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Gene for Key Spider-Silk Protein Found

Stefan Lovgren
for National Geographic News
August 2, 2005
 
The strength and elasticity of spider silk makes it the toughest natural
fiber. Its characteristics could have applications in areas ranging from
medicine to ballistics.

Yet only a fraction of the known silk types have been identified at the molecular level. Most of the research has focused on a type of silk known as dragline, which spiders use to construct the frame of their web.

But now scientists have identified the gene for the main protein that female spiders use to make their silken egg cases. The finding could help engineers in their quest to develop applications using artificial spider silk.

"It's likely that the egg case spider silk will have properties that are unique to it because of its specific function," said Jessica Garb, a post doctoral researcher at the University of California, Riverside.

The findings, which also shed light on the evolution of spiders, are reported this week in the journal Proceedings of the National Academies of Science.

Evolutionary Origin

There are more than 37,000 known spider species, and probably thousands more that have not been described.

Spiders use silk to move, trap, and store food and to reproduce. Different proteins are made and mixed in silk glands, creating a silk suited to each task.

All spiders have silk genes, which provide the biological instructions for making different proteins. Yet scientists have only identified a few of the silk genes from a small number of species.

Garb and Cheryl Hayashi, a UC Riverside biology professor, isolated a protein called TuSp1 used by 12 types of orb-weaving spiders to make egg case silk.

The egg case silk was shown to be composed of nearly identical, repeating sequences. These gene repeats also share strong similarities across spider species that diverged more than 125 million years ago. The findings point to what is known as concerted evolution among these genes.

"The different elements of a gene sequence typically evolve independently of each other," Garb said. "What happens with concerted evolution is that different segments within the same gene are all evolving together, creating a repetitive sequence."

Bulletproof Vests

Scientists want to understand the molecular structure of different types of spider silks in order to engineer artificial silk that could be used for a broad range of medical and industrial applications. These include artificial tendons and ligaments, biodegradable fishing lines, parachute cords, and bulletproof vests.

"First, we really have to understand how these things work," said Ward Wheeler, curator of invertebrates at the American Museum of Natural History in New York.

"As we understand how these [spider silks] are structured, we can talk about perhaps engineering them to either amplify desirable properties or perhaps minimize other, less desirable properties," he said.

Individual spider species are capable of making different types of silks, which have different functional purposes.

Tubuliform glands, located in female spiders' abdomens, produce the fibers used to construct egg cases. But not all species' cases look the same. Black widow spiders, for example, produce an egg case that looks like a white, fluffy ball. Other spider species make egg cases that are flattened.

"The egg case silk protein is very different [from other proteins], and it appears to be exclusively used for constructing these egg cases," Garb said.

Scientists do not know much about the mechanical properties of egg case silk. But Garb hypothesizes that it might have unique properties based on its ability to protect hundreds of eggs. One possible trait, Garb notes, is the ability to block ultraviolet radiation, which could be useful for future spider silk applications.

Many more spider silk genes are waiting to be found.

As Wheeler, of the American Museum of Natural History, noted, there are "hundreds of million of years of biological experimentation in the tens of thousands of spider species we have today."

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