KAIST Researchers Uncover Molecular Switch Linking Amino Acid Sensing to mTORC1 Activation, Opening New Avenues for Cancer Therapy
Key Insights
A KAIST (search)-Yonsei University team has identified the molecular mechanism by which amino acid signals trigger mTORC1 (search)-dependent cell growth via phosphorylation and release of LARS1 (search) from the multi-tRNA synthetase complex.
Cryo-EM structural analysis revealed that nutrient-induced phosphorylation of LARS1 (search) weakens its binding to IARS1 (search), allowing LARS1 to dissociate from the MSC and activate mTORC1 (search).
The findings suggest a more selective anticancer strategy that targets upstream growth signaling rather than directly inhibiting mTORC1 (search), potentially sparing normal cells.
A collaborative research team from KAIST (search) and Yonsei University has mapped, in atomic detail, the molecular switch that converts amino acid availability into a cellular growth signal—a discovery that could reshape strategies for developing more selective anticancer therapies.
The study, published online in Nature Communications on June 11, reveals how the multi-tRNA synthetase complex (MSC) serves as a nutrient-sensing hub that controls the activation of mTORC1 (search) (mammalian Target of Rapamycin Complex 1), the central regulator of cell growth and metabolism.
A long-sought link between nutrients and growth
Cells continuously monitor the availability of amino acids—the building blocks of proteins—and adjust growth, protein synthesis, and energy expenditure accordingly. At the heart of this regulatory system sits mTORC1 (search), a protein complex that promotes cell growth and metabolism when nutrients and energy are abundant.
When mTORC1 (search) becomes excessively active, however, cells can grow and proliferate beyond normal requirements—a pattern of dysregulation observed in numerous cancers. For this reason, mTORC1 has long been considered a prime target for anticancer drug development. Yet exactly how cells detect external nutrient cues and translate them into mTORC1 activation has remained incompletely understood.
The research team, led by Professors Hee-Sung Park and Jin Young Kang from KAIST (search)'s Department of Chemistry, working with Professor Sunghoon Kim's team from Yonsei University, focused on the MSC—a large protein assembly composed of multiple aminoacyl-tRNA synthetases and scaffold proteins.
LARS1 (search): the field agent that flips the growth switch
The key player within the MSC turned out to be LARS1 (search) (leucyl-tRNA synthetase 1), an enzyme that attaches leucine to its corresponding tRNA and also functions as an intracellular leucine sensor. When cells receive a signal that nutrients are sufficient, LARS1 undergoes phosphorylation—a modification in which a small chemical tag is attached to the protein, altering its function and binding behavior.
The relationship can be pictured as follows: the MSC is a control center where multiple proteins wait on standby, and LARS1 (search) is the field agent dispatched to flip on the growth switch. When nutrients become abundant, LARS1 receives a phosphorylation "deployment signal," dissociates from IARS1 (search)—the protein that anchors LARS1 to the MSC—and is thereby released from the complex. The freed LARS1 then goes on to activate mTORC1 (search).
"When nutrients are scarce, LARS1 (search) stays bound within the MSC and the growth signal remains off. Once nutrients become sufficient, LARS1 is released from the MSC and switches on mTORC1 (search)," the researchers explained.
Cryo-EM reveals the structural basis of the switch
To investigate the structural basis of this process, the team employed cryo-electron microscopy (cryo-EM), a technique that visualizes protein complexes in three dimensions at near-atomic resolution by rapidly freezing samples at extremely low temperatures. This allowed the researchers to determine how LARS1 (search) and IARS1 (search) bind to each other and to structurally explain how phosphorylation could disrupt their interaction.
The results showed that LARS1 (search) and IARS1 (search) are normally bound tightly, but amino acid stimulation induces the phosphorylation of LARS1, weakening its interaction with IARS1. This allows LARS1 to dissociate from the MSC and activate mTORC1 (search).
The researchers also engineered phosphomimetic LARS1 (search) variants—mutant proteins designed to imitate the phosphorylated state—and found that these variants substantially enhanced mTORC1 (search) activity. This confirmed that the phosphorylation of LARS1 functions as the key molecular switch converting a nutrient signal into a cell growth signal.
Toward more selective anticancer strategies
The significance of this study lies in mapping, in concrete molecular detail, how cells sense amino acids and use that information to activate their growth switch. In particular, the study revealed that, upon receiving nutrient signals, the MSC—a complex involved in protein synthesis—releases its constituent protein LARS1 (search), which then activates cellular growth signaling.
Some existing anticancer drugs work by directly inhibiting mTORC1 (search). However, because mTORC1 is also required for normal cellular growth and metabolism, its direct inhibition may also affect normal cells—a limitation that has constrained the therapeutic window of mTORC1-targeted agents.
The research team expects that further identifying the kinase responsible for phosphorylating LARS1 (search), along with its regulatory mechanism, could enable a more precise anticancer strategy—one that intercepts the growth signal further upstream, before it reaches mTORC1 (search), rather than blocking mTORC1 itself.
Professor Park noted, "This study clarifies, at the molecular level, how cells sense their nutritional status and activate growth signals." Professor Kang added, "We hope this will lead to therapeutic strategies that more selectively block abnormally activated growth signaling in cancer (search) cells."
The study was co-first-authored by Youjin Kim and Joo-Chan Kim from KAIST (search)'s Department of Chemistry and was supported by the National Research Foundation of Korea.