Generation of multiple forms of methionyl-tRNA synthetase from the multi-enzyme complex of mammalian aminoacyl-tRNA synthetases by endogenous proteolysis

Structure and Dynamics of the Human Multi-tRNA Synthetase Complex

Aminoacyl-tRNA synthetases (ARSs) are essential enzymes that ligate amino acids to their cognate tRNAs during protein synthesis. A growing body of scientific evidence acknowledges that ubiquitously expressed ARSs act as crossover mediators of biological processes, such as immunity and metabolism, beyond translation. In particular, a cytoplasmic multi-tRNA synthetase complex (MSC), which consists of eight ARSs and three ARS-interacting multifunctional proteins in humans, is recognized to be a central player that controls the complexity of biological systems. Although the role of the MSC in biological processes including protein synthesis is still unclear, maintaining the structural integrity of MSC is essential for life. This chapter deals with current knowledge on the structural aspects of the human MSC and its protein components. The main focus is on the regulatory functions of MSC beyond its catalytic activity.

A novel pathway for the conversion of homocysteine to methionine in eukaryotes

Activation of amino acid homocysteine was compared with that of methionine in rabbit crude liver extracts and purified multi-enzyme complex of aminoacyl-tRNA synthetases. Activation was studied by measuring the incorporation of radioactive amino acid into unlabelled trichloroacetic-acid insoluble materials in the absence of protein synthesis. Homocysteine synthetase activity was found in the crude extract and in the purified multi-enzyme complex of aminoacyl-tRNA synthetases. On a molar basis, the activation of methionine by the crude extract was five times higher than the activation of homocysteine. There was a partial loss of Hcy-tRNA synthetase activity in the purified multi-enzyme complex. Preliminary reconstitution experiments indicated a requirement for an additional factor for Hcy-tRNA synthetase activity. TLC of the amino acid released from tRNA charged with [14C]homocysteine, revealed radioactivity in homocysteine, methionine and homocysteine thiolactone, indicating a conversion of tRNA-attached homocysteine to methionine. Total tRNA was separated on a benzoylated cellulose column into a fraction enriched in initiator tRNA and a methionine-accepting, but initiator tRNA-deficient, fraction. Homocysteine-accepting activity was present only in the initiator tRNA-enriched fraction. Based on the above data we propose that homocysteine activation in reticulocyte lysates, reported previously, also occurs in liver. Activated homocysteine is attached to initiator tRNA and then converted to methionine by a methylating enzyme. In the absence of methylation, tRNA-attached homocysteine is hydrolysed to produce homocysteine thiolactone.

Engineering mammalian aspartyl-tRNA synthetase to probe structural features mediating its association with the multisynthetase complex

Aspartyl-tRNA synthetase from higher eukaryotes is a component of a multienzyme complex comprising nine aminoacyl-tRNA synthetases. The cDNA encoding cytoplasmic rat liver aspartyl-tRNA synthetase was previously cloned and sequenced. This work reports the identification of structural features responsible for its association within the multisynthetase complex. Mutant and chimeric proteins have been expressed in mammalian cells and their structural behavior analyzed. A wild-type rat liver aspartyl-tRNA synthetase, expressed in Chinese hamster ovary (CHO) cells, associates within the complex from CHO cells, whereas a mutant enzyme with a deletion of 34 amino acids from its amino-terminal extremity does not. A chimeric enzyme, made of the amino-terminal moiety of rat liver aspartyl-tRNA synthetase fused to the catalytic domain of yeast lysyl-tRNA synthetase, has been expressed in Lys-101 cells, a CHO cell line with a temperature-sensitive lysyl-tRNA synthetase. The fusion protein is stable in vivo, does not associate within the multisynthetase complex and cannot restore normal growth of the mutant cells. These results establish that the 3.7-kDa amino-terminal moiety of mammalian aspartyl-tRNA synthetase mediates its association with the other components of the complex. In addition, the finding that yeast lysyl-tRNA synthetase cannot replace the aspartyl-tRNA synthetase component of the mammalian complex, indicates that interactions between neighbouring enzymes also play a prominent role in stabilization of this multienzyme structure and strengthened the view that the multisynthetase complex is a discrete entity with a well-defined structural organization.

The NH2-terminal extension of rat liver arginyl-tRNA synthetase is responsible for its hydrophobic properties

Rat liver arginyl-tRNA synthetase is found in extracts either as a component (Mr = 72,000) of the multienzyme aminoacyl-tRNA synthetase complex or as a low molecular weight (Mr = 60,000) free protein. The two forms are thought to be identical except for an extra peptide extension at the NH2-terminus of the larger form which is required for its association with the complex, but is unessential for catalytic activity. It has been suggested that interactions among synthetases in the multienzyme complex are mediated by hydrophobic domains on these peptide extensions of the individual proteins. To test this model we have purified to homogeneity the larger form of arginyl-tRNA synthetase and compared its hydrophobicity to that of its low molecular weight counterpart. We show that whereas the smaller protein displays no hydrophobic character, the larger protein demonstrates a high degree of hydrophobicity. No lipid modification was found on the high molecular weight protein indicating that the amino acid sequence itself is responsible for its hydrophobic properties. These findings support the proposed model for synthetase association within the multienzyme complex.

