Biochemical and immunological comparison of bovine butyrophilin with a butyrophilin-like glycoprotein in guinea pig milk-fat-globule membrane

Abstract

A glycoprotein of apparent Mr 63,000 in the fat globule membrane (FGM) of guinea pig milk has been purified, characterized and compared with bovine butyrophilin. The two proteins resist extraction from FGM with non-ionic detergents, they share common epitopes and have similar but not identical amino acid compositions and N-terminal sequences. Digestion of the proteins with specific proteinases leads to the release of some similar peptides. The guinea pig protein therefore appears to be closely related to bovine butyrophilin. The distribution of the guinea pig protein was determined in tissues and milk by a combination of immunoblotting techniques and immunofluorescence microscopy. The guinea pig protein was detected in lactating mammary tissue and accounted for approximately 0.3% of tissue proteins. No evidence for the expression of this protein was obtained in mammary tissue from virgin or midpregnant animals, or in kidney, uterus, spleen, heart, pancreas, intestine, liver, lung, brain, ovary, sublingual salivary gland, skin, anal apocrine sweat glands or blood cells. The 63,000 Da guinea pig protein is therefore a member of a family of butyrophilin-like proteins which are specifically expressed in mammary epithelial cells during lactation. The results are discussed with reference to the possible role of butyrophilin in milk secretion, and the use of butyrophilin as a tissue-specific marker of differentiation in the mammary gland.