Tryptophanyl-tRNA synthetase is a major soluble protein species in bovine pancreas

Tryptophanyl-tRNA Synthetase as a Potential Therapeutic Target

Tryptophanyl-tRNA synthetase (WRS) is an essential enzyme that catalyzes the ligation of tryptophan (Trp) to its cognate tRNA^trp during translation via aminoacylation. Interestingly, WRS also plays physiopathological roles in diseases including sepsis, cancer, and autoimmune and brain diseases and has potential as a pharmacological target and therapeutic. However, WRS is still generally regarded simply as an enzyme that produces Trp in polypeptides; therefore, studies of the pharmacological effects, therapeutic targets, and mechanisms of action of WRS are still at an emerging stage. This review summarizes the involvement of WRS in human diseases. We hope that this will encourage further investigation into WRS as a potential target for drug development in various pathological states including infection, tumorigenesis, and autoimmune and brain diseases.

Non-canonical functions of human cytoplasmic tyrosyl-, tryptophanyl- and other aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases catalyze the aminoacylation of their cognate tRNAs. Here we review the accumulated knowledge of non-canonical functions of human cytoplasmic aminoacyl-tRNA synthetases, especially tyrosyl- (TyrRS) and tryptophanyl-tRNA synthetase (TrpRS). Human TyrRS and TrpRS have an extra domain. Two distinct cytokines, i.e., the core catalytic "mini TyrRS" and the extra C-domain, are generated from human TyrRS by proteolytic cleavage. Moreover, the core catalytic domains of human TyrRS and TrpRS function as angiogenic and angiostatic factors, respectively, whereas the full-length forms are inactive for this function. It is also known that many synthetases change their localization in response to a specific signal and subsequently exhibit alternative functions. Furthermore, some synthetases function as sensors for amino acids by changing their protein interactions in an amino acid-dependent manner. Further studies will be necessary to elucidate regulatory mechanisms of non-canonical functions of aminoacyl-tRNA synthetases in particular, by analyzing the effect of their post-translational modifications.

Identification of a residue crucial for the angiostatic activity of human mini tryptophanyl-tRNA synthetase by focusing on its molecular evolution

Human tryptophanyl-tRNA synthetase (TrpRS) exists in two forms: a full-length TrpRS and a mini TrpRS. We previously found that human mini, but not full-length, TrpRS is an angiostatic factor. Moreover, it was shown that the interaction between mini TrpRS and the extracellular domain of vascular endothelial (VE)-cadherin is crucial for its angiostatic activity. However, the molecular mechanism of the angiostatic activity of human mini TrpRS is only partly understood. In the present study, we investigated the effects of truncated (mini) form of TrpRS proteins from human, bovine, or zebrafish on vascular endothelial growth factor (VEGF)-stimulated chemotaxis of human umbilical vein endothelial cells (HUVECs). We show that both human and bovine mini TrpRSs inhibited VEGF-induced endothelial migration, whereas zebrafish mini TrpRS did not. Next, to identify residues crucial for the angiostatic activity of human mini TrpRS, we prepared several site-directed mutants based on amino acid sequence alignments among TrpRSs from various species and demonstrated that a human mini K153Q TrpRS mutant cannot inhibit VEGF-stimulated HUVEC migration and cannot bind to the extracellular domain of VE-cadherin. Taken together, we conclude that the Lys153 residue of human mini TrpRS is a VE-cadherin binding site and is therefore crucial for its angiostatic activity.

An appended domain results in an unusual architecture for malaria parasite tryptophanyl-tRNA synthetase

Specific activation of amino acids by aminoacyl-tRNA synthetases (aaRSs) is essential for maintaining fidelity during protein translation. Here, we present crystal structure of malaria parasite Plasmodium falciparum tryptophanyl-tRNA synthetase (Pf-WRS) catalytic domain (AAD) at 2.6 Å resolution in complex with L-tryptophan. Confocal microscopy-based localization data suggest cytoplasmic residency of this protein. Pf-WRS has an unusual N-terminal extension of AlaX-like domain (AXD) along with linker regions which together seem vital for enzymatic activity and tRNA binding. Pf-WRS is not proteolytically processed in the parasites and therefore AXD likely provides tRNA binding capability rather than editing activity. The N-terminal domain containing AXD and linker region is monomeric and would result in an unusual overall architecture for Pf-WRS where the dimeric catalytic domains have monomeric AXDs on either side. Our PDB-wide comparative analyses of 47 WRS crystal structures also provide new mechanistic insights into this enzyme family in context conserved KMSKS loop conformations.

