TRANSLOCATION OF PDK-1 TO THE PLASMA-MEMBRANE IS IMPORTANT IN ALLOWING PDK-1 TO ACTIVATE PROTEIN-KINASE-B

Background: Protein kinase B (PKB) is involved in the regulation of ap optosis, protein synthesis and glycogen metabolism in mammalian cells. Phosphoinositide-dependent protein kinase (PDK-1) activates PKB in a manner dependent on phosphatidylinositol 3,4,5-trisphosphate (Ptdlns(3 ,4,5)P-3), which is also needed for the translocation of PKB to the pl asma membrane. It has been proposed that the amount of PKB activated i s determined exclusively as a result of its translocation, and that a constitutively active pool of membrane-associated PDK-1 simply phospho rylates all the PKB made available. Here, we have investigated the eff ects of membrane localisation of PDK-1 on PKB activation. Results: Ect opically expressed PDK-1 translocated to the plasma membrane in respon se to platelet-derived growth factor (PDGF) and translocation was sens itive to wortmannin, an inhibitor of phosphoinositide 3-kinase. Transl ocation of PDK-1 also occurred upon its co-expression with constitutiv ely active phosphoinositide 3-kinase, but not with an inactive form. O verexpression of PDK-1 enhanced the ability of PDGF to activate PKB. P DK-1 disrupted in the pleckstrin homology (PH) domain which did not tr anslocate to the membrane did not increase PKB activity in response to PDGF, whereas membrane-targeted PDK-1 activated PKB to the extent tha t it could not be activated further by PDGF. Conclusions: In response to PDGF, binding of Ptdlns(3,4,5)P-3 and/or Ptdlns(3,4)P-2 to the PH d omain of PDK-1 causes its translocation to the plasma membrane where i t co-localises with PKB, significantly contributing to the scale of PK B activation.