| Akt1 (also termed protein kinase B alpha) is a serine/threonine kinase of the AGC family that functions as a central signaling hub downstream of phosphoinositide 3-kinase, integrating growth factor, cytokine, and nutrient inputs into coordinated control of cell survival, proliferation, metabolism, and growth. The protein contains an N‑terminal pleckstrin homology domain that binds phosphatidylinositol 3,4,5‑trisphosphate at the plasma membrane, a central catalytic kinase domain with the conserved activation loop threonine, and a C‑terminal regulatory tail harboring a hydrophobic motif serine and other residues that tune activity and interactions with upstream kinases and phosphatases. Activation begins when receptor tyrosine kinases, G protein–coupled receptors, or other receptors stimulate class I PI3K to generate phosphatidylinositol 3,4,5‑trisphosphate, which recruits Akt1 and its upstream kinase PDK1 to the membrane via their pleckstrin homology domains; PDK1 phosphorylates the activation loop threonine and mTOR complex 2 phosphorylates the C‑terminal hydrophobic motif serine, together producing full Akt1 catalytic activity. Protein phosphatases such as PP2A and PHLPP reverse these phosphorylations, while PTEN and related lipid phosphatases suppress Akt1 indirectly by dephosphorylating phosphatidylinositol 3,4,5‑trisphosphate, establishing a balance between kinase and phosphatase activities that defines Akt1 signaling amplitude and duration. Once active, Akt1 phosphorylates substrates containing an RxRxxS/T motif across multiple compartments, including the pro-apoptotic Bcl‑2 family member Bad, FoxO transcription factors, and components of the intrinsic caspase machinery, which shifts signaling toward survival and resistance to apoptotic triggers. Akt1 also modulates metabolism and nutrient handling through targets such as AS160 and glycolytic regulators, couples insulin and growth factor signaling to glucose transport and glycogen synthesis via inhibition of GSK‑3 isoforms, and regulates cell-cycle progression by phosphorylating p21 and p27 to influence their stability and localization. Direct phosphorylation of TSC2 and other elements of the TSC1–TSC2–mTOR axis links Akt1 to mTOR complex 1 activation, eIF4F assembly, and protein synthesis, placing this kinase at the core of growth and anabolic signaling networks. |
