Making the match and breaking it: values, perceptions, and obstacles of trainees applying into physician-scientist training programs

BACKGROUND

Replenishing the physician-scientist workforce constitutes a central mission of medical education, but the loss of qualified trainees to non-academic positions remains an ongoing threat. Among the barriers facing physician-scientists today is the game-like model of U.S. medical residency matching through the National Research Matching Program (NRPM), which applies several assumptions regarding the comparability of applicant qualifications, cohort size, and the institutional breadth of applicants' training needs.

METHODS

The current report therefore summarizes the survey-based views and experiences of physician-scientist trainees obtained following the 2021-2022 application cycle for research-oriented residency programs, or physician-scientist training programs (PSTPs). From among this small cohort of applicants, we obtained survey-based feedback of 27 PSTP applicants across 17 U.S. medical universities, among whom 85% (23/27) matched into a PSTP.

RESULTS

Among these PSTP applicants, 25/27 (93%) recognized "scientific community" as the most important feature of a postgraduate training program, with applicants identifying as female placing a higher value on the program's infrastructure of personal and/or family support. Most (18/27) respondents found "waiting for interviews" as the most stressful phase of their application cycle, and roughly half of all respondents encountered at least one NRMP policy violation through post-interview communication. Specifically, 93% (25/27) respondents were contacted by at least one PSTP following interviews, and 1/3 of them admitted to feeling pressured into sharing their ranking preferences.

CONCLUSION

We highlight many previously unrecognized priorities among applicants to PSTPs, which include fostering community among its trainees and reinforcing structured mentoring. We uncover an inconsistency among PSTPs regarding the post-interview process, which represents an opportunity to better support applicants seeking to gauge programs according to their clinical, scientific, and academic interests as physician-scientists, while still adhering to NRMP policies.

Inhibition of the AT1R agonistic autoantibody in a rat model of preeclampsia improves fetal growth in late gestation

Placenta ischemia, the initiating event in preeclampsia (PE), is associated with fetal growth restriction. Inhibition of the agonistic autoantibody against the angiotensin type 1 receptor AT1-AA, using an epitope-binding inhibitory peptide ('n7AAc') attenuates increased blood pressure at gestational day (G)19 in the clinically relevant reduced uterine perfusion pressure (RUPP) model of PE. Thus we tested the hypothesis that maternal administration of 'n7AAc' does not transfer to the fetus, improves uterine blood flow and fetal growth, and attenuates elevated placental expression of miRNAs implicated in PE and FGR. Sham or RUPP surgery was performed at G14 with vehicle or 'n7AAc' (144 µg/day) administered via an osmotic pump from G14 to G20. Maternal plasma levels of the peptide on G20 were 16.28 ± 4.4 nM, and fetal plasma levels were significantly lower at 1.15 ± 1.7 nM (P = 0.0007). The uterine artery resistance index was significantly elevated in RUPP (P < 0.0001) but was not increased in 'n7AAc'-RUPP or 'n7AAc'-Sham versus Sham. A significant reduction in fetal weight at G20 in RUPP (P = 0.003) was not observed in 'n7AAc'-RUPP. Yet, percent survival was reduced in RUPP (P = 0.0007) and 'n7AAc'-RUPP (P < 0.0002). Correlation analysis indicated the reduction in percent survival during gestation was specific to the RUPP (r = 0.5342, P = 0.043) and independent of 'n7AAc'. Placental miR-155 (P = 0.0091) and miR-181a (P = 0.0384) expression was upregulated in RUPP at G20 but was not elevated in 'n7AAc'-RUPP. Collectively, our results suggest that maternal administration of 'n7AAc' does not alter fetal growth in the RUPP implicating its potential as a therapeutic for the treatment of PE.NEW & NOTEWORTHY The seven amino acid inhibitory peptide to the AT1-AA ('n7AAc') has limited transfer to the fetus at gestational day 20, improves uterine blood flow and fetal growth in the reduced uterine perfusion pressure model of preeclampsia (PE), and does not impair fetal survival during gestation in sham-operated or placental ischemic rats. Collectively, these findings suggest that maternal administration of 'n7AAc' as an effective strategy for the treatment of PE is associated with improved outcomes in the fetus.

Elastin-Like Polypeptide: VEGF-B Fusion Protein for Treatment of Preeclampsia

[Figure: see text].

Abstract 25: Inhibition Of The Agonistic Ang II Type 1 Receptor Autoantibody In A Rat Model Of Preeclampsia Improves Uteroplacental Perfusion And Fetal Growth In Late Gestation

Placenta ischemia, the initiating factor in preeclampsia (PE), is associated with intrauterine growth restriction (IUGR) and increased blood pressure (BP) in offspring. Yet, the only treatment for PE is delivery of the baby and placenta. The Reduced Uterine Perfusion Pressure (RUPP) rat model induced by placental ischemia at gestational day 14 (G14) mimics many facets of human PE including pregnancy-specific hypertension, an increase in the agonistic ANG II Type 1 receptor autoantibody (AT1-AA), IUGR and increased BP in the offspring. Inhibition of AT1-AA using an epitope-binding inhibitory peptide ('n7AAc') attenuates increased BP at gestational day 19 in the RUPP. Yet, whether use of ‘n7aac’ improves fetal growth and mitigates increased BP in the offspring is unknown. Thus, we tested the hypothesis that maternal administration of ‘n7aac’ improves fetal growth by attenuating reduced uterine blood flow and impaired placental remodeling. Sham or RUPP surgery was performed at G14 with administration of vehicle or ‘n7aac’ (144μg/day) via mini osmotic pump until gestational day 20 (G20). At G20 uterine artery resistance index was significantly elevated in vehicle RUPP (0.69±0.02 mm/s n=10) compared to vehicle Sham (0.48±0.02 mm/s n=8) (P<0.0001) and not increased in treated RUPP (0.49±0.02 mm/s n=10) or treated Sham (0.48±0.02 mm/s n=9). Fetal weight was significantly reduced in vehicle RUPP (3.24±0.2 g) compared to vehicle Sham (3.92±0.05 g) (P=0.013) and not decreased in treated RUPP (3.70±0.04 g) or Sham (3.98±0.10 g). Litter size of viable pups at G20 was only reduced in treated RUPP (5.3±1.4) compared to vehicle Sham (11.56±0.7) (P=0.003). Importantly, using in vivo imaging, little to no auto fluorescence of rhodamine-labeled peptide (480 μg/kg/day, n=4) was detectable in the pups at G20. Thus, our results demonstrate that maternal treatment with ‘n7aac’ in the RUPP rat model of PE improve UARI, which is associated with improved fetal weight at G20 in response to placental ischemia. Whether this benefit continues to birth and mitigates increased BP in IUGR offspring is unknown but is the focus of future studies. In conclusion, inhibition of the AT1AA during PE may not only provide benefit to the mother, but may also be associated with benefit in the offspring.

