It seemed likely that the major difference between these structures would be the arrangement of the domain equivalent to TyeA as this is a separate polypeptide in the Yersinia structure. Surprisingly, the most striking difference instead involves the long á-helix (α9) in the central domain of MxiC, which is straight in all of our MxiC structures, but possesses a sharp kink in YopN when complexed with TyeA (Fig. 3b). It is the straightening of this helix that results in a reorientation of the C-terminal domain of MxiC, compared with TyeA, and opens one face of the molecule. As the sequence at the hinge is not conserved, and all MxiC molecules possess the straight helix conformation, these structures may represent genuine differences between species. Alternatively, it may represent a conformational switch that, in this case, was captured due to the very different pH for the YopN–TyeA structure (pH 10.5) compared with the MxiC structures (pH 6.5–7.5).