Ribosomal S6 kinase signaling and the control of translation

The highly homologous 40S ribosomal protein S6 kinases (S6K1 and S6K2) play a key role in the regulation of cell growth by controlling the biosynthesis of translational components which make up the protein synthetic apparatus, most notably ribosomal proteins. In the case of S6K1, at least eight phosphorylation sites are believed to mediate kinase activation in a hierarchical fashion. Activation is initiated by phosphatidylinositide-3OH kinase (PI3K)-mediated phosphorylation of key residues in the carboxy-terminus of the kinase, allowing phosphorylation of a critical residue residing in the activation loop of the catalytic domain by phosphoinositide-dependent kinase 1 (PDK1). The kinases responsible for phosphorylating the carboxy-terminal sites have yet to be identified. Additionally, S6 kinases are under the control of the PI3K relative, mammalian Target Of Rapamycin (mTOR), which may serve an additional function as a checkpoint for amino acid availability. In this review we set out to discuss the present state of knowledge regarding upstream signaling components which have been implicated in the control of S6K1 activation and the role of the kinase in controlling cell growth through regulating ribosome biogenesis at the translational level.