GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PROTEINS IN HUMAN AND PIGS MILK

Glycosylphosphatidylinositols (GPIs) are a recently discovered class o f glycoconjugates that anchor either proteins, polysaccharides or smal l oligosaccharides to cellular membranes via a covalent linkage. To in vestigate the presence of soluble GPIs, individual human milk samples and mature pig's milk were defatted and casein removed by acid precipi tation and ultracentrifugation. Soluble proteins were subjected to FPL C gel filtration (Superdex 200) and high molecular weight proteins wer e separated into fractions I-V. Immunological studies have been perfor med using Western blotting of whole milk, casein and whey fractions, a nd Superdex fractions I-V followed by incubation with a monoclonal ant ibody against the purified GPI anchor of the variant surface glycoprot ein 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 gr eater than or equal to 200 (at least 3 components), 90 and 80 kD. In p ig's milk, the staining pattern was found to be different from human m ilk. Similar to other GPI-anchored cell structures the functions of GP I-containing proteins in human milk and pig's milk as well as the spec ific components carrying these anchors remain to be investigated.