Akt (Protein Kinase B)
Akt (protein kinase B), a serine/threonine kinase, has emerged as a critical enzyme in signal transduction pathways involved in cell proliferation, apoptosis, angiogenesis, and diabetes. In mammals three isoforms of Akt (a, b, g or Akt 1, 2, 3) are reported that exhibit a high degree of homology, but differ slightly in the localization of their regulatory phosphorylation sites. Akta is the predominant isoform in most tissues, whereas the highest expression of Aktb is observed in the insulin-responsive tissues, and Aktg is abundant in brain tissue. Each Akt isoform is composed of three functionally distinct regions: an N-terminal pleckstrin homology (PH) domain that provides a lipid-binding module to direct Akt to PIP2 and PIP3, a central catalytic domain, and a C-terminal hydrophobic motif.
Akt is constitutively phosphorylated at Ser124, in the region between the PH and catalytic domains, and on Thr450, in the C-terminal region (in Akta, the most widely studied isoform) in unstimulated cells. Activation of Akt involves growth factor binding to a receptor tyrosine kinase and activation of PI 3-K, which phosphorylates membrane bound PIP2 to generate PIP3. The binding of PIP3 to the PH domain anchors Akt to the plasma membrane and allows its phosphorylation and activation by PDK1. Akt is fully activated following its phosphorylation at two regulatory residues, a threonine residue on the kinase domain and a serine residue on the hydrophobic motif, which are structurally and functionally conserved within the AGC kinase family. Phosphorylation at Thr308 and Ser473 is required for the activation of Akta, while phosphorylation at Thr309 and Ser474 activates Aktb. Phosphorylation at Thr305 activates Aktg. Phosphorylation of a threonine residue on the kinase domain, catalyzed by PDK1, is essential for Akt activation. It causes a charge-induced conformational change, allowing substrate binding and increased rate of catalysis. Akt activity is augmented about 10-fold by phosphorylation at the serine residue by PDK2. DNA-PK and PKCbII are reported to phosphorylate the serine residue on the regulatory subunit. Without threonine phosphorylation, the hydrophobic motif of Akt is more susceptible to the action of phosphatases; however, the dually phosphorylated and fully active enzyme is stable, allowing its localization to the nucleus and other sites. The activity of Akt is negatively regulated by PTEN and SHIP. |
