Immunoelectron microscopic localization of glutamyl-/ prolyl-tRNA synthetase within the eukaryotic multisynthetase complex

A high molecular mass complex of aminoacyl-tRNA synthetases is readily isolated from a variety of eukaryotes. Although its composition is well characterized, knowledge of its structure and organization is still quite limited. This study uses antibodies directed against prolyl-tRNA synthetase for immunoelectron microscopic localization of the bifunctional glutamyl-/prolyl-tRNA synthetase. This is the first visualization of a specific site within the multisynthetase complex. Images of immunocomplexes are presented in the characteristic views of negatively stained multisynthetase complex from rabbit reticulocytes. As described in terms of a three domain working model of the structure, in "front" views of the particle and "intermediate" views, the primary antibody binding site is near the intersection between the "base" and one "arm." In "side" views, where the particle is rotated about its long axis, the binding site is near the midpoint. "Top" and "bottom" views, which appear as square projections, are also consistent with the central location of the binding site. These data place the glutamyl-/prolyl-tRNA synthetase polypeptide in a defined area of the particle, which encompasses portions of two domains, yet is consistent with the previous structural model.     

Interaction of the C-terminal domain of p43 and the alpha subunit of ATP synthase. Its functional implication in endothelial cell proliferation.

Human p43 is associated with macromolecular tRNA synthase complex and known as a precursor of endothelial monocyte-activating polypeptide II (EMAP II). Interestingly, p43 is also secreted to induce proinflammatory genes. Although p43 itself seems to be a cytokine working at physiological conditions, most of the functional studies have been obtained with its C-terminal equivalent, EMAP II. To gain an insight into the working mechanism of p43/EMAP II, we used EMAP II and searched for an interacting cell surface molecule. The level of EMAP II-binding molecule(s) was significantly increased in serum-starved tumor cells. Thus, the EMAP II-binding molecule was isolated from the membrane of the serum-starved CEM cell. The isolated protein was determined to be the alpha subunit of ATP synthase. The interaction of EMAP II and alpha-ATP synthase was confirmed by enzyme-linked immunosorbent assay and in vitro pull down assays and blocked with the antibodies raised against EMAP II and alpha-ATP synthase. The binding of EMAP II to the surface of serum-starved cells was inhibited in the presence of soluble alpha-ATP synthase. EMAP II inhibited the growth of endothelial cells, and this effect was relieved by soluble alpha-ATP synthase. Anti-alpha-ATP synthase antibody also showed an inhibitory effect on the proliferation of endothelial cells mimicking the activity of EMAP II. These results suggest the potential interaction of p43/EMAP II with alpha-ATP synthase and its role in the proliferation of endothelial cells.     

A novel anti-tumor cytokine contains an RNA binding motif present in aminoacyl-tRNA synthetases

Endothelial monocyte-activating polypeptide II (EMAP II) is a novel pro-apoptotic cytokine that shares sequence homology with the C-terminal regions of several tRNA synthetases. Pro-EMAP II, the precursor of EMAP II, is associated with the multi-tRNA synthetase complex and facilitates aminoacylation activity. The structure of human EMAP II, solved at 1.8 A resolution, revealed the oligomer-binding fold for binding different tRNAs and a domain that is structurally homologous to other chemokines. The similar structures to the RNA binding motif of EMAP II was previously observed in the anticodon binding domain of yeast Asp-tRNA synthetase (AspRSSC) and the B2 domain of Thermus thermophilus Phe-tRNA synthetase. The RNA binding pattern of EMAP II is likely to be nonspecific, in contrast to the AspRSSC. The peptide sequence that is responsible for cytokine activity is located, for the most part, in the beta1 strand. It is divided into two regions by a neighboring loop.     

Isolation and characterization of human nuclear and cytosolic multisynthetase complexes and the intracellular distribution of p43/EMAPII

In this study, the human multienzyme aminoacyl-tRNA synthetase "core" complex has been isolated from the nuclear and cytosolic compartments of human cells and purified to near homogeneity. It is clear from the polypeptide compositions, stoichiometries, and three-dimensional structures that the cytosolic and nuclear particles are very similar to each other and to the particle obtained from rabbit reticulocytes. The most significant difference observed via aminoacylation activity assays and densitometric analysis of electrophoretic band patterns is a lower amount of glutaminyl-tRNA synthetase in the human particles. However, this is not enough to cause major changes in the three-dimensional structures calculated from samples negatively stained with either uranyl acetate or methylamine vanadate. Indeed, the latter samples produce volumes that are highly similar to an initial structure previously calculated from a frozen hydrated sample of the rabbit multisynthetase complex. New structures in this study reveal that the three major structural domains have discrete subsections. This information is an important step toward determination of specific protein interactions and arrangements within the multisynthetase core complex and understanding of the particle's cellular function(s). Finally, gel filtration and immunoblot analysis demonstrate that a major biological role for the cytokine precursor p43 is as an integral part of the multisynthetase complex.     

