M. Frodin et al., A phosphoserine-regulated docking site in the protein kinase RSK2 that recruits and activates PDK1, EMBO J, 19(12), 2000, pp. 2924-2934
The 90 kDa ribosomal S6 kinase-2 (RSK2) is a growth factor-stimulated prote
in kinase with two kinase domains. The C-terminal kinase of RSK2 is activat
ed by ERK-type MAP kinases, leading to autophosphorylation of RSK2 at Ser38
6 in a hydrophobic motif. The N-terminal kinase is activated by 3-phosphoin
ositide-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 recruitmen
t of PDK1 to the Ser386-phosphorylated hydrophobic motif and phosphorylatio
n of RSK2 at Ser227. A RSK2-S386K mutant showed no interaction with PDK1 or
phosphorylation at Ser227, Interaction with Ser386-phosphorylated RSK2 ind
uced autophosphorylation of PDK1. Addition of a synthetic phosphoSer386 pep
tide (RSK2(373-396)) increased PDK1 activity 6-fold in vitro. Finally, muta
nts of RSK2 and MSK1, a RSK-related kinase, with increased affinity for PDK
1, were constitutively active in vivo and phosphorylated histone H3. Our re
sults suggest a novel regulatory mechanism based on phosphoserine-mediated
recruitment of PDK1 to RSK2, leading to coordinated phosphorylation and act
ivation of PDK1 and RSK2.