Smart silk from an unusual source

For thousands of years, the proteins from moths and butterflies, particularly silkworms, have been used to make natural textiles. Huge industries manufacture cultivated silks around the world, but the scale of production is dependent on one thing: silkworms can be domesticated.

More recently the amazing properties of spider silk have stirred scientific interest. With the equivalent strength of bullet-proof materials like Kevlar, spider silk has one important difference - it is also extremely elastic. However, unlike silkworms, spiders cannot be domesticated. Encouraging them to make silk in sufficient quantities to manufacture bullet-proof, light weight clothing is simply impossible.

Putting spider genes into fermenting bacteria such as yeast, or other transgenic systems, such as plants, otherwise known as crop biofactories, is one method being trialed to create spider silk proteins. This type of system is not new.

"All enzymes that go into washing detergents contain enzymes made from proteins in transgenic systems," CSIRO's Dr Tara Sutherland says.

The problem with this approach to creating spider silk is that the proteins are very large, containing long repetitive strands of over 5,000 amino acids. Bacteria that have been genetically engineered to produce spider silk proteins simply run out of steam, giving up half way through.

So Sutherland and her group from CSIRO Entomology are looking at silks produced by the larvae of bees and ants. They reported their recent work in the journal Molecular Biology and Function.

"Most people are unaware that bees and ants produce silk," Sutherland says. "There is a long period where the larvae are defenseless, so they produce silk to use in structures like cocoons to protect themselves.

"Its molecular structure is very different to that of the large protein, sheet structure of moth and spider silk."

With between 300 and 400 amino acids, the silk proteins in bee and ant silks are less than a tenth of the size of spider silk proteins. They make structures called 'coiled coils', where multiple helices wind around each other.

"The structure is like three or four springs, which fit together with a bit of a twist," Sutherland says.

While she and her team do not fully understand how these silks get their incredible strength, the coiled structure gives the silk its amazing elasticity, as it can unwind and coil back again.

The relatively small size of these silks also makes them much easier to replicate in transgenic systems like bacteria and plants, and in the future it may be possible to harvest the silk from bacteria or non-food crop plants.

---PB--- Silk and sociability

The structure of the silk is essentially identical across the species of social insects that Sutherland and her team have looked at.

It also appears that there is an evolutionary link between the coiled coil silk produced by these insects and their social nature. The coiled coil silks evolved around 155 million years ago and are very tough and stable compared with the classical sheet silks.

"In the group of insects that includes ants and bees, silk arose at the time that sociality arose," she says.

"This silk is more stable and allows them to build communal hives or nests that last longer."

It is therefore probable that the evolution of the coiled coil silks has underpinned the success of the social Hymenoptera, the order of insects which contains the groups being looked at by Sutherland and her team.

Sutherland is also keen to stress that this research into bee and ant silks opens the door to a whole new way of producing materials and fibres. She says there is currently no control over the individual building blocks, such as those from petrochemical sources, when chemical synthesis is used to make different materials and fibres.

"In transgenic systems we can control the building blocks at every amino acid in the sequence. The protein sequence is encoded by DNA, which is like a book on how to make it. Using modern molecular biology, we can develop smart materials and precisely control existing matter."

The silk research is part of a joint CSIRO and Grains Research & Development Corporation Crop Biofactories Initiative.

Thermal regulation

Honeybee larvae produce silk to reinforce the wax cells in which they pupate, bulldog ant larvae spin solitary cocoons for protection during pupation, bumblebee larvae spin cocoons within wax hives which are reused to store pollen and honey, and weaver (or green) ants use their larvae as 'tools' to fasten fresh plant leaves together to form large communal nests.

"Green ants no longer make a cocoon, they have large communal nests and are completely protected in the nest," Sutherland says.

``Bees still have individual cocoons, each generation weaves another layer of silk, which helps the wax keep its structure and also keeps the hive at the right temperature. So the silk contributes to the thermal regulation and mechanical strength of the hive."