[Comparative study of localization of tryptophanyl-tRNA-synthetase and components of high molecular weight aminoacyl-tRNA-synthetase complex in animal cells]

A comparative study on the localization of cytosolic Trp-tRNA synthetase (TrpRS), aminoacyl-tRNA synthetases associated in a multienzyme complex (Glu-tRNA synthetase (GluRS), and Arg-tRNA synthetase (ArgRS)) and polypeptides p37 and p43 from the multienzyme complex was carried out on ultrathin sections of cultured rabbit cells RK-1 by means of immunogold technique. It is shown that GluRS, ArgRS, and polypeptide p43 have approximately the same distribution in the cell as TrpRS. The data obtained evidences in favour of a multienzyme structure of most (or, may be all) aminoacyl-tRNA synthetases in intact cells. A statistical analysis of enzyme distribution in different cell organelles showed nonrandom, compartmentalized distribution of studied synthetases in the mammalian cell. Aminoacyl-tRNA synthetases were found in the cell nucleus in the vicinity of interchromatin granules and in the regions of diffused chromatin. This fact points to a role which these proteins may play in active chromatin functions (transcription, processing, transfer of gene products, etc.) and needs special attention. Detection of ArgRS and GluRS in the nucleus allows one to suggest that either multienzyme synthetase complexes are present not only in the cytoplasm, but also in the nucleus, or these enzymes can dissociate from the complex and pass to the nucleus as individual proteins.

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.

Carbohydrates in mammalian tryptophanyl-tRNA synthetase

Homogeneous preparations of bovine tryptophanyl-tRNA synthetase (EC 6.1.1.2) contain monosaccharides (mannose, fucose, galactose, N-acetylglucosamine) as revealed by liquid chromatography. Their content comprises 2.5-3.0% (w/w) of the enzyme composed of two subunits (60 kDa x 2). The same set of sugars was detected in elastase and CNBr-generated fragments (with molecular masses of approx. 40 kDa and 30 kDa, respectively). It is concluded that bovine tryptophanyl-tRNA synthetase, in addition to being a metallo- and phosphoprotein, is also a glycoprotein.

Human interferon gamma potently induces the synthesis of a 55-kDa protein (gamma 2) highly homologous to rabbit peptide chain release factor and bovine tryptophanyl-tRNA synthetase

An interferon gamma (IFN-gamma)-inducible protein, gamma 2, was identified by two-dimensional gel electrophoresis of transformed human amnion (AMA) cell proteins. cDNA clones coding for this protein have been isolated and characterized as encoding a polypeptide with a predicted molecular weight of 53,165 and a pI of 6.16. Both values are in good agreement with those observed in two-dimensional gel electrophoresis. The gamma 2 protein is found to be highly induced by IFN-gamma, whereas no induction was seen after addition of IFN-alpha to AMA cells. A gamma 2-specific 2.7-kilobase mRNA was likewise seen to accumulate selectively in response to IFN-gamma in these cells. Comparison of the predicted amino acid sequence of gamma 2 to proteins in GenBank data bases revealed that gamma 2 is highly homologous to rabbit peptide chain release factor [Lee, C. C., Craigen, W. J., Muzny, D. M., Harlow, E. & Caskey, C. T. (1990) Proc. Natl. Acad. Sci. USA 87, 3508-3512] and bovine tryptophanyl-tRNA synthetase [M. Garret, V. Trezeguet, B. Pajot, J. C. Gandar, M. Merle, M. Guegiev, J. P. Benedetto, C. Sarger, J. Alteriot, J. La Bouessec, J. Labouesse, and J. Bonnet (1990), GenBank accession no. X52113]. Amino acid sequence similarities of 94% and 97%, respectively, are found, which in general would indicate that gamma 2 represents the human equivalent to either of these two mammalian genes. Based on these sequence similarities, the current data raise the possibility that tryptophanyl-tRNA charging and peptide chain release are carried out by the same enzyme. The gamma 2 protein is shown to possess tryptophan-dependent aminoacyl-tRNA synthetase activity and thus constitutes an enzymatic activity involved in the biological activity of IFN-gamma.

