Identification of a major secretory glycoprotein from rat epididymis: interaction with spermatozoa

A polypeptide with molecular mass of 17 kDa has been partially purified and identified as a major secretory glycoprotein in the rat epididymis. It is phosphorylated and contains high mannose-type oligosaccharides with 5 and 6 mannose units predominantly. These sugar residues are sufficiently exposed in the molecule to be released by endo-beta-N-acetylglucosaminidase H without prior denaturation or protease digestion. Specific binding of the glycoprotein to testicular spermatozoa was demonstrated with Ka 0.2 x 10(9) M-1 and 17,200 sites per cell, while no binding to epididymal spermatozoa was detectable. Direct labeling of surface proteins on cauda epididymis spermatozoa revealed the presence of a major band of 16.2 kDa, which may be equivalent to GP17. The interaction of the epididymal secretory protein with sperm suggests a possible role in the maturation process.