Biologists at the University of California, Riverside, have uncovered the molecular structure of the gene for the protein that female spiders use to make their silken egg cases. The discovery will help biotechnologists develop applications for spider silk and will shed light on spider evolution.

Assistant Professor of Biology Cheryl Hayashi and postdoctoral researcher Jessica Garb characterized the variants of the protein (TuSp1) used by 12 species of spiders to make egg-case silk. They found strong similarities in the lengthy amino acid sequences of the proteins among species that diverged at least 125 million years ago.

The findings are important, in part, because the mechanical properties of the various types of spider silk - their elasticity, tensile and breaking strength - are dependent on the sequence of amino acids that form the silk proteins.

"Collectively, spider silks are some of the toughest natural fibers known," Hayashi said. "Imagine a fabric made from such a substance? It would be incredibly strong, flexible and ultimately, biodegradable."

Spider silks have just begun to be considered in the improvement of a wide variety of products such as super-strong body armor, specialty rope, and surgical microsutures.

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. For instance, web-weaving spiders use dragline silk, which is very strong, as a frame for their wagon-wheel-like webs and a different type of silk, known as capture silk, to fill in the web. Capture silk is more elastic than the dragline variety, and is sticky to entrap prey. Of the seven types of silk spiders produce, the fibers used to encase spider eggs are of exceptional strength and durability.

"The protein of the egg-case fibers has a different function altogether from that of the other silks such as dragline or capture silks," Garb said. "Egg-case silk has to last a long time and therefore must be durable under a wide variety of conditions, from freezing to very high temperatures. It needs to be strong enough to protect the eggs from threats such as predators, parasites and molds."

Despite all this, the molecular sequences of the genes that encode spider silks are only partially known. Garb and Hayashi suggest there are many more spider silk genes waiting to be found.

Spider silk genes are composed of long repeating sequences, or modules, and a mutation in one repeat can be spread to adjacent repeats, an example of concerted evolution. Cracking the molecular structure for silk is important not only for the development of products but for those like Hayashi and Garb who study the evolutionary biology of spiders.

"The egg-case silk is the product of millions of years of evolution and the amino acid modules can serve as a biochemical blueprint," Hayashi said.
Comparison with 25 other spider silk genes showed few similarities, implying that the protein TuSp1 arose by gene duplication followed by substantial sequence evolution.