Proteomics reveal a link between the endoplasmic reticulum and lipid secretory mechanisms in mammary epithelial cells

The synthesis and secretion of lipids by mammary epithelial cells is a high ly ordered process that involves several distinct steps. Triacylglycerols a re synthesized in the endoplasmic reticulum and incorporated into microlipi d droplets which coalesce into cytoplasmic lipid droplets. These are vector ially transported to the apical plasma membrane where they are secreted int o the milk surrounded by a membrane bilayer. The origin of this membrane as well as the mechanism by which cytoplasmic lipid droplets form and become surrounded by membrane is poorly understood. Proteomic analysis of the prot ein composition of milk fat globules and cytoplasmic lipid droplet has reve aled that the endoplasmic reticulum is not only involved in the synthesis o f the lipid but also potentially contributes to the membrane component of m ilk fat globules. The proteins identified suggest possible mechanisms of mu ltiple steps during this process. Completion of the proteome of milk fat gl obule membranes and cytoplasmic lipid droplets will provide the necessary r eporter molecules to follow and dissect the mechanisms of the sorting and u ltimate secretion of cytoplasmic lipid droplets.