Structural and functional aspects of three major glycoproteins of the human milk fat globule membrane

The MUC1 mucin, lactadherin, and butyrophilin are 3 major components of the human milk fat globule membrane. The mucin inhibits binding of S-fimbriated Escherichia coli to buccal epithelial cells, and lactadherin prevents symptomatic rotavirus infection in breast-fed infants. Butyrophilin has been suggested to be a structural component of the human milk fat globule (HMFG) membrane and to have receptor functions, but has no known anti-infective activity. These HMFG glycoproteins also are present in skimmed milk, possibly associated with phospholipid micelles, while mucin is also in a soluble form. Mucin and lactadherin resist digestion in the stomach of milk-fed infants, while butyrophilin is rapidly degraded. The MUC1 mucin is an extended rod-like structure forming part of the glycocalyx on the surface of many epithelial cells and membranes of milk, and may act as a decoy for binding of infective agents. The extracellular segment of butyrophilin has homology to Ig superfamily receptors and an intracellular domain with homology to developmentally regulated proteins. Lactadherin is a laterally mobile cell adhesion molecule that interacts with integrins and has a novel means of membrane-association involving specific binding to phosphatidylserine. The structural and functional aspects of these glycoproteins are discussed with regard to their role in human milk for breast-fed infants.