A basic NH2-terminal extension of rat liver arginyl-tRNA synthetase required for its association with high molecular weight complexes

Regulation of ex-translational activities is the primary function of the multi-tRNA synthetase complex

During mRNA translation, tRNAs are charged by aminoacyl-tRNA synthetases and subsequently used by ribosomes. A multi-enzyme aminoacyl-tRNA synthetase complex (MSC) has been proposed to increase protein synthesis efficiency by passing charged tRNAs to ribosomes. An alternative function is that the MSC repurposes specific synthetases that are released from the MSC upon cues for functions independent of translation. To explore this, we generated mammalian cells in which arginyl-tRNA synthetase and/or glutaminyl-tRNA synthetase were absent from the MSC. Protein synthesis, under a variety of stress conditions, was unchanged. Most strikingly, levels of charged tRNA^Arg and tRNA^Gln remained unchanged and no ribosome pausing was observed at codons for arginine and glutamine. Thus, increasing or regulating protein synthesis efficiency is not dependent on arginyl-tRNA synthetase and glutaminyl-tRNA synthetase in the MSC. Alternatively, and consistent with previously reported ex-translational roles requiring changes in synthetase cellular localizations, our manipulations of the MSC visibly changed localization.

Distinct pathogenic mechanisms of various RARS1 mutations in Pelizaeus-Merzbacher-like disease

Mutations of the genes encoding aminoacyl-tRNA synthetases are highly associated with various central nervous system disorders. Recurrent mutations, including c.5A>G, p.D2G; c.1367C>T, p.S456L; c.1535G>A, p.R512Q and c.1846_1847del, p. Y616Lfs*6 of RARS1 gene, which encodes two forms of human cytoplasmic arginyl-tRNA synthetase (hArgRS), are linked to Pelizaeus-Merzbacher-like disease (PMLD) with unclear pathogenesis. Among these mutations, c.5A>G is the most extensively reported mutation, leading to a p.D2G mutation in the N-terminal extension of the long-form hArgRS. Here, we showed the detrimental effects of R512Q substitution and ΔC mutations on the structure and function of hArgRS, while the most frequent mutation c.5A>G, p.D2G acted in a different manner without impairing hArgRS activity. The nucleotide substitution c.5A>G reduced translation of hArgRS mRNA, and an upstream open reading frame contributed to the suppressed translation of the downstream main ORF. Taken together, our results elucidated distinct pathogenic mechanisms of various RARS1 mutations in PMLD.

Structure of the ArgRS-GlnRS-AIMP1 complex and its implications for mammalian translation

In higher eukaryotes, one of the two arginyl-tRNA synthetases (ArgRSs) has evolved to have an extended N-terminal domain that plays a crucial role in protein synthesis and cell growth and in integration into the multisynthetase complex (MSC). Here, we report a crystal structure of the MSC subcomplex comprising ArgRS, glutaminyl-tRNA synthetase (GlnRS), and the auxiliary factor aminoacyl tRNA synthetase complex-interacting multifunctional protein 1 (AIMP1)/p43. In this complex, the N-terminal domain of ArgRS forms a long coiled-coil structure with the N-terminal helix of AIMP1 and anchors the C-terminal core of GlnRS, thereby playing a central role in assembly of the three components. Mutation of AIMP1 destabilized the N-terminal helix of ArgRS and abrogated its catalytic activity. Mutation of the N-terminal helix of ArgRS liberated GlnRS, which is known to control cell death. This ternary complex was further anchored to AIMP2/p38 through interaction with AIMP1. These findings demonstrate the importance of interactions between the N-terminal domains of ArgRS and AIMP1 for the catalytic and noncatalytic activities of ArgRS and for the assembly of the higher-order MSC protein complex.

Aminoacyl-tRNA synthetase complexes: molecular multitasking revealed

The accurate synthesis of proteins, dictated by the corresponding nucleotide sequence encoded in mRNA, is essential for cell growth and survival. Central to this process are the aminoacyl-tRNA synthetases (aaRSs), which provide amino acid substrates for the growing polypeptide chain in the form of aminoacyl-tRNAs. The aaRSs are essential for coupling the correct amino acid and tRNA molecules, but are also known to associate in higher order complexes with proteins involved in processes beyond translation. Multiprotein complexes containing aaRSs are found in all three domains of life playing roles in splicing, apoptosis, viral assembly, and regulation of transcription and translation. An overview of the complexes aaRSs form in all domains of life is presented, demonstrating the extensive network of connections between the translational machinery and cellular components involved in a myriad of essential processes beyond protein synthesis.

