AKT3 (v-akt murine thymoma viral oncogene homolog 3, Protein Kinase B gamma)

	Diagram of the AKT3 protein in scale. The numbers represent specific residues. The domains are PH (Pleckstrin Homology), a short helical region, Kinase (Catalytic Kinase), and Regulatory (Regulatory Region). Indicated are the two phosphorylation sites shown to be essential for activation of AKT3: Threonine 305 and serine 472. C: Carboxyl-terminal; N: Amino-terminal.
Description	The AKT3 serine/threonine kinase consists of 479 amino acids with a calculated molecular weight of 55.8 kDa (approximate molecular weight of 59-60 kDa seen on a Western blot). The protein contains three important regions: the PH domain at the N terminus (residues 1-107), a kinase domain (residues 148-405), and a C-terminal regulatory domain (residues 406-479). A 465-amino acid splice variant lacking the serine 472 residue has been identified, which results from alternative splicing of an exon at the C terminus.
Expression	Northern blot analysis of AKT3 indicated that it is expressed in virtually all tissues, predominantly in the brain and fetal heart and at lower levels in liver and skeletal muscle. RT-PCR analysis also indicated expression of AKT3 in a wide variety of tissues.
Localisation	Predominantly cytoplasmic; also found at the plasma membrane and in the nucleus following its activation.
Function	AKT family members are serine/threonine kinases activated following stimulation by growth factors, hormones and the extracellular matrix. AKT kinases play a key role in proliferation, cell survival, and tumorigenesis. Binding of ligands (e.g., EGF) to tyrosine kinase receptors or G-protein coupled receptors leads to the recruitment and activation of the Class 1A and Class 1B PI3K (Phosphatidylinositol 3-Kinase), respectively. The pleckstrin homology domain of AKT kinases has affinity for the 3'-phosphorylated phosphoinositides 3,4,5-trisphosphate (PI-3,4,5-P3) and PI-3,4-P2 produced by PI3K, and they are activated specifically by the latter lipid. Phospholipid binding triggers the translocation of AKT kinases to the plasma membrane. There, AKT is activated through phosphorylation of a threonine (T308 in AKT1, T309 in AKT2 and T305 in AKT3) by PDK1 and on a serine (S473 on AKT1, S474 on AKT2, and S472 on AKT3) by PDK2. Activated AKT then phosphorylates a number of different substrates involved in survival, cell cycle progression, and other pathways implicated in tumorigenesis.
Homology	AKT3 is a serine/threonine kinase and is a member of the AKT family that also includes AKT1 and AKT2. At the protein level, AKT3 shows overall 83.6% identity with AKT1 and 78% identity with AKT2. The three AKT kinases are identical in the ATP binding region, except for one residue: Ala 230 of AKT1 is conserved in AKT2 (Ala 232), but switches to Val 228 in AKT3.

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