The complete primary structure of the boar spermadhesin AQN‐1, a carbohydrate‐binding protein involved in fertilization

Gamete recognition and adhesion are essential steps in the complex process of fertilization. In mammals and in other species, increasing evidence indicates that carbohydrate‐binding proteins on the sperm surface play a pivotal role as counter‐receptors for certain oligosaccharide moieties attached to the oocyte zona pellucida glycoproteins. Although different sperm‐associated zona‐pellucida‐binding proteins have been identified in a number of species, few of them have been isolated and structurally characterized. In this paper we report the primary structural characterization of AQN‐1, a 12‐kDa boar‐sperm‐associated carbohydrate‐binding and zona‐pellucida‐binding protein. The molecular mass of AQN‐1 was determined by time‐of‐flight plasma‐desorption mass spectrometry. Determination of its amino acid sequence and location of disulphide bridges were accomplished by a combination of proteochemical and mass spectrometric methods. The primary structure of AQN‐1 failed to show any significant similarity to the protein structures deposited with the Martinsried Institute for Protein Sequences data bank, indicating that it may belong to a novel protein family involved in fertilization. AQN‐1 shares extensive structural, as well as functional, similarity with two other boar sperm zona‐pellucida‐binding proteins, AQN‐3 and AWN, which we have recently characterized. To name this protein family, we have coined the term spermadhesin. Our data may be relevant for identification of spermadhesins in other species, and thus may contribute to a better understanding of the species‐specific sperm‐egg recognition mechanism.