Spider silk is prized for its strength. But spiders don’t make enough of the tough fibers to harvest for industrial uses. So researchers and biotech firms have turned to bioengineering silk with genetically modified cells or animals. These methods produce water-insoluble silk proteins, which then need to be spun into fibers in the presence of harsh solvents—a process quite unlike the one spiders use.
A team of Swedish researchers led by Anna Rising and Jan Johansson of the Swedish University of Agricultural Sciences and the Karolinska Institute has now come up with a process for making silk that more closely mimics what spiders do (Nat. Chem. Biol. 2017, DOI: 10.1038/nchembio.2269).
Spider silk proteins are made of an N-terminal domain and a C-terminal domain bracketing a region of repeated amino acids, which gives the silk its strength. “The N-terminal domains are generally quite water soluble, but some C-terminal domains aren’t very soluble at all,” Johansson says. To make their artificial silk, the researchers used the N-terminal and repetitive regions from one spider species and a relatively water-soluble C-terminal region from another species.
The team then engineered bacteria to produce the protein, which was soluble at concentrations as high as 500 mg/mL. To “spin” nearly a kilometer of silk fiber, the researchers pumped a pH 7.5 solution of their protein through a glass capillary into a pH 5 aqueous buffer, mimicking the process spiders use, where the low pH in a spider’s spinning ducts helps trigger fiber assembly.
Spider silk expert Randy Lewis of Utah State University says the team’s use of a pieced-together silk sequence was novel, but “it remains to be seen” if it will work for spider proteins with longer repetitive regions.