Engineered silkworms spin unusual amino acids into silk

Researchers have engineered silkworms to incorporate an unnatural amino acid, 4-azido-l-phenylalanine (shown, below), into silk. The azido group on this amino acid serves as a ready target for click chemistry reactions that could produce silks with a range of novel properties (ACS Synth. Biol. 2018, DOI: 10.1021/acssynbio.7b00437). “Some people said worms might not be able to survive” incorporation of an unnatural amino acid, Hidetoshi Teramoto of the National Agriculture & Food Research Organization says. First, the researchers had to get the silkworm’s protein-making machinery to pick up azidophenylalanine in addition to the usual phenylalanine and incorporate it into the major silk protein, fibroin. They generated a pool of less selective variants of tRNA synthetase, the enzyme that attaches phenylalanine to transfer RNA for transport to the ribosome. Coauthor Kensaku Sakamoto of RIKEN Center for Life Science Technologies says they then used a bacterial screening system to rapidly identify cells that incorporated azidophenylalanine into their proteins. With four candidates, the team created transgenic silkworm strains that produced the enzyme variants in their silk glands. The team analyzed the resulting cocoons, finding that in two strains, over 6% of the phenylalanine had been replaced by azidophenylalanine. Then the researchers demonstrated that the azidophenylalanine in the silk could be functionalized by conjugating the silk to fluorescent molecules using click chemistry. The cocoons glowed.