Casein Kinase from the Golgi Apparatus of Lactating Mammary Gland

A casein kinase that catalyzes the phosphorylation of dephosphorylated aSI-casein by ATP has been found in the Golgi apparatus of lactating rat mammary gland. Dephosphorylated /3and K-Caseins are also phosphorylated by this enzyme, while other milk proteins (P-lactoglobulin, a-lactalbumin, native CY,~-, fi-, and K-case& and proteins of the fat globule membrane) are phosphorylated to a limited extent. Histones, phosvitin, and lysozyme are not appreciably phosphorylated. The optimum pH for phosphate incorporation into dephosphorylated casein is 7.6. The K, for dephosphorylated a,,-casein is 12 pM (27 mg per ml), whereas the K,,, for ATP is 80 PM. The casein kinase requires a divalent cation for maximum activity; both Ca2+ and Mg2f can satisfy this requirement. Adenosine 3’: 5’.monophosphate (cyclic AMP) has no effect on casein kinase activity. The protein inhibitor from rabbit skeletal muscle, which inhibits adenosine 3’:5’-monophosphate-dependent protein kinases, is also without effect. These findings suggest that phosphorylation of casein, a food protein, is quite different from the phosphorylation of cellular enzymes, which requires cyclic AMP, is inhibited by Ca2+ and is involved in control mechanisms.