PDK1 (3 Phosphoinositide Dependent Protein Kinase 1) phosphorylates AGC kinases. PDK1 activates conventional PKC and PKC zeta through phosphorylation of critical threonine residues in the activation loop. PDK1 also phosphorylates Protein Kinase B (PKB) at threonine 308 in the presence of phosphatidylinositol-3,4,5-trisphosphate. Active Akt inactivates Glycogen Synthase Kinase 3 (GSK3), eventually leading to the dephosphorylation and activation of glycogen synthase and the stimulation of glycogen synthesis. Because of the role that PDK plays in insulin-induced glycogen synthesis and PKC activation it is a potentially important target for metabolic drug research. There are three named isoforms.