Phosphorylation of the modulator protein of the ATP,Mg‐dependent protein phosphatase by casein kinase TS

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Abstract
The phosphorylation by casein kinase TS (II) of the modulator protein of the ATP,Mg‐dependent phosphatase increases after preineubation with the PCSH1 phosphatase or with the catalytic subunit of the ATP,Mg‐dependent phosphatase. Dephosphorylation by the two phosphatases combined leads to the incorporation of 2 mol phosphate per mol modulator (at Ser residues). Occupancy of the ATP,Mg‐dependent phosphatase phosphorylation site(s) is a negative determinant in the phosphorylation of the modulator by kinase TS. Among the PCS phosphatases PCSh1 shows the highest activity toward the 32P‐Ser residues labeled by kinase TS in untreated or previously dephosphorylated modulator, while the ATP,Mg‐dependent phosphatase is totally ineffective. Protamine stimulates all phosphatase activities, so that the catalytic subunit of the ATP,Mg‐dependent phosphatase becomes almost as effective as the PCSC phosphatase in dephosphorylating the kinase TS sites.