DIFFERENTIAL EFFECT OF BREFELDIN-A ON PHOSPHORYLATION OF THE CASEINS IN LACTATING MOUSE MAMMARY EPITHELIAL-CELLS

The major milk proteins, the caseins, contain multiple phosphorylation sites. Phosphorylation of the caseins is necessary to allow Ca2+ bind ing and aggregation of the caseins to form micelles. We have followed the phosphorylation of the caseins in isolated acini from lactating mo use mammary gland. Incubation of mammary cells with [P-32]orthophospha te revealed that phosphorylation of newly synthesised caseins was comp lete within 20 minutes of synthesis. Extensive secretion of alpha-, be ta- and gamma-caseins occurred over a 2 hour period. Activation of the regulated secretory pathway using ionomycin over the last hour result ed in a preferential increase in secretion of alpha- and gamma-caseins . Brefeldin A (BFA) inhibited protein secretion and synthesis in mamma ry cells in prolonged incubations. An examination of short-term treatm ents with BFA on P-32 incorporation into the caseins revealed a differ ential effect of BFA in which the drug inhibited phosphorylation of be ta- and gamma- but not alpha-caseins. These results suggest that phosp horylation of alpha-casein normally occurs in Golgi cisternae whereas that of beta- and gamma-caseins occurs in the trans-Golgi network. Pho sphorylation of specific secretory proteins may, therefore, occur in d ifferent Golgi compartments.