Glycosylphosphatidylinositol-anchored proteins in human and pig's milk.

Glycosylphosphatidylinositols (GPIs) are a recently discovered class of glycoconjugates that anchor either proteins, polysaccharides or small oligosaccharides to cellular membranes via a covalent linkage. To investigate the presence of soluble GPIs, individual human milk samples and mature pig's milk were defatted and casein removed by acid precipitation and ultracentrifugation. Soluble proteins were subjected to FPLC gel filtration (Superdex 200) and high molecular weight proteins were separated into fractions I-V. Immunological studies have been performed using Western blotting of whole milk, casein and whey fractions, and Superdex fractions I-V followed by incubation with a monoclonal antibody against the purified GPI anchor of the variant surface glycoprotein from Trypanosoma brucei brucei. We found that significant amounts of GPI-anchored proteins are secreted into human milk and pig's milk. The antibody which reacted only with components in the whey fractions bound to 5 different human milk proteins with a molecular weight of >/=200 (at least 3 components), 90 and 80 kD. In pig's milk, the staining pattern was found to be different from human milk. Similar to other GPI-anchored cell structures the functions of GPI-containing proteins in human milk and pig's milk as well as the specific components carrying these anchors remain to be investigated.