Phosphorylation of the catalytic subunit of type‐1 protein phosphatase by the v‐abl tyrosine kinase

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Abstract
The catalytic subunit of type‐1 protein phosphatase (PP1) was phosphorylated by the tyrosine kinase v‐abl as follows: (i) cytosolic PP1 was phosphorylated more (0.73 mol/mol) than PP1 obtained from the glycogen particles (0.076 mol/mol), while free catalytic subunit isolated in the active or inactive form from cytosolic PP1 was phosphorylated even less and catalytic subunit complexed with inhibitory was not phosphorylated; (ii) phosphorylation stoichiometry was dependent on the concentration of PP1 and 3 h incubation at 30°C was required for maximal phosphorylation; (iii) phosphorylation was on a tyrosine residue located in the C‐terminal region of PP1 which is lost during proteolysis; (iv) phosphorylation did not affect enzyme activity but allowed conversion from the active to the inactive form upon incubation with inhibitory of a PP1 form that in its dephospho‐form did not convert.