Auto-crosslinking sporesilk fibers promote endospore and Cry toxin clustering

Bacillus thuringiesis infects and kills insect larvae via its spores and associated entomotoxin-containing parasporal bodies (PSBs). We show that endospores and PSBs of B. thuringiensis Sv. Israëlensis are covered in a ‘sporesilk’ matrix that consist of 8 nm wide fibers with a double helical symmetry, formed by protofilaments of stacked alphahelical hairpins. These alpha endospore appendages (‘A-ENA’) are stabilized by up to ten autocatalytic and proximity-induced intermolecular isopeptide bonds per subunit, forming a continuous covalent polymer with remarkable chemical and physical robustness. We show A-ENA functions as B. thuringiensis virulence factor, increasing insecticidal activity by clustering spores and PSBs into an infectious clumps. Moreover, we demonstrate the recombinant production and self-assembly of A-ENA nanofibers, and show that the exogenic addition of A-ENA fibers to B. thuringiensis strains natively lacking sporesilks results in spore-PSB clustering and gain of virulence, enabling the rational, non-GMO functionalization of these biological pest control agents. Authors reveal that protein nanofibrils govern assembly of bacterial spores and extrasporal toxins into insecticidal aggregates. Spores and parasporal bodies are incased in a “sporesilk” matrix that drives clustering of infectious co-assemblies. This work provides a non-GMO strategy for developing biological pest-control agents.