Polymer size affects biodistribution and placental accumulation of the drug delivery biopolymer elastin-like polypeptide in a rodent pregnancy model

INTRODUCTION

Fusion of therapeutic agents to Elastin-like Polypeptide (ELP) is a novel drug delivery strategy for prevention of placental drug transfer. Previous studies have used a 60 kDa ELP tag for this purpose. However, placental transfer of ELP may be size dependent. The goal of this study was to measure the effects of ELP polymer size on pharmacokinetics, biodistribution, and placental transfer of ELP.

METHODS

Three ELPs ranging from 25 to 86 kDa (4.1-6.8 nm hydrodynamic radius) were fluorescently labeled and administered by i.v. bolus to pregnant Sprague Dawley rats on gestational day 14. Plasma levels were monitored for 4 h, organ levels and placental transfer determined by ex vivo fluorescence imaging, and placental localization determined by confocal microscopy.

RESULTS

Increasing ELP size resulted in slower plasma clearance and increased deposition in all major maternal organs, except in the kidneys where an opposite effect was observed. Placental levels increased with an increase in size, while in the pups, little to no ELP was detected.

DISCUSSION

Pharmacokinetics and biodistribution of ELPs during pregnancy are size dependent, but all ELPs tested were too large to traverse the placental barrier. These studies verify that ELP fusion is a powerful method of modulating half-life and preventing placental transfer of cargo molecules. The tunable nature of the ELP sequence makes it ideal for drug delivery applications during pregnancy, where it can be used to target drugs to the mother while preventing fetal drug exposure.

A kidney-selective biopolymer for targeted drug delivery

Improving drug delivery to the kidney using renal-targeted therapeutics is a promising but underdeveloped area. We aimed to develop a kidney-targeting construct for renal-specific drug delivery. Elastin-like polypeptides (ELPs) are nonimmunogenic protein-based carriers that can stabilize attached small-molecule and peptide therapeutics. We modified ELP at its NH₂-terminus with a cyclic, seven-amino acid kidney-targeting peptide (KTP) and at its COOH-terminus with a cysteine residue for tracer conjugation. Comparative in vivo pharmacokinetics and biodistribution in rat and swine models and in vitro cell binding studies using human renal cells were performed. KTP-ELP had a longer plasma half-life than ELP in both animal models and was similarly accumulated in kidneys at levels fivefold higher than untargeted ELP, showing renal levels 15- to over 150-fold higher than in other major organs. Renal fluorescence histology demonstrated high accumulation of KTP-ELP in proximal tubules and vascular endothelium. Furthermore, a 14-day infusion of a high dose of ELP or KTP-ELP did not affect body weight, glomerular filtration rate, or albuminuria, or induce renal tissue damage compared with saline-treated controls. In vitro experiments showed higher binding of KTP-ELP to human podocytes, proximal tubule epithelial, and glomerular microvascular endothelial cells than untargeted ELP. These results show the high renal selectivity of KTP-ELP, support the notion that the construct is not species specific, and demonstrate that it does not induce acute renal toxicity. The plasticity of ELP for attachment of any class of therapeutics unlocks the possibility of applying ELP technology for targeted treatment of renal disease in future studies.

Purification and properties of cysteinyl-tRNA synthetase from rabbit liver

Cysteinyl-tRNA synthetase (CRS) from rabbit liver was purified 8300-fold to a constant specific activity. SDS-PAGE revealed the presence of two polypeptides of 86 kDa and 92 kDa, in the proportions of 60% and 40% respectively. The SDS-electrophoretic migration of the major 86 kDa component was indistinguishable from that of the single polypeptide previously found in CRS from S. cerevisiae. The two polypeptides from rabbit CRS were inaccessible to Edman degradation, but internal peptides generated from each by in-gel proteolysis after SDS-electrophoretic separation, yielded sequences found in the deduced protein sequence of human CRS. Moreover, subjecting the two polypeptides separated by SDS-PAGE to a renaturation treatment showed that CRS activity was associated with both. The structure of the native enzyme was probed by limited proteolysis with elastase. The strikingly simple degradation pattern observed supported a model according to which the two polypeptides derive from the same gene, differing only by a approximately 6 kDa extension located at the C-terminal extremity of the 92 kDa component. Moreover, the finding that notwithstanding the presence of the two polypeptides, the behaviour of rabbit CRS upon gel-filtration or chemical cross-linking was indistinguishable from that of homodimeric yeast CRS, indicated that the 6 kDa C-terminal extension on the 92 kDa polypeptide does not impede dimerisation. The origin of the two components of rabbit CRS is discussed in light of the deduced protein sequence of human CRS derived from the published cDNA sequence and the recently released genomic sequence of the human enzyme.

Cysteinyl-tRNA synthetase from Saccharomyces cerevisiae. Purification, characterization and assignment to the genomic sequence YNL247w

Cysteinyl-tRNA synthetase (CRS) from Saccharomyces cerevisiae was purified 2300-fold with a yield of 33%, to a high specific activity (kcat4.3 s-1 at 25 degrees C for the aminoacylation of yeast tRNACys). SDS-PAGE revealed a single polypeptide corresponding to a molecular mass of 86 kDa. Polyclonal antibodies to the purified protein inactivated CRS activity and detected only one polypeptide of 86 kDa in a yeast extract subjected to SDS-PAGE followed by immunoblotting. In contrast to bacterial CRS which is a monomer of about 50 kDa, the native yeast enzyme behaved as a dimer, as assessed by gel filtration and cross-linking. Its subunit molecular mass is in good agreement with the value of 87.5 kDa calculated for the protein encoded by the yeast genomic sequence YNL247w. The latter was previously tentatively assigned to CRS, based on limited sequence similarities to the corresponding enzyme from other sources. Determination of the amino acid sequence of internal polypeptides derived from the purified yeast enzyme confirmed this assignment. Alignment of the primary sequences of prokaryotic and yeast CRS reveals that the larger size of the latter is accounted for mostly by several insertions within the sequence.

