Functional characterization of human RSK4, a new 90-kDa ribosomal S6 kinase, reveals constitutive activation in most cell types.
The 90-kDa ribosomal S6 kinases (RSK1-3) are important mediators of growth factor stimulation of cellular proliferation, survival, and differentiation and are activated via coordinated phosphorylation by ERK and 3-phosphoinositide-dependent protein kinase-1 (PDK1). Here we performed the functional characterization of a predicted new human RSK homologue, RSK4. We showed that RSK4 is a predominantly cytosolic protein with very low expression and several characteristics of the RSK family kinases, including the presence of two functional kinase domains and a C-terminal docking site for ERK. Surprisingly, however, in all cell types analyzed, endogenous RSK4 was maximally (constitutively) activated under serum-starved conditions where other RSKs are inactive due to their requirement for growth factor stimulation. Constitutive activation appeared to result from constitutive phosphorylation of Ser232, Ser372, and Ser389, and the low basal ERK activity in serum-starved cells appeared to be sufficient for induction of approximately 50% of the constitutive RSK4 activity. Finally experiments in mouse embryonic stem cells with targeted deletion of the PDK1 gene suggested that PDK1 was not required for phosphorylation of Ser232, a key regulatory site in the activation loop of the N-terminal kinase domain, that in other RSKs is phosphorylated by PDK1. The unusual regulation and growth factor-independent kinase activity indicate that RSK4 is functionally distinct from other RSKs and may help explain recent findings suggesting that RSK4 can participate in non-growth factor signaling as for instance p53-induced growth arrest.
Dümmler
Bettina A
BA
Department of Clinical Biochemistry, Glostrup Hospital, 2600 Glostrup, Denmark.
Hauge
Camilla
C
Silber
Joachim
J
Yntema
Helger G
HG
Kruse
Lars S
LS
Kofoed
Birte
B
Hemmings
Brian A
BA
Alessi
Dario R
DR
Frödin
Morten
M
D064413:3-Phosphoinositide-Dependent Protein Kinases; D000595:Amino Acid Sequence; D000818:Animals; D002070:Butadienes / Q000378:metabolism; D002460:Cell Line; D016895:Culture Media, Serum-Free; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000378:metabolism; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D009570:Nitriles / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000235:genetics / Q000378:metabolism*; D016415:Sequence Alignment; D012694:Serine / Q000378:metabolism; D014018:Tissue Distribution
D002070:Butadienes; D016895:Culture Media, Serum-Free; D004791:Enzyme Inhibitors; D009570:Nitriles; D011994:Recombinant Proteins; C113580:U 0126; D012694:Serine; D064413:3-Phosphoinositide-Dependent Protein Kinases; C578562:PDPK1 protein, human; C578564:Pdpk1 protein, mouse; D017346:Protein Serine-Threonine Kinases; C501305:RPS6KA6 protein, human; D038744:Ribosomal Protein S6 Kinases, 90-kDa