Activation and inhibition of the C-terminal kinase domain of p90 ribosomal S6 kinases

The p90 ribosomal S6 kinases (RSKs) contain two distinct catalytic kinase domains, the N-terminal and C-terminal kinase domains (NTKD and CTKD, respectively). The activation of CTKD is regulated by phosphorylation by extracellular signal-regulated kinase (ERK1/2) and an autoinhibitory αL helix. Through a mutational seriesin vitroof the RSK CTKDs, we found a complex mechanism lifting autoinhibition that led us to design constitutively active RSK CTKDs. These are based on a phosphomimetic mutation and a C-terminal truncation (e.g. RSK2 T577E D694*) where a high activity in absence of ERK phosphorylation is obtained. Using these constructs, we characterize IC50values of ATP-competitive inhibitors and provide a setup for determining specificity constants (kinact/Ki) of covalent CTKD inhibitors.