Reversal of the inhibitory action of the hemin-controlled translational repressor by a post-ribosomal supernatant factor from rabbit reticulocyte lysate

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.

Regulation of protein synthesis by hemin: effect of dithiothreitol on the formation and activity of the hemin-controlled translational repressor

Previous studies have demonstrated that the hemin-controlled translational repressor (HCR), a high molecular weight protein inhibitor of polypeptide chain initiation in rabbit reticulocyte lysate, is formed from a presynthesized prorepressor over a period of 12--18 h in three stages denoted reversible, intermediate, and irreversible. The prorepressor can, however, be completely converted to irreversible HCR within 2 min by incubation with such sulfhydryl reagents as N-ethylmaleimide. The results in this report demonstrate that dithiothreitol, which stabilizes thiol groups, will, like hemin, prevent the conversion of the prorepressor to HCR and will inactivate reversible HCR. Unlike hemin, dithiothreitol also inactives the intermediate form of HCR. Neither dithiothreitol nor hemin has any effect on the activity of irreversible HCR. Since the prorepressor used in these experiments had been separated from the supernatant factor (a soluble protein that reverses the inhibition of protein synthesis due to HCR), the effect of dithiothreitol and of hemin is independent of this factor and may be mediated by direct interaction with the prorepressor and HCR. Dithioerythritol, the erythro isomer of dithiothreitol, is as effective as dithiothreitol in preventing the formation of HCR, whereas glutathione and beta-mercaptoethanol have little or no effect.

Control of protein synthesis by hemin. Isolation and characterization of a supernatant factor from rabbit reticulocyte lysate

The regulation of protein synthesis by hemin in rabbit reticulocyte lysates is mediated by a hemin-controlled translational repressor protein (HCR) that inhibits polypeptide chain initiation. The effect of this translational inhibitor can be reversed by a high molecular weight protein in the post-ribosomal supernatant fraction. This supernatant factor has been purified approx. 700-fold. It is as effective in reversing the inhibition of protein synthesis due to an early form of HCR (intermediate HCR) as it is in stimulating protein synthesis in the absence of hemin. It is progressively less effective at reversing the inhibition of protein synthesis due to a late from of HCR (irreversible HCR), double-stranded RNA, and oxidized glutathione. The supernatant factor is chromatographically different from the initiation factor IF-MP, isolated from reticulocyte ribosomes, that can also overcome the inhibitory effect of HCR. The supernatant factor does not require hemin for activity, and its action is somewhat suppressed by a level of hemin that is optimal for protein synthesis.

On the mechanism of delayed inhibition of protein synthesis in heme-defecient rabbit reticulocyte lysates

In the absence of added hemin, protein synthesis in a rabbit reticulocyte lysate declines abruptly (shuts off) after about 5 min at 30 degrees. In these studies we have examined the basis for the lag period preceding shut-off. The initiation factor that binds Met-tRNAf, previously shown to be rate-limiting in inhibited, heme-deficient lysates, is found to be used stoichiometrically in the presence of excess inhibitor. We suggest that a principal effect of the inhibitor is to impair the recycling of the Met-tRNAf-binding factor; the lag period is attributable largely to the presence of a pool of excess Met-tRNAf-binding factor, which, once used in initiation, cannot be recycled because of the action of the inhibitor.

Protein synthesis in dystrophic muscle. Activity of the pH 5 supernatant fraction of muscle in dystrophic mice

The pH 5 supernatant fractions prepared from homogenates of tissues of normal and dystrophic mice were used to study the incorporation of [14C]phenylalanyl-tRNA into peptide. The incorpoation was markedly reduced using the muscle pH 5 supernatant fraction from dystrophic animals but no reduction was seen with brain, liver or heart preparations from dystrophic mice. The lower incorporation with dystrophic muscle pH 5 supernatant was not due to altered activity of ribonuclease, elongation factors, proteolytic enzymes, GTP or sulfhydryl reagents, but was attributable to the presence of activity that was inhibitory to protein synthesis.