Molecular and cellular studies of tryptophanyl-tRNA synthetase using monoclonal antibodies. Evaluation of a common antigenic determinant in eukaryotic, prokaryotic and archaebacterial enzymes which maps outside the catalytic domain

Regulation of angiogenesis by aminoacyl-tRNA synthetases

In addition to their canonical roles in translation the aminoacyl-tRNA synthetases (ARSs) have developed secondary functions over the course of evolution. Many of these activities are associated with cellular survival and nutritional stress responses essential for homeostatic processes in higher eukaryotes. In particular, six ARSs and one associated factor have documented functions in angiogenesis. However, despite their connection to this process, the ARSs are mechanistically distinct and exhibit a range of positive or negative effects on aspects of endothelial cell migration, proliferation, and survival. This variability is achieved through the appearance of appended domains and interplay with inflammatory pathways not found in prokaryotic systems. Complete knowledge of the non-canonical functions of ARSs is necessary to understand the mechanisms underlying the physiological regulation of angiogenesis.

RNA-protein interactions at the initial and terminal stages of protein biosynthesis as investigated by Lev Kisselev (on the occasion of his 70th anniversary)

This review highlights studies by Lev L. Kisselev and his colleagues on the initial and terminal stages of protein biosynthesis, which cover the period of the last 45 years (1961-2006). They investigated spatial structure of tRNAs, structure and functions of aminoacyl-tRNA-synthetases of higher organisms, and the final step of protein synthesis, termination of translation. L. Kisselev and his team have made three major contributions to these fields of molecular biology; (i) they proposed the hypothesis on the role of anticodon triplet of tRNA in recognition by cognate aminoacyl-tRNA synthetase, which has been experimentally confirmed and is now included in textbooks; (ii) identified primary structures and functions of two eukaryotic protein factors (eRF1 and eRF3) playing a pivotal role in translation termination; (iii) characterized a structural basis for stop codon recognition by eRF1 within the ribosome and discovered the negative structural elements of eRF1, limiting its recognition of one or two stop-codons.

Mapping and molecular characterization of novel monoclonal antibodies to conformational epitopes on NH2 and COOH termini of mammalian tryptophanyl-tRNA synthetase reveal link of the epitopes to aggregation and Alzheimer's disease

Tryptophanyl-tRNA synthetase (TrpRS) is an interferon-induced phosphoprotein with autoantigenic and cytokine activities detected in addition to its canonical function in tRNA aminoacylation. The availability of monoclonal antibodies (mAbs) specific for TrpRS is important for development of tools for TrpRS monitoring. A molecular characterization of two mAbs raised in mice, using purified, enzymatically active bovine TrpRS as the inoculating antigen, is presented in this report. These IgG1 antibodies are specific for bovine, human and rabbit but not E. coli TrpRS. Immunoreactivity and specificity of mAbs were verified with purified recombinant hTrpRS expressed in E. coli and TrpRS-derived synthetic peptides. One of the mAbs, 9D7 is able to disaggregate fibrils formed by Ser32-Tyr50 TrpRS-peptide. Epitope mapping revealed that disaggregation ability correlates with binding of 9D7 to this peptide in ELISA and immunocytochemistry. This epitope covers a significant part of N-terminal extension that suggested to be proteolytically deleted in vivo from the full-length TrpRS whereas remaining COOH-fragment possesses a cytokine activity. For epitope mapping of mAb 6C10, the affinity selected phage-displayed peptides were used as a database for prediction of conformational discontinuous epitopes within hTrpRS crystal structure. Using computer algorithm, this epitope is attributed to COOH-terminal residues Asp409-Met425. In immunoblotting, the 6C10 mAb reacts preferably with (i) oligomer than monomer, and (ii) bound than free TrpRS forms. The hTrpRS expression was shown to correlate with growth rates of neuroblastoma and pancreatic cancer cells. Immunohistochemically both mAbs revealed extracellular plaque-like aggregates in hippocampus of Alzheimer's disease brain.

