Sarbassov, D

Sarbassov, D. Rictor-T1135 phosphorylation does not appear to regulate mTORC2-mediated effects on SGK1 or PKC. While the precise molecular mechanism affecting Akt is unknown, phosphorylation of T1135 stimulates binding of Rictor to 14-3-3 proteins. We provide evidence that Rictor-T1135 phosphorylation functions in parallel with other mTORC1-dependent feedback mechanisms, such as those affecting IRS-1 signaling to PI3K, to regulate the response of Akt to insulin. The mammalian target of rapamycin (mTOR) is usually a protein kinase conserved in all eukaryotes that controls key cellular processes, including growth, proliferation, survival, and metabolism (66). Its activity is dependent on association with regulatory proteins that cooperate to form functional kinase complexes that are directed toward specific downstream substrates. At least two mTOR-containing complexes exist in eukaryotic cells, mTOR complexes 1 and 2 (mTORC1 and -2), which consist of distinct units of proteins and appear to perform nonoverlapping functions (34). In mammals, both complexes contain mTOR and mLST8, along with core components that are essential for the specific functions of each complex: Raptor (16, 30) in mTORC1 and Rictor (28, 50) and mSIN1 (11, 70) in mTORC2. In addition, these complexes contain other interacting proteins that are not functionally essential to Rabbit Polyclonal to TAS2R49 the core complex, including PRAS40 in mTORC1 (10, 41, 49, 58, 64) and Protor/PRR5 in mTORC2 (42, 58, 65). Finally, both complexes have been found to interact with a negative regulator dubbed DEPTOR (44). While there are likely to be many direct substrates of the mTOR complexes, specific members of the protein kinase A, G, and C (AGC) family are among the best-characterized downstream effectors of mTOR signaling (examined in reference 29). These kinases include the mTORC1 substrates S6K1 and S6K2 (5, 32) and the mTORC2 substrates Akt1/2/3, SGK1, and PKC (8, 12, 22, 26, 52). Many AGC kinases share a requirement for phosphorylation of three conserved regions for their full activation and/or stability: the activation loop within the kinase domain name, a linker Carteolol HCl region termed the change motif, and a C-terminal hydrophobic motif (examined in reference 39). Phosphorylation of the activation loop is essential for kinase Carteolol HCl activity and is invariably phosphorylated by PDK1 (37). Phosphorylation of the hydrophobic motif, on the other hand, can either produce a docking site for PDK1 to facilitate activation loop phosphorylation, as it appears to do in S6K1 and SGK1 (4), or stabilize the active conformation of the kinase, as it does in Akt (69). Phosphorylation of the change motif in AGC Carteolol HCl kinases generally occurs as part of protein maturation and is thought to stabilize the kinase (20). The hydrophobic motif on S6K1 (T389) is the best-characterized substrate of mTORC1, while this same motif on Akt (S473), SGK1 (S422), and PKC (S657) is usually phosphorylated in an mTORC2-dependent manner (29). mTORC2 is also essential for change motif phosphorylation of Akt (T450) and PKC (S638) (8, 26). How mTORC2 controls the phosphorylation of both growth factor-responsive sites, such as the hydrophobic motifs on Akt and SGK1, and constitutive growth factor-independent sites, such as the Carteolol HCl change motifs on Akt and PKC, is currently unknown (examined in reference 2). Due primarily to its acute sensitivity to rapamycin, most studies over the past 15 years have focused on mTORC1, and therefore, much more is known about its upstream regulation than mTORC2’s. Many of the upstream signaling pathways (e.g., Akt, Erk, AMPK, and Wnt, etc.) that regulate mTORC1 do so by affecting the phosphorylation state of a protein complex comprised of the tuberous sclerosis complex tumor suppressors TSC1 and TSC2, which functions as a key unfavorable regulator of mTORC1 (examined in recommendations 7 and 24). In addition,.