Heat shock protein 90 mediates protein-protein interactions between human aminoacyl-tRNA synthetases

Beyond controlling cell size: functional analyses of S6K in tumorigenesis

As a substrate and major effector of the mammalian target of rapamycin complex 1 (mTORC1), the biological functions of ribosomal protein S6 kinase (S6K) have been canonically assigned for cell size control by facilitating mRNA transcription, splicing, and protein synthesis. However, accumulating evidence implies that diverse stimuli and upstream regulators modulate S6K kinase activity, leading to the activation of a plethora of downstream substrates for distinct pathobiological functions. Beyond controlling cell size, S6K simultaneously plays crucial roles in directing cell apoptosis, metabolism, and feedback regulation of its upstream signals. Thus, we comprehensively summarize the emerging upstream regulators, downstream substrates, mouse models, clinical relevance, and candidate inhibitors for S6K and shed light on S6K as a potential therapeutic target for cancers.

EPRS/GluRS promotes gastric cancer development via WNT/GSK-3β/β-catenin signaling pathway

BACKGROUND

Glutamyl-prolyl-tRNA synthetase (EPRS/GluRS) is primarily part of the multi-synthetase complex that may play a key role in cancer development. However, the biological function, molecular mechanism, and inhibitor of EPRS have not been investigated in gastric cancer (GC).

METHODS

Immunohistochemistry was performed to detect the expression of EPRS in human gastric tumor tissues. Knocking down of EPRS, cell-derived xenograft mouse model, and patient-derived xenograft mouse model was used to identify the biological function of EPRS. Immunoprecipitation was applied to elucidate the interaction between EPRS and SCYL2. Computer docking model and multiple in vitro and in vivo experiments were conducted to discover EPRS inhibitors.

RESULTS

Here, we report that EPRS is frequently overexpressed in GC tissues compared to that adjacent controls and its overexpression predicts poor prognosis in GC patients. Functionally, high expression of EPRS positively co-relates with GC development both in vitro and in vivo. Mechanistically, EPRS directly binds with SCYL2 to enhance the activation of WNT/GSK-3β/β-catenin signaling pathway and the accumulation of β-catenin in the nuclear, leading to GC cell proliferation and tumor growth. Moreover, we identified that xanthoangelol (XA) and 4-hydroxyderricin (4-HD) can directly bind to EPRS to block WNT/GSK-3β/β-catenin signaling pathway. More importantly, XA and 4-HD restrain gastric cancer patient-derived xenograft tumor growth and Helicobacter pylori combined with alcohol-induced atrophic gastritis and gastric tumorigenesis.

CONCLUSION

These findings unveil a promising strategy for GC prevention and therapy by targeting EPRS-mediated WNT/GSK-3β/β-catenin cascades. Moreover, XA and 4-HD may be effective reagents used for GC prevention and therapy.

A novel heat shock protein alpha 8 (Hspa8) molecular network mediating responses to stress- and ethanol-related behaviors

Genetic differences mediate individual differences in susceptibility and responses to stress and ethanol, although, the specific molecular pathways that control these responses are not fully understood. Heat shock protein alpha 8 (Hspa8) is a molecular chaperone and member of the heat shock protein family that plays an integral role in the stress response and that has been implicated as an ethanol-responsive gene. Therefore, we assessed its role in mediating responses to stress and ethanol across varying genetic backgrounds. The hippocampus is an important mediator of these responses, and thus, was examined in the BXD family of mice in this study. We conducted bioinformatic analyses to dissect genetic factors modulating Hspa8 expression, identify downstream targets of Hspa8, and examined its role. Hspa8 is trans-regulated by a gene or genes on chromosome 14 and is part of a molecular network that regulates stress- and ethanol-related behaviors. To determine additional components of this network, we identified direct or indirect targets of Hspa8 and show that these genes, as predicted, participate in processes such as protein folding and organic substance metabolic processes. Two phenotypes that map to the Hspa8 locus are anxiety-related and numerous other anxiety- and/or ethanol-related behaviors significantly correlate with Hspa8 expression. To more directly assay this relationship, we examined differences in gene expression following exposure to stress or alcohol and showed treatment-related differential expression of Hspa8 and a subset of the members of its network. Our findings suggest that Hspa8 plays a vital role in genetic differences in responses to stress and ethanol and their interactions.

