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Smart silk from an unusual source
Posted by: Prof. Dr. M. Raupp (IP Logged)
Date: April 10, 2008 08:25AM

By Ruth Beran
CSIRO research is paving the way for the production of novel
biomaterials from bee and ant silk.
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.


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."


[www.biotechnews.com.au]



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