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 in cell lines resistant to tryptophan analogs

Bovine kidney cell lines resistant to tryptamine and tryptophanol (tryptophan analogs) were selected. The content of tryptophanyl-tRNA synthetase (WRS, EC 6.1.1.2) was assayed by measuring the binding of monospecific polyclonal antibodies to the 35S-labeled enzyme in detergent-soluble and -insoluble forms and measuring the enzyme activity. Both the enzyme content and activity were elevated in the resistant cells. As was found by immunoelectron microscopy, the initial and resistant cells contained WRS in most of their cellular compartments: on free polyribosomes, as large conglomerates in the cytoplasm, on polysomes bound to the rough endoplasmic reticulum membranes and to the outer nuclear membrane, on the cytoskeleton, and in the detergent-insoluble nuclear matrix. Immunochemically stained tangles of filaments were found in the resistant cells, but not in the control cells. WRS was less phosphorylated in the resistant than in the original Madin Darby bovine kidney cells. Karyological and morphometric analysis revealed that, in tryptamine-resistant cells, the marker acrocentric chromosome was longer and the frequency of its duplication rose to 96%. The results of this work indicate that the cultivated cells have become resistant to tryptophan analogs because of an elevated WRS concentration in the cells, possibly due to amplification of the WRS gene.

[Immunochemical study of tryptophanyl-tRNA-synthetase]

The immunochemical approach used extensively in medicine and in some fields of biology has not yet been systematically applied to enzymology, in particular, to the study of a large, functionally significant group of enzymes, such as aminoacyl-tRNA synthetases (EC 6.1.1). The present investigation was aimed at the analysis of applicability of polyclonal and monoclonal antibodies against bovine tryptophanyl-tRNA synthetase in the study of functional properties of this enzyme as well as of its distribution inside the cell and among organs and tissues of various animals. The general conclusions one may draw from these data are as follows. i) Tryptophanyl-tRNA synthetase of eukaryotes, eubacteria and archaebacteria share one common structural element (antigenic determinant) that is not essential for the catalytic activity. The evolutionary conservative nature of this element suggests that the enzyme may implement functions other than catalysis of tryptophanyl-tRNA formation. ii) Tryptophanyl-tRNA synthetase shows an anomalous distribution among mammalian organs: its content is far greater in the exocrine part of the ruminant animal pancreas in comparison with their other organs (liver) or with other mammalian orders. This finding suggests that the enzyme or its fragments may play a role in the digestive function of ruminant animals. iii) Tryptophanyl-tRNA synthetase was found in considerable quantities in diffuse chromatin of mammalian cell nuclei. This fact indicates that the enzyme may participate in such processes in the nucleus as transcription, processing, transport, etc. It may thus be concluded that tryptophanyl-tRNA synthetase of higher organisms, besides catalyzing the formation of aminoacyl-tRNAs can exert some other, yet unknown, noncanonical functions.

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.

[Aminoacyl-tRNA synthetases (codases) and their noncanonical functions]

The aim of this review is to summarize the data obtained in the author's laboratory during the last decade. The main objects of these investigations were mammalian aminoacyl-tRNA synthetases, mainly bovine tryptophanyl-tRNA synthetase (EC 6.1.1.2). The data are discussed and compared with those described in literature. In the course of these studies it turned out that some properties of mammalian aminoacyl-tRNA synthetases for instance, nuclear location of some of the synthetases, presence of extra-domain in bovine tryptophanyl-tRNA synthetase capable of catalyzing hydrolysis of ATP and GTP in the absence of Zn2+ ions and normal aminoacylation capacity, ability to bind to one of the glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase, formation of aminoacylated and pyrophosphorylated forms of tryptophanyl-tRNA synthetase etc., seem to be unrelated to the main function of the synthetases, catalysis of aminoacyl-tRNA formation, and, therefore, might be classified as noncanonical ones. Comparison of prokaryotic and eukaryotic aminoacyl-tRNA synthetases indicates the multipotential nature of the latter.

