Milk lipid globule precursor release from endoplasmic reticulum reconstituted in a cell-free system

Lipid droplet precursors of milk lipid globules are believed to be derived from elements of endoplasmic reticulum in milk-secreting mammary epithelial cells. Endoplasmic reticulum isolated from mammary gland was able to generate small droplets morphologically resembling microlipid droplet precursors of milk lipid globules. Droplet generation was time and temperature dependent and required a cytosol fraction of Mr greater than 10,000. Droplet generation was enhanced by, but did not require, addition of nucleoside triphosphates, fatty acids, coenzyme A, glycerol-3-phosphate, and dithiothreitol. Microlipid droplets generated in this cell-free system were enriched in triacylglycerols and resembled microlipid droplets formed within mammary epithelial cells in polar lipid and polypeptide composition. Endoplasmic reticulum immobilized onto nitrocellulose retained activity in generation of putative microlipid droplets, and this immobilization method provided a facile means for separation of the donor from the generated products.