Posttranslational NH2-terminal aminoacylation

Post-translational arginylation in the bovine lens

This study demonstrates post-translational arginylation of bovine serum albumin and endogenous lens proteins by bovine lens arginyl-tRNA:protein transferase. This reaction has been proposed to be the first step in marking specific proteins for degradation by the non-lysosomal, ATP-dependent, ubiquitin-mediated proteolytic pathway. The transferase was obtained by the method used for isolation of the same enzyme from reticulocytes (Ferber and Ciechanover, 1987, Nature 326, 808-11). Incorporation of [3H]Arg was linear for at least 2 hr at 37 degrees C. The amount of incorporation was directly proportional to the amount of lens enzyme or substrate added. Arginylation was ATP-dependent. A requirement for tRNA was demonstrated by inhibition upon pretreatment of the enzyme preparation with nuclease to hydrolyse endogenous tRNA, and restoration of activity upon replacement of tRNA. [3H]Leu, [3H]Lys and [3H]His were not incorporated, demonstrating specificity of the reaction for arginine. This is the first demonstration of post-translational modification of proteins by arginylation in the lens.

Existence of two forms of rat liver arginyl-tRNA synthetase suggests channeling of aminoacyl-tRNA for protein synthesis

Arginyl-tRNA synthetase (arginine-tRNA ligase, EC 6.1.1.19) is found in extracts of mammalian cells both as a free protein (Mr = 60,000) and as a component (Mr approximately 72,000) of the high molecular weight aminoacyl-tRNA synthetase complex (Mr greater than 10(6). Several pieces of evidence indicate that the low molecular weight free form is not a proteolytic degradation product of the complex-bound enzyme but that it preexists in vivo: (i) the endogenous free form differs in size from the active proteolytic fragment generated in vitro, (ii) conditions expected to increase or decrease the amount of proteolysis do not alter the ratio of the two forms of the enzyme, and (iii) the free form contains an NH2-terminal methionine residue. A model is presented that provides a rationale for the existence of two forms of arginyl-tRNA synthetase in cells. In this model the complexed enzyme supplies arginyl-tRNA for protein synthesis, whereas the free enzyme provides arginyl-tRNA for the NH2-terminal arginine modification of proteins by arginyl-tRNA:protein arginyltransferase. This latter process targets certain proteins for removal by the ubiquitin-dependent protein degradation pathway. The necessity for an additional pool of arginyl-tRNA for the modification reaction leads to the conclusion that the arginyl-tRNA destined for protein synthesis (and/or protein modification) is channeled and unavailable for other processes. Other evidence supporting channeling in protein synthesis is discussed.

Quantities of individual aminoacyl-tRNA families and their turnover in Escherichia coli

The cellular content of all 20 aminoacyl-tRNA species was determined in small cultures of Escherichia coli by labeling cells with 3H-amino acids and extraction of 3H-amino acid-labeled nucleic acid by standard procedures. Of 3H-amino acid-labeled material, 25 to 90% was identified as 3H-aminoacyl-tRNA by the following criteria: sensitivity to base hydrolysis with expected kinetics; association of 3H counts released by base treatment of the 3H-amino acid-labeled nucleic acid with amino acid standards upon paper chromatography of the hydrolysate; and changes in the amount of 3H-amino acid-labeled nucleic acid recovered from cells as a function of time. Individual aminoacyl-tRNA content was determined with as few as 8 X 10(7) to 4 X 10(8) E. coli cells. Although the total number of aminoacyl-tRNA molecules per cell varied only by 10 to 20% among various strains of E. coli, some individual aminoacyl-tRNA families varied two- to threefold among strains. For a given amino acid, the number of aminoacyl-tRNA molecules per cell in E. coli strain K38 growing with a doubling time of 60 min varied from 730 (glutamyl-tRNA) to 7,910 (valyl-tRNA) with a mean value of 3,200. The total number of aminoacyl-tRNA molecules per cell (6.4 X 10(4)) in E. coli K38 was 5.5-fold higher than the number of ribosomes and was equal to 84% of the amount of elongation factor Tu molecules per cell. The ratio of aminoacyl-tRNA to synthetase for 10 amino acids varied from about 1 to 15 with a mean value of 4.7. The turnover of individual aminoacyl-tRNA families in E. coli cells was estimated to be in the range of 1.7 to 8.1 s-1 with a mean value of 3.7 s-1. An estimate of minimum in vivo molecular activity of aminoacyl-tRNA synthetases gives values of 2 to 48 s-1 for individual enzymes.

Reinitiation of translation from the triplet next to the amber termination codon in the absence of ribosome-releasing factor

Ribosome releasing factor (RR factor) releases ribosomes from mRNA at the termination codon in Escherichia coli. In the absence of this factor, polypeptides with molecular weights very close to the molecular weight of bacteriophage R17 coat protein were synthesized in vitro under the direction of a mutant R17 phage RNA having an amber mutation at codon 7 of the coat cistron. The major coat-protein-like product shared all the R17 coat protein sequence except that the seven NH2-terminal amino acids were missing. The minor product had the complete coat protein sequence starting from formylmethionine except for a probable amino acid substitution at codon 7 (UAG). Addition of RR factor inhibited the synthesis of the major protein. These results indicate that, in the absence of RR factor, the ribosome that has released the NH2-terminal hexapeptide at the amber codon stays on the mRNA and subsequently reinitiates translation "in phase" immediately after the amber codon without formylmethionine.