Structural Mechanisms of mTORC1 activation by RHEB and inhibition by PRAS40

The mechanistic target of rapamycin complex 1 (mTORC1) controls cell growth and metabolism in response to nutrients, energy levels and growth factors. It contains the atypical kinase mTOR and the RAPTOR subunit that binds to the TOS motif of substrates and regulators. mTORC1 is activated by the small GTPase RHEB and inhibited by PRAS40. Here we present the 3.0 Å cryo-EM structure of mTORC1 and the 3.4 Å structure of activated RHEB-mTORC1. RHEB binds to mTOR distally from the kinase active site, yet causes a global conformational change that allosterically realigns active-site residues, accelerating catalysis. Cancer-associated hyperactivating mutations map to structural elements that maintain the inactive state, and we provide biochemical evidence that they mimic RHEB relieving auto-inhibition. We also present crystal structures of RAPTOR-TOS motif complexes that define the determinants of TOS recognition, of an mTOR FKBP12-rapamycin-binding (FRB) domain–substrate complex that establishes a second substrate-recruitment mechanism, and of a truncated mTOR-PRAS40 complex that reveals PRAS40 inhibits both substrate-recruitment sites.