A tRNA synthetase is an amino acid sensor for TOR in plants

PROJECT SUMMARY TARGET OF RAPAMYCIN (TOR) is a deeply conserved protein kinase that regulates eukaryotic metabolism. TOR senses and integrates upstream signals, especially nutrient availability, to coordinate metabolism and promote growth only when conditions are favorable. TOR dysregulation causes or contributes to a broad range of human diseases, including cancers, age-related health disorders, and metabolic disorders, which are the major causes of morbidity and mortality in the United States. Therefore, a major goal for biomedical research is to develop therapeutic treatments that specifically target components of the TOR signaling network without broadly disrupting metabolism and homeostasis in healthy cells that rely on TOR. Recently, there have been significant advances to that goal with the discovery of several putative amino acid sensors for TOR. Conflicting reports about the relative contributions, importance, and molecular mechanisms of these sensors have stymied these advances, however. This project uses an innovative approach to bring fresh perspective to these ongoing debates by shifting focus to the other major eukaryotic lineage, plants. In my lab’s ongoing work to elucidate the TOR signaling network in plants, I discovered a novel amino acid sensor for TOR, an aminoacyl tRNA synthetase (aaRS). This aaRS is necessary to maintain TOR activity and sufficient to stimulate TOR in plant cells. Using a combination of biochemical, molecular, genetic, and systems-level approaches, I propose to precisely define how the aaRS activates TOR in plant cells through three independent aims. In Aim 1, I propose to mutate key enzymatic residues and structural features of the aaRS to determine the molecular features it requires to activate TOR. In Aim 2, I propose to map the signal transduction pathway mediating aaRS-TOR activation using robust orthogonal interactomic approaches. Putative signal transduction components will then be validated using reciprocal assays and functional genetics to comprehensively define how aaRS-TOR interactors contribute to TOR regulation. In Aim 3, I propose to establish the selective sensitivity of TOR for specific amino acids and determine whether the aaRS is a bona fide amino acid sensor for TOR. Taken together, these three aims will define the molecular mechanisms underlying the putative amino acid-aaRS-TOR signaling axis and open new directions for future research on metabolic regulation in eukaryotes. Moreover, this pathway will serve as a model for understanding how tRNA synthetases have evolved functions beyond translation in signal transduction pathways and illuminate how the complex TOR signaling network evolved to integrate diverse physiological cues in humans. Long-term, our findings will make significant contributions to a major goal of contemporary biomedical research: fine-tuning TOR signaling networks to improve and lengthen healthy human lifespans.