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.