The "eRF" clone corresponds to tryptophanyl-tRNA synthetase, not mammalian release factor

A mammalian tryptophanyl-tRNA synthetase is associated with protein kinase activity

Bovine Trp-tRNA synthetase is a dimer with subunit molecular mass of 60 kDa (p60) which catalyzes ATP-dependent formation of tryptophanyl-tRNA. Evidence is presented that Trp-tRNA synthetase whose homogeneity had been proven by SDS/PAGE and silver staining of the gel is autophosphorylated in vitro. Anti-(Trp-tRNA synthetase) antibodies, whose specificity was verified by using a combination of different approaches, were able to effectively inhibit and immunoprecipitate the Trp-tRNA-synthetase-associated kinase activity. The two-dimensional tryptic phosphopeptide map of autophosphorylated p60 Trp-tRNA synthetase was found to be similar to that of its major 40-kDa degradation fragment bearing resemblance to previously demonstrated unlabeled peptide patterns of the Trp-tRNA synthetase forms. Trp-tRNA synthetase which had undergone denaturation during SDS/PAGE, regained serine/threonine specific protein kinase activity (PK 60) after guanidine treatment. Trp-tRNA synthetase induced phosphorylation of specific substrate such as 100-kDa protein in non-immune but not in anti-(Trp-tRNA synthetase) sera which distinguishes Trp-tRNA-synthetase-associated kinase from other protein kinases. Sequence analysis permitted the identification of regions of bovine Trp-tRNA synthetase sharing similarity with the catalytic domains of known protein kinases. These findings suggest that PK 60 and Trp-tRNA synthetase (p60) are either closely related or identical.

Evolution of the aminoacyl-tRNA synthetase family and the organization of the Drosophila glutamyl-prolyl-tRNA synthetase gene. Intron/exon structure of the gene, control of expression of the two mRNAs, selective advantage of the multienzyme complex

In Drosophila, glutamyl-prolyl-tRNA synthetase is a multifunctional synthetase encoded by a unique gene and composed of three domains: the amino- and carboxy-terminal domains catalyze the aminoacylation of glutamic acid and proline tRNA species, respectively, and the central domain is made of 75 amino acids repeated six times amongst which 46 are highly conserved and constitute the repeated motifs [Cerini, C., Kerjan, P., Astier, M., Gratecos, D., Mirande, M. & Sémériva, M. (1991) EMBO J. 10, 4267-4277]. The intron/exon organization of the Drosophila gene reveals the presence of six exons among which four are in the 5'-end encoding glutamic acid activity. Only one exon encodes the repeated motifs. A comparison of introns positions, intron classes and intron/exon boundaries in the Drosophila gene and in its human counterpart is compatible with the intron-early hypothesis presiding, at least in part, to the evolution of the synthetases. The full-length fly protein is encoded by a 6.1-kb mRNA which is expressed throughout development. In addition, a shorter transcript encompasses the 3'-end of the cDNA and it is especially abundant in 5-10-h embryos until the first larval stage. Expression of these two mRNAs seems to be controlled by two independent promoters. The 6.1-kb mRNA promoter is probably localized in the 5'-end of the gene. The small mRNA promoter resides in the 4th intron and evidence is provided that the mRNA encodes only the domain corresponding to prolyl-tRNA synthetase and is functional in vivo. Finally, transgenic flies have been established by using constructs corresponding to the three domains of the protein. Overexpression of the repeated motifs leads to a sterility of the flies that suggests a role of these motifs in linking the multienzyme complex to an, as yet, unknown structure of the protein synthesis apparatus.

Regulation of translation termination: conserved structural motifs in bacterial and eukaryotic polypeptide release factors

Translation termination requires codon-dependent polypeptide release factors. The mechanism of stop codon recognition by release factors is unknown and holds considerable interest since it entails protein-RNA recognition rather than the well-understood mRNA-tRNA interaction in codon-anticodon pairing. Bacteria have two codon-specific release factors and our picture of prokaryotic translation is changing because a third factor, which stimulates the other two, has now been found. Moreover, a highly conserved eukaryotic protein family possessing properties of polypeptide release factor has now been sought. This review summarizes our current understanding of the structural and functional organization of release factors as well as our recent findings of highly conserved structural motifs in bacterial and eukaryotic polypeptide release factors.