Long-range coupling regulates stator dynamics in the bacterial flagellar motor

The bacterial flagellar motor is driven by stator complexes that couple ion flux to torque generation. Active stators dynamically exchange with a membrane pool in a load-dependent manner, with off-rates decreasing as motor load increases. Each stator comprises of MotA, which engages the rotor, and MotB, whose periplasmic domain anchors the complex to the peptidoglycan. But how external load regulates this anchoring remains unclear. Here, we show that long-range couplings within the Escherichia coli MotB periplasmic domain regulate stator dynamics. Computational modeling revealed that the flexible loops believed to anchor the stator to the peptidoglycan and P-ring are dynamically coupled to distant residues. Coevolutionary analysis reinforced these couplings, highlighting conserved communication pathways within the domain. Guided by these predictions, we introduced point mutations at key sites and assayed motility of cells harboring these mutations. Most mutants remained motile but displayed distinct swimming phenotypes. In agreement with computational predictions, measurements of swimming speed at different stator expression levels showed that several mutations altered stator dynamics. Finally, molecular dynamics simulations revealed that variation in dynamic flexibility of the loops strongly correlates with the observed swimming speeds in vivo. Together, these results demonstrate that flexibility and long-range coupling within MotB tune stator anchoring, providing new insight into mechanosensitive remodeling of the flagellar motor.