ABC toxins are large, tripartite protein toxins secreted by a wide range of bacteria. They are the dominant virulence factors in many insecticidal bacteria, where they function as cytotoxins, targeting the actin cytoskeleton of susceptible host cells and triggering apoptotic cell death. Genes encoding related toxins have also been identified in bacterial pathogens of significance to human health. We have recently combined single particle EM, X ray crystallography, small angle X-ray scattering and homology modelling to solve the first composite structure of a prototypical ABC toxin isolated from the bacterium Yersinia entomophaga.1,2,3 The structure is a highly novel assembly comprised of 6 proteins associated in a 5:5:5:5:1:1 stoichiometry for a total of 22 polypeptide chains and a molecular mass of 2.3 MDa. The complex contains a pentameric, channel forming subunit, capable of introducing a transmembrane pore, which recognises specific cell surface receptors, and also incorporates a cooperatively folded chaperonin which encapsulates the active cytotoxin. Our studies have revealed a completely novel and unanticipated mechanism by which bacteria manufacture, store and target cytotoxic molecules to specific, extracellular locations. Our structure is also the first example of an RHS (rearrangement hot spot)-repeat-containing protein, a widely distributed protein family whose function has been a mystery until now. Finally, similarities between the ABC toxin machinery and a family of neuronal signalling molecules are suggestive of a previously unknown functional parallel with the way in which neuronal proteins package inter-cellular signalling molecules.