Characterization of human seminal plasma proteins homologous to boar AQN spermadhesins

Spermadhesins, proteins secreted by the boar sexual accessory glands, are believed to play an important role in sperm capacitation and primary contact of sperm and egg. We have previously found human seminal plasma proteins immunobiochemically related to boar AQN and AWN spermadhesins. In this study, we characterized further the AQN spermadhesin-related proteins, here designated as hSA (human spermadhesin-like) proteins. On Western blot, we immunodetected 14, 16 and 18 kDa forms of hSA proteins (hSA-14, hSA-16 and hSA-18, respectively) cross-reacting with rabbit antibody against AQN spermadhesins. Each relative molecular-mass form of hSA comprised three isoelectric isoforms (6.0, 6.8 and 8.4) as shown by 2D-PAGE. Glycoprotein analysis revealed that all hSA-16 and hSA-18 isoforms were N-glycosylated, and those of hSA-14 were non-glycosylated. Two isoforms of hSA-14 (pI 6.0 and 8.4) had affinity to heparin. Size-exclusion chromatography of human seminal plasma indicated that hSA proteins formed high molecular-mass complexes either with other hSA proteins or with seminal plasma lactoferrin and/or its fragments. Similarity of biochemical properties (relative molecular masses, isoelectric points and existence of non- and N-glycosylated forms) of hSA proteins and those of boar AQN spermadhesins, together with a previously described N-terminal amino acid sequence of one hSA protein identical to AQN spermadhesins, imply that hSA proteins are structurally related to boar AQN spermadhesins. However, localization of hSA proteins on the sperm tail and neck suggests that their biological role differs from that of boar AQN spermadhesins located on the sperm head.