Expression of rat aspartyl-tRNA synthetase in Saccharomyces cerevisiae. Role of the NH2-terminal polypeptide extension on enzyme activity and stability

Cytoplasmic aspartyl-tRNA synthetase from mammals is one of the components of a multienzyme complex comprising nine synthetase activities. The presence of an amino-terminal extension composed of about 40 residues is a characteristic of the eukaryotic enzyme. We report here the expression in the yeast Saccharomyces cerevisiae of a native form of rat aspartyl-tRNA synthetase and of two truncated derivatives lacking 20 or 36 amino acid residues from their amino-terminal polypeptide extension. The three recombinant enzyme species were purified to homogeneity. They behave as alpha2 dimers and display catalytic parameters in the tRNA aminoacylation reaction identical to those determined for the native, complex-associated form of aspartyl-tRNA synthetase isolated from rat liver. Because the dimer dissociation constant of rat AspRS is much higher than that of its bacterial and yeast counterparts, we could establish a direct correlation between dissociation of the dimer and inactivation of the enzyme. Our results clearly show that the monomer is devoid of amino acid activation and tRNA aminoacylation activities, indicating that dimerization is essential to confer an active conformation on the catalytic site. The two NH2-terminal truncated derivatives were fully active, but proved to be more unstable than the recombinant native enzyme, suggesting that the polypeptide extension fulfills structural rather than catalytic requirements.

Polyanion-induced alpha-helical structure of a synthetic 23-residue peptide representing the lysine-rich segment of the N-terminal extension of yeast cytoplasmic aspartyl-tRNA synthetase

Conformational studies were performed on the synthetic tricosapeptide N-acetyl-SKKALKKLQKEQEKQRKKEERAL-amide, representing the highly basic segment (residues 30-52) of the N-terminal extension of yeast cytoplasmic aspartyl-tRNA synthetase. Circular dichroism experiments show that, in aqueous solution at neutral pH, the peptide adopts a random conformation. The effects of pH, temperature, addition of trifluoroethanol (TFE), and titration with polyanions on the conformation of the peptide were studied. In TFE or in the presence of an equimolar concentration of (phosphate)18, the peptide adopts a 100% alpha-helical conformation. A partially alpha-helical conformation is induced by (phosphate)4 or d(pT)8 (respectively 40% and 35% helical content). Raising the pH in aqueous solution promotes 75% alpha-helicity, with a transition pK of 9.9 reflecting deprotonation of lysine residues. On the basis of these results, nuclear magnetic resonance studies were carried out in TFE as well as in aqueous solution in the presence of (phosphate)18, to determine the structure of the molecule. Complete 1H resonance assignments were obtained by conventional two-dimensional NMR techniques. A total of 138 interproton constraints derived from NOESY experiments were used to calculate the three-dimensional structure by a two-stage distance geometry/simulated annealing procedure. The two deduced structures were highly similar and show that nine cationic residues are segregated on one face of a helical structure, providing an ideal polycationic interface for binding to polyanionic surfaces.

Reconstitution in vitro of the valyl-tRNA synthetase-elongation factor (EF) 1 beta gamma delta complex. Essential roles of the NH2-terminal extension of valyl-tRNA synthetase and of the EF-1 delta subunit in complex formation

Valyl-tRNA synthetase from mammalian cells is isolated exclusively as a complex with elongation factor (EF) 1H (the "heavy" form of eukaryotic EF-1, composed of subunits alpha, beta, gamma, and delta). In a previous study, the 140-kDa valyl-tRNA synthetase subunit dissociated from the purified rabbit liver complex was shown to display hydrophobic properties, unlike the corresponding yeast cytoplasmic enzyme of 125 kDa (Bec, G., and Waller, J.-P. (1989) J. Biol. Chem. 264, 21138-21143). Compared to the sequence of yeast cytoplasmic valyl-tRNA synthetase, that of the human enzyme displays an NH2-terminal extension of approximately 200 amino acid residues that bears strong sequence similarity to the NH2-terminal moiety of EF-1 gamma (Hsieh, S. L., and Campbell, R. D. (1991) Biochem. J. 278, 809-816). We now show that this NH2-terminal extension can be selectively excised by elastase treatment of the isolated rabbit valyl-tRNA synthetase, without impairing catalytic activity. To examine the role of the NH2-terminal extension of mammalian valyl-tRNA synthetase in complex formation and to identify the subunit(s) of EF-1H responsible for binding the enzyme, reconstitution experiments were undertaken. Native or truncated valyl-tRNA synthetases were incubated with the isolated EF-1 subunits beta gamma and delta, either separately or in combination, and the ensuing products were analyzed by chromatography on DEAE-Sepharose FF and Superose 6. The results demonstrate that the NH2-terminal extension of valyl-tRNA synthetase is required for complex formation and that the enzyme-binding site(s) resides on the EF-1 delta subunit. Moreover, although the EF-1 beta gamma binary complex does not bind valyl-tRNA synthetase, it is nevertheless required for assembly of a complex of defined quaternary structure by preventing the formation of high molecular weight aggregates generated in the presence of EF-1 delta alone.

Mammalian prolyl-tRNA synthetase corresponds to the approximately 150 kDa subunit of the high-M(r) aminoacyl-tRNA synthetase complex

The high-M(r) aminoacyl-tRNA synthetase complex previously purified from sheep liver differed from those isolated from several other mammalian sources by the absence of prolyl-tRNA synthetase activity and the presence of glutamyl tRNA synthetase as a polypeptide of 85 kDa instead of 150 kDa. Using a milder extraction procedure that minimizes proteolysis, we now report the isolation of a sheep liver complex that contains both prolyl-tRNA synthetase activity and the 150-kDa polypeptide. The correspondence between prolyl-tRNA synthetase and the 150-kDa polypeptide, inferred from the results of several approaches reported in this study, was further demonstrated by showing that antibodies to a free form of sheep liver prolyl-tRNA synthetase generated by endogenous proteolysis, specifically reacted with the 150-kDa components of the complexes from sheep and rabbit, but failed to react with the previously purified complex from sheep that contained neither prolyl-tRNA synthetases activity nor the 150-kDa component. Moreover, we show that the 150-kDa polypeptide is also recognized by antibodies to the 85-kDa polypeptide previously assigned to glutamyl-tRNA synthetase. The possibility that the largest subunit of the mammalian high-M(r) complexes may be a bifunctional protein encoding both glutamyl- and prolyl-tRNA synthetase activities is considered and discussed in light of the recently published sequence of the corresponding polypeptide from HeLa cells. In accordance with this prediction, we show that the amino acid sequence of the carboxyl-terminal moiety of this bifunctional polypeptide shows significant similarity to the sequence of prolyl-tRNA synthetase from Escherichia coli.

