Comparative studies on phosphorylation of synthetic peptide analogue of ribosomal protein S6 and 40‐S ribosomal subunits between Ca2 +/phospholipid‐dependent protein kinase and its protease‐activated form

Ca2+/phospholipid-dependent protein kinase (protein kinase C) and trypsin-activated protein kinase C (protein kinase M) phosphorylated the synthetic peptide R1-A13 (Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala-Ser-Thr-Ser-Lys-Ala) which contains both cAMP- and insulin-regulated phosphorylation sites in rat liver ribosomal protein S6 [Wettenhall, R. E. H. & Morgan, F. J. (1984) J. Biol. Chem. 259, 2084-2091]. Both enzymes showed essentially the same kinetic properties; V and apparent Km were determined to be 0.16 mumol min-1 mg-1 and 30 microM, respectively. At first, tryptic phosphopeptides were prepared at the early stage of phosphorylation and purified by high-performance liquid chromatography (HPLC). Through these analyses, four radioactive peptides were isolated. When protein kinase C was employed, phosphorylation was observed on all four peptides in a Ca2+/phospholipid-dependent manner. Irrespective of the protein kinase employed, phosphate incorporation into these peptides increased linearly with time; the peptide concentration did not affect the ratio of phosphate distribution into these four peptides. Analysis of amino acid composition and phosphoamino acid of radioactive peptides obtained after extensive phosphorylation showed that phosphates were incorporated into Ser-4, Ser-5, Ser-9 and Ser-11. The latter three serine residues were major phosphorylated sites. When rat liver 40-S ribosomal subunits were employed as substrate for protein kinases C and M, a radioactive protein with Mr,app = 31,000, which corresponded to S6 protein, was detected on an autoradiogram of a sodium dodecyl sulfate/polyacrylamide slab gel. The rate of phosphorylation with protein kinase M was twice as fast as that with protein kinase C. The elution profile of radioactive tryptic peptides in HPLC suggest that phosphorylation occurred on the sites in S6 protein corresponding to Ser-5, Ser-9 and Ser-11 as major sites and Ser-4 as the minor one. These results indicate that protein kinase C has an ability to recognize at least four sites derived from hormone-dependent phosphorylation sites in ribosomal protein S6 irrespective of the mode of activation of this enzyme.