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Unexpected metabolic function of a tRNA synthetase

Glutamyl(E)-prolyl(P) tRNA Synthetase (EPRS) is an exception to the “one amino acid–one aminoacyl-tRNA synthetase (AARS)” rule. Class I ERS and class II PRS are fused into a single polypeptide that aminoacylates Glu and Pro, respectively, to cognate tRNAs for protein synthesis. They were joined via a non-catalytic linker in a unicellular animal-like organism »800 Mya, and have remained fused with limited exceptions. EPRS is a constituent of the cytosolic multi-AARS complex (MSC) containing 9 synthetases and 3 non-synthetase proteins in mice and humans. The linker region is central to several EPRS noncanonical activities beyond aminoacylation. For example, interferon-g (IFN-g) induces phosphorylation of Ser in the linker, inducing translation-inhibition of inflammation-related mRNAs by the GAIT system in myeloid cells. Also, Ser phosphorylation in the linker directs antiviral defense upon infection. Finally, we have recently reported that insulin-stimulated Ser phosphorylation in adipocytes influences adiposity and aging. In all cases, phosphorylation induces EPRS release from the parental MSC for execution of its noncanonical activities, supporting the quarter century-old hypothesis that AARS phosphorylation might induce extra-aminoacylation activities. Insulin in adipocytes and IFN-g in myeloid cells stimulate mammalian target of rapamycin complex 1 (mTORC1) to activate ribosomal protein S6 kinase 1 (S6K1) that in turn phosphorylates EPRS at Ser (Fig. 1, left). Utilizing knock-in mice bearing phospho-deficient (Ser-to-Ala) and phosphomimetic (Ser-to-Asp) mutations in Eprs, we observed, unexpectedly, that EPRS links mTOR signaling and obesity-related metabolic processes. Homozygous Eprs mice on standard rodent diet displayed several loss-of-function phenotypes observed in S6K1 and adipocyte-specific raptor (mTORC1constituent)-deficient mice including low body weight, reduced fat mass, and increased life-span (Fig. 1, right). The body weight phenotype of S6K1 was substantially rescued by introduction of the phosphomimetic Eprs allele, providing in vivo, epistatic evidence for pathway linearity. Investigation of the underlying molecular mechanism revealed that insulin induces EPRS phosphorylation and consequent release from the MSC in cultured adipocytes. Freed EPRS binds fatty acid transport protein 1 (FATP1) and conveys it to the plasma membrane where it facilitates long-chain fatty acid (LCFA) uptake for increased triglyceride synthesis (Fig. 1, left). This newly discovered pathway complements other adipogenic, mTOR-dependent pathways, e.g., SREBP-1-induced lipid synthesis. In mice, the tissue-specific metabolic phenotype resulting from mutation of a single amino acid in an AARS was not anticipated. Our results extended the mTORC1-S6K1 signaling axis that influences adiposity and aging to include 2 new constituents – EPRS and FATP1. However, the relationship between the phenotypes of Eprs and FATP1 mice is equivocal due to conflicting reports regarding adiposity in the latter. Moreover, our studies do not illuminate the somewhat contentious mechanism of action of FATP1 – early studies suggested it facilitated inward LCFA transport, but later studies suggested intracellular metabolic trapping driven by a coenzyme A ligase activity inherent in FATP1, or in a bound enzyme. These results raise 2 important conundrums: What is the origin of the observed tissue specificity of EPRS-mediated LCFA influx as the insulin-activated TOR pathway is operative in non-adipose tissues? Equally surprising is the Ser phospho-site, which is completely unrelated to the consensus S6K1 recognition sequence, RXRXXS/T. A clue to both puzzles might be found by elucidating the role of cyclin-dependent kinase 5 (Cdk5), which is found primarily in neuronal cells, but also in adipocytes and monocytes, and is essential for EPRS Ser phosphorylation in monocytes. The Eprs mice do not precisely phenocopy S6K1 mice with respect to the earlier onset of body weight reduction in the latter. Moreover, breeding the S6K1 with mice bearing an Eprs mutation incompletely normalized body weight. These observations suggest as-yet unidentified targets of S6K1 might contribute to the metabolic phenotype. The mTORC1-S6K1 axis regulates a myriad of downstream pathways involved in functions that are most likely EPRS-independent. For example, the relatively early appearance of low body weight in S6K1 mice might be due to reduced H2B phosphorylation which