A sperm cell latches on to its target with spiky filaments that emerge from its head, say scientists at the University of Virginia (UVA) School of Medicine in the US whose 14 years of research led to the discovery.
Their finding is featured on the cover of the scientific journal Andrology, in which their corresponding paper was published.
"This finding has really captured our imagination," says UVA reproduction researcher John Herr, PhD, of the Department of Cell Biology, adding that they have come up with an entirely new hypothesis about what happens at conception.
The research has implications for our understanding of the protein architecture of the sperm head, specifically in an organelle called the acrosomal matrix.
"One of the major proteins that is abundant in the acrosome (in the anterior region of the sperm head) is crystallizing into filaments, and we now postulate they are involved in penetrating the egg," says Dr Herr.
This protein was discovered years ago in Dr Herr's lab in collaboration with the lab of Wladek Minor, PhD and they named it "sperm lyzosyme-like protein" (SLLP1).
Minor and his team captured the protein within a static crystal, which they chilled to cryogenic temperatures (approximately below 180 degrees C or below 292 degrees F) for protection, then zapped it with X-rays.
Observing the direction of the X-rays, the scientists were able to determine the protein's shape in a process they compare to mapping out a shipwreck with a sonar.
It's the first protein from that part of the sperm to have a crystal structure and it's the first mammalian sperm protein that can bind to eggs, says Heping Zheng, the lead author of the paper. Spring-boarding from their new understanding of the protein, the researchers aim to explore how fertilization works in the most precise detail.
The study builds on previous groundbreaking research from Herr's lab — that time featured on the cover of the journal Biology of Reproduction, which published the corresponding paper in March.
Here, it was reported that the protein ESP1 (as it relates to the gene SPESP1) remains intact during fusion, acting as a stabilizer, while the rest of the sperm head undergoes dramatic changes.
"Getting at the molecular components of the fertilization event has a lot of practical applications — as well as intellectual value — because you want to account for all the major components involved in the essential events of the fertilization cascade," says Dr. Herr.