CHARACTERIZATION OF 2 GLYCOSYLATED BOAR SPERMADHESINS

Boar spermadhesins AQN-1, AQN-3 and AWN form a recently described prot ein family, synthesized by the sexual accessory glands, and become ass ociated with the sperm head upon ejaculation. They contain 109-133 ami no acid residues, two conserved disulphide bridges, are not glycosylat ed, and have 40-60% primary structure identity. These boar polypeptide s are multifunctional proteins, which possess heparin-, serine-proteas e-inhibitor- and/or zona-pellucida-glycoprotein-binding capability and have, therefore, been implicated in sperm capacitation and sperm-oocy te attachment. AQN-2 (18-20 kDa), however, is unique among boar sperma dhesins in that it is the only member of the family which is known to be glycosylated and which possesses weak zona-pellucida-binding but no t seminal-plasma-inhibitor-binding ability. In this study we report th e structural and functional characterization of the two glycoproteins contained in the AQN-2 fraction. One component is identical with PSP-1 , a major porcine seminal plasma protein whose function has not yet be en identified, while the second protein is a glycosylated isoform of A QN-3. Here we show that the inability of the glycosylated boar spermad hesins to bind seminal-plasma protease inhibitors as well as the weak binding of glycosylated AQN-3 to zona pellucida glycoproteins is due t o the presence of the oligosacharide chain on a conserved asparagine r esidue. This indicates that modification of a spermadhesin polypeptide framework may serve to modulate its ligand-binding capabilities.