Interaction of microtubule-associated proteins with microtubules: yeast lysyl- and valyl-tRNA synthetases and tau 218-235 synthetic peptide as model systems

The respective contributions of electrostatic interaction and specific sequence recognition in the binding of microtubule-associated proteins (MAPs) to microtubules have been studied, using as models yeast valyl- and lysyl-tRNA synthetases (VRS, KRS) that carry an exposed basic N-terminal domain, and a synthetic peptide reproducing the sequence 218-235 on tau protein, known to be part of the microtubule-binding site of MAPs. VRS and KRS bind to microtubules with a KD in the 10(-6) M range, and tau 218-235 binds with a KD in the 10(-4) M range. Binding of KRS and tau 218-235 is accompanied by stabilization and bundling of microtubules, without the intervention of an extraneous bundling protein. tau 218-235 binds to microtubules with a stoichiometry of 2 mol/mol of assembled tubulin dimer in agreement with the proposed binding sequences alpha[430-441] and beta[422-434]. Binding stoichiometries of 2/alpha beta S tubulin and 1/alpha S beta S tubulin were observed following partial or complete removal of the tubulin C-terminal regions by subtilisin, which localizes the site of subtilisin cleavage upstream residue alpha-441 and downstream residue beta-434. Quantitative measurements show that binding of MAPs, KRS, VRS, and tau 218-235 is weakened but not abolished following subtilisin digestion of the C-terminus of tubulin, indicating that the binding site of MAPs is not restricted to the extreme C-terminus of tubulin.

Valyl-tRNA synthetase from rabbit liver. II. The enzyme derived from the high-Mr complex displays hydrophobic as well as polyanion-binding properties

The preceding paper (Bec, G., Kerjan, P., Zha, X.D., and Waller, J.P. (1989) J. Biol. Chem. 264, 21131-21137) described the purification to apparent homogeneity from rabbit liver, of a heterotypic complex comprising valyl-tRNA synthetase and Elongation Factor 1H. In the present study, valyl-tRNA synthetase was dissociated and separated from the other components of this complex by hydroxylapatite chromatography in the presence of 0.5 M NaSCN. The properties of the homogeneous mammalian enzyme were compared to those of the corresponding enzyme from yeast. Both behaved as monomeric entities, with apparent molecular masses of 140 and 125 kDa, respectively. Furthermore, both displayed strong affinity toward the polyanionic support heparin-Ultrogel, a property not manifested by the corresponding prokaryotic enzyme. However, unlike the yeast enzyme, that of mammalian origin additionally exhibited hydrophobic properties, as reflected by its affinity toward phenyl-Sepharose. A structural model is proposed according to which mammalian valyl-tRNA synthetase has conserved the polycationic N-terminal domain that distinguishes the corresponding lower eukaryotic enzyme from its prokaryotic counterpart, while acquiring a hydrophobic domain most likely responsible for its association to Elongation Factor 1H.

Valyl-tRNA synthetase from rabbit liver. I. Purification as a heterotypic complex in association with elongation factor 1

Valyl-tRNA synthetase occurs as a high molecular mass entity of approximately equal to 700 kDa in the crude extract from rabbit liver. The enzyme was purified as a heterotypic complex comprising four polypeptides of 140, 50, 35, and 27 kDa in the molar proportions of 1:2:1:1, respectively, as determined by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Co-purification of these components at each step of the purification supports the conclusion that they are physically associated within the same complex. In addition to valyl-tRNA synthetase activity, which was assigned to the 140-kDa component, the purified complex exhibits a potent Elongation Factor 1 activity, determined by its ability to sustain poly(U)-dependent polyphenylalanine synthesis in the presence of Elongation Factor 2. Our results are essentially in agreement with those from a recent report (Motorin, Y., Wolfson, A., Orlovsky, A., and Gladilin, K. (1988) FEBS Lett. 238, 262-264), according to which the polypeptides other than that assigned to valyl-tRNA synthetase correspond to the subunits of Elongation Factor 1H.

Molecular cloning and primary structure of cDNA encoding the catalytic domain of rat liver aspartyl-tRNA synthetase

A cDNA clone encoding rat liver aspartyl-tRNA synthetase was isolated by probing a lambda gt11 recombinant cDNA expression library with antibodies directed against the corresponding polypeptide from sheep liver. The 1930-base pairs-long cDNA insert allowed the expression in Escherichia coli of an active enzyme of mammalian origin. The nucleotide sequence of that cDNA, corresponding to the DRS1 gene, was determined. The open reading frame of DRS1 corresponds to a protein of Mr = 57,061, in good agreement with the previously determined molecular weight of the purified enzyme. The deduced amino acid sequence shows extensive homologies with that of yeast cytoplasmic aspartyl-tRNA synthetase, more than 50% of the residues being identical. In rat liver, aspartyl-tRNA synthetase occurs in two distinct forms: a dimeric enzyme and a component of a multienzyme complex comprising the nine aminoacyl-tRNA synthetases specific for arginine, aspartic acid, glutamic acid, glutamine, isoleucine, leucine, lysine, methionine, and proline. The primary structure of the DRS1 gene product is discussed in relation to the occurrence of two distinct forms of that enzyme.

The yeast lysyl-tRNA synthetase gene. Evidence for general amino acid control of its expression and domain structure of the encoded protein

The nucleotide sequence of a 3.6-kilobase pair DNA fragment containing the structural gene for yeast cytoplasmic lysyl-tRNA synthetase (KRS1) and its flanking regions was determined. The encoded protein of 67,881 kDa displays a cluster of 11 lysines within a 29-amino acid residue segment at its amino-terminal extremity. Evidence is presented that this segment is responsible for the affinity displayed by the native enzyme toward polyanionic carriers. The transcription initiation sites of the KRS1 gene were determined. Upstream from the TATA box, putative control elements corresponding to the concensus sequences for the RPG box and the general amino acid control system were identified. Evidence for transcriptional induction of the KRS1 gene via the general amino acid control system is presented.

Structure and expression of the genes encoding the alpha and beta subunits of yeast phenylalanyl-tRNA synthetase

The two genes FRS1 and FRS2 encoding, respectively, the large (alpha) and small (beta) subunits of cytoplasmic phenylalanyl-tRNA synthetase from bakers' yeast have been cloned and sequenced. The derived protein primary structures are confirmed by peptide sequences evenly distributed along the reading frames. These predict a subunit Mr of 67,347 for alpha and 57,433 for beta, in good agreement with earlier determinations carried out on the purified protein. These subunit sequences have been compared to those of Escherichia coli phenylalanyl-tRNA synthetase as well as to the small beta subunit of the corresponding yeast mitochondrial enzyme; limited but significant homology was found between the two alpha subunits on the one hand and between the three beta subunits on the other hand. The results suggest that these three enzymes, from E. coli, yeast cytoplasm, and yeast mitochondria, have strongly diverged from one another. The initiation sites of transcription have been determined for both yeast genes. Their 5'-upstream regions show no sequence similarities that would have indicated a coordinate control of gene expression at the transcriptional level. Measurements of steady-state levels of FRS-mRNAs in overproducing strains indicate that there is no restriction in mRNA synthesis. Therefore the control of gene expression, leading to a balanced synthesis of alpha and beta subunits, is likely to occur at the translational level.

