Researchers say they have discovered tiny “harpoons” on sperm that allow them to latch onto eggs and other sperm during fertilization. The finding, the result of 14 years of research, is published in the journal Andrology.
According to the scientists from the University of Virginia (UVA), sperm are able to form spiky protein filaments on their head. The protein believed to form the filaments, sperm lysozyme-like protein 1 (SLLP1), was already known but the study suggests a new role. It resides in the acrosomal matrix, which is a structure in the sperm head. The exact part played by the filaments during fertilization is not entirely clear, but it is thought to help the sperm enter an egg.
"This finding has really captured our imagination," said UVA reproduction researcher John Herr of the Department of Cell Biology in a statement. "One of the major proteins that is abundant in the acrosome is crystallizing into filaments, and we now postulate they're involved in penetrating the egg – that's the new hypothesis emerging from the finding, which leads to a whole new set of questions and new hypotheses about the very fine structure of molecular events during fertilization."
The discovery was made by capturing the protein in a static crystal. This crystal was then cooled to cryogenic temperatures to prevent degradation and blasted with X-rays. Measuring the refraction of the X-rays, the team could work out the shape of the protein – which, from the 3D models produced in the study, is cylinder-like and hollow in the middle.
The discovery could now give scientists a better picture of the process taking place during fertilization. "At the very fundamental level, understanding that fine molecular architecture leads me, the biologist, to be able to posit new functions for this family of proteins my lab discovered in the acrosome," said Herr in the statement.