A three-dimensional working model of the multienzyme complex of aminoacyl-tRNA synthetases based on electron microscopic placements of tRNA and proteins

It has become evident that the process of protein synthesis is performed by many cellular polypeptides acting in concert within the structural confines of protein complexes. In multicellular eukaryotes, one of these assemblies is a multienzyme complex composed of eight proteins that have aminoacyl-tRNA synthetase activities as well as three non-synthetase proteins (p43, p38, and p18) with diverse functions. This study uses electron microscopy and three-dimensional reconstruction to explore the arrangement of proteins and tRNA substrates within this "core" multisynthetase complex. Binding of unfractionated tRNA establishes that these molecules are widely distributed on the exterior of the structure. Binding of gold-labeled tRNA(Leu) places leucyl-tRNA synthetase and the bifunctional glutamyl-/prolyl-tRNA synthetase at the base of this asymmetric "V"-shaped particle. A stable cell line has been produced that incorporates hexahistidine-labeled p43 into the multisynthetase complex. Using a gold-labeled nickel-nitrilotriacetic acid probe, the polypeptides of the p43 dimer have been located along one face of the particle. The results of this and previous studies are combined into an initial three-dimensional working model of the multisynthetase complex. This is the first conceptualization of how the protein constituents and tRNA substrates are arrayed within the structural confines of this multiprotein assembly.     

A recurrent general RNA binding domain appended to plant methionyl-tRNA synthetase acts as a cis-acting cofactor for aminoacylation

The cDNA encoding rice methionyl-tRNA synthetase was isolated. The protein exhibited a C-terminal polypeptide appended to a classical MetRS domain. This supplementary domain is related to endothelial monocyte activating polypeptide II (EMAPII), a cytokine produced in mammals after cleavage of p43, a component of the multisynthetase complex. It is also related to Arc1p and Trbp111, two tRNA binding proteins. We expressed rice MetRS and a derivative with a deletion of its EMAPII-like domain. Band-shift analysis showed that this extra-domain provides MetRS with non-specific tRNA binding properties. The EMAPII-like domain contributed a 10-fold decrease in K:(M) for tRNA in the aminoacylation reaction catalyzed by the native enzyme, as compared with the C-terminally truncated MetRS. Consequently, the EMAPII domain provides MetRS with a better catalytic efficiency at the free tRNA concentration prevailing in vivo. This domain binds the acceptor minihelix of tRNA(Met) and facilitates its aminoacylation. These results suggest that the EMAPII module could be a relic of an ancient tRNA binding domain that was incorporated into primordial synthetases for aminoacylation of RNA minihelices taken as the ancestor of modern tRNA.     

1H, 13C, and 15N chemical shifts assignments for human endothelial monocyte-activating polypeptide EMAP II

Endothelial and monocyte-activating polypeptide II (EMAP II) is a cytokine that plays an important role in inflammation, apoptosis and angiogenesis processes in tumour tissues. Structurally, the EMAP II is a 169 amino acid residues long C-terminal domain (residues 147–312) of auxiliary tRNA binding protein p43. In spite of existence in pdb databank of two X-ray structures there are some important aspects of EMAP II cytokine function which are still not fully understood in detail. To obtain information about 3D structure and backbone dynamic processes in solution we perform structure evaluation of human EMAP II cytokine by NMR spectroscopy. The standard approach to sequence-specific backbone assignment using 3D NMR data sets was not successful in our studies and was supplemented by recently developed 4D NMR experiments with random sampling of evolution time space. Here we report the backbone and side chain ¹H, ¹³C, and ¹⁵N chemical shifts in solution for recombinant EMAP II cytokine together with secondary structure provided by TALOS + software.     

