The mechanism of action of Tc toxins at high resolution | New Voices in Infection Biology

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Talk abstract:

Bacterial toxins play important roles in infectious diseases and can serve as alternative to insecticides. The Toxin Complex (Tc) of the insect pathogen Photorhabdus luminescens is a modular toxin system that is composed of a pentameric membrane translocation syringe (TcA) and a cocoon (TcB, TcC) that encapsulates a deadly toxic enzyme, an ADP-ribosyltransferase (TcC C-terminus). After target cell binding, the Tc toxin is endocytosed and the pH-dependent conformational transition of TcA from the soluble prepore to the membrane-embedded pore follows. Subsequently, the toxic enzyme is translocated from the cocoon through the TcA channel into the cytoplasm.

Here, I present mechanistic details of the individual steps of the intoxication mechanism. The first step, holotoxin assembly, requires the unfolding and refolding of a b-propeller that forms the TcA-TcB interface and acts as a gatekeeper for the transport of the toxic enzyme. The second step, cell binding, is facilitated by two different types of glycans as receptors for Tc. Cryo-EM structures of TcA-glycan-complexes reveal different glycan binding sites, which suggest a two-step cell association model. The third step, prepore-to-pore transition, is driven by the contraction of five linker domains in the TcA pentamer. Using a combination of structural biology, single molecule Förster resonance energy transfer and electron paramagnetic resonance spectroscopy, we revealed the timing and probabilities of two individual steps of prepore-to-pore transition, shell destabilization and channel insertion of TcA. Following channel insertion into the target cell membrane, the final step, translocation of the toxic enzyme, proceeds without the necessity of additional factors, as shown by cryo-EM structures of Tc in lipid nanodiscs. Finally, we explored the limits and prerequisites to turn the Tc toxin into a universal protein translocation device by replacing the toxic enzyme in the cocoon with heterologous proteins.