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#sea urchin fertilization

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Sea urchin -l

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.

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Sea urchin -l

Egg jelly in sea urchins primarily functions in:

Egg jelly is thick gelatinous extracellular mucoid coat outside vitelline envelope composed of sulfated fucan polysaccharides intermingled with small peptides such as resact and speract. Its principal biological functions include long-range sperm chemoattraction and preparation for fusion. Diffusible peptides establish chemotactic gradient guiding motile sperm toward egg, while polysaccharides interact with sperm receptors inducing Ca2+-dependent acrosomal reaction ensuring membrane fusogen bindin becomes exposed precisely before reaching vitelline membrane. Although it contributes to species filtering, core specificity arises later at bindin-receptor binding step. It does not block polyspermy nor supply metabolic nutrition; polyspermy prevented by electrical shift and fertilization envelope.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Egg jelly role - sperm attraction and acrosome induction.

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