Identification of proteins associated with Aha1 in HeLa cells by quantitative proteomics

The identification of the activator of heat shock protein 90 (Hsp90) ATPase's (Aha1) protein-protein interaction (PPI) network will provide critical insights into the relationship of Aha1 with multi-molecular complexes and shed light onto Aha1's interconnections with Hsp90-regulated biological functions. Flag-tagged Aha1 was over-expressed in HeLa cells and isolated by anti-Flag affinity pull downs, followed by trypsin digestion and identification co-adsorbing proteins by liquid chromatography-tandem mass spectroscopy (LC-MS/MS). A probability-based identification of Aha1 PPIs was generated from the LC-MS/MS analysis by using a relative quantification strategy, spectral counting (SC). By comparing the SC-based protein levels between Aha1 pull-down samples and negative controls, 164 Aha1-interacting proteins were identified that were quantitatively enriched in the pull-down samples over the controls. The identified Aha1-interacting proteins are involved in a wide number of intracellular bioprocesses, including DNA maintenance, chromatin structure, RNA processing, translation, nucleocytoplasmic and vesicle transport, among others. The interactions of 33 of the identified proteins with Aha1 were further confirmed by Western blotting, demonstrating the reliability of our affinity-purification-coupled quantitative SC-MS strategy. Our proteomic data suggests that Aha1 may participate in diverse biological pathways to facilitate Hsp90 chaperone functions in response to stress.

Protein-protein interactions and multi-component complexes of aminoacyl-tRNA synthetases

Protein-protein interaction occurs transiently or stably when two or more proteins bind together to mediate a wide range of cellular processes such as protein modification, signal transduction, protein trafficking, and structural folding. The macromolecules involved in protein biosynthesis such as aminoacyl-tRNA synthetase (ARS) have a number of protein-protein interactions. The mammalian multi-tRNA synthetase complex (MSC) consists of eight different enzymes: EPRS, IRS, LRS, QRS, MRS, KRS, RRS, and DRS, and three auxiliary proteins: AIMP1/p43, AIMP2/p38, and AIMP/p18. The distinct ARS proteins are also connected to diverse protein networks to carry out biological functions. In this chapter we first show the protein networks of the entire MSC and explain how MSC components interact with or can regulate other proteins. Finally, it is pointed out that the understanding of protein-protein interaction mechanism will provide insight to potential therapeutic application for diseases related to the MSC network.

Identification of Linkage Disequilibrium SNPs from a Kidney-Yang Deficiency Syndrome Pedigree

In order to probe the genetic traits of Kidney-yang Deficiency Syndrome (KDS), we employed a national standard of KDS diagnosis for the collection of KDS subjects. Each candidate KDS subject from a typical family was diagnosed by 5 independent physicians of Traditional Chinese Medicine (TCM), and repeated for 3 years, all on the first Saturday of December. Fifteen samples of genomic DNA were isolated and genotyped by Affymetrix 100 K arrays of single nucleotide polymorphism (SNP). Then appropriate tools were used for the analysis of linkage disequilibrium (LD) and bioinformatic mining of LD SNPs. The results indicated that our procedure of TCM diagnosis can effectively collect KDS subjects and therefore provide substantial basis for the linkage analysis of KDS. Five SNPs (i.e. rs514207, rs1054020, rs7685923, rs10515889 and rs10516202) were identified as LD SNPs from this KDS family, representing an unprecedented set of LD SNPs derived from TCM syndrome. These SNPs demonstrate midrange linkage disequilibrium within the KDS family. Two genes with established functions were identified within 100 bp of these SNPs. One is Homo sapiens double cortin domain containing 5, which interacts selectively with mono-, di- or tri-saccharide carbohydrate and involves certain signaling cascades. Another one, leucyl-tRNA synthetase, is also a pleiotropic gene response to cysteinyl-tRNA aminoacylation and protein biosynthesis. In conclusion, KDS is involved in special SNP linkage disequilibrium in the intragenic level, and genes within the flanks of these SNPs suggest some essential symptoms of KDS. However, definitive evidence to confirm or exclude these loci and to establish their biological activities will be required.

