AQN proteins mediate the binding of boar spermatozoa to component(s) of the egg's zona pellucida by a carbohydrate-binding mechanism. AQN proteins are secretory components of the male accessory glands being coated to the sperm surface at the time of ejaculation. They possess as well heparin-, serine-protease-inhibitor-binding capability. The residue at position 85 was identified as a methylhistidine by mass spectrometry. Belongs to the spermadhesin family.