The PIF-binding pocket in PDK1 is essential for activation of S6K and SGK,but not PKB

PKB/Akt, S6K1 and SGK are related protein kinases activated in a PI 3-kinas e-dependent manner in response to insulin/growth factors signalling. Activa tion entails phosphorylation of these kinases at two residues, the T-loop a nd the hydrophobic motif. PDK1 activates S6K, SGK and PKB isoforms by phosp horylating these kinases at their T-loop. We demonstrate that a pocket in t he kinase domain of PDK1, termed the 'PIF-binding pocket', plays a key role in mediating the interaction and phosphorylation of S6K1 and SGK1 at their T-loop motif by PDK1. Our data indicate that prior phosphorylation of S6K1 and SGK1 at their hydrophobic motif promotes their interaction with the PI F-binding pocket of PDK1 and their T-loop phosphorylation. Thus, the hydrop hobic motif phosphorylation of S6K and SGK converts them into substrates th at can be activated by PDK1. In contrast, the PIF-binding pocket of PDK1 is not required for the phosphorylation of PKB alpha by PDK1. The PIF-binding pocket represents a substrate recognition site on a protein kinase that is only required for the phosphorylation of a subset of its physiological sub strates.