The GPI-anchored CD52 antigen of the sperm surface interacts with semenogelin and participates in clot formation and liquefaction of human semen.

CD52 is a human glycosylphosphatidylinositol (GPI)-anchored antigen exclusively expressed in leukocytes and epididymal cells. It is also present in sperm, being inserted in their plasma membrane as they pass through the epididymis. In a previous paper we identified a new CD52 form without GPI anchor by fast performance liquid chromatography (FPLC) fractionation of semen components. The form has a lower negative charge than the GPI-anchored form and occurs as the only CD52 form in prostasome-free seminal plasma. It was also found associated with the ejaculated sperm, but in contrast to the GPI-anchored one, it is lost during the capacitation process. In this paper we indicate that (1) the GPI-anchored CD52 of the sperm surface serves as receptor for semenogelin I during clot formation, (2) liquefaction involves cleavage of the GPI anchor from certain CD52 molecules, releasing sperm from the clot and the soluble antigen bound to semenogelin fragments into the seminal plasma and (3) the clot is a sponge-like structure housing sperm. Soluble CD52 was immunopurified from the soluble CD52-containing FPLC fraction using CAMPATH-1G and was found to be complexed with a semenogelin-derived peptide of the carboxyl terminal portion of semenogelin I, having the sequence SQTEKLVAGKQI and starting from amino acid 376. Immunoprecipitation and immunoblot analyses using CAMPATH-1G and anti-semenogelin as immunoprecipitating antibodies and anti-gp20 and anti-semenogelin as immunoblot detectors of the corresponding antigens, confirmed that the soluble CD52 formed a complex with semenogelin. The semenogelin-CD52 soluble form was found to be a direct consequence of the liquefaction process since only the GPI-anchored CD52 was recovered in uniquefied semen after recovering sperm and seminal plasma by urea solubilization of the clot.