12. Successful fertilization in sea urchin demands specific interaction between proteins and receptors of sperms and eggs. In view of the above, which one of the following combinations is correct?

(1) Bindin in acrosomes and bindin receptors on egg vitelline membrane
(2) Bindin in egg membrane and bindin receptors in acrosomes
(3) Resact on egg jelly and bindin on sperm membrane
(4) Proteasomes on egg membranes and complex sugars on sperm membranes

 

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Introduction

Successful fertilization in sea urchins requires a highly specific interaction between proteins on the sperm and receptors on the egg. This molecular recognition ensures species specificity and efficient fusion of gametes, preventing cross-species fertilization. Central to this process is the sperm protein Bindin and its corresponding receptor on the egg’s vitelline membrane.

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What is Bindin?

Bindin is an adhesive protein localized in the acrosomal vesicle of the sea urchin sperm. Upon undergoing the acrosome reaction, Bindin is exposed on the sperm surface. It plays a pivotal role by binding specifically to receptors on the egg’s vitelline membrane, thereby mediating sperm attachment to the egg.

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Egg Vitelline Membrane Receptors

The egg vitelline membrane contains specialized receptors for Bindin, such as the Egg Bindin Receptor 1 (EBR1). This receptor is highly glycosylated and facilitates species-specific binding with sperm Bindin, ensuring that sperm from the same species effectively recognize and adhere to the egg.

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Why The Other Options Are Incorrect

• Bindin in egg membrane and bindin receptors in acrosomes: Bindin is a sperm protein, not located in the egg membrane.

• Resact on egg jelly and bindin on sperm membrane: Resact functions as a chemoattractant released by eggs to guide sperm, not as a binding protein for adhesion.

• Proteasomes on egg membranes and complex sugars on sperm membranes: Proteasomes are intracellular protein complexes involved in protein degradation, not in sperm-egg adhesion; complex sugars on sperm are not the primary binding elements in sea urchin fertilization.

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The Importance of Bindin-Receptor Interaction

This interaction restricts fertilization to conspecific gametes, supporting reproductive isolation and species continuity. Experimental knockouts of Bindin gene render sperm incapable of fertilizing eggs without affecting sperm morphology, highlighting Bindin’s indispensable role in fertilization.

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Conclusion

The correct pairing in sea urchin fertilization is

• Bindin located in sperm acrosomes

• Specific bindin receptors on the egg’s vitelline membrane

This binding governs the initial and species-specific adhesion crucial for fertilization success.

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This article highlights the molecular basis of species-specific sperm-egg interaction in sea urchins, emphasizing the critical role of Bindin and its egg receptors.