Identification of betacellulin as a major peptide growth factor in milk: purification, characterization and molecular cloning of bovine betacellulin

Betacellulin (BTC), a member of the epidermal growth factor (EGF) family of peptide growth factors, was purified from a growth-factor-enriched whey fraction of bovine milk by a combination of ion-exchange chromatography, gel-filtration chromatography, affinity chromatography and reverse-phase HPLC. Bovine ETC (bBTC) had an apparent molecular mass of 21-22 kDa on SDS/PAGE and exists in a glycosylated form. The cDNA encoding bBTC was obtained by a combination of 5' and 3' rapid amplification of cDNA ends ('RACE'). The primary translation product consists of 178 amino acid residues containing a putative signal sequence, a transmembrane domain, the mature ETC domain and a cytoplasmic domain containing a highly hydrophilic Arg-Lys-rich region similar to that of mouse ETC and human ETC. The amino acid sequence of the bBTC precursor was 88% identical with human ETC and 79% identical with mouse ETC. The bBTC gene was found to be expressed in a wide range of tissues, including the mammary gland. The identification of ETC in milk raises the possibility that it has a major role in the growth and development of the neonatal gastrointestinal tract.