Transgenic Silk

Silk is an amazing biomaterial, cultivated and prized for more than 5,000 years. The silk threads that we weave into our shiny fabrics are actually enormous protein crystals produced by insects. This industrial silk that you can buy at the mall is made by silkworms, which use the silk to form the cocoon that protects them as they transform into moths. Many other species of insect also produce silk proteins to protect themselves or their eggs, get around, or catch their prey, but none in such enormous quantity in such easy to harvest packages as the silkworm. Silks from different species are similar in many of the protein crystalline properties, with a range of elasticity and strength depending on the use of the silk, but rather than being the result of evolutionary relatedness between different insects, the different silk genes themselves are all the result of convergent evolution, independently evolved in multiple lineages. The genes that code for the silk protein, enormous and incredibly repetitive monstrosities, arose in all the separate insect species through many gene duplication events of shorter sequences, the silk protein becoming stronger and more elastic with each doubling. The protein sequences alternate between the amino acids glycine, alanine, and serine, whose small size allows them to pack tightly against each other into the crystalline form of the silk fiber.

These repeats in the silk gene give the protein its powerful characteristics, but make it extremely difficult to work with using molecular biology techniques. Even the sequencing of such a repetitive gene is a herculean task--sequencing of large stretches of DNA is typically done by breaking up large DNA strands and sequencing many much shorter chunks, using a computer to assemble the overlapping sequences into the full-length gene sequence at the end. When almost all the fragments overlap, it's nearly impossible to assemble a sequence. The tools used to cut, copy, and paste DNA pieces together for molecular biology and recombinant DNA experiments rely on the existence of unique DNA sequences that can be recognized and amplified by enzymes. Without any unique sequences the enzymes get confused and you end up with DNA gibberish.

There has been some progress in the last ten years, however, in manipulating and engineering silk proteins in insects and in transplanting pieces of silk genes into other types of cells. Silkworm silk is made up of two silk fibers, the incredibly repetitive fibroin heavy chain which makes the bulk of the crystal, and the much smaller and more genetically and chemically diverse fibroin light chain which plays a supporting role in the fiber structure. Because the light chain gene can be easily copied, cut, and pasted into different arrangements and fused to other genes, many researchers have worked on attaching different proteins to the light chain and then replacing the natural light chain gene with the new fusion. This creates silkworms with different kinds of proteins expressed in the silk fiber. Fluorescent silks can be made by fusing a green fluorescent protein (GFP) to the light chain gene and injecting the gene into a mutant strain of silkworms that is missing the light chain gene. The mutant strain produces very weak, brown, non-fluorescent cocoons (figure b), but the silkworms that have the fusion gene re-introduced produce fluorescent, but otherwise normal cocoons (figure c). When woven into tapestries, these fluorescent silks can be used to encode secret messages and designs, seen only under UV light!

A lot of scientists are also working on a different kind of transgenic silk, one produced by cells in vats that can be grown at industrial scale like yeasts used in brewing. This is especially the case for spider silks, the strongest material on earth, natural or human-made, but which cannot be produced industrially. An industrial-scale source of such a material would be useful for producing many different kinds of powerful but biodegradable products. However, spiders cannot be kept in captivity the way that silkworms can (and thank goodness for that, I am actually very glad that there aren't any factories full of spiders anywhere in the world!), and any attempt to breed and domesticate them leads to spiders killing themselves and each other. Pieces of the enormous and repetitive spider silk genes have been expressed in many different cells (it's still impossible to grab the whole gene), including E. coli, Salmonella, mammalian cells, insect cells, even goats that secrete the silk fibers in their milk! Insect cells in culture expressing the spider silk gene fragments will actually start to fill up with strands of protein fiber (you can see these fibers inside of a single cell in the figure up there on the left). As scientists develop better techniques for synthesizing and manipulating large and repetitive genes, and better techniques for collecting and spinning the fibers formed inside these transgenic silk cells, the synthetic spider silk gets closer and closer to the natural super-strong spider silk, but we're still not quite there yet.

Biological fibers like cotton, linen, silk, and wool (to name just a few) are versatile, renewable, biodegradable and have been useful for thousands of years. Genetic engineering tools can make different kinds of biomaterials that can perhaps replace some of the materials made with fossil fuels today, or lead to entirely new biotechnological applications, like silk scaffolds for tissue and organ engineering, implantable medical devices, bendy and biodegradable electronics, and other new connections between the soft world of biology and the hard world of industry.