Lysyl-tRNA synthetase interacts with EF1alpha, aspartyl-tRNA synthetase and p38 in vitro

The functions of evolved mammalian supramolecular assemblies and extensions of enzymes are not well understood. Human lysyl-tRNA synthetase (hKRS) only upon the removal of the amino-terminal extension (hKRSDelta60) bound to EF1alpha and was stimulated by EF1alphain vitro. HKRS and hKRSDelta60 were also differentially stimulated by aspartyl-tRNA synthetase (AspRS) from the multi-synthetase complex. The non-synthetase protein from the multi-synthetase complex p38 alone did not affect hKRS lysylation but inhibited the AspRS-mediated stimulation of hKRS. These results revealed the functional interactions of hKRS and shed new lights on the functional significance of the structural evolution of multienzyme complexes and appended extensions.

Proteasomes and RARS modulate AIMP1/EMAP II secretion in human cancer cell lines

The aminoacyl t-RNA synthetase interacting multifunctional protein (AIMP1) is the precursor of the multifunctional inflammatory cytokine endothelial monocyte-activating polypeptide II (EMAP II). We previously demonstrated that AIMP1 secretion by pituitary adenomas is inversely correlated with tumor diameter and with RARS expression, suggesting that a high amount of RARS associated with AIMP1 might prevent the secretion of the latter cytokine. In this study, we investigated the role of RARS in modulating the secretion of AIMP1 in HeLa and MCF7 cell lines and investigated the possible role of the multicatalytic protease in the cleavage of AIMP1 to generate EMAP II. Our data show that RARS over-expression impairs AIMP1 secretion by both HeLa and MCF7 cells. Moreover, proteasome inhibition impairs AIMP1 cleavage to produce EMAP II. These data indicate that RARS over-expression associates with a reduced AIMP1 secretion and that the multicatalytic protease is involved in the generation of the mature cytokine, EMAP II.

Two forms of human cytoplasmic arginyl-tRNA synthetase produced from two translation initiations by a single mRNA

Human cytoplasmic arginyl-tRNA synthetase (ArgRS) is a component of a macromolecular complex consisting of at least nine tRNA synthetases and three auxiliary proteins. In mammalian cells, ArgRS is present as a free protein as well as a component of the complex. Via an alignment of ArgRSs from different vertebrates, the genes encoding full-length human cytoplasmic ArgRS and an N-terminal 72-amino acid deletion mutant (hcArgRS and DeltaNhcArgRS, respectively) were subcloned and expressed in Escherichia coli. The two ArgRS products were expressed as a soluble protein in E. coli. The level of production of DeltaNhcArgRS in E. coli and its specific activity were higher than those for hcArgRS. By Western blot analysis, using an antibody against the purified DeltaNhcArgRS, the two forms of ArgRS were detected in three human cell types. The 5'-end cDNA sequence, as confirmed by 5'RACE (5'-rapid amplification of cDNA ends), contained three start codons. Through mutation of the three codons, the two human cytoplasmic ArgRSs were found to be produced in different amounts, indicating that they resulted from two different translation initiation events. Here we show evidence that two forms of human cytoplasmic ArgRS were produced from two translational initiations by a single mRNA.

The C-terminal appended domain of human cytosolic leucyl-tRNA synthetase is indispensable in its interaction with arginyl-tRNA synthetase in the multi-tRNA synthetase complex

Human cytosolic leucyl-tRNA synthetase is one component of a macromolecular aminoacyl-tRNA synthetase complex. This is unlike prokaryotic and lower eukaryotic LeuRSs that exist as free soluble enzymes. There is little known about it, since the purified enzyme has been unavailable. Herein, human cytosolic leucyl-tRNA synthetase was heterologously expressed in a baculovirus system and purified to homogeneity. The molecular mass (135 kDa) of the enzyme is close to the theoretical value derived from its cDNA. The kinetic constants of the enzyme for ATP, leucine, and tRNA(Leu) in the ATP-PP(i) exchange and tRNA leucylation reactions were determined, and the results showed that it is quite active as a free enzyme. Human cytosolic leucyl-tRNA synthetase expressed in human 293 T cells localizes predominantly to the cytosol. Additionally, it is found to have a long C-terminal extension that is absent from bacterial and yeast LeuRSs. A C-terminal 89-amino acid truncated human cytosolic leucyl-tRNA synthetase was constructed and purified, and the catalytic activities, thermal stability, and subcellular location were found to be almost identical to native enzyme. In vivo and in vitro experiments, however, show that the C-terminal extension of human cytosolic leucyl-tRNA synthetase is indispensable for its interaction with the N-terminal of human cytosolic arginyl-tRNA synthetase in the macromolecular complex. Our results also indicate that the two molecules interact with each other only through their appended domains.

