by competition with eIF-2.GDP, the complete reversal of inhibition of GEF activity in heat-shocked and serum-deprived cells indicates that inhibition is due solely to phosphorylation of eIF-2 alpha. In glutamine-deprived cells phosphorylation of eIF-2 alpha was increased modestly and GEF activity was reduced but GEF activity could not be fully reversed by addition of eIF-2.GDP, suggesting that GEF may also be regulated in other ways. There are greater amounts of GEF relative to eIF-2 in Ehrlich cells (approximately 50%) compared with rabbit reticulocytes (approximately 20%). This explains the efficient rates of protein synthesis in control Ehrlich cells even though they have 30% of their eIF-2 phosphorylated which is enough to inhibit GEF and initiation in reticulocytes completely but only enough to trap approximately 60% of the GEF in Ehrlich cells.

Structure and regulation of eukaryotic initiation factor eIF-2. Sequence of the site in the alpha subunit phosphorylated by the haem-controlled repressor and by the double-stranded RNA-activated inhibitor

Eukaryotic protein synthesis initiation factor 2 (eIF-2) can be phosphorylated on its alpha subunit by two well-characterised protein kinases, termed the haem-controlled repressor (HCR) and the double-stranded RNA-activated inhibitor (dsI). Phosphorylation of eIF-2 by these kinases is thought to be important in the regulation of peptide-chain initiation. We report the location of the serine residue in the alpha subunit, which is phosphorylated by both these enzymes. Limited tryptic digestion and subsequent cyanogen bromide treatment of rat liver eIF-2 phosphorylated by HCR yielded one major phosphopeptide. This peptide had the sequence Ile-Leu-Leu-Ser-Glu-Leu-Ser(P)-Arg-Arg. The same major phosphopeptide was obtained from rabbit reticulocyte eIF-2 phosphorylated by HCR or dsI as judged by its behaviour on two-dimensional mapping and reverse-phase chromatography. In all cases the phosphorylated residue was found to be serine-7, and not serine-4, of the above sequence as determined from sequence analysis and by subdigestion of the peptide with Staphylococcus aureus V8 proteinase.

Glutathione Status and Protein Synthesis during Drought and Subsequent Rehydration in Tortula ruralis

Glutathione status and its relationship to protein synthesis during water deficit and subsequent rehydration have been examined in the drought-tolerant moss, Tortula ruralis. During slow drying there is a small decrease in total glutathione but the percentage of oxidized glutathione (GSSG) increases. During rapid drying there is little change in total glutathione but a small increase in GSSG. On rehydration of slowly dried moss, GSSG rapidly declines to normal level. But when rapidly dried moss is rehydrated, there is an immediate, sharp increase in GSSG as a percentage of total glutathione. After 2 hours of rehydration GSSG starts declining and reaches a normal level in about 6 hours. When an increasing degree of steady state water deficit is imposed on the moss tissue with polyethylene glycol 6000, there is a progressive decrease in protein synthesis but an increase in oxidized glutathione. When 5 millimolar GSSG is supplied exogenously during rehydration of rapidly dried or slowly dried moss, protein synthesis is strongly inhibited. In vitro protein synthesis supported by moss mRNA is also inhibited by more than 85% by 150 micromolar GSSG. The role of glutathione status in water deficit-induced inhibition of protein synthesis is discussed.

Effects on in vitro brain protein synthesis of a translational inhibitor isolated from rabbit brain following intravenous administration of LSD

Inhibition of brain protein synthesis following intravenous administration of d-lysergic acid diethylamide (LSD) is accompanied by generation of a translational inhibitor protein in the postribosomal supernatant of cerebral hemispheres. Addition of an enriched preparation of this factor to a brain cell-free translation system resulted in a selective reduction in the level of phosphorylation of proteins of molecular weight 55K, 41K, and 25K. A similar set of changes was also observed in a brain cell-free system prepared 1 hr subsequent to drug injection. The brain inhibitor reduced the translational capacity of a messenger RNA-dependent reticulocyte lysate programmed with brain polysomes isolated from saline-injected animals however little effect was apparent when polysomes were prepared from LSD-treated animals. The translational inhibitor did not affect the spectrum of translation products from either set of polysomes.

