Dairy Chemistry And Biochemistry

MILK PROTEINS 207

4.14.6 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.

Lumina at the nuclear face of the Golgi apparatus (Figure 4.33) are

termed cis cisternae; those at the apical face trans cisternae. Proteins appear

to enter the complex at the cis face and progress, undergoing post-transla-

tional modification, towards the trans face. Transfer between adjacent Golgi

cisternae is thought to be achieved by budding and subsequent fusion of

vesicles.

Milk fat globule

Signet

Figure 4.33 Schematic representation of a mammary secretory cell as interpreted from electron
micrographs; c, cis face of Golgi apparatus; t, trans face (from Mepham, 1987).