Phosphorylation of the phosphatase modulator subunit (inhibitor‐2) by casein kinase‐1 Identification of the phosphorylation sites

Abstract
The isolated modulator subunit of the inactive protein phosphatase‐1 is phosphorylated in vitro by casein kinase‐1 at two different site Ser‐86 and Ser‐174. The Ser‐86 site is a common target for casein kinase‐1 and casein kinase‐2, but is preferentially phosphorylated by the former enzyme. The Ser‐174 site seems to be specific for casein kinase‐1, and is phosphorylated at a slower rate. These results give a new insight into the in vitro phosphorylation pattern of the modulator subunit of the phosphatase and provides additional data on the specificity of casein kinase‐1.