Sperm harpoons: X-rays reveal all

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  • Published: Sep 1, 2015
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Sperm harpoons: X-rays reveal all

Fertilisation, the name of the game

The SLLP1 filament viewed along the side with each neighboring monomer coloured alternatively

Sperm may have a secret weapon to use in their battle to fertilise the egg, according to a new X-ray study - they are, it seems, armed with tiny harpoons. Researchers at the University of Virginia School of Medicine have studied a protein, sperm lysozyme-like protein 1, or SLLP1, that is found within the head of the sperm and forms microscopic spikes on the surface. The team suggests that these tiny filaments may help bind sperm to egg thus facilitating fertilisation.

"The filamentous structure of the protein crystals has led to a new hypothesis and that the hunt is on to verify if the filaments actually form at some stage during the fertilization cascade," Herr told SpectroscopyNOW. "Finding the filaments in vivo is needed to validate the new hypothesis."

Details of the work, which is the culmination of 14 years of effort, are published in the journal Andrology and feature on its cover, represents a significant step forward in the fine dissection of the protein architecture of the sperm's acrosomal matrix, an organelle in the sperm head, and suggests a new hypothesis concerning what happens during fertilization. The discovery is the result of a longstanding collaboration between the laboratories of John Herr  and crystallographer Wladek Minor. Herr's team originally discovered the protein SLLP1 in the acrosome that the collaborators have now shown to form these microscopic filaments revealed at 2.15 Å resolution.

The team describes the structure as follows: "mSLLP1 monomer adopts a structural fold similar to that of chicken/mouse lysozymes retaining all four canonical disulfide bonds. mSLLP1 is distinct from c-lysozyme by substituting two essential catalytic residues (E35T/D52N), exhibiting different surface charge distribution, and by forming helical filaments approximately 75 Å in diameter with a 25 Å central pore comprised of six monomers per helix turn repeating every 33 Å." The team also modelled the interaction between SLLP1 monomer and its oolemmal receptor SAS1B using protein-protein docking algorithms.

Sperm whaling?

"This finding has really captured our imagination," says Herr. "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'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 protein has a high affinity for the oolemmal receptor, SAS1B, the team says.

Heping Zheng, lead author on the paper adds, "This is an important protein, because it's the first crystal structure from a protein within the sperm acrosome. It is also the first structure of a mammalian sperm protein with a specific oocyte-side binding partner characterized. To our knowledge, only nine proteins specifically obtained from mammalian sperm have known structures."

Reproductive mapping

Reproductive biologists, including Herr, can now use this structure to help them map in greater detail the process of fertilisation. "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," Herr explains. The current study involved mouse SLLP1, but the equivalent proteins are present in human sperm too.

Related Links

Androl 2015, online: "Sperm Lysozyme-Like Protein 1 (SLLP1), an intra-acrosomal oolemmal-binding sperm protein, reveals filamentous organization in protein crystal form"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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