Dephosphorylation of ribosomal protein S6 phosphorylated via the cAMP-mediated signaling pathway in rat parotid gland: Effect of okadaic acid and Zn²⁺

The particulate proteins of 34, 26, and 22 kDa are phosphorylated on serine residues in the rat parotid gland by activation of the cAMP-mediated signaling system. The 34 kDa protein was identified as ribosomal protein S6 by immunoprecipitation with anti S6 peptide antibody. The dephosphorylation of S6 was observed by incubation of the particulate fraction of the saponin-permeabilized cells labeled with [γ-³²P]ATP in the presence of cAMP/3-isobutyl-1-methylxanthine. The dephosphorylation of S6 was inhibited by either okadaic acid, a potent inhibitor of protein phosphatase, or Zn²⁺, however, neither Ca²⁺ nor Mg²⁺ showed significant effect. S6 phosphatase activities detected by using the ³²P-labeled S6 peptide as a substrate were inhibited by both okadaic acid and Zn²⁺. These results suggest that the dephosphorylation of S6 is mediated by the okadaic acid and Zn²⁺-sensitive phosphatases in the rat parotid gland.