Identification and characterization of human mitochondrial tryptophanyl-tRNA synthetase

A full-length cDNA clone encoding the human mitochondrial tryptophanyl-tRNA synthetase (h(mt)TrpRS) has been identified. The deduced amino acid sequence shows high homology to both the mitochondrial tryptophanyl-tRNA synthetase ((mt)TrpRS) from Saccharomyces cerevisiae and to different eubacterial forms of tryptophanyl-tRNA synthetase (TrpRS). Using the baculovirus expression system, we have expressed and purified the protein with a carboxyl-terminal histidine tag. The purified His-tagged h(mt)TrpRS catalyzes Trp-dependent exchange of PP(i) in the PP(i)-ATP exchange assay. Expression of h(mt)TrpRS in both human and insect cells leads to high levels of h(mt)TrpRS localizing to the mitochondria, and in insect cells the first 18 amino acids constitute the mitochondrial localization signal sequence. Until now the human cytoplasmic tryptophanyl-tRNA synthetase (hTrpRS) was thought to function as the h(mt)TrpRS, possibly in the form of a splice variant. However, no mitochondrial localization signal sequence was ever detected and the present identification of a different (mt)TrpRS almost certainly rules out that possibility. The h(mt)TrpRS shows kinetic properties similar to human mitochondrial phenylalanyl-tRNA synthetase (h(mt)PheRS), and h(mt)TrpRS is not induced by interferon-gamma as is hTrpRS.

Tryptophanyl-tRNA synthetase as a human autoantigen

Autoantibodies to highly purified tryptophanyl-tRNA synthetase, consisting of two approximately 60-kDa subunits (6.1.1.2, TrpRS), were detected in some sera of donors and patients with various diagnosis using the newly developed 125I-TrpRS-radiodot, 125I-TrpRS-radioblot, ELISA and Western immunoblotting. The percentage of positive sera appears to be dependent upon the method of sera testing. The autoimmune sera recognized both the native and denatured TrpRS forms. The binding of the human serum to the 60-kDa band of tissue extract was demonstrable by the 125I-TrpRS-blot as well as Western blot techniques. The possible role of infections in the induction of anti-TrpRS antibodies and maintenance of the autoimmune response is discussed.

A broadly applicable continuous spectrophotometric assay for measuring aminoacyl-tRNA synthetase activity

We describe a convenient, simple and novel continuous spectrophotometric method for the determination of aminoacyl-tRNA synthetase activity. The assay relies upon the measurement of inorganic pyrophosphate generated in the first step of the aminoacylation of a tRNA. Pyrophosphate release is coupled to inorganic pyrophosphatase, to generate phosphate, which in turn is used as the substrate of purine nucleoside phosphorylase to catalyze the N-glycosidic cleavage of 2-amino 6-mercapto 7-methylpurine ribonucleoside. Of the reaction products, ribose 1-phosphate and 2-amino 6-mercapto 7-methylpurine, the latter has a high absorbance at 360 nm relative to the nucleoside and hence provides a spectrophotometric signal that can be continuously followed. The non-destructive nature of the spectrophotometric assay allowed the re-use of the tRNAs in question in successive experiments. The usefulness of this method was demonstrated for glutaminyl-tRNA synthetase (GlnRS) and tryptophanyl-tRNA synthetase. Initial velocities measured using this assay correlate closely with those assayed by quantitation of [3H]Gln-tRNA or [14C]Trp-tRNA formation respectively. In both cases amino acid transfer from the aminoacyl adenylate to the tRNA represents the rate determining step. In addition, aminoacyl adenylate formation by aspartyl-tRNA synthetase was followed and provided a more sensitive means of active site titration than existing techniques. Finally, this novel method was used to provide direct evidence for the cooperativity of tRNA and ATP binding to GlnRS.

Nucleoside triphosphatase activity associated with the N-terminal domain of mammalian tryptophanyl-tRNA synthetase

Bovine tryptophanyl-tRNA synthetase (EC 6.1.1.2) deprived of Zn2+ by chelation with the phosphonate analog of Ap4A hydrolyzed ATP(GTP) to ADP(GDP) although its ability to form tryptophanyl adenylate was impaired. This hydrolytic activity is stimulated by Mg2+ and Mn2+ ions and inhibited by Zn2+. Monoclonal antibody Am1 against the N-terminal domain of the enzyme completely abolished ATP(GTP)ase activity. The core peptide generated after proteolytic splitting of the N-domain lacks this activity. We suggest that the nucleotide binding site(s) different from ATP sites involved in aminoacylation reaction reside(s) at the N-terminal domain(s) of the enzyme.

Are the tryptophanyl-tRNA synthetase and the peptide-chain-release factor from higher eukaryotes one and the same protein?

