Which of these is true for bindin protein?
Bindin is not component of egg jelly or fertilization envelope but major cell adhesion protein uniquely sequestered inside intact sperm acrosomal vesicle. Upon Ca2+-triggered acrosome reaction it becomes prominently exposed on surface of extended acrosomal process where it performs dual functions: mediating species-specific attachment to EBR1 receptor complex of vitelline envelope through lectin-like domain, and also possessing hydrophobic fusogenic sequence promoting merger of sperm and egg lipid bilayers after adhesion. Rapid sequence divergence among Strongylocentrotus species establishes potent prezygotic reproductive barrier. Biochemically insoluble collagen-like protein, bindin represents definitive species-specific fusogenic adhesion protein essential for productive gamete recognition and membrane fusion.
Ref: Vacquier & Moy, J Biol Chem 1997, Bindin fusogenic function; Gilbert Chapter 7: Bindin as species-specific fusogen.