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‐kinase‐dependent manner in response to insulin/growth factors signalling. Activ ation entails phosphorylation of these kinases at two residues, the T‐loop and the hydrophobic motif. PDK1 activates S6K, SGK and PKB isoforms by phosphorylating these kinases at their T‐loop. We demonstrate that a pocket in the 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 PIF‐binding pocket of PDK1 and their T‐loop phosphorylation. Thus, the hydrophobic motif phosphorylation of S6K and SGK converts them into substrates that can be activated by PDK1. In contrast, the PIF‐binding pocket of PDK1 is not required for the phosphorylation of PKBα 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 substrates.

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