Insects use the fibrous proteins we know as silks in many different ways. Spiders weave them into webs, while larvae of butterflies and moths often wrap themselves up inside them to pupate and undergo metamorphosis. One species of moth found across Asia, the the paulownia bagworm, Eumeta variegata, uses silk at all life stages including the larval development stage in which it uses silk to wrap itself in leaf and twig fragments and other detritus from its environment for protection from invertebrate predators and to hang by silken threads during this stage until it emerges as an adult Psychid moth. Moreover, the female, vermiform (wormlike) adults remain in their bag during mating. The silk threads are also then used for ballooning the freshly hatched larvae.
Writing in the journal Communications Biology, a team from Keio University, in Yamagata, Japan, describe this quite unique adaptation as inspirational: "An understanding at the molecular level of bagworm silk, which enables such unique purposes, is an opportunity to expand the possibilities for artificial biomaterial design," they write. [Kono, N. et al., Commun. Biol. (2019) 2, 148; DOI: 10.1038/s42003-019-0412-8]
The bar is set high for this endeavor as very little was known until now about the bagworm fibroin proteins. As such, the team has gone back to basics, unraveling the bagworm genome, all 700 million base pairs of it. Within that double helix of genetic information therein lie the genes for bagworm fibroin, the protein silk.
Intriguingly, the fibroin gene they have identified for this moth species has a unique repetitive motif. It is this repetitive motif that translates to a repetitive structural motif in the protein, of course, and endows the bagworm's silk with its particular mechanical properties. The team's tests of those mechanical properties demonstrate what they refer to as a phylogenetic relationship between the repetitive motif in this remarkable long silk gene and the tensile strength of the express protein, the silk.
"The poly-alanine and alternating glycine-alanine motifs constitute a beta-sheet structure, and the crystal structure makes silk that is strong, rigid and tensile in exchange for its elasticity," the team reports. The motifs are related to those in other moths of the family as well as being present in spider silk, spidroins. Materials science is perhaps a long way from emulating the wondrous properties of natural silks, but every step taken in research, especially with such a multi-tasking bagworm silk offers inspiration and its tensile strength of 636 ±55 megapascals and its extensibility of 19.5 ±4.5%.