Cyclodepsipeptide toxin promotes the degradation of Hsp90 client proteins through chaperone-mediated autophagy

Promoting the degradation of Hsp90 client proteins by inhibiting Hsp90, an important protein chaperone, has been shown to be a promising new anticancer strategy. In this study, we show that an oxazoline analogue of apratoxin A (oz-apraA), a cyclodepsipeptide isolated from a marine cyanobacterium, promotes the degradation of Hsp90 clients through chaperone-mediated autophagy (CMA). We identify a KFERQ-like motif as a conserved pentapeptide sequence in the kinase domain of epidermal growth factor receptor (EGFR) necessary for recognition as a CMA substrate. Mutation of this motif prevents EGFR degradation by CMA and promotes the degradation of EGFR through the proteasomal pathway in oz-apraA–treated cells. Oz-apraA binds to Hsc70/Hsp70. We propose that apratoxin A inhibits Hsp90 function by stabilizing the interaction of Hsp90 client proteins with Hsc70/Hsp70 and thus prevents their interactions with Hsp90. Our study provides the first examples for the ability of CMA to mediate degradation of membrane receptors and cross talks of CMA and proteasomal degradation mechanisms.

The tandem PDZ protein Syntenin interacts with the aminoacyl tRNA synthetase complex in a lysyl-tRNA synthetase-dependent manner

Syntenin-1 is a tandem PDZ protein that binds a diverse array of signaling molecules that are often associated with cell adhesion and intracellular trafficking. With the use of a MS-based functional proteomics approach, we identified several members of the aminoacyl-tRNA synthetase macromolecular (ARS) complex in a syntenin-1 pull down assay. Interaction of these proteins with syntenin-1 was confirmed by co-immunoprecipitation from cultured cells. We demonstrate a direct interaction of syntenin-1 with lysyl-tRNA synthetase (KRS), which contains a PDZ binding motif at its C-terminus. This motif is important for the interaction of the entire complex with syntenin-1. A point mutation in the PDZ2 domain of syntenin-1 abrogates interaction with KRS. As a result, other components of the ARS complex no longer co-immunoprecipitate with syntenin-1. We further show that syntenin-1 regulates KRS activity. These findings suggest that syntenin-1 is an adaptor modulating the activity of KRS.

Peroxin Pex21p interacts with the C-terminal noncatalytic domain of yeast seryl-tRNA synthetase and forms a specific ternary complex with tRNA(Ser)

The seryl-tRNA synthetase from Saccharomyces cerevisiae interacts with the peroxisome biogenesis-related factor Pex21p. Several deletion mutants of seryl-tRNA synthetase were constructed and inspected for their ability to interact with Pex21p in a yeast two-hybrid assay, allowing mapping of the synthetase domain required for complex assembly. Deletion of the 13 C-terminal amino acids abolished Pex21p binding to seryl-tRNA synthetase. The catalytic parameters of purified truncated seryl-tRNA synthetase, determined in the serylation reaction, were found to be almost identical to those of the native enzyme. In vivo loss of interaction with Pex21p was confirmed in vitro by coaffinity purification. These data indicate that the C-terminally appended domain of yeast seryl-tRNA synthetase does not participate in substrate binding, but instead is required for association with Pex21p. We further determined that Pex21p does not directly bind tRNA, and nor does it possess a tRNA-binding motif, but it instead participates in the formation of a specific ternary complex with seryl-tRNA synthetase and tRNA(Ser), strengthening the interaction of seryl-tRNA synthetase with its cognate tRNA(Ser).

Functional expansion of aminoacyl-tRNA synthetases and their interacting factors: new perspectives on housekeepers

Aminoacyl-tRNA synthetases (ARSs) are essential enzymes that join amino acids to tRNAs, thereby linking the genetic code to specific amino acids. Once considered a class of 'housekeeping' enzymes, ARSs are now known to participate in a wide variety of functions, including transcription, translation, splicing, inflammation, angiogenesis and apoptosis. Three nonenzymatic proteins--ARS-interacting multi-functional proteins (AIMPs)--associate with ARSs in a multi-synthetase complex of higher eukaryotes. Similarly to ARSs, AIMPs have novel functions unrelated to their support role in protein synthesis, acting as a cytokine to control angiogenesis, immune response and wound repair, and as a crucial regulator for cell proliferation and DNA repair. Evaluation of the functional roles of individual ARSs and AIMPs might help to elucidate why these proteins as a whole contribute such varied functions and interactions in complex systems.

