Species-specific Sperm Adhesion in Sea Urchins

Bindin, a protein component of the acrosomal vesicle of sea urchin sperm, has been isolated from Arbacia punctulata and Strongylocentrotus purpuratus . Using this isolated bindin, we have devised a quantitative assay for bindin-mediated egg agglutination and compared the agglutination of bindin and eggs from A . punctulata and S. purpuratus . Bindin-mediated agglutination is species-specific in both species, although a measurable degree of heterotypic interaction is observed . Homotypic bindin-egg interactions differ significantly from heterotypic interactions both in the extent of agglutination and the size of the resulting aggregates . We also provide direct evidence that bindin particles agglutinate eggs by adhering to the surfaces of adjacent eggs . Although the A . punctulata bindin preparation displays the same functional properties and consists of one major polypeptide of the same apparent molecular weight as S . purpuratus bindin, its morphology is very different . Unlike the spherical aggregates observed with S. purpuratus bindin, A . punctulata bindin exists as lamellar vesicles and binds significant amounts of phospholipid and Triton X-100, suggesting that it may be tightly associated with the acrosomal membrane . Having defined a number of the basic parameters of bindin-mediated agglutination, we examined the effect of a number of saccharides and glycopeptides on bindin-mediated egg agglutination . Carbohydrate-containing components derived from the egg cell surface by proteolysis were found to inhibit bindin-mediated egg agglutination at low concentrations, but this inhibition is not species-specific.