[Phosphorylation of tryptophanyl-tRNA-synthetase by casein kinase type II]

Incubation of tryptophanyl-tRNA synthetase from bovine pancrease with [gamma-32P]ATP of [gamma-32P]GTP and casein kinase II from rabbit liver leads to the incorporation of labeled phosphate into serine residues of synthetase polypeptide. The maximal level of 32P incorporation into synthetase polypeptide (Mr = 60 kDa) 0.15 moles of 32P per 1 mole of polypeptide was observed. Electrophoretic analysis according to O'Farrell showed that kinase phosphorylates exclusively the most acidic polypeptides (pI 4.9) of the synthetase preparation. Pretreatment of synthetase with animal acidic and alkaline phosphatases had no influence on the level of 32P incorporation in synthetase during subsequent incubation in the presence of casein kinase II.

[Phosphorylation of tryptophanyl-tRNA-synthase in extracts of bovine pancreas]

The polypeptide with a mobility of the tryptophanyl-tRNA-synthetase subunit can be labeled in bovine pancreas extracts from [gamma-32P]ATP. Immunoprecipitation analysis with monospecific polyclonal antibodies against the enzyme as well as identification of [32P]phosphoamino acids in the immunoprecipitate revealed that in bovine pancreas extracts tryptophanyl-tRNA-synthetase undergoes phosphorylation at serine residues. The level of phosphorylation does not change in the presence of activity modulators of cAMP-, cGMP- and Ca2(+)-dependent protein kinases, decreases after addition of phosphoseryl/phosphothreonyl-protein phosphatase inhibitors and increases in the presence of their activators. It was supposed that phosphorylation of tryptophanyl-tRNA-synthetase catalyzed by seryl/threonyl-specific protein kinase depends on the activity of phosphoseryl/phosphothreonyl-phosphatase.

Relationship between protein synthesis and concentrations of charged and uncharged tRNATrp in Escherichia coli

We have continuously monitored Trp-tRNA(Trp) concentrations in vivo and, in the same cultures, measured rates of protein synthesis in isogenic stringent and relaxed strains. We have also manipulated cellular charged and uncharged [tRNA(Trp)] by two means: (i) the strain used contains a Trp-tRNA synthetase mutation that increases the Km for Trp; thus, varying exogenous Trp varies cellular Trp-tRNA(Trp); and (ii) we have introduced into the mutant strain a plasmid containing the tRNA(Trp) gene behind an inducible promoter; thus, total [tRNA(Trp)] also can be varied depending on length of induction. The use of these conditions, combined with a previously characterized assay system, has allowed us to demonstrate that (i) the rate of incorporation of Trp into protein is proportional to the fraction of tRNA(Trp) that is charged; for any given total [tRNA(Trp)], this rate is also proportional to the [Trp-tRNA(Trp)]; (ii) uncharged tRNA(Trp) inhibits incorporation of Trp into protein; and (iii) rates of incorporation into protein of at least two other amino acids, Lys and Cys, are also sensitive to [Trp-tRNA(Trp)] and are inhibited by uncharged tRNA(Trp). Our results are consistent with models of translational control that postulate modulating polypeptide chain elongation efficiency by varying concentrations of specific tRNAs.

[Amino acid sequence of various peptides of tryptophanyl-tRNA-synthetase from bovine pancreas]

Method of isolation of the bovine pancreas tryptophanyl-tRNA synthetase is improved and a protein with greater than or equal to 99% purity, according to PAGE-SDS, is obtained. The pure enzyme is digested with clostripain and the hydrolysate is separated by FPLC anion-exchange chromatography followed by reversed phase HPLC. Amino acid sequences of 6 individual peptides, including C-terminal one, were determined by the automated Edman degradation. A peptide is also revealed which is encoded with the low degeneracy.

Bovine tryptophanyl-tRNA synthetase and glyceraldehyde-3-phosphate dehydrogenase form a complex

Bovine tryptophanyl-tRNA synthetase is able to form a complex with glyceraldehyde-3-phosphate dehydrogenase. The complex formation (i) does not influence the tryptophan-dependent PPi-ATP exchange reaction and (ii) involves predominantly the N-terminal dispensable domain of the synthetase. Glyceraldehyde-3-phosphate dehydrogenase was shown to be capable of interacting simultaneously with tryptophanyl-tRNA synthetase and with ribosomal RNA to form a ternary complex. It is proposed that compartmentation of some aminoacyl-tRNA synthetases in certain cases might be achieved via 'adapter' molecules which can bind at once to ribonucleic acids and to aminoacyl-tRNA synthetases.

