The recipe for black widow dragline silk is surprisingly simple–there are only two ingredients, according to a new paper in PLoS One. Dragline silk is made of two proteins, called MaSp1 and MaSp2. And for the first time, researchers have sequenced the spider genes that are responsible for making the dragline proteins.

Dragline silk is “half the strength of something like steel, but silk is much more elastic,” says Nadia Ayoub, a postdoctoral researcher in the department of biology at the University of California – Riverside and the first-author of the paper. Because of the elasticity, she adds, “it’s actually tougher than steel or Kevlar.”

Although pieces of spider-silk genes have been sequenced, knowing the full code means that Ayoub and her colleagues, Jessica Garb, Robin Tinghitella, Matthew Collin, and Cheryl Hayashi all of UC Riverside, now can recreate the full proteins, and presumably the silk itself.

Ayoub says people are interested in fabricating the super-tough silk into material to make sutures, ropes, artificial tendons and even ultra-light body armor, “but because of the stretchiness of the silk we don’t know that we could actually make something like [armor] out of the fiber,” Ayoub says.

The shape of the proteins that make up the silk may explain the silk's toughness. The components of the proteins, the amino acids, are arranged highly repetitively. Long stretches of the amino acid alanine stack on top of each other to make what are called beta sheets: these sheets are thought to make the silk strong, Ayoub says. Glycine and proline amino acids form kinks in the protein, which may explain why the silk is stretchy. Collagen contains proteins with similar structures.

Ayoub and her colleagues found that the genes that code for the silk are unusual. The genes are long—almost 10,000 base pairs—and yet they have no introns, regions sometimes referred to as junk DNA. Ayoub says: “That’s very unusual. You’d rarely see a gene of that size not have an intron.”

There is some evidence to suggest that highly-expressed genes—ones that are used more often—have smaller and fewer introns. Introns may slow down transcription. “So because these genes, we think, are highly expressed—they are making a lot of silk—it could be that there is selection against having introns, but that’s just a theory,” says Ayoub.

When will dragline silk hit the market? The question is finding the right surrogate silk-maker. “We can take the silk gene and put it in another organism,” Ayoub explains. For instance, the gene can be inserted in a tomato plant and the tomato can be instructed to make the proteins in its seeds. Ayoub says: “Then we can use some kind of artificial spinning apparatus that mimics the spider to spin a fiber from that extracted protein.”

-Flora Lichtman

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