The key difference between dry silks from moths and butterflies and wet silks from caddisflies is that the serines in the silk from caddisflies are "phosphorylated," meaning phosphates are added to the serines as the fibroin silk protein is synthesized.
"Phosphates are well-known adhesion promoters used in dental fixtures such as crowns or fillings," says Stewart. "They are also in latex paints that are water-based, and the phosphates increase the adhesion of those paints. The paint industry discovered this fairly recently. Caddisflies have been doing this for at least 150 million years."
The phosphates attached to the serines are negatively charged. Other amino acids in the protein are positively charged. Stewart found that is a key factor in making silk underwater. Chains of proteins each with alternating regions of positive and negative charges line up in parallel with positive and negative charges attracting each other.
"Imagine those chains aligned side-by-side, but staggered so the pluses and minuses are lined up, which then forms silk fibers with lots and lots of these protein chains in one fiber," Stewart says. "You wouldn't be able to make shirts out of it, but you might be able to make wet Band-Aids."
Stewart made a counterintuitive finding about how wet silks are made. "These fibroin proteins that make up the silks are water-soluble because of the electrical charges. Ironically and this is our hypothesis for now the association of those plus or minus charges makes them water-insoluble. This is how you make a silk fiber under water."
Comparison with amino acids from three other caddisfly species found great similarities, suggesting other caddisflies also use phosphorylation to spin silk underwater.
Stewart says caddisfly silk and sandcastle worm glue are similar: their proteins are heavily phosphorylated and have a large number of po
|Contact: Lee Siegel|
University of Utah