Scientists have applied genetic engineering to create proteins that can be used to create electronics. They've used the tools of molecular biology and principles of evolution to find proteins that can make new structures of silicon dioxide, commonly found in computer chips, and titanium dioxide, often used in solar cells.
Traditional genetic engineering involves sticking a foreign gene into bacteria and using the bacteria as tiny factories to make the protein encoded by that gene. This approach wouldn’t work for all silica-forming proteins found in marine sponges. The minerals produced by these proteins, which the researchers want to study, can kill the cells.
So Daniel Morse, of the University of California, Santa Barbara, and his colleagues looked to another protein making strategy: synthetic cells with a tiny plastic bead nucleus surrounded by a bubble of oil that acts as a cell membrane.
The scientists attached a piece of DNA to each of the beads, encoding a unique silica-forming protein, or silicatein. This DNA is a random combination of genes from two related silicateins, interspersed with random mutations.
Then the scientists soaked the beads in watery mixture of the bacterial proteins necessary to turn the DNA into silicateins and covered each bead with a thin layer of oil, trapping water and the enzymes inside. With the artificial cell complete, the interior enzymes made the silicateins, which stuck to antibodies covering the bead’s surface.
Next the scientists triggered a mineral-forming reaction. They broke open the “cells,” soaked them in a solution containing the silicon or titanium molecule used by these proteins, and captured them with a new oil layer.
The silicatein proteins gathered either silicon dioxide or titanium dioxide inside the oil bubble, depending on which mineral precursor they were fed. Then the cells were subjected to two “selection pressures” to weed out non-functional genes and identified those that coded for proteins which made extra strong minerals.
The scientists sorted the beads by size, collecting the largest beads with the thickest mineral layers. Then they shook the beads to break up the mineral coating. Beads that survived this process contained genes for proteins that made minerals of intermediate strength.
From the cells that survived the selection process, the scientists randomly picked 30 genes from either the silicon or titanium dioxide-forming proteins and sequenced them. Not surprisingly, the researchers found sequences common to the two original silicateins. But in each group, they also found a gene completely different from the starting proteins.
The scientists synthesized the proteins coded for by these new genes and studied the minerals produced by each one. The standard protein, silicatein α, makes clumps of silica particles. Both new proteins, however, produced dispersed nanoparticles containing the metal oxides. And the new silica-forming protein, named silicatein X1, could even make folded sheets of silica-protein fibers.
Directed evolution is not limited to these silica-forming proteins, as other organisms have proteins to make interesting materials too. Some marine sponges produce fiberglass that could be used as optical wave guides. And some bacteria build magnetic nanoparticles.
In this work, the scientists demonstrated that directed evolution of a mineral-producing protein could create materials with never-before seen structures. The next challenge is to learn how to change the selection pressures to evolve a specific property, such as semiconductor performance. “This approach will begin to allow the same DNA-based evolutionary processes that have created seashells and skeletons to be harnessed to advance human technologies,” they write.