A cofactor of tRNA synthetase, p43, is secreted to up-regulate proinflammatory genes

An auxiliary factor of mammalian multi-aminoacyl-tRNA synthetases, p43, is thought to be a precursor of endothelial monocyte-activating polypeptide II (EMAP II) that triggers proinflammation in leukocytes and macrophages. In the present work, however, we have shown that p43 itself is specifically secreted from intact mammalian cells, while EMAP II is released only when the cells are disrupted. Secretion of p43 was also observed when its expression was increased. These results suggest that p43 itself should be a real cytokine secreted by an active mechanism. To determine the cytokine activity and active domain of p43, we investigated tumor necrosis factor (TNF) and interleukin-8 (IL-8) production from human monocytic THP-1 cells treated with various p43 deletion mutants. The full length of p43 showed higher cytokine activity than EMAP II, further supporting p43 as the active cytokine. p43 was also shown to activate MAPKs and NFkappaB, and to induce cytokines and chemokines such as TNF, IL-8, MCP-1, MIP-1alpha, MIP-1beta, MIP-2alpha, IL-1beta, and RANTES. Interestingly, the high level of p43 was observed in the foam cells of atherosclerotic lesions. Therefore, p43 could be a novel mediator of atherosclerosis development as well as other inflammation-related diseases.     

The EMAPII cytokine is released from the mammalian multisynthetase complex after cleavage of its p43/proEMAPII component

Endothelial-monocyte-activating polypeptide II (EMAPII) is an inflammatory cytokine released under apoptotic conditions. Its proEMAPII precursor proved to be identical to the auxiliary p43 component of the aminoacyl-tRNA synthetase complex. We show here that the EMAPII domain of p43 is released readily from the complex after in vitro digestion with caspase 7 and is able to induce migration of human mononuclear phagocytes. The N terminus of in vitro-processed EMAPII coincides exactly with that of the mature cytokine isolated from conditioned medium of fibrosarcoma cells. We also show that p43/proEMAPII has a strong tRNA binding capacity (K(D) = 0.2 microm) as compared with its isolated N or C domains (7.5 microm and 40 microm, respectively). The potent general RNA binding capacity ascribed to p43/proEMAPII is lost upon the release of the EMAPII domain. This suggests that after onset of apoptosis, the first consequence of the cleavage of p43 is to limit the availability of tRNA for aminoacyl-tRNA synthetases associated within the complex. Translation arrest is accompanied by the release of the EMAPII cytokine that plays a role in the engulfment of apoptotic cells by attracting phagocytes. As a consequence, p43 compares well with a molecular fuse that triggers the irreversible cell growth/cell death transition induced under apoptotic conditions.     

Web-ware bioinformatical analysis and structure modelling of N-terminus of human multisynthetase complex auxiliary component protein p43

Human multisynthetase complex auxiliary component, protein p43 is an endothelial monocyte-activating polypeptide II precursor. In this study, comprehensive sequence analysis of N-terminus has been performed to identify structural domains, motifs, sites of post-translation modification and other functionally important parameters. The spatial structure model of full-chain protein p43 is obtained.     

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.     

The tRNA-interacting factor p43 associates with mammalian arginyl-tRNA synthetase but does not modify its tRNA aminoacylation properties

Arginyl-tRNA synthetase (ArgRS) is one of the nine synthetase components of a multienzyme complex containing three auxiliary proteins as well. We previously established that the N-terminal moiety of the auxiliary protein p43 associates with the N-terminal, eukaryotic-specific polypeptide extension of ArgRS. Because p43 is homologous to Arc1p, a yeast general RNA-binding protein that associates with MetRS and GluRS and plays the role of tRNA-binding cofactor in the aminoacylation reaction, we analyzed the functional significance of p43-ArgRS association. We had previously showed that full-length ArgRS, corresponding to the ArgRS species associated within the multisynthetase complex, and ArgRS with a deletion of 73 N-terminal amino acid residues, corresponding to a free species of ArgRS, both produced in yeast, have similar catalytic parameters (Lazard, M., Kerjan, P., Agou, F., and Mirande, M. (2000) J. Mol. Biol. 302, 991-1004). However, a recent study had suggested that association of p43 to ArgRS reduces the apparent K(M) of ArgRS to tRNA (Park, S. G., Jung, K. H., Lee, J. S., Jo, Y. J., Motegi, H., Kim, S., and Shiba, K. (1999) J. Biol. Chem. 274, 16673-16676). In this study, we analyzed in detail, by gel retardation assays and enzyme kinetics, the putative role of p43 as a tRNA-binding cofactor of ArgRS. The association of p43 with ArgRS neither strengthened tRNA-binding nor changed kinetic parameters in the amino acid activation or in the tRNA aminoacylation reaction. Furthermore, selective removal of the C-terminal RNA-binding domain of p43 from the multisynthetase complex did not affect kinetic parameters for ArgRS. Therefore, p43 has a dual function. It promotes association of ArgRS to the complex via its N-terminal domain, but its C-terminal RNA-binding domain may act as a tRNA-interacting factor for an as yet unidentified component of the complex.     

