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
"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,"
Comparison with 25 other spider silk genes showed few
similarities, implying that the protein TuSp1 arose by gene duplication
followed by substantial sequence evolution.