Abstract
Bindin is a 24kD protein, contained within the acrosomal granule of the sea urchin sperm, which is secreted upon contact of the sperm with the egg surface and mediates the species-specific attachment of sperm to the vitelline envelope. Early studies indicated that the egg surface receptor for bindin is a glycoprotein that contains sulfated fucan polysaccharide chains. Sulfated fucans isolated from the egg surface or from algal sources bind to bindin with moderately high affinity(Kd 5 x 10-9M) and inhibit the agglutination of eggs by bindin in vitro. Other naturally occurring sulfated polysaccharides (heparin, chondroitin sulfate, carrageenan) do not bind to bindin or inhibit bindin-mediated egg agglutination leading to the suggestion that bindin is a lectin-like protein specific for α 1, 2-fucose polysaccharides. More recent studies indicate that bindin is not a traditional lectin, but instead specifically recognizes sulfate esters on the polysaccharide backbone. The selectivity of bindin for sulfated fucans is apparently due to the complementary location of the sulfate esters on the fucan polysaccharide backbone. The species specific properties of the egg surface receptor for bindin are believed to reside in the core protein sequence of vitelline envelope glycoconjugates.
