Cloning and analysis of cDNA encoding bovine butyrophilin, an apical glycoprotein expressed in mammary tissue and secreted in association with the milk-fat globule membrane during lactation

Butyrophilin is a glycoprotein expressed on the apical surfaces of secretory cells in lactating mammary tissue, which may function in the secretion of milk-fat droplets (Franke, W. W., Heid, H. W., Grund, C., Winter, S., Freudenstein, C., Schmid, E., Jarasch, E.-D., and Keenan, T. W. (1981) J. Cell Biol. 89, 485-494). A cDNA clone encoding bovine butyrophilin was isolated and the primary structure of the protein deduced from the DNA sequence. Bovine butyrophilin contains 526 amino acids with a putative signal peptide of 26 amino acids. Hydropathy analysis predicts the existence of a single membrane-spanning region with the amino terminus facing the exoplasmic space. Butyrophilin cDNA hybridized to mRNA of 2.9 kilobases in bovine mammary tissue taken from pregnant or lactating animals, but specific mRNA was not detected in a 2-year-old virgin cow. The amount of message was maximal in lactating tissue. Butyrophilin mRNA could not be detected in bovine heart, intestine, kidney, liver, ovary, or uterus. A search of the GenBank and EMBL data banks showed closest homology was between the C termini of butyrophilin and "ret finger protein" (Takahashi, M., Inaguma, Y., Hiai, H., and Hirose, F. (1988) Mol. Cell. Biol. 8, 1853-1856). The ret-finger protein gene is expressed in a variety of tumor cell lines, mouse testis and embryonic tissue, which like the mammary gland undergo periods of rapid cell division and development. The possible significance of this homology and the possible function of butyrophilin in milk-lipid secretion are discussed.