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

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.