Equilibration of methionine into the Met-tRNA pool of rabbit reticulocyte lysates

Use of an antibody to characterize and determine the role of the major Met-tRNAf deacylase from rabbit reticulocyte ribosomes

Inhibition of polypeptide chain initiation in rabbit reticulocyte lysate by phosphorylation of eukaryotic initiation factor-2(alpha) results, secondarily, in the enzymatic deacylation of Met-tRNAf on the 48 S initiation complexes that accumulate. We have prepared an antibody to a highly purified preparation of the major Met-tRNAf deacylase activity on rabbit reticulocyte ribosomes, termed deacylase II. Antibody, but not similarly purified normal IgG, completely neutralizes the activity of Met-tRNAf deacylase II and has no effect on Met-tRNAf deacylase I, a separate, minor, reticulocyte activity with the same substrate specificity but very different physical and enzymatic properties, strongly suggesting that deacylase I and II are distinct proteins. We partially purified Met-tRNAf deacylase activities from rabbit liver, myocardium and bone marrow ribosomes and found them to be similar to each other and to reticulocyte deacylase I in their enzymatic properties and insensitivity to anti-deacylase II, suggesting that deacylase I may be a general form of this enzyme, present in many cells, while deacylase II may be induced specifically during erythroid differentiation. Addition of the antibody to reticulocyte lysate incubated in the absence of hemin or presence of hemin plus 0.1 microgram/ml poly(I X C) did not reverse the inhibition of protein synthesis but did reduce the rate of turnover/utilization of Met-tRNAf and increase the level of Met-tRNAf bound to 48 S initiation complexes, demonstrating that the deacylase does not directly inhibit protein synthesis under these conditions but does mediate the deacylation, loss, and thus greater than expected turnover of Met-tRNAf in the 48 S complexes that accumulate.

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.