Cell-based analysis of pairwise interactions between the components of the multi-tRNA synthetase complex

Cardiomyocyte-Specific Loss of Glutamyl-prolyl-tRNA Synthetase Leads to Disturbed Protein Homeostasis and Dilated Cardiomyopathy

Glutamyl-prolyl-tRNA synthetase (EPRS1), an aminoacyl-tRNA synthetase (ARS) ligating glutamic acid and proline to their corresponding tRNAs, plays an essential role in decoding proline codons during translation elongation. The physiological function of EPRS1 in cardiomyocytes (CMs) and the potential effects of the CM-specific loss of Eprs1 remain unknown. Here, we found that heterozygous Eprs1 knockout in CMs does not cause any significant changes in CM hypertrophy induced by pressure overload, while homozygous knockout leads to dilated cardiomyopathy, heart failure, and lethality at around 1 month after Eprs1 deletion. The transcriptomic profiling of early-stage Eprs1 knockout hearts suggests a significantly decreased expression of multiple ion channel genes and an increased gene expression in proapoptotic pathways and integrated stress response. Proteomic analysis shows decreased protein expression in multi-aminoacyl-tRNA synthetase complex components, fatty acids, and branched-chain amino acid metabolic enzymes, as well as a compensatory increase in cytosolic translation machine-related proteins. Immunoblot analysis indicates that multiple proline-rich proteins were reduced at the early stage, which might contribute to the cardiac dysfunction of Eprs1 knockout mice. Taken together, this study demonstrates the physiological and molecular outcomes of loss-of-function of Eprs1 in vivo and provides valuable insights into the potential side effects on CMs, resulting from the EPRS1-targeting therapeutic approach.

Cardiomyocyte-specific Loss of Glutamyl-prolyl-tRNA Synthetase Leads to Disturbed Protein Homeostasis and Dilated Cardiomyopathy

Glutamyl-prolyl-tRNA synthetase (EPRS1), an aminoacyl-tRNA synthetase (ARS) ligating glutamic acid and proline to their corresponding tRNAs, plays an essential role in decoding proline codons during translation elongation. The physiological function of EPRS1 in cardiomyocytes (CMs) and the potential effects of CM-specific loss of EPRS1 remain unknown. Here, we found that heterozygous Eprs1 knockout in CMs does not cause any significant changes in CM hypertrophy induced by pressure overload, while homozygous knockout leads to dilated cardiomyopathy, heart failure, and lethality at around 1 month after Eprs1 deletion. Transcriptomic profiling of early-stage Eprs1 knockout hearts suggests a significantly decreased expression of multiple ion channel genes and an increased gene expression in proapoptotic pathways and integrated stress response. Proteomic analysis shows decreased protein expression of multi-aminoacyl-tRNA synthetase complex components, fatty acid, and branched-chain amino acid metabolic enzymes, as well as a compensatory increase in cytosolic translation machine-related proteins. Immunoblot analysis indicated that multiple proline-rich proteins were reduced at the early stage, which might contribute to cardiac dysfunction of Eprs1 knockout mice. Taken together, this study demonstrates the physiological and molecular outcome of loss-of-function of EPRS1 in vivo and provides valuable insights into the potential side effects on CMs resulting from the EPRS1-targeting therapeutic approach.

Multimodal cotranslational interactions direct assembly of the human multi-tRNA synthetase complex

Amino acid ligation to cognate transfer RNAs (tRNAs) is catalyzed by aminoacyl-tRNA synthetases (aaRSs)-essential interpreters of the genetic code during translation. Mammalian cells harbor 20 cytoplasmic aaRSs, out of which 9 (in 8 proteins), with 3 non-aaRS proteins, AIMPs 1 to 3, form the ∼1.25-MDa multi-tRNA synthetase complex (MSC). The function of MSC remains uncertain, as does its mechanism of assembly. Constituents of multiprotein complexes encounter obstacles during assembly, including inappropriate interactions, topological constraints, premature degradation of unassembled subunits, and suboptimal stoichiometry. To facilitate orderly and efficient complex formation, some complexes are assembled cotranslationally by a mechanism in which a fully formed, mature protein binds a nascent partner as it emerges from the translating ribosome. Here, we show out of the 121 possible interaction events between the 11 MSC constituents, 15 are cotranslational. AIMPs are involved in the majority of these cotranslational interactions, suggesting they are not only critical for MSC structure but also for assembly. Unexpectedly, several cotranslational events involve more than the usual dyad of interacting proteins. We show two modes of cotranslational interaction, namely a "multisite" mechanism in which two or more mature proteins bind the same nascent peptide at distinct sites and a second "piggy-back" mechanism in which a mature protein carries a second fully formed protein and binds to a single site on an emerging peptide. Multimodal mechanisms of cotranslational interaction offer a diversity of pathways for ordered, piecewise assembly of small subcomplexes into larger heteromultimeric complexes such as the mammalian MSC.

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.

Structural basis for the dynamics of human methionyl-tRNA synthetase in multi-tRNA synthetase complexes

In mammals, eight aminoacyl-tRNA synthetases (AARSs) and three AARS-interacting multifunctional proteins (AIMPs) form a multi-tRNA synthetase complex (MSC). MSC components possess extension peptides for MSC assembly and specific functions. Human cytosolic methionyl-tRNA synthetase (MRS) has appended peptides at both termini of the catalytic main body. The N-terminal extension includes a glutathione transferase (GST) domain responsible for interacting with AIMP3, and a long linker peptide between the GST and catalytic domains. Herein, we determined crystal structures of the human MRS catalytic main body, and the complex of the GST domain and AIMP3. The structures reveal human-specific structural details of the MRS, and provide a dynamic model for MRS at the level of domain orientation. A movement of zinc knuckles inserted in the catalytic domain is required for MRS catalytic activity. Depending on the position of the GST domain relative to the catalytic main body, MRS can either block or present its tRNA binding site. Since MRS is part of a huge MSC, we propose a dynamic switching between two possible MRS conformations; a closed conformation in which the catalytic domain is compactly attached to the MSC, and an open conformation with a free catalytic domain dissociated from other MSC components.

The 3'tsRNAs are aminoacylated: Implications for their biogenesis

Emerging evidence indicates that tRNA-derived small RNAs (tsRNAs) are involved in fine-tuning gene expression and become dysregulated in various cancers. We recently showed that the 22nt LeuCAG3´tsRNA from the 3´ end of tRNALeu is required for efficient translation of a ribosomal protein mRNA and ribosome biogenesis. Inactivation of this 3´tsRNA induced apoptosis in rapidly dividing cells and suppressed the growth of a patient-derived orthotopic hepatocellular carcinoma in mice. The mechanism involved in the generation of the 3´tsRNAs remains elusive and it is unclear if the 3´-ends of 3´tsRNAs are aminoacylated. Here we report an enzymatic method utilizing exonuclease T to determine the 3´charging status of tRNAs and tsRNAs. Our results showed that the LeuCAG3´tsRNA, and two other 3´tsRNAs are fully aminoacylated. When the leucyl-tRNA synthetase (LARS1) was inhibited, there was no change in the total tRNALeu concentration but a reduction in both the charged tRNALeu and LeuCAG3´tsRNA, suggesting the 3´tsRNAs are fully charged and originated solely from the charged mature tRNA. Altering LARS1 expression or the expression of various tRNALeu mutants were also shown to affect the generation of the LeuCAG3´tsRNA further suggesting they are created in a highly regulated process. The fact that the 3´tsRNAs are aminoacylated and their production is regulated provides additional insights into their importance in post-transcriptional gene regulation that includes coordinating the production of the protein synthetic machinery.