RSK

The p90 ribosomal protein S6 kinases RSK1 through RSK4 (RPS6KA1-6, MAPKAP-K1) are terminal effectors of the ERK1/2 MAPK cascade and possess a unique tandem-kinase architecture. The C-terminal kinase domain is phosphorylated by ERK, then intramolecularly activates the N-terminal kinase domain, while PDK1 also phosphorylates the NTKD activation loop, integrating growth-factor and MAPK inputs. Active RSKs phosphorylate CREB, c-Fos, ribosomal protein S6, BAD, and p27Kip1, driving immediate-early gene induction, protein synthesis, and survival. RSK2 loss-of-function mutations cause Coffin-Lowry syndrome, an X-linked intellectual disability with skeletal defects, and RSK activity contributes to proliferation and therapy resistance in cancers downstream of oncogenic RAS and FGFR signaling, making the RSK branch an active oncology target.