The δ Isoform of Protein Phosphatase Type 1 Is Localized in Nucleolus and Dephosphorylates Nucleolar Phosphoproteins

The immunolocalization and substrates of protein phosphatases present in nucleolus were investigated using Swiss 3T3 cells and Novikoff hepatoma ascites cells. The protein phosphatase activity was detected in the extract of the isolated nucleoli and its activity was inhibited by okadaic acid with IC50 value of 160 nM. Immunoblotting assay indicated that PP1cδ but not PP1cα, PP1cγ1, and PP2Ac was localized in the isolated nucleoli. Confocal microscopy showed that PP1cδ was localized in nucleoli, nuclei, and cytosol, though the intensity of fluorescence at the nucleoli was stronger than that of the cytosol or nuclei. PP1cδ was co-localized with the major nucleolar phosphoprotein B23 at nucleoli. The phosphatase was capable of dephosphorylating several proteins in the nucleolus, including B23. The Km of PP1 for the recombinant B23.1, phosphorylated by endogenous kinase(s), was 3.5 μM. These results indicate that PP1cδ is the major serine/threonine phosphatase present in nucleolus and it dephosphorylates nucleolar phosphoproteins, including B23.