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 (MotB_C). However, no direct evidence of PG binding coupled with channel activation has been presented. Here, we report the structural rearrangements of MotB_C responsible for this coupling process. A MotB_C fragment with the L119P replacement, which is known to cause channel activation, was able to bind PG. Nuclear magnetic resonance analysis of MotB_C and the crystal structure of the MotB_C-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 MotB_C 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.

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