Phosphorylation of K-casein by glycogen synthase kinase-3 from rabbit skeletal muscle

Glycogen synthase kinase-3 (ATP: protein phosphotransferase, EC 2.7.1.37) phosphorylated K-casein 20-fold more rapidly than β-casein, while α_S1-casein was not a substrate. This distinguished it from casein kinase-I and casein kinase-II, which phosphorylate the β-casein variant preferentially. Glycogen synthase kinase-3 phosphorylated a serine residue(s) in the C-terminal cyanogen bromide fragment on K-casein. In contrast, cyclic AMP-dependent protein kinase phosporylated the N-terminal fragment, and phosphorylase kinase the N-terminal and intermediate cyanogen bromide fragments. The results emphasize the potential value of casein phosphorylation as a means of classifying protein kinases.