Activation of S6K1 (p70 ribosomal protein S6 kinase 1) requires an initial calcium-dependent priming event involving formation of a high-molecular-mass signalling complex

The mitogen-stimulated protein kinase p70 ribosomal protein S6 kinase 1 (S6K1) is a key enzyme in the regulation of cell growth and proliferation. Activation of S6K1 requires a complex, ordered series of conformational changes and phosphorylation reactions. While the role of sequential, multi-site phosphorylation has been extensively detailed, characterization of the priming step required to initiate this cascade has remained elusive. In the present study we show for the first time that this priming process is dependent on calcium. Calcium-dependent regulation of S6K1 did not specifically target Thr-229 and Thr-389, the key regulatory phosphorylation sites; rather, calcium chelation resulted in a global inhibition of S6K1 phosphorylation. Mutation of individual phosphorylation sites in the auto-inhibitory and hydrophobic domains to acidic residues (to mimic phosphorylation) yields a kinase that remains sensitive to calcium chelation, while the combined mutations alleviate the requirement for calcium. Furthermore, deletion of the C-terminal residues (398502) also renders the kinase insensitive to calcium. We hypothesize that the initial calcium-dependent process is required to release an inhibitory interaction between the C- and N-termini of S6K1, thus allowing phosphorylation of these key domains. The requirement for this priming step can only be overcome by mutations mimicking the phosphorylation of both the autoinhibitory and hydrophobic domains. We further propose that the priming event involves formation of a calcium-dependent protein complex that releases the interaction between the N- and C-termini. S6K1 is then accessible for activation by the kinases that target the known regulatory phosphorylation sites. Consistent with this hypothesis, serum stimulation of S6K1 activity is associated with its incorporation into a calcium-dependent high-molecular-mass complex.