Tryptophanyl-tRNA synthetase from Bacillus subtilis. Characterization and role of hydrophobicity in substrate recognition

The tryptophanyl-tRNA synthetase from Bacillus subtilis was purified to homogeneity and characterized. It has an alpha 2 subunit structure and a molecular weight of 77,000. Tryptophanyl-tRNA synthetase does not catalyze any significant proofreading. It activates tryptophan as well as the three fluorinated analogues, DL-4-fluoro-, DL-5-fluoro-, or DL-6-fluorotryptophan (4F-, 5F-, and 6F-Trp), in the ATP-pyrophosphate exchange reaction. In the aminoacylation reaction, the fluorotryptophans act as competitive inhibitors of Trp. Their relative activities follow the same order in both reactions: Trp greater than 4F-Trp greater than 6F-Trp greater than 5F-Trp. This order is the inverse of the order of relative hydrophobicities of these compounds, pointing to the importance of hydrophobic interactions in the selective recognition by tryptophanyl-tRNA synthetase among this group of substrates. To define the physical basis of the relative hydrophobicities, the crystallographic structure of 4F-Trp was determined and compared to that of trptophan. Charge distributions calculated for tryptophan and its different fluoroanalogues on the basis of molecular structures were supported by their carbon-13 NMR spectra. Correlations between charge distributions and relative hydrophobicities suggest that the polarity of the C-F bond represents an underlying factor determining the hydrophobicities of 4F-, 5F-, and 6F-Trp, thus relating tryptophanyl-tRNA synthetase selectivity toward tryptophan and its fluoroanalogues directly to their electronic configurations.

Probing the substrate-binding sites of aminoacyl-tRNA synthetases with the procion dye green HE-4BD

A reactive bis-dichloro derivative of the Procion dye Green HE-4BD was shown to inactivate irreversibly methionyl-tRNA synthetase (MTS) from Escherichia coli and also tryptophyl-tRNA synthetase (WTS) and tyrosyl-tRNA synthetase (YTS) from Bacillus stearothermophilus at pH 8.5 and 37 degrees C. At a 5-fold excess of reactive dye over enzyme subunit concentration MTS was quantitatively inactivated within 20 min in the ATP/pyrophosphate exchange assay, whereas WTS and YTS show an 80% loss of activity over the same time period. The inactivation is affected by the addition of substrates, which either protect (WTS and YTS) or promote (YTS with tyrosine) the dye-mediated enzyme inactivation. Green HE-4BD-OH was shown to be a competitive inhibitor of MTS with respect to MgATP, methionine and tRNA substrates.

[Hydrolytic activity of bovine tryptophanyl-tRNA-synthetase cause by removal of Zn2+]

Bovine tryptophanyl-tRNA synthetase (E.C.6.1.1.2) lacking Zn2+ ions removed by chelation with phosphonate analog of P1,P4-bis-(5'-adenosyl)tetraphosphate (Ap4A) was obtained (E-Zn). E-Zn lost the ability to form tryptophanyl adenylate, however it hydrolyses ATP to ADP and further on to AMP and Pi. GTP serves as a substrate with Km approximately 0.6 mM. It is proposed that the hydrolysable nucleotides bind to a nucleotide binding site(s) distinguishable from the substrate (catalytic) ones. After incubation of E-Zn with Zn2+ and Mg2+ the initial catalytic activity (ATP-PPi exchange and amino-acylation reactions) is restored whereas the hydrolytic activity becomes fully suppressed.

