THE DEPHOSPHORYLATION OF 22-KDA PHOSPHOPROTEIN BY TYPE 2B PROTEIN PHOSPHATASE IN RAT PAROTID ACINAR-CELLS

In saponin-permeabilized rat parotid acinar cells, cyclic AMP and 3-isobutyl-1-methylxanthine stimulated the phosphorylation of three particulate proteins with molecular masses of 34, 26 and 22 kDa. The particulate fractions containing 22-kDa phosphoprotein were isolated from the cells labelled with [gamma-P-32]ATP and used to study the dephosphorylation of the 22-kDa phosphoprotein. When the labelled fractions were incubated at 30 degrees C in the presence of 0.3 mM CaCl2 and 10 mu g calmodulin, dephosphorylation of the 22-kDa phosphoprotein was evoked. Further addition of the type 2B phosphatase (Ca2+/calmodulin-stimulated protein phosphatase purified from bovine brain) resulted in a remarkable dephosphorylation of the 22-kDa phosphoprotein. Western immunoblotting showed that type 2B protein phosphatase exists in rat parotid acinar cells. These results suggest that type 2B protein phosphatase in those cells is involved in the dephosphorylation of the 22-kDa phosphoprotein.