The amino acid sequence of AQN-3, a carbohydrate-binding protein isolated from boar sperm. Location of disulphide bridges.

Gamete recognition and adhesion are essential steps in fertilization. Among others, carbohydrate-binding proteins on the sperm surface have been recognized to play a central role in the initial interaction of the male gamete with components of the zona pellucida of the homologous investing oocyte. We have isolated several members of a carbohydrate- and zona pellucida-binding protein family from ejaculated sperm. Here we report the biological origin and structural characterization of AQN-3, a component of this carbohydrate-binding family. The molecular weight of purified AQN-3 was determined by plasma desorption mass spectrometry. The protein was chemically and enzymatically degraded, the proteolytic fragments isolated and characterized by N-terminal sequencing and fast atom bombardment mass spectrometry. In this manner we established the complete amino acid sequence of AQN-3 and the location of its two disulphide bonds. No analogous protein sequence could be found in the MIPS protein sequence data bank, indicating that AQN-3 may belong to a novel mammalian carbohydrate-binding protein family.