Elaborate uORF/IRES features control expression and localization of human glycyl-tRNA synthetase

The canonical activity of glycyl-tRNA synthetase (GARS) is to charge glycine onto its cognate tRNAs. However, outside translation, GARS also participates in many other functions. A single gene encodes both the cytosolic and mitochondrial forms of GARS but 2 mRNA isoforms were identified. Using immunolocalization assays, in vitro translation assays and bicistronic constructs we provide experimental evidence that one of these mRNAs tightly controls expression and localization of human GARS. An intricate regulatory domain was found in its 5'-UTR which displays a functional Internal Ribosome Entry Site and an upstream Open Reading Frame. Together, these elements hinder the synthesis of the mitochondrial GARS and target the translation of the cytosolic enzyme to ER-bound ribosomes. This finding reveals a complex picture of GARS translation and localization in mammals. In this context, we discuss how human GARS expression could influence its moonlighting activities and its involvement in diseases.