Seeking the secrets of spider silk

A few years ago I wrote a column to which, much to my delight, I was able to assign the rather B-movie-ish title “Spider-Goat Clones of Montreal.”

The column described how a Montreal company called Nexia Technologies had cloned goats that had been genetically engineered to produce spider-silk proteins in their milk. Nexia retrieved the proteins and used it to make a synthetic form of spider silk called BioSteel. Their hope was to produce bulk spider-silk fibers for use in all kinds of applications, from body armor to medical sutures.

They’re still struggling with the project–among other things, the company has recently undergone restructuring, which isn’t doing their research efforts any good.

But other researchers are still on the spider-silk case, because the potential uses are enormous. Drag-line spider-silk–the silk spiders dangle on specifically to scare the living daylights out of you when you happen to walk into them–is six times stronger than steel fiber of equal diameter. Just as importantly, it’s immensely stretchy. It could be used for extremely thin sutures for eye or nerve surgery, or to make artificial ligaments and tendons, body armor, ropes, fishing nets–even airbags that are less punishing to the people they’re protecting.

Nexia was able to create a spider-silk-protein-producing goat because a team of researchers at the University of Wyoming successfully discovered and mapped the genes responsible for spider silk in 1989. Those researchers, led by Randy Lewis, a professor of molecular biology, are undoubtedly the world’s leading authorities on spider-silk genes. Since 1989, they’ve cloned and sequenced the genes from 26 different species of spiders.

The team inserts small segments of spider DNA they hoped contain the silk-producing genes into bacteria, then waits to see if the bacterial colonies produce silk proteins. Unfortunately, while bacteria produce enough silk proteins for research purposes, they don’t produce enough for commercial purposes. Hence the Nexia spider-goat clones.

Even the goat-milk method is less than ideal, though. Suppose you wanted to use Nexia’s special goat milk to produce a 2.25-kilogram bullet-proof vest (spider silk is tougher than Kevlar but weighs only a third as much). You’d better get milking: you need around 2,280 litres of milk to recover enough silk. That’s the daily production of 200 goats.

The University of Wyoming team, not surprisingly, is looking for other means of producing spider silk. They’re considering introducing the silk gene into alfalfa, for example. Other researchers have looked at introducing the silk genes into tobacco plants, an attractive idea because it could allow tobacco farmers to stay in business without contributing to a public health nuisance.

In November an Israeli-German scientific team announced it had succeeded in producing spider-silk fiber in cultures of insect cells. They placed spider-silk genes in an insect-infecting virus, then used the virus to infect cultures of cells from a type of caterpillar called the fall armyworm.

(And if you’re wondering, “Why can’t you just farm spiders?”, there’s a very simple answer: spiders are cannibalistic. If you put two spiders into an enclosure, before long you only have one fat spider–and there goes your silk production.)

Dr. Lewis and his team are interested in more than just producing everyday spider silk. They want to not only duplicate nature’s work, but improve on it. They’ve been able to produced synthetic genes that result in spider silk that’s even stronger and more flexible than that produced by real spiders. Eventually, spider silk manufacturers may be able to dial up silk with precisely the tensile strength and elasticity required by customers.

Although, even there, the researchers may be duplicating nature. As Thomas Scheibel of the Department of Chemistry of the Technische Universität in Munich points out, there are more than 34,000 known species of spider, and each creates its own different kinds of silk with different mechanical properties for different purposes.

And despite all these promising developments, researchers have yet to figure out the best way to spin spider silk. That’s because they don’t really know how spiders do it: somehow, with their spinnerets, they apply physical force to a liquid protein and turn it into silk. Natural spider silk is only 2.5 to 4 micrometres in diameter; the ones scientists have managed to spin are anywhere from 10 to 60 micrometres in diameter.

Nature, it seems, still has a leg up–or, in this case, eight legs–on us.

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