Spider silk proteins have medical and research uses and are remarkably soluble.
Spider silk is lightweight, strong, and nontoxic. It has biomedical applications in surgery, transplants, and as a matrix for culturing cells. The genes for spidroins, which are spider silk proteins, are known. Generating fibers is challenging, however. To make spider silk in vitro, spidroins must be kept soluble at concentrations as high as 50% weight/volume, then quickly converted to threads.
A process for making kilometer-long fibers of spider silk was recently described in Nature Chemical Biology by Senior Researcher Anna Rising and Professor Jan Johansson, Swedish University of Agricultural Sciences in Uppsala and Karolinska Institute in Stockholm. In earlier studies, the scientists and their colleagues studied spidroin structures and identified pH and mechanical conditions to convert spidroins into fibers. They used this knowledge to make silk fibers in the lab.
“We found a way to design new spider silk proteins,” says Rising, “so they can be expressed in bacteria and kept in solutions as high as 50%, as they are in spider silk glands.” The N-terminal and C-terminal ends of spidroins regulate solubility and the middle domains are involved in aggregating and forming fibers. The researchers surveyed spidroin genes from different spiders and selected N- and C-terminal domains that would generate a hypersoluble recombinant “minispidroin”.
To create fibers, the researchers mimicked a spider’s physiological mechanism by pumping a minispidroin solution through a capillary into a low-pH buffer. This method can turn the recombinant minispidroin from 1 liter of bacterial culture into an unbroken fiber of 1 km or more. Chemical and physical analyses confirmed that the fibers have the conformation of natural spider silk, and bear stress and strain in a similar way.
Spidroin solubility domains
The fibers are ideal for medical and research uses, because they are produced using physiological buffers and conditions. The scientists are now finding ways to turn the fibers into matrices and three-dimensional structures for research and clinical applications.
They are also studying spidroin domains because, as Johansson says, “Spiders are really good at keeping proteins soluble in high concentrations.” People who generate proteins industrially spend a lot of time, energy, and money preventing protein aggregation. The researchers are now collaborating with the Italian pharmaceutical company Chiesi to produce a surfactant protein with spider silk domains for solubility, as a treatment for the lung surfaces of premature infants.
Finally, spidroins are an excellent model for some human diseases. “By studying the mechanisms that keep spidroin in solution,” Johansson says, “we can also learn about protein aggregation, as it occurs in neurodegenerative disease.”
Andersson M, Jia Q, Abella A, Lee X-Y, Landreh M, Purhonen P, Hebert H, Tenje M, Robinson CV, Meng Q, Plaza GR, Johansson J, Rising A. Biomimetic spinning of artificial spider silk from a chimeric minispidroin. Nature Chemical Biology 13: 262–264
Photo: Kerstin Nordling (From left: Anna Rising and Jan Johansson)