Dramatic domain orientation differences in the PilT C2 hexamer. (A) 6-fold symmetry is broken. Subunits E and B (light and dark blue, respectively) are displaced from the center of the hexamer. (B) Adjacent subunits E (blue) and F (orange) were superimposed over CTD residues 116–361. The NTDs of these monomers are then related by a 69° rotation about an axis through the domain linker. Helices C are labeled to highlight the exent of the movement. (C) Close-up view of the nucleotide-binding site at the three-way interface of NTDF (orange, below) and CTDF (orange, above) from the closed, liganded subunit and the CTDE (blue) from the adjacent open subunit, with refined ADP and modeled Pi shown in Fo-Fc omit electron density calculated without nucleotide (contoured at 2.5σ (blue) and 5σ (red)). Note the arginine wire. Given the low resolution of the data used for this refinement, it is appropriate to consider this figure as one model that is consistent with our data. (D) At the CTDE:CTDF interface, arginine finger residues are engaged and 950 Å2 are buried.