Milk Lipids | Milk Fat Globule Membrane

Abstract

In this article, the cellular formation and composition of the milk fat globule membrane (MFGM) are discussed. Milk lipid droplets acquire the MFGM during their assembly and secretion from mammary epithelial cells during lactation. Elements of the MFGM may be derived from the endoplasmic reticulum, other intracellular organelles, the cytoplasm, and apical plasma membrane. The secreted membrane has a tripartite structure consisting of an inner layer of phospholipids and proteins contiguous with the triacylglycerol core, an interstitial protein coat, and an outer membrane layer with an associated glycocalyx. Major lipid components of the MFGM include triacylglycerols, phospholipids, glycosphingolipids, sterols, and sterol esters. Well over a hundred proteins have been identified in bovine MFGM, including eight that are relatively abundant, namely, the mucins MUC-1 and MUC-15, the redox enzyme xanthine oxidoreductase, CD36, butyrophilin 1A1, PAS 6/7 (lactadherin), adipophilin, and fatty acid-binding protein. Many of the minor proteins function in membrane trafficking in cells, including a large cohort of small-molecular-weight GTP-binding proteins and other proteins that function in vesicle targeting and fusion, including cytoskeletal elements, SNAREs, and annexins. A large number of cytoplasmic proteins are present in minor amounts, most of which were probably entrained with the globules in cytoplasmic inclusions (‘crescents’) during the secretion process.