Role of phosphatidylinositol 3,4,5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1

3-Phosphoinositide-dependent protein kinase-1 (PDK1) interacts stereoselect ively with the D-enantiomer of PtdIns(3,4,5)P-3 (K-D 1.6 nM) and PtdIns(3,4 )P-2 (K-D 5.2 nM), but binds with lower affinity to PtdIns3P or PtdIns(4,5) P-2. The binding of PtdIns(3,4,5)P-3 to PDK1 was greatly decreased by makin g specific mutations in the pleckstrin homology (PH) domain of PDK1 or by d eleting it. The same mutations also greatly decreased the rate at which PDK 1 activated protein kinase B alpha (PKB alpha) in vitro in the presence of lipid vesicles containing PtdIns(3,4,5)P-3, but did not affect the rate at which PDK1 activated a PKB alpha mutant lacking the PH domain in the absenc e of PtdIns(3,4,5)P-3. When overexpressed in 293 or PAE cells, PDK1 was loc ated at the plasma membrane and in the cytosol, but was excluded from the n ucleus. Mutations that disrupted the interaction of PtdIns(3,4,5)P-3 or Ptd Ins(4,5)P-2 with PDK1 abolished the association of PDK1 with the plasma mem brane. Growth-factor stimulation promoted the translocation of transfected PKB alpha to the plasma membrane, but had no effect on the subcellular dist ribution of PDK1 as judged by immunoelectron microscopy of fixed cells. Thi s conclusion was also supported by confocal microscopy of green fluorescent protein-PDK1 in live cells. These results, together with previous observat ions, indicate that PtdIns(3,4,5)P-3 plays several roles in the PDK1-induce d activation of PKB alpha. First, it binds to the PH domain of PKB, alterin g its conformation so that it can be activated by PDK1. Secondly, interacti on with PtdIns(3,4,5)P-3 recruits PKB to the plasma membrane with which PDK 1 is localized constitutively by virtue of its much stronger interaction wi th PtdIns(3,4,5)P-3 or PtdIns(4,5)P-2. Thirdly, the interaction of PDK1 wit h PtdIns(3,4,5)P-3 facilitates the rate at which it can activate PKB.