Ribosomal S6 kinase 1 (RSK1) activation requires signals dependent on and independent of the MAP kinase ERK

Background: The rsk1 gene encodes the 90 kDa ribosomal S6 kinase 1 (RSK1) p rotein, which contains two kinase domains. RSK1, which is involved in regul ating cell survival and proliferation, lies at the end of the signaling cas cade mediated by the extracellular signal-regulated kinase (ERK) subfamily of mitogen-activated protein (MAP) kinases. ERK activation and subsequent p hosphorylation of the RSK1 carboxy-terminal catalytic loop stimulates phosp hotransferase activity in the RSK1 amino-terminal kinase domain. When activ ated, RSK1 phosphorylates both nuclear and cytoplasmic substrates through t his amino-terminal catalytic domain. It is thought that stimulation of the ERK/MAP kinase pathway is sufficient for RSK1 activation, but how ERK phosp horylation activates the RSK1 amino-terminal kinase domain is not known. Results: The individual isolated RSK1 kinase domains were found to be under regulatory control. In vitro kinase assays established that ERK phosphoryl ates RSK1 within the carboxy-terminal kinase domain, and the phosphoinositi de-dependent kinase 1 (PDK1) phosphorylates RSK1 within the amino-terminal kinase domain. In transiently transfected HEK 293E cells, PDK1 alone stimul ated phosphotransferase activity of an isolated RSK1 amino-terminal kinase domain. Nevertheless, activation of full-length RSK1 in the absence of seru m required activation by both PDK1 and ERK. Conclusions: RSK1 is phosphorylated by PDK1 in the amino-terminal kinase-ac tivation loop, and by ERK in the carboxy-terminal kinase-activation loop. A ctivation of phosphotransferase activity of full-length RSK1 in vivo requir es both PDK1 and ERK. RSK1 activation is therefore regulated by both the mi togen-stimulated ERK/MAP kinase pathway and a PDK1-dependent pathway.