Identification of MYPT1 and NIPP1 as subunits of protein phosphatase 1 in rat liver cytosol

Various studies have provided evidence for the existence of spontaneously a ctive cytosolic species of protein phosphatase 1, but these enzymes have ne ver been purified and characterized. We have used chromatography on microcy stin-Sepharose and Resource Q to purify cytosolic protein phosphatases from rat liver. Two of the isolated enzymes were identified by Western analysis and peptide sequencing as complexes of the catalytic subunit of protein ph osphatase 1 and either the inhibitor NIPP1 or the myosin-binding subunit MY PT1, which reportedly is not present in chicken liver. In contrast, PCR clo ning revealed the expression of two MYPT1 splice variants in rat liver. (C) 1999 Federation of European Biochemical Societies.