Dairy Chemistry And Biochemistry

(Steven Felgate) #1
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).

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