Recently, cDNA clones encoding the bovine (b) [M. Garret, B. Pajot, V. Trézéguet, J. Labouesse, M. Merle, J.-C. Gandar, J.-P. Benedetto, M.-L. Sallafranque, J. Alterio, M. Gueguen, C. Sarger, B. Labouesse and J. Bonnet (1991) Biochemistry 30, 7809-7817] and human (h) [L. Yu. Frolova, M. A. Sudomoina, A. Yu. Grigorieva, O. L. Zinovieva and L. L. Kisselev (1991) Gene 109, 291-296] tryptophanyl-tRNA synthetases (TrpRS) were sequenced; the deduced amino acid sequences exhibit typical structural features of class I aminoacyl-tRNA synthetases [G. Eriani, M. Delarue, O. Poch, J. Gangloff and D. Moras (1990) Nature 237, 203-206] and limited, although significant, similarity with bacterial TrpRS. Independently, it was shown that a major protein whose synthesis is stimulated in human cell cultures by interferon gamma [J. Fleckner, H. H. Rasmussen and J. Justesen (1991) Proc. Natl Acad. Sci. USA 88, 11,520-11,524], and interferons gamma or alpha [B. Y. Rubins, S. L. Anderson, L. Xing, R. J. Powell and W. P. Tate (1991) J. Biol. Chem. 226, 24,245-24,248], exhibits TrpRS activity and an amino acid sequence identical to that of hTrpRS. The amino acid sequences of bTrpRS and hTrpRS are highly similar and are surprisingly very similar to the amino acid sequence deduced from a cloned and sequenced cDNA reported to encode rabbit (r) peptide-chain-release factor (RF) [C. C. Lee, W. J. Craigen, D. M. Muzny, E. Harlow and C. T. Caskey (1990) Proc. Natl Acad. Sci. USA 87, 3508-3512]. This close similarity between mammalian TrpRS and cloned RF is unexpected given the distinct functional properties of these proteins. Consequently, the question arises as to whether the mammalian TrpRS and RF activities reside on identical or very similar polypeptides. Alternatively, one may assume that the cloned rabbit cDNA encodes a protein other than rRF. Several properties (immunochemical, biochemical and physico-chemical) of mammalian TrpRS and RF have been compared. rTrpRS and rRF have distinct thermostability behaviours, and dissimilar chromatographic profiles on phosphocellulose. Both the anti-bTrpRS polyclonal antibodies and the monoclonal antibody Am2 strongly inhibit the bTrpRS and hTrpRS aminoacylation activities, but not the rRF activity. In addition, neither bTrpRS nor hTrpRS exhibit RF activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Mammalian tryptophanyl-tRNA synthetases

Aminoacyl-tRNA synthetases of higher organisms are far less studied compared to their prokaryotic and unicellular eukaryotic counterparts. However, many aminoacyl-tRNA synthetases from multi-cellular organisms exhibit certain features not yet described for the same enzymes of bacteria or yeast. Tryptophanyl-tRNA synthetases (TrpRS) are among the most thoroughly studied mammalian enzymes of this group. TrpRS are Zn(2+)-dependent, dimeric, class I aminoacyl-tRNA synthetases with known amino acid sequence for four different mammalian orders. TrpRS is not associated in a stable multi-synthetase complex, although it exhibits a long N-terminal extension absent from bacterial TrpRS. The human gene encoding TrpRS belongs to the interferon-responsive gene family and TrpRS activity drastically increases after interferon gamma induction. For unknown reasons TrpRS is overproduced in pancreas of Ruminantia. Other data on TrpRS available so far are summarized and briefly discussed here.

Switching tRNA(Gln) identity from glutamine to tryptophan

The middle base (U35) of the anticodon of tRNA(Gln) is a major element ensuring the accuracy of aminoacylation by Escherichia coli glutaminyl-tRNA synthetase (GlnRS). An opal suppressor of tRNA(Gln) (su+2UGA) containing C35 (anticodon UCA) was isolated by genetic selection and mutagenesis. Suppression of a UGA mutation in the E. coli fol gene followed by N-terminal sequence analysis of purified dihydrofolate reductase showed that this tRNA was an efficient suppressor that inserted predominantly tryptophan. Mutations of the 3-70 base pair (U70 and A3U70) were made. These mutants of su+2UGA are less efficient suppressors and inserted predominantly tryptophan in vivo; alanine insertion was not observed. Mutations of the discriminator nucleotide (A73, U73, C73) result in very weak opal suppressors. Aminoacylation in vitro by E. coli TrpRS of tRNA(Gln) transcripts mutated in the anticodon demonstrate that TrpRS recognizes all three nucleotides of the anticodon. The results show the interchangeability of the glutamine and tryptophan identities by base substitutions in their respective tRNAs. The amber suppressor (anticodon CUA) tRNA(Trp) was known previously to insert predominantly glutamine. We show that the opal suppressor (anticodon UCA) tRNA(Gln) inserts mainly tryptophan. Discrimination by these synthetases for tRNA includes position 35, with recognition of C35 by TrpRS and U35 by GlnRS. As the use of the UGA codon as tryptophan in mycoplasma and in yeast mitochondria is conserved, recognition of the UCA anticodon by TrpRS may also be maintained in evolution.