A proteomic snapshot of the human heat shock protein 90 interactome

Heat shock protein 90 (Hsp90) is a molecular chaperone which modulates several signalling pathways within a cell. By applying co-immunoprecipitation with endogeneous Hsp90, we were able to identify 39 novel protein interaction partners of this chaperone in human embryonic kidney cells (HEK293). Interestingly, levels of DNA-activated protein kinase catalytic subunit, an Hsp90 interaction partner found in this study, were found to be sensitive to Hsp90 inhibitor treatment only in HeLa cells but not in HEK293 cells referring to the tumorgenicity of this chaperone.

Association between Archaeal prolyl- and leucyl-tRNA synthetases enhances tRNA(Pro) aminoacylation

Aminoacyl-tRNA synthetase-containing complexes have been identified in different eukaryotes, and their existence has also been suggested in some Archaea. To investigate interactions involving aminoacyl-tRNA synthetases in Archaea, we undertook a yeast two-hybrid screen for interactions between Methanothermobacter thermautotrophicus proteins using prolyl-tRNA synthetase (ProRS) as the bait. Interacting proteins identified included components of methanogenesis, protein-modifying factors, and leucyl-tRNA synthetase (LeuRS). The association of ProRS with LeuRS was confirmed in vitro by native gel electrophoresis and size exclusion chromatography. Determination of the steady-state kinetics of tRNA(Pro) charging showed that the catalytic efficiency (k(cat)/K(m)) of ProRS increased 5-fold in the complex with LeuRS compared with the free enzyme, whereas the K(m) for proline was unchanged. No significant changes in the steady-state kinetics of LeuRS aminoacylation were observed upon the addition of ProRS. These findings indicate that ProRS and LeuRS associate in M. thermautotrophicus and suggest that this interaction contributes to translational fidelity by enhancing tRNA aminoacylation by ProRS.

Aminoacyl-tRNA synthetase complexes: beyond translation

Although aminoacyl-tRNA synthetases (ARSs) are housekeeping enzymes essential for protein synthesis, they can play non-catalytic roles in diverse biological processes. Some ARSs are capable of forming complexes with each other and additional proteins. This characteristic is most pronounced in mammals, which produce a macromolecular complex comprising nine different ARSs and three additional factors: p43, p38 and p18. We have been aware of the existence of this complex for a long time, but its structure and function have not been well understood. The only apparent distinction between the complex-forming ARSs and those that do not form complexes is their ability to interact with the three non-enzymatic factors. These factors are required not only for the catalytic activity and stability of the associated ARSs, such as isoleucyl-, methionyl-, and arginyl-tRNA synthetase, but also for diverse signal transduction pathways. They may thus have joined the ARS community to coordinate protein synthesis with other biological processes.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.

Downregulation of FUSE-binding protein and c-myc by tRNA synthetase cofactor p38 is required for lung cell differentiation

p38 is associated with a macromolecular tRNA synthetase complex. It has an essential role as a scaffold for the complex, and genetic disruption of p38 in mice causes neonatal lethality. Here we investigated the molecular mechanisms underlying lethality of p38-mutant mice. p38-deficient mice showed defects in lung differentiation and respiratory distress syndrome. p38 was found to interact with FUSE-binding protein (FBP), a transcriptional activator of c-myc. Binding of p38 stimulated ubiquitination and degradation of FBP, leading to downregulation of c-myc, which is required for differentiation of functional alveolar type II cells. Transforming growth factor-beta (TGF-beta) induced p38 expression and promoted its translocation to nuclei for the regulation of FBP and c-myc. Thus, this work identified a new activity of p38 as a mediator of TGF-beta signaling and its functional importance in the control of c-myc during lung differentiation.

