Butyrophilin is expressed in mammary epithelial cells from a single-sized messenger RNA as a type I membrane glycoprotein

We investigated the expression of butyrophilin in eukaryotic cells with a view to determining the number of mRNA species, the incorporation of the peptide chain into microsomes, and the topology of the processed protein in biological membranes. Butyrophilin is synthesized from a single sized mRNA in both bovine and murine lactating mammary tissue and associates with microsomal membranes with a type I topology (Nexo.Ccyto) via a single hydrophobic anchor in the middle of the sequence. Several isoelectric variants of the protein were detected in cellular membranes from lactating bovine mammary tissue and in the milk-fat-globule membrane. We found no evidence for soluble forms of butyrophilin in postmicrosomal supernatants. The 66-kDa protein appears to be subjected to limited proteolysis, giving rise to a 62-kDa fragment lacking the C terminus and to other more minor fragments of lower Mr in the milk-fat-globule membrane. Antipeptide antibodies to epitopes within the N- and C-terminal domains were used to show that butyrophilin retains a type I topology in plasma membranes when expressed in insect cells from a baculovirus vector, and in secreted milk-fat globules. These data do not agree with previous suggestions that butyrophilin may exist in cytoplasmic soluble forms, or be reorganized in the plane of the lipid bilayer during secretion in lipid droplets from mammary cells. The results are discussed with reference to the role butyrophilin may play as the principal scaffold for the assembly of a complex with xanthine oxidase and other proteins that functions in the budding and release of milk-fat globules from the apical surface during lactation.

The major fat-globule membrane proteins, bovine components 15/16 and guinea-pig GP 55, are homologous to MGF-E8, a murine glycoprotein containing epidermal growth factor-like and factor V/VIII-like sequences

The glycoproteins, Components 15/16 and GP 55, were partially purified from bovine and guinea-pig fat-globule membrane (FGM), respectively. Peptides prepared from immunoblots of Component 16 and GP 55, were shown by sequencing, to be similar to mouse MFG-E8 and a human milk protein of M(r) 46,000. Both of these latter proteins are associated with the FGM and contain epidermal growth factor-like domains and/or factor V/VIII-like sequences. These data complement earlier biochemical studies on the biosynthesis, tissue distribution and topology of Components 15/16 and GP 55, and indicate that MFG-E8-like proteins are expressed on the apical surfaces of mammary epithelial cells and are secreted both in a soluble form and as major components of the FGM in diverse species. The possible function of these proteins in lactation and in mammary and neonatal development is discussed.

Affinity purification of polyclonal antibodies from antigen immobilized in situ in sodium dodecyl sulfate-polyacrylamide gels

A procedure for the preparation of affinity-purified antibody is described. Protein mixtures are separated by electrophoresis in sodium dodecyl sulfate (SDS)--polyacrylamide gels. Individual bands of protein are cut from the gel and fixed in situ with glutaraldehyde. The gel pieces are then homogenized and washed extensively with buffered solutions and chaotropic agents. The washed gels can then be used as immunoadsorbents to purify antibodies from crude antisera. This method should be especially useful for the preparation of small amounts of antibody to proteins that are difficult to purify by conventional means, that are available only in limited quantity, or that cannot be blotted to immunoadsorbents such as nitrocellulose or diazotized paper.