Secretion of milk-specific proteins

Following synthesis in the ribosomes and vectorization into the ER lumen, the polypeptides are transferred to Golgi lumina. The route of transfer from ER to Golgi has not been established with certainty. It is possible that lumina of the ER and Golgi apparatus are connected, or that small vesicles bud from the ER and subsequently fuse with the Golgi membranes. In either case, casein molecules aggregate in the Golgi cisternal lumina in the form of micelles. 

Secretory vesicles seem to become attached to the cytoplasmic face of the apical plasmalemma. The vesicles have a distinctive coat on their outer surface which appears to react with appropriate receptors on the apical membrane, forming a series of regularly spaced bridges. Presumably, these bridges, and the contiguous vesicle and apical membrane material, are subsequently eliminated and the vesicular contents released, but the process seems to be very rapid and it has proved difficult to visualize the details of the sequence by EM. However, secretory vesicle membrane becomes incorporated, however briefly, into the apical membrane as a consequence of exocytosis. 

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