Site-specific phosphorylation of a synthetic peptide derived from ribosomal protein S6 by human placenta protein kinases.

A synthetic peptide S6-21 (AKRRRLSSLRASTSKSESSQK) which contains the phosphorylated residues in the ribosomal protein S6 has been used as a substrate for two partially purified human placenta protein kinases. Two distinct classes of protein kinases which catalyze either amino terminal (AKRRRLSS) or carboxyl terminal (LRASTSKSESSQK) peptide phosphorylation were identified. Multiple sites were phosphorylated in each domain. A single protein kinase which catalyzed phosphorylation of sites in both domains was identified. Although growth factors are known to promote phosphorylation of S6 at five serine sites, no enzyme which could modify S6-21 to that extent was observed.