Overexpression of mammalian phenylalanyl-tRNA synthetase upon phenylalanine restriction

The effect of phenylalanine restriction on the level of expression of phenylalanyl-tRNA synthetase from cultured Chinese hamster ovary cells was investigated. By lowering the phenylalanine concentration from 200 to 2 microM, cell growth was arrested, tRNAPhe aminoacylation level was rapidly and specifically decreased and phenylalanyl-tRNA synthetase was derepressed. The progressive 2-fold elevation of phenylalanyl-tRNA synthetase level was determined by activity measurement and immunotitration. None of the other aminoacyl-tRNA synthetases tested were significantly affected.

Expression of the aminoacyl-tRNA synthetase complex in cultured Chinese hamster ovary cells. Specific depression of the methionyl-tRNA synthetase component upon methionine restriction

Cultured Chinese hamster ovary cells were subjected to amino acid restriction to examine its effects on the level of expression of the nine aminoacyl-tRNA synthetase components of the multienzyme complex which was previously characterized (Mirande, M., Le Corre, D., and Waller, J.-P. (1985) Eur. J. Biochem. 147, 281-289). Lowering the methionine concentration in the medium from 100 to 1 microM led to growth arrest, rapid deacylation of tRNAMet, and progressive 2-fold elevation of the methionyl-tRNA synthetase level, as assessed by specific activity measurements and immunotitration. The levels of the other eight aminoacyl-tRNA synthetases were not affected. Total methionine deprivation led to the additional derepression of the leucyl- and isoleucyl-tRNA synthetase components, whereas the corresponding tRNAs remained fully acylated. These pleiotropic responses to total methionine restriction were abolished in the presence of 2 mM methioninol, suggesting that amino acid transport systems may play a role in the regulation of aminoacyl-tRNA synthetase expression. The effect of total deprivation of arginine, glutamine, isoleucine, leucine, lysine, or proline from the culture medium on the level of expression of the corresponding aminoacyl-tRNA synthetases was also examined. In all cases, no elevation of the level of the corresponding synthetase was observed. The behavior of methionyl-tRNA synthetase from Chinese hamster ovary cells displaying a 2-fold increased level of the enzyme due to methionine restriction was examined in detail. Failure to detect a free form of the enzyme by gel filtration, as well as the finding that the isolated complex displayed twice the amount of methionyl-tRNA synthetase relative to the other components, indicates that this multienzyme structure can accommodate at least one additional copy of one of its components.

Cloning of yeast lysyl- and phenylalanyl-tRNA synthetase genes

Cloning of yeast lysyl- and phenylalanyl-tRNA synthetase genes was accomplished by probing a lambda gt11 recombinant DNA expression library with antibodies directed against the purified enzymes. Several DNA clones encoding either the alpha or the beta subunit of phenylalanyl-tRNA synthetase were isolated. In each case, the inserted DNA was oriented in the same direction with respect to the lambda gt11 lacZ transcription unit, giving rise to the expression of hybrid proteins. The corresponding DNA fragments constitute suitable hybridization probes for the isolation of complete nucleotide sequences encoding the alpha and beta subunits of the enzyme. Recombinant DNA lambda gt11 clones encoding lysyl-tRNA synthetase were also selected. One of these contained yeast DNA inserted with the opposite orientation with respect to lacZ. The lysogen corresponding to that recombinant DNA phage produced an active, native lysyl-tRNA synthetase. The 3.6 kbp DNA insert contained all the information necessary for the expression of yeast lysyl-tRNA synthetase in E. coli.

Purification and characterization of the isoleucyl-tRNA synthetase component from the high molecular weight complex of sheep liver: a hydrophobic metalloprotein

Native isoleucyl-tRNA synthetase and a structurally modified form of methionyl-tRNA synthetase were purified to homogeneity following trypsinolysis of the high molecular weight complex from sheep liver containing eight aminoacyl-tRNA synthetases. The correspondence between purified isoleucyl-tRNA synthetase and the previously unassigned polypeptide component of Mr 139 000 was established. It is shown that dissociation of this enzyme from the complex has no discernible effect on its kinetic parameters. Both isoleucyl- and methionyl-tRNA synthetases contain one zinc ion per polypeptide chain. In both cases, removal of the metal ion by chelating agents leads to an inactive apoenzyme. As the trypsin-modified methionyl-tRNA synthetase has lost the ability to associate with other components of the complex [Mirande, M., Kellermann, O., & Waller, J. P. (1982) J. Biol. Chem. 257, 11049-11055], the zinc ion is unlikely to be involved in complex formation. While native purified isoleucyl-tRNA synthetase displays hydrophobic properties, trypsin-modified methionyl-tRNA synthetase does not. It is suggested that the assembly of the amino-acyl-tRNA synthetase complex is mediated by hydrophobic domains present in these enzymes.

Leucyl-tRNA and lysyl-tRNA synthetases, derived from the high-Mr complex of sheep liver, are hydrophobic proteins

The leucyl-tRNA and lysyl-tRNA synthetase components of the multienzyme complex from sheep liver were selectively dissociated by hydrophobic interaction chromatography on hexyl-agarose and purified to homogeneity. Conservation of activities during the purification required the presence of Triton X-100. The homogeneous enzymes corresponded to a monomer of Mr 129000 and a dimer of Mr 2 X 79000, respectively. Both were strongly adsorbed to the hydrophobic support phenyl-Sepharose, in conditions where the corresponding purified enzymes from yeast and Escherichia coli were not bound. Moreover, like the corresponding enzymes from yeast but unlike those of prokaryotic origin, the purified leucyl-tRNA and lysyl-tRNA synthetases derived from the complex displayed affinity for polyanionic supports. It is shown that proteolytic conversion of lysyl-tRNA synthetase to a fully active dimer of Mr 2 X 64000, leads to loss of both the hydrophobic and the polyanion-binding properties. These results support the view that each subunit of lysyl-tRNA synthetase is composed of a major catalytic domain, similar in size to the subunit of the prokaryotic enzyme, contiguous to a chain extension which carries both cationic charges and hydrophobic residues. The implications of these findings on the structural organization of the complex are discussed in relation to its other known properties.

Multiple forms of arginyl- and lysyl-tRNA synthetases in rat liver: a re-evaluation

The size distribution of lysyl- and arginyl-tRNA synthetases in crude extracts from rat liver was re-examined by gel filtration. It is shown that irrespective of the addition or not of several proteinase inhibitors, lysyl-tRNA synthetase was present exclusively as a high-Mr entity, while arginyl-tRNA synthetase occurred as high- and low-Mr forms, in the constant proportions of 2:1, respectively. The polypeptide molecular weights of the arginyl-tRNA synthetase in these two forms were 74000 and 60000, respectively. The high-Mr forms of lysyl- and arginyl-tRNA synthetases were co-purified to yield a multienzyme complex, the polypeptide composition of which was virtually identical to that of the complexes from rabbit liver and from cultured Chinese hamster ovary cells. Of the nine aminoacyl-tRNA synthetases, specific for lysine, arginine, methionine, leucine, isoleucine, glutamine, glutamic and aspartic acids and proline, which characterize the purified complex, each, except prolyl-tRNA synthetase, was assigned to the constituent polypeptides by the protein-blotting procedure, using the previously characterized antibodies to the aminoacyl-tRNA synthetase components of the corresponding complex from sheep liver.

