A phosphoserine-regulated docking site in the protein kinase RSK2 that recruits and activates PDK1

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Abstract
The 90 kDa ribosomal S6 kinase‐2 (RSK2) is a growth factor‐stimulated protein kinase with two kinase domains. The C‐terminal kinase of RSK2 is activated by ERK‐type MAP kinases, leading to autophosphorylation of RSK2 at Ser386 in a hydrophobic motif. The N‐terminal kinase is activated by 3‐phosphoinositide‐dependent protein kinase‐1 (PDK1) through phosphorylation of Ser227, and phosphorylates the substrates of RSK. Here, we identify Ser386 in the hydrophobic motif of RSK2 as a phosphorylation‐dependent docking site and activator of PDK1. Treatment of cells with growth factor induced recruitment of PDK1 to the Ser386‐phosphorylated hydrophobic motif and phosphorylation of RSK2 at Ser227. A RSK2‐S386K mutant showed no interaction with PDK1 or phosphorylation at Ser227. Interaction with Ser386‐phosphorylated RSK2 induced autophosphorylation of PDK1. Addition of a synthetic phosphoSer386 peptide (RSK2373–396) increased PDK1 activity 6‐fold in vitro . Finally, mutants of RSK2 and MSK1, a RSK‐related kinase, with increased affinity for PDK1, were constitutively active in vivo and phosphorylated histone H3. Our results suggest a novel regulatory mechanism based on phosphoserine‐mediated recruitment of PDK1 to RSK2, leading to coordinated phosphorylation and activation of PDK1 and RSK2.