Phosphorylation of ribosomal protein S6 is dependent on cyclic AMP in Dictyostelium discoideum

Extracts of aggregation-competent cells of Dictyostelium discoideum have an S6 protein kinase activity which is inhibited in the presence of the inhibitor of the cAMP-dependent protein kinase. The phosphorylation of S6 is rapid, and decays rapidly. The S6 kinase activity is detectable in the 150,000g supernatant only in the presence of phosphatase inhibitors known for preserving the S6 kinase in other systems, indicating that the activated form of the enzyme is phosphorylated by the cAMP-dependent protein kinase. S6 kinase elutes as a peak from DEAE-Sephacel at 100 mM NaC1, with an activity that is cAMP-dependent.