Life at the tide's edge is never dull. Some species take refuge in rock pools as the waves recede, while others secure themselves with glue to rocks,for fear of being washed away by the returning tide. Marsh dwelling periwinkles, on the other hand, have a different dilemma. They tether themselves high up on grasses to evade the returning waves, preventing themselves from being blown free with an adhesive gel. Andy Smith is fascinated by the biomechanical properties of the periwinkle's flexible,gel-like glue. He explains that while even the most flexible commercial glues are essentially solid, the periwinkle's adhesive is often 97% water, and the molluscs adhere as well to soggy surfaces as they do to dry. So how do these amazing adhesives work? After all, at first glance, the glue looks like the gooey slime left by roving molluscs! But when Smith took a closer look at the glue's molecular components, he discovered that the glue had one extra, vital ingredient; a protein, which made up 50% of the glue's solid material. Intrigued by the mystery ingredient, Smith decided to discover whether the protein was all it took for molluscs to convert goo to glue(p. 1127).

But first he had to get enough of the tacky adhesive to be able to isolate the tiny amount of protein each periwinkle produced. Scraping the glue from the glass walls of an aquarium, the team redissolved the glue and isolated the protein from the glue's slimy components, ready to test its gelling activity. Dissolving pectin in a 0.1% solution of the protein, Smith was delighted when the mixture quickly turned into a clear lump of gel. And when Jan Pawlicki,Laura Pease and Yuanming Zhang tested the glue's stiffness in a rheometer,they found that it was much stiffer than a gel made from pectin alone. Not only did the protein cause the glue to gel, but it reinforced it too.

However, Smith needed much more gelling protein than the tiny periwinkles could produce, to get to grips with the gluey material. He needed prolific glue producers, so he turned to terrestrial slugs and snails.

Comparing glue and trail goo from both species, the team isolated a 15kDa glue protein from the slug, and three large glue proteins from the snail. And when the team tested the effect of the snail's protein on a selection of polysaccharides, they were surprised; the protein's effects were non-specific,causing almost all of the negatively charged polysaccharides that they tested to gel. However, when they tested uncharged polysaccharides with the gel-forming proteins, they barely gelled at all.

All of which could hold a clue to the protein's function. Smith suspects that the proteins act as electrostatic cross linkers between the charged polysaccharide chains, allowing the gel to form quickly. Smith also noticed that the protein improves the glue solution's wetting properties, allowing it to spread more effectively over a surface before gelling, to get a good grip.

So next time you're clambering around at the seashore, spare a thought for the humble molluscs, which anchor down there, and their amazing adhesive proteins.

References

Pawlicki, J. M., Pease, L. B., Pierce, C. M., Startz, T. P.,Zhang, Y. and Smith, A. M. (
2004
). The effect of molluscan glue proteins on gel mechanics.
J. Exp. Biol.
207
,
1127
-1135.