Carbohydrate-binding properties of boar sperm proacrosin and assessment of its role in sperm-egg recognition and adhesion during fertilization

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Abstract
Recent evidence indicates that a carbohydrate recognition mechanism is involved in the early stages of sperm-egg interaction in mammals. In this communication, we describe a potential zona-ligand molecule on boar spermatozoa that has the capacity to recognize and bind to carbohydrate moieties of zona pellucida glycoproteins as well as neoglycoproteins, BSA-fucose and BSA-mannose. The molecule has broad specificity for carbohydrate binding and there is a requirement for a polysaccharide structure or for ‘clustering’ of saccharides on a protein backbone. The molecule has been identified as proacrosin, the zymogen form of the acrosomal protease acrosin. Strong similarities exist between proacrosin and ‘bindin’, the lectin-like protein that is thought to mediate sperm-egg adhesion in echinoderms. An hypothesis is proposed for spermegg interaction in mammals in which proacrosin, released during the early stages of the acrosome reaction, mediates secondary or consolidated binding of spermatozoa to the zona pellucida by virtue of its carbohydrate-binding capacity. The localized proteolytic action of active acrosin on the zona enhances this interaction in a manner analogous to the requirement for trypsinization of erythrocytes before agglutination by certain lectins. This hypothesis, which is supported by evidence from in vitro fertilization experiments, is discussed in relation to current concepts on sperm-egg recognition.