Pro-EMAP II is not primarily cleaved by caspase-3 and -7

Endothelial monocyte-activating polypeptide (EMAP) II is a unique cytokine, also known as p43, the active mature form of which exhibits antiangiogenic properties in vivo and in vitro. The proteolytic enzymes associated with the cleavage and release of the active mature form, however, remain unclear. Here we show that, in contrast to prior observations, purified pro-EMAP II is not cleaved by either caspase-3 or -7 in vivo or in vitro. Thus other proteolytic processes, which allow it to induce apoptosis via caspase-3 activation in migrating and dividing endothelium, may be involved in the release of the active mature EMAP II.     

The N Terminus of Pro-endothelial Monocyte-activating Polypeptide II (EMAP II) Regulates Its Binding with the C Terminus,Arginyl-tRNA Synthetase,and Neurofilament Light Protein

Pro-endothelial monocyte-activating polypeptide II (EMAP II), one component of the multi-aminoacyl tRNA synthetase complex, plays multiple roles in physiological and pathological processes of protein translation, signal transduction, immunity, lung development, and tumor growth. Recent studies have determined that pro-EMAP II has an essential role in maintaining axon integrity in central and peripheral neural systems where deletion of the C terminus of pro-EMAP II has been reported in a consanguineous Israeli Bedouin kindred suffering from Pelizaeus-Merzbacher-like disease. We hypothesized that the N terminus of pro-EMAP II has an important role in the regulation of protein-protein interactions. Using a GFP reporter system, we defined a putative leucine zipper in the N terminus of human pro-EMAP II protein (amino acid residues 1–70) that can form specific strip-like punctate structures. Through GFP punctum analysis, we uncovered that the pro-EMAP II C terminus (amino acids 147–312) can repress GFP punctum formation. Pulldown assays confirmed that the binding between the pro-EMAP II N terminus and its C terminus is mediated by a putative leucine zipper. Furthermore, the pro-EMAP II 1–70 amino acid region was identified as the binding partner of arginyl-tRNA synthetase, a polypeptide of the multi-aminoacyl tRNA synthetase complex. We also determined that the punctate GFP pro-EMAP II 1–70 amino acid aggregate colocalizes and binds to the neurofilament light subunit protein that is associated with pathologic neurofilament network disorganization and degeneration of motor neurons. These findings indicate the structure and binding interaction of pro-EMAP II protein and suggest a role of this protein in pathological neurodegenerative diseases.     

Highly differentiated motifs responsible for two cytokine activities of a split human tRNA synthetase.

While native human tyrosyl-tRNA synthetase (TyrRS) is inactive as a cell-signaling molecule, it can be split into two distinct cytokines. The enzyme is secreted under apoptotic conditions in culture where it is cleaved into an N-terminal fragment that harbors the catalytic site and into a C-domain fragment found only in the mammalian enzymes. The N-terminal fragment is an interleukin-8 (IL-8)-like cytokine, whereas the released C-domain is an endothelial-monocyte-activating polypeptide II (EMAP II)-like cytokine. Although the IL-8-like activity of the N-fragment depends on an ELR motif found in alpha-chemokines and conserved among mammalian TyrRSs, here we show that a similar (NYR) motif in the context of a lower eukaryote TyrRS does not confer the IL8-like activity. We also show that a heptapeptide from the C-domain has EMAP II-like chemotaxis activity for mononuclear phagocytes and polymorphonuclear leukocytes. Eukaryote proteins other than human TyrRS that have EMAP II-like domains have variants of the heptapeptide motif. Peptides based on these sequences are inactive as cytokines. Thus, the cytokine activities of split human TyrRS depend on highly differentiated motifs that are idiosyncratic to the mammalian system.     

