Sestrin2 is identified as a leucine sensor for the mTORC1 pathway. Leucine disrupts the interaction between Sestrin2 and GATOR2, a negative regulator of mTORC1, by binding to Sestrin2 with a Kd of 20 μM. This binding is required for leucine to activate mTORC1 in cells. Sestrin2 mutants that cannot bind leucine are unable to signal the presence of leucine to mTORC1, and loss of Sestrin2 and its homologs renders the mTORC1 pathway insensitive to leucine deprivation. The study suggests that Sestrin2 and Sestrin1 are likely leucine sensors upstream of mTORC1, and that these proteins may serve as targets for developing small molecule modulators of the mTORC1 pathway.Sestrin2 is identified as a leucine sensor for the mTORC1 pathway. Leucine disrupts the interaction between Sestrin2 and GATOR2, a negative regulator of mTORC1, by binding to Sestrin2 with a Kd of 20 μM. This binding is required for leucine to activate mTORC1 in cells. Sestrin2 mutants that cannot bind leucine are unable to signal the presence of leucine to mTORC1, and loss of Sestrin2 and its homologs renders the mTORC1 pathway insensitive to leucine deprivation. The study suggests that Sestrin2 and Sestrin1 are likely leucine sensors upstream of mTORC1, and that these proteins may serve as targets for developing small molecule modulators of the mTORC1 pathway.