A short peptide insertion crucial for angiostatic activity of human tryptophanyl-tRNA synthetase

Human tryptophanyl-tRNA synthetase (TrpRS) is secreted into the extracellular region of vascular endothelial cells. The splice variant form (mini TrpRS) functions in vascular endothelial cell apoptosis as an angiostatic cytokine. In contrast, the closely related human tyrosyl-tRNA synthetase (TyrRS) functions as an angiogenic cytokine in its truncated form (mini TyrRS). Here, we determined the crystal structure of human mini TrpRS at a resolution of 2.3 A and compared the structure with those of prokaryotic TrpRS and human mini TyrRS. Deletion of the tRNA anticodon-binding (TAB) domain insertion, consisting of eight residues in the human TrpRS, abolished the enzyme's apoptotic activity for endothelial cells, whereas its translational catalysis and cell-binding activities remained unchanged. Thus, we have identified the inserted peptide motif that activates the angiostatic signaling.

A human aminoacyl-tRNA synthetase as a regulator of angiogenesis

Aminoacyl-tRNA synthetases catalyze the first step of protein synthesis. It was shown recently that human tyrosyl-tRNA synthetase (TyrRS) can be split into two fragments having distinct cytokine activities, thereby linking protein synthesis to cytokine signaling pathways. Tryptophanyl-tRNA synthetase (TrpRS) is a close homologue of TyrRS. A natural fragment, herein designated as mini TrpRS, was shown by others to be produced by alternative splicing. Production of this fragment is reported to be stimulated by IFN-gamma, a cytokine that also stimulates production of angiostatic factors. Mini TrpRS is shown here to be angiostatic in a mammalian cell culture system, the chicken embryo, and two independent angiogenesis assays in the mouse. The full-length enzyme is inactive in the same assays. Thus, protein synthesis may be linked to the regulation of angiogenesis by a natural fragment of TrpRS.

A root-specific iron-regulated gene of tomato encodes a lysyl-tRNA-synthetase-like protein

The tomato mutant chloronerva exhibits a defect in iron-uptake regulation. Despite high apoplastic and symplastic iron concentrations, the mutant shows characteristic symptoms of iron deficiency. Using a subtractive-hybridisation approach, we have screened for cDNA clones specific for genes with altered expression in wild-type versus mutant root tissue. Based on this clone collection, we have isolated and characterised a 2075-bp full-length cDNA encoding a lysyl-tRNA-synthetase-like protein. The corresponding gene is localised as a single copy on chromosome 10. Its expression is strongly induced by changes in the iron status of the plant. This iron-dependent regulation is superimposed upon a strict root specificity of gene expression. Possible functions of the gene product other than in protein biosynthesis will be discussed.

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.

Gamma interferon potently induces tryptophanyl-tRNA synthetase expression in human keratinocytes

Incubation of human keratinocytes with gamma interferon (gamma-IFN) induces the synthesis of a 53-kDa protein of unknown nature and function. We report the identification of this protein through amino acid microsequencing. The NH2-terminal amino acid sequence of the 53-kDa antigen demonstrated that this protein was tryptophanyl-tRNA synthetase (Frolova et al, Gene 109:291-296, 1991, Genbank accession number 61715). This result was validated by the sequencing of tryptic peptides. Identification of the 53-kDa gamma-IFN-induced protein was confirmed by immunoblotting with an antiserum directed against beef pancreas tryptophanyl-tRNA synthetase. Northern blot analysis using a synthetic oligonucleotidic 32P-labeled probe evidenced a 3.1-kb transcript in gamma-IFN-treated cells indicating that the gene was regulated at the pre-translational level. These data show that gamma-IFN potently induces in keratinocytes the expression of an enzyme directly involved in protein biosynthesis. Elevated levels of tryptophanyl-tRNA synthetase in treated cultured keratinocytes might be involved in the cell-growth-inhibitory activity of gamma interferon.

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.

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.

A mammalian tryptophanyl-tRNA synthetase shows little homology to prokaryotic synthetases but near identity with mammalian peptide chain release factor

Determination of the amino acid sequence of beef pancreas tryptophanyl-tRNA synthetase was undertaken through both cDNA and direct peptide sequencing. A full-length cDNA clone containing a 475 amino acid open reading frame was obtained. The molecular mass of the corresponding peptide chain, 53,728 Da, was in agreement with that of beef tryptophanyl-tRNA synthetase, as determined by physicochemical methods (54 kDa). Expression of this clone in Escherichia coli led to tryptophanyl-tRNA synthetase activity in cell extracts. The open reading frame included two sequences analogous to the consensus sequences, HIGH and KMSKS, found in class I aminoacyl-tRNA synthetases. The homology with prokaryotic and yeast mitochondrial tryptophanyl-tRNA synthetases was low and was limited to the regions of the consensus sequences. However, a 90% homology was observed with the recently described rabbit peptide chain release factor (eRF) [Lee et al. (1990) Proc. Natl. Acad. Sci. 87, 3508-3512]. Such a strong homology may reveal a new group of genes deriving from a common ancestor, the products of which could be involved in tRNA aminoacylation (tryptophanyl-tRNA synthetase) or translation termination (eRF).

