The AKT serine-threonine kinases are the human homologue to the viral oncogene v-akt. They are also known as protein kinase B (PKB) due to their similarity to protein kinase A (PKA) and protein kinase C (PKC) (Manning BD et al.2007).
Three Akt genes (AKT-1, AKT -2 and AKT -3) have been identified in mammals, and the expression of the different proteins varies in tissues. The serine-threonine kinases Akt-1 and Akt-2 can be detected in almost all tissues. In contrast, the expression of the serine-threonine kinase Akt-3 is limited to some tissue types. In particular, this kinase is highly expressed in the brain, lung, heart and kidney (Koseoglu S et al. 2007).
Akt is an important effector protein in this signaling cascade and is phosphorylated at the cell membrane by phosphoinositide-dependent kinase-1 (PDK1) at the amino acid threonine 308 (Akt-1), 309 (Akt-2), and 305 (Akt-3), respectively, depending on the isoform. For complete activation, phosphorylation at the amino acid serine 473 (Akt-1), 474 (Akt-2) or 472 (Akt-3) is additionally required. This is carried out by phosphoinositide-dependent kinase-2 (PDK2. This is a group of different kinases that can phosphorylate Akt.
They include
- the integrin linked kinase (ILK)
- the Mammalian target-of-rapamycin complex 2(mTORC2)
- the protein kinase Cβ2
- the DNA-dependent protein kinase (DNA-PK)
- and the kinase "Ataxia telangiectasia mutated" (ATM).