The phosphorylation of eukaryotic ribosomal protein S6 by protein kinase C

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Abstract
Purified Ca2+-dependent and phospholipid-dependent protein kinase (protein kinase C) from bovine brain catalyzed the phosphorylation of ribosomal protein S6 when incubated with 40S 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 3 sites on ribosomal protein S6 when incubated with unphosphorylated ribosomes, and increased the extent of phosphorylation of ribosomes previously phosphorylated predominantly on 2 sites by cAMP-dependent protein kinase, converting some molecules to the tetraphosphorylated or pentaphosphorylated form. Protein kinase C can phosphorylate sites on ribosomal protein S6 other than those phosphorylated by the cAMP-dependent protein kinase, and this conclusion was confirmed by analysis of tryptic phosphopeptides. Protein kinase C may be involved in catalyzing the multisite phosphorylation of ribosomal protein S6 in certain circumstances in vivo.