Aminoacyl-tRNA synthetase-induced cleavage of tRNA

Aminoacyl-tRNA synthetases interact with their cognate tRNAs in a highly specific fashion. We have examined the phenomenon that upon complex formation E. coli glutaminyl-tRNA synthetase destabilizes tRNA(Gln) causing chain scissions in the presence of Mg2+ ions. The phosphodiester bond cleavage produces 3'-phosphate and 5'-hydroxyl ends. This kind of experiment is useful for detecting conformational changes in tRNA. Our results show that the cleavage is synthetase-specific, that mutant and wild-type tRNA(Gln) species can assume a different conformation, and that modified nucleosides in tRNA enhance the structural stability of the molecule.

Cloning and nucleotide sequence of the structural gene encoding for human tryptophanyl-tRNA synthetase

A structural gene encoding bovine (b) tryptophanyl-tRNA synthetase (WRS) has recently been cloned and sequenced [Garret et al., Biochemistry 30 (1991) 7809-7817]. Using part of this sequence as a hybridisation probe we have cloned and sequenced a structural gene encoding human polypeptide highly homologous with two mammalian proteins, bWRS [Garret et al., Biochemistry 30 (1991) 7809-7817; EMBL accession No. X52113] and rabbit peptide chain release factor [Lee et al., Proc. Natl. Acad. Sci. USA 87 (1990) 3508-3512]. Identification of the sequence encoding a human WRS is based on (i) the presence of 'HIGH' and 'KMSKS' structural motifs typical for class-I aminoacyl-tRNA synthetases [Eriani et al., Nature 347 (1990) 203-206]; (ii) coincidence of the number of SH groups per subunit estimated experimentally [Muench et al., Science 187 (1975) 1089-1091] and deduced from the cDNA sequence (six in both cases); (iii) close resemblance of two WRS polypeptides sequenced earlier [Muench et al., Science 187 (1975) 1089-1091] and the predicted structure in two different regions.

Immuno-chemical non-cross-reactivity between eukaryotic and prokaryotic seryl-tRNA synthetases

Monospecific polyclonal antibodies (pAbs) against highly purified bovine seryl-tRNA synthetase (SerRS, EC 6.1.1.1) were prepared and their specificity tested. The interactions of pAbs with SerRS from different organisms were investigated by protein immunoblotting and ELISA methods. pAbs inhibit eukaryotic SerRS aminoacylating activity and exert no effect on SerRS activity from prokaryotes. It is proposed that prokaryotic and eukaryotic SerRS evolve from different ancestor genes.

Molecular and cellular studies of tryptophanyl-tRNA synthetases using monoclonal antibodies. Remarkable variations in the content of tryptophanyl-tRNA synthetase in the pancreas of different mammals

The content of Trp-tRNA synthetase in pancreas and liver of cattle, sheep, swine, rat, rabbit and man was assayed by direct radioimmunoblotting with a 125I-labelled monoclonal antibody Am1, specifically interacting with any eukaryotic Trp-tRNA synthetase. Its content in the organs studied, with the exception of bovine and sheep pancreas, was found to be 0.002-0.012% of total proteins. The enzyme content in bovine pancreas was about 0.2% of total proteins, i.e. 70 times higher than in bovine liver; similar correlations were found for sheep. The Trp-tRNA synthetase levels in each organ varied from animal to animal of the same species by not more than a factor of four; these individual variations cannot affect the conclusion about the profound differences in the levels of the enzyme in pancreases of Ruminantia and of the other mammalians. As shown by indirect immunofluorescence technique, bovine Trp-tRNA synthetase is mainly located in the exocrine part of the pancreas. Moreover, the immunoreactive material is detectable also in bovine (not human) pancreatic juice. The abnormally high Trp-tRNA synthetase content in the ruminant pancreas may be connected with unknown function(s) of this protein somehow related to the peculiarities of digestion of these mammals.