The endothelial monocyte-activating polypeptide II (EMAP II) is a substrate for caspase-7

Endothelial monocyte-activating polypeptide II (EMAP II) is a proinflammatory cytokine and a chemoattractant for leukocytes. The mature cytokine is formed in apoptotic cells by cleavage of the precursor proEMAP II. Here we show that caspase-7 is capable of cleaving proEMAP II in vitro. A proEMAP II mutant, in which the ASTD cleavage site was changed to the sequence ASTA, was not processed by caspase-7. The caspase-7-mediated generation and release of mature EMAP II may provide a mechanism for leukocyte recruitment to sites of programmed cell death, and thus may link apoptosis to inflammation.     

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.     

Structure of the EMAPII domain of human aminoacyl-tRNA synthetase complex reveals evolutionary dimer mimicry

The EMAPII (endothelial monocyte-activating polypeptide II) domain is a tRNA-binding domain associated with several aminoacyl-tRNA synthetases, which becomes an independent domain with inflammatory cytokine activity upon apoptotic cleavage from the p43 component of the multisynthetase complex. It comprises a domain that is highly homologous to bacterial tRNA-binding proteins (Trbp), followed by an extra domain without homology to known proteins. Trbps, which may represent ancient tRNA chaperones, form dimers and bind one tRNA per dimer. In contrast, EMAPII domains are monomers. Here we report the crystal structure at 1.14 Angstroms of human EMAPII. The structure reveals that the Trbp-like domain, which forms an oligonucleotide-binding (OB) fold, is related by degenerate 2-fold symmetry to the extra-domain. The pseudo-axis coincides with the dyad axis of bacterial TtCsaA, a Trbp whose structure was solved recently. The interdomain interface in EMAPII mimics the intersubunit interface in TtCsaA, and may thus generate a novel OB-fold-based tRNA-binding site. The low sequence homology between the extra domain of EMAPII and either its own OB fold or that of Trbps suggests that dimer mimicry originated from convergent evolution rather than gene duplication.     

A component of the multisynthetase complex is a multifunctional aminoacyl-tRNA synthetase.

In higher eukaryotes, nine aminoacyl-tRNA synthetases are associated within a multienzyme complex which is composed of 11 polypeptides with molecular masses ranging from 18 to 150 kDa. We have cloned and sequenced a cDNA from Drosophila encoding the largest polypeptide of this complex. We demonstrate here that the corresponding protein is a multifunctional aminoacyl-tRNA synthetase. It is composed of three major domains, two of them specifying distinct synthetase activities. The amino and carboxy-terminal domains were expressed separately in Escherichia coli, and were found to catalyse the aminoacylation of glutamic acid and proline tRNA species, respectively. The central domain is made of six 46 amino acid repeats. In prokaryotes, these two aminoacyl-tRNA synthetases are encoded by distinct genes. The emergence of a multifunctional synthetase by a gene fusion event seems to be a specific, but general attribute of all higher eukaryotic cells. This type of structural organization, in relation to the occurrence of multisynthetase complexes, could be a mechanism to integrate several catalytic domains within the same particle. The involvement of the internal repeats in mediating complex assembly is discussed.     

Two conserved threonines collaborate in the Escherichia coli leucyl-tRNA synthetase amino acid editing mechanism

The aminoacyl-tRNA synthetases covalently link transfer RNAs to their cognate amino acids. Some of the tRNA synthetases have employed an editing mechanism to ensure fidelity in this first step of protein synthesis. The amino acid editing active site for Escherichia coli leucyl-tRNA synthetase resides within the CP1 domain that folds discretely from the main body of the enzyme. A portion of the editing active site is lined with conserved threonines. Previously, we identified one of these threonine residues (Thr(252)) as a critical amino acid specificity factor. On the basis of X-ray crystal structure information, two other nearby threonine residues (Thr(247) and Thr(248)) were hypothesized to interact with the editing substrate near its cleavage site. Single mutations of either of these conserved threonine residues had minimal effects on amino acid editing. However, double mutations that deleted the hydroxyl group from the neighboring threonine residues abolished amino acid editing activity. We propose that these threonine residues, which are also conserved in the homologous isoleucyl-tRNA synthetase and valyl-tRNA synthetase editing active sites, play a central role in amino acid editing. It is possible that they collaborate in stabilizing the transition state.