miR-15a and miR-16-1 down-regulation in pituitary adenomas

Micro RNAs (miRs) are small noncoding RNAs, functioning as antisense regulators of other RNAs. miR-15a and miR-16-1 genes are located at chromosome 13q14, a region which is frequently deleted in pituitary tumors. An inverse correlation has been shown in B cell chronic lymphocytic leukemia (B-CLL) between miR-15a and miR-16-1 expression and the expression levels of arginyl-tRNA synthetase (RARS), an enzyme which associates with the cofactor p43 in the aminoacyl-tRNA synthetase complex. When secreted, p43 regulates local inflammatory response and macrophage chemotaxis, and seems to have anti-neoplastic properties in mice. We explored miR-15a and miR-16-1 expression in 10 GH-secreting and in 10 PRL-secreting pituitary macroadenomas by Northern blot, and investigated the possible correlation with in vivo and in vitro characteristics. We found that miR-15a and miR-16-1 are expressed at lower levels in pituitary adenomas as compared to normal pituitary tissue. Moreover, their expression inversely correlates with tumor diameter and with RARS expression (P < 0.05), but directly correlates with p43 secretion (P < 0.02). Therefore, miR15 and miR16 down-regulation in pituitary adenomas correlates with a greater tumor diameter and a lower p43 secretion, suggesting that these genes may, at least in part, influence tumor growth.

Macromolecular assemblage of aminoacyl-tRNA synthetases: quantitative analysis of protein-protein interactions and mechanism of complex assembly

The structure of the mammalian multi-synthetase complex was investigated in vitro using qualitative and quantitative approaches. This macromolecular assemblage comprises the bifunctional glutamyl-prolyl-tRNA synthetase, the seven monospecific isoleucyl, leucyl, methionyl, glutaminyl, lysyl, arginyl and aspartyl-tRNA synthetases, and the three auxiliary p43, p38 and p18 proteins. The scaffold p38 protein was expressed in Escherichia coli and purified to homogeneity as a His-tagged protein. The different components of the complex were shown to associate in vitro with p38 immobilized on Ni(2+)-coated plates. Interactions between peripheral enzymes and p38 are referred to as central interactions, as opposed to lateral interactions between peripheral enzymes. Kinetic parameters of the interactions were determined by the means of a biosensor-based approach. The two dimeric proteins LysRS and AspRS were found to tightly bind to p38, with a K(d) value of 0.3 and 4.7 nM, respectively. These interactions involved the catalytic core of the enzymes. By contrast, binding of ArgRS or GlnRS to p38 was much weaker (>5 microM). ArgRS and p43, two peripheral components, were shown to interact with moderate affinity (K(d)=93 nM). Since all the components of the complex are tightly associated within this particle, lateral interactions were believed to contribute to the stabilization of this assemblage. Using an in vitro binding assay, concomitant association of several components of the complex on immobilized p38 could be demonstrated, and revealed the involvement of synergistic effects for association of weakly interacting proteins. Taking into account the possible synergy between central and lateral contributions, a sub-complex containing p38, p43, ArgRS and GlnRS was reconstituted in vitro. These data provide compelling evidence for an ordered and concerted mechanism of complex assembly.

Aspartyl-tRNA synthetase from rat: in vitro functional analysis of its assembly into the multisynthetase complex

In mammalian cells, nine aminoacyl-tRNA synthetases, including aspartyl-tRNA synthetase, are associated within a multienzyme complex. Rat aspartyl-tRNA synthetase has a N-terminal polypeptide extension of about 40 amino acid residues which can be removed without impairing its catalytic activity. Earlier, in vivo studies showed that enzymes deprive of this N-terminal segment behave in vivo as free entities. We designed an experimental in vitro approach, based on the exchange of the complexed endogenous enzyme by free recombinant species, to assess the contribution of that domain in the association of aspartyl-tRNA synthetase to the complex. A phosphorylation site was introduced at the N-terminus of rat aspartyl-tRNA synthetase. The enzyme served as a reporter protein to evaluate the dissociation constants of native and N-terminal-truncated species towards the complex. Our data show that a moderate but significant drop in affinity is inferred by the removal of the N-terminal domain. The results suggest that this domain binds to another component of the complex, but might primarily serve a targeting function absolutely required in vivo for the assembly within the multienzyme structure.

The C-terminal extension of yeast seryl-tRNA synthetase affects stability of the enzyme and its substrate affinity

Saccharomyces cerevisiae seryl-tRNA synthetase (SerRS) contains a 20-amino acid C-terminal extension, which is not found in prokaryotic SerRS enzymes. A truncated yeast SES1 gene, lacking the 60 base pairs that encode this C-terminal domain, is able to complement a yeast SES1 null allele strain; thus, the C-terminal extension in SerRS is dispensable for the viability of the cell. However, the removal of the C-terminal peptide affects both stability of the enzyme and its affinity for the substrates. The truncation mutant binds tRNA with 3.6-fold higher affinity, while the Km for serine is 4-fold increased relative to the wild-type SerRS. This indicates the importance of the C-terminal extension in maintaining the overall structure of SerRS.

