The Helix Rearrangement in the Periplasmic Domain of the Flagellar Stator B Subunit Activates Peptidoglycan Binding and Ion Influx.

The stator of the bacterial flagellar motor couples ion flow with torque generation. The ion-conducting stator channel opens only when incorporated into and anchored around the rotor via the peptidoglycan (PG) binding domain of the B subunit (MotBC). However, no direct evidence of PG binding coupled with channel activation has been presented. Here, we report the structural rearrangements of MotBC responsible for this coupling process. A MotBC fragment with the L119P replacement, which is known to cause channel activation, was able to bind PG. Nuclear magnetic resonance analysis of MotBC and the crystal structure of the MotBC-L119P dimer revealed major structural changes in helix α1. In vivo crosslinking results confirm that a major rearrangement occurs. Our results suggest that, upon stator incorporation into the motor, helix α1 of MotBC changes into an extended non-helical structure. We propose that this change allows the stator both to bind PG and to open its proton channel.