Purification and characterization of protein phosphatase 2C in rat parotid acinar cells: Two forms of Mg²⁺-activated histone phosphatase and phosphorylation by cAMP-dependent protein kinase

Two forms of Mg²⁺-activated histone phosphatase activities were partially purified from rat parotid acinar cells using Mono Q and gel filtration chromatography. Both enzymes activities were dependent on the presence of Mg²⁺, showing little activity in the presence of EDTA. The activities fractionated on the Mono Q column into two peaks: the first was a minor peak of histone phosphatase activity; the second was a major peak. These two peaks eluted at distinct positions on the gel filtration column. The molecular masses of the two peak fractions corresponded to 46 and 55 kDa, respectively on SDS-gels. The first 46-kDa peak immunoreacted with anti- PP2Cα phosphatase antibody and like PP2Cα phosphatase could be phosphorylated by cAMP-dependent protein kinase. The second 55-kDa peak showed neither reactivity with anti-PP2Cα phosphatase antibody nor phosphorylability by cAMP-dependent protein kinase, but retained a Mg²⁺ or Mn²⁺ dependence for its histone phosphatase activity. Ca²⁺ showed a strong inhibition on this activity. On the basis of these observations, we have identified the first peak enzyme as PP2Cα phosphatase and the second peak as a novel PP2C-like phosphatase.