Molecular network and functional implications of macromolecular tRNA synthetase complex

Understanding the complex network and multi-functionality of proteins is one of the main objectives of post-genome research. Aminoacyl-tRNA synthetases (ARSs) are the family of enzymes that are essential for cellular protein synthesis and viability that catalyze the attachment of specific amino acids to their cognate tRNAs. However, a lot of evidence has shown that these enzymes are multi-functional proteins that are involved in diverse cellular processes, such as tRNA processing, RNA splicing and trafficking, rRNA synthesis, apoptosis, angiogenesis, and inflammation. In addition, mammalian ARSs form a macromolecular complex with three auxiliary factors or with the elongation factor complex. Although the functional meaning and physiological significance of these complexes are poorly understood, recent data on the molecular interactions among the components for the multi-ARS complex are beginning to provide insights into the structural organization and cellular functions. In this review, the molecular mechanism for the assembly and functional implications of the multi-ARS complex will be discussed.

Regulation of signaling protein function and trafficking by the hsp90/hsp70-based chaperone machinery

Nearly 100 proteins are known to be regulated by hsp90. Most of these substrates or "client proteins" are involved in signal transduction, and they are brought into complex with hsp90 by a multiprotein hsp90/hsp70-based chaperone machinery. In addition to binding substrate proteins at the chaperone site(s), hsp90 binds cofactors at other sites that are part of the heterocomplex assembly machinery as well as immunophilins that connect assembled substrate*hsp90 complexes to protein-trafficking systems. In the 5 years since we last reviewed this subject, much has been learned about hsp90 structure, nucleotide-binding, and cochaperone interactions; the most important concept is that ATP hydrolysis by an intrinsic ATPase activity results in a conformational change in hsp90 that is required to induce conformational change in a substrate protein. The conformational change induced in steroid receptors is an opening of the steroid-binding cleft so that it can be accessed by steroid. We have now developed a minimal system of five purified proteins-hsp90, hsp70, Hop, hsp40, and p23- that assembles stable receptor*hsp90 heterocomplexes. An hsp90*Hop*hsp70*hsp40 complex opens the cleft in an ATP-dependent process to produce a receptor*hsp90 heterocomplex with hsp90 in its ATP-bound conformation, and p23 then interacts with the hsp90 to stabilize the complex. Stepwise assembly experiments have shown that hsp70 and hsp40 first interact with the receptor in an ATP-dependent reaction to produce a receptor*hsp70*hsp40 complex that is "primed" to be activated to the steroid-binding state in a second ATP-dependent step with hsp90, Hop, and p23. Successful use of the five-protein system with other substrates indicates that it can assemble signal protein*hsp90 heterocomplexes whether the substrate is a receptor, a protein kinase, or a transcription factor. This purified system should facilitate understanding of how eukaryotic hsp70 and hsp90 work together as essential components of a process that alters the conformations of substrate proteins to states that respond in signal transduction.

Identifying Pex21p as a protein that specifically interacts with yeast seryl-tRNA synthetase

The interaction of Saccharomyces cerevisiae seryl-tRNA synthetase (SerRS) with peroxin Pex21p was identified in a two-hybrid screen with SerRS as bait. This was confirmed by an in vitro binding assay with truncated Pex21p fused to glutathione S-transferase. Furthermore, purified Pex21p acts as an activator of yeast seryl-tRNA synthetase in aminoacylation in vitro, revealing the functional significance of the Pex21p-SerRS interaction. Pex21p is a protein involved in the peroxisome biogenesis [Purdue, P.E., Yang, X. and Lazarow, P.B., J. Cell Biol. 143 (1998) 1859-1869]. Since eukaryotic aminoacyl-tRNA synthetases are known to participate in assembles with other synthetases and non-synthetase proteins, we propose that this unusual interaction reflects another function of the peroxin.

p38 is essential for the assembly and stability of macromolecular tRNA synthetase complex: implications for its physiological significance

Mammalian tRNA synthetases form a macromolecular complex with three nonenzyme factors: p43, p38, and p18. Here we introduced a mutation within the mouse p38 gene to understand its functional significance for the formation of the multi-tRNA synthetase complex. The complex was completely disintegrated by the deficiency of p38. In addition, the protein levels and catalytic activities of the component enzymes and cofactors were severely decreased. A partial truncation of the p38 polypeptide separated the associated components into different subdomains. The mutant mice showed lethality within 2 days of birth. Thus, this work provides the first evidence, to our knowledge, that p38 is essential for the structural integrity of the multi-tRNA synthetase complex and mouse viability.

