90-kDa ribosomal S6 kinase is phosphorylated and activated by 3-phosphoinositide-dependent protein kinase-1

90-kDa ribosomal S6 kinase-2 (RSK2) belongs to a family of growth factor-ac tivated serine/threonine kinases composed of two kinase domains connected b y a regulatory linker region. The N-terminal kinase of RSK2 is involved in substrate phosphorylation. Its activation requires phosphorylation of the l inker region at Ser(369), catalyzed by extracellular signal-regulated kinas e (ERK), and at Ser(386), catalyzed by the C-terminal kinase, after its act ivation by ERK. In addition, the N-terminal kinase must be phosphorylated a t Ser(227) in the activation loop by an as yet unidentified kinase. Here, w e show that the isolated N-terminal kinase of RSK2 (amino acids 1-360) is p hosphorylated at Ser(227) by PDK1, a constitutively active kinase, leading to 100-fold stimulation of kinase activity. In COS7 cells, ectopic PDK1 ind uced the phosphorylation of full-length RSK2 at Ser(227) and Ser(386), with out involvement of ERK, leading to partial activation of RSK2. Similarly, t wo other members of the RSK family, RSK1 and RSK3, were partially activated by PDK1 in COS7 cells. Finally, our data indicate that full activation of RSK2 by growth factor requires the cooperation of ERK and PDK1 through phos phorylation of Ser(227), Ser(369), and Ser(386). Our study extend recent fi ndings which implicate PDK1 in the activation of protein kinases B and C an d p70(S6K), suggesting that PDK1 controls several major growth factor-activ ated signal transduction pathways.