Sperm cells attach to eggs with tiny 'harpoons'

Scientists have discovered that a protein in the head of sperm cells forms spiky filaments, which could act as tiny 'harpoons' that anchor the sperm to an egg.

The finding was the result of a 14-year effort at the University of Virginia. Dr John Herr of the School of Medicine first discovered the protein – sperm lysozyme-like protein 1 (SLLP1) – in 2001 but encountered huge technical problems in determining its structure. The problem has finally been cracked by Professor Wladek Minor, an x-ray crystallographer, and his team in the Department of Molecular Physiology and Biological Physics.

The protein is found in the sperm's acrosome, a structure in the head of the sperm cell containing enzymes that aid in drilling through the gelatinous coating of the egg.

'This finding has really captured our imagination,' said Dr Herr. 'One of the major proteins that is abundant in the acrosome is crystallising into filaments, and we now postulate they're involved in penetrating the egg – that’s the new hypothesis emerging from the finding.'

To determine the structure of the protein it was first separated, trapped within a crystal, cryogenically frozen and then bombarded with x-rays. The patterns formed by the diffraction of these x-rays by the crystalline protein allowed the scientists to determine its size, shape and structure. To date, only nine mammalian sperm proteins have known structures.

'This is an important protein, because it's the first crystal structure from a protein within the sperm acrosome,' said Dr Heping Zheng, also at the University of Virginia and the lead author of the paper, which was published in the journal Andrology. 'It is also the first structure of a mammalian sperm protein with a specific oocyte-side binding partner characterised.'

Herr's team hope to find the structure of more proteins in the acrosome to discover more about the specific mechanisms involved in the process of fertilisation.