Do yeast aminoacyl-tRNA synthetases exist as soluble enzymes within the cytoplasm?

The aminoacyl-tRNA synthetases from a crude extract of yeast were shown to bind to heparin-Ultrogel through ionic interactions, in conditions where the corresponding enzymes from Escherichia coli did not. The behaviour of purified lysyl-tRNA synthetases from yeast and E. coli was examined in detail. The native dimeric enzyme from yeast (Mr 2 X 73000) strongly interacted with immobilized heparin or tRNA, as well as with negatively charged liposomes, in conditions where the corresponding native enzyme from E. coli (Mr 2 X 65000) displayed no affinity for these supports. Moreover, the aptitude of the native enzyme from yeast to interact with polyanionic carriers was lost on proteolytic conversion to a fully active modified dimer of Mr 2 X 65500. A structural model is proposed, according to which each subunit of yeast lysyl-tRNA synthetase is composed of a functional domain similar in size to that of the prokaryotic enzyme, contiguous to a 'binding' domain responsible for association to negatively charged carriers. The evolutionary acquisition of this property by lower eukaryotic aminoacyl-tRNA synthetases suggests that it fulfils an important function in vivo, unrelated to catalysis. We propose that it promotes the compartmentalization of these enzymes within the cytoplasm, through associations with as yet unidentified, negatively charged components, by electrostatic interactions too fragile to withstand the usual extraction conditions.

A complex from cultured Chinese hamster ovary cells containing nine aminoacyl-tRNA synthetases. Thermolabile leucyl-tRNA synthetase from the tsH1 mutant cell line is an integral component of this complex

The size distribution of the 20 aminoacyl-tRNA synthetases from wild-type Chinese hamster ovary (CHO) cells and from the mutant cell line tsH1, containing a temperature-sensitive leucyl-tRNA synthetase, was determined by gel filtration. Nine aminoacyl-tRNA synthetases, specific for arginine, aspartic acid, glutamic acid, glutamine, isoleucine, leucine, lysine, methionine and proline, which coeluted as high-Mr entities (Mr approximately 1.2 X 10(6)), were further co-purified to yield a multienzyme complex, the polypeptide composition of which was identical to that previously determined for the complex from rabbit liver. Immunoprecipitates obtained from crude extracts of wild-type and tsH1 mutant cells, using specific antibodies directed to the lysyl-tRNA or methionyl-tRNA synthetase components of the complex, displayed the same polypeptide compositions as that of the purified complex, thereby establishing the heterotypic nature of this complex. Although the activity of leucyl-tRNA synthetase from the mutant cells, grown at a permissive temperature, was low compared to that from the wild-type, the polypeptide of Mr 129 000, corresponding to this enzyme, was present in similar amounts and occurred exclusively as a component of the high-Mr complex. Finally, we report that attempts to demonstrate phosphorylation of the components of the complex from cultured CHO, HeLa and C3 cells were unsuccessful.

Association of an aminoacyl-tRNA synthetase complex and of phenylalanyl-tRNA synthetase with the cytoskeletal framework fraction from mammalian cells

The intracellular distribution of several mammalian aminoacyl-tRNA synthetases was investigated by biochemical and immunocytological approaches. The fraction of amino-acyl-tRNA synthetases bound to the detergent-insoluble cytoskeletal framework obtained after extraction of NRK cells by 0.1% Triton X-100 was estimated, by activity measurements, to about 80% for phenylalanyl-tRNA synthetase and 40% for the high-molecular-weight (HMW) complex containing the seven aminoacyl-tRNA synthetases specific for glutamic acid, isoleucine, leucine, methionine, glutamine, lysine, and arginine. This association was shown to be salt-dependent. The subcellular localization of these enzymes was examined using an immunocytological approach. When cultured cells were fixed with paraformaldehyde and then permeabilized with Triton X-100, a fairly uniform cytoplasmic labelling was observed with antibodies directed to the aminoacyl-tRNA synthetase complex or to phenylalanyl-tRNA synthetase. By contrast, when cells were extracted with 0.1% Triton X-100 prior to fixation with paraformaldehyde, the staining patterns obtained with antibodies to aminoacyl-tRNA synthetases were very similar to that obtained with antibodies to rough endoplasmic reticulum, as assessed by single or double indirect immunofluorescence microscopy. These results suggest that free and bound forms of these aminoacyl-tRNA synthetases may coexist within the cell. In addition to cytoplasmic labelling, antibodies directed to phenylalanyl-tRNA synthetase stained the nucleus of rapidly growing cells. The possible significance of this finding is discussed.

Phenylalanyl-tRNA synthetases from sheep liver and yeast. Correlation between net charge and binding to ribosomes

Unlike phenylalanyl-tRNA synthetase from lower eukaryotes, the corresponding enzyme from higher eukaryotes displays a pronounced tendency to associate with ribosomes in vitro. To attempt to uncover the structural features responsible for this difference in behavior, a comparative study of the enzymes purified to homogeneity from sheep liver and yeast was undertaken. The two alpha 2 beta 2-type enzymes displayed remarkably similar subunit molecular masses (71 and 63 kDa for sheep, 74 and 63 kDa for yeast), yet differed markedly in their isoelectric points (8.0 and 5.6 pH units, respectively). Mild tryptic digestion of the enzyme from sheep led to preferential degradation of the 63-kDa beta subunit into two major fragments of 35 and 25 kDa, respectively, with concomitant loss of activity. The isoelectric points of the denatured fragments were found to be distinctly lower than that of the denatured beta subunit, implying that the residues responsible for the basic net charge of the original beta subunit are mainly clustered in a small portion of the polypeptide chain which was excised during proteolysis. Despite their different isoelectric points, the enzymes from yeast and sheep displayed identical requirements for aminoacylation of tRNA at optimal rates. Moreover, the incidence of variations in pH and ionic strength on the kinetic parameters of the two enzymes was indistinguishable. Interpreted in terms of the polyelectrolyte theory, these results support the view that the residues responsible for the basic net charge of the mammalian enzyme are located in a region distal from the active site. It is suggested that the cationic charge of the enzyme allows anchorage to a cellular component carrying negative charges, possibly the ribosome.

