Regulation of Ex-Translational Activities Is Primary Function of Multi-tRNA Synthetase Complex

During mRNA translation, tRNAs are charged by aminoacyl-tRNA synthetases (aaRS) and subsequently used by ribosomes. A multi-enzyme aminoacyl-tRNA synthetase complex (MSC) has long been proposed to increase protein synthesis efficiency by passing charged tRNAs directly to ribosomes. An alternative is that the MSC repurposes specific synthetases for ex-translational functions that are activated by cues that direct specific enzymes to novel targets. To explore this question, we generated mammalian cell clones in which ArgRS and GlnRS were absent from the MSC to give a stable complex lacking the two enzymes (MSC?RQ). Protein synthesis, under a variety of stress conditions, was unchanged in MSC?RQ cells. Most strikingly, levels of charged tRNAGln and tRNAArg remained unchanged and no ribosome pausing was observed at codons for Arg and Gln. Thus, increasing or regulating protein synthesis efficiency is not dependent on ArgRS and GlnRS in the MSC. Alternatively, and consistent with previously reported ex-translational roles, we found manipulations that do not affect protein synthesis but instead MSC cellular localization. Overall design: Ribosome footprints of wildtype, ArgRS deltaLZ, and delta LZ + LZ mCer HEK 293T cells.