Distribution of reversing factor in reticulocyte lysates during active protein synthesis and on inhibition by heme deprivation or double-stranded RNA

We have recently shown a direct correlation between protein synthetic activity and the function of reversing factor (RF) as a catalyst of GDP-GTP exchange in whole reticulocyte lysates under normal conditions and on inhibition of protein synthesis by heme deficiency, double-stranded RNA, or oxidized glutathione. In this paper we report that RF is detectable as a nonribosomal complex with eukaryotic initiation factor 2 phosphorylated in its alpha subunit [eIF-2(alpha P)] in whole lysates inhibited by heme deprivation or by double-stranded RNA. The complex contains no unphosphorylated eIF-2 alpha, and the GDP present is freely dissociable. All nonribosomal eIF-2(alpha P) is complexed with RF in fully inhibited lysates; we have not detected free eIF-2(alpha P). RF in this [RF X eIF-2(alpha P)] complex is unavailable to catalyze the release of GDP from eIF-2-GDP. Dephosphorylation of eIF-2(alpha P) present in nonribosomal fractions releases active RF, which is able to carry out its normal guanine nucleotide exchange function.

Methylation of elongation factor 1 alpha in mouse 3T3B and 3T3B/SV40 cells

Two-dimensional gel electrophoretic (NEPHGE) analysis of proteins from mouse 3T3B and 3T3B/SV40 cells labelled with [methyl-3H]methionine in the presence of cycloheximide have revealed that the elongation factor 1 alpha (EF-1 alpha) in these cells is methylated and that the extent of methylation is higher in the SV40 transformed cell type. It is suggested that methylation may account for differences in growth properties for the different cell types.

Effects of skeletal muscle protein phosphatase inhibitor-2 on protein synthesis and protein phosphorylation in rabbit reticulocyte lysates

Reticulocyte lysates contain two major classes of protein phosphatase activities, designated type 1 and type 2. These designations are based on criteria derived from the analyses of protein phosphatase species in other tissues. The criteria include (i) chromatographic elution profiles on DEAE-cellulose; (ii) specificity of lysate phosphatases toward [(32)P]phosphorylase a and [(32)P]phosphorylase kinase; (iii) sensitivity of lysate phosphatases to Mg(2+) ATP; and (iv) sensitivity to the heat-stable protein phosphatase inhibitor-2. The lysate phosphatase species are similar to those described in rabbit skeletal muscle and rabbit liver. Reticulocyte protein phosphatase type 1, but not type 2, is inhibited by heat-stable protein phosphatase inhibitor-1 and -2 which have been characterized from rabbit skeletal muscle. We have initiated a study on the function and specificity of lysate protein phosphatase activities involved in the regulation of protein synthesis by examining the effects of protein phosphatase inhibitor-2 on reticulocyte protein synthesis and protein phosphorylation. Our findings are as follows. (a) Protein phosphatase inhibitor-2 inhibits protein chain initiation in hemin-supplemented lysates. (b) Inhibition is characterized by biphasic kinetics and is reversed by the delayed addition of purified reticulocyte eukaryotic initiation factor 2 (eIF-2). (c) Inhibition of protein synthesis by inhibitor-2 is accompanied by the phosphorylation of the alpha-subunit (38,000 daltons) of eIF-2 (eIF-2alpha) and of two heat-stable polypeptides of 29,000 and 44,000 daltons. (d) The 29,000-dalton component is phosphorylated in lysates under conditions of protein synthesis and appears to be inhibitor-2, but the physiological significance of this modification of inhibitor-2 is not clear. (e) Inhibitor-2 has no effect on the activation in vitro of isolated heme-regulated or double-stranded RNA-dependent eIF-2alpha kinases. We propose that the inhibition of protein synthesis in hemin-supplemented lysates by added inhibitor-2 is due at least in part to the inhibition of a type 1 eIF-2alpha phosphatase activity, which permits a basal eIF-2alpha kinase activity to be expressed leading to the accumulation of phosphorylated eIF-2alpha and an inhibition of protein synthesis.

