Extended Data Fig. 7: Bidirectional rotation of the C-ring by MotA/B.

MotA subunits are labeled A – E in the panels. (a) Comparison of MotA/B^64,65 and MotA⁷⁹ cryoEM structures identifies varying levels of asymmetry that change the width of the cleft between subunits. The most symmetric structure is that of isolated MotA⁷⁹. In the context of past biochemistry and structures of MotA/B, the MotA/B stator binds to the torque helix on FliG_D5. A compelling model would use a cleft between MotA subunits, a concept with parallels to interlocking cogwheels in macroscopic motors. However, these molecular cogwheels in the chemotaxis machinery undergo shape changes during function, which may benefit from the symmetry mismatch of MotA/B. One possibility is that the open MotA/B clefts^64,65 allow rapid binding or release of the torque helix without the need for a rate-limiting induced-fit process. Cleft closure would grasp the FliG_D5 torque helix tightly. (b) Complementary electrostatics and sterics of the torque helix of FliG_D5 and the MotA/B stator. An electrostatic surface representation of the FliG_D5 domain shows that the torque helix is presented as an isolated feature and is negatively charged. (c) A schematic mechanism for bidirectional MotA/B-dependent rotation of the C-ring by moving the MotA/B binding site from the outside to the inside of the ring.