Valyl-tRNA synthetase from Artemia. Purification and association with elongation factor 1

Two components of the protein biosynthetic machinery, valyl-transfer RNA synthetase (VRS) and elongation factor 1 (EF-1), have been isolated as a complex from several mammalian tissues. However, yeast VRS, which lacks an amino-terminal extension, does not associated with EF-1. We purified VRS from the brine shrimp Artemia and investigated its interaction with EF-1. Western blotting of crude Artemia extracts revealed the presence of two forms of VRS, differing in size and capacity to associate with EF-1. About 80% of the total VRS corresponds to a polypeptide of 130 kDa which behaves as a monomer upon gel filtration. Only the larger form of 140 kDa coelutes, cosediments and co-immunoprecipitates with the EF-1 alpha 2 beta gamma delta complex. The ratio of the two forms of VRS remains constant throughout early development. The possible origin and mode of expression of the two forms of VRS present in Artemia are discussed.

Protein translation components are colocalized in granules in oligodendrocytes

The intracellular distribution of various components of the protein translational machinery was visualized in mouse oligodendrocytes in culture using high resolution fluorescence in situ hybridization and immunofluorescence in conjunction with dual channel confocal laser scanning microscopy. Arginyl-tRNA synthetase, elongation factor 1a, ribosomal RNA, and myelin basic protein mRNA were all co-localized in granules in the processes, veins and membrane sheets of the cell. Colocalization was evaluated by dual channel cross correlation analysis to determine the correlation index (% colocalization) and correlation distance (granule radius), and by single granule ratiometric analysis to determine the distribution of the different components in individual granules. Most granules contained synthetase, elongation factor, ribosomal RNA and myelin basic protein mRNA. These results indicate that several different components of the protein synthetic machinery, including aminoacyl-tRNA synthetases, elongation factors, ribosomes and mRNAs, are colocalized in granules in oligodendrocytes. We propose that these granules are supramolecular complexes containing all of the necessary macromolecular components for protein translation and that they represent a heretofore undescribed subcellular organization of the protein synthetic machinery. This spatial organization may increase the efficiency of protein synthesis and may also provide a vehicle for transport and localization of specific mRNAs within the cell.

Cloning and analysis of a cDNA encoding mammalian arginyl-tRNA synthetase, a component of the multisynthetase complex with a hydrophobic N-terminal extension

In mammalian cells, the nine aminoacyl-tRNA synthetases (aaRS) specific for the amino acids (aa) Glu, Pro, Ile, Leu, Met, Gln, Lys, Arg and Asp are associated within a multienzyme complex. Arginyl-tRNA synthetase (ArgRS) is characterized by the occurrence of two structurally distinct forms of that enzyme: a complexed (approximately 74 kDa) and a free (approximately 60 kDa) form. The cDNA encoding the 74-kDa species of ArgRS from Chinese hamster ovary cells has been isolated and sequenced. The deduced aa sequence shows 38% identity to the homologous bacterial enzyme but displays an N-terminal polypeptide extension composed of 73 aa, which is absent in the free form of mammalian ArgRS. Two regions of this extension are predicted to be alpha-helical, leading to the clustering of Leu and Ile residues on one side of the helices. This suggests that the N-terminal domain is involved in the assembly of the 74-kDa species of ArgRS within the multisynthetase complex through hydrophobic interactions. By using the isolated cDNA, a Northern blot analysis showed a single mRNA species. Thus, there is a possibility that the free and complexed forms of ArgRS are encoded by the same gene.

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.

Glutaminyl-tRNA synthetase as a component of the high-molecular weight complex of human aminoacyl-tRNA synthetases. An immunological study

The human glutaminyl-tRNA synthetase is three times larger than the corresponding bacterial and twice as large as the yeast enzyme. It is possible that the additional sequences of the human glutaminyl-tRNA synthetase are required for the formation of the multienzyme complex which is known to include several of aminoacyl-tRNA synthetases in mammalian cells. To address this point we prepared antibodies against three regions of the human glutaminyl-tRNA synthetase, namely against its enzymatically important core region, and against two sections in its large C-terminal extension. In intact multienzyme complexes the core region was accessible to specific antibody binding. However, the C-terminal sections became available to specific antibody binding only when certain components of the multienzyme complex were either absent or degraded. These findings allow first conclusions as to the relative position of some components in the mammalian aminoacyl-tRNA synthetase complex.