PRIMARY STRUCTURE OF EPV20, A SECRETORY GLYCOPROTEIN CONTAINING A PREVIOUSLY UNCHARACTERIZED TYPE OF DOMAIN

A 20-kDa glycoprotein, EPV20, was isolated from bovine milk and charac terized. The primary structure was determined by cDNA and protein sequ encing combined with mass spectrometry. EPV20 is a 130-residue polypep tide synthesized with a 19-residue signal peptide. The function of EPV 20 is unknown, but it displays 79% sequence similarity to a putative p rotein deduced from a human testis cDNA sequence designated HE1 (human epididymis clone 1) (Kirchhoff, C., 1992. EMBL/GeneBank/DDBJ Database s, accession number X67698). Northern blot analysis showed the bovine EPV20 to be expressed in kidney, spleen, liver and mammary gland, but remarkably not in bovine testis. The six Cys residues of EPV20 were fo und to be disulfide-linked in a 1-6, 2-3 and 4-5 pattern. This disulfi de arrangement has been observed in other proteins, e.g. in human pros tatic acid phosphatase, but the spacing between the cystines differs. Therefore, EPV20 represents a new structure among the large group of p roteins containing domains with three disulfide bonds.