(A) tomographic slice through the sub-tomogram average of the motor complex. SL, S-layer; PD; periplasmic densities; CM, cell membrane; MC, motor complex; CR, cytosolic ring; PC, polar cap. Arrowheads indicate two of six narrow connections between MC and CM. (B–D) segmented 3D surface representation of the sub-tomogram average of the MC (multiple colours) as seen from the side (B), the cytosol (C) and the cell membrane (D). Yellow, blue, green, central complex; purple, cytosolic ring. (E,F) S. acidocaldarius FlaI (PDB-4IHQ, gold) and Symmdoc model of S. acidocaldarius FlaH (PDB-4YDS, cyan) fitted into the MAC density in side view (E) and in cross-section through FlaH (F); position of cross-section shown as dotted line in E. Scale bars, 20 nm.