The ABC toxin complexes (Tc) produced by certain bacteria are of interest due to their potent insecticidal activity 1,2 and potential role in human disease 3 . These complexes comprise at least three proteins (A, B and C), which must assemble to be fully toxic 4 . The carboxy-terminal region of C is the main cytotoxic component 5 , and is poorly conserved between different Tcs. A general model of action has been proposed, in which the Tc binds to the cell surface via the A protein, is endocytosed, and subsequently forms a pH-triggered channel, allowing the translocation of C into the cytoplasm, where it can cause cytoskeletal disruption in both insect and mammalian cells 5 . Tc complexes have been visualised using single particle electron microscopy 6,7 , but no high-resolution structures of the components are available, and the role of the B protein in the mechanism of toxicity remains unknown. Here we report the three-dimensional structure of the complex formed between the B and C proteins, determined to 2.5 Å by X-ray crystallography. These proteins assemble to form an unprecedented, large hollow structure that encapsulates and sequesters the cytotoxic, carboxy-terminal region of the C protein like the shell of an egg. The shell is decorated on one end with a -propeller domain, which mediates attachment of the B/C heterodimer to the A protein in the native complex. The structure reveals how C auto-proteolyses when folded in complex with B. The C protein is the first example of a structure that contains RHS (rearrangement hot spot) repeats 8 , and illustrates a striking structural architecture that is likely conserved across both this widely distributed bacterial protein family and the related eukaryotic YD-repeatcontaining protein family, which includes the teneurins 9 . The structure provides the first clues about the function of these protein repeat families, and suggests a generic mechanism for protein encapsulation and delivery.ABC toxins were first identified, and have been best characterised, in the bacterium Photorhabdus luminescens 1 . However, the entomopathogenic bacterium Yersinia entomophaga contains a related Tc locus that includes an A component encoded by two ORFs (yenA1 and yenA2), a single B gene (yenB) and two C genes (yenC1 and yenC2), 10 the products of which associate independently with the A and B proteins, giving rise to two Tcs from one genetic locus. The C proteins of this and other Tcs are similar to the "polymorphic toxins" described by Zhang et al. 11 as they have a conserved RHS-repeat-containing amino-terminal region and a variable carboxyterminal region 10 . The carboxy-terminal regions (CTRs) of the Y. entomophaga C proteins are predicted to have different toxic activities: the C1 CTR is homologous to cytotoxic necrotising factor 1 (CNF1) from Escherichia coli 12 , whereas the C2 CTR is homologous to the deaminase YwqJ from Bacillus subtilis 13 . As in related complexes 3 , when the B subunit is co-expressed with either of the C subunits, C is cleaved at the boundary between th...

Toxin complex (Tc) proteins are a class of bacterial protein toxins that form large, multisubunit complexes. Comprising TcA, B, and C components, they are of great interest because many exhibit potent insecticidal activity. Here we report the structure of a novel Tc, Yen-Tc, isolated from the bacterium Yersinia entomophaga MH96, which differs from the majority of bacterially derived Tcs in that it exhibits oral activity toward a broad range of insect pests, including the diamondback moth ( Plutella xylostella ). We have determined the structure of the Yen-Tc using single particle electron microscopy and studied its mechanism of toxicity by comparative analyses of two variants of the complex exhibiting different toxicity profiles. We show that the A subunits form the basis of a fivefold symmetric assembly that differs substantially in structure and subunit arrangement from its most well characterized homologue, the Xenorhabdus nematophila toxin XptA1. Histopathological and quantitative dose response analyses identify the B and C subunits, which map to a single, surface-accessible region of the structure, as the sole determinants of toxicity. Finally, we show that the assembled Yen-Tc has endochitinase activity and attribute this to putative chitinase subunits that decorate the surface of the TcA scaffold, an observation that may explain the oral toxicity associated with the complex.

ABC toxins are pore-forming virulence factors produced by pathogenic bacteria. YenTcA is the pore-forming and membrane binding A subunit of the ABC toxin YenTc, produced by the insect pathogen Yersinia entomophaga . Here we present cryo-EM structures of YenTcA, purified from the native source. The soluble pre-pore structure, determined at an average resolution of 4.4 Å, reveals a pentameric assembly that in contrast to other characterised ABC toxins is formed by two TcA-like proteins (YenA1 and YenA2) and decorated by two endochitinases (Chi1 and Chi2). We also identify conformational changes that accompany membrane pore formation by visualising YenTcA inserted into liposomes. A clear outward rotation of the Chi1 subunits allows for access of the protruding translocation pore to the membrane. Our results highlight structural and functional diversity within the ABC toxin subfamily, explaining how different ABC toxins are capable of recognising diverse hosts.