Practice question

Bindin is major insoluble protein sequestered inside acrosomal vesicle and exposed exclusively after Ca2+-dependent acrosomal exocytosis, localized on tip of extended acrosomal process. It possesses lectin-like carbohydrate recognition domain that binds sulfated polysaccharides on vitelline envelope and amphipathic sequences mediating fusion between egg and sperm lipid bilayers. Sequence analyses reveal hypervariable domains determining species-specific recognition, functioning as prezygotic isolation mechanism preventing interspecies hybridization between Strongylocentrotus species. Antibody blocking or recombinant bindin competition inhibits fertilization in species-specific manner. Its dedicated role is selective binding and membrane fusion ensuring accurate gamete union.