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

The effects of haem on translational control of protein synthesis in cell-free extracts from fed and lysine-derived Ehrlich ascites tumour cells

1. Addition of haem to cell-free extracts from Ehrlich ascites tumour cells stimulates protein synthesis only in extracts from cells previously incubated in nutritionally complete conditions. Extracts from amino-acid-deprived cells do not respond to haem. The stimulation of protein synthesis in fed cell extracts is due to increased initiation on endogenous mRNA mediated by an increase in the levels of 40-S-subunit X Met-tRNA initiation complexes. Extracts from starved cells exhibit a defect in 40-S initiation complex formation which cannot be overcome by haem. 2. Experiments to test for the presence of an inhibitor of initiation in Ehrlich cell extracts by monitoring effects on translation in haem-supplemented reticulocyte lysates have revealed that extracts from both fed and starved cells contain one or more inhibitory activities which shut off protein synthesis, dissagregate polysomes and reduce the level of 40-S initiation complexes in the lysate. Extracts from starved cells are more inhibitory for protein synthesis than those from fed cells. 3. Initiation factor eIF-2 is phosphorylated by an endogenous Ehrlich cell protein kinase in vitro, but this occurs to the same extent in extracts from fed and starved cells. 4. We propose a possible model for the role of eIF-2 in the control of protein synthesis by amino acid supply in Ehrlich cells.

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.

Regulation of translational initiation ina temperature-sensitive mutant of BHK cells

The role of various essential components in mammalian cell protein chain initiation, such as the 40S ribosomal subunit and the initiation factors, have been studied largely in fractionated systems and little is known about the mechanisms of their regulation. The process was studied in an unfractionated system derived from temperature-sensitive (ts) 422E cells. The mutant cells fail to assemble 60S ribosomal subunits at the nonpermissive temperature with little effect on the production of 40S ribosomal subunits. Although the resulting imbalance in ribosomal subunits has little immediate effect on the overall rate of protein synthesis, the nascent 40S subunits in mutant cells appear to be markedly unstable at the nonpermissive temperature. The initial step in protein synthesis, initiation factor elF-2-promoted binding of [35S]Met-tRNAf Met to 40S ribosomal subunits, remains unimparied in ts 422E cells at the nonpermissive temperature. However, most of the nascent 40S ribosomal subunits in ts 422E cells band at a higher density, suggesting their failure to bind initiation factor elF-3. These newly synthesized 40S subunits in the mutant cells are unstable, and may be degraded before their appearance in the polysomes.

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.

A new purification scheme for elongation factor 1 from rabbit reticulocytes and investigation of the homology of the subunits with those of initiation factor 2

The aim of this work was to compare the subunits of the elongation factor EF-1 and the initiation factor eIF-2 from rabbit reticulocytes. We devised a simple procedure for the purification of EF-1: stepwise chromatography on heparin-Sepharose, separation of the heavy form by sucrose gradient centrifugation, and a final step of stepwise chromatography on RNA-Sepharose. The heparin-Sepharose column also clearly separated EF-1 and EF-2 within one chromatographic step. The EF-1 was 350-fold puried and the yield was 10%. This preparation showed after electrophoresis on polyacylamide gels in the presence of sodium dodecyl sulfate three bands corresponding to those described by others as the subunits, with Mr of 54000, 49000 and 29200. An additional band of Mr 34000 was present but no others. The 49000-Mr and 34000-Mr bands corresponded exactly in molecular weight to two of three subunits of eIF-2. A more detailed comparison was therefore made of all subunits of EF-1 and eIF-2. This was done by examination of chymotryptic fingerprints on polyacrylamide gel electrophoresis. No evidence for homology between EF-1 and eIF-2 was found. However, the two larger subunits of eIF-2 had a majority of chymotryptic fragments in common, thus indicating some homology between these polypeptides.

Further studies on the mode of action of the heme-controlled translational inhibitor

We have isolated [de Haro, C. & Ochoa, S. (1978) Proc. Natl. Acad. Sci. USA 75, 2713-2716] a protein factor (eIF-2 stimulating protein, ESP) that is essential for formation of ternary and 40S initiation complexes by the eukaryotic polypeptide chain initiation factor 2 (eIF-2) at the low concentrations of eIF-2 present in reticulocyte lysates. The fact that stimulation of complex formation by ESP is virtually abolished when the small (38,000 daltons) subunit of eIF-2 is phosphorylated by ATP in the presence of eIF-2 kinase (heme-controlled inhibitor, HCI) is consistent with the notion that HCI inhibits translation in lysates by blocking the interaction of eIF-2 with ESP. Our present work, with highly purified eIF-2 and ESP, has additionally established that, unlike phosphorylation of the small subunit, phosphorylation of the middle (52,000 daltons) subunit of eIF-2, which does not lead to translational inhibition in lysates, does not affect eIF-2-ESP interaction. This provides further support for our model of translational inhibition by HCI.

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.

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.

Mode of action of the hemin-controlled inhibitor of protein synthesis: studies with factors from rabbit reticulocytes

Previously [de Haro, C., Datta, A & Ochoa, S. (1978) Proc. Natl. Acad. Sci. USA 75, 243--247] it was shown with initiation factors from Artemia salina embryos that the activity of the initiator methionyl-tRNA binding factor eIF-2 is stimulated by another factor (ESP, for eIF-2 stimulating protein) present, like eIF-2, in ribosomal salt washes. Incubation of eIF-2 with translational inhibitor from rabit reticulocytes, in the presence of ATP, abolished the ESP effect. At physiological concentrations eIF-2 was virtually inactive without ESP. These observations indicated that the translational inhibitor acts by converting eIF-2 to a form that is not stimulated by ESP. The same observations have now been made with eIF-2 and ESP from rabbit reticulocytes but, in this case, the dependence of eIF-2 activity on ESP is much more pronounced than with the A. salina factors. eIF-2 from reticulocytes interacts with ESP from A. salina and conversely.