Tryptophanyl-tRNA synthetase-like immunoreactivity in the central nervous system and midgut of the migratory locust. Comparisons with gastrin-cholecystokinin-like and octopamine-like immunoreactivity

A tryptophanyl-tRNA synthetase (TrpRS)-immunoreactivity is localized in various neurosecretory cells of all ganglia of the central nervous system of the Orthoptera Locusta migratoria, except in deutocerebrum, and in endocrine cells of the midgut. It has been observed that TrpRS-like material never co-localizes either with CCK-like or octopamine-like material. TrpRS immunoreactive perikarya and processes that ramify extensively throughout the neuropiles have been detected in the protocerebrum, optic lobes, tritocerebrum, suboesophageal, thoracic and abdominal ganglia. In the lateral protocerebrum, a particular TrpRS pathway different from the lateral gastrin cholecystokinin (CCK-8(s] pathway is revealed, certain of these processes terminating in the glandular part of the corpora cardiaca. In the metathoracic ganglion, have been observed numerous immunoreactive cell bodies and processes in the neuropiles. Some of them constitute a major pathway and which are distinct from octopamine (OA) cells but in close vicinity with the latter. In the midgut immunopositive TrpRS-like cells are dispersed among the regenerative and digestive cells of the epithelium; they are different from gastrin-cholecystokinin positive cells. The various TrpRS-like immunoreactivities identified in Locusta indicate that TrpRS-like material may occur in different tissues of organisms other than Vertebrates. These results suggest also that TrpRS-like enzyme could be involved in functions other than aminoacylation, as in Vertebrates.

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.

Control of the levels of alanyl-, glycyl-, and seryl-tRNA synthetases in the silkgland of Bombyx mori

We have examined the levels of glycyl-, alanyl-, and seryl-tRNA synthetases and the levels of their corresponding translatable mRNAs in the posterior and middle silkglands of the silkworm, Bombyx mori. Analysis of Western blots reveals that the change in the abundance of these enzymes during the fifth instar in crude extracts derived from posterior and middle silkgland correlates with changes in enzymatic activity; most of the change in activity for seryl- and alanyl-tRNA synthetases can be accounted for by the corresponding change in enzyme concentration, while the apparent specific activity of glycyl-tRNA synthetase appears to be elevated in the posterior silkgland. Accompanying the changes in enzyme activity and enzyme concentration are changes in the levels of the corresponding mRNAs as determined by immunoprecipitation of in vitro translation products. The levels of all three enzymes are 10 to 20 times higher in the posterior and middle silkglands than in ovarian tissue. A form of alanyl-tRNA synthetase with a slightly higher apparent molecular weight is detected in the posterior silkgland early in the fifth instar and in ovarian tissue.

Aminoacyl-tRNA synthetases catalyze AMP----ADP----ATP exchange reactions, indicating labile covalent enzyme-amino-acid intermediates

Aminoacyl-tRNA synthetases (amino acid-tRNA ligases, EC 6.1.1.-) catalyze the aminoacylation of specific amino acids onto their cognate tRNAs with extraordinary accuracy. Recent reports, however, indicate that this class of enzymes may play other roles in cellular metabolism. Several aminoacyl-tRNA synthetases are herein shown to catalyze the AMP----ADP and ADP----ATP exchange reactions (in the absence of tRNAs) by utilizing a transfer of the gamma-phosphate of ATP to reactive AMP and ADP intermediates that are probably the mixed anhydrides of the nucleotide and the corresponding amino acid. AMP and ADP produce active intermediates with amino acids by entering the back-reaction of amino acid activation, reacting with labile covalent amino acid-enzyme intermediates. Gramicidin synthetases 1 and 2, which are known to activate certain amino acids through the formation of intermediate thiol-esters of the amino acids and the enzymes, catalyze the same set of reactions with similar characteristics. Several lines of evidence suggest that these activities are an inherent part of the enzymatic reactions catalyzed by the aminoacyl-tRNA synthetases and gramicidin synthetases and are not due to impurities of adenylate kinase, NDP kinase, or low levels of tRNAs bound to the enzymes. The covalent amino acid-enzyme adducts are likely intermediates in the aminoacylation of their cognate tRNAs. The use of gramicidin synthetases has thus helped to illuminate mechanistic details of amino acid activation catalyzed by the aminoacyl-tRNA synthetases.