Scrutinizing Stator Rotation in the Bacterial Flagellum: Reconciling Experiments and Switching Models

The bacterial flagellar motor is one of the few known rotary motors, powering motility and chemotaxis. The mechanisms underlying its rotation and the switching of its rotational direction are fundamental problems in biology that are of significant interest. Recent high-resolution studies of the flagellar motor have transformed our understanding of the motor, revealing a novel gear mechanism where a membranous pentamer of MotA proteins rotates around a cell wall-anchored dimer of MotB proteins to turn the contacting flagellar rotor. A derivative model suggests that significant changes in rotor diameter occur during switching, enabling each MotA₅MotB₂ stator unit to shift between internal and external gear configurations, causing clockwise (CW) and counterclockwise (CCW) motor rotation, respectively. However, recent structural work favors a mechanism where the stator units dynamically swing back and forth between the two gear configurations without significant changes in rotor diameter. Given the intricate link between the switching model and the gear mechanism for flagellar rotation, a critical evaluation of the underlying assumptions is crucial for refining switching models. This review scrutinizes key assumptions within prevailing models of flagellar rotation and switching, identifies knowledge gaps, and proposes avenues for future biophysical tests.