Serine/threonine-protein kinase that acts downstream of mTOR signaling in response to growth factors and nutrients to promote cell proliferation, cell growth and cell cycle progression. Regulates protein synthesis through phosphorylation of EIF4B, RPS6 and EEF2K, and contributes to cell survival by repressing the pro-apoptotic function of BAD. Under conditions of nutrient depletion, the inactive form associates with the EIF3 translation initiation complex. Upon mitogenic stimulation, phosphorylation by the mammalian target of rapamycin complex 1 (mTORC1) leads to dissociation from the EIF3 complex and activation. The active form then phosphorylates and activates several substrates in the pre-initiation complex, including the EIF2B complex and the cap-binding complex component EIF4B. Also controls translation initiation by phosphorylating a negative regulator of EIF4A, PDCD4, targeting it for ubiquitination and subsequent proteolysis. Promotes initiation of the pioneer round of protein synthesis by phosphorylating POLDIP3/SKAR. In response to IGF1, activates translation elongation by phosphorylating EEF2 kinase (EEF2K), which leads to its inhibition and thus activation of EEF2. Also plays a role in feedback regulation of mTORC2 by mTORC1 by phosphorylating RICTOR, resulting in the inhibition of mTORC2 and AKT1 signaling. Mediates cell survival by phosphorylating the pro-apoptotic protein BAD and suppressing its pro-apoptotic function. Phosphorylates mitochondrial RMP leading to dissociation of a RMP:PPP1CC complex. The free mitochondrial PPP1CC can then dephosphorylate RPS6KB1 at Thr-412, which is proposed to be a negative feedback mechanism for the RPS6KB1 anti-apoptotic function. Mediates TNF-alpha-induced insulin resistance by phosphorylating IRS1 at multiple serine residues, resulting in accelerated degradation of IRS1. In cells lacking functional TSC1-2 complex, constitutively phosphorylates and inhibits GSK3B. May be involved in cytoskeletal rearrangement through binding to neurabin. Phosphorylates and activates the pyrimidine biosynthesis enzyme CAD, downstream of MTOR (By similarity) (PubMed:11493700, PubMed:11500364, PubMed:15060135, PubMed:18952604). Following activation by mTORC1, phosphorylates EPRS and thereby plays a key role in fatty acid uptake by adipocytes and also most probably in interferon-gamma-induced translation inhibition (PubMed:28178239). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activation requires multiple phosphorylation events on serine/threonine residues. Activation appears to be first mediated by phosphorylation of multiple sites in the autoinhibitory domain, which facilitates phosphorylation at Thr-412, disrupting the autoinhibitory mechanism and allowing phosphorylation of Thr-252 by PDPK1. The active conformation of the kinase is believed to be stabilized by a mechanism involving three conserved phosphorylation sites located in the kinase domain activation loop (Thr-252) and in the AGC-kinase C-terminal domain (Ser-394 in the middle of the tail/linker region and Thr-412 within a hydrophobic motif at its end). Activated by mTORC1; isoform Alpha I and isoform Alpha II are sensitive to rapamycin, which inhibits activating phosphorylation at Thr-412. Activated by PDPK1 (By similarity). Interacts with PPP1R9A/neurabin-1. Interacts with RPTOR. Interacts with IRS1. Interacts with EIF3B and EIF3C. Interacts with TRAF4. Interacts with POLDIP3. Interacts (via N-terminus) with IER5. Colocalizes with URI1 at mitochondrion. The autoinhibitory domain is believed to block phosphorylation within the AGC-kinase C-terminal domain and the activation loop. The TOS (TOR signaling) motif is essential for activation by mTORC1. Phosphorylation at Thr-412 is regulated by mTORC1. The phosphorylation at this site is maintained by an agonist-dependent autophosphorylation mechanism. Activated by phosphorylation at Thr-252 by PDPK1. Dephosphorylation by PPP1CC at Thr-412 in mitochondrion (By similarity). Impairment of body growth. Lethal in combination with Rps6kb2 deficiency. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.