Izumo and Juno are two proteins that are expressed in sperm and egg, respectively and play critical roles in mediating interactions between them. The following experiments were carried out with Izumo and Juno:
A. Sperms from a male mouse, where Izumo has been knocked out, could not fertilize eggs from a normal female.
B. Two independent kidney cell lines were developed, one expressing Izumo and the other expressing Juno. If the two cells are mixed, the cells will tend to aggregate with each other.
C. Whole mount immunostaining for Izumo and Juno indicated specific staining on the sperm and egg, respectively.
Which one of the following options lists experiment(s) that demonstrate(s) interactions between Izumo and Juno?

Correct option is B

Answer: b) B only

Explanation
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Izumo (in sperm) and Juno (in egg) are complementary receptor–ligand proteins essential for sperm–egg membrane fusion during fertilization.

Now, let’s analyze each experiment:

A. Sperms from Izumo-knockout males cannot fertilize eggs from normal females.

This shows Izumo is essential for fertilization, but does not directly prove interaction with Juno.

It only demonstrates the functional requirement of Izumo, not the physical interaction.

B. Kidney cells expressing Izumo and Juno aggregate when mixed.

This clearly indicates that Izumo binds directly to Juno, leading to cell–cell adhesion.

Hence, this experiment directly demonstrates their molecular interaction.

C. Immunostaining shows Izumo on sperm and Juno on eggs.

This only demonstrates localization, not interaction.

It confirms where the proteins are expressed, not whether they bind.

Therefore, only experiment B provides direct evidence of interaction between Izumo and Juno.

Information Booster

Izumo1: A sperm surface protein named after a Japanese marriage shrine ("Izumo Taisha").

Juno: The egg receptor for Izumo, named after the Roman goddess of marriage.

After fertilization, Juno is lost from the egg membrane, preventing polyspermy.