Structural insights into Tc toxins from human and insect pathogenic bacteria
Creators: name: Leidreiter, Franziska
Publisher: name: Technische Universität Dortmund
Year: 2019
Subjects: subject: Cryo-EM
subject: Bacterial toxins
subject: Pore-forming toxins
subject: Structural biology
subject: 570
subject: 540
subject: Mikrobiologie
subject: Toxin

description: Many pathogenic bacteria produce virulence factors that act as pore-forming toxins (PFTs) on the cytoplasmic membrane of target hosts. PFTs can be distinguished in binary (AB) toxins that actively transport a toxic counterpart into the cell and in conventional PFTs, which perforate the host membrane and translocate ions. Upon membrane interaction, the PFTs provoke different effects such as abolishing the transmembrane potential, altering signal pathways or disruption of the cell cytoskeletal. Ultimately the attack of PFTs leads to cell death. Amongst the PFTs is the family of the tripartite toxin complex (Tc) toxins that act as Alpha-helical PFTs. Tc toxins are composed of three components: namely TcA, TcB and TcC; and only the tripartite complex (A5BC) is biologically active. Tcs use a special syringe-like mechanism to attack the respective host cell and translocate a toxic enzyme into the host cytosol. TcA forms the membrane-perforating translocation channel and the toxic enzyme is located at the TcBTcC dimer. Tc toxins exist in a wide range of bacteria including insect, plant and human pathogenic organisms. However, so far, only the structure of the entomopathogenic Photorhabdus luminescens Tc toxins has been studied in great detail. Therefore, it is still unclear if other insect or human pathogenic homologues have a similar architecture and use a common mechanism. To unravel the mechanism of action and host specificity of these toxins, here the near-atomic resolution structures of three TcAs from human and insect pathogenic bacteria were solved using electron cryo microscopy; Yp-TcaATcaB from Yersinia pseudotuberculosis (3.3 Å), Mm-TcdA4 from Morganella morganii (3.3 Å) and Xn-XptA1 from Xenorhabdus nematophila (2.8 Å). The structures revealed that the overall composition and domain organization of the three TcAs is similar to the well-studied TcdA1 from P. luminescens. They assemble as a pentameric bottle-shaped structure with a central Alpha-helical channel surrounded by an outer shell composed of a conserved Alpha-helical domain and more variable Beta-sheet receptor-binding domains, implying species-specific receptor interactions. Functionally crucial structural features for the mechanism of action of P. luminescens TcdA1, i.e. the linker domain, the translocation channel, a molecular knot and the TcB-binding domain show high structural similarities in all analyzed TcAs. The analyzed TcAs form ion conductive pores and the pore state could be induced by a pH shift in vitro for all studied TcAs. Interestingly, fully functional chimeric holotoxins can be formed, combining TcAs and TcBTcC cocoons from different organisms. The previously described electrostatic lock at the neuraminidase-like domain of TcdA1 is not conserved within the here studied TcAs. However, one conserved ionic interaction pair was identified that stabilizes the complex in the shell domain. It might act as a latch that lead to the pH-induced opening of the shell after other interacting protomer interfaces are destabilized upon pH shift to pH 4 or 11. Altogether, results obtained from this work show that Tc toxins from different organisms share a common mechanism of action, while the variability in receptor-binding domains and formation of the active pore state enable targeting of different hosts. Thus, this work presents the first detailed structural insights into multiple TcAs derived from various prokaryotic organisms.