Casein kinase activity in rat mammary gland Golgi vesicles. Phosphorylation of endogenous caseins.

A Golgi vesicle preparation isolated from the mammary tissue of rats in mid-lactation has been shown to contain the caseins of rat milk. These proteins were phosphorylated when the Golgi vesicles were incubated in the presence of [gamma-32P]ATP. Although this phosphorylation occurred when the physical integrity of the vesicles was maintained, it was markedly increased when the membrane structure was disrupted by hypoosmotic conditions or by use of detergents. The kinase responsible has been shown to be responsive to the intravesicular concentration of Ca2+ and to the extravesicular concentration of Mg2+. These results have been interpreted in terms of a model suggesting a transmembrane location for the enzyme with binding sites on the cytosolic membrane face for Mg2+ and possibly also for ATP and on the luminal surface for Ca2+ and the caseins. Others have postulated that the assembly of caseins into micelles occurs in Golgi vesicles and requires both prior phosphorylation of the proteins and the presence of Ca2+. In this investigation we demonstrate that treatments which increase the intravesicular casein phosphorylation also alter the Ca2+ balance within the vesicle lumen. These results are discussed in relation to the ATP-dependent accumulation of Ca2+ by the mammary gland Golgi vesicles.