Regulation of double-stranded RNA-activated eukaryotic initiation factor 2 alpha kinase by type 2 protein phosphatase in reticulocyte lysates

Protein synthesis initiation in reticulocyte lysates is inhibited by low concentrations (1-20 ng/ml) of double-stranded RNA (ds RNA) due to the activation of a ds RNA-dependent cAMP-independent protein kinase (ds I) that phosphorylates the alpha subunit of the eukaryotic initiation factor eIF-2. In lysates, ds I is present in the latent inactive form and is associated with the ribosome complement. Latent ds I is solubilized by extraction with high-salt buffers and can be purified in its latent form. Activation of purified latent ds I requires ds RNA and ATP and is accompanied by the ds RNA-dependent autophosphorylation of a polypeptide doublet of 70,000 and 72,000 daltons ("70k/72k"), which represent different phosphorylated states of the same polypeptide. These are phosphorylated in the sequence 70k-->72k; increased phosphorylation of 72k is associated with increased ds I activation. Lysates (or Sepharose 6B ribosomes) treated with ds RNA display a similar ds I phosphoprotein profile, and this is accompanied by the phosphorylation of endogenous eIF-2alpha (38,000 daltons). Delayed (32)P pulses in ds RNA-inhibited lysates indicate that the phosphates on ds I and eIF-2alpha turn over. Under defined conditions, activated ds I in lysates is selectively dephosphorylated by endogenous protein phosphatase(s), and this is accompanied by the dephosphorylation of eIF-2alpha. Similarly, purified activated ds I is rapidly dephosphorylated by unfractionated lysate protein phosphatase(s) and by type 2 protein phosphatase but not by type 1 protein phosphatase. The dephosphorylation of ds I occurs in the sequence 72k-->70k and is correlated with ds I inactivation. The heat-stable protein phosphatase inhibitor-2, which selectively blocks type 1 protein phosphatase, does not significantly affect the dephosphorylation of ds I by type 2 protein phosphatase or by unfractionated lysate phosphatases. The data support the conclusion that a ds I phosphatase activity with type 2 characteristics is involved in the regulation of ds I activity.

Regulation of protein synthesis by phosphorylation of eukaryotic initiation factor 2 alpha in intact reticulocytes and reticulocyte lysates

Studies in intact rabbit reticulocytes and reticulocyte lysates provide further evidence of a functional role for the phosphorylation of eukaryotic initiation factor 2 alpha (eIF-2 alpha) in the regulation of initiation of protein synthesis in eukaryotic cells. In intact reticulocytes treated with isonicotinic acid hydrazide to inhibit heme synthesis, the phosphorylation of eIF-2 alpha was significantly greater than in control cells. In heme-deficient reticulocyte lysates and in lysates treated with double-stranded RNA, significant phosphorylation of eIF-2 alpha occurred prior to the onset of inhibition of protein synthesis; a large proportion, however, of the total eIF-2 alpha remained unphosphorylated. These findings indicate that a modest concentration of phosphorylated eIF-2 alpha can suffice to inhibit initiation, and they suggest that one of the factors with which eIF-2 must interact may be rate limiting, especially when eIF-2 alpha is phosphorylated.

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.

Purification and properties of the double-stranded RNA-activated eukaryotic initiation factor 3 kinase from rabbit reticulocytes

The double-stranded RNA (dsRNA)-activated protein kinase (DAI) that phosphorylates the alpha subunit of the eukaryotic initiation factor eIF-2 and inhibits chain initiation has been isolated from rabbit reticulocyte lysates. The nonactivated enzyme or the enzyme partially activated by incubation with low levels of dsRNA (pro-DAI) could be purified only to a slight extent. However, the enzyme that was fully activated by incubation with both dsRNA and ATP was purified to near homogeneity. Active DAI is a phosphoprotein with an apparent subunit mass of 68,000 daltons. It can phosphorylate histone as well as the alpha subunit of eIF-2. Our results suggest that, after interaction with dsRNA, the enzyme phosphorylates itself and is thereby activated to phosphorylate alpha eIF-2 and histone.

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.