PURIFICATION AND CHARACTERIZATION OF P99, A NUCLEAR MODULATOR OF PROTEIN PHOSPHATASE-1 ACTIVITY

We have purified a form of protein phosphatase 1 (PP1) from HeLa cell nuclei, in which the phosphatase is complexed to a regulatory subunit termed p99, We report here the cloning and characterisation of the p99 component, p99 mRNA is widely expressed in human tissues and immunofl uorescence analysis with anti-p99 antibodies showed a punctate nucleop lasmic staining with additional accumulations within the nucleolus, Th e C-terminus of p99 contains seven RGG RNA-binding motifs, followed by eleven decapeptide repeats containing six or more of the following co nserved residues (GHRPHEGPGG), and finally a putative zinc finger doma in. Recombinant p99 suppresses the phosphorylase phosphatase activity of PP1 by > 90% and the canonical PP1-binding motif on p99 (residues 3 96-401) is unusual in that the phenylalanine residue is replaced by tr yptophan. (C) 1997 Federation of European Biochemical Societies.