Second, the specificity and tenacity with which epididymal glycoproteins bind to the sperm plasma membrane, and ultimately their structure within it, remain problematic. Some glycoproteins are readily dissociated from the sperm surface by high ionic strength solutions (e.g., ASF, HIS-50 ), others are only released after treatment with phospha-tidylinositol-specific phospholipase C indicating the presence of a GPI anchor (e.g., CD52 ), while most require detergents for complete solubilization. Third, it is not known what directs binding of secreted glycoproteins to specific regions of the sperm surface, that is, whether it is a property of the glycoprotein itself or of the region of the plasma membrane involved.
To address these questions we have investigated the properties of secreted epididymal antigens recognized by the monoclonal antibody (McAb) 2D6. In previous work it was shown that during the early stages of fertilization a 24-kDa antigen on cauda spermatozoa recognized by 2D6 McAb is transferred to, and mixes with, components of the oolemma. We have hypothesized that this glycoprotein may be involved in the initial stages of egg activation by forming complexes with egg components to initiate, for example, calcium oscillations in the cytoplasm. Thus, glycoproteins of epididymal origin would have a direct effect on egg activation. The present results, however, show that the 24-kDa antigen on cauda spermatozoa is a member of the p-defensin superfamily of small pore-forming peptides of which several are secreted specifically by the epididymis and have putative antibacterial functions.