Researchers unravel how spiders store their silk

Spider silk is stronger than steel by weight and as stretchy as rubber. Scientists want to understand how spiders spin their silk so they can make artificial versions of the gossamer threads that are as tough as the real thing. In a key insight to the spinning process, researchers have found that spiders store silk proteins as complex, tiered nanostructures inside their silk glands (Proc. Natl. Acad. Sci. USA 2018, DOI: 10.1073/pnas.1810203115). The findings explain how the arachnids pack the proteins at a super-high concentration and how they start the fiber-making process inside the glands.

Each glossy fiber of spider silk is a string of large protein molecules. In the spider’s glands, these proteins mix with water and salt to form a highly concentrated gel-like fluid. Researchers have been trying to copy spider silk’s remarkable traits to make materials for medical implants, bulletproof vests, and sports gear. But mimicking the way spiders spin fibers from the concentrated protein solution has been a challenge.

One key question has been how the spiders store the proteins inside this silk gland fluid. Fifteen years ago, researchers hypothesized that the proteins form spherical nanostructures. “But nobody knew if these things were real,” says Gregory P. Holland, a chemist at San Diego State University. The nanoparticle hypothesis could explain some of spider silk’s mechanical properties. “Instead of having a solution of individual proteins, if you spin from a nanoparticle, you could potentially get tougher fibers,” says Northwestern University chemist Nathan Gianneschi who worked with Holland.

To find out if the nanoparticles existed, Holland, Gianneschi, and colleagues extracted fluid from the glands of euthanized black widow spiders. This species boasts one of the stronger silks, Holland says. The team used nuclear magnetic resonance to show that the proteins were packed into volumes with diameters of about 300 nm in the fluid. Then they used cryogenic transmission electron microscopy to get the first-ever images of the protein nanostructures.

The images showed that the protein molecules assemble into 50-nm-wide, 25-nm-thick flakes. Multiple flakes interact to form a network with densely packed regions and void spaces that are likely occupied by solvent.

The group also studied what happens to the proteins under the strain they would experience when extruded from the spider’s spinning duct. The scientists put the protein solution into a micropipette and pumped it up and down. The force of passing through the pipette opening stretched the flakes into narrow fibers that are about 100 nm long and 5–15 nm wide.

“The findings are important for the understanding of the spider silk spinning process,” says Anna Rising of the Karolinska Institute. “In order to recapitulate the properties of the native spider silk fiber, we need to understand and replicate this process in vitro.”

This study is just the start, Holland says. In the spider spinning ducts, the protein fluid becomes more acidic, ions get exchanged, and water is removed. “Now we can test what happens to these superstructures as we change biochemical conditions.”