by Katie Galloway
In the 2008 comedy Get Smart, Maxwell Smart, aka Agent 86, is armed with an array of crazy spy gadgets, including a grappling gun outfitted with super strong spider silk line. The tech guys at CONTROL explain to Smart how they made this light-weight yet sturdy cable, “It’s a time-consuming, labor-intensive technology. The spiders have to be individually milked…and they do not like it!”
Spider silk has long represented the holy grail of biomimetic materials. By weight this thread is three times stronger than steel. However, as alluded to by the technicians in Get Smart, our “inability to domesticate spiders has driven numerous attempts to artificially manufacture spider dragline silks for industrial and medical applications.”1 In other words, spider wrangling has never been a viable profession.
Overcoming the bioengineering challenge of synthesizing spider silks has proven difficult at many levels. First there is the challenge of isolating and cloning the spider silk genes. Given the high number of repetitive regions that give the silks their strength and elasticity, traditional cloning strategies are prone to failure. Second, once these genes have been cloned within bacterial hosts, expressing and collecting the proteins is no simple matter. Most bacteria aren’t optimized to deal with highly repetitive sequences and consequently generate truncated versions of these proteins. Third, even after the proteins are expressed and purified, they’re not spider silk but merely a blob of protein with potential called “unspun silk dope.”
Once spiders have their unspun silk dope stored in their glands, they spin it by pulling it through a narrow gland called a spinarette in a process engineers call extrusion. By extruding the silk, water is removed from the dope as the silk makes the transition from a gel to a solid fiber about 2.5–4 microns thick (about 30 times thinner than the width of a human hair.) Spiders are uniquely equipped to spin these mighty mite threads, but so far synthetic spinning by extrusion has generated threads that are no smaller than 10–60 microns thick.
Despite the challenges, the amazing properties of these biomaterials make them an attractive target for bioengineers. Not only are spider silks highly elastic, lightweight, and extremely strong, some have even been shown to facilitate nerve regrowth in mammalian cells. Consequently, teams of people around the country and the world continue working to generate synthetic spider silks.
In one of the more promising recent attempts, an international team described how they generated and spun recombinant spider-like proteins. Spider silk proteins alternate in composition between crystalline and amorphous regions. The exact sequence of these regions dictates the mechanical properties of the spider silk. Starting with spider silk-like crystalline domains flanked by elastic and helical regions, the researchers cloned various proteins into Escherichia coli expression vectors. They were then able to express (produce) the silk proteins by introducing these DNA cassettes called vectors into the E. coli bacterial host. To allow for sequence diversity and easy scale-up, the researchers used a special cloning trick to make repetitive cloning of these subunits simple. This neat scheme allowed them to “mix-n-match” domains to create a wide array of protein combinations from the subunits.
To purify the spider silk-like proteins from all the other proteins produced by the bacteria, the team cloned a special removable protein sequence called a polyhistidine-tag to the end of the protein. This sequence of ten histidine amino acids binds to metals such as nickel and cobalt. The silk proteins were separated from the bulk of the cellular proteins through a process called immobilized metal affinity chromatography (IMAC). Running the cell lysates (broken, open cells) over a nickel resin with the histidine-tag immobilizes the silk-like proteins on the column while the non-specific proteins were washed away. The silk-like proteins were then recovered by adding a protease solution that released them from the column-bound histidine-tag.
The team then dissolved the proteins into deionized water and HFIP (Hexafluoro-2-propano), an organic solvent commonly used to solubilize biopolymers. The silk dope was extruded through a stainless-steel spinarette into an isopropanol bath to create uniform spider silk-like fibers. This process took a mere forty days from start to finish but would be longer if researchers wanted more than two “mix-n-matched” domains in the final silk. However, once the desired sequence is cloned into in bacterial vectors, the harvesting and spinning process can be completed in approximately 15 days.
Finding ways to readily produce spider-like silks will continue to be a hot topic in the fields of biomimetics and biomaterials. The value of this special material, combined with the difficulty encountered by spider wranglers in obtaining natural silks, demands a synthetic alternative. However, as we learn from nature and attempt to repeat the wonders found therein, the intricacy of the solutions is astounding. A simple garden spider can achieve in minutes what takes teams of people using toxic chemicals and rigorous protocols weeks to accomplish. Even Hollywood movies recognize the challenge of making spider silk.
But spiders don’t need a whole host of technicians and scientists to catch their next meal. God has endowed them with unique abilities to fulfill their role as predators of the insect world. By learning from nature, not only can we obtain new technologies to benefit mankind, but we can see how God has provided for even the lowliest of creatures by granting them extraordinary traits. In Matthew 6 Jesus reminds us how valuable we are in comparison to the birds and the flowers. In light of how the Creator has provided for the spiders, we might paraphrase the analogy, “Consider the spiders of the attic and the wild, they have no formal training, yet they weave beautiful webs that yield them dinner. How much more will God provide for you?”
- Florence Teul et al., “A Protocol for the Production of Recombinant Spider Silk-Like Proteins for Artificial Fiber Spinning,” Nature Protocols 4 (February 1, 2009): 341–55.