Sperm carry tiny harpoons for fertilisation

WASHINGTON: A protein that acts like a tiny harpoon to allow the sperm to latch onto the egg and fertilise it has been discovered by researchers, including those of Indian-origin.

Researchers from the University of Virginia discovered that the protein within the head of the sperm forms spiky filaments, suggesting that these tiny filaments may lash together the sperm and its target.

"This finding has really captured our imagination," said reproduction researcher John Herr of the Department of Cell Biology.

"One of the major proteins that is abundant in the acrosome [in the anterior region of the sperm head] is crystallising 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 fertilisation," Herr said.

The discovery is the result of a longstanding collaboration between Herr's lab and the lab of Wladek Minor of the Department of Molecular Physiology and Biological Physics.

Years ago, Herr's lab discovered the protein that has now been shown to form the filaments, which they dubbed sperm lysozyme-like protein 1, or SLLP1. This protein is a member of a family of proteins now known to reside inside the acrosome.

To determine the shape and structure of the protein, Minor's team had to capture the protein within a static crystal, cool the crystal to cryogenic temperatures to prevent decay and then blast it with X-rays.

By examining how those X-rays were refracted, they could calculate the shape of the protein.

"This is an important protein, because it's the first crystal structure from a protein within the sperm acrosome," said Heping Zheng, the lead author of the paper outlining the discovery.

"It is also the first structure of a mammalian sperm protein with a specific oocyte-side binding partner characterised. To our knowledge, only nine proteins specifically obtained from mammalian sperm have known structures," Zheng said.

The new understanding of the structure will now act as a map for Herr and other reproductive biologists exploring how fertilisation occurs.

Zheng detailed the structure in an article published in the journal Andrology along with Arabinda Mandal, Igor A Shumilin, Mahendra D Chordia, Subbarayalu Panneerdoss, John Herr and Wladek Minor.