Sedimentation behaviour of aminoacyl-tRNA synthetases from mixed lysates of yeast and rabbit liver

The subcellular distribution of five aminoacyl-tRNA synthetases from yeast, including lysyl-, arginyl- and methionyl-tRNA synthetases known to exist as high-molecular-weight complexes in lysates from higher eukaryotes, was investigated. To minimize the risks of proteolysis, spheroplasts prepared from exponentially grown yeast cells were lysed in the presence of several proteinase inhibitors, under conditions which preserved the integrity of the proteinase-rich vacuoles. The vacuole-free supernatant was subjected to sucrose density gradient centrifugation. No evidence for multimolecular associations of these enzymes was found. In particular, phenylalanyl-tRNA synthetase activity was not associated with the ribosomes, whereas purified phenylalanyl-tRNA synthetase from sheep liver, added to the yeast lysate prior to centrifugation, was entirely recovered in the ribosomal fraction. A mixture of lysates from yeast and rabbit liver was also subjected to sucrose gradient centrifugation and assayed for methionyl- and arginyl-tRNA synthetase activities, under conditions which allowed discrimination between the enzymes originating from yeast and rabbit. The two enzymes from rabbit liver were found to sediment exclusively as high-molecular-weight complexes, in contrast to the corresponding enzymes from yeast, which displayed sedimentation properties characteristic of free enzymes. The preservation of the complexed forms of mammalian aminoacyl-tRNA synthetases upon mixing of yeast and rabbit liver extracts argues against the possibility that failure to observe complexed forms of these enzymes in yeast was due to uncontrolled proteolysis. Furthermore, this result denies the presence, in the crude extract from liver, of components capable of inducing artefactual aggregation of the yeast aminoacyl-tRNA synthetases, and thus indirectly argues against an artefactual origin of the multienzyme complexes encountered in lysates from mammalian cells.

Seven mammalian aminoacyl-tRNA synthetases associated within the same complex are functionally independent

A heterotypic multienzyme complex from sheep liver containing seven aminoacyl-tRNA synthetases specific for isoleucine, leucine, methionine, glutamine, glutamic acid, lysine and arginine was subjected to kinetic analyses to examine the possibility that association of these enzymes may impart kinetic properties which differ from those of their unassociated counterparts. The evidence obtained by two different approaches leads to the conclusion that the associated enzymes are functionally independent. Firstly, the kinetic constants of the methionyl-tRNA and lysyl-tRNA synthetase components of the complex do not differ significantly from those of their unassociated counterparts obtained after controlled proteolysis of the complex. Secondly, the methionyl-tRNA synthetase component of the complex displays identical kinetic constants, whether assayed in the presence of [14C]methionine, ATP and highly enriched tRNAMet alone, or in the additional presence of the substrates required for unlabeled aminoacyl-tRNA formation by each of the other six enzymes. Similarly, the initial rates of [14C]aminoacyl-tRNA formation catalyzed by any of the six other enzymes was unaffected by the concomitant functioning of the other aminoacyl-tRNA synthetases. The sedimentation behaviour of the aminoacyl-tRNA synthetase components of the complex under conditions prevailing in the tRNA aminoacylation assay indicates that they remain associated under these conditions. The implications of these findings on the structural organization of the enzymes within the complex are discussed.

Macromolecular complexes from sheep and rabbit containing seven aminoacyl-tRNA synthetases. II. Structural characterization of the polypeptide components and immunological identification of the methionyl-tRNA synthetase subunit

The extensively purified multienzyme complexes from sheep and rabbit livers containing seven aminoacyl-tRNA synthetases specific for Ile, Leu, Met, Gln, Glu, Lys, and Arg displayed characteristic one-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoretic patterns composed of 11 and 10 major polypeptide components, respectively. Their polypeptide compositions revealed by two-dimensional electrophoresis, including isoelectric focusing in 9 M urea, were not significantly more complex. The isoelectric point of each component from the two complexes fell within the pH range of 6.2 to 7.1, with the notable exception of the common polypeptide of Mr = 43,000 which was distinctly basic. The apparent molecular weight of each component from both complexes was determined by SDS-polyacrylamide gel electrophoresis. Four polypeptides, corresponding to molecular weights of 139,000, 129,000, 43,000, and 38,000 were common to both complexes. The other components from the two complexes displayed similar yet clearly distinct molecular weights. The molar ratios of the polypeptides, estimated by densitometry scanning of stained SDS-polyacrylamide gels, indicated that several components from each complex may be present as more than one copy. Following SDS-polyacrylamide gel electrophoresis, the methionyl-tRNA synthetase component from each complex was identified by the protein blotting procedure, using specific antibodies and 125I-labeled protein A. The unique labeled bands from the complexes of sheep and rabbit precisely matched the major polypeptides of Mr = 103,000 and 108,000, respectively. Mild trypsin treatment of the two native complexes generated fully active forms of methionyl-tRNA synthetase, with molecular weights of 68,000 and 69,500, respectively. The kinetics of proteolysis showed that modification proceeded sequentially through discrete intermediates.

Macromolecular complexes from sheep and rabbit containing seven aminoacyl-tRNA synthetases. I. Species specificity of the polypeptide composition

Using a three-step procedure designed to minimize the risks of proteolysis, high molecular weight complexes containing the same seven aminoacyl-tRNA synthetases specific for isoleucine, leucine, methionine, lysine, arginine, glutamic acid, and glutamine were purified from sheep liver and spleen, as well as from rabbit reticulocytes and liver. The polypeptide composition of these complexes, as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, is characteristic of the animal species from which they are derived. The complexes from sheep liver and spleen display indistinguishable polypeptide patterns composed of 11 major components. Of the 10 common components which characterize the complexes of rabbit reticulocytes and liver, 4 are also shared by the complexes from sheep, while 6 have distinctly different electrophoretic mobilities. Furthermore, in the case of the complex from rabbit reticulocytes, it is shown that the enzyme and polypeptide composition of the complex is independent of the purification method employed. The isolation of high molecular weight complexes of identical aminoacyl-tRNA synthetase and polypeptide compositions from two cell types as radically different as rabbit reticulocytes and hepatocytes suggests that these multienzyme complexes do not arise as artifacts of preparation and supports the view that they reflect a structural organization existing within the cell.

Seven mammalian aminoacyl-tRNA synthetases co-purified as high molecular weight entities are associated within the same complex

Seven aminoacyl-tRNA synthetases from sheep liver were co-purified as high mol. wt. entities to constant specific activities. The purified multienzyme preparation displayed an apparent mol. wt. of approximately 10(6) and was composed of 11 distinct polypeptides, as revealed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). To test the assumption that all of these components were physically associated within the same complex, the purified preparation was subjected to immunoprecipitation by antibodies raised against its lysyl- or methionyl-tRNA synthetase component. Depending on the limiting concentrations of the specific antibodies used, from 5 to 40% of the input protein was recovered in the immunoprecipitate. Its polypeptide composition, as revealed by SDS-PAGE, was indistinguishable from that of the original material. The immunoprecipitation reaction was highly specific, as attested by the observation that IgG from nonimmunized rabbit failed to precipitate any of the 11 polypeptides, even when used in 30-fold molar excess over input protein. We conclude that co-precipitation of all of these polypeptides by antibodies directed against a single component of the purified preparation is a consequence of their physical association within the same multienzyme complex.

