Isoleucine-to-valine substitutions support cellular physiology during isoleucine deprivation

Aminoacyl-tRNA synthetases (ARSs) couple tRNAs with their corresponding amino acids. While ARSs can bind structurally similar amino acids, extreme specificity is ensured by subsequent editing activity. Yet, we found that upon isoleucine (I) restriction, healthy fibroblasts consistently incorporated valine (V) into proteins at isoleucine codons, resulting from misacylation of tRNAIle with valine by wildtype IARS1. Using a dual-fluorescent reporter of translation, we found that valine supplementation could fully compensate for isoleucine depletion and restore translation to normal levels in healthy, but not IARS1 deficient cells. Similarly, the antiproliferative effects of isoleucine deprivation could be fully restored by valine supplementation in healthy, but not IARS1 deficient cells. This indicates I > V substitutions help prevent translational termination and maintain cellular function in human primary cells during isoleucine deprivation. We suggest that this is an example of a more general mechanism in mammalian cells to preserve translational speed at the cost of translational fidelity in response to (local) amino acid deficiencies.