Effects of the ligands of beef tryptophanyl-tRNA synthetase on the elementary steps of the tRNA(Trp) aminoacylation

Tryptophanyl-tRNA synthetase catalyzed formation of Trp-tRNA(Trp) has been studied by mixing tRNA(Trp) with a preformed bis(tryptophanyl adenylate)-enzyme complex in the 0-60-ms time range, on a quenched-flow apparatus. Analyzing the data gives an association rate constant ka = (1.22 +/- 0.47) X 10(8) M-1 S-1, a dissociation rate constant kd = 143 +/- 73 S-1, and a dissociation constant Kd = 1.34 +/- 0.80 microM for tRNA(Trp). The maximum rate constant of tryptophan transfer to tRNA(Trp) is kt = 33 +/- 3 S-1. When starting the aminoacylation reaction with a mono(tryptophanyl adenylate)-enzyme complex, one obtains different kinetic profiles than when using a bis(tryptophanyl adenylate)-enzyme complex. Over a 0-400-ms time range, the monoadenylate-enzyme complex yields an apparent first-order reaction, while the bis-adenylate-enzyme complex yields a biphasic aminoacylation of tRNA(Trp). Analysis of Trp-tRNA(Trp) formation from both complexes according to simple reaction schemes shows that the dissociation of tRNA(Trp) from an enzyme subunit carrying no adenylate is 6.9-fold slower than from an enzyme subunit carrying an adenylate. The apparent rate constant of dissociation of nascent tryptophanyl-tRNA(Trp) is 4.9 S-1 in the absence of free tryptophan, which is much slower than its rate of formation (33 S-1).(ABSTRACT TRUNCATED AT 250 WORDS)

[Eukaryotic tryptophanyl-tRNA synthetase acquires affinity to high molecular weight RNA after limited proteolysis]

It is shown that the native tryptophanyl-tRNA synthetase (Mr 2X60 kDa) isolated from bovine pancreas does not interact with high-molecular weight RNAs (E. coli rRNAs). The enzyme acquires the affinity for high-molecular weight RNAs after the cleavage of the 20 kDa fragment from each of the subunits upon digestion by protease. The possible functional significance of the discovered phenomenon is discussed.

Binding stoichiometry of tRNATrp and tryptophanyl-tRNA synthetase from bovine pancreas under pH conditions of maximum activity. Analysis by ultracentrifugation, fluorescence quenching and chemical modification

The binding stoichiometry of tRNATrp and tryptophanyl-tRNA synthetase (EC 6.1.1.2) from beef is examined by three approaches, under pH conditions of maximum activity (pH 8.0). (1) Analytical ultracentrifugation evidences the binding of a single mol of tRNATrp in a 2.5-10 microM concentration range. (2) tRNATrp quenches the fluorescence of the enzyme. The dependence of this fluorescence quenching on the tRNATrp concentration (0.1-4 microM) reflects also the binding of 1 mol of tRNA per mol of enzyme, with a Kd value of 0.19 +/- 0.02 microM. (3) tRNATrp protects the enzyme against derivatization by oxidized ATP. Out of the two fast-reacting lysine residues of the native enzyme, only one is prevented from reacting by tRNATrp in the 0.5-110 microM concentration range. This protection can be significantly analyzed only by assuming a one-to-one complex between the enzyme and tRNA. These results, obtained at pH 8.0 and 25 degrees C, are in contrast with the stoichiometry of 2 mol of tRNA to 1 mol of enzyme, previously observed at pH 6.0 and 4 degrees C.

[A method for detecting protein-protein interactions. Detection of proteins with the molecular weight of approximately 37 kD binding with tryptophanyl-tRNA-synthetase]

A procedure for detecting protein-protein interactions is proposed. It is based on blotting of electrophoretically separated protein mixtures followed by detection of the proteins interacting with a given 125I-labelled protein used as a probe. Application of this approach to lysates of cultured mammalian cells enabled us to reveal a unique 37 kDa polypeptide interacting with 125I-labelled bovine tryptophanyl-tRNA synthetase (EC 6.1.1.2).

