Differential effect of Mn2+ on the hemin-controlled translational repressor and the double-stranded RNA-activated inhibitor.

The inhibition of protein synthesis that occurs when rabbit reticulocyte lysate is incubated in the absence of hemin is due to the activation of a protein kinase termed the hemin-controlled translational repressor, and that occurring when reticulocyte lysate is incubated with a low level of double-stranded RNA is mediated by the activation of a separate protein kinase termed the double-stranded RNA-activated inhibitor. Both the hemin-controlled translational repressor and the double-stranded RNA-activated inhibitor act by phosphorylating the Mr = 35,000 (alpha) subunit of eIF-2. MnCl2 (0.5 mM) partly reverses the inhibition of protein synthesis produced by hemin deficiency but not that induced by double-stranded RNA. In addition, Mn2+ reverses the inhibition of binding of [35S]Met-tRNAf to reticulocyte ribosomal components, isolated on Sepharose 6B, produced by the hemin-controlled translational repressor but not by the double-stranded RNA-activated inhibitor. The effect of Mn2+ is mediated at the level of activation and eIF-2 alpha kinase activity of these two regulatory protein kinases. Specifically, Mn2+ inhibits activation of the hemin-controlled translational repressor in the absence of hemin and the phosphorylation of eIF-2 alpha by pre-activated translational repressor. In contrast, the phosphorylation of eIF-2 alpha by the double-stranded RNA-activated inhibitor is not suppressed by Mn2+, and the activation and autophosphorylation of this inhibitor is enhanced by Mn2+. Finally, while the activation and inactivation of the hemin-controlled translational repressor does not appear to be mediated by autophosphorylation and dephosphorylation, the activation of the double-stranded RNA-activated inhibitor does appear to require autophosphorylation.

Isolation and characterization of two forms of Met-tRNAf deacylase from rabbit reticulocyte ribosomes.

We have separated and purified two forms of Met-tRNAf deacylase (or two separate enzymes), an activity that mediates in part the suppression of polypeptide chain initiation that occurs in heme deficiency or with double-stranded RNA, 1000-fold from the 0.5 M KCl wash of rabbit reticulocyte ribosomes. Deacylase I is a minor activity with an S20,w of 5.9, D20,w of 4.9 and Mr of 110 000, while deacylase II is the major activity with an S20,w of 3.3, D20,w of 7.1 and Mr of 43 000. Both convert crude reticulocyte or pure yeast, wheat germ, and E. coli [35S]Met-tRNAf to [35S]methionine and tRNAfMet and have no effect on reticulocyte [35S]fMet-tRNAf, [3H]Ala-tRNA or [3H]Lys-tRNA. However, while deacylase I has similar activity throughout the pH range of 6.1-8.1, deacylase II has a sharp pH optimum at 7.9 and is almost completely inactive at 6.1. In addition, deacylase II shows a much greater affinity for pure Met-tRNAf than deacylase I (Km of 1.5-3 nM vs. 100 nM), and, while deacylase II is selectively inhibited by tRNAfMet, deacylase I is inhibited similarly by any added tRNA.

Characterization of a rabbit reticulocyte supernatant factor that reverses the translational inhibition of hemin deficiency.

The control of protein synthesis by hemin in rabbit reticulocyte lysate is mediated by the formation of a high molecular weight protein inhibitor (or translational repressor) of polypeptide chain initiation from a precursor (prorepressor), which acts by phosphorylating the Mr 35,000 (alpha) subunit of eIF-2. We originally isolated a post-ribosomal supernatant factor from reticulocyte lysate, distinct from soluble eIF-2, that could completely reverse the inhibition of protein synthesis that occurs when reticulocyte lysate is incubated in the absence of hemin. We have found that this supernatant factor promotes the inactivation of the intermediate form of the translational repressor (generated within 1 h of incubation of the prorepressor in the absence of hemin), but only in the presence of GTP. This inactivation is not seen with the irreversible form of the translational repressor (generated after prolonged incubation of the prorepressor) and does not occur with other nucleoside triphosphates, dGTP or GDP. In addition, the inactivation reaction is not dependent upon Mg2+ and is not mediated by cyclic GMP. When lysate samples were pulsed with [gamma-32P]ATP at 15 min of incubation, the addition of the supernatant factor at zero time or after 7 min was associated with preventing or reversing the phosphorylation of eIF-2 alpha. In contrast, another soluble reticulocyte protein termed reversing factor can be separated from the supernatant factor on phosphocellulose, partly overcomes the effect of the irreversible translational repressor, and is associated with increased phosphorylation of eIF-2 alpha.

Equilibration of methionine into the Met-tRNA pool of rabbit reticulocyte lysates. Additional evidence that deacylation of ribosomal subunit-bound Met-tRNAf does occur in heme-deficiency.

