Fig. 4.

TolC, a natural variant on the α-cylinder structure. (A and B) Orthogonal views of the TolC trimer showing the barrel-like structures (Koronakis et al. 2000). The three protomer chains are distinguished by shading. (C) The two helical domains (upper and lower) of a TolC monomer, highlighting the many knobs-into-holes of the two-stranded antiparallel coiled coils of the lower barrel compared with the small number of such interactions in the upper barrel. The orientation of this figure is the same as that used in Figures 1B and 1C ▶ of the original paper describing the TolC structure (Koronakis et al. 2000). Thus, from left to right in the upper barrel, the helices are H7, H6, H3, and H2. Otherwise, the legend is the same as for Figure 3 ▶. A kinemage has also been prepared for this structure.