αS1-casein, which is essential for efficient ER-to-Golgi casein transport, is also present in a tightly membrane-associated form

These experiments reveal for the first time the existence of a membrane-associated form of alphaS1-casein in the endoplasmic reticulum and in more distal compartments of the secretory pathway of mammary epithelial cells. Our data suggest that alphaS1-casein, which is required for efficient export of the other caseins from the endoplasmic reticulum, plays a key role in early steps of casein micelle biogenesis and casein transport in the secretory pathway.