The animal world – including some of its ickiest parts – never ceases to amaze. According to researchers in Canada and Singapore, velvet worm slime contains an ingredient that could revolutionize the design of high-performance polymers, making them far more sustainable than current versions.
“We have been investigating velvet worm slime as a model system for inspiring new adhesives and recyclable plastics because of its ability to reversibly form strong fibres,” explains Matthew Harrington, the McGill University chemist who co-led the research with Ali Miserez of Nanyang Technological University (NTU). “We needed to understand the mechanism that drives this reversible fibre formation, and we discovered a hitherto unknown feature of the proteins in the slime that might provide a very important clue in this context.”
The velvet worm (phylum Onychophora) is a small, caterpillar-like creature that lives in humid forests. Although several organisms, including spiders and mussels, produce protein-based slimy material outside their bodies, the slime of the velvet worm is unique. Produced from specialized papillae on each side of the worm’s head, and squirted out in jets whenever the worm needs to capture prey or defend itself, it quickly transforms from a sticky, viscoelastic gel into stiff, glassy fibres as strong as nylon.
When dissolved in water, these stiff fibres return to their biomolecular precursors. Remarkably, new fibres can then be drawn from the solution – implyimg that the instructions for fibre self-assembly are “encoded” within the precursors themselves, Harrington says.
High-molecular-weight protein identified
Previously, the molecular mechanisms behind this reversibility were little understood. In the present study, however, the researchers used protein sequencing and the AI-guided protein structure prediction algorithm AlphaFold to identify a specific high-molecular-weight protein in the slime. Known as a leucine-rich repeat, this protein has a structure similar to that of a cell surface receptor protein called a Toll-like receptor (TLR).
In biology, Miserez explains, this type of receptor is involved in immune system response. It also plays a role in embryonic or neural development. In the worm slime, however, that’s not the case.
“We have now unveiled a very different role for TLR proteins,” says Miserez, who works in NTU’s materials science and engineering department. “They play a structural, mechanical role and can be seen as a kind of ‘glue protein’ at the molecular level that brings together many other slime proteins to form the macroscopic fibres.”
Miserez adds that the team found this same protein in different species of velvet worms that diverged from a common ancestor nearly 400 million years ago. “This means that this different biological function is very ancient from an evolutionary perspective,” he explains.
“It was very unusual to find such a protein in the context of a biological material,” Harrington adds. “By predicting the protein’s structure and its ability to bind to other slime proteins, we were able to hypothesize its important role in the reversible fibre formation behaviour of the slime.”
The team’s hypothesis is that the reversibility of fibre formation is based on receptor-ligand interactions between several slime proteins. While Harrington acknowledges that much work remains to be done to verify this, he notes that such binding is a well-described principle in many groups of organisms, including bacteria, plants and animals. It is also crucial for cell adhesion, development and innate immunity. “If we can confirm this, it could provide inspiration for making high-performance non-toxic (bio)polymeric materials that are also recyclable,” he tells Physics World.
The study, which is detailed in PNAS, was mainly based on computational modelling and protein structure prediction. The next step, say the McGill researchers, is to purify or recombinantly express the proteins of interest and test their interactions in vitro.
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