Scientists reveal one of nature’s strongest bonds
When a sperm finally reaches an egg, its journey is far from over.
Before a new life can begin, the two cells must hold tightly together long enough for their membranes to merge.
Now, scientists from ETH Zurich and the University of Basel have discovered why this connection is so strong.
Their study, published in Nature Communications, reveals that the sperm and egg use a special kind of molecular grip known as a “catch bond” — one of the strongest types of adhesion found in living organisms.
The bond forms between two key proteins: Izumo, found on the sperm’s surface, and Juno, located on the egg’s outer membrane. Like a lock and key, these two proteins fit perfectly together.
But unlike ordinary protein bonds, which weaken when pulled, the Juno–Izumo connection becomes stronger under tension.
This is exactly what happens when the sperm’s tail, called a flagellum, lashes back and forth after reaching the egg. The pulling motion actually makes the bond tighter, ensuring the sperm stays attached while the membranes prepare for fusion.
To understand this powerful attachment, the research team used an atomic force microscope to measure how individual Juno and Izumo proteins interact.
The test works a bit like a “finger-wrestling” game—two linked molecules are pulled apart until the connection breaks.
Surprisingly, when tension increased, the bond between Juno and Izumo didn’t give way easily. Instead, it held on longer, revealing the telltale signature of a catch bond.
“Most protein bonds act like Velcro—you can pull them apart more easily the harder you tug,” said Viola Vogel, professor at ETH Zurich. “But this one does the opposite. It gets stronger when stretched.”
Computer simulations run at the Swiss National Supercomputing Center in Lugano helped explain how this happens.
As the two proteins are pulled apart, some atomic connections between them snap, but at the same time, the proteins twist and shift closer together. Juno even rotates by about a quarter turn, forming new atomic links that reinforce the attachment. This dynamic twisting makes the bond last much longer, especially under the physical strain of the sperm’s movements.
The strength of the Juno–Izumo bond rivals that of the protein structures that hold muscle fibers together, preventing them from tearing. Similar catch bonds are also used by immune cells that cling to blood vessel walls or skin cells that attach to wounds during healing.
The researchers also studied a known genetic mutation that affects about one in 600 women worldwide. This mutation slightly changes the Juno protein’s structure and is linked to reduced fertility.
When the scientists recreated this mutation in the lab, they found that the modified protein couldn’t hold on as long under tension. “It seems the bond breaks too soon,” said Vogel. “That means the sperm and egg may separate before they have time to fuse, preventing fertilization.”
By uncovering this mechanism, researchers now have a clearer picture of how sperm and egg binding works and how certain mutations may cause infertility. This knowledge could lead to new genetic tests or treatments to help couples struggling to conceive.