Macromolecular complexes of aminoacyl-tRNA synthetases from eukaryotes. 1. Extensive purification and characterization of the high-molecular-weight complex(es) of seven aminoacyl-tRNA synthetases from sheep liver

Starting from homogenates of sheep liver, extensive co-purification of seven aminoacyl-tRNA synthetases to high specific activities was achieved by a three-step procedure involving fractional precipitation by poly(ethylene glycol) 6000, gel filtration on 6% agarose and chromatography on Sepharose-bound tRNA. The purified material is composed of nine major protein components as revealed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and has an apparent molecular weight of about 10(6) estimated by gel filtration on 6% agarose. It contains aminoacyl-tRNA synthetase activities specific for methionine, lysine, arginine, leucine, isoleucine, glutamine and glutamic acid. The rigorous co-elution of these seven enzymes at each chromatographic step suggests, but does not conclusively prove, that they are physically associated within the same complex. The enzyme composition of the high-molecular-weight complex purified from sheep liver is identical to that of the complex previously isolated from human placenta by Denney in 1977 (Arch. Biochem. Biophys. 183, 156--167).

Cobalt(III) labeling of methionyl-tRNA synthetase from Escherichia coli

Native and trypsin-modified methionyl-tRNA synthetases from Escherichia coli were found to be inactivated by incubation in the presence of Co(III) complexes of ATP, stabilized either by imidazole or phenanthroline, or by oxidation in situ to Co(III) of the substrate ATP-Co(II). It has been shown that the inactivation proceeds by specific labeling of the catalytic ATP-Mg(II) site of the synthetases. The enzymes are completely inactivated by the incorporation of one cobalt atom and one ATP molecule per active site. The inactivated enzymes may be stored for a long period without significant reactivation or removal of the cobalt label. In the presence of dithiothreitol or 2-mercaptoethanol, the labeled enzymes recover full activity with concomittant release of the bound label molecules.

Methionyl-tRNA synthetase from sheep liver. Purification of a fully active monomer derived from high-molecular-weight complexes by trypsin treatment. Evidence for immunological cross-reaction with the corresponding enzyme from sheep mammary gland

The size distribution of methionyl-tRNA synthetase in extracts from sheep liver is compared to that of lysyl-tRNA, isoleucyl-tRNA, leucyl-tRNA and seryl-tRNA synthetases by gel filtration on Biogel A-5m. Extraction conditions are described which lead to isolation of methionyl-tRNA synthetase exclusively in the form of complexes of molecular weight close to 10(6). Limited trypsin treatment of these aggregates releases a fully active low-molecular-weight form of methionyl-tRNA synthetase which was purified to a specific activity of 674 units/mg at 25 degrees C with a yield of 40%. The homogeneous enzyme appears to be undistinguishable from the corresponding enzyme derived from sheep lactating mammary gland, as judged by acrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and by titration with antibodies raised against the enzyme purified from liver.

Methionyl-tRNA synthetase from sheep mammary gland. Purification of a fully active monomeric enzyme derived from high-molecular-weight complexes by controlled proteolysis

Methionyl-tRNA synthetase from sheep lactating mammary gland is found predominantly in the form of high-molecular-weight complexes. Controlled proteolysis of these aggregates generates a low-molecular-weight species of the enzyme with full maintenance of activity as assessed by the rate of aminoacylation of tRNA. The product of proteolysis, which has been purified to homogeneity with a yield of 23%, is a monomeric enzyme of molecular weight 78 000. It has a specific activity of 405 units/mg at 25 degrees C. These findings clearly demonstrate that the aggregated state of methionyl-tRNA synthetase is not a prerequisite for full expression of catalytic activity. Furthermore, the results emphasize the need to provide effective protection against proteolytic damage in studies dealing with the characterization of high-molecular-weight complexes of aminoacyl-tRNA synthetases.

Enhanced level and metabolic regulation of methionyl-transfer ribonucleic acid synthetase in different strains of Escherichia coli K-12

The methionyl-transfer ribonucleic acid (tRNA) synthetase of Escherichia coli K-12 eductants carrying P2-mediated deletions in the region of the structural gene of this enzyme was investigated. No structural alteration of this enzyme was observed in three eductants examined. These were isolated from strain AB311, which had a threefold higher level of methionyl-tRNA synthetase than most haploid strains examined. In two of the three eductants studied, the level of this enzyme was twofold higher than in their parental strain regardless of growth conditions used. In contrast, isoleucyl-, leucyl-, and valyl-tRNA synthetases had similar levels in all strains examined. Like valyl-tRNA synthetase, but to a lesser extent, methionyl-tRNA synthetase was subject to metabolic regulation. Coupling between the level of methionyl-tRNA synthetase and growth rate was observed even in strains that had an enhanced level of methionyl-tRNA synthetase. These results suggest that the formation of methionyl-tRNA synthetase remains subject to metabolic regulation even when the repression-like mechanism that controls the synthesis of this enzyme is altered. In addition, we report that in the merodiploid strain EM20031, which was haploid for the valyl-tRNA synthetase structural gene and diploid for the structural genes of methionyl-tRNA synthetase and D-serine deaminase, the levels of these latter two enzymes varied to a minor yet significant extent with the phosphate concentration of the culture medium; under the same conditions, the level of valyl-tRNA synthetase remained unchanged. Moreover, no variation of the levels of these three enzymes in response to phosphate was observed in the haploid strain HfrH. These results indicate that in the merodiploid strain EM20031, which carries the episome F32, the number of episomes per chromosome varies to some extent according to the phosphate concentration of the culture medium.

The mechanism of action of methionyl-tRNA synthetase from Escherichia coli. Inhibition by adenosine and 8-aminoadenosine of the amino-acid activation reaction

Adenosine and 8-aminoadenosine, both competitive inhibitors of ATP-Mg2+ in the ATP-PPi exchange reaction catalyzed by methionyl-tRNA synthetase, are used to investigate the active center for methionyl-adenylate formation. Resolution of the kinetics parameters of the reaction indicates that methionine markedly enhances the affinity of the nucleosides for the enzyme, providing evidence for coupling between the sites for amino acid and the nucleoside moiety of ATP. Furthermore, occupation of both of these sites is a prerequisite for binding of pyrophosphate. Introduction of an amino group in position 8 of the adenine ring strongly increases the affinity constants for the nucleoside and for pyrophosphate in the coupled reactions described above.