Structural basis for a phosphoinositide-driven mTORC2-AKT positive feedback loop
The mammalian target of rapamycin complex 2 (mTORC2) regulates metabolism, growth, survival and cytoskeletal organization, yet its activation mechanism is poorly understood. We show that mTORC2 is directly activated by membranes and our cryo-electron tomography structure of membrane-bound mTORC2 reveals the N-HEAT region of mTOR is at the major membrane interface. mTORC2 is further potently activated by a positive feedback loop involving reciprocal phosphorylation of mTORC2 and its substrate kinase AKT. Cryo-EM structures of dephosphorylated, autophosphorylated and AKT-phosphorylated mTORC2 reveal structural changes in the SIN1 subunit, regulating an autoinhibitory anchor. Reconstitution of the PDK1-AKT-mTORC2 hub on PIP3-containing membranes shows that PDK1/PIP3-dependent AKT activation drives SIN1-T86 phosphorylation, enabling mTORC2 to phosphorylate S473 of AKTs hydrophobic motif, establishing a PI3K-dependent, phosphorylation-driven positive feedback loop at the membrane.