Structures of a Tc holotoxin reveal the mechanism of activation and translocation

Daniel Roderer1, Christos Gatsogiannis1, Felipe Merino1, David Balchin2, Evelyn Schubert1, Anne Kuhlee1, Manajit Hayer-Hartl2, Stefan Raunser1

1 Max Planck Institute of Molecular Physiology, Stuctural Biochemistry, Otto-Hahn-Str. 11, 44227 Dortmund
2 Max Planck Institut of Biochemistry

Bacterial toxins play important roles in infectious diseases and can serve as alternative to insecticides. The heterotrimeric Toxin Complex (Tc) of the insect-pathogen Photorhabdus luminescens is a modular toxin system that is composed of a 1.4 MDa, pentameric membrane translocator (TcA) and a 270 kDa cocoon encapsulating the toxic enzyme, an ADP-ribosyltransferase (TcB and TcC). Upon toxin activation, TcA binds to the target cell, and a pH shift triggers the conformational transition from the soluble pre-pore form to the membrane-embedded pore. Subsequently the ADP-ribosyltransferase is translocated through the channels of TcB and TcA and is released into the cytoplasm.

Here we report the overall structures of the 1.7 MDa holotoxin in the pre-pore form and in the pore form in lipid nanodiscs, obtained by electron cryo-microscopy. We show that binding of the TcB-TcC cocoon to the TcA channel triggers the opening of the cocoon and the specific loading of the toxic enzyme into the channel. The presence of the toxic enzyme inside the cocoon is essential for holotoxin formation. The enzyme passes through a narrow constriction site with alternating negative charges and hydrophobic stretches inside the cocoon, acting as an extruder that releases the unfolded protein with its C-terminus first into the translocation channel.

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