Characterization of two glycosylated boar spermadhesins

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Abstract
Boar spermadhesins AQN‐1, AQN‐3 and AWN form a recently described protein family, synthesized by the sexual accessory glands, and become associated with the sperm head upon ejaculation. They contain 109–133 amino acid residues, two conserved disulphide bridges, are not glycosylated, and have 40–60% primary structure identity. These boar polypeptides are multifunctional proteins, which possess heparin‐, serine‐protease‐inhibitor‐ and/or zona‐pellucida‐glycoprotein‐binding capability and have, therefore, been implicated in sperm capacitation and sperm‐oocyte attachment. AQN‐2 (18–20 kDa), however, is unique among boar spermadhesins in that it is the only member of the family which is known to be glycosylated and which possesses weak zona‐pellucida‐binding but not seminal‐plasma‐inhibitor‐binding ability. In this study we report the structural and functional characterization of the two glycoproteins contained in the AQN‐2 fraction. One component is identical with PSP‐I, a major porcine seminal plasma protein whose function has not yet been identified, while the second protein is a glycosylated isoform of AQN‐3. Here we show that the inability of the glycosylated boar spermadhesins to bind seminal‐plasma protease inhibitors as well as the weak binding of glycosylated AQN‐3 to zona pellucida glycoproteins is due to the presence of the oligosacharide chain on a conserved asparagine residue. This indicates that modification of a spermadhesin polypeptide framework may serve to modulate its ligand‐binding capabilities.