Interaction network of human aminoacyl-tRNA synthetases and subunits of elongation factor 1 complex

Aminoacyl-tRNA synthetases (ARSs) ligate amino acids to their cognate tRNAs. It has been suggested that mammalian ARSs are linked to the EF-1 complex for efficient channeling of aminoacyl tRNAs to ribosome. Here we systemically investigated possible interactions between human ARSs and the subunits of EF-1 (alpha, beta, gamma, and delta) using a yeast two-hybrid assay. Among the 80 tested pairs, leucyl- and histidyl-tRNA synthetases were found to make strong and specific interaction with the EF-1gamma and beta while glu-proly-, glutaminyl-, alanyl-, aspartyl-, lysyl-, phenylalanyl-, glycyl-, and tryptophanyl-tRNA synthetases showed moderate interactions with the different EF-1 subunits. The interactions of leucyl- and histidyl-tRNA synthetase with the EF-1 complex were confirmed by immunoprecipitation and in vitro pull-down experiments. Interestingly, the aminoacylation activities of these two enzymes, but not other ARSs, were stimulated by the cofactor of EF-1, GTP. These data suggest that a systematic interaction network may exist between mammalian ARSs and EF-1 subunits probably to enhance the efficiency of in vivo protein synthesis.

The renaissance of aminoacyl-tRNA synthesis

The role of tRNA as the adaptor in protein synthesis has held an enduring fascination for molecular biologists. Over four decades of study, taking in numerous milestones in molecular biology, led to what was widely held to be a fairly complete picture of how tRNAs and amino acids are paired prior to protein synthesis. However, recent developments in genomics and structural biology have revealed an unexpected array of new enzymes, pathways and mechanisms involved in aminoacyl-tRNA synthesis. As a more complete picture of aminoacyl-tRNA synthesis now begins to emerge, the high degree of evolutionary diversity in this universal and essential process is becoming clearer.

Hsp90: chaperoning signal transduction

Hsp90 is an ATP dependent molecular chaperone involved in the folding and activation of an unknown number of substrate proteins. These substrate proteins include protein kinases and transcription factors. Consistent with this task, Hsp90 is an essential protein in all eucaryotes. The interaction of Hsp90 with its substrate proteins involves the transient formation of multiprotein complexes with a set of highly conserved partner proteins. The specific function of each component in the processing of substrates is still unknown. Large ATP-dependent conformational changes of Hsp90 occur during the hydrolysis reaction and these changes are thought to drive the chaperone cycle. Natural inhibitors of the ATPase activity, like geldanamycin and radicicol, block the processing of Hsp90 substrate proteins. As many of these substrates are critical elements in signal transduction, Hsp90 seems to introduce an additional level of regulation.

Aminoacyl-tRNA synthetases database

Aminoacyl-tRNA synthetases (AARSs) are at the center of the question of the origin of life. They constitute a family of enzymes integrating the two levels of cellular organization: nucleic acids and proteins. AARSs arose early in evolution and are believed to be a group of ancient proteins. They are responsible for attaching amino acid residues to their cognate tRNA molecules, which is the first step in the protein synthesis. The role they play in a living cell is essential for the precise deciphering of the genetic code. The analysis of AARSs evolutionary history was not possible for a long time due to a lack of a sufficiently large number of their amino acid sequences. The emerging picture of synthetases' evolution is a result of recent achievements in genomics [Woese,C., Olsen,G.J., Ibba,M. and Söll,D. (2000) Microbiol. Mol. Biol. Rev., 64, 202-236]. In this paper we present a short introduction to the AARSs database. The updated database contains 1047 AARS primary structures from archaebacteria, eubacteria, mitochondria, chloroplasts and eukaryotic cells. It is the compilation of amino acid sequences of all AARSs known to date, which are available as separate entries via the WWW at http://biobases.ibch.poznan.pl/aars/.