Structural features of mTORC2 that control substrate-specific activities.
mTORC2 is a multi-subunit kinase complex that is central to multiple essential signaling pathways. Two core subunits, Rictor and mSin1 distinguish it from its relative, mTORC1 and support context-dependent phosphorylation of its substrates. mTORC2 structures have been determined previously, however, important questions remain, particularly regarding structural determinants of substrate specificity and context dependent activities. We used cryo-EM to obtain high resolution structures of the human mTORC2 apo-complex, as well as structures in the presence of substrates, Akt and SGK1. Specific predictions suggested by substrate-induced structural changes were tested in functional assays. First, side chain interactions between Rictor and mTOR that prevent recruitment of mTORC1 substrates and confer resistance to the mTORC1 inhibitor rapamycin were visualized for the first time in the apo-state, demonstrating the steric occlusion that prevents mTORC2 interaction with mTORC1 substrates and rapamycin. Also in the apo-state, mSin1 was seen to form extensive contacts with Rictor, including a pair of short -helices nestled between two Rictor helical repeat clusters, followed by an extended strand, which makes multiple weak contacts with Rictor helical cluster 1. In co-complex structures, SGK1, but not Akt, markedly altered the conformation of the mSin1 N-terminal extended strand, disrupting multiple weak interactions while inducing a large rotation of mSin1/Arg-83, which comes to interact with a negative patch within Rictor. Mutation of Arg-83 to Ala selectively disrupted mTORC2 dependent phosphorylation of SGK1 but not of Akt, supporting context-dependent substrate selection. These findings provide new structural and functional insights into mTORC2 specificity and context-dependent activities.