Probing the function and structure of SwrD, a potential flagellar motor power enhancer

The bacterial flagellar motor is a dynamic molecular machine composed of at least 70 proteins. The basal body of the motor plays critical roles in torque generation, force transduction, directional rotation, and export of flagella filament components. Active stator complexes convert proton-motive force into rotation of the filament and proteins that aide with in stator formation and/or stabilization are now being investigated. SwrD/FlbD is a small (8 kDa) protein conserved among several classes of bacteria, including spirochetes and firmicutes, that currently have no known structure or function, but are coded within flagellar operons. Motility experiments have suggested that SwrD increases the torque of the flagellar motor in Bacillus subtilis, potentially through influencing the stability of the stator complex. Herein, we probe the function and structure of SwrD from Bacillus subtills and Borrelia burgdorferi with high-resolution crystal-structures that yield a novel fold that shares a remarkable similarity to the SM family of RNA chaperones. We also provide evidence that SwrD is a highly tunable scaffold.