Reaction of tryptophanyl-tRNA synthetase from beef pancreas with periodate-oxidized ATP

Tryptophanyl-tRNA synthetase from beef pancreas reacts with periodate-oxidized ATP according to biphasic kinetics. A rapid phase involves two groups of the protein, presumably lysine side-chains. The slow phase corresponds to the reaction of a larger number of groups. The time-course of the partial losses of the ATP-PPi isotopic exchange and of the aminoacylation activities of the enzyme follow the labelling of the two fast-reacting groups. However, the ability of the enzyme to form a bis(tryptophanyladenylate)-enzyme complex is not lost after reaction of these two groups with the reagent. The affinity for ATP is also unaffected by this initial labelling of the protein, as seen from the Km values of this substrate in the ATP-PPi isotopic exchange reaction. These data suggest that, in this fast initial reaction, oxidized ATP reacts neither with specific ATP-binding groups of the enzyme nor with any major catalytic residue of the tryptophan-activation site. In contrast with this first step, the further slow labelling of lysine residues leads to a disappearance of the aminoacylation ability of the enzyme, while it does not further affect the ATP-PPi exchange activity. The behaviour of beef tryptophanyl-tRNA synthetase during derivatization with oxidized ATP is therefore at variance with that which has been described for the homologous E. coli enzyme.

[Autoadenylation of tryptophanyl-tRNA synthetase]

Incubation of bovine tryptophanyl-tRNA synthetase (EC 6.1.1.2) deprived of endogenous tryptophan, with [14C]ATP and without [gamma-32P]ATP, causes an appearance of radioactivity in protein due to formation of adenylated enzyme, [14C]AMP-E. Examination of the properties of the [14C]AMP-E thus obtained led to the conclusion that AMP is linked to the protein molecule via a macroergetic phosphoanhydride bond. ATP is formed when AMP-E is incubated with PPi. However, no tryptophanyl adenylate formation was observed when AMP-E was treated with tryptophan. The functional role of AMP-E remains obscure.

Kinetic evidence for half-of-the-sites reactivity in tRNATrp aminoacylation by tryptophanyl-tRNA synthetase from beef pancreas

The aminoacylation reaction catalyzed by the dimeric tryptophanyl-tRNA synthetase from beef pancreas was studied under pre-steady-state conditions by the quenched-flow method. The transfer of tryptophan to tRNATrp was monitored by using preformed enzyme-bis(tryptophanyl adenylate) complex. Combinations of either unlabeled or L-[14C]tryptophan-labeled tryptophanyl adenylate and of aminoacylation incubation mixtures containing either unlabeled tryptophan or L-[14C]tryptophan were used. We measured either the formation of a single labeled aminoacyl-tRNATrp per enzyme subunit or the turnover of labeled aminoacyl-tRNATrp synthesis. Four models were proposed to analyze the experimental data: (A) two independent and nonequivalent subunits; (B) a single active subunit (subunits presenting absolute "half-of-the-sites reactivity"); (C) alternate functioning of the subunits (flip-flop mechanism); (D) random functioning of the subunits with half-of-the-sites reactivity. The equations corresponding to the formation of labeled tryptophanyl-tRNATrp under each labeling condition were derived for each model. By use of least-squares criteria, the experimental curves were fitted with the four models, and it was possible to disregard models B and C as likely mechanisms. Complementary experiments, in which there was no significant excess of ATP-Mg over the enzyme-adenylate complex, emphasized an activator effect of free L-tryptophan on the rate of aminoacylation. This result disfavored model A. Model D was in agreement with all data. The analyses showed that the transfer step was not the major limiting reaction in the overall aminoacylation process.

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

Besides their central role in protein synthesis, aminoacyl-tRNA synthetases have been found or thought to be involved in other processes. We present here a study showing that tryptophanyl-tRNA synthetase has a surprising tissular distribution. Indeed, immunochemical determinations showed that in several bovine organs such as liver, kidney and heart, tryptophanyl-tRNA synthetase constitutes, as expected, about 0.02% of soluble proteins. In spleen, brain cortex, stomach, cerebellum or duodenum, this amount is about 10-times higher, and in pancreas it is 100-fold. There is no correlation between these amounts and the RNA content of the organs. Moreover, the concentration of another aminoacyl-tRNA synthetase (methionyl-tRNA synthetase) is higher in liver than in pancreas, while the amount of tRNATrp is not higher in pancreas than in liver as compared to other tRNAs. Among several interpretations, it is possible that tryptophanyl-tRNA synthetase is involved in a function other than tRNA aminoacylation. This unknown function would be specific to the differentiated organs, since fetal cerebellum and fetal pancreas contain the same amount of tryptophanyl-tRNA synthetase as adult liver.