Mechanism of Tc toxin action revealed in molecular detail

Dominic Meusch, Christos Gatsogiannis, Rouslan G. Efremov, Alexander E. Lang, Oliver Hofnagel, Ingrid R. Vetter, Klaus Aktories, Stefan Raunser
2014 Nature  
Tripartite Tc toxin complexes of bacterial pathogens perforate the host membrane and translocate toxic enzymes into the host cell, including in humans. The underlying mechanism is complex but poorly understood. Here we report the first, to our knowledge, high-resolution structures of a TcA subunit in its prepore and pore state and of a complete 1.7 megadalton Tc complex. The structures reveal that, in addition to a translocation channel, TcA forms four receptor-binding sites and a
more » ... like region, which are important for its host specificity. pH-induced opening of the shell releases an entropic spring that drives the injection of the TcA channel into the membrane. Binding of TcB/TcC to TcA opens a gate formed by a six-bladed b-propeller and results in a continuous protein translocation channel, whose architecture and properties suggest a novel mode of protein unfolding and translocation. Our results allow us to understand key steps of infections involving Tc toxins at the molecular level. Tc toxin complexes are virulence factors of many bacteria such as the plague pathogen Yersinia pestis and the insect pathogen Photorhabdus luminescens 1-3 . Excreted by the bacteria as soluble proteins, Tcs bind to the cell surface, are endocytosed and perforate the host endosomal membrane by forming channels that translocate toxic enzymes into the host 4,5 . This damages and ultimately kills the target cells. Tcs with specificity for insects are receiving great interest as potential biopesticides for expression in transgenic plants as alternatives to Bacillus thuringiensis toxins 6,7 . Tc toxins are composed of TcA, TcB and TcC subunits 4,8 . TcB and TcC together form a closed cage, in which the cytotoxic domain of TcC is autoproteolytically cleaved 9 . TcA forms a large bell-shaped pentameric structure 5,10,11 and enters the membrane like a syringe, forming a translocation channel through which the cytotoxic domain is probably transported into the cytoplasm 5 . This unique mechanism differs from that of typical pore-forming toxins 12 or other toxins that form translocation pores, for example, diphtheria 13 and anthrax toxin 14 . However, little is known about the force that drives this mechanism and how the three components interact with each other and with the host membrane to unfold and translocate the toxic domain. A greater understanding requires knowledge of the structures of the Tc complex and its subunits in different states, that is, before and after membrane insertion. We use a hybrid approach combining X-ray crystallography and electron cryomicroscopy (cryo-EM) to determine the crystal structures of the prepore TcA (TcdA1) subunit and TcB-TcC (TcdB2-TccC3) subunits, the cryo-EM structures of TcA in its pore state and the complete Tc toxin complex PTC3.
doi:10.1038/nature13015 pmid:24572368 fatcat:ik4vew5gyndfzm27leokq4ebzm