Differential effect of brefeldin A on phosphorylation of the caseins in lactating mouse mammary epithelial cells

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Abstract
The major milk proteins, the caseins, contain multiple phosphorylation sites. Phosphorylation of the caseins is necessary to allow Ca2+ binding and aggregation of the caseins to form micelles. We have followed the phosphorylation of the caseins in isolated acini from lactating mouse mammary gland. Incubation of mammary cells with [32P]orthophosphate revealed that phosphorylation of newly synthesised caseins was complete within 20 minutes of synthesis. Extensive secretion of-, - and -caseins occurred over a 2 hour period. Activation of the regulated secretory pathway using ionomycin over the last hour resulted in a preferential increase in secretion of - and -caseins. Brefeldin A (BFA) inhibited protein secretion and synthesis in mam-mary cells in prolonged incubations. An examination of short-term treatments with BFA on 32P incorporation into the caseins revealed a differential effect of BFA in which the drug inhibited phosphorylation of - and -but not -caseins. These results suggest that phospho-rylation of -casein normally occurs in Golgi cisternae whereas that of - and -caseins occurs in the trans-Golgi network. Phosphorylation of specific secretory proteins may, therefore, occur in different Golgi com-partments.