Akt/PKB (Protein kinase B), a serine/threonine protein kinase with antiapoptotic activity, is one of the major downstream targets of PtdIns(3,4,5)P3 signaling pathway. It contains a pleckstrin homology domain (PH domain) that specifically binds PtdIns(3,4,5)P3 on the plasma membrane. Akt phosphorylation and activation are directly determined by the level of PtdIns(3,4,5)P3 on the plasma membrane, which is regulated by PI3K. Akt consists of three isoforms: PKBα/Akt1, PKBβ/Akt2 and PKBγ/Akt3. Akt isoforms have an N-terminal PH (pleckstrin homology) domain and a kinase domain, which are separated by a 39-amino-acid hinge region. Catalytically active Akt regulates the function of numerous substrates involved in cell survival, growth, proliferation, metabolism and protein synthesis. Akt is a crucial mediator of cell survival and its deactivation is implicated in various stress-induced pathological cell death and degenerative diseases.
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