Gene cloned for stretchiest spider silk

To snag a speeding insect, the resilient silk at the center of a spider's web may stretch to almost three times its original length. Now, researchers have cloned the gene for this most elastic of spider silks and unraveled its protein structure.

 The extreme elasticity of this natural miracle fiber, called capture silk, comes from long spirals in the protein's configuration, propose researchers from the University of Wyoming in Laramie. Figuring out what makes silk stretchy and what makes it strong will ultimately enable scientists to design genes to control the manufacture of silks, says Randolph V. Lewis, a coauthor of the report in the Feb. 6 Journal of Molecular Biology. This finding, he says, "Gives us the tools to say, 'If you want to make an elastic silk, this is what you've gotta have.'"

 The researchers obtained the gene from a gland of the golden orb-weaving spider, Nephila clavipes. They found that capture silk protein, a chain of thousands of amino acids, contains regions in which a sequence of five amino acids is repeated over and over, as many as 63 times.

 The researchers suggest that the segments of the protein with the repeating blocks form long, springlike shapes. At the end of each five-amino-acid block, the protein kinks back on itself in a 180@ turn, Lewis says. The series of turns eventually forms a spiral that "looks exactly like a molecular spring."

 Spiders make as many as seven different types of silk, says coauthor Cheryl Y. Hayashi. Insects get entangled in the sticky web, she explains, because the stretchiness of capture silk lets the web oscillate back and forth after the insect hits it. If the web were stiff, the insect might just bounce off.

 Researchers have cloned several genes for dragline silk, the type that the nursery rhyme spider must have spun to lower itself down beside Miss Muffet. Spiders use dragline silk to form the guylines and framework for wheel-shaped orb webs. It is stronger than capture silk but less flexible. In fact, Lewis says, dragline silk is only one-fifth as elastic as capture silk.

 Dragline silk proteins and capture silk proteins have similar turn-forming blocks of amino acids. However, the researchers found that these blocks repeat an average of 43 times in the capture silk, compared to only 9 times in the dragline silk. That fivefold difference in length corresponds to the difference in elasticity between the two proteins, Lewis says.

"When you put the math to it," he notes, "it looks pretty good." 

 The stretchy section of the protein may not spiral in the way Lewis describes, cautions John Gosline, a biomechanic at the University of British Columbia in Vancouver. An alternative theory suggests that the zigzag turns may simply allow the protein to flex and bend, Gosline says. Rather than assuming a specific, organized shape, the stretchy parts of the protein may flop around at random. 

 Gosline adds that he has no doubt that the Wyoming group has identified the correct gene for capture silk protein. 

"I think it's interesting," he says. "We're actually a bit jealous."