Abstract:
ABC toxin complexes (Tcs) are a class of large, multi-subunit protein complexes produced by bacteria. They are widespread in pathogens of insects, and are also present in several mammalian pathogens. These complexes typically contain three major proteins: TcA, TcB, and TcC. The active toxin complex is made up of a pentameric assembly of the TcA protein plus a single copy of TcB and TcC, with the C-terminal domain of TcC harbouring the toxin functionality. This study characterises the structure of the TcB and TcC proteins from Yersinia entomophaga (YenB, YenC1, and YenC2), as well as two associated chitinases (Chi1 and Chi2), by X-ray crystallography. These structures are then docked into the lower-resolution electron microscopy map of the complete toxin complex produced by collaborators. The structures of Chi1 and Chi2 show that these chitinases adopt a TIM-barrel fold, typical of glycosyl hydrolases. The active site residues are intact, and these enzymes are bound to the complex in such a way that leaves the chitin-binding groove exposed and allows the proteins to function as chitinases while remaining part of the complex. The TcB protein (YenB) and either of the TcC proteins (YenC1 or YenC2) form a complex when co-expressed, and the TcC protein is cleaved at a specific residue into N- and C-terminal regions. At low pH, the C-terminal region dissociates from the complex and precipitates, and the structure of the remaining YenB/YenC2NTR complex has been determined. These proteins form a large, hollow shell composed of a long, continuous strip of β-sheet that spirals around a central cavity. This is a new protein fold, and represents the first high-resolution structure of a TcB protein, a TcC protein, and of any protein that contains rearrangement-hot-spot (RHS) repeats. As such, this structure has far-reaching implications for the understanding of a widespread and diverse family of proteins.