The phosphorylation of eukaryotic ribosomal protein S6 by protein kinase C (original) (raw)

Purified Ca2 +-dependent and phospholipid-dependent protein kinase (protein kinase C) from bovine brain catalysed the phosphorylation of ribosomal protein S 6 when incubated with 40 S ribosomal subunits from rat liver or from hamster fibroblasts. The phosphorylation was dependent on Ca2 + and phospholipid, and occurred under ionic conditions similar to those which support protein biosynthesis in vitro. Protein kinase C phosphorylated at least three sites on ribosomal protein S 6 when incubated with unphosphorylated ribosomes, and increased the extent of phosphorylation of ribosomes previously phosphorylated predominantly on two sites by cyclic-AMP-dependent protein kinase, converting some molecules to the tetraphosphorylated or pentaphosphorylated form. This indicates that protein kinase C can phosphorylate sites on ribosomal protein S6 other than those phosphorylated by the cyclic-AMP-dependent protein kinase, and this conclusion was confirmed by analysis of tryptic phosphopeptides. These results strengthen the possibility that protein kinase C might be involved in catalysing the multisite phosphorylation of ribosomal protein S 6 in certain circumstances in vivo.