We have measured the rate of equilibration of [35S]methionine into the Met-tRNA pool of rabbit reticulocyte lysate as in [FEBS Lett. (1982) 143, 301-305]. Our results indicate that hemin-deficiency inhibits the equilibration of methionine into the tRNA pool much less than protein synthesis or the equilibration of alanine into the tRNA pool, whereas cycloheximide inhibits these processes similarly. This finding is consistent with our previous data and supports the hypothesis that with hemin-deficiency much of the Met-tRNAf that becomes bound to 40 S subunits subsequently undergoes enzymatic deacylation.

Evidence that the primary effect of phosphorylation of eukaryotic initiation factor 2(alpha) in rabbit reticulocyte lysate is inhibition of the release of eukaryotic initiation factor-2.GDP from 60 S ribosomal subunits.

The phosphorylation of eukaryotic initiation factor (eIF) 2 alpha that occurs when rabbit reticulocyte lysate is incubated in the absence of hemin or with poly(I.C) causes inhibition of polypeptide chain initiation by preventing a separate factor (termed RF) from promoting the exchange of GTP for GDP on eIF-2. When lysate was incubated in the presence of hemin and [14C] eIF-2 or [alpha-32P]GTP, we observed binding of eIF-2 and GDP or GTP to 60 S ribosomal subunits that was slightly greater than that bound to 40 S subunits and little binding to 80 S ribosomes. When incubation was in the absence of hemin or in the presence of hemin plus 0.1 microgram/ml poly(I.C), eIF-2 and GDP binding to 60 S subunits was increased 1.5- to 2-fold, that bound to 80 S ribosomes was almost as great as that bound to 60 S subunits, and that bound to 40 S subunits was unchanged. Our data indicate that about 40% of the eIF-2 that becomes bound to 60 S subunits and 80 S ribosomes in the absence of hemin or with poly(I.C) is eIF-2(alpha-P) and suggest that the eIF-2 and GDP bound is probably in the form of a binary complex. The accumulation of eIF-2.GDP on 60 S subunits occurs before binding of Met-tRNAf to 40 S subunits becomes reduced and before protein synthesis becomes inhibited. The rate of turnover of GDP (presumably eIF-2.GDP) on 60 S subunits and 80 S ribosomes in the absence of hemin is reduced to less than 10% the control rate, because the dissociation of eIF-2.GDP is inhibited. Additional RF increases the turnover of eIF-2.GDP on 60 S subunits and 80 S ribosomes to near the control rate by promoting dissociation of eIF-2.GDP but not eIF-2(alpha-P).GDP. Our findings suggest that eIF-2.GTP binding to and eIF-2.GDP release from 60 S subunits may normally occur and serve to promote subunit joining. The phosphorylation of eIF-2 alpha inhibits polypeptide chain initiation by preventing dissociation of eIF-2.GDP from either free 60 S subunits (thus inhibiting subunit joining directly) or the 60 S subunit component of an 80 S initiation complex (thereby blocking elongation and resulting in the dissociation of the 80 S complex).

Effect of Met-tRNAf deacylase on polypeptide chain initiation in rabbit reticulocyte lysate.

The possible role of Met-tRNAf deacylase in the regulation of protein synthesis in rabbit reticulocyte lysate by the hemin-controlled translational repressor (HCR) or the double-stranded RNA-activated inhibitor (dsI) has been examined. Inhibition of protein synthesis by either HCR or dsI is associated with a marked increase in the steady state level of 48 S initiation complexes, containing a 40 S ribosomal subunit, globin mRNA, and a reduced level of Met-tRNAf, suggesting that the rate of 60 S subunit addition may be inhibited and that subunit-bound Met-tRNAf may become deacylated by Met-tRNAf deacylase. The addition of highly purified Met-tRNAf deacylase to lysate samples incubated with HCR or dsI reduces the [35S]Met-tRNAf labeling of 48 S complexes to even a lower level but has no effect on the high level of [35S]Met-tRNAf associated with 43 S complexes in the plus hemin control. The effect of added deacylase on the labeling of 48 S complexes with [35S]Met-tRNAf can be overcome by adding eIF-5 or a soluble reticulocyte protein that has been termed the reversing factor, but not by the addition of eIF-2. Added deacylase has no effect on the level of mRNA in 48 S complexes or the labeling of these complexes with [35S]fMet-tRNAf. When lysate samples were labeled with Met-tRNAf, purified from wheat germ or yeast, and doubly labeled with 32P at the 5' end and [35S]methionine aminoacylation, HCR reduced the level of 32P and 35S-labeled tRNAMetf in 48 S complexes to a similar degree, suggesting that once it has become deacylated, tRNAMetf dissociates from the 40 S subunit.