Evidence for a ribosome-associated thiol protease cleaving wheat germ methionyl-tRNA synthetase

A wheat germ protease is responsible for Mr 105,000 methionyl-tRNA synthetase hydrolysis, generating two fragments of Mr 82,000 (harbouring the catalytic domain) and 20,000, respectively. Specificity of the protease was sought for using different kinds of protein substrates. It turned out that charged peptides were preferentially cleaved and that no proteolysis occurred when proteins were replaced by small synthetic substrates, harbouring target sites similar to those cleaved in proteins. The protease could be a ribosomal protein, since it remained associated to ribosomal structure, even after treatment by deoxycholate, Triton X-100, 800 mM KC1 and puromycin. Nevertheless, it was still active after ribonuclease treatment of the ribosomes. An identical protease activity was found in rat liver, but not in E. coli.

Co-purification of the aminoacyl-tRNA synthetase complex with the elongation factor eEF1

A multi-enzyme complex of mammalian aminoacyl-tRNA synthetases was isolated from rabbit reticulocytes, and purified by polyethylene glycol fractionation and gel filtration on Biogel A15m and affinity chromatography on tRNA-Sepharose. The synthetase complex contains nine synthetase activities, and the corresponding proteins as analyzed by SDS polyacrylamide gel electrophoresis. Three of the proteins showed the identical subunit molecular weights to those of the reticulocyte's elongation factor eEF1H. The eEF1 alpha protein could not be removed by second tRNA-Sepharose column chromatography, or gel filtration on Biogel A5m or Biogel A15m. Antibodies against eEF1 alpha react with the purified synthetase complex on the basis of dot blot analysis. This finding should provide new clues for elucidating the structural organization of the mammalian protein biosynthetic machinery.

[Methionyl-tRna synthetase from wheat germ. Effect of an endogenous protease and correlations between structural characteristics and catalytic properties]

Methionyl-tRNA synthetase (MetRS) has been described as a free monomeric or oligomeric enzyme; or included in a multienzyme complex. Moreover, on limited tryptic digestion, it can generate shorter forms. So, when purified from wheat-germ lysate, the possible presence of proteases able to hydrolyse this enzyme was investigated. When extraction was performed with sulfhydryl-blocking reagents, an active monomeric MetRS of Mr 105,000 was purified. This enzyme form was identical to the structure exhibiting methionyl-tRNA synthetase activity in multienzyme complexes. Without this inhibitor, MetRS was purified as an active dimeric form of Mr 165,000 with identical subunits of Mr 82,000. A protease inhibited by sulfhydryl-blocking reagents and included in a complex of Mr 2.10(6) was isolated from this wheat-germ lysate. This protease was able to hydrolyse different proteins (albumin, casein), but was without activity for a trypsin substrate, such as N-alpha-benzoyl-DL-arginine p-nitroanilide. When added to a solution of Mr-105,000 MetRS, it yielded an inactive peptide of Mr 20,000, containing numerous charged amino acids and a protein of Mr 82,000, able to give an active dimeric enzyme of Mr 165,000. Amino acid analysis of this last form, indicated an identical structure with the active dimeric MetRS of Mr 165,000, purified in the absence of protease inhibitors. Moreover, the affinity for methionine was the same for the monomeric enzyme of Mr 105,000 and the dimeric form of Mr 165,000, probably because proteolysis did not affect the catalytic domain. When enzymic activity of the proteolyzed form (Mr 2 x 82,000) was studied versus enzyme concentration, a decrease in specific activity, at low concentrations, was seen. This phenomenon was analysed on the basis of the existence of an equilibrium between an active dimer and two inactive monomers. With the active monomeric form of Mr 105,000, no change in specific activity with decreasing enzyme concentration occurred.

Mammalian high molecular weight and monomeric forms of valyl-tRNA synthetase

Valyl-tRNA synthetase from rat liver sediments at 15.5 S with a Stokes radius of 90 A, corresponding to a native molecular weight of 585,000. Purification of valyl-tRNA synthetase to homogeneity by a combination of conventional and affinity column chromatography yields a fully active monomeric form of valyl-tRNA synthetase with a sedimentation coefficient of 7.7 S and a Stokes radius of 45 A. The subunit molecular weight of the monomeric valyl-tRNA synthetase is 140,000, as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. In the presence of 400 mM KCl, the purified monomeric valyl-tRNA synthetase associates to a high molecular weight form. The high molecular weight valyl-tRNA synthetase in the homogenate can be readily converted to the monomeric form by controlled trypsinization. The kinetic parameters of the two forms are nearly identical. The results suggest that the high molecular weight valyl-tRNA synthetase is a homotypic tetramer and converts to the monomeric valyl-tRNA synthetase after the cleavage of a small peptide.

The microheterogeneity of the crystallizable yeast cytoplasmic aspartyl-tRNA synthetase

Yeast aspartyl-tRNA synthetase is a dimeric enzyme (alpha 2, Mr 125,000) which can be crystallized either alone or complexed with tRNAAsp. When analyzed by electrophoretic methods, the pure enzyme presents structural heterogeneities even when recovered from crystals. Up to three enzyme populations could be identified by polyacrylamide gel electrophoresis and more than ten by isoelectric focusing. They have similar molecular masses and mainly differ in their charge. All are fully active. This microheterogeneity is also revealed by ion-exchange chromatography and chromatofocusing. Several levels of heterogeneity have been defined. A first type, which is reversible, is linked to redox effects and/or to conformational states of the protein. A second one, revealed by immunological methods, is generated by partial and differential proteolysis occurring during enzyme purification from yeast cells harvested in growth phase. As demonstrated by end-group analysis, the fragmentation concerns exclusively the N-terminal end of the enzyme. The main cleavage points are Gln-19, Val-20 and Gly-26. Six minor cuts are observed between positions 14 and 33. The present data are discussed in the perspective of the crystallographic studies on aspartyl-tRNA synthetase.