Nature Communications, Published online: 11 June 2026; doi:10.1038/s41467-026-74085-x Human LARS1 senses leucine to activate mTORC1, but its dissociation mechanism from the MSC remains unclear. Here the authors determine the cryo-EM structure of the LARS1–IARS1 complex, revealing phosphorylation as a switch regulating LARS1 dissociation and mTORC1 activation.