Casein phosphatase dependent dephosphorylation of 32P-casein fractions labelled by two protein kinases.

The dephosphorylation by a rat liver "Casein phosphatase" of 32P-casein preparations labelled by two protein kinases displaying different specificities for casein residues and subfractions has been studied. Although the Pi released by the phosphatase accounts for no more than 20% of the total casein bound P, it includes pratically all the 32P incorporated into thr-32P and ser-32P residues by casein kinases TS and S respectively. Such radioactive residues preferentially hydrolyzed by the phosphatase are located in two different minor casein fractions distinct from the main αs1, β and k-caseins. On the contrary the ser-32P residues labelled by casein kinase TS and largely accounted for by αS1-casein, are unaffected by the phosphatase. These results suggest that the minor casein fractions preferentially dephosphorylated by casein phosphatase share structural similarities with the still unknown endogenous substrate (s) of this enzyme. © 1977.