Distinct activation mechanisms of protein kinase B by growth-factor stimulation and heat-shock treatment

Protein kinase B (PKB) alpha, having the pleckstrin homology (PH) and catalytic domains in its amino- and carboxyl-terminal regions, respectively, is activated in the signaling pathway of growth factors as a downstream target of phosphatidylinositol 3-kinase and becomes an active form in heat-shocked cells in a manner independent of the lipid kinase. Therefore, the activation mechanisms of PKBalpha were compared in platelet-derived growth factor (PDGF)-stimulated and heat-shocked cells by monitoring the protein kinase activity and phosphorylation of the mutant molecules expressed in COS-7 cells. In heat-shocked cells, PKBalpha was activated to a certain level without phosphorylation on Thr-308 in the activation loop and on Thr-450 and Ser-473 in the carboxyl-terminal end region, which is critical for growth-factor-induced activation of PKBalpha. Metabolic labeling with P-32-orthophosphate in the transfected cells revealed that there is no major phosphorylation site other than the three residues in PKBalpha. PKBalpha activated by heat shock was more stable than the enzyme stimulated by PDGF in the cells, and PKBalpha recovered from heat-shocked cells was resistant to the protein phosphatase treatment, whereas the enzyme obtained from the growth-factor-stimulated cells was inactivated by dephosphorylation. Heat shock also enhanced the association of the PH-domain fragment to the full-length PKBalpha in the transfected cells. On the other hand, the PH-domain fragment of PKBalpha, which moves from the cytosol to the plasma membrane upon PDGF stimulation by the interaction with the phosphatidylinositol 3-kinase products, did not translocate but stayed in the cytosol in heat-shocked NIH 3T3 cells. Furthermore, PKBalpha was associated with the nuclear region in heat-shocked cells, which is not observed in growth-factor-stimulated cells. These results indicate that heat shock induces the conformational change of PKBalpha that accompanies the protein complex formation and perinuculear/nuclear localization of the enzyme, to generate an active form by a mechanism distinct from that in the growth-factor-signaling pathway.