Structure of the NheA Component of the Nhe Toxin from Bacillus cereus: Implications for Function

The structure of NheA, a component of the Bacillus cereus Nhe tripartite toxin, has been solved at 2.05 A resolution using selenomethionine multiple-wavelength anomalous dispersion (MAD). The structure shows it to have a fold that is similar to the Bacillus cereus Hbl-B and E. coli ClyA toxins, and it is therefore a member of the ClyA superfamily of alpha-helical pore forming toxins (alpha-PFTs), although its head domain is significantly enlarged compared with those of ClyA or Hbl-B. The hydrophobic beta-hairpin structure that is a characteristic of these toxins is replaced by an amphipathic beta-hairpin connected to the main structure via a beta-latch that is reminiscent of a similar structure in the beta-PFT Staphylococcus aureus a-hemolysin. Taken together these results suggest that, although it is a member of an archetypal alpha-PFT family of toxins, NheA may be capable of forming a beta rather than an alpha pore.