Sperm cannot open the egg without this ancient molecular key

They are the original odd couple: one is huge, spherical, and immobile. The other is small in size, has a tail, and never stops swimming. However, the union of egg and sperm is crucial to every sexually reproducing animal on Earth.

Exactly how this union occurred has been a mystery to scientists for a long time. A He studies The results of the study, published Thursday in the journal Cell, relied on the Nobel Prize-winning artificial intelligence technique, which showed that an interlocking package of three proteins is the key that allows sperm and egg to bind together. This crucial package is shared by distantly related animals such as fish and mammals, likely including humans.

For almost all animals on Earth, life begins with a sperm cell that makes its way to the egg cell membrane. Somehow, the two cells recognize each other and bond together. Then, in the blink of an eye, the sperm’s head passes inside the egg, as if walking through a door. Now the fused cell is a zygote and ready to grow into a new animal.

In previous research, scientists found four proteins in mammalian sperm, which are also found in fish sperm and are essential for fertilization. But no one knew if or how they could work as a team to insert the egg.

In the new study, Andrea Pauli, a molecular and developmental biologist at the Research Institute for Molecular Pathology in Vienna, and collaborators from several institutions asked how sperm proteins cooperate during fertilization.

The researchers relied on AlphaFold technology, which shared the Nobel Prize in Chemistry last week. It uses artificial intelligence to predict the shape of the protein. Using AlphaFold, the team was able to compare the four sperm proteins shared between mammals and fish against a library of about 1,400 other proteins found on the cell surfaces of zebrafish testes, looking for potential partners.

“We wanted to find something that we knew would be in the right place at the right time,” said Victoria Deneke, a postdoctoral researcher in Dr. Pauli’s lab.

Even for AlphaFold, this was a challenge. “It was working for two or three weeks,” Dr. Deneke said, monopolizing the campus’ computing resources.

“Other people at the institute were not so happy,” Dr. Pauli added.

Finally, AlphaFold predicted that two common native sperm proteins would bind to each other, along with a third, previously unknown protein, creating a team of three.

Laboratory experiments confirmed the program’s guess: male zebrafish lacking the newly discovered third protein were infertile, as were male mice. Their sperm swim normally but cannot fuse with the egg. The scientists also found biochemical evidence that the three sperm proteins function as a single unit, in both zebrafish and humans.

Dr Pauley said it was likely that the same crucial bundle was present in many – or all – animals with spines.

She described the sperm’s protein bundle as a kind of key, which fits into the egg cell’s lock. In fish, this lock is a protein called a bouncer, which is convenient because the sperm head cannot enter the egg without it.

Previous research has also identified a lock molecule in mammalian eggs, which binds to one of the proteins in the three-protein bundle. Oddly enough, the mammal lock is not the guard. It’s an unrelated protein called Juno.

This means that somewhere in history, animals must have evolved different egg proteins to bind to the sperm protein package. This presents a mystery: The lock has changed, but somehow, “the sperm key has remained the same,” Dr. Pauli said.

“We would like to know the answer,” she added.

Amber Kruchounas, a reproductive biologist at the University of Delaware who was not involved in the new research, called the new study “really exciting.”

Earlier this year, a different research group independently used AlphaFold anticipation The same triple package of protein exists in mammals. “The fact that two independent groups reached the same conclusions certainly increases our confidence in the results,” Dr. Krochounas said.

However, she said, “there is definitely more work to be done to uncover the secrets of fertilization.” For example, some sperm proteins are known to be shared between mammals and fish but are not part of this package; What are they doing?

“This is a fundamental question with very few molecular answers,” Dr. Pauli said. “It’s amazing.”