Induction, partial purification and characterization of a hamster fibroblast protein kinase activity that phosphorylates ribosomal protein S6

When BHK cells were grown to confluence and the growth medium replenished, there was a large and rapid increase in the phosphorylation of ribosomal protein S6. In postribosomal extracts of these cells, prepared in the presence of glycerol 2‐phosphate and EGTA, a ribosomal protein S6 kinase was detected. The increase in activity of this protein kinase broadly reflected the increase in phosphorylation of ribosomal protein S6 observed in vivo . This ribosomal protein S6 kinase activity was substantially purified by a combination of phosphocellulose, DEAE‐cellulose, Mono Q and heparin‐Sepharose chromatography, and some of its characteristics were examined. When the products of phosphorylation of 40S ribosomal subunits by purified enzyme in vitro were analysed using two‐dimensional gel electrophoresis, monophosphorylated and diphosphorylated